The following stormwater management standards
shall be applied to all subdivision and site plans.
A. Untreated stormwater.
(1) No new stormwater conveyances may discharge untreated
stormwater directly to or cause erosion into wetlands or water bodies.
(2) Rooftop runoff is considered uncontaminated for the
purposes of these standards and therefore does not require treatment.
B. Postdevelopment peak discharge rates.
(1) Stormwater management systems must be designed so
that the ten-year, twenty-four-hour postdevelopment peak discharge
rate does not exceed the ten-year, twenty-four-hour predevelopment
peak discharge rates.
(2) In order to meet this standard, controls must be developed
for the two-year, ten-year and fifty-year, twenty-four-hour storm
events. The hundred-year, twenty-four-hour storm event must be evaluated
to demonstrate that there will not be increased flooding impacts off
site.
[Amended 6-26-2018 by Ord. No. O-18-015]
(3) Measurement of peak discharge rates shall be calculated
using point of discharge or the downgradient property boundary. The
topography of the site may require evaluation at more than one location
if flow leaves the property in more than one direction. An applicant
may demonstrate that a feature beyond the property boundary is more
appropriate as a design point.
C. Recharge to groundwater. Annual groundwater recharge
rates shall be maintained by providing infiltration by the use of
structural and nonstructural methods. The annual recharge from post
development site conditions shall mimic the annual recharge from predevelopment
site conditions. Best management practice (BMP) techniques to achieve
recharge requirements include infiltration, bioretention, dry swale,
and nonstructural techniques. Alternative techniques may be used if
they meet the performance criteria stated herein and are approved
by the Planning Board.
D. Water quality.
(1) For discharges to the Conservation Zone within the Water Supply Protection District as defined in §
190-24, the runoff volume to be treated for water quality is calculated as 1.0 inch of runoff multiplied by the total impervious area of the postdevelopment project site.
(2) For all other discharges, the runoff volume to be
treated for water quality is calculated as 0.5 inch of runoff multiplied
by the total impervious area of the postdevelopment project site.
(3) Removal of 80% of the total suspended solids (TSS),
floatables, greases, and oils. For new developments, stormwater management
systems shall be designed to remove 80% of the average annual load
of total suspended solids (TSS), floatables, greases, and oils after
the site is developed. This standard is met when:
(a)
The Planning Board determines that suitable
nonstructural practices for source control and pollution prevention
are implemented;
(b)
Stormwater management best management practices
(BMPs) capture the prescribed runoff volume; and
(c)
Stormwater management BMPs are maintained as
designed.
E. Critical areas land uses with higher potential pollutant
loads require the use of specific stormwater management BMPs, as detailed
in this section. The use of infiltration practices without pretreatment
is prohibited.
[Amended 6-26-2018 by Ord. No. O-18-015]
(1) The following uses are considered to create high potential
pollutant loads:
(a)
Any use requiring a National Pollutant Discharge
Elimination System (NPDES) stormwater discharge permit associated
with industrial activity;
(b)
Auto salvage yards/auto recycler facilities;
(c)
Auto fueling facilities/gas stations;
(d)
Fleet storage areas (cars, buses, trucks, public
works);
(e)
Vehicle service, maintenance and equipment cleaning
areas;
(g)
Road salt storage or loading areas if exposed
to rainfall;
(i)
Metal rooftops, including roofs made from aluminum,
tin, galvanized steel, copper, or rooftops which contribute significant
pollutant loads;
(j)
Outdoor storage and loading/unloading areas
of hazardous substances;
(k)
SARA 312 generators if materials or containers
are exposed to rainfall; and
(l)
The service, repainting, and hull maintenance
areas of marinas.
(2) The following measures are required in addition to BMPs described in Subsection
E(1) above, within areas with high potential pollutant loads:
(3) The following are prohibited within areas with high
potential pollutant loads located in a Conservation Zone:
(4) The following restrictions apply to certain BMPs within
areas of high potential pollutant loads: Sand or organic filters,
detention basins, wet ponds or constructed wetlands may be used only
if sealed or lined.
F. Wetland or water bodies.
(1) BMPs approved for use within 300 feet of a wetland or water body unless otherwise prohibited by §
190-24 are limited to:
(a)
Extended detention basins;
(h)
Infiltration trenches; and
(i)
Deep sump and hooded catch basins (used with
other BMPs).
(2) Stormwater management systems should incorporate designs
which allow for shutdown and containment in the event of an emergency
spill or other unexpected contamination event.
G. Redevelopment. Redevelopment of previously developed sites must meet
the stormwater management standards to the maximum extent possible
to be determined by the Planning Board. The goal is to reduce impervious
area by a minimum of 20% or meet the open space requirement in that
zoning district. The application shall include a certification by
a registered professional engineer as to compliance with this standard.
[Amended 6-26-2018 by Ord. No. O-18-015]
H. Erosion and sedimentation plans.
[Amended 6-26-2018 by Ord. No. O-18-015]
(1) Erosion and sedimentation controls must be implemented to prevent
impacts during construction or post-construction or land disturbance
activities, and shall be properly installed prior to soil disturbance
in the contributing drainage area, and a plan shall contain the following:
(a)
A project narrative including a description of the development.
(b)
Locus map showing property boundaries.
(c)
North arrow, scale, date.
(e)
Structures, roads, and utilities.
(f)
Topographic contours at two-foot intervals.
(h)
Within the project area and within 50 feet of the project boundary
based on Nashua GIS, surface waters, wetlands, and drainage patterns
and watershed boundaries.
(j)
Extent of 100-year floodplain boundaries if published or determined.
(l)
Areas of soil disturbance.
(n)
Locations of equipment storage and staging.
(o)
Highlighted areas of poorly and very poorly drained soils.
(p)
Highlighted areas of poorly and/or very poorly drained soils
proposed to be filled.
(q)
Location, descriptions, details, and design criteria and calculations
for all structural, nonstructural, permanent, and temporary erosion
and sedimentation control measures and BMPs.
(r)
Identification of all permanent control measures.
(s)
Identification of permanent snow storage areas.
(t)
Identification of snow management measures during construction.
(u)
Description of the combination of sediment and erosion control
measures which are required to achieve maximum pollutant removal such
as:
[1]
Sediment basins. For common drainage that serves an area with
10 or more acres disturbed at one time, a temporary (or permanent)
sediment basin must provide storage for a calculated volume of runoff
from a drainage area from a two-year, twenty-four-hour storm, or equivalent
control measures must be provided, where attainable, until final stabilization
of the site. Where no such calculation has been performed, a temporary
(or permanent) sediment basin providing 3,600 cubic feet of storage
per acre drained, or equivalent control measures, must be provided
where attainable until final stabilization of the site. When computing
the number of acres draining to a common location, it is not necessary
to include flows from off-site areas and flows from on-site areas
that are either undisturbed or have undergone final stabilization
where such flows are diverted around both the disturbed areas and
the sediment basin. In determining whether a sediment basin is attainable,
the operators may consider such factors as site soils, slope, available
area on-site, etc. In any event, the operator must consider public
safety, especially as it relates to children, as a design factor for
the sediment basin, and alternative sediment controls must be used
where site limitations would preclude a safe design.
[2]
Drainage locations which serve 10 or more disturbed acres at
one time and where temporary sediment basins or equivalent controls
are not attainable, smaller sediment basins and/or sediment traps
should be used. At a minimum, silt fence, vegetative buffer strips,
or equivalent sediment controls are required for all downslope boundaries
(and for those side slope boundaries deemed appropriate as dictated
by individual site conditions).
[3]
Drainage locations serving less than 10 acres may utilize smaller
sediment basins and/or sediment traps. At a minimum, silt fence, vegetative
buffer strips, or equivalent sediment controls are required for all
downslope boundaries (and for those side slope boundaries deemed appropriate
as dictated by individual site conditions) of the construction area
unless a sediment basin providing storage for a calculated volume
of runoff from a two-year, twenty-four-hour storm or 3,600 cubic feet
of storage per acre drained is provided.
(2) Whenever practical, natural vegetation shall be retained, protected
or supplemented. Priority shall be given to preserving natural drainage
systems, including perennial and intermittent streams, wetlands, swales,
and drainage ditches for conveyance of runoff leaving the project
area.
(3) Examples of BMPs for erosion and sedimentation control are staked
straw bales, filter fences, hydroseeding, and phased development.
Many stormwater BMP technologies (e.g., infiltration technologies)
are not designed to handle high concentrations of sediments typically
found in construction runoff and must be protected from construction-related
sediment loadings. Construction BMPs must be maintained while construction
or land disturbance activities continue.
(4) Measures shall meet as a minimum the best management practices set
forth in the "New Hampshire Stormwater Manual, Volumes 1 through 3"
and any published DES regulations.
(5) Off-site surface water and runoff from undisturbed areas shall be
diverted away from disturbed areas where feasible or carried nonerosively
through the project area. Integrity of downstream drainage systems
shall be maintained.
(6) All temporary erosion and sediment control measures shall be removed
after final site stabilization. Trapped sediment and other disturbed
soil areas resulting from the removal of temporary measures shall
be permanently stabilized within 30 days.
I. Stormwater management measures.
(1) Structural stormwater management measures to achieve recharge, water
quality, and peak discharge control shall be structural BMPs designed
in accordance with the requirements of the following:
(a)
New Hampshire Stormwater Manual, Volumes 1 through 3.
[Amended 6-26-2018 by Ord. No. O-18-015]
(b)
Best Management Practices for Urban Stormwater Runoff, published
by NH DES, 1996, as amended.
(c)
Innovative Stormwater Treatment Technologies Best Management
Practices Manual, published by NH DES, May 2002, as amended [Reference:
http://www.des.state.nh.us/wmb/was/manual/]
(2) The performance criteria specified in the Design Manual with regard
to general feasibility, conveyance, pretreatment, environment and
landscaping, and maintenance shall be considered in the selection
of a structural BMP.
(3) Structural stormwater management practices shall be selected to accommodate
the unique hydrologic and geologic conditions of the site. Design
computations must document these conditions.
(4) Nonstructural management measures may reduce or eliminate the need for structural BMPs to meet recharge, water quality, and peak discharge control requirements. These techniques shall include runoff prevention methods (RPMs), are encouraged and shall be designed in accordance with the guidelines in the documents listed in Subsection
I(1) above. These techniques may include disconnection of rooftop and nonrooftop runoff, vegetated bio-cells and bio-islands, infiltration edges, dividers, or islands, planters, and raingardens.
J. Allowable nonstormwater discharges. The following nonstormwater discharges
are authorized, provided it has been determined that they are not
significant contributors of pollutants to the MS4. If these discharges
are identified as significant contributors to the MS4, they must be
addressed in the illicit discharge detection and elimination minimum
control measure described.
[Added 6-26-2018 by Ord.
No. O-18-015]
(5)
Uncontaminated groundwater infiltration [as defined at 40 CFR
35.2005(20)];
(6)
Uncontaminated pumped groundwater;
(7)
Discharge from potable water sources;
(9)
Air-conditioning condensation;
(10)
Irrigation water, springs;
(11)
Water from crawl space pumps;
(14)
Individual resident car washing;
(15)
Flows from riparian habitats and wetlands;
(16)
Dechlorinated swimming pool discharges;
(18)
Residential building wash waters, without detergents; and
(19)
Discharges or flows from firefighting activities that occur
during emergency situations. The permittee is not expected to evaluate
firefighting discharges with regard to pollutant contributions. Therefore,
these discharges are authorized as allowable nonstormwater discharges,
unless identified by EPA as significant sources of pollutants to waters
of the United States.
K. Litter, debris, yard waste, and all other nonstormwater discharges except for those listed in Subsection
J above are prohibited.
[Added 6-26-2018 by Ord.
No. O-18-015]
Where a subdivision is traversed by or requires
the construction of a watercourse or a drainage way, an easement of
adequate width shall be provided for such purpose.
The Planning Board shall indicate approval of
the stormwater management and erosion control plan, as filed, if it
complies with the requirements and objectives of this article. Such
approval shall be a component of subdivision or site plan approval.
If disapproved, a list of plan deficiencies and the procedure for
filing a revised plan will be given to the applicant.