Current through Register No. 13, March 27, 2025
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; or
b. 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; and
c. 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:
1.
Taken from the NRCS county-wide web soil survey at
http://websoilsurvey.nrcs.usda.gov;
or
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; and
d. 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; and
f.
Impervious cover shall be shaded gray;
(3) The map symbol identifying the soil
mapped; and
(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)
