Statutes, codes, and regulations
Delaware Administrative Code
Title 7 - Natural Resources and Environmental ControlDivision of Watershed Stewardship
Sediment and Stormwater Regulations
Section 5101-11.0 - Post Construction BMP Standards and Specifications

7 Del. Admin. Code § 5101-11.0

Download
PDF
Current through Reigster Vol. 28, No. 6, December 1, 2024
Section 5101-11.0 - Post Construction BMP Standards and Specifications
11.1 Infiltration Practices
11.1.1 Infiltration practices are defined as practices that capture and temporarily store the design storm volume before allowing it to infiltrate into the soil over a two day period. Design variants include:
11.1.1.1 Infiltration Trench;
11.1.1.2 Infiltration Basin; and
11.1.1.3 Underground Infiltration.
11.1.2 Infiltration practices receive 100% retention volume credit (Rv) for the volume stored and infiltrated by the practice.
11.1.2.1 The RPv runoff reduction performance credit for stormwater infiltration is 100% of the retention storage.
11.1.2.2 The Cv runoff reduction performance credit for stormwater infiltration is 100% of the retention storage.
11.1.2.3 The Fv runoff reduction performance credit for stormwater infiltration is 100% of the retention storage.
11.1.2.4 The total nitrogen pollutant reduction performance credit for stormwater infiltration is 100% of the load reduction.
11.1.2.5 The total phosphorus pollutant reduction performance credit for stormwater infiltration is 100% of the load reduction.
11.1.2.6 The total suspended solids pollutant reduction performance credit for stormwater infiltration is 100% of the load reduction.
11.1.3 Infiltration Feasibility Criteria
11.1.3.1 Infiltration practices shall be located a minimum horizontal distance of 200 feet from down-gradient slopes greater than 20% unless slope stability calculations demonstrate stable conditions.
11.1.3.2 A minimum vertical distance of two feet must be provided between the bottom of the infiltration practice and the seasonal high water table as determined by subsection 12.1 or bedrock layer. The minimum vertical distance of two feet may be relaxed if a groundwater mounding analysis or piezometer testing has been performed by a qualified professional.
11.1.3.3 Native soils, in proposed infiltration areas, must have a minimum infiltration rate of one inch per hour. Designers must verify soil permeability by using the on-site soil investigation methods provided in subsection 12.1.
11.1.4 Infiltration Conveyance Criteria
11.1.4.1 Infiltration practices must be designed to pass the maximum design storm event (Fv) if the Fv is being routed through the practice rather than bypassing. An earthen emergency spillway designed to convey the Fv shall be cut in natural ground or, if cut in fill, shall be constructed and stabilized with methods to prevent erosion and structural failure.
11.1.4.2 Infiltration basins constructed to meet regulatory stormwater management requirements in the State of Delaware shall be designed and constructed in accordance with the USDA NRCS Pond Code 378, as amended.
11.1.5 Infiltration Pretreatment Criteria
11.1.5.1 Every inlet into an infiltration system shall have pretreatment.
11.1.5.2 Exit velocities from the pretreatment shall be non-erosive during the largest design storm that is routed through the facility.
11.1.6 Infiltration Design Criteria
11.1.6.1 Infiltration basin side-slopes shall be no steeper than 4H:1V.
11.1.6.2 Stone, when used for infiltration trenches or underground infiltration systems, shall consist of clean, washed aggregate with a maximum of 2.0% passing the #200 sieve. Stone shall have a maximum diameter of 2.5 inches and a minimum diameter of 0.5 inches. A porosity value of 0.4 shall be used in the design of stone reservoirs, although a larger value may be used if underground retention chambers are installed within the reservoir.
11.1.6.3 Infiltration trenches and underground infiltration practices shall include an inspection port to facilitate periodic inspection and maintenance.
11.1.6.4 Geotextile fabric, when used to separate stone from native soil, in an infiltration trench or underground infiltration system shall have a flow rate of 110 gallons per minute per square foot (gal/min/sf) or greater.
11.1.6.5 For design purposes, the field verified infiltration rate shall have a factor of safety applied in accordance with subsection 12.1 to account for potential compaction during construction and to approximate long term infiltration rates.
11.1.6.6 Infiltration practices shall be designed so that the RPv infiltrates within 48 hours.
11.1.6.7 Infiltration practices shall be designed so that they will:
11.1.6.7.1 Infiltrate the Fv within 72 hours, or
11.1.6.7.2 Dewater the Fv within 72 hours, or
11.1.6.7.3 Manage the Fv on site with no adverse impact.
11.1.6.8 All Infiltration practices must be designed so as to be accessible for maintenance.
11.1.6.8.1 A maintenance right-of-way or easement must extend to the Infiltration practice from a public or private road.
11.1.6.8.2 Adequate maintenance access must extend to the perimeter of the Infiltration practice and outlet structure, if applicable.
11.1.6.8.3 Maintenance access must meet the following criteria:
11.1.6.8.3.1 Minimum width of fifteen feet;
11.1.6.8.3.2 Profile grade that does not exceed 10H:1V; and
11.1.6.8.3.3 Minimum 10H:1V cross slope.
11.1.7 Infiltration Construction Criteria
11.1.7.1 During site construction, steps shall be taken to prevent compaction and sedimentation of the infiltration practice unless extensive design and construction methods are employed to protect the infiltration practices' ability to infiltrate.
11.1.7.2 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.1.7.2.1 Pre-construction meeting.
11.1.7.2.2 Initial site preparation including installation of erosion and sediment controls and sensitive area protection surrounding infiltration practice locations.
11.1.7.2.3 Construction of the embankment, including installation of the principal spillway and the outlet structure. as applicable for infiltration basins.
11.1.7.2.4 Excavation and grading including interim and final elevations. Confirmatory infiltration testing and verification must be completed prior to stone placement for infiltration trenches and underground infiltration.
11.1.7.2.5 Implementation of required stabilization.
11.1.7.2.6 Final construction review including development of a punch list for facility acceptance.
11.1.7.3 The infiltration rate and separation from groundwater of the constructed infiltration practice shall be verified prior to completion of construction in accordance withsubsection 12.1 . The results shall be included with the Post Construction Verification Documentation upon project completion.
11.1.7.4 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the infiltration practice has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. Allowable tolerances for infiltration practices are as follows:
11.1.7.4.1 The constructed top of bank elevation may be no lower than the design elevation for top of bank.
11.1.7.4.2 The constructed area of the infiltrating surface shall be no less than 90% of the design surface area.
11.1.7.4.3 The constructed volume of the infiltration practice surface storage shall be no less than 90% of the design volume.
11.1.7.4.4 The constructed elevation of any structure shall be within 0.15 foot of the design.
11.1.8 The infiltrating surface shall never be covered by an impermeable material, such as asphalt or concrete.
11.2 Bioretention
11.2.1 Bioretention is defined as practices that capture and store stormwater runoff and pass it through a bed of engineered soil media comprised of sand, lignin and organic matter, known as biosoil. Filtered runoff may be collected and returned to the conveyance system, or allowed to infiltrate into the soil. Design variants include:
11.2.1.1 Traditional Bioretention;
11.2.1.2 In-Situ Bioretention including Rain Gardens;
11.2.1.3 Streetscape Bioretention;
11.2.1.4 Engineered Tree Boxes;
11.2.1.5 Stormwater Planters; and
11.2.1.6 Advanced Bioretention systems.
11.2.2 Bioretention Stormwater Credit Calculations
11.2.2.1 The volume management credit for bioretention practices shall be based upon the volume of runoff that is either slowly released or infiltrated from the practice.
11.2.2.2 Advanced Bioretention Systems shall be evaluated on a case-by-case basis and assigned performance credits as deemed appropriate by the Department.
11.2.2.3 Bioretention with underdrain
11.2.2.3.1 The RPv, Cv and Fv runoff reduction performance credits for bioretention with underdrain is 100% of the detention storage.
11.2.2.3.2 The total nitrogen pollutant reduction performance credit for bioretention with underdrain is not less than 30% removal efficiency.
11.2.2.3.3 The total phosphorus pollutant reduction performance credit for bioretention with underdrain is not less than 40% removal efficiency.
11.2.2.3.4 The total suspended solids pollutant reduction performance credit for bioretention with underdrain is not less than 80% removal efficiency.
11.2.2.4 Bioretention with infiltration
11.2.2.4.1 The RPv, Cv and Fv runoff reduction performance credits for bioretention with infiltration is 100% of the retention storage.
11.2.2.4.2 The total nitrogen, total phosphorous and total suspended solids pollutant reduction performance credits for bioretention with infiltration is 100% of the load reduction.
11.2.3 Bioretention Feasibility Criteria
11.2.3.1 A minimum vertical distance of two feet must be provided between the bottom of the infiltrating bioretention practice and the seasonal high water table as determined by the procedures in subsection 12.1 or bedrock layer. The minimum vertical distance of two feet may be relaxed if a groundwater mounding analysis or piezometer testing has been performed by a qualified professional.
11.2.3.2 Underdrains are required if the permeability of the underlying soils does not have a minimum field-verified infiltration rate of one inch per hour.
11.2.3.3 An impermeable bottom liner and an underdrain system must be employed when a bioretention facility will receive untreated hotspot runoff.
11.2.3.4 Infiltrating bioretention practices shall be located a minimum horizontal distance of 200 feet from down-gradient slopes greater than 20% unless slope stability calculations demonstrate stable conditions.
11.2.4 Bioretention Conveyance Criteria
11.2.4.1 Bioretention practices must be designed to pass the maximum design storm event (Fv) if the Fv is being routed through the practice rather than bypassing. An earthen emergency spillway designed to convey the Fv shall be cut in natural ground or, if cut in fill, shall be constructed and stabilized with methods to prevent erosion and structural failure.
11.2.4.2 Bioretention practices constructed to meet regulatory stormwater management requirements in the State of Delaware shall be designed and constructed in accordance with the USDA NRCS Pond Code 378 as amended.
11.2.4.3 An overflow structure shall be incorporated into on-line designs to safely convey larger storms through the bioretention facility.
11.2.4.4 The maximum design discharge velocity shall be checked for a non-erosive condition at the outlet point. Outlet protection shall be provided as necessary.
11.2.5 Bioretention Pretreatment Criteria
11.2.5.1 Every inlet into a bioretention practice shall have pretreatment.
11.2.5.2 Exit velocities from the pretreatment shall be non-erosive during the largest design storm that is routed through the facility.
11.2.6 Bioretention Design Criteria
11.2.6.1 Bioretention practices shall have energy dissipation provided at all inlets.
11.2.6.2 Bioretention practices shall be designed so that the RPv either infiltrates or discharges within 48 hours.
11.2.6.3 Bioretention practices shall be designed so that they will:
11.2.6.3.1 Infiltrate the Fv within 72 hours, or
11.2.6.3.2 Dewater the Fv within 72 hours, or
11.2.6.3.3 Manage the Fv on site with no adverse impact.
11.2.6.4 Traditional and advanced bioretention facilities and rain gardens shall be constructed with side slopes above biosoil media of 3:1 or flatter.
11.2.6.5 Biosoil Media
11.2.6.5.1 The Biosoil-14 soil mixture shall have the following volumetric composition:
11.2.6.5.1.1 60% coarse concrete sand having a Fineness Modulus > 2.75
11.2.6.5.1.2 30% triple shredded hardwood mulch
11.2.6.5.1.3 10% aged, STA certified compost, meeting the requirements of Delaware Erosion and Sediment Control Handbook Appendix A-6 Compost Material Properties.
11.2.6.5.2 Biosoil media must be obtained from a Department approved vendor.
11.2.6.5.3 The design permeability rate for biosoil media shall be 2.83 inches per hour.
11.2.6.5.4 The biosoil media bed depth shall be a minimum of 24 inches for traditional bioretention and advanced bioretention systems.
11.2.6.5.5 Gravel layers used for extending the bioretention facility into a more permeable layer shall meet the same requirements as those for an underdrain design.
11.2.6.6 A surface cover shall be provided over the biosoil media.
11.2.6.6.1 Mulch, if used as a surface cover, shall be triple shredded hardwood aged for a minimum of six months.
11.2.6.6.2 Use of alternative surface cover shall be shown on the approved plan.
11.2.6.7 Underdrains
11.2.6.7.1 For bioretention designs that require an underdrain, the underdrain shall be a minimum of 4-inch perforated corrugated polyethylene pipe (CPP).
11.2.6.7.2 The underdrain shall be encased in a layer of clean, washed nominal 1/4 inch gravel with a maximum of 2.0% passing the #200 sieve with a minimum of 3" of cover.
11.2.6.7.3 The gravel layer in traditional bioretention shall be extended a minimum of 2' below the invert of the underdrain.
11.2.6.8 All traditional and advanced bioretention systems shall include at least one inspection port or cleanout pipe.
11.2.6.9 Traditional sizing approaches using design volume considering void ratio of the stone and biosoil media shall be used when sizing bioretention.
11.2.6.10 All Bioretention practices must be designed so as to be accessible for maintenance.
11.2.6.10.1 A maintenance right-of-way or easement must extend to the Bioretention practice from a public or private road.
11.2.6.10.2 Adequate maintenance access must extend to the perimeter of the bioretention practice and outlet structure.
11.2.6.10.3 Maintenance access must meet the following criteria:
11.2.6.10.3.1 Minimum width of fifteen feet.
11.2.6.10.3.2 Profile grade that does not exceed 10H:1V.
11.2.6.10.3.3 Minimum 10H:1V cross slope.
11.2.7 Bioretention Landscaping Criteria
11.2.7.1 A planting plan shall be provided for all bioretention facilities. Minimum elements of a planting plan include the following:
11.2.7.1.1 The proposed bioretention template to be used;
11.2.7.1.2 Delineation of planting areas;
11.2.7.1.3 Size and spacing of plant material; and
11.2.7.1.4 The planting sequence, including post-nursery care and initial maintenance requirements.
11.2.7.2 Planting plans must be certified by a qualified professional.
11.2.8 Bioretention Construction Criteria
11.2.8.1 When a bioretention system is used as a sediment trap or basin during construction, the Sediment & Stormwater Plan must include notes and graphic details specifying that:
11.2.8.1.1 The maximum excavation depth of the trap or basin at the construction stage must be at least one foot higher than the final invert or bottom of the facility, and
11.2.8.1.2 The bottom of the facility shall be ripped, tilled or otherwise scarified upon final excavation.
11.2.8.2 The plan shall include the proper procedures for converting the temporary sediment control practice to a permanent bioretention facility, including dewatering, cleanout and stabilization.
11.2.8.3 For infiltrating bioretention systems, confirmatory infiltration testing and verification must be completed prior to completion of construction in accordance with subsection 12.1 . The results shall be included with the Post Construction Verification Documentation upon project completion.
11.2.8.4 The final bottom elevation of any bioretention facility shall not be traversed by construction equipment.
11.2.8.5 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.2.8.5.1 Pre-construction meeting.
11.2.8.5.2 Initial site preparation including installation of erosion and sediment controls, sensitive area protection surrounding bioretention locations, and blockage of inlets to bioretention locations
11.2.8.5.3 Excavation and grading including interim and final elevations. For infiltrating bioretention systems, confirmatory infiltration testing and a verification must be completed prior to gravel and biosoil media placement.
11.2.8.5.4 Construction of the underdrain, including inspection ports and installation of the overflow structure, as applicable.
11.2.8.5.5 Installation of gravel and biosoil media.
11.2.8.5.6 Implementation of required stabilization and planting plan.
11.2.8.5.7 Final construction review including development of a punch list for facility acceptance.
11.2.8.6 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the bioretention practice has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. Allowable tolerances for bioretention practices are as follows:
11.2.8.6.1 The constructed top of bank elevation may be no lower than the design elevation for top of bank.
11.2.8.6.2 The constructed area of the bioretention surface shall be no less than 90% of the design surface area.
11.2.8.6.3 The constructed volume of the bioretention storage shall be no less than 90% of the design volume.
11.2.8.6.4 The constructed elevation of any structure shall be within 0.15 foot of the design.
11.2.9 Supplemental fertilizer applications shall consist of a 0% phosphorus formulation only as needed to maintain plant vigor.
11.3 Permeable Pavement Systems
11.3.1 Permeable Pavement Systems are defined as paving surfaces that capture and temporarily store stormwater by filtering runoff through voids in the pavement surface into an underlying reservoir. Design variants include:
11.3.1.1 Porous Asphalt (PA)
11.3.1.2 Pervious Concrete (PC)
11.3.1.3 Permeable interlocking concrete Pavers (PP) or Concrete grid Pavers (CP)
11.3.1.4 Plastic Grid Pavers (GP)
11.3.2 Permeable Pavement Systems receive 100% retention volume credit (Rv) for the volume stored and infiltrated by the practice.
11.3.2.1 The RPv runoff reduction performance credit for permeable pavement is 100% of the retention storage.
11.3.2.2 The Cv runoff reduction performance credit for permeable pavement is 100% of the retention storage.
11.3.2.3 The Fv runoff reduction performance credit for permeable pavement is 100% of the retention storage.
11.3.2.4 The total nitrogen pollutant reduction performance credit for permeable pavement is 100% of the load reduction.
11.3.2.5 The total phosphorus pollutant reduction performance credit for permeable pavement is 100% of the load reduction.
11.3.2.6 The total suspended solids pollutant reduction performance credit for permeable pavement is 100% of the load reduction.
11.3.3 Permeable Pavement Feasibility Criteria
11.3.3.1 Drainage Area. For parking lots and roadways configurations:
11.3.3.1.1 The contributing drainage area to permeable pavement shall not exceed five times the surface area of the permeable pavement.
11.3.3.1.2 Pervious areas shall be diverted from the permeable pavement area such that the total contributing drainage area is at least 80% impervious.
11.3.3.2 Soils and Overdrains. Parking lot and roadway configurations utilizing permeable pavement shall meet the following requirements:
11.3.3.2.1 Infiltration testing in accordance with subsection 12.1 shall be required.
11.3.3.2.2 Overdrains are required if the permeability of the underlying soils does not have a minimum infiltration rate of one inch per hour.
11.3.3.3 The surface slope shall be no greater than 5.0%.
11.3.3.4 The bottom slope of a permeable pavement installation shall be no greater than 1.0%.
11.3.3.5 If an overdrain is not provided, a separation distance of two feet is required between the bottom of the reservoir layer and the seasonal high water table as determined in accordance with subsection 12.1.
11.3.3.6 Permeable pavements shall not be used to treat hotspot runoff.
11.3.4 Permeable pavement designs shall include methods to safely convey the Cv and Fv.
11.3.5 Permeable Pavement Design Criteria
11.3.5.1 Permeable pavement shall be designed according to DelDOT specifications or the product manufacturer's recommendations as applicable.
11.3.5.2 Internal Geometry and Drawdown
11.3.5.2.1 For design purposes, the field verified infiltration rate shall have a factor of safety applied in accordance with subsection 12.1 to account for potential compaction during construction and to approximate long term infiltration rates.
11.3.5.2.2 Permeable pavement practices shall be designed so that the RPv infiltrates within 48 hours.
11.3.5.2.3 Permeable pavement practices shall be designed so that they will:
11.3.5.2.3.1 Infiltrate the Fv within 72 hours, or
11.3.5.2.3.2 Dewater the Fv within 72 hours, or
11.3.5.2.3.3 Manage the Fv on site with no adverse impact.
11.3.5.3 Reservoir layer
11.3.5.3.1 The suitability of the soil subgrade shall be determined by a qualified geotechnical engineer.
11.3.5.3.2 The reservoir layer shall be composed of clean, washed gravel with a maximum of 2.0% passing the #200 sieve and sized for both the maximum storm event to be managed and the structural requirements of the expected traffic loading.
11.3.5.3.3 The depth of the reservoir layer shall be a minimum of six inches.
11.3.5.3.4 If an overdrain is not provided, a separation distance of two feet is required between the bottom of the reservoir layer and the seasonal high water table as determined in accordance with subsection 12.1.
11.3.5.3.5 Overdrains shall be a minimum of four inches in diameter.
11.3.5.4 All parking lot and roadway configurations shall include inspection ports.
11.3.5.5 The permeable pavement shall be designed to support the maximum anticipated traffic load.
11.3.5.6 The reservoir layer shall be sized to temporarily store and then infiltrate the RPv.
11.3.6 Permeable Pavement Construction
11.3.6.1 All permeable pavement areas shall be fully protected from sediment intrusion by silt fence or construction fencing to prevent construction traffic tracking.
11.3.6.2 During site construction, steps shall be taken to prevent compaction of the underlying soil and sedimentation of the permeable pavement practice.
11.3.6.3 The infiltration rate and separation from groundwater of the constructed permeable pavement practice shall be verified prior to completion of construction in accordance with subsection 12.1 . The results shall be included with the Post Construction Verification Documentation upon project completion.
11.3.6.4 During construction, care shall be taken to avoid tracking sediments onto any permeable pavement surface to avoid clogging.
11.3.6.5 When locating a sediment basin on an area intended for permeable pavement is unavoidable, the invert of the sediment basin must be a minimum of one foot above the final design elevation of the bottom of the reservoir course.
11.3.6.6 Permeable pavement shall be installed according to DelDOT specifications or the product manufacturer's recommendations as applicable.
11.3.6.7 Construction of the permeable pavement shall only begin after the entire contributing drainage area has been stabilized.
11.3.6.8 The proposed permeable pavement area shall be kept free from sediment during the entire construction process.
11.3.6.9 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.3.6.9.1 Pre-construction meeting;
11.3.6.9.2 Initial site preparation including installation of erosion and sediment controls, sensitive area protection surrounding permeable pavement locations;
11.3.6.9.3 Excavation and grading including interim and final elevations. Observation of infiltrating surface and permeable pavement practice verification must be completed prior to gravel placement;
11.3.6.9.4 Construction of the overdrain, including inspection ports and installation of the overflow structure, as applicable;
11.3.6.9.5 Installation of gravel;
11.3.6.9.6 Implementation of required stabilization; and
11.3.6.9.7 Final construction review including development of a punch list for facility acceptance.
11.3.6.10 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the permeable pavement has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. Allowable tolerances for permeable pavement are as follows:
11.3.6.10.1 The constructed permeable pavement surface area shall be no less than the design permeable pavement surface area.
11.3.6.10.2 The contributing drainage area as constructed shall be no greater than the design contributing drainage area.
11.3.6.10.3 The constructed storage volume of the reservoir layer shall be no less than 90% of the design volume.
11.3.6.10.4 The constructed elevation of the overdrain or any structure shall be within 0.15 foot of the design.
11.3.6.11 In the event that the allowable tolerances are exceeded for permeable pavement surface area or volume or structure elevations, supplemental calculations and provisions of adequate maintenance must be submitted to the approval agency to determine if the permeable pavement, as constructed, meets the design requirements.
11.3.7 Activities that have the potential to clog the permeable pavement surface, including but not limited to sanding, re-sealing, re-surfacing, storage of snow piles containing sand, storage of mulch or soil material, or construction staging, shall be prohibited.
11.4 Vegetated Roofs
11.4.1 Vegetated Roofs are defined as practices on top of buildings that capture and store rainfall in an engineered growing media, which is designed to support plant growth. A portion of the captured rainfall evaporates or is taken up by plants, which helps reduce runoff volumes, peak runoff rates, and pollutant loads on development sites. Design variants include:
11.4.1.1 Extensive Vegetated Roofs which contain shallow growth media with drought resistant plants, such as Sedum.
11.4.1.2 Intensive Vegetated Roofs which contain deep growth media with a wide range of plant varieties and typically include irrigation.
11.4.2 Vegetated Roofs receive annual runoff reduction credit (RR) for the contributing roof area, along with associated pollutant removals as follows:
11.4.2.1 The RPv runoff reduction performance credit for Extensive Vegetated Roof is 50% annual runoff reduction.
11.4.2.2 The Cv runoff reduction performance credit for Extensive Vegetated Roof is 5% of the RPv allowance.
11.4.2.3 The Fv runoff reduction performance credit for Extensive Vegetated Roof is 1% of the RPv allowance.
11.4.2.4 The RPv runoff reduction performance credit for Intensive Vegetated Roof is 75% annual runoff reduction.
11.4.2.5 The Cv runoff reduction performance credit for Intensive Vegetated Roof is 8% of the RPv allowance.
11.4.2.6 The Fv runoff reduction performance credit for Intensive Vegetated Roof is 2% of the RPv allowance.
11.4.2.7 The total nitrogen pollutant reduction performance credit for Extensive Vegetated Roof and Intensive Vegetated Roof is not less than 0%.
11.4.2.8 The total phosphorus pollutant reduction performance credit for Extensive Vegetated Roof and Intensive Vegetated Roof is not less than 0%.
11.4.2.9 If the phosphorous (P) content of mature growth media is 2.0 mg/l (Saturated Paste Extraction), or less, then the Vegetated Roof will be assumed to be neutral with respect to P loadings. If the P content of mature growth media exceeds 2.0 mg/l, a supplemental phosphorus-reducing BMP, such as an activated alumina or hematite filter, will be required.
11.4.3 Vegetated Roof Feasibility Criteria
11.4.3.1 The designer shall demonstrate that the building will be able to support the additional live and dead structural load. Structural capability of the roof must be assessed by a qualified licensed professional and included with building permit documentation.
11.4.3.2 Safe access to the Vegetated Roof shall be available to allow for delivery of construction materials and performance of routine maintenance reviews and maintenance operations.
11.4.3.3 A permanent source of water shall be provided to all Vegetated Roof areas.
11.4.3.4 A minimum one foot wide vegetation-free zone is required along the perimeter of all Vegetated Roofs and around all roof penetrations.
11.4.3.5 The Vegetated Roof design must comply with all federal, state and local building codes.
11.4.4 Vegetated Roof Conveyance Criteria
11.4.4.1 The Vegetated Roof drainage layer shall convey flow from under the growth media layer to an outlet or overflow system.
11.4.4.2 All drains and scuppers shall be accessible through enclosures that include lids that are level with the surface of the growth media layer.
11.4.4.3 Emergency drains or emergency scuppers shall have inverts that are high enough above the waterproofing surface to prevent discharge during the RPv event.
11.4.5 Vegetated Roof Design Criteria
11.4.5.1 All Vegetated Roof systems must include an effective and reliable waterproof membrane to prevent water damage to the building structure.
11.4.5.2 Protection Layer shall have a puncture resistance in accordance with ASTM D4833 >220 lbs.
11.4.5.3 Waterproof membranes designed for burial in Vegetated Roofs do not require supplemental root barriers; however root barriers are required in combination with some waterproof membranes.
11.4.5.3.1 Chemical root barriers or physical root barriers that have been impregnated with pesticides, metals or other chemicals shall not be used as those chemicals can leach into stormwater runoff and will slowly lose effectiveness over time.
11.4.5.3.2 To insure that a building is adequately protected against damage from roots, the waterproof membrane or the root-barrier shall be certified as root-resistant based on the two-year ANSI/SPRI VR-1; Procedure for Investigating the Root Penetration Resistance of Vegetated Roofs or the two-year European FLL root-security test (Forschungsgesellschaft Landschaftsentwicklung Landschaftsbau e.V., Richlinien fuer die Planung, Ausfuehrung und Pflege von Dachbegruenung). If certification is not available at the time the system is permitted, an HDPE membrane, 30 mil or thicker, with seams overlapped at least three inches and continuously hot-air welded shall be provided as a root barrier.
11.4.5.4 A drainage layer shall underlie the growth media to control the release of water that percolates into the Vegetated Roof assembly.
11.4.5.5 Roof drains and any emergency overflow shall be designed in accordance with state and local building codes.
11.4.5.6 Growth Media
11.4.5.6.1 The growth media shall be the uppermost layer in a Vegetated Roof assembly.
11.4.5.6.2 The designer shall certify that growth media meets or exceeds all required specifications prior to placement.
11.4.5.6.3 The growth media must maintain minimum 6.0% air filled porosity (ASTM E2399) to avoid anoxic conditions when wetted.
11.4.5.6.4 The upper one inch of a growth media profile shall consist of an engineered mineral soil. To minimize the potential for clogging of fabrics and migration of fine particles, the silt-size fraction (0.063mm) of engineered mineral soils shall not exceed 15% and the clay fraction (2 micron) shall not exceed 5.0%.
11.4.5.6.5 Organic amendments used in preparing growth media must be stable. The respiration rate of organic ingredients must be 3.0 mg CO2-C/g OM/day (TMECC 05.08.B), or less.
11.4.5.6.6 For a Vegetated Roof to be regarded as neutral with respect to phosphorous (P) loading in runoff, the P content of the growth media must be controlled.
11.4.5.6.6.1 The initial available P content of the growth media may not exceed 200 ppm dry weight (Mehlich III).
11.4.5.6.6.2 Chemical fertilizers containing phosphorous may not be added to the growth media during blending or used subsequently during maintenance unless a phosphorus deficiency in the plants has been documented. Only nitrogen fertilizers may be used and these must be applied according to soil test. If the P limit for the media layer cannot meet compliance, the Vegetated Roof must be treated as a P source and a supplemental water quality BMP must be introduced.
11.4.5.6.7 If trees are included in the Vegetated Roof landscape plan, the growth media must be at least 30 inches deep.
11.4.5.7 Plant cover
11.4.5.7.1 Initial planting plan shall be designed such that mature plant coverage within 24 months of initial planting shall be:
11.4.5.7.1.1 Minimum 75% warm season plant coverage for Extensive Vegetated Roofs.
11.4.5.7.1.2 Minimum 90% plant coverage for turf intensive Vegetated Roofs.
11.4.5.7.1.3 Uniform cover with vigorous plants conforming to the design plant density for non-turf Intensive Vegetated Roofs.
11.4.5.7.2 The planting plan for Vegetated Roofs must be certified by a qualified professional.
11.4.5.8 Vegetated Roof sizing
11.4.5.8.1 The size of the Vegetated Roof, both Extensive and Intensive, shall be a minimum 66% of the total contributing drainage area.
11.4.5.8.2 No runoff reduction credit shall be given for runoff from bare areas of the roof that do not come in contact with root zone.
11.4.6 Vegetated Roof Landscaping Criteria
11.4.6.1 Irrigation is required for Vegetated Roofs for the first year after planting.
11.4.6.2 The minimum plant coverage shall be achieved 24 months after initial planting and maintained throughout the life of the Vegetated Roof.
11.4.7 Vegetated Roof Construction
11.4.7.1 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.4.7.1.1 Pre-construction meeting;
11.4.7.1.2 During placement of the waterproof membrane, to ensure that it is properly installed and watertight;
11.4.7.1.3 During placement of the drainage layer and drainage system, to prevent future ponding water;
11.4.7.1.4 During placement of the growing media, to confirm that it meets the approved plan;
11.4.7.1.5 Upon installation of plants, to ensure they conform to the planting plan; and
11.4.7.1.6 Final construction review including development of a punch list for facility acceptance.
11.4.7.2 Post Construction Verification Documentation
11.4.7.2.1 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the Vegetated Roof has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. Allowable tolerances for Vegetated Roofs are as follows:
11.4.7.2.1.1 Growth media thickness within 15% of design thickness.
11.4.7.2.1.2 Plant density no less than specified on the planting plan.
11.4.7.2.1.3 No less than 66% of the total contributing drainage area shall be Vegetated Roof.
11.4.7.2.2 The post construction verification shall confirm that temporary or permanent irrigation has been installed in accordance with the approved plan.
11.4.7.2.3 Certification of growth media shall be submitted with post construction verification.
11.4.8 Vegetated Roof Maintenance Criteria
11.4.8.1 Maintenance Reviews
11.4.8.1.1 A minimum of one maintenance review is required each year for Vegetated Roofs.
11.4.8.1.2 Maintenance reviews shall be performed by a qualified reviewer.
11.4.8.1.3 The completed maintenance review report shall be sent to the Department or the appropriate Delegated Agency.
11.4.8.1.4 Vegetated Roofs must be reviewed at the end of the first 24 months to confirm minimum vegetated surface cover specified in the Operation and Maintenance Plan has been achieved, and to look for leaks, drainage problems and any rooftop structural concerns.
11.4.8.2 Growth media shall be routinely tested for P as part of the Operation and Maintenance Plan. Phosphorous-removing BMPs (e.g., activated alumina or hematite filters) must be installed for Vegetated Roofs, 24 months and older, with phosphorous concentrations that are consistently higher than 2.0 mg/l.
11.5 Rainwater Harvesting
11.5.1 Rainwater harvesting is defined as systems that intercept, divert, store and release rainfall for future use. Rainwater that falls onto impervious surfaces is collected and conveyed into an above- or below-ground cistern, where it can be used for non-potable water uses and on-site stormwater disposal or infiltration. Runoff collected and temporarily stored in more traditional stormwater management practices constructed in accordance with the Post Construction Stormwater BMP Standards and Specifications, such as wet ponds, can also be used for irrigation purposes to achieve these same goals. Design variants include:
11.5.1.1 Seasonal Rainwater Harvesting Systems; and
11.5.1.2 Continuous Rainwater Harvesting Systems.
11.5.2 Rainwater Harvesting Stormwater Credit Calculations
11.5.2.1 If not protected from freezing, Rainwater Harvesting systems must be taken offline for the winter and credited as seasonal systems.
11.5.2.2 Seasonal Rainwater Harvesting Performance Credits
11.5.2.2.1 The RPv runoff reduction performance credit for Seasonal Rainwater Harvesting systems is 50% of the retention storage.
11.5.2.2.2 The Cv and Fv runoff reduction performance credit for Seasonal Rainwater Harvesting systems is 0%.
11.5.2.2.3 The total nitrogen, total phosphorus, and total suspended solids pollutant reduction performance credits for Seasonal Rainwater Harvesting is 100% of the load reduction.
11.5.2.3 Continuous Rainwater Harvesting Performance Credits
11.5.2.3.1 The RPv runoff reduction performance credit for Continuous Rainwater Harvesting systems is 75% of the retention storage.
11.5.2.3.2 The Cv and Fv runoff reduction performance credit for Continuous Rainwater Harvesting systems is 0%.
11.5.2.3.3 The total nitrogen, total phosphorus, and total suspended solids pollutant reduction performance credits for Continuous Rainwater Harvesting is 100% of the load reduction.
11.5.3 Rainwater Harvesting Feasibility Criteria
11.5.3.1 Designers and plan reviewers shall consult all applicable local, State and Federal regulations to determine the allowable indoor uses and required treatment for harvested rainwater.
11.5.3.2 Pipes and spigots using rainwater must be clearly labeled as non-potable.
11.5.3.3 The final invert of the outlet pipe from the cistern must be at an elevation that will not allow water from the discharge point to backflow into the cistern.
11.5.3.4 In areas where a below-ground cistern will be buried partially below the water table, buoyancy calculations must be conducted for the empty cistern and special design features must be employed, as applicable, to secure the cistern.
11.5.3.5 Cisterns must be installed according to the manufacturer's specifications.
11.5.3.6 The bearing capacity of the soil upon which the full cistern will be placed must be considered.
11.5.3.7 Cisterns shall be designed to be watertight.
11.5.4 Rainwater Harvesting Conveyance Criteria
11.5.4.1 All conveyance pipes to the cistern, including gutters and downspouts, must be kept clean and free of sediment, debris and rust.
11.5.4.2 Overflow
11.5.4.2.1 An overflow mechanism must be included in the Rainwater Harvesting system design to handle flows that exceed the capacity of the cistern.
11.5.4.2.2 Overflow pipes must have a capacity equal to or greater than the total capacity of the inflow pipes and have a diameter and slope sufficient to drain the cistern before it reaches full capacity.
11.5.4.2.3 The overflow pipe must be screened to prevent access to the cistern by rodents and birds.
11.5.5 Rainwater Harvesting Pretreatment Criteria
11.5.5.1 Pretreatment is required to keep sediment, leaves, and other debris from the system.
11.5.5.2 Small cistern systems of 2,500 gallons or less shall have leaf screens or gutter guards for pretreatment as a minimum.
11.5.5.3 Large cistern systems of greater than 2,500 gallons shall include a pretreatment system capable of treating and conveying the flow rate generated by the RPv from the contributing impervious surface drainage area without creating a backup or bypass condition.
11.5.6 Rainwater Harvesting Design Criteria
11.5.6.1 The following factors that must be considered when designing a Rainwater Harvesting system and selecting a cistern:
11.5.6.1.1 Rainwater Harvesting Systems shall comply with all applicable local, State, and Federal regulations.
11.5.6.1.2 Above-ground cisterns must be impact resistant or protected from impact using bollards or other physical barriers.
11.5.6.1.3 Below-ground cisterns must be designed to support the overlying soil and any other anticipated loads.
11.5.6.1.4 Below-ground cisterns must have a standard size manhole or equivalent opening to allow access for cleaning, inspection, maintenance, and repair purposes.
11.5.6.1.5 Cisterns must be screened to discourage mosquito breeding and reproduction.
11.5.6.1.6 A suitable foundation must be provided to support the cistern when it is filled to capacity.
11.5.6.1.7 Dead storage below the outlet to the distribution system and an air gap at the top of the cistern must be added to the total volume.
11.5.6.1.8 Any hookup to a municipal backup water supply must have a backflow prevention device to keep municipal water separate from stored rainwater.
11.5.6.2 Distribution Systems
11.5.6.2.1 The Rainwater Harvesting system must be equipped with an appropriately-sized pump, if necessary, that produces sufficient pressure for all intended end-uses.
11.5.6.2.2 A backflow preventer is required to separate harvested rainwater from the main potable water distribution lines.
11.5.6.2.3 Distribution lines for Continuous Rainwater Harvesting Systems must be buried beneath the frost line.
11.5.6.2.4 A drain plug or cleanout sump, also draining to a pervious area, must be installed to allow the system to be completely emptied, if needed.
11.5.6.2.5 Above-ground outdoor pipes must be insulated or heat-wrapped to prevent freezing and ensure uninterrupted operation during winter.
11.5.6.2.6 Distribution lines and above ground outdoor pipes for Seasonal Rainwater Harvesting Systems shall be drained or otherwise winter-proofed during the non-operational period.
11.5.6.3 Rainwater Harvesting Material Specifications
11.5.6.3.1 Cisterns must be structurally sound, watertight, and sealed using a water-safe, non-toxic material. Re-purposed tanks used to store rainwater for reuse must be acceptable for potable water or food-grade products.
11.5.6.3.2 Above-ground cisterns must be UV resistant and opaque to prevent the growth of algae in the tank.
11.5.6.3.3 Below-ground cisterns shall be located below the frost line.
11.5.6.4 Sizing of Rainwater Harvesting Systems
11.5.6.4.1 For seasonal rainwater harvesting systems, weekly irrigation demand shall be at least 50% of the stored volume.
11.5.6.4.2 For Continuous Rainwater Harvesting Systems, a minimum of 50% of the demand shall be met through non-irrigation needs, such as plumbing, process water, car washing, or other uses that are present throughout the year.
11.5.6.5 All Rainwater Harvesting Systems must be designed so as to be accessible for maintenance.
11.5.6.5.1 A maintenance right-of-way or easement must extend to the Rainwater Harvesting System from a public or private road.
11.5.6.5.2 Adequate maintenance access must extend to all components of the Rainwater Harvesting System.
11.5.6.5.3 Maintenance access must meet the following criteria:
11.5.6.5.3.1 Minimum width of fifteen feet.
11.5.6.5.3.2 Profile grade that does not exceed 10H:1V.
11.5.6.5.3.3 Minimum 10H:1V cross slope.
11.5.7 If the harvested rainwater is to be used for irrigation, the design plan must include the delineation of the proposed planting areas to be irrigated and quantification of the expected water demand based upon the area to be irrigated.
11.5.8 Rainwater Harvesting Construction
11.5.8.1 Rainwater Harvesting system components connecting to the internal plumbing system shall be installed by a licensed plumber.
11.5.8.2 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.5.8.2.1 Pre-construction meeting;
11.5.8.2.2 Initial site preparation including installation of erosion and sediment controls;
11.5.8.2.3 Excavation and grading including interim and final elevations for cistern foundations;
11.5.8.2.4 Installation of cistern, pretreatment system and conveyance system;
11.5.8.2.5 Implementation of required stabilization; and
11.5.8.2.6 Final construction review including development of a punch list for facility acceptance.
11.5.8.3 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the rainwater harvesting practice has been constructed in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. Items to be checked and verified are as follows:
11.5.8.3.1 Presence of a pretreatment device.
11.5.8.3.2 Capacity of any cisterns matches the design plan.
11.5.8.3.3 For ponds, the constructed volume shall be no less than 90% of the design volume.
11.5.8.3.4 For continuous systems, all pumps, controls, and other appurtenances installed in accordance with the plan.
11.5.8.3.5 For irrigation systems, area of coverage is within 90% of that shown on the plan.
11.5.9 Operation and Maintenance Plans shall clearly outline how Rainwater Harvesting Systems will be managed taking into account seasonal variations and cistern location.
11.6 Restoration Practices
11.6.1 Restoration Practices include Regenerative Stormwater Conveyance Systems (RSCS), also known as Coastal Plain Outfalls, and other practices that restore existing degraded natural systems to their former functional condition. Streambank stabilization is also included as a Restoration Practice.
11.6.2 Design variants for Restoration Practices include:
11.6.2.1 Step Pool RSCS
11.6.2.2 Seepage Wetland RSCS
11.6.2.3 Streambank Stabilization
11.6.3 Runoff reduction and pollutant reduction performance credits for Restoration Practices shall be determined by the Department on a case-by-case basis.
11.6.4 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the restoration practice has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency.
11.7 Rooftop Disconnection
11.7.1 Rooftop Disconnection involves managing runoff close to its source by intercepting, infiltrating, filtering, treating, or reusing it as it moves from the rooftop to the drainage system. Rooftop Disconnection can reduce the volume of runoff that enters the combined or separate storm sewer systems.
11.7.2 Rooftop Disconnection that meets the minimum full disconnection length shall receive the following annual runoff reduction and pollutant reduction credits:
11.7.2.1 Rooftop Disconnection receives 100% annual runoff reduction performance credit for the RPv.
11.7.2.2 The Cv runoff reduction performance credit for Rooftop Disconnection is 10% of the RPv allowance.
11.7.2.3 The Fv runoff reduction performance credit for Rooftop Disconnection is 1% of the RPv allowance.
11.7.2.4 The total nitrogen pollutant reduction performance credit for Rooftop Disconnection is 100% of the load reduction.
11.7.2.5 The total phosphorus pollutant reduction performance credit for Rooftop Disconnection is 100% of the load reduction.
11.7.2.6 The total suspended solids pollutant reduction performance credit for Rooftop Disconnection is 100% of the load reduction.
11.7.3 Partial RPv runoff reduction credit shall be based on the ratio of the disconnection length provided to the full disconnection length.
11.7.4 Rooftop Disconnection Feasibility Criteria
11.7.4.1 If being used for RPv credit in a residential subdivision, a Record Plan shall include a not identifying Rooftop Disconnection as a BMP.
11.7.4.2 The disconnection area shall have a maximum slope of 25%.
11.7.4.3 The maximum impervious rooftop area treated may not exceed 1,000 square feet per downspout.
11.7.4.4 Receiving area shall be graded away from the structure per local requirements.
11.7.5 Rooftop Disconnection areas shall be designed to safely convey all design storm events (Rpv, Cv, and Fv) over the receiving area without causing erosion.
11.7.6 A downspout energy dissipater shall be located at the discharge point of the downspout.
11.7.7 Rooftop Disconnection Design Criteria
11.7.7.1 Regardless of rooftop area collected the available pervious disconnection area at the point of discharge for any downspout must be at least 15 feet wide.
11.7.7.2 A five foot long transition section from the downspout point of discharge shall be provided prior to the beginning of the disconnection area.
11.7.7.3 The minimum full disconnection lengths shall be as follows:
11.7.7.3.1 For projects above the C&D Canal, the full disconnection length shall be 75 feet.
11.7.7.3.2 For projects below the C&D Canal, the full disconnection length shall be 60 feet.
11.7.7.4 Impervious areas shall not be constructed within the area designated as the pervious rooftop disconnection area.
11.7.7.5 The pervious rooftop disconnection area must be stabilized with vegetation for a non-erosive condition.
11.7.8 All pervious disconnection areas receiving rooftop runoff shall be vegetatively stabilized to prevent erosion or transport of sediment to receiving practices or drainage systems.
11.7.9 The post construction verification for Rooftop Disconnection shall visually verify that no impervious surface exists within the rooftop disconnection area.
11.7.10 The Sediment and Stormwater Plan shall include the following operation and maintenance notes for Rooftop Disconnection:
11.7.10.1 The rooftop disconnection area shall be maintained in a stabilized vegetative condition.
11.7.10.2 Ensure that downspouts remain disconnected and pervious filtering or infiltrating areas are not converted to impervious surface.
11.8 Vegetated Channels
11.8.1 Vegetated channels are open conveyances planted with grass or other suitable vegetation and having a shallow depth of flow to allow runoff to be filtered and recharged along the length of the channel. Design variants include:
11.8.1.1 Bioswale
11.8.1.2 Grassed Channel
11.8.2 Vegetated channels receive the following annual runoff reduction and pollutant reduction credits.
11.8.2.1 The RPv runoff reduction performance credit for a Bioswale on A/B soil or compost amended C soil is 50% annual runoff reduction.
11.8.2.2 The RPv runoff reduction performance credit for a Grassed Channel on A/B soil or compost amended C soil is 20% annual runoff reduction.
11.8.2.3 The RPv runoff reduction performance credit for a Bioswale on C/D soil is 25% annual runoff reduction.
11.8.2.4 The RPv runoff reduction performance credit for a Grassed Channel on C/D soil is 10% annual runoff reduction.
11.8.2.5 The Cv runoff reduction performance credit for Vegetated Channels is 10% of the RPv allowance.
11.8.2.6 The Fv runoff reduction performance credit for Vegetated Channels is 1% of the RPv allowance.
11.8.2.7 The total nitrogen pollutant reduction performance credit for Vegetated Channels is 100% of the load reduction.
11.8.2.8 The total phosphorous pollutant reduction performance credit for Vegetated Channels is 100% of the load reduction.
11.8.2.9 The total suspended solids pollutant reduction performance credit for Vegetated Channels is 100% of the load reduction.
11.8.3 Vegetated Channel Feasibility Criteria
11.8.3.1 The bottom of vegetated channels shall be above the seasonal high water table.
11.8.3.2 Approval from the applicable utility company or agency is required if utility lines will run below the vegetated channel.
11.8.4 Vegetated Channel Conveyance Criteria
11.8.4.1 The bottom width and slope of a vegetated channel shall be designed such that the flow depth based on 50% of RPv peak flow rate, does not exceed four inches.
11.8.4.2 Vegetated channels shall convey the Cv and Fv peak flow rate at non-erosive velocities for the soil and vegetative cover provided.
11.8.5 Every inlet into a vegetated channel system shall have pretreatment.
11.8.6 Vegetated Channel Design Criteria
11.8.6.1 Channel Geometry
11.8.6.1.1 The bottom width of a trapezoidal channel shall be a minimum of two feet wide to ensure that an adequate surface area exists along the bottom of the channel for filtering.
11.8.6.1.2 If a channel bottom will be wider than eight feet, benches, check dams, level spreaders, or multi-level cross sections shall be incorporated to prevent braiding and erosion along the channel bottom.
11.8.6.1.3 Vegetated channel side slopes shall be no steeper than 3H:1V.
11.8.6.2 Check Dams
11.8.6.2.1 Check dams must be firmly anchored into the side-slopes to prevent outflanking; check dams must also be anchored into the channel bottom to prevent hydrostatic head from pushing out the underlying soils.
11.8.6.2.2 Check dams must be designed to pass the Cv design storm peak flow.
11.8.6.2.3 Check dams shall be composed of wood, concrete, stone, or other non-erodible material.
11.8.6.2.4 Each check dam shall have a weep hole or similar drainage feature, to allow for dewatering following a storm event.
11.8.6.3 All seeded vegetated channels require a minimum SSM-III biodegradable erosion control matting conforming to Delaware Erosion and Sediment Control Handbook.
11.8.6.4 The soil amendments, when used, shall extend over the length and width of the channel bottom, and the compost shall be incorporated to the depth as shown on the approved plan.
11.8.6.5 Adequate conveyance and treatment capacity shall be provided in accordance with the following guidelines:
11.8.6.5.1 Hydraulic capacity shall be verified using Manning's equation or an accepted equivalent method, such as tractive forces and vegetal retardance.
11.8.6.5.2 Design storm flow depth based on 50% of RPv peak flow rate shall be maintained at four inches or less.
11.8.6.5.3 Manning's "n" value for vegetated channels shall be 0.2 for flow depths up to four inches, decreasing to 0.03 above four inches of flow depth. If alternative vegetation is used to increase the Manning's "n" value and decrease the resulting channel width, material specifications and construction oversight shall be provided to ensure that the denser vegetation is actually established.
11.8.6.5.4 Peak flow rates for the Cv and Fv storms shall be non-erosive.
11.8.6.5.5 The Cv peak flow rate shall be contained within the channel banks.
11.8.6.5.6 If the Fv storm event is not contained within the channel, the area of inundation shall be shown.
11.8.6.5.7 The total peak discharge at the outlet shall be used to calculate the depth of flow and velocity for the channel unless lateral flow along the channel is calculated incrementally.
11.8.6.5.8 Hydraulic residence time is the time for runoff to travel the full length of the channel. For both Bioswales and Grassed Channels hydraulic residence time is computed based upon 50% of the RPv peak flow rate.
11.8.6.5.8.1 For Bioswales, the hydraulic residence time shall be a minimum of nine minutes. If flow enters the channel at several locations, a nine minute minimum hydraulic residence time shall be demonstrated for each entry point.
11.8.6.5.8.2 For Bioswales, adjusted RPv runoff reduction credit based on the ratio of the computed residence to the minimum residence time shall be applied to Bioswales that meet the maximum depth of flow criteria. The maximum adjusted RPv runoff reduction credit is 75% for HSG A/B soils and 40% for HSG C/D soils. Adjusted RPv reduction credit shall not be granted for computed residence times of less than five minutes.
11.8.6.5.8.3 For Grassed Channels, the hydraulic residence time for concentrated flow entering the Grassed Channel shall be a minimum of five minutes.
11.8.6.5.8.4 Lateral flow entering the Grassed Channel as sheet flow may be excluded from residence time calculations but shall be accounted for in the channel depth and velocity calculations.
11.8.6.5.8.5 For Grassed Channels with in-line culverts, the proportion of grassed channel flow length shall be a minimum of 80% of the total flow length.
11.8.6.6 All Vegetated Channels must be designed so as to be accessible for maintenance.
11.8.6.6.1 A maintenance right-of-way or easement must extend to the Vegetated Channel from a public or private road.
11.8.6.6.2 Adequate maintenance access must extend to the full Vegetated Channel length.
11.8.6.6.3 Maintenance access must meet the following criteria:
11.8.6.6.3.1 Minimum width of 15 feet.
11.8.6.6.3.2 Profile grade that does not exceed 10H:1V.
11.8.6.6.3.3 Minimum 10H:1V cross slope.
11.8.7 Vegetated Channel Landscaping Criteria
11.8.7.1 A planting plan must be provided that indicates the methods used to establish and maintain vegetative stabilization of the vegetated channel.
11.8.7.2 Vegetated channels shall be established at such a density to achieve a 90% vegetated cover for project completion.
11.8.7.3 All seeded vegetated channels require a minimum SSM-III biodegradable erosion control matting conforming to Delaware Erosion and Sediment Control Handbook.
11.8.8 Vegetated Channel Construction
11.8.8.1 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.8.8.1.1 Pre-construction meeting;
11.8.8.1.2 Initial site preparation including installation of erosion and sediment controls, sensitive area protection surrounding vegetated channel locations, and blockage of inlets to vegetated channels;
11.8.8.1.3 Excavation and grading including interim and final elevations;
11.8.8.1.4 Construction of check dams and pretreatment practices, as applicable;
11.8.8.1.5 Implementation of required stabilization and planting plan; and
11.8.8.1.6 Final construction review including development of a punch list for facility acceptance.
11.8.8.2 Post Construction Verification Documentation.
11.8.8.2.1 Upon facility completion, the owner shall submit post construction verification documents as follows to demonstrate that the vegetated channel has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency:
11.8.8.2.1.1 Spot elevations of top of bank, bottom of bank, and centerline of the vegetated channel every 25 feet throughout the length of the channel;
11.8.8.2.1.2 Additional spot elevations that demonstrate positive downstream drainage beyond the end of the vegetated channel;
11.8.8.2.1.3 Cross section of the vegetated channel at the midpoint; and
11.8.8.2.1.4 Photo documentation of the vegetated channel depicting the channel bottom width and verification of achievement of the required 90% vegetated cover.
11.8.8.2.2 The constructed slope, bottom width, depth, and length of the vegetated channel shall be within 90% of the design geometrics for those parameters.
11.8.8.2.3 In the event that the constructed allowable tolerances are exceeded for the vegetated channel, supplemental calculations shall be submitted to determine if the vegetated channel, as constructed, meets the design requirements. The computed residence time rounded to the nearest minute shall be no less than the minimum design residence time.
11.8.8.2.4 Performance of a vegetated channel shall be evaluated by the Department or Delegated Agency if requested in writing to determine if reconstruction of a vegetated channel that exceeds allowable tolerances is necessary.
11.9 Sheet Flow to Vegetated Filter Strip or Vegetated Open Space
11.9.1 Vegetated areas can treat sheet flow delivered from adjacent impervious and managed turf areas by slowing runoff velocities and allowing sediment and attached pollutants to settle or be filtered by the vegetation. Vegetation can consist of grasses, planted trees, or existing forest. Design variants include:
11.9.1.1 Sheet Flow to Grassed Filter Strip;
11.9.1.2 Sheet Flow to Afforested Filter Strip;
11.9.1.3 Sheet Flow to Forested Filter Strip;
11.9.1.4 Sheet Flow to Grassed Open Space;
11.9.1.5 Sheet Flow to Afforested Open Space; and
11.9.1.6 Sheet Flow to Forested Open Space.
11.9.2 Sheet Flow practices receive the following annual runoff reduction and pollutant reduction credits.
11.9.2.1 The RPv runoff reduction performance credit for a Vegetated Filter Strip on A/B soil or compost amended C soil is as follows:
11.9.2.1.1 Grassed filter strips receive 25% annual runoff reduction performance credit.
11.9.2.1.2 Afforested filter strips receive 30% annual runoff reduction performance credit.
11.9.2.1.3 Forested filter strips receive 40% annual runoff reduction performance credit.
11.9.2.2 The RPv runoff reduction performance credit for a Vegetated Filter Strip on C/D soil is as follows:
11.9.2.2.1 Grassed filter strips receive 10% annual runoff reduction performance credit.
11.9.2.2.2 Afforested filter strips receive 15% annual runoff reduction performance credit.
11.9.2.2.3 Forested filter strips receive 20% annual runoff reduction performance credit.
11.9.2.3 The RPv runoff reduction performance credit for Sheet Flow to Vegetated Open Space on A/B soil or compost amended C soil is as follows:
11.9.2.3.1 Grassed open space receives 50% annual runoff reduction performance credit.
11.9.2.3.2 Afforested open space receives 60% annual runoff reduction performance credit.
11.9.2.3.3 Forested open space receives 65% annual runoff reduction performance credit.
11.9.2.4 The RPv runoff reduction performance credit for Sheet Flow to Vegetated Open Space on C/D soil is as follows:
11.9.2.4.1 Grassed open space receives 20% annual runoff reduction performance credit.
11.9.2.4.2 Afforested open space receives 30% annual runoff reduction performance credit.
11.9.2.4.3 Forested open space receives 40% annual runoff reduction performance credit.
11.9.2.5 The Cv runoff reduction performance credit for all Sheet Flow practices is 10% of the RPv allowance.
11.9.2.6 The Fv runoff reduction performance credit for all Sheet Flow practices is 1% of the RPv allowance.
11.9.2.7 The total nitrogen pollutant reduction performance credit for all Sheet Flow practices is 100% of the load reduction.
11.9.2.8 The total phosphorous pollutant reduction performance credit for all Sheet Flow practices is 100% of the load reduction.
11.9.2.9 The total suspended solids pollutant reduction performance credit for all Sheet Flow Practices is 100% of the load reduction.
11.9.3 Sheet Flow Feasibility Criteria
11.9.3.1 To maintain sheet flow through the practice, maximum slope for Filter Strips shall be 8.0% unless additional calculations are submitted showing the maximum depth and minimum residence time can be met.
11.9.3.2 Filter Strips shall not be used in structural fill areas where material must be compacted to meet specific structural criteria.
11.9.3.3 To maintain sheet flow through the practice, maximum slope for Open Space shall be 3.0%.
11.9.4 Sheet Flow Design Criteria
11.9.4.1 Stormwater shall enter the filter strip or open space as sheet flow.
11.9.4.1.1 Sheet flow length from impervious surfaces shall be limited to 150 feet.
11.9.4.1.2 A gravel diaphragm or other level spreading device shall be provided for impervious sheet flow lengths greater than 75 feet.
11.9.4.1.3 When the inflow is from a pipe or channel, an engineered level spreader or other device shall be used to convert the concentrated flow to sheet flow.
11.9.4.2 Vegetated Filter Strip
11.9.4.2.1 The maximum length of a Vegetated Filter Strip shall be 100 feet.
11.9.4.2.2 Vegetated Filter Strips shall have the following minimum lengths, measured in the direction of flow, unless calculations are provided in accordance with the Computational Method of Compliance.
11.9.4.2.2.1 Vegetated Filter Strips having slopes less than 3.0% shall have a minimum length of 20 feet.
11.9.4.2.2.2 Vegetated Filter Strips having slopes between 3.0% and 8.0% shall have a minimum length of 30 feet.
11.9.4.3 Vegetated Open Space
11.9.4.3.1 Vegetated Open Space shall have a maximum slope of 3.0%.
11.9.4.3.2 The minimum area of the Vegetated Open Space shall be equivalent to the impervious area of the contributing drainage area to the Vegetated Open Space.
11.9.4.4 Computational Method of Compliance.
11.9.4.4.1 Vegetated Filter Strips using the Computational Method of Compliance shall meet the following criteria in order to receive RPv runoff reduction credits listed above:
11.9.4.4.1.1 The maximum depth of flow shall be 0.5 inches (0.04 feet).
11.9.4.4.1.2 The minimum residence time shall be 2.5 minutes.
11.9.4.4.2 Adjusted RPv runoff reduction credit based on the ratio of the computed residence to the minimum residence time shall be applied to Vegetated Filter Strips that meet the maximum depth of flow criteria.
11.9.4.4.2.1 The maximum adjusted RPv runoff reduction credit is 75% for HSG A/B soils and 30% for HSG C/D soils.
11.9.4.4.2.2 RPv runoff reduction credit shall not be adjusted for lengths greater than 100 feet.
11.9.4.5 Soil amendments, when used, shall extend over the length and width of the Vegetated Filter Strip or Vegetated Open Space, and compost shall be incorporated to the depth as shown on the approved plan.
11.9.4.6 All Vegetated Filter Strips and Vegetated Open Spaces must be designed so as to be accessible for maintenance.
11.9.5 Sheet Flow Landscaping Criteria
11.9.5.1 Grassed Filter Strips and Grassed Open Space shall be established at such a density to achieve a 90% vegetated cover for project completion.
11.9.5.2 Afforested Filter Strips and Afforested Open Space shall be planted in accordance with Afforestation requirements.
11.9.5.3 Forested Filter Strips and Forested Open Space shall have no grading or clearing of native vegetation and shall have at least 80% tree canopy coverage.
11.9.5.4 All Vegetated Filter Strips and Vegetated Open Spaces must be stabilized to prevent erosion or transport of sediment to receiving practices or drainage systems.
11.9.5.5 A planting plan shall be provided that indicates the methods used to establish and maintain vegetative stabilization of the Vegetated Filter Strip or Vegetated Open Space.
11.9.6 Sheet Flow Construction
11.9.6.1 No clearing or grading shall take place in Vegetated Open Space except temporary disturbances associated with incidental utility construction, restoration operations, or management of nuisance vegetation. The Vegetated Open Space area shall not be stripped of topsoil.
11.9.6.2 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.9.6.2.1 Pre-construction meeting;
11.9.6.2.2 Initial site preparation including installation of erosion and sediment controls and sensitive area protection surrounding vegetated filter strip locations;
11.9.6.2.3 Excavation and grading including interim and final elevations;
11.9.6.2.4 Implementation of required stabilization and planting plan; and
11.9.6.2.5 Final construction review including development of a punch list for facility acceptance.
11.9.6.3 Post Construction Verification Documentation.
11.9.6.3.1 Upon facility completion, the owner shall submit Post Construction Verification Documents at the discretion of the Department or Delegated Agency as follows to demonstrate that the Vegetated Filter Strip or Vegetated Open Space has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. The following items, as applicable, shall be included in the Post Construction Verification Documentation for Sheet Flow Practices:
11.9.6.3.1.1 Dimensions of Vegetated Filter Strips (length and width).
11.9.6.3.1.2 Area of Vegetated Open Space.
11.9.6.3.1.3 Cross-slope.
11.9.6.3.1.4 Elevations of any structural components, such as gravel diaphragms or engineered level spreaders.
11.9.6.3.1.5 Photo documentation of the grassed filter strip or grassed open space providing verification of achievement of the required 90% vegetated cover.
11.9.6.3.2 Constructed allowable tolerances for vegetated filter strips and vegetated open spaces, if disturbed, shall be within the tolerances of design geometrics for the following parameters:
11.9.6.3.2.1 Slope shall be no greater than 2.0% steeper than design slope;
11.9.6.3.2.2 Length shall be no less than 90% of design length;
11.9.6.3.2.3 Width shall be no less than 90% of design width; and
11.9.6.3.2.4 Elevations of any structural components shall be within 0.15 feet of design elevation.
11.9.6.3.3 In the event that the constructed allowable tolerances are exceeded for the vegetated filter strip, supplemental calculations shall be submitted to determine if the vegetated filter strip, as constructed, meets the minimum residence time. The computed residence time rounded to the nearest minute shall be no less than the minimum design residence time.
11.9.6.3.4 Performance of a vegetated filter strip shall be evaluated by the Department or Delegated Agency if requested in writing to determine if reconstruction of a vegetated filter strip that exceeds allowable tolerances is necessary.
11.10 Detention Practices
11.10.1 Detention Practices are storage practices that are explicitly designed to provide stormwater detention for the Conveyance Event, Cv (10-year) and Flooding Event, Fv (100-year). Design variants include:
11.10.1.1 Dry Detention Pond
11.10.1.1.1 The RPv runoff reduction performance credit for this variant is 0%.
11.10.1.1.2 The Cv runoff reduction performance credit for this variant is 0%.
11.10.1.1.3 The Fv runoff reduction performance credit for this variant is 0%.
11.10.1.1.4 The total nitrogen pollutant reduction performance credit for this variant is not less than 5%.
11.10.1.1.5 The total phosphorous pollutant reduction performance credit for this variant is not less than 10%.
11.10.1.1.6 The total suspended solids pollutant reduction performance credit for this variant is not less than 10%.
11.10.1.2 Dry Extended Detention (ED) Basin
11.10.1.2.1 The RPv runoff reduction performance credit for this variant is 100%.
11.10.1.2.2 The Cv runoff reduction performance credit for this variant is 1%.
11.10.1.2.3 The Fv runoff reduction performance credit for this variant is 0%.
11.10.1.2.4 The total nitrogen pollutant reduction performance credit for this variant is not less than 20%.
11.10.1.2.5 The total phosphorous pollutant reduction performance credit for this variant is not less than 20%.
11.10.1.2.6 The total suspended solids pollutant reduction performance credit for this variant is not less than 60%.
11.10.1.3 Underground Detention Facilities
11.10.1.3.1 Underground Detention Facilities not achieving 48-hour ED of the full RPv shall have RPv, Cv, and Fv runoff reduction performance credits and total nitrogen, total phosphorous, and total suspended solids pollutant reduction performance credits in accordance with that of Dry Detention Pond.
11.10.1.3.2 Underground Detention Facilities achieving 48-hour ED of the full RPv shall have RPv, Cv, and Fv runoff reduction performance credits and total nitrogen, total phosphorous, and total suspended solids pollutant reduction performance credits in accordance with that of Dry Extended Detention (ED) Basin.
11.10.2 Full runoff reduction credit is given for detention practices that provide 48-hour extended detention of the full RPv.
11.10.3 Detention Practices Feasibility Criteria
11.10.3.1 Depth-to-Water Table and Bedrock.
11.10.3.1.1 Dry Detention Ponds or Dry ED Basins shall not be allowed if the seasonal high water table or bedrock will be within one foot of the floor of the pond.
11.10.3.1.2 Non-watertight Underground Detention Facilities shall be no lower than the seasonal high water table and two feet above bedrock.
11.10.3.1.3 For watertight Underground Detention Facilities, an anti-flotation analysis is required to check for buoyancy problems in seasonal high water table areas.
11.10.3.1.4 Subsection 12.1 shall be followed for testing.
11.10.3.2 Underground Detention Facilities must meet structural requirements for bearing capacity, overburden support, and traffic loading as determined by a licensed design professional, and based upon manufacturer's recommendations where applicable.
11.10.4 Detention Practice Conveyance Criteria
11.10.4.1 Principal Spillway
11.10.4.1.1 The principal spillway must be accessible from dry land.
11.10.4.1.2 A structure-pipe spillway shall be designed with anti-flotation, anti-vortex and trash rack devices on the structure.
11.10.4.1.3 The outfall pipe and all connections to the outfall structure shall be made watertight. Soil tight only joints are not acceptable.
11.10.4.1.4 Anti-seep collars shall be used in accordance with USDA NRCS Delaware Pond Code 378, as amended.
11.10.4.1.5 When the principal spillway is composed of a weir wall discharging to a channel, the channel below the weir must be reinforced with riprap or other acceptable material to prevent scour.
11.10.4.2 Non-Clogging Outlet
11.10.4.2.1 For Dry ED Basins, the control structure must include an outlet that will slowly release the RPv over a 48-hour period.
11.10.4.2.2 When a low flow orifice is specified, it must be adequately protected from clogging by either an acceptable external trash rack or by internal orifice protection. Orifice diameters shall not be less than three inches unless internal orifice control is provided.
11.10.4.3 The design shall specify an outfall that can discharge the maximum design storm event in a non-erosive manner at the project point of discharge.
11.10.4.4 Emergency Spillway.
11.10.4.4.1 Dry Detention Ponds and Dry ED Basins must be designed to pass the maximum design storm event (Fv) if the Fv is being routed through the Dry Detention Ponds and Dry ED Basins rather than bypassing.
11.10.4.4.2 An earthen emergency spillway designed to convey the Fv shall be cut in natural ground or, if cut in fill, shall be constructed and stabilized with methods to prevent erosion and structural failure.
11.10.4.5 Inflow Points.
11.10.4.5.1 Inflow points into the Dry Detention Ponds and Dry ED Basins must be stabilized to ensure that non-erosive conditions exist during storm events up to the conveyance event (Cv).
11.10.4.5.2 A forebay shall be provided at each inflow location that provides 10% or greater of the total RPv inflow to the Dry Detention Ponds and Dry ED Basins.
11.10.4.6 In the event that the embankment is a regulated dam, the designer must verify that the appropriate Dam Safety Permit has been approved by the Department's Dam Safety Program.
11.10.4.7 For Underground Detention Facilities, an internal or external high flow bypass or overflow shall be included in the design to safely pass the Fv.
11.10.5 Detention Practices Pretreatment Criteria
11.10.5.1 A forebay must be located at each major inlet to trap sediment and preserve the capacity of the main treatment cell. The following criteria apply to forebay design:
11.10.5.1.1 A major inlet is defined as an individual storm drain inlet pipe or open channel conveying at least 10% of the Dry Detention Pond's and Dry ED Basin's contributing RPv runoff volume.
11.10.5.1.2 The forebay shall be no deeper than three feet.
11.10.5.1.3 The forebay must be sized to contain 10% of the volume of runoff from the contributing drainage area for the Resource Protection event.
11.10.5.1.4 Discharge from the forebay shall be non-erosive.
11.10.5.2 Every underground detention practice shall have pretreatment mechanisms to protect the long term integrity of the practice.
11.10.6 Detention Practices Design Criteria
11.10.6.1 Dry Detention Ponds and Dry ED Basins constructed to meet regulatory stormwater management requirements in the State of Delaware shall be designed and constructed in accordance with the USDA NRCS Delaware Pond Code 378 as amended.
11.10.6.2 Detention Practice Sizing.
11.10.6.2.1 For RPv compliance, a Dry ED Basin or Underground Detention Facility must provide 48 hours extended detention for the RPv runoff volume.
11.10.6.2.2 Detention time shall be based on the time of initial inflow to time of final outflow from the facility. In order to simulate a baseflow condition to the extent practicable, the peak discharge for the outflow hydrograph shall not exceed five times the average discharge rate.
11.10.6.3 Earthen side slopes shall be designed and constructed no steeper than 3H:1V.
11.10.6.4 Retaining walls around Dry Detention Ponds and Dry ED Basins shall be limited to no more than 50% of the pond perimeter based upon the peak elevation of the Cv. In order to maintain the safety requirements, retaining walls shall be configured as follows:
11.10.6.4.1 The retaining wall shall have a maximum height of three feet.
11.10.6.4.2 Any additional retaining walls shall have a maximum height of two feet and provide a minimum 10-foot level terrace from a lower retaining wall.
11.10.6.5 Safety Features
11.10.6.5.1 Any inflow opening 12 inches or greater discharging to a closed drainage system shall include safety grates.
11.10.6.5.2 The emergency spillway must be located so that downstream structures will not be adversely impacted by spillway discharges.
11.10.6.5.3 The emergency spillway exit channel must be designed to direct runoff to a point of discharge without adverse impact to downstream structures.
11.10.6.6 All Detention Practices shall be designed so as to be accessible for maintenance.
11.10.6.6.1 Adequate maintenance access must extend to the pretreatment, riser, and outlet structure. Adequate maintenance access must also be provided for all Underground Detention Facilities.
11.10.6.6.2 A maintenance right-of-way or easement must extend to the Detention Practice from a public or private road.
11.10.6.6.3 Maintenance access must meet the following criteria:
11.10.6.6.3.1 Minimum width of 15 feet.
11.10.6.6.3.2 Profile grade that does not exceed 10H:1V.
11.10.6.6.3.3 Minimum 10H:1V cross slope.
11.10.6.7 Maintenance Set-Aside Area
11.10.6.7.1 The maintenance set-aside area shall accommodate the volume of 50% of the collective forebay volume.
11.10.6.7.2 The maximum depth of the set aside volume shall be one foot.
11.10.6.7.3 The slope of the set aside area shall not exceed 5.0%.
11.10.6.8 Detention Vault and Tank Materials. All construction joints and pipe joints shall be water tight. Cast-in-place wall sections must be designed as retaining walls.
11.10.6.9 For watertight Underground Detention Facilities, anti-flotation analysis is required to check for buoyancy problems in the high water table areas. Anchors shall be designed to counter the pipe and structure buoyancy by at least a 1.2 factor of safety.
11.10.7 Detention Practices Landscaping Criteria
11.10.7.1 Woody vegetation shall not be planted or allowed to grow within 15 feet of the embankment and 10 feet on either side of principal spillway or inflow pipes.
11.10.7.2 For Dry Detention Ponds and Dry ED Basins, a planting plan shall be provided that indicates the methods used to establish and maintain vegetative coverage within the Detention Practice and its vegetated perimeter area. Minimum elements of a plan include seed mixes by botanical and common names as well as percentages by weight or volume.
11.10.8 Detention Practices Construction
11.10.8.1 Construction of proprietary Underground Detention Facilities must be in accordance with manufacturer's specifications.
11.10.8.2 Underground Detention Facilities must be inspected and cleaned of sediment after the site is stabilized.
11.10.8.3 Use of Dry Detention Pond or Dry ED Basin for Erosion and Sediment Control.
11.10.8.3.1 Approval from the Department or the appropriate Delegated Agency must be obtained before any planned Dry Detention Ponds and Dry ED Basins can be used as a sediment basin.
11.10.8.3.2 If a Dry Detention Pond or Dry ED Basin serves as a sediment basin during project construction, the volume of the sediment basin must be based on the more stringent sizing rule.
11.10.8.3.3 When the sediment basin is being converted into a Dry Detention Pond or Dry ED Basin, the sediment basin shall be dewatered in accordance with the approved plan and appropriate details from the Delaware Erosion and Sediment Control Handbook prior to removing accumulated sediment and regrading the pond bottom.
11.10.8.3.4 The Sediment and Stormwater Plan must include conversion steps from sediment basin to permanent Dry Detention Ponds and Dry ED Basins in the construction sequence.
11.10.8.3.4.1 The Department or Delegated Agency must be notified and provide approval prior to conversion from sediment basin to the final configuration of the Dry Detention Pond or Dry ED Basin.
11.10.8.3.4.2 Appropriate procedures must be implemented to prevent discharge of turbid waters when the sediment basin is being converted into a Dry Detention Pond or Dry ED Basin.
11.10.8.4 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.10.8.4.1 Pre-construction meeting.
11.10.8.4.2 Initial site preparation including installation of erosion and sediment controls.
11.10.8.4.3 Construction of the embankment, including installation of the principal spillway and the outlet structure.
11.10.8.4.4 For Dry Detention Pond and Dry ED Basin, excavation and grading including interim and final elevations.
11.10.8.4.5 For Underground Detention, subgrade, placement of stone, system components in accordance with manufacturer's recommendations and backfill.
11.10.8.4.6 Implementation of the planting plan and vegetative stabilization.
11.10.8.4.7 Final inspection including development of a punch list for facility acceptance.
11.10.8.5 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the Detention Practice has been constructed within allowable tolerances and in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency.
11.10.8.5.1 Allowable tolerances for Dry Detention Pond and Dry ED Basin are as follows:
11.10.8.5.1.1 The constructed top of bank elevation may be no lower than the design elevation for top of bank.
11.10.8.5.1.2 The constructed volume of the dry pond surface storage shall be no less than 90% of the design volume.
11.10.8.5.1.3 The constructed elevation of any structure shall be within 0.15 foot of the design.
11.10.8.5.2 Allowable tolerances for Underground Detention Facilities are as follows:
11.10.8.5.2.1 Grate and invert elevations of all structures, including weirs shall be within 0.15 foot of the design.
11.10.8.5.2.2 Diameter of all pipes or dimensions of chambers within underground detention facility shall be as shown on the plan.
11.10.8.5.2.3 Dimension of any weirs shall be within 10% of the design.
11.10.8.5.3 When the allowable tolerances are exceeded for volume or structure elevations, supplemental calculations must be submitted to the approval agency to demonstrate that the Detention Practice, as constructed, meets the design requirements.
11.10.9 Detention Practices Maintenance Criteria
11.10.9.1 Repair of critical structural features such as embankments and risers shall be performed by responsible personnel that have successfully completed the Department Contractor Training Program.
11.10.9.2 Sediment removal in the Dry Detention Pond or Dry ED Basin pretreatment practice must occur when 50% of total forebay capacity has been lost.
11.11 Stormwater Filtering Systems
11.11.1 Stormwater Filter Systems are practices that capture and temporarily store the design storm volume and pass it through a filter media or material. Filtered runoff may be collected and returned to the conveyance system, or allowed to partially infiltrate into the soil. Design variants include:
11.11.1.1 Non-Structural Sand Filter;
11.11.1.2 Surface Sand Filter;
11.11.1.3 Three-Chamber Underground Sand Filter; and
11.11.1.4 Perimeter Sand Filter.
11.11.2 Stormwater Filtering Systems shall be combined with a separate facility to provide stormwater detention.
11.11.3 Proprietary stormwater filtering systems shall be verified by the Department for adequate performance, sizing, and longevity.
11.11.4 Stormwater Filtering Systems Stormwater Credits
11.11.4.1 Stormwater Filtering Systems receive no runoff reduction performance credit.
11.11.4.2 Stormwater Filtering Systems sized in accordance with the design criteria shall receive the following pollutant reduction performance credits:
11.11.4.2.1 The total nitrogen pollutant reduction performance credit for Stormwater Filtering Systems is not less than 40% removal efficiency.
11.11.4.2.2 The total phosphorus pollutant reduction performance credit for Stormwater Filtering Systems is not less than 60% removal efficiency.
11.11.4.2.3 The total suspended solids pollutant reduction performance credit for Stormwater Filtering Systems is not less than 80% removal efficiency.
11.11.5 Stormwater Filtering Systems Feasibility Criteria
11.11.5.1 Depth to Water Table and Bedrock
11.11.5.1.1 A minimum vertical distance of two feet must be provided between the bottom of the non-structural sand filter or surface sand filter and the seasonal high water table as determined by the procedures in subsection 12.1 or bedrock layer.
11.11.5.1.2 The minimum vertical distance of two feet may be relaxed if a groundwater mounding analysis or piezometer testing has been performed by a qualified professional.
11.11.5.1.3 Three-chamber underground sand filter and perimeter sand filter require no minimum separation to seasonal high water table or bedrock.
11.11.5.2 Stormwater Filtering Systems shall not be located on slopes greater than 6.0%.
11.11.6 Stormwater Filtering Systems Conveyance Criteria
11.11.6.1 On-line stormwater filtering systems' designs, shall demonstrate that the filter will safely pass the largest design storm event to a stabilized water course without resuspending or flushing previously trapped material.
11.11.6.2 All Stormwater Filtering Systems shall be designed to drain or dewater within 48 hours after a storm event.
11.11.7 Every inlet into a Stormwater Filtering System shall have a pretreatment mechanism to trap sediment, preserve the capacity of the main treatment area, and protect the long term integrity of the practice.
11.11.8 Stormwater Filtering Systems Design Criteria
11.11.8.1 Stormwater Filtering Systems shall be designed to drain the design storm volume from the filter chamber within 48 hours after each rainfall event.
11.11.8.2 Filter
11.11.8.2.1 The filter media shall consist of clean, washed AASHTO M-6/ASTM C-33 medium aggregate concrete sand with individual grains between 0.02 and 0.04 inches in diameter.
11.11.8.2.2 A minimum filter bed depth of 12" is required.
11.11.8.3 Underdrain
11.11.8.3.1 The underdrain shall be a minimum of four inch perforated corrugated polyethylene pipe (CPP).
11.11.8.3.2 The underdrain shall be encased in a layer of clean, washed nominal 1/4 inch gravel with a maximum of 2.0% passing the #200 sieve with a minimum of three inches of cover.
11.11.8.3.3 When an underdrain is specified a needled, non-woven, polypropylene geotextile having a flow rate (ASTM D4491) [GREATER THEN OR EQUAL TO]110 gallons per minute per square foot (gal/min/sf) and an apparent opening size (ASTM D4751) of US #70 or #80 sieve shall be placed beneath the filter media and above the underdrain gravel layer.
11.11.8.4 All Stormwater Filtering Systems must be designed so as to be accessible for maintenance.
11.11.8.4.1 A maintenance right-of-way or easement must extend to the Stormwater Filtering System from a public or private road.
11.11.8.4.2 Adequate maintenance access must extend to the perimeter of the Stormwater Filtering System pretreatment area and the filter bed
11.11.8.4.3 Maintenance access must meet the following criteria:
11.11.8.4.3.1 Minimum width of fifteen feet.
11.11.8.4.3.2 Profile grade that does not exceed 10H:1V.
11.11.8.4.3.3 Minimum 10H:1V cross slope.
11.11.8.4.4 Access to Underground Sand Filters must be provided by manholes at least 30 inches in diameter, along with steps to the areas where maintenance will occur.
11.11.8.5 The Stormwater Filtering System including pretreatment shall be sized to contain a minimum of 75% of the RPv prior to filtration.
11.11.9 Vegetative cover shall be established over the contributing pervious drainage areas before runoff can be accepted into the Stormwater Filtering System.
11.11.10 Stormwater Filtering Systems Construction Criteria
11.11.10.1 Erosion and Sediment Control.
11.11.10.1.1 No runoff shall be allowed to enter the Stormwater Filtering System prior to completion of all construction activities, including revegetation and final site stabilization.
11.11.10.1.2 Construction runoff shall be treated in separate sedimentation basins and routed to bypass the filter system. Should construction runoff enter the filter system prior to final site stabilization, all contaminated materials shall be removed and replaced with new clean filter materials before a regulatory inspector approves its completion.
11.11.10.1.3 The approved Sediment & Stormwater Plan shall include specific measures to provide for the protection of the filter system before the final stabilization of the site.
11.11.10.2 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.11.10.2.1 Pre-construction meeting.
11.11.10.2.2 Initial site preparation including installation of erosion and sediment controls, sensitive area protection, and blockage of inlets to stormwater filtering system locations.
11.11.10.2.3 Excavation and grading to design dimensions and elevations.
11.11.10.2.4 Installation of the filter structure, including the water tightness test as applicable.
11.11.10.2.5 Installation of the underdrain and filter bed.
11.11.10.2.6 Check that stabilization in contributing area is adequate to bring the stormwater filtering system online.
11.11.10.2.7 Final construction review after a rainfall event to ensure that it drains properly and all pipe connections are watertight. Develop a punch list for facility acceptance.
11.11.10.3 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the stormwater filtering system has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. Allowable tolerances for stormwater filtering systems are as follows:
11.11.10.3.1 The constructed surface area of the filter bed shall be no less than 90% of the design surface area.
11.11.10.3.2 The constructed volume of the surface storage shall be no less than 90% of the design volume.
11.11.10.3.3 Depth of filter media shall be no less than 12 inches.
11.11.10.3.4 The constructed elevation of any structure shall be within 0.15 foot of the design.
11.12 Constructed Wetlands
11.12.1 Constructed Wetlands are practices that mimic natural wetland areas to treat urban stormwater by incorporating permanent pools with shallow storage areas. Constructed Wetlands may provide stormwater detention for larger storms (Cv and Fv) above the RPv storage. Design variants include:
11.12.1.1 Traditional Constructed Wetlands;
11.12.1.2 Wetland Swales;
11.12.1.3 Ephemeral Constructed Wetlands; and
11.12.1.4 Submerged Gravel Wetlands.
11.12.2 Constructed Wetland Stormwater Credits
11.12.2.1 Traditional Constructed Wetlands Performance Credits
11.12.2.1.1 Traditional Constructed Wetlands receive 100% runoff reduction credit for the RPv.
11.12.2.1.2 Traditional Constructed Wetlands receive not less than 1% of the RPv allowance for the Cv.
11.12.2.1.3 Traditional Constructed Wetlands receive not less than 0% runoff reduction credit for the Fv.
11.12.2.1.4 Traditional Constructed Wetlands receive not less than 30% pollutant removal efficiency for total nitrogen reduction.
11.12.2.1.5 Traditional Constructed Wetlands receive not less than 40% pollutant removal efficiency for total phosphorous reduction.
11.12.2.1.6 Traditional Constructed Wetlands receive not less than 80% pollutant removal efficiency for total suspended solids reduction.
11.12.2.2 Wetland Swale Performance Credits
11.12.2.2.1 Wetland Swales receive the following runoff reduction credits:
11.12.2.2.1.1 Wetland Swales located in HSG A or B soils receive 15% annual runoff reduction for the RPv.
11.12.2.2.1.2 Wetland Swales located in HSG C or D soils receive 10% annual runoff reduction for the RPv.
11.12.2.2.1.3 Wetland Swales receive not less than 1% of the RPv allowance for the Cv.
11.12.2.2.1.4 Wetland Swales receive not less than 0% runoff reduction credit for the Fv.
11.12.2.2.2 Wetland Swales receive 100% of the load reduction plus not less than 20% pollutant removal efficiency for total nitrogen reduction.
11.12.2.2.3 Wetland Swales receive 100% of the load reduction plus not less than 30% pollutant removal efficiency for total phosphorous reduction.
11.12.2.2.4 Wetland Swales receive 100% of the load reduction plus not less than 60% pollutant removal efficiency for total suspended solids reduction.
11.12.2.3 Ephemeral Constructed Wetland Performance Credits
11.12.2.3.1 Ephemeral Constructed Wetlands receive the following runoff reduction credits:
11.12.2.3.1.1 Ephemeral Constructed Wetlands located in HSG A or B soils receive 40% annual runoff reduction for the RPv.
11.12.2.3.1.2 Ephemeral Constructed Wetlands located in HSG C or D soils receive 10% annual runoff reduction for the RPv.
11.12.2.3.1.3 Ephemeral Constructed Wetlands receive not less than 1% of the RPv allowance for the Cv.
11.12.2.3.1.4 Ephemeral Constructed Wetlands receive not less than 0% runoff reduction credit for the Fv.
11.12.2.3.2 Ephemeral Constructed Wetlands receive 100% of the load reduction plus not less than 20% pollutant removal efficiency for total nitrogen reduction.
11.12.2.3.3 Ephemeral Constructed Wetlands receive 100% of the load reduction plus not less than 30% pollutant removal efficiency for total phosphorous reduction.
11.12.2.3.4 Ephemeral Constructed Wetlands receive 100% of the load reduction plus not less than 60% pollutant removal efficiency for total suspended solids reduction.
11.12.2.4 Submerged Gravel Wetland Performance Credits
11.12.2.4.1 The RPv, Cv and Fv runoff reduction performance credit for Submerged Gravel Wetlands is 100% of the detention storage.
11.12.2.4.2 Submerged Gravel Wetlands receive not less than 30% pollutant removal efficiency for total nitrogen reduction.
11.12.2.4.3 Submerged Gravel Wetlands receive not less than 40% pollutant removal efficiency for total phosphorous reduction.
11.12.2.4.4 Submerged Gravel Wetlands receive not less than 80% pollutant removal efficiency for total suspended solids reduction.
11.12.3 Constructed Wetlands shall not be located within existing jurisdictional wetlands.
11.12.4 Constructed Wetland Conveyance Criteria
11.12.4.1 Principal Spillway
11.12.4.1.1 The principal spillway must be accessible from dry land.
11.12.4.1.2 A structure-pipe spillway shall be designed with anti-flotation, anti-vortex and trash rack devices on the structure.
11.12.4.1.3 The outfall pipe and all connections to the outfall structure shall be made watertight. Soil tight only joints are not acceptable.
11.12.4.1.4 Anti-seep collars shall be used in accordance with Pond Code 378, as amended.
11.12.4.1.5 When the principal spillway is composed of a weir wall discharging to a channel, the channel below the weir must be reinforced with riprap or other acceptable material to prevent scour.
11.12.4.1.6 When a low flow orifice is specified, it must be adequately protected from clogging by either an acceptable external trash rack or by internal orifice protection. Orifice diameters shall not be less than three inches.
11.12.4.2 The design shall specify an outfall that can discharge the maximum design storm event in a non-erosive manner at the project point of discharge.
11.12.4.3 Constructed Wetlands must be designed to pass the maximum design storm event (Fv) if the Fv is being routed through the Constructed Wetland rather than bypassing. An earthen emergency spillway designed to convey the Fv shall be cut in natural ground or, if cut in fill, shall be constructed and stabilized with methods to prevent erosion and structural failure.
11.12.4.4 Inflow Points
11.12.4.4.1 Inflow points into the Constructed Wetland must be stabilized to ensure that non-erosive conditions exist during storm events up to the conveyance event (Cv).
11.12.4.4.2 For Submerged Gravel Wetlands, the inflow volume shall enter the gravel substrate directly via a pipe manifold or inflow chimneys or as sheet flow through connected gravel layer.
11.12.5 Constructed Wetland Pretreatment Criteria
11.12.5.1 Every inlet into a Constructed Wetland shall have pretreatment.
11.12.5.2 Exit velocities from the pretreatment shall be non-erosive during the largest design storm that is routed through the Constructed Wetland.
11.12.5.3 A forebay shall be located at each major inlet to trap sediment and preserve the capacity of the main treatment cell.
11.12.5.4 The following criteria apply to forebay design:
11.12.5.4.1 A major inlet is defined as an individual storm drain inlet pipe or open channel conveying at least 10% of the Constructed Wetland's contributing RPv runoff volume.
11.12.5.4.2 The forebay must be sized to contain 10% of the volume of runoff from the contributing drainage area for the Resource Protection event.
11.12.5.4.3 Discharge from the forebay shall be non-erosive.
11.12.6 Constructed Wetland Design Criteria
11.12.6.1 Constructed Wetlands constructed to meet regulatory stormwater management requirements shall be designed and constructed in accordance with the USDA NRCS Pond Code 378 as amended.
11.12.6.2 Constructed Wetlands shall be designed so that they will:
11.12.6.2.1 Dewater the Fv within 72 hours, or
11.12.6.2.2 Manage the Fv on site with no adverse impact. The extents of the Fv shall be clearly delineated.
11.12.6.3 The lowest discharge elevation on the outlet device shall be located no lower than the seasonal high groundwater table as determined by subsection 12.1.
11.12.6.4 Liners
11.12.6.4.1 All Traditional Constructed Wetlands shall be evaluated for feasibility and ability to maintain permanent pool, including the need for a liner, by a qualified, licensed geotechnical engineer or geologist. If the design professional chooses not to follow the recommendations of the geotechnical professional, a signed, sealed and dated letter from the design professional providing justification for removal of the liner from the design shall be provided to the Department or Delegated Agency.
11.12.6.4.2 When the geotechnical engineer recommends a liner, acceptable options include the following:
11.12.6.4.2.1 A clay liner having a minimum compacted thickness of six inches with an additional six inch layer of engineered wetland soil mix containing a minimum of 35% organic material above it. Clay used as a liner must meet the following specifications:
11.12.6.4.2.1.1 Permeability of 1x10-6 centimeters per second (cm/sec) using ASTM D-2434 procedure;
11.12.6.4.2.1.2 Plasticity index of not less than 15% using ASTM D-423/424 procedures;
11.12.6.4.2.1.3 Liquid limit of not less than 30% using ASTM D-2216 procedure;
11.12.6.4.2.1.4 Clay particles passing not less than 30% using ASTM D-422 procedure; and
11.12.6.4.2.1.5 Compaction of 95% of standard proctor density using ASTM D-2216 procedure.
11.12.6.4.2.2 Other acceptable measures as recommended by a qualified geotechnical professional.
11.12.6.5 Trash Racks.
11.12.6.5.1 Trash racks shall be provided for low-flow pipes and for all riser structure openings.
11.12.6.5.2 All metal trash racks shall be coated with a rust inhibitor to increase longevity of the device.
11.12.6.6 When a riser is used, it must be located such that it is accessible from the side slope for the purposes of inspection and maintenance.
11.12.6.7 Safety Features
11.12.6.7.1 Any opening 12 inches or greater discharging to a closed drainage system shall include safety grates.
11.12.6.7.2 The emergency spillway must be located so that downstream structures will not be adversely impacted by spillway discharges.
11.12.6.7.3 The emergency spillway exit channel must be designed to direct runoff to a point of discharge without adverse impact to downstream structures.
11.12.6.8 All Constructed Wetlands must be designed so as to be accessible for maintenance.
11.12.6.8.1 Adequate maintenance access must extend to the forebays, micropools, riser, and outlet structure.
11.12.6.8.2 A maintenance right-of-way or easement must extend to the Constructed Wetland from a public or private road.
11.12.6.8.3 Maintenance access must meet the following criteria:
11.12.6.8.3.1 Minimum width of 15 feet.
11.12.6.8.3.2 Profile grade that does not exceed 10H:1V.
11.12.6.8.3.3 Minimum 10H:1V cross slope.
11.12.6.9 Maintenance Set-Aside Area
11.12.6.9.1 The maintenance set-aside area shall accommodate the volume of 50% of the collective forebay volume.
11.12.6.9.2 The maximum depth of the set aside area shall be one foot.
11.12.6.9.3 The slope of the set aside area shall not exceed 5.0%.
11.12.6.10 Traditional Constructed Wetlands
11.12.6.10.1 The permanent pool volume, or the volume below the normal water surface elevation, shall be equivalent to a minimum of 50% of the RPv volume.
11.12.6.10.2 Traditional Constructed Wetlands shall be sized so that the RPv has a maximum ponding depth of 12 inches above the normal water surface elevation.
11.12.6.10.3 The RPv shall dewater within 48 hours.
11.12.6.10.4 The Cv maximum ponding depth shall not exceed 12 inches above the normal water surface elevation for more than 12 hours.
11.12.6.10.5 Flow Path
11.12.6.10.5.1 The total length of the flow path compared to the linear length through the Traditional Constructed Wetland shall be a minimum ratio of 2:1.
11.12.6.10.5.2 When an inlet is located near the outlet, the ratio of the shortest flow path through the system to the overall length shall be a minimum of 0.5:1.
11.12.6.10.5.3 The drainage area served by any inlets located less than a 0.5:1 ratio shall constitute no more than 20% of the total contributing drainage area.
11.12.6.10.6 Traditional Constructed Wetlands shall be composed of the following zones:
11.12.6.10.6.1 Zone 1: Deep Pools.
11.12.6.10.6.1.1 The volume of water stored in the deep pools, also referred to as micropools, shall be a minimum of 20% of the RPv volume.
11.12.6.10.6.1.2 A minimum of two deep pools in addition to the forebay shall be provided, one of which shall be located prior to the outlet location to provide for additional sediment deposition.
11.12.6.10.6.1.3 Deep pools shall range from a minimum of 30 inches to a maximum of six feet in depth below the normal water surface elevation and shall be designed to remain permanently saturated.
11.12.6.10.6.1.4 The deep pools shall be hydraulically connected within the water flow path.
11.12.6.10.6.1.5 The deep pools shall be designed with a side slope not steeper than 3:1.
11.12.6.10.6.1.6 A safety bench is required for deep pool depths greater than four feet.
11.12.6.10.6.2 Zone 2: Transition Zone.
11.12.6.10.6.2.1 Zone 2 is a short transition zone between the deeper pools and the low marsh zone, and ranges from a minimum of six inches to a maximum of 30 inches below the normal water surface elevation.
11.12.6.10.6.2.2 The volume of water stored in the transition zone shall be a minimum of 20% of the RPv volume.
11.12.6.10.6.2.3 The transition zone shall have a maximum side slope of 3:1 from the deep pool to the low marsh zone.
11.12.6.10.6.3 Zone 3: Low Marsh Zone.
11.12.6.10.6.3.1 The low marsh zone ranges from a maximum of six inches below the normal water surface elevation to the normal water surface elevation.
11.12.6.10.6.3.2 The volume of water stored in the low marsh zone shall be a minimum of 10% of the RPv volume.
11.12.6.10.6.3.3 The side slope within the low marsh zone shall not be steeper than 4:1.
11.12.6.10.6.4 Zone 4: High Marsh Zone.
11.12.6.10.6.4.1 The upper end of the marsh zone is the high marsh zone, which ranges from the normal water surface elevation to a maximum of 12 inches above the normal water surface elevation, allowing the RPv to inundate to the top of the high marsh zone.
11.12.6.10.6.4.2 The side slope within the high marsh zone shall not be steeper than 4:1.
11.12.6.10.6.5 Zone 5: Floodplain.
11.12.6.10.6.5.1 A low floodplain shall range between a minimum of 12 inches and a maximum of 18 inches above the normal water surface elevation and be planted with plants suited for infrequent to temporary saturations.
11.12.6.10.6.5.2 The side slope within the floodplain shall not be steeper than 4:1.
11.12.6.10.7 A minimum 10-foot-wide vegetated perimeter around the wetland area shall be planted with appropriate grasses, trees, and shrubs.
11.12.6.10.8 A simple water balance calculation shall be performed to ensure that the deep pools will not go completely dry during a 30-day summer drought.
11.12.6.11 Wetland Swales
11.12.6.11.1 Wetland Swale Sizing.
11.12.6.11.1.1 Wetland swales shall contain the Cv event.
11.12.6.11.1.2 If the Fv event is not contained within the Wetland swale top of bank, then the area of inundation and discharge route shall be delineated.
11.12.6.11.1.3 The maximum RPv water surface elevation shall be no greater than six inches above the normal water surface elevation.
11.12.6.11.2 The average groundwater elevation as determined by subsection 12.1 shall be below the bottom of the Wetland Swale. Only the seasonal high groundwater as determined by the Soil Investigation Procedures may intersect the bottom of the Wetland Swale.
11.12.6.11.3 Wetland Swales shall not have side slopes steeper than 3:1.
11.12.6.11.4 The maximum longitudinal slope shall be an average of 1.0%.
11.12.6.11.5 A minimum 10-foot-wide vegetated perimeter on both sides of the wetland swale shall be planted with appropriate grasses, trees and shrubs.
11.12.6.12 Ephemeral Constructed Wetlands
11.12.6.12.1 Ephemeral Constructed Wetland Sizing.
11.12.6.12.1.1 The RPv event shall pond a minimum of six inches and a maximum of 12 inches of water above the ground surface of the Ephemeral Constructed Wetland.
11.12.6.12.1.2 The Fv water surface shall be a maximum of 30 inches above the ground surface of the Ephemeral Constructed Wetland.
11.12.6.12.1.3 The average groundwater elevation as determined by the Soil Investigation Procedures shall be below the wetland bottom of the Ephemeral Constructed Wetland. Only the seasonal high groundwater as determined by subsection 12.1 may intersect the bottom of the Ephemeral Constructed Wetland. If the seasonal high groundwater intersects the bottom of the Ephemeral Constructed Wetland, the wetland shall be modeled considering the elevation of the seasonal high groundwater.
11.12.6.12.2 The side slopes of the buffer area and within the wetland shall be 4:1 or flatter.
11.12.6.12.3 A minimum 10-foot-wide vegetated perimeter around the wetland area shall be planted with appropriate grasses, trees, and shrubs.
11.12.6.13 Submerged Gravel Wetlands
11.12.6.13.1 Submerged Gravel Wetland Sizing.
11.12.6.13.1.1 The maximum surface ponding depth for the RPv shall not be greater than the tolerance depths of the wetland plantings selected, or two feet, whichever is less.
11.12.6.13.1.2 The Submerged Gravel Wetland shall store the RPv volume within the stone substrate and wetland soils and above the soils in surface ponding.
11.12.6.13.1.3 Submerged Gravel Wetlands shall have no minimum detention time.
11.12.6.13.2 Gravel substrate.
11.12.6.13.2.1 The gravel substrate shall be a minimum of two feet and a maximum of four feet in depth.
11.12.6.13.2.2 The gravel substrate shall be sized to contain a minimum of 25% of the RPv volume considering 40% void ratio.
11.12.6.13.2.3 The gravel substrate shall be composed of clean washed gravel, with a maximum of 2.0% passing the #200 sieve. Gravel shall have a maximum diameter of 2.5 inches and a minimum diameter of 0.5 inches. A porosity value of 0.4 shall be used for areas of stone in the design of gravel substrate. Sand shall not be an acceptable substitute for gravel.
11.12.6.13.3 An engineered wetland soil layer containing a minimum of 15% organic material and a maximum of 15% clay content shall be included on the surface of the Submerged Gravel Wetland. The wetland soil layer shall be a minimum of eight inches thick.
11.12.6.13.4 A minimum four inch thick layer of clean, washed nominal 1/4 inch gravel with a maximum of 2.0% passing the #200 sieve shall be installed between the gravel substrate and the wetland soil layer.
11.12.6.13.5 Underdrain.
11.12.6.13.5.1 An underdrain shall be provided at an elevation three inches above the invert of the gravel substrate.
11.12.6.13.5.2 The underdrain shall be a minimum of four inch perforated high density polyethylene pipe (HDPE) or polyvinyl chloride pipe (PVC).
11.12.6.13.5.3 The underdrain shall connect to the outlet structure. The discharge elevation shall be four inches below the wetland soil surface.
11.12.6.13.6 There shall be a minimum of 15 feet separation distance between all gravel substrate inflow points and all underdrain outlet points.
11.12.6.13.7 Side slopes above the gravel substrate shall not be steeper than 3:1.
11.12.7 Constructed Wetland Landscaping Criteria
11.12.7.1 A planting plan is required for all Constructed Wetlands. Invasive species shall not be specified within Constructed Wetlands.
11.12.7.2 The planting plan shall be certified by a qualified professional with demonstrated knowledge in wetland species.
11.12.7.3 Plants used in Constructed Wetlands shall be supplied by a certified wetland nursery using plants selected for the region.
11.12.8 Constructed Wetland Construction
11.12.8.1 Approval from the Department or the appropriate Delegated Agency must be obtained before any planned Constructed Wetlands can be used as a sediment basin. If a Constructed Wetlands serves as a sediment basin during project construction, the volume of the sediment basin must be based on the more stringent sizing rule.
11.12.8.2 The Sediment and Stormwater Plan must include conversion steps from sediment basin to permanent Constructed Wetlands in the construction sequence.
11.12.8.2.1 The Department or Delegated Agency must be notified and provide approval prior to conversion from sediment basin to the final configuration of the Constructed Wetlands.
11.12.8.2.2 Appropriate procedures must be implemented to prevent discharge of turbid waters when the sediment basin is being converted into a Constructed Wetland.
11.12.8.3 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.12.8.3.1 Pre-construction meeting;
11.12.8.3.2 Initial site preparation including installation of erosion and sediment controls;
11.12.8.3.3 Construction of the embankment, including installation of the principal spillway and the outlet structure as applicable;
11.12.8.3.4 Excavation and grading including interim and final elevations;
11.12.8.3.5 Construction of wetland features including grading of the various zones, introduction of soil amendments and staking of planting zones;
11.12.8.3.6 Construction of the underdrain, installation of gravel substrate and wetland soils as applicable;
11.12.8.3.7 Implementation of the planting plan and vegetative stabilization; and
11.12.8.3.8 Final inspection including development of a punch list for facility acceptance.
11.12.8.4 All areas surrounding the Constructed Wetlands that are graded or denuded during construction must be planted with turf grass, native plantings, or other approved methods of soil stabilization.
11.12.8.5 Outside of optimum seeding and planting dates, temporary seed, such as annual rye or winter wheat, may be used to stabilize the soil within the Constructed Wetland, but permanent species shall then be planted or seeded during the next optimum planting period.
11.12.8.6 Stabilization matting shall be utilized in Wetland Swales and in all areas of concentrated flow or slopes 3:1 or steeper.
11.12.8.7 Upon facility completion, the owner shall submit Post Construction verification documents to demonstrate that the Constructed Wetlands has been constructed within allowable tolerances in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. Allowable tolerances for Constructed Wetlands practices are as follows:
11.12.8.7.1 The constructed top of bank elevation may be no lower than the design elevation for top of bank.
11.12.8.7.2 The constructed volume of the Constructed Wetlands surface storage and any other required volumes shall be no less than 90% of the design volume.
11.12.8.7.3 The constructed volume of the gravel substrate storage for Submerged Gravel Wetlands shall be no less than 90% of the design volume.
11.12.8.7.4 The constructed elevation of any structure shall be within 0.15 foot of the design.
11.12.8.8 When the allowable tolerances are exceeded for Constructed Wetlands surface area or volume or structure elevations, supplemental calculations must be submitted to the approval agency to determine if the Constructed Wetlands, as constructed, meets the design requirements.
11.12.9 Constructed Wetland Maintenance Criteria
11.12.9.1 During the first two years following construction, the Constructed Wetland shall be reviewed twice each year by a qualified professional with demonstrated knowledge of wetland species, once in the spring and once in the fall after a storm event that exceeds one-half inch of rainfall. The Operation and Maintenance Plan shall outline a detailed schedule for the monitoring and possible reinstallation of vegetation in the wetland and its buffer for the first two years of establishment.
11.12.9.2 Repair of critical structural features such as embankments and risers shall be performed by responsible personnel that have successfully completed the Department Contractor Training Program.
11.12.9.3 Project closeout shall not occur until a minimum of 70% of the wetland area is permanently vegetated.
11.12.9.4 Sediment removal in the pretreatment forebay shall occur when 50% of total forebay capacity has been lost.
11.12.9.5 The Department or the Delegated Agency shall be notified before a Constructed Wetland is drained.
11.13 Wet Ponds
11.13.1 Wet Ponds are stormwater storage practices that consist of a combination of a permanent pool, micropool, or shallow marsh that promote a good environment for gravitational settling, biological uptake and microbial activity. Wet Ponds are widely applicable for most land uses and are best suited for larger drainage areas. Runoff from each new storm enters the wet pond and partially displaces pool water from previous storms. The pool also acts as a barrier to re-suspension of sediments and other pollutants deposited during prior storms. When sized properly, Wet Ponds have a residence time that ranges from many days to several weeks, which allows numerous pollutant removal mechanisms to operate. Wet Ponds can also provide storage above the permanent pool to help meet stormwater management requirements for larger storms. Design variants include:
11.13.1.1 Wet Quantity Management Pond
11.13.1.1.1 The RPv runoff reduction performance credit for this variant is 0%.
11.13.1.1.2 The Cv runoff reduction performance credit for this variant is 0%.
11.13.1.1.3 The Fv runoff reduction performance credit for this variant is 0%.
11.13.1.1.4 The total nitrogen pollutant reduction performance credit for this variant is not less than 0%.
11.13.1.1.5 The total phosphorus pollutant reduction performance credit for this variant is not less than 0%.
11.13.1.1.6 The total suspended solids pollutant reduction performance credit for this variant is not less than 0%.
11.13.1.2 Wet Extended Detention (ED) Pond
11.13.1.2.1 The RPv runoff reduction performance credit for this variant having 48 hours of extended detention is 100%.
11.13.1.2.2 The Cv runoff reduction performance credit for this variant having 48 hours of extended detention is 1%.
11.13.1.2.3 The Fv runoff reduction performance credit for this variant having 48 hours of extended detention is 0%.
11.13.1.2.4 The total nitrogen pollutant reduction performance credit for this variant having 48 hours of extended detention is not less than 30%.
11.13.1.2.5 The total phosphorus pollutant reduction performance credit for this variant having 48 hours of extended detention is not less than 55%.
11.13.1.2.6 The total suspended solids pollutant reduction performance credit for this variant having 48 hours of extended detention is not less than 60%.
11.13.2 Wet Ponds constructed to meet regulatory stormwater management requirements shall be designed and constructed in accordance with the USDA NRCS Pond Code 378 as amended.
11.13.3 Wet Pond Feasibility Criteria
11.13.3.1 Soil investigations must be conducted in accordance with subsection 12.1 to determine the suitability of the soils to meet recommended embankment and permanent pool criteria.
11.13.3.2 Locating Wet Ponds within perennial streams will require all appropriate state or federal permits.
11.13.4 Wet Pond Conveyance Criteria
11.13.4.1 Principal Spillway
11.13.4.1.1 The principal spillway must be accessible from dry land.
11.13.4.1.2 A structure-pipe spillway shall be designed with anti-flotation, anti-vortex and trash rack devices on the structure.
11.13.4.1.3 The outfall pipe and all connections to the outfall structure shall be made watertight. Soil tight only joints are not acceptable.
11.13.4.1.4 Anti-seep collars shall be used in accordance with Pond Code 378, as amended.
11.13.4.1.5 When the principal spillway is composed of a weir wall discharging to a channel, the channel below the weir must be reinforced with riprap or other acceptable material to prevent scour.
11.13.4.1.6 When a low flow orifice is specified, it must be adequately protected from clogging by either an acceptable external trash rack or by internal orifice protection. Orifice diameters shall not be less than three inches unless internal orifice control is provided.
11.13.4.2 The design shall specify an outfall that can discharge the maximum design storm event in a non-erosive manner at the project point of discharge.
11.13.4.3 Wet Ponds must be designed to pass the maximum design storm event (Fv) if the Fv is being routed through the Wet Pond rather than bypassing. An earthen emergency spillway designed to convey the Fv shall be cut in natural ground or, if cut in fill, shall be constructed and stabilized with methods to prevent erosion and structural failure.
11.13.4.4 Inflow Points
11.13.4.4.1 Inflow points into the Wet Pond must be stabilized to ensure that non-erosive conditions exist during storm events up to the conveyance event (Cv).
11.13.4.4.2 A forebay shall be provided at each inflow location that provides 10% or greater of the total RPv inflow to the Wet Pond.
11.13.4.5 In the event that the embankment is a regulated dam, the designer must verify that the appropriate Dam Safety Permit has been approved by the Department's Dam Safety Program.
11.13.5 Wet Pond Pretreatment Criteria
11.13.5.1 A forebay must be located at each major inlet to trap sediment and preserve the capacity of the main treatment cell.
11.13.5.2 The following criteria apply to forebay design:
11.13.5.2.1 A major inlet is defined as an individual storm drain inlet pipe or open channel conveying at least 10% of the Wet Pond's contributing RPv runoff volume.
11.13.5.2.2 A safety bench is required at the pond shoreline for forebay depths greater than three feet.
11.13.5.2.3 The forebay must be sized to contain 10% of the volume of runoff from the contributing drainage area for the Resource Protection event.
11.13.5.2.4 Discharge from the forebay shall be non-erosive.
11.13.6 Wet Pond Design Criteria
11.13.6.1 For RPv compliance, a Wet ED Pond must provide 48 hours extended detention for the RPv runoff volume. Detention shall be based on the time of initial inflow to time of final outflow from the facility. In order to simulate a base flow condition to the extent practicable, the peak discharge for the outflow hydrograph shall not exceed five times the average discharge rate.
11.13.6.2 The minimum depth to prevent the permanent pool area from being overtaken by undesirable vegetation is four feet.
11.13.6.3 The maximum depth of the permanent pool shall not exceed eight feet for safety reasons.
11.13.6.4 Earthen side slopes for Wet Ponds both above and below permanent pool shall be no steeper than 3H:1V.
11.13.6.5 Excluding areas containing retaining walls, when Wet Pond side slopes above permanent pool are steeper than 4H:1V, a ten foot wide safety bench shall be constructed one foot above the permanent pool. The maximum slope of the safety bench shall be 5.0%.
11.13.6.6 A 10 foot wide aquatic bench shall be provided one foot below permanent pool.
11.13.6.7 Retaining walls
11.13.6.7.1 Retaining walls around Wet Ponds shall be limited to no more than 50% of the pond perimeter based upon the peak elevation of the Cv.
11.13.6.7.2 In order to maintain the safety requirements, retaining walls shall be configured as follows:
11.13.6.7.2.1 The retaining wall at the permanent pool shall have a maximum height of three feet above the aquatic bench.
11.13.6.7.2.2 Any additional retaining walls shall have a maximum height of two feet and provide a minimum 10-foot level terrace from a lower retaining wall.
11.13.6.8 Liners
11.13.6.8.1 All Wet Ponds shall be evaluated for feasibility and ability to maintain permanent pool, including the need for a liner, by a qualified, licensed geotechnical engineer or geologist. If the design professional chooses not to follow the recommendations of the geotechnical professional, a signed, sealed and dated letter from the design professional providing justification for removal of the liner from the design shall be provided to the Department or Delegated Agency.
11.13.6.8.2 When the geotechnical engineer recommends a liner, acceptable options include the following:
11.13.6.8.2.1 A clay liner having a minimum compacted thickness of 12 inches with an additional 12 inch layer of compacted soil above it. Clay used as a pond liner must meet the following specifications:
11.13.6.8.2.1.1 Permeability of 1x10-6 centimeters per second (cm/sec) using ASTM D-2434 procedure.
11.13.6.8.2.1.2 Plasticity index of not less than 15% using ASTM D-423/424 procedures.
11.13.6.8.2.1.3 Liquid limit of not less than 30% using ASTM D-2216 procedure.
11.13.6.8.2.1.4 Clay particles passing not less than 30% using ASTM D-422 procedure.
11.13.6.8.2.1.5 Compaction of 95% of standard proctor density using ASTM D-2216 procedure.
11.13.6.8.2.2 A 30 mil poly-liner; or
11.13.6.8.2.3 Other acceptable measures as recommended by a qualified geotechnical professional.
11.13.6.9 Trash racks shall be provided for low-flow pipes and for all riser structure openings.
11.13.6.9.1 All metal trash racks shall be coated with a rust inhibitor to increase longevity of the device.
11.13.6.9.2 The low flow extended detention orifice shall be protected from clogging by an external trash rack.
11.13.6.10 When a riser is used, it must be located such that it is accessible from the pond side slope or safety bench for the purposes of inspection and maintenance.
11.13.6.11 All materials used in construction of a Wet Quantity Management Pond or Wet ED Pond shall meet the material specifications in USDA NRCS Pond Code 378 as amended.
11.13.6.12 Safety Features
11.13.6.12.1 Any opening 12 inches or greater discharging to a closed drainage system shall include safety grates.
11.13.6.12.2 The emergency spillway and exit channel must be designed to direct runoff to a point of discharge without adversely impacting downstream structures.
11.13.6.13 All Wet Ponds must be designed so as to be accessible for maintenance.
11.13.6.13.1 Adequate maintenance access must extend to the pretreatment, safety bench, riser, and outlet structure.
11.13.6.13.2 A maintenance right-of-way or easement must extend to the Wet Pond from a public or private road.
11.13.6.13.3 Maintenance access must meet the following criteria:
11.13.6.13.3.1 Minimum width of 15 feet.
11.13.6.13.3.2 Profile grade that does not exceed 10H:1V.
11.13.6.13.3.3 Minimum 10H:1V cross slope.
11.13.6.14 Maintenance Set-Aside Area
11.13.6.14.1 The maintenance set-aside area shall accommodate the volume of 50% of the collective forebay volume.
11.13.6.14.2 The maximum depth of the set aside area shall be one foot.
11.13.6.14.3 The slope of the set aside area shall not exceed 5.0%.
11.13.7 Wet Pond Landscaping Criteria
11.13.7.1 Woody vegetation shall not be planted or allowed to grow within 15 feet of the embankment and 10 feet on either side of principal spillway or pipes.
11.13.7.2 A planting plan must be provided that indicates the methods used to establish and maintain vegetative coverage in the Wet Pond and its vegetated perimeter. Minimum elements of a planting plan include the following:
11.13.7.2.1 Delineation of zones within both the Wet Pond and vegetated perimeter area;
11.13.7.2.2 Selection of corresponding plant species; and
11.13.7.2.3 Size and spacing of plant material or application rate of seed mixes, as applicable.
11.13.7.2.3.1 Native plant material shall be specified by botanical and common name.
11.13.7.2.3.2 Seed mixes shall be specified by botanical and common names as well as percentages by weight or volume.
11.13.8 Wet Pond Construction
11.13.8.1 Use of Wet Ponds for Erosion and Sediment Control
11.13.8.1.1 Approval from the Department or the appropriate Delegated Agency must be obtained before any planned Wet Quantity Management Pond or Wet ED Pond can be used as a sediment basin.
11.13.8.1.2 If a Wet Pond serves as a sediment basin during project construction, the volume of the sediment basin must be based on the more stringent sizing rule.
11.13.8.1.3 The Sediment and Stormwater Plan must include conversion steps from sediment basin to permanent Wet Pond in the construction sequence.
11.13.8.1.3.1 The Department or Delegated Agency must be notified and provide approval prior to conversion from sediment basin to the final configuration of the Wet Quantity Management Pond or Wet ED Pond.
11.13.8.1.3.2 Appropriate procedures must be implemented to prevent discharge of turbid waters when the sediment basin is being converted into a Wet Pond.
11.13.8.2 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.13.8.2.1 Pre-construction meeting.
11.13.8.2.2 Initial site preparation including installation of erosion and sediment controls.
11.13.8.2.3 Construction of the embankment, including installation of the principal spillway and the outlet structure.
11.13.8.2.4 Excavation and grading including interim and final elevations.
11.13.8.2.5 Implementation of the planting plan and vegetative stabilization.
11.13.8.2.6 Final construction review including development of a punch list for facility acceptance.
11.13.8.3 All areas surrounding the Wet Pond that are graded or denuded during construction must be planted with turf grass, native plantings, or other approved methods of soil stabilization.
11.13.8.4 Upon facility completion, the owner shall submit post construction verification documents to demonstrate that the wet pond has been constructed within allowable tolerances and in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency. Allowable tolerances for wet pond practices are as follows:
11.13.8.4.1 The constructed top of bank elevation may be no lower than the design elevation for top of bank.
11.13.8.4.2 The constructed volume of the wet pond surface storage shall be no less than 90% of the design volume.
11.13.8.4.3 The constructed elevation of any structure shall be within 0.15 foot of the design.
11.13.8.5 When the allowable tolerances are exceeded for wet pond surface area or volume or structure elevations, supplemental calculations must be submitted to the approval agency to determine if the wet pond, as constructed, meets the design requirements.
11.13.9 Wet Pond Maintenance Criteria
11.13.9.1 Repair of critical structural features, such as embankments and risers, shall be performed by responsible personnel that have successfully completed the Department Contractor Training Program.
11.13.9.2 The Department or the Delegated Agency shall be notified before a Wet Pond is drained.
11.13.9.3 Sediment removal in the Wet Pond pretreatment forebay must occur when 50% of total forebay capacity has been lost.
11.14 Soil Amendments
11.14.1 Soil Amendment, also called soil restoration, is a technique applied after construction to till compacted soils and restore their porosity by amending them with compost. Soil amendments reduce the generation of runoff from compacted urban lawns and may also enhance the performance of impervious cover disconnections and grass channels.
11.14.2 Soil Amendment Stormwater Credit Calculations
11.14.2.1 Soil Amendments receive the following runoff reduction performance credits. Runoff reduction allowances are applied to the amendment area only.
11.14.2.1.1 The RPv runoff reduction performance credit for Soil Amendments is based upon the hydrologic soil group (HSG) of the existing soil:
11.14.2.1.1.1 HSG A receives 48% annual runoff reduction credit.
11.14.2.1.1.2 HSG B receives 50% annual runoff reduction credit.
11.14.2.1.1.3 HSG C receives 29% annual runoff reduction credit.
11.14.2.1.1.4 HSG D receives 13% annual runoff reduction credit.
11.14.2.1.2 The Cv runoff reduction performance credit for soil amendments is 10% of the RPv allowance.
11.14.2.1.3 The Fv runoff reduction performance credit for soil amendments is 1% of the RPv allowance.
11.14.2.2 The total nitrogen, total phosphorus, and total suspended solids pollutant reduction performance credit for soil amendments is 100% of the load reduction.
11.14.3 Soil Amendments shall not be applied where:
11.14.3.1 The water table or bedrock is located within two feet of the soil surface. Subsection 12.1 shall be followed for determination of depth to the limiting layer.
11.14.3.2 Slope of soil to be amended exceeds 10%.
11.14.3.3 Soil to be amended is saturated or seasonally wet.
11.14.4 Soil Amendment Design Criteria
11.14.4.1 Soil testing shall be conducted during two stages of the Soil Amendment process.
11.14.4.1.1 The first test shall be performed to determine soil properties to a depth one foot below the proposed soil amendment area, with respect to saturation, bulk density, pH, salts, and soil nutrients. The initial test shall determine what soil amendments are needed.
11.14.4.1.2 The second soil test shall be conducted at least one week after compost has been incorporated into the soils to determine whether any further nutritional requirements, pH adjustment, and organic matter adjustments are necessary for plant growth.
11.14.4.2 When Soil Amendments are used to either adjust the hydrologic soil group of the amended area to lower the curve number of the site, or receive the annual runoff reduction performance credits for the amendment area, the soil amendment area shall receive no impervious cover runoff and shall place three inches of compost into the soil amendment area to a minimum incorporation depth of six inches using a tiller.
11.14.4.3 When Soil Amendments are used within the footprint of a BMP such as sheet flow to filter strip, sheet flow to open space, or vegetated channels to adjust the hydrologic soil group of the amended area and receive the runoff reduction performance credits for those BMPs, the following criteria apply:
11.14.4.3.1 Soil amendment areas having a contributing impervious cover (square feet) to surface area of compost amendment (square feet) ratio of up to 0.5 shall place four inches of compost into the soil amendment area to a minimum incorporation depth of eight inches using a tiller.
11.14.4.3.2 Soil amendment areas having a contributing impervious cover (square feet) to surface area of compost amendment (square feet) ratio of 0.51 to 0.75 shall place six inches of compost into the soil amendment area to a minimum incorporation depth of 15 inches using an excavation and mixing method.
11.14.4.3.3 Soil amendment areas having a contributing impervious cover (square feet) to surface area of compost amendment (square feet) ratio greater than 0.75 shall place eight inches of compost into the soil amendment area to a minimum incorporation depth of 20 inches using an excavation and mixing method.
11.14.4.4 Compost incorporation depths greater than 12 inches require removal of the existing soil down to the incorporation depth and physically mixing existing soil with compost.
11.14.4.5 Compost used for soil amendment shall be STA certified compost, meeting the requirements of Delaware Erosion and Sediment Control Handbook Appendix A-6 Compost Material Properties.
11.14.5 Soil Amendment Construction Criteria
11.14.5.1 For compost incorporation depths up to 12 inches:
11.14.5.1.1 The proposed incorporation area shall be deep tilled to a depth of two to three feet using a tractor and sub-soiler. This deep-tilling step may be omitted when soil amendment is used for filter strip widths of 20 feet or less in the direction of flow.
11.14.5.1.2 Existing soils shall be in dry condition prior to incorporating compost.
11.14.5.1.3 The compost layer shall be placed on surface of proposed amendment area to the depth specified and then incorporated into the soil using a roto-tiller or similar equipment.
11.14.5.1.4 Conduct soil test to determine whether any further nutrient requirements, pH adjustment, and organic matter adjustments are necessary for plant growth.
11.14.5.2 For compost incorporation depths 12 inches or greater:
11.14.5.2.1 The proposed amendment area shall be excavated to the required incorporation depth, as follows:
11.14.5.2.1.1 Remove topsoil and stockpile for later use.
11.14.5.2.1.2 Excavate subsoil working in strips perpendicular to the slope and flow path using multiple lifts.
11.14.5.2.1.3 Separate and remove a minimum of 25% of the subsoil, taking the most densely compacted soils for removal. Stockpile remaining subsoil next to excavated area, separately from topsoil.
11.14.5.2.1.4 Scarify bottom of excavated area.
11.14.5.2.2 Amended soil shall be returned to the soil amendment area as follows. The number of lifts may vary depending on the capabilities of the equipment being used, but a minimum of two lifts is required.
11.14.5.2.2.1 Replace subsoils by loosening, aerating, and mixing subsoil.
11.14.5.2.2.2 Replace stockpiled topsoil.
11.14.5.2.2.3 Incorporate required layer of compost, such that compost is uniformly incorporated throughout. Existing soils shall be in dry condition prior to incorporating compost.
11.14.5.2.2.4 Repeat above steps for each lift.
11.14.5.2.3 Rake to level and remove surface woody debris and rocks larger than one inch.
11.14.5.2.4 The finished grade of the combination of replaced subsoil, topsoil and compost shall be a minimum of four inches above the existing grade to account for settlement, but must be adjusted to account for field conditions and soil texture, such that a final settled grade at three months post-installation matches the original grade.
11.14.5.2.5 Conduct soil test to determine whether any further nutritional requirements, pH adjustment, and organic matter adjustments are necessary for plant growth.
11.14.5.3 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.14.5.3.1 Pre-construction meeting;
11.14.5.3.2 Initial site preparation including installation of erosion and sediment controls;
11.14.5.3.3 Deep tillage using subsoiler or excavation of existing subsoil;
11.14.5.3.4 Incorporation of compost amendment into existing soil including verification of the depth of compost amendment;
11.14.5.3.5 Implementation of required stabilization and planting plan; and
11.14.5.3.6 Final construction review including development of a punch list for facility acceptance.
11.14.5.4 Upon project completion, the owner shall submit Post Construction verification documents, including but not limited to compost delivery tickets and photo documentation of construction, to demonstrate that the soil amendment has been constructed within in accordance with the approved Sediment and Stormwater Management Plan and accepted by the approving agency.
11.15 Proprietary Practices
11.15.1 Proprietary Practices are manufactured stormwater treatment practices that utilize settling, filtration, absorptive/adsorptive materials, vortex separation, vegetative components, or other appropriate technology to manage the impacts caused by stormwater runoff.
11.15.2 Proprietary Practices Stormwater Credit Calculations
11.15.2.1 Proprietary Practices receive no runoff reduction credits unless approved by the Department.
11.15.2.2 Practices may receive pollutant reduction credits as determined by the Department on a case-by-case basis.
11.15.3 All proprietary practices shall be designed to safely overflow or bypass flows from larger storm events to downstream drainage systems.
11.15.4 Pretreatment shall be provided in accordance with manufacturer's recommendations for individual Proprietary Practices.
11.15.5 Proprietary Practice Design Criteria
11.15.5.1 Design criteria for Proprietary Practices shall be proposed by the manufacturer and approved by the Department.
11.15.5.2 All Proprietary Practices must be designed so as to be accessible for maintenance.
11.15.5.2.1 A maintenance right-of-way or easement must extend to the Proprietary Practice from a public or private road.
11.15.5.2.2 Adequate maintenance access must extend to the all components of the Proprietary Practice.
11.15.5.2.3 Maintenance access must meet the following criteria:
11.15.5.2.3.1 Minimum width of fifteen feet.
11.15.5.2.3.2 Profile grade that does not exceed 10H:1V.
11.15.5.2.3.3 Minimum 10H:1V cross slope.
11.15.6 Landscaping shall be provided in accordance with manufacturer's recommendations for individual Proprietary Practices.
11.15.7 Proprietary Practice Construction Sequence
11.15.7.1 Construction and installation of Proprietary Practices shall be conducted in accordance with manufacturer's recommendations for individual Proprietary Practices.
11.15.7.2 Construction reviews are required during the following stages of construction, and shall be noted on the plan in the sequence of construction:
11.15.7.2.1 Pre-construction meeting;
11.15.7.2.2 Initial site preparation including installation of erosion and sediment controls;
11.15.7.2.3 Construction of the Proprietary Practice in accordance with manufacturer's recommendations;
11.15.7.2.4 Implementation of required stabilization and planting plan as applicable; and
11.15.7.2.5 Final construction review including development of a punch list for facility acceptance.
11.15.7.3 Upon project completion, the owner shall submit Post Construction verification documents to demonstrate that the Proprietary Device has been installed in accordance with manufacturer's recommendations.
11.15.7.4 All Proprietary Practices shall be inspected and maintained in accordance with the manufacturer's instructions and recommendations.
11.16 Source Controls
11.16.1 Source Control consists of measures to prevent pollutants from coming into contact with stormwater runoff. Preventing pollutant exposure to rainfall and runoff is an important management technique that can reduce the amount of pollutants in runoff and the need for stormwater treatment.
11.16.2 Design variants for Source Controls include:
11.16.2.1 Nutrient Management
11.16.2.2 Street Sweeping
11.16.3 Source Controls Stormwater Credit Calculations
11.16.3.1 Source controls receive no runoff reduction credits.
11.16.3.2 Nutrient Management receives the following pollutant reduction credits:
11.16.3.2.1 The total nitrogen pollutant reduction performance credit is not less than 17%.
11.16.3.2.2 The total phosphorous pollutant reduction performance credit is not less than 22%.
11.16.3.2.3 The total suspended solids pollutant reduction performance credit is not less than 0%.
11.16.3.3 Street Sweeping receives the following pollutant reduction credits:
11.16.3.3.1 The total nitrogen pollutant reduction performance credit is not less than 3%.
11.16.3.3.2 The total phosphorous pollutant reduction performance credit is not less than 3%.
11.16.3.3.3 The total suspended solids pollutant reduction performance credit is not less than 9%.
11.16.4 Source Controls Design Summary
11.16.4.1 To receive nutrient management pollutant reduction performance credits, sites must fully comply with the requirements of the Delaware Nutrient Management Law through implementation of a nutrient management plan.
11.16.4.2 To receive street sweeping pollutant reduction performance credits, sites must submit to the Department or Delegated Agency a plan documenting the street sweeping frequency. Annual street sweeping tracking shall be submitted to the Department or Delegated Agency.
11.17 Afforestation
11.17.1 Afforestation includes practices that mimic the hydrologic benefits of a natural forest utilizing a regeneration process within the landscape by selectively planting tree seedlings (less than one inch DBH) or saplings (greater than one inch DBH). Afforestation can be used as both a runoff reduction practice by converting non-forested areas to forested areas as well as a mitigation practice for offsetting the clearing of forested areas during the development process. Design variants for afforestation include:
11.17.1.1 Afforestation. RPv credit based on runoff reduction from open space (good) condition to wooded (good) condition.
11.17.1.2 Urban Tree Planting. For areas planted in trees that do not meet the design criteria for afforestation, credit shall be an equivalent 1/200th of an acre per tree.
11.17.2 Afforestation Design Criteria
11.17.2.1 The minimum size of the afforestation area shall be 10,000 square feet with a minimum width of 50 feet.
11.17.2.2 The proposed afforestation area shall be upland.
11.17.2.3 At the end of the second year there shall be at least 200 live plants six inches or higher.
11.17.3 Afforestation Landscaping Criteria
11.17.3.1 The planting density shall account for mortality, which over time can result in more random arrangement of the trees.
11.17.3.2 Final stabilization shall meet EPA requirements at the end of the second year.
11.17.4 Afforestation Construction Sequence
11.17.4.1 The sizes and types of plantings shall be in accordance with the planting schedule developed for the site.
11.17.4.2 Construction reviews are necessary for the success of any phase of a project, including Pre-construction meeting; Planting Phase (with designer and installation contractor); and Final Review (punch list of corrections for acceptance).
11.17.5 Afforestation Maintenance Criteria
11.17.5.1 Following planting, a period of maintenance and monitoring will begin. The afforestation planting will be considered successful if the survival of trees at the end of the second year is at least 200 combined live, planted or volunteer, trees per acre.

7 Del. Admin. Code § 5101-11.0

22 DE Reg. 680 (2/1/2019) (Final)