Section 37.03.06.050 - NEW DAMS AND RESERVOIRSThe following criteria shall be used by the Director as a basis to evaluate the design of new embankment dams and reservoirs. These guidelines are intended for a broad range of circumstances, and engineers should not consider them as a restriction to the use of other sound engineering design principles. Exclusion from these established criteria will be considered by the Director on a case-by-case basis during design review of plans, drawings, reports, and specifications submitted for approval prior to commencing construction. Structures which are or will be constructed of other materials, for example concrete, timber, steel, or combinations thereof shall comply with these criteria as found appropriate by the Director, and with other engineering design methods and construction standards of care approved by the Director.
01.Embankment Stability. Slope stability analyses shall determine the appropriate upstream and downstream slopes. Unless a discrete slope stability analysis determines otherwise, the embankment slopes of earthen dams shall comply with the following: Upstream slope | 3:1 or flatter |
Downstream slope | 2.5:1 or flatter |
a. Embankments shall be designed, constructed, and maintained to assure stability under static loads and prevent instability due to seepage or uplift forces, rapid drawdown conditions, and applied seismic loads.b. The design analysis shall consider the need for installing filters, including but not limited to chimney drains, blanket drains, or toe drains, to avoid developing saturated conditions and to protect against piping of the embankment fill material. Transmission of seepage through the embankment, abutments, and foundation shall be controlled to prevent internal erosion, the removal of material, or the creation of instability.c. The minimum factor of safety for a steady state loading condition shall be one point five (1.5.) The minimum factor of safety for rapid drawdown loading shall be one point two (1.2.) The minimum factor of safety for seismic loading shall be one point zero (1.0.)d. Seismic Stability. i. The stability of an embankment subjected to earthquake ground motions may be analyzed by the engineer using either a dynamic response or pseudo-static analyses. Pseudo-static analyses are acceptable for embankment dams and foundations composed of non-liquifiable soils that preclude the generation of excess pore water pressures due to shaking. Otherwise, the stability analysis shall employ a dynamic response method.ii. Slope deformation analyses are required for structures that are constructed of cohesionless soils exhibiting fine grain-size gradation and/or on foundations that may be subject to liquefaction.iii. The design analysis for regulated dams shall include in the seismic stability analysis peak ground accelerations obtained from Seismic Hazard Maps published by the United States Geological Survey (USGS) using a minimum return interval of 2 percent (2%) probability of exceedance in fifty (50) years, or greater interval, as determined by the Director.iv. The design analyses for large and high hazard dams shall include a report or report(s) covering geology, geologic hazard, and seismicity. The report(s) shall identify the location of faults, evaluate landslide potential, and include a history of seismicity. A comparison using deterministic and probabilistic analyses to calculate peak ground acceleration at the dam site may be required for geographic areas of the state showing evidence of seismic faults or faulting, as determined by the Director. e. Where in the opinion of the Director, embankment design or conditions warrant, the owner may be required to instrument their embankment or foundation.02.Top Width. The minimum top width for any embankment shall be twelve (12) feet to allow safe access by wheeled vehicles or tracked equipment for maintenance or repair.03.Cutoff Trenches or Walls. Cutoff trenches shall be excavated into competent foundation material to bear on an approved stratum or zone, as site conditions require and when employed. a. The cutoff trench shall be backfilled with suitable material free from organic matter and debris and compacted to the specified moisture and density. The cutoff trench shall extend up the sides of both abutments to the design maximum storage elevation.b. Cutoff trenches shall be wide enough to allow the free movement of excavation and compaction equipment. To provide for proper compaction side slopes shall be no steeper than one to one (1:1) for shallow depths up to twelve (12) feet, and no steeper than one and one half to one (1.5:1) for greater depths. Flatter slopes may be required for safety and stability, as determined by the Director.c. Concrete cutoff walls may be used in a similar manner as cutoff trenches, with the base firmly entrenched in the underlying foundation material. Where suitable bedrock or suitable foundation material exists, concrete cutoff walls shall be doweled with steel rebar a minimum depth and spacing determined by the engineer necessary to create a structural bond with the underlying foundation. Concrete walls shall have a minimum vertical projection above the foundation surface of three (3) feet, oriented perpendicular to the surface, and shall have a minimum thickness of twelve (12) inches. Reinforcement of the concrete may be required in addition to being doweled into suitable foundation material(s).04.Impermeable Core Material. Soils used to construct the inner sectional core of an embankment shall consist of relatively impermeable cohesive materials approved by the engineer and compacted in strict accordance with the approved plans and specifications. A minimum ninety-five percent (95%) maximum dry density compacted in accordance with the American Society Testing Materials (ASTM) D-698 is required. The use of other relatively impermeable however non-cohesive material is subject to approval by the Director on a case-by-case basis.05.Drains. Toe, blanket, or chimney drains consisting of approved free draining material or approved manufactured drainage geotextile shall be installed where necessary to maintain the phreatic line at or near the design level(s) within the embankment. a. Filter design for toe, blanket, or chimney drains, or any combination thereof shall be included in the design plans and specifications submitted by the engineer for review and approval by the Director.b. Perforated and slotted drainpipes must be four (4) inches diameter or greater and shall be surrounded by permeable drainage material to a distance equal to or greater than the outside pipe diameter. The maximum particle size of the drainage material shall be between one-half (1/2) inch to three-fourths (3/4) inch, or as specified by the design engineer based on the drainage filter analysis. Underdrains and collection pipes must be constructed of noncorrosive material, taking care to ensure slots and perforations are appropriately sized to avoid long-term migration of the drain material into the pipe.06.Freeboard. The elevation of the top of the embankment shall be constructed and maintained above the design flood surcharge level, including the vertical height of wind generated waves estimated for the greatest distance of open water measured perpendicular to the major axis of the dam. Camber estimated for post-construction settlement shall be included in the design and incorporated in the construction of the top of the embankment.a. The minimum freeboard shall be two (2) feet plus wave height as calculated for the design spillway flow capacity during passage of the one percent (1%) flood, or greater.07.Riprap. All embankments which are subject to erosion on either the upstream and downstream slope(s) shall be protected using riprap or other approved material. Pipes, cables, brush, tree growth, dead growth, logs, or floating debris are not acceptable substitutes for approved riprap. The engineer, with approval of the Director, shall determine the extent of slope protection as deemed necessary for existing site, seasonal, and operating conditions. a. Where rock riprap or other approved material is used for erosion protection on the upstream slope, it shall be placed on an approved thickness of well-graded and free-draining granular bedding material. Riprap or other approved erosion protection material shall extend up the slope a sufficient height.08.Outlet Conduits. All reservoirs impounding water shall have an outlet conduit of sufficient capacity to prevent interference with natural streamflow through the reservoir to the injury of downstream appropriators. In addition to any natural flow releases, the outlet conduit should be of sufficient capacity to pass at the same time, the maximum water requirement of the owner. A larger outlet conduit may be required to provide adequate release capacity as determined by the Director. Upon recommendation n of the design engineer, the Director may waive this requirement for off channel reservoirs.a. Outlet conduits shall be laid on a firm and stable foundation material to avoid the likelihood of differential settlement or consolidation causing the separation or misalignment of the conduit. Outlet conduits shall be encased on all sides by concrete of approved compressive strength and having a minimum thickness of twelve (12) inches. During construction outlet conduits shall be properly aligned on an established grade and adequately supported to prevent movement or damage caused by placement of concrete or by compaction equipment.b. Unless otherwise required, the outlet conduit shall have a minimum inside diameter of twelve (12) inches. The conduits shall consist of approved material and composition as approved by the Director. Exceptions may be made only where conditions warrant, but in no case shall the reasonable life expectancy of the pipe be less than the design life of the embankment.09.Gates and Valves. a. Conduits shall be gated on the upstream end to avoid pressurizing the conduit inside the embankment. Designed pressurized conduits shall be fitted with both a guard gate and a control gate or valve.b. All conduits shall be vented directly behind the gate.c. All gate stem pedestals shall be securely founded to prevent future movement.d. At least one (1) of the sides of the inlet structure shall be open to allow water to flow into the outlet conduit. The opening shall be covered with a trash rack.e. Trash racks should be designed to facilitate cleaning of trash and debris. If fish screens are used, they shall be placed over the trash rack and shall be removable for cleaning or be self-cleaning.10.Outlet Controls. Outlet controls shall be installed at a stable location, on the crest or on an elevated platform, or within an enclosure when required, but secured to prevent unauthorized operation. Reservoirs storing water during the winter and subject to severe freezing conditions shall have inclined gate stems or other controlling mechanical or hydraulic features enclosed in a protective sleeve which is buried beneath the upstream slope to suitable depth, to prevent damage or movement caused by ice.11.Release Capacity. Based on the size of the dam and the downstream hazard classification assigned by the Director, the release capacity shall equal or exceed the inflow design flood as set forth in the following table. Where the table specifies an inflow design flood range, the governing inflow design flood shall be determined by the professional engineer in responsible charge of design and IDWR based on a site-specific review of the proposed dam, watershed conditions, and downstream hazard potential. The minimum flow capacity of the emergency spillway(s) shall be sized using the one-percent (1%) rate of flow (i.e., Q100 cfs) calculated for the contributing watershed upstream from the dam, plus two (2) feet of freeboard, plus wave height. Hazard Classification | Dam Size Classification | Inflow Design Flood (IDF) |
Low | All Sizes | Q100 |
Significant | Small | Q100 |
| Intermediate | Q100 to Q500 |
| Large | Q500 |
High | Small | Q100 to Q500 |
| Intermediate | Q500 |
| Large | Q500 to PMF |
a. All spillways shall be stabilized for the discharge of flow using concrete, masonry, riprap, or sod, if not constructed in resistant rock.b. For embankment dams, where site conditions allow, the spillway shall be constructed independent of the embankment. The spillway(s) shall guide the discharge of water away from the embankment.c. The minimum base width of an open-channel spillway shall be ten (10) feet, or greater to allow access by mechanical equipment. Siphon pipes or pumps are not acceptable substitutes for an open-channel spillway.d. The effective flow capacity of spillways shall be undiminished by bridges, fences, pipelines, or other obstructions.e. The installation of stop logs or flashboards in the spillway is prohibited unless they are part of an approved design and included as an integral part of an approved operation plan.12.Reservoir Site. Prior to filling the reservoir, the site shall be cleared of all woody material, growth or debris that is large enough to lodge in the spillway, or outlet works.13.Inspection and Completion Reports. As construction proceeds, it is the responsibility of the engineer to submit test reports (e.g., soil material analyses, density tests, concrete strength tests, etc.) along with periodic inspection and progress reports to the Director.a. Upon completion of construction the owner or their engineer shall provide the Director a written narrative account of all items of construction. Record drawings (i.e., as-builts or as-constructed drawings) and revised specifications shall be submitted to the Director to accurately reflect the completed project works.b. The engineer, acting on behalf of and representing the owner, shall certify that the construction, reconstruction, enlargement, replacement, or repair of the embankment and appurtenances was completed in accordance with the record drawings and specifications.Idaho Admin. Code r. 37.03.06.050