Section Env-Wq 1504.09 - Stormwater Drainage Report; Drainage Area Plans; Hydrologic Soil Group Plans A stormwater drainage report, associated drainage area plans, and associated hydrologic soil group plans shall include the following:
(a) A narrative with the following information:(1) A description of the pre-development and post-development conditions affecting drainage;(2) The total impervious area assumed per lot, as applicable;(3) A discussion of how the proposed development is likely to impact hydrologically-connected surface waters of the state, and wetlands as defined in RSA 482-A, and other properties;(4) A comparison between the pre-development peak discharge rates and the post-development peak discharge rates, for the one-year, 2-year, 10-year, and 50-year, 24-hour storms;(5) A discussion of how treatment criteria will be met in accordance with Env-Wq 1507.03; and(6) A discussion of how groundwater recharge is met in accordance with Env-Wq 1507.04;(b) Calculations for pre- and post-construction stormwater drainage, for 24-hour duration storms with minimum return frequencies of once in one year, 2 years, 10 years, and 50 years using the NRCS "National Engineering Handbook", Part 630, Hydrology, or Win TR-20, version 3.20.0000, as developed by the NRCS for determining the rate of runoff, both available as noted in Appendix B, subject to the following: (1) Depth of precipitation shall be determined using either of the following, both available as noted in Appendix B:a. "Extreme Precipitation in New York and New England", version 2.0, published by the USDA, NRCS http://websoilsurvey.nrcs.usda.gov, and Cornell University's Northeast Regional Climate Center; orb. NOAA's National Weather Service, Hydrometeorological Design Studies Center, Precipitation Frequency Data Server; (2) The time of concentration shall be determined as follows: a. If using the Lag method for determining the time of concentration, identify the hydraulic length in feet and provide calculations for determining the average land slope in percent for each sub-basin;b. If using the velocity method for determining the time of concentration, identify the sheet flow path(s), the shallow concentrated flow path(s), and the channel flow path(s) for each sub-basin; andc. The sheet flow path identified in b. above, shall be limited to 100 feet;(3) Except as required for solar arrays pursuant to Env-Wq 1511.05, the curve number for each sub-basin shall be calculated as follows:a. For proposed areas of disturbance, including lot development, the hydrologic condition for woods, meadows, or pastures shall be assumed to be "good", as defined by the National Engineering Handbook referenced in (b) above; b. For proposed areas of disturbance, including lot development, soil types shall be identified in accordance with the Society of Soil Scientists of Northern New England (SSSNNE) Special Publication No. 3, Version 7.0, Site-Specific Soil Mapping Standards for New Hampshire and Vermont, July 2021, available as noted in Appendix B;c. For all other areas that contribute runoff to the project site, soil types shall be:2. Identified in accordance with SSSNNE Special Publication No. 3, Version 7.0, Site-Specific Soil Mapping Standards for New Hampshire and Vermont, July 2021, available as noted in Appendix B; andd. The hydrologic soil group shall be assigned in accordance with SSSNNE Special Publication No. 5, K Values for New Hampshire Soils, September 2009, as available in Appendix B; and(4) If the calculations are done using software that provides error messages, warnings, or other such indicators, such as HydroCAD®, a copy of the calculations shall be submitted that shows all error messages, warnings, and other such indicators;(c) WQV, WQF, and GRV calculations;(d) Calculations for designing outlet protection;(e) Drainage area plans for pre- and post-construction that delineate each sub-basin, including off-site areas which flow onto the project area, at a scale for off-site areas of one inch equals 100 feet, or at a scale of one inch equals 2,000 feet if delineated from a USGS map, and at a scale of one inch equals 50 feet for on-site areas, identifying the following:(1) The location of sub-basins, reaches, ponds, and all points of interest, as modeled in the stormwater drainage report;(2) The hydraulic length or time of concentration flow path;(3) Contours for on-site areas at the same intervals as the plans prepared pursuant to Env-Wq 1504.02 through Env-Wq 1504.05, as applicable, and contours for off-site areas at the same interval or at the intervals on the applicable USGS map;(4) Roadway station numbering, if applicable; and(5) Drainage structures such as detention basins, culverts, and treatment practices;(f) If the plans prepared pursuant to (e), above, for the on-site areas comprise more than 3 sheets at the specified scale, an overview sheet which shows the location of each 50-foot scale sheet and an outline of the area to be impacted by the proposed project; and(g) Hydrologic soil group plans for pre- and post-construction that delineate each sub-basin, including off-site areas which flow onto the project area, identifying the following: (1) The location of sub-basins, as modeled in the stormwater drainage report;(2) The locations of the different soil groups using the following color-coding: a. Hydrologic soil group A soils shall be shaded green;b. Hydrologic soil group B soils shall be shaded yellow;c. Hydrologic soil group C soils shall be shaded orange;d. Hydrologic soil group D soils shall be shaded red;e. Open water features shall be shaded blue; andf. Impervious cover shall be shaded gray; (3) The map symbol identifying the soil mapped; andN.H. Admin. Code § Env-Wq 1504.09
(See Revision Note #1 at chapter heading for Env-Wq 1500) #9343, eff 1-1-09; (See Revision Note #2 at chapter heading for Env-Wq 1500) amd by #9817-A, eff 12-1-10 (formerly Env-Wq 1504.08)
Amended by Volume XXXVII Number 32, Filed August 10, 2017, Proposed by #12342, Effective 8/15/2017, Expires 8/15/2027.Amended by Volume XLII Number 2, Filed January 13, 2022, Proposed by #13317, Effective 12/22/2021, Expires 12/22/2031.Amended by Number 41, Filed October 12, 2023, Proposed by #13758, Effective 9/28/2023, Expires 9/28/2033 (see Revision Note #4 at chapter heading for Env-Wq 1500).