[Ord. 303, 3/3/2014]
1.
Applicants proposing regulated activities in Wrightstown Township that do not fall under the exemption criteria shown in §§ 25-106 and 25-107 shall submit a stormwater management (SWM) site plan consistent with this chapter to the municipality for review. The SWM criteria of this chapter shall apply to the total proposed development even if development is to take place in stages. Preparation and implementation of an approved SWM site plan is required. No regulated activities shall commence until the municipality issues written approval of an SWM site plan, which demonstrates compliance with the requirements of this chapter.
2.
SWM site plans approved by the municipality, in accordance with Part
4, shall be on site throughout the duration of the regulated activity.
3.
The municipality may, after consultation with the Department of Environmental
Protection (PADEP), approve measures for meeting the state water quality
requirements other than those in this chapter, provided that they
meet the minimum requirements of, and do not conflict with, state
law, including, but not limited to, the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
4.
For all regulated earth disturbance activities, erosion and sediment
(E&S) control best management practices (BMPs) shall be designed,
implemented, operated, and maintained during the regulated earth disturbance
activities (e.g., during construction) to meet the purposes and requirements
of this chapter and to meet all requirements under Title 25 of the
Pennsylvania Code and the Clean Streams Law. Various BMPs and their
design standards are listed in the Erosion and Sediment Pollution
Control Program Manual, No. 363-2134-008 (April 15, 2000), as amended
and updated.
5.
For all regulated activities in the Neshaminy Creek watershed, implementation of the volume controls in § 25-303 of this chapter is required.
6.
Impervious Areas:
A.
The measurement of impervious areas shall include all of the impervious
areas in the total proposed development even if development is to
take place in stages.
B.
For development taking place in stages, the entire development plan
must be used in determining conformance with this chapter.
C.
For projects that add impervious area to a parcel, the total impervious
area on the parcel is subject to the requirements of this chapter.
7.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written notification
of the adjacent property owner(s) by the developer. Such stormwater
flows shall be subject to the requirements of this chapter.
8.
All regulated activities shall include such measures as necessary
to:
A.
Protect health, safety, and property.
B.
Meet the water quality goals of this chapter by implementing measures
to:
C.
To the maximum extent practicable, incorporate the techniques for
low-impact development practices (e.g., protecting existing trees,
reducing area of impervious surface, cluster development, and protecting
open space) described in the Pennsylvania Stormwater Best Management
Practices Manual, Pennsylvania Department of Environmental Protection
(PADEP) No. 363-0300-002 (2006). See Appendix E for a summary description.[3]
[3]
Editor's Note: The appendixes are included as attachments
to this chapter.
9.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize the use of natural on-site infiltration features
while still meeting the other requirements of this chapter.
10.
The design of all facilities over karst shall include an evaluation
of measures to minimize the risk of adverse effects.
11.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
and not more than 72 hours from the end of the design storm.
12.
The design storm volumes to be used in the analysis of peak rates
of discharge should be obtained from the Precipitation-Frequency Atlas
of the United States, Atlas 14, Volume 2, Version 3.0, United States
Department of Commerce, National Oceanic and Atmospheric Administration
(NOAA), National Weather Service, Hydrometeorological Design Studies
Center, Silver Spring, Maryland. NOAA’s Atlas 14 can be accessed
at http://hdsc.nws.noaa.gov/hdsc/pfds/
13.
For all regulated activities, SWM BMPs shall be designed, implemented,
operated, and maintained to meet the purposes and requirements of
this chapter and to meet all requirements under Title 25 of the Pennsylvania
Code, the Clean Streams Law, and the Storm Water Management Act.
14.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (PA BMP Manual).
[Ord. 303, 3/3/2014]
Approvals issued and actions taken under this chapter do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
[Ord. 303, 3/3/2014]
1.
Volume controls will mitigate increased runoff impacts, protect stream
channel morphology, maintain groundwater recharge, and contribute
to water quality improvements. Stormwater runoff volume control methods
are based on the net change in runoff volume for the two-year storm
event.
2.
Volume controls shall be implemented using the Design Storm Method
in Subsection 2A or the Simplified Method in Subsection 2B below.
For regulated activities equal to or less than one acre, this chapter
establishes no preference for either methodology; therefore, the applicant
may select either methodology on the basis of economic considerations,
the intrinsic limitations of the procedures associated with each methodology,
and other factors. All regulated activities greater than one acre
must use the Design Storm Method.
A.
Design-Storm Method (Any Regulated Activity): This method requires detailed modeling based on site conditions. For modeling assumptions, refer to § 25-305, Subsection 1.
(1)
Post-development total runoff should not be increased from predevelopment
total runoff for all storms equal to or less than the two-year, twenty-four-hour
duration precipitation.
(2)
The following applies in order to estimate the increased volume
of runoff for the two-year, twenty-four-hour duration precipitation
event:
To calculate the runoff volume (cubic feet) for existing site
conditions (predevelopment) and for the proposed developed site conditions
(post-development), it is recommended to use the Soil Cover Complex
Method as shown following. Table B-3 in Appendix B is available to
guide a qualified professional and/or an applicant to calculate the
stormwater runoff volume.[1] The calculated volume shall be either reused, evapotranspired,
or infiltrated through structural or nonstructural means.
|
Soil Cover Complex Method:
| |
|---|---|
|
Step 1: Runoff (inches) = Q = (P - 0.2S)2/(P + 0.8S)
|
|
Where:
| ||||
|
P
|
=
|
Two-year rainfall (inches).
| ||
|
S
|
=
|
(1,000/CN) - 10, the potential maximum retention (including
initial abstraction, Ia).
| ||
|
Step 2: Runoff Volume (cubic feet) = Q x Area x 1/12
| ||||
|
Where:
| ||||
|
Q
|
=
|
Runoff (inches).
| ||
|
Area
|
=
|
SWM area (square feet).
| ||
[1]
Editor's Note: The appendixes are included as attachments
to this chapter.
B.
Simplified Method (Regulated Activities Less Than or Equal to One
Acre):
(1)
Stormwater facilities shall capture the runoff volume from at
least the first two inches of runoff from all new impervious surfaces.
|
Volume (cubic feet) = (2 inches runoff/12 inches) * impervious
surface (square feet)
|
(2)
At least the first inch of runoff volume from the new impervious
surfaces shall be permanently removed from the runoff flow, i.e.,
it shall not be released into the surface waters of the commonwealth.
The calculated volume shall be either reused, evapotranspired or infiltrated
through structural or nonstructural means.
|
Volume (cubic feet) = (1 inch runoff/12 inches) * impervious
surface (square feet)
|
(3)
Infiltration facilities should be designed to accommodate the
first half inch of the permanently removed runoff.
(4)
No more than one inch of runoff volume from impervious surfaces
shall be released from the site. The release time must be over 24
to 72 hours.
C.
Stormwater Control Measures. The applicant must demonstrate how the
required volume is controlled through stormwater best management practices
(BMPs), which shall provide the means necessary to capture, reuse,
evaporate, transpire or infiltrate the total runoff volume.
(1)
If natural resources exist on the site, the applicant is required
to submit an SWM site plan and shall determine the total acreage of
protected area where no disturbance is proposed. The acreage of the
protected area should be subtracted from the total site area and not
included in the stormwater management site area acreage used in determining
the volume controls.
|
Stormwater Management Site Area =
| |
|---|---|
|
{Total Site Area (for both pre- and post-development conditions)
- Protected Area}
| |
[2]
Editor's Note: The appendixes are included as attachments
to this chapter.
(3)
Volume controls provided through nonstructural BMPs should be
subtracted from the required volume to determine the necessary structural
BMPs.
|
Required
|
Nonstructural
|
Structural
| |
|---|---|---|---|
|
Volume Control (ft3) -
|
Volume Control (ft3) -
|
Volume Requirement (ft3)
|
(5)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on the suitability of soils and site
conditions (see Table B-6 in Appendix B for a list of infiltration
BMPs). Infiltration BMPs shall be constructed on soils that have the
following characteristics:
(a)
A minimum soil depth of 24 inches between the bottom of the
infiltration BMPs and the top of bedrock or seasonally high water
table.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests. A minimum
of 0.2 inch per hour (in/hr) should be utilized, and for acceptable
rates a safety factor of 50% should be applied for design purposes
(e.g., for soil which measured 0.4 inch per hour, the BMP design should
use 0.2 inch per hour to ensure safe infiltration rates after construction).
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate runoff volume within three days (72 hours) from the start
of the design storm.
(6)
Soils. A soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. All regulated
activities are required to perform a detailed soils evaluation by
a qualified design professional which at a minimum addresses soil
permeability, depth to bedrock, and subgrade stability. The general
process for designing the infiltration BMP shall be:
(a)
Analyze hydrologic soil groups as well as natural and man-made
features within the site to determine general areas of suitability
for infiltration practices. In areas where development on fill material
is under consideration, conduct geotechnical investigations of subgrade
stability; infiltration may not be ruled out without conducting these
tests.
(b)
Provide field tests, such as double-ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface),
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
(c)
Design the infiltration structure based on field-determined
capacity at the level of the proposed infiltration surface and based
on the safety factor of two.
(d)
If on-lot infiltration structures are proposed, it must be demonstrated
to the municipality that the soils are conducive to infiltrate on
the lots identified.
(e)
An impermeable liner will be required in detention basins where
the possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the municipality.
[Ord. 303, 3/3/2014]
1.
Peak rate controls for large storms, up to the one-hundred-year
event, are essential in order to protect against immediate downstream
erosion and flooding. The following peak rate controls have been determined
through hydrologic modeling of the Neshaminy Creek and Delaware River
South watersheds.
A.
Standards for managing runoff from each subarea in the Neshaminy
Creek and Delaware River South watersheds for the two- , five- , ten-
, twenty-five- , fifty- , and one-hundred-year design storms are shown
in Tables 304.1 and 304.2. Development sites located in each of the
management districts must control proposed development conditions'
runoff rates to existing conditions' runoff rates for the design storms
in accordance with Tables 304.1 and 304.2 following .
|
Table 304.1
| |||
|---|---|---|---|
|
Peak Rate Runoff Control Standards by Stormwater Management
Districts in the Neshaminy Creek Watershed
| |||
|
District
|
Design Storm Post-Development (Proposed Conditions)
|
Design Storm Predevelopment (Existing Conditions)
| |
|
A
|
2-year
|
1-year
| |
|
5-year
|
5-year
| ||
|
10-year
|
10-year
| ||
|
25-year
|
25-year
| ||
|
50-year
|
50-year
| ||
|
100-year
|
100-year
| ||
|
B
|
2-year
|
1-year
| |
|
5-year
|
2-year
| ||
|
10-year
|
5-year
| ||
|
25-year
|
10-year
| ||
|
50-year
|
25-year
| ||
|
100-year
|
50-year
| ||
|
C
|
2-year
|
2-year
| |
|
5-year
|
5-year
| ||
|
10-year
|
10-year
| ||
|
25-year
|
25-year
| ||
|
50-year
|
50-year
| ||
|
100-year
|
100-year
| ||
|
Table 304.2
| |||
|---|---|---|---|
|
Peak Rate Runoff Control Standards by Stormwater Management
Districts in the Delaware River South Watershed
| |||
|
District
|
Design Storm Post-Development
(Proposed Conditions)
|
Design Storm Predevelopment
(Existing Conditions)
| |
|
A
|
2-year
|
1-year
| |
|
5-year
|
5-year
| ||
|
10-year
|
10-year
| ||
|
25-year
|
25-year
| ||
|
50-year
|
50-year
| ||
|
100-year
|
100-year
| ||
|
B
|
2-year
|
1-year
| |
|
5-year
|
2-year
| ||
|
10-year
|
5-year
| ||
|
25-year
|
10-year
| ||
|
50-year
|
50-year
| ||
|
100-year
|
100-year
| ||
B.
General. Proposed conditions' rates of runoff from any regulated
activity shall not exceed the peak release rates of runoff from existing
conditions for the design storms specified on the Stormwater Management
District Watershed Map (Appendix D)[1] and in this section of the chapter.
[1]
Editor's Note: The appendixes are included as attachments
to this chapter.
C.
District Boundaries. The boundaries of the stormwater management
districts are shown on official maps and are available for inspection
at the municipal office and county planning offices. A copy of the
Wrightstown Township Area Map with zoomed-in extents is included in
Appendix D. The exact location of the stormwater management district
boundaries as they apply to a given development site shall be determined
by mapping the boundaries using the two-foot topographic contours
(or most-accurate data required) provided as part of the SWM site
plan.
D.
Sites Located in More than One District. For a proposed development
site located within two or more stormwater management district category
subareas, the peak discharge rate from any subarea shall meet the
management district criteria for the district in which the discharge
is located.
E.
Off-Site Areas. When calculating the allowable peak runoff rates,
developers do not have to account for runoff draining into the subject
development site from an off-site area. On-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
F.
Site Areas. The stormwater management site area is the only area
subject to the management district criteria. Nonimpacted areas or
nonregulated activities bypassing the stormwater management facilities
would not be subject to the management district criteria.
G.
Alternate Criteria for Redevelopment Sites. For redevelopment sites,
one of the following minimum design parameters shall be accomplished,
whichever is most appropriate for the given site conditions as determined
by Wrightstown Township:
[Ord. 303, 3/3/2014]
1.
The following criteria shall be used for runoff calculations:
A.
For development sites not considered redevelopment, the ground cover
used to determine the existing conditions' runoff volume and flow
rate shall be as follows:
(1)
Wooded sites shall use a ground cover of “woods in good
condition.” A site is classified as wooded if a continuous canopy
of trees exists over 1/4 acre.
(2)
The undeveloped portion of the site, including agriculture,
bare earth, and fallow ground, shall be considered as “meadow
in good condition,” unless the natural ground cover generates
a lower curve number (CN) or Rational “c” value (i.e.,
woods) as listed in Table B-4 or B-7 in Appendix B of this chapter.[1]
[1]
Editor's Note: The appendixes are included as attachments
to this chapter.
B.
For sites considered redevelopment, the ground cover used to determine
the existing conditions' runoff volume and flow rate for the developed
portion of the site shall be based upon actual land cover conditions.
If the developed site contains impervious surfaces, 20% of the impervious
surface area shall be considered meadow in the model for existing
conditions.
2.
Stormwater runoff peak discharges from all development sites with
a drainage area equal to or greater than 200 acres shall be calculated
using a generally accepted calculation technique that is based on
the NRCS Soil Cover Complex Method. Table 305.1 summarizes acceptable
computation methods. The method selected by the design professional
shall be based on the individual limitations and suitability of each
method for a particular site. The municipality may allow the use of
the Rational Method (Q=CIA) to estimate peak discharges from drainage
areas that contain less than 200 acres.
|
Q
|
=
|
Peak flow rate, cubic feet per second (cfs).
| |
|
C
|
=
|
Runoff coefficient, dependent on land use/cover.
| |
|
I
|
=
|
Design rainfall intensity, inches per hour.
| |
|
A
|
=
|
Drainage area, acres.
|
3.
All calculations consistent with this chapter using the Soil Cover
Complex Method shall use the appropriate design rainfall depths for
the various return-period storms according to the National Oceanic
and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding
to the Doylestown rain gage, seen in Table B-1 in Appendix B.[2] The SCS Type II rainfall curve from NOAA is found on Figure
B-1 in Appendix B. This data may also be directly retrieved from the
NOAA Atlas 14 website: hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html.
If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS is used
for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours.
|
Table 305.1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|
Method
|
Method Developed By
|
Applicability
| |
|
TR-20
(or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
TR-55
(or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-1/HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
| |
|
Rational Method
(or commercial computer package based on Rational Method)
|
Emil Kuichling (1889)
|
For sites less than 200 acres, or as approved by the municipality
and/or Municipal Engineer
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer
| |
[2]
Editor's Note: The appendixes are included as attachments
to this chapter.
4.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration for overland flow
and return periods from NOAA Atlas 14, Volume 2, Version 2.1. Times-of-concentration
for overland flow shall be calculated using the methodology presented
in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55
(as amended or replaced from time to time by NRCS). Times-of-concentration
for channel and pipe flow shall be computed using Manning’s
Equation.
7.
Runoff from proposed sites graded to the subsoil will not have the
same runoff conditions as the site under existing conditions because
of soil compaction, even after top-soiling or seeding. The proposed
conditions' “CN” or “C” shall increase by
5% to better reflect proposed soil conditions.
8.
The Manning Equation is preferred for one-dimensional, gradually
varied, open channel flow. In other cases, appropriate, applicable
methods should be applied; however, early coordination with the municipality
is necessary.
9.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using the generally
accepted hydraulic analysis technique or method of the municipality.
10.
The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than 200 acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. The municipality may approve the use of
any generally accepted full hydrograph approximation technique that
shall use a total runoff volume that is consistent with the volume
from a method that produces a full hydrograph.
[Ord. 303, 3/3/2014]
[Ord. 303, 3/3/2014]
1.
In addition to the performance standards and design criteria requirements
of this Part, the applicant shall comply with the following water
quality requirements of this Part:
A.
Adequate storage and treatment facilities will be provided to capture and treat stormwater runoff from developed or disturbed areas. The recharge volume computed under § 25-308 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than recharge/infiltration BMPs. The required water quality volume (WQv) is the storage capacity needed to capture and to treat a portion of stormwater runoff from the developed areas of the site produced from 90% of the average annual rainfall (P).
|
To achieve this goal, the following criterion is established:
| |
|
The following calculation formula is to be used to determine
the water quality storage volume (WQv) in acre-feet of storage for
the Delaware River South watershed:
|
|
Equation: 307.1
| |
|
WQv = [(P)(Rv)(A)]/12
| |
|
Where:
| |
|
WQv = Water quality volume (acre-feet).
| |
|
P = Rainfall amount equal to 90% of events producing this rainfall
(inches).
| |
|
A = Area of the project contributing to the water quality BMP
(acres).
| |
|
Rv = 0.05 + 0.009(I), where I is the percent of the area that
is impervious surface (impervious area/A*100).
|
|
The P value for the five PennDOT rainfall regions is shown in
Figure B-2 in Appendix B of the Model Ordinance within the Plan. Since
the Delaware River South is in PennDOT Region 5, the P value to be
utilized to meet this requirement is 2.04 inches.
|
B.
Design of BMPs used for water quality control shall be in accordance
with design specifications outlined in the Stormwater Best Management
Practices Manual or other applicable manuals. The following factors
shall be considered when evaluating the suitability of BMPs used to
control water quality at a given development site:
(1)
Total contributing drainage area.
(2)
Permeability and infiltration rate of the site soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high-water table.
(5)
Proximity to building foundations and wellheads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Stream bank erosion.
(10)
Efficiency of the BMPs to mitigate potential water quality problems.
(11)
The volume of runoff that will be effectively treated.
(12)
The nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
(16)
Enhancement of aesthetic and property value.
C.
To accomplish the above, the applicant shall submit original and
innovative designs to the municipality for review and approval. Such
designs may achieve the water quality objectives through a combination
of BMPs (best management practices).
D.
No regulated earth disturbance activities within the municipality
shall commence until approval by the municipality of a plan which
demonstrates that compliance with state water quality requirements
post-construction is complete.
E.
The BMPs shall be designed, implemented and maintained to meet state
water quality requirements and any other more-stringent requirements
as determined by the municipality.
F.
To control post-construction stormwater impacts from regulated earth
disturbance activities, state water quality requirements can be met
by BMPs, including site design, which provide for replication of preconstruction
stormwater infiltration and runoff conditions, so that post-construction
stormwater discharges do not degrade the physical, chemical or biological
characteristics of the receiving waters. As described in the DEP Comprehensive
Stormwater Management Policy (No. 392-0300-002, September 28, 2002),
this may be achieved by the following:
(1)
Infiltration: replication of preconstruction stormwater infiltration
conditions;
(2)
Treatment: use of water quality treatment BMPs to ensure filtering
out of the chemical and physical pollutants from the stormwater runoff;
and
(3)
Stream Bank and Streambed Protection: management of volume and
rate of post-construction stormwater discharges to prevent physical
degradation of receiving waters (e.g., from scouring).
G.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the municipality. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of § 25-303, Subsection 2C.
[Ord. 303, 3/3/2014]
1.
Infiltration BMPs shall meet the following minimum requirements:
A.
Regulated activities will be required to recharge (infiltrate) a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be recharged shall be determined from § 25-308, Subsection 1A(2)(a) or (b), depending upon demonstrated site conditions.
(1)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on suitability of soils and site conditions
(see Table B-6 in Appendix B for a list of Infiltration BMPs)[1] and shall be constructed on soils that have the following
characteristics:
(a)
A minimum depth of 24 inches between the bottom of the BMP and
the limiting zone.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and drain completely as determined by field tests conducted by
the applicant’s design professional. A minimum of 0.2 inch/hour
(in/hr) should be utilized, and for acceptable rates a safety factor
of 50% should be applied for design purposes (e.g., for soil which
measured 0.4 in/hr, the BMP design should use 0.2 in/hr to ensure
safe infiltration rates after construction).
(c)
The recharge facility shall be capable of completely infiltrating
the recharge volume within four days (96 hours).
(d)
Pretreatment shall be provided prior to infiltration.
(e)
The requirements for recharge are applied to all disturbed areas,
even if they are ultimately to be an undeveloped land use such as
grass, since studies have found that compaction of the soils during
disturbance reduces their infiltrative capacity.
[1]
Editor's Note: The appendixes are included as attachments
to this chapter.
(2)
The recharge volume (Rev) shall be computed by first obtaining the infiltration requirement using methods in either § 25-308, Subsection 1A(2)(a) or (b), then multiplying by the total proposed impervious area. The overall required recharge volume for a site is computed by multiplying total impervious area by the infiltration requirement.
(a)
NRCS Curve Number Equation.
|
The following criteria shall apply.
| |
|
The NRCS runoff shall be utilized to calculate infiltration
requirements (P) in inches.
|
|
For zero runoff:
| ||||
|
Where:
| ||||
|
Equation: 308.1
| ||||
|
P
|
=
|
I (Infiltration) = (200/CN) - 2
| ||
|
P
|
=
|
I = infiltration requirement (inches)
| ||
|
CN
|
=
|
SCS (NRCS) curve number of the existing conditions contributing
to the recharge facility
| ||
|
This equation can be displayed graphically in, and the infiltration
requirement can also be determined from, Figure 308-1.
| |
|
The recharge volume (Rev) required would
therefore be computed as:
| |
|
Equation: 308.2
| |
|
Rev = I * impervious area (SF) /12 =
cubic feet (CF)
|
(b)
Annual Recharge Water Budget Approach
|
It has been determined that infiltrating 0.5 inch of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of § 25-308, Subsection 1A(2)(a) cannot be achieved, then 0.5 inch of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions' curve number of 81. Above a curve number of 81, Equation 308.1 or the curve in Figure 308.1 should be used to determine the infiltration requirement.
|
|
Where:
| ||||
|
I
|
=
|
0.5 inch
| ||
|
The recharge volume (Rev) required would
therefore be computed as:
| ||||
|
Rev
|
=
|
I * percent impervious area (SF)/12 = cubic feet (CF)
| ||
B.
The recharge values derived from these methods are the minimum volumes
the applicant must control through an infiltration/recharge BMP facility.
However, if a site has areas of soils where additional volume of infiltration
can be achieved, the applicant is encouraged to recharge as much of
the stormwater runoff from the site as possible.
|
Figure 308-1. Infiltration Requirement Based Upon NRCS
Curve Number
 |
2.
The general process for designing the infiltration BMP shall be:
A.
A detailed soils evaluation of the project site shall be required
to determine the suitability of recharge facilities. The evaluation
shall be performed by a qualified professional and, at a minimum,
address soil permeability, depth to bedrock, and subgrade stability.
(1)
Analyze hydrologic soil groups as well as natural and man-made
features within the watershed to determine general areas of suitability
for infiltration practices.
(2)
Provide field tests, such as double-ring infiltration tests,
at the level of the proposed infiltration surface to determine the
appropriate hydraulic conductivity rate.
(3)
Design the infiltration structure for the required storm volume
based on field-determined capacity at the level of the proposed infiltration
surface.
(4)
Where the recharge volume requirement cannot be physically accomplished
due to the results of the field soils testing, supporting documentation
and justification shall be supplied to the municipality with the drainage
plan.
(5)
If on-lot infiltration structures are proposed by the applicant’s
design professional, it must be demonstrated to the municipality that
the soils are conducive to infiltration on the lots identified.
3.
Extreme caution shall be exercised where infiltration is proposed
in geologically susceptible areas, such as strip mine or limestone
areas. Extreme caution shall also be exercised where salt or chloride
would be a pollutant, since soils do little to filter this pollutant
and it may contaminate the groundwater. Extreme caution shall be exercised
where infiltration is proposed in source water protection areas. The
qualified design professional shall evaluate the possibility of groundwater
contamination from the proposed infiltration/recharge facility and
perform a hydrogeologic justification study if necessary. The infiltration
requirement in high-quality/exceptional-value waters shall be subject
to the DEP’s Title 25, Chapter 93, Antidegradation Regulations.
The municipality may require the installation of an impermeable liner
in BMP and/or detention basins where the possibility of groundwater
contamination exists. A detailed hydrogeologic investigation may be
required by the municipality.
4.
The municipality shall require the applicant to provide safeguards
against groundwater contamination for uses which may cause groundwater
contamination, should there be a mishap or spill.
5.
Recharge/infiltration facilities shall be used in conjunction with
other innovative or traditional BMPs, stormwater control facilities,
and nonstructural stormwater management alternatives.
[Ord. 303, 3/3/2014]
1.
In addition to the water quality volume, to minimize the impact
of stormwater runoff on downstream stream bank erosion, the requirement
is to design the BMP to detain the post-development two-year, twenty-four-hour
design storm to the predevelopment one-year flow using the SCS Type
II distribution. Additionally, provisions shall be made (such as adding
a small orifice at the bottom of the outlet structure) so that the
post-development one-year storm takes a minimum of 24 hours to drain
from the facility from a point where the maximum volume of water from
the one-year storm is captured (i.e., the maximum water surface elevation
is achieved in the facility).
2.
Release of water can begin at the start of the storm (i.e., the invert
of the water quality orifice is at the invert of the facility). The
design of the facility shall consider and minimize the chances of
clogging and sedimentation. Orifices smaller than three inches in
diameter are not recommended. However, if the design engineer can
provide proof that the smaller orifices are protected from clogging
by use of trash racks, etc., smaller orifices may be permitted.
