[Ord. No. 509, 6/18/2014]
1. Applicants proposing regulated activities in the Neshaminy Creek watershed that do not fall under the exemption criteria shown in §
26-106 shall submit a stormwater management (SWM) site plan consistent with the Neshaminy Creek Watershed SWM Plan to the Borough of Hatfield for review. The SWM criteria of this Part 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 Borough of Hatfield issues written approval of an SWM site plan, which demonstrates compliance with the requirements of this Part.
2. SWM site plans approved by the Borough of Hatfield, in accordance
with Subpart D, shall be on site throughout the duration of the regulated
activity.
3. The Borough of Hatfield may, after consultation with the Department
of Environmental Protection (PADEP), approve measures for meeting
the state water quality requirements other than those in this Part,
provided that they meet the minimum requirements of, and do not conflict
with, state law, including, but not limited to, the Clean Streams
Law.
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 Part 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, implementation of the volume controls in §
26-123 of this Part 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 Part.
C. For projects that add impervious area to a parcel, the total impervious
area on the parcel is subject to the requirements of this Part.
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) from the developer. Such stormwater
flows shall be subject to the requirements of this Part.
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 Part by implementing measures
to:
(1)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(2)
Create, maintain, repair or extend riparian buffers.
(3)
Avoid erosive flow conditions in natural flow pathways.
(4)
Minimize thermal impacts to waters of this commonwealth.
(5)
Disconnect impervious surfaces (i.e., disconnected impervious
areas, DIAs) by directing runoff to pervious areas, wherever possible.
See Appendix F for detail on DIAs.
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.
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 Part.
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 Part 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. No. 509, 6/18/2014]
1. Approvals issued and actions taken under this Part do not relieve
the applicant of the responsibility to secure other required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
A. General. The following permit requirements may apply to certain regulated
earth disturbance activities and must be met prior to commencement
of regulated earth disturbance activities, as applicable:
(1)
All regulated earth disturbance activities subject to permit
requirements by DEP under regulations at 25 Pa. Code Chapter 102.
(2)
Work within natural drainageways subject to permit by DEP under
25 Pa. Code Chapter 105.
(3)
Any stormwater management facility that would be located in
or adjacent to surface waters of the commonwealth, including wetlands,
subject to permit by DEP under 25 Pa. Code Chapter 105.
(4)
Any stormwater management facility that would be located on
a state highway right-of-way, or require access from a state highway,
shall be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
(5)
Culverts, bridges, storm sewers or any other facilities which
must pass or convey flows from the tributary area and any facility
which may constitute a dam subject to permit by DEP under 25 Pa. Code
Chapter 105.
B. Erosion and Sediment Control During Regulated Earth Disturbance Activities.
(1)
No regulated earth disturbance activities within the Borough
of Hatfield shall commence until approval by the Borough of Hatfield
of an erosion and sediment control plan for construction activities.
(2)
The DEP has regulations that require an erosion and sediment
control plan for any earth disturbance activity of 5,000 square feet
or more, under 25 Pa. Code § 102.4(b).
(3)
In addition, under 25 Pa. Code Chapter 92, a DEP NPDES construction activities permit is required
for regulated earth disturbance activities.
(4)
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or County Conservation District must be provided to the Borough of Hatfield. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] may satisfy the requirements of Subsection
1B(1).
(5)
A copy of the erosion and sediment control plan and any required
permit, as required by DEP regulations, shall be available at the
project site at all times.
C. Water Quality Requirements After Regulated Earth Disturbance Activities
Are Complete.
(1)
No regulated earth disturbance activities within the Borough
of Hatfield shall commence until approval by the Borough of Hatfield
of a plan which demonstrates compliance with state water quality requirements
after construction is complete.
(2)
The BMPs must be designed, implemented and maintained to meet
state water quality requirements and any other more-stringent requirements
as determined by the Borough of Hatfield.
(3)
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:
(a)
Infiltration: replication of preconstruction stormwater infiltration
conditions.
(b)
Treatment: use of water quality treatment BMPs to ensure filtering
out of the chemical and physical pollutants from the stormwater runoff.
(c)
Stream Bank and Stream Bed protection: management of volume
and rate of post-construction stormwater discharges to prevent physical
degradation of receiving waters (e.g., from scouring).
(4)
The DEP has regulations that require municipalities to ensure
design, implementation and maintenance of best management practices
(BMPs) that control runoff from new development and redevelopment
after regulated earth disturbance activities are complete. These requirements
include the need to implement post-construction stormwater BMPs with
assurance of long-term operations and maintenance of those BMPs.
(5)
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate DEP regional office must be provided
to the Borough of Hatfield. The issuance of an NPDES construction
permit [or permit coverage under the statewide general permit (PAG-2)]
may satisfy the requirements of Subsection 1.C(1).
(6)
BMP operations and maintenance requirements are described in
Subpart G of this Part.
[Ord. No. 509, 6/18/2014]
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 2.A or the Simplified Method in Subsection 2.B below.
For regulated activities equal to or less than one acre, this Part
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 §
26-125.
(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. The calculated volume shall be either reused, evapotranspired,
or infiltrated through structural or nonstructural means.
Soil Cover Complex Method:
Step 1: Runoff (in.) = Q = (P - 0.2S)2/(P + 0.8S)
|
Where:
|
|
P = Two-year rainfall (in.).
|
|
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 (in.).
|
|
Area = SWM area (sq. ft.).
|
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 (sq. ft.)
(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 (sq. ft.).
(3)
Infiltration facilities should be designed to accommodate the
first 1/2 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 and an SWM site plan
submission is required for the regulated activity, the applicant shall
determine and display the total acreage of protected area where no
disturbance is proposed on the plan. 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]
Natural resource areas should be calculated based upon the Borough
of Hatfield's own Natural Resource Protection Ordinance. If no ordinance
exists, see Table B-2 in Appendix B for guidance to assess the total
protected area. For additional reference see Chapter
5, Section 5.4.1, of the PA BMP Manual.
(2)
Calculate the volume controls provided through nonstructural
BMPs. Table B-5 in Appendix B is recommended as guidance.
(3)
Volume controls provided through nonstructural BMPs should be
subtracted from the required volume to determine the necessary structural
BMPs.
(4)
Calculate the volume controls provided through structural BMPs. Table B-6 in Appendix B is recommended as guidance. See PA BMP Manual Chapter
6 for description of the BMPs.
(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 in./hr., the BMP design should
use 0.2 in./hr. 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 50%.
(d)
If on-lot infiltration structures are proposed, it must be demonstrated
to the Borough of Hatfield 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 Borough of Hatfield.
[Ord. No. 509, 6/18/2014]
1. Peak rate controls for large storms, up to the 100-year event, is
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 Watershed.
A. Standards for managing runoff from each subarea in the Neshaminy
Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and
100-year design storms are shown in Table 26-124.1. 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 Table 26-124.1 following.
Table 26-124.1
|
---|
Peak Rate Runoff Control Standards by Stormwater Management
Districts in the Neshaminy Creek Watershed (includes
Little Neshaminy Creek)
|
---|
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
|
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) and in this section of this Part.
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
map at a reduced scale, and four other maps with zoomed-in extents,
are 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 the Borough of Hatfield:
(1)
Meet the full requirements specified by Table 26-124.1 and §
26-124, Subsection 1.A through F; or
(2)
Reduce the total impervious surface on the site by at least
20% based upon a comparison of existing impervious surface to proposed
impervious surface.
H. Areas Covered By a Release Rate Map From an Alternative Approved
Act 167 Stormwater Management Plan: For the one-, two-, five-, ten-,
twenty-five-, fifty-, and 100-year storms, the post-development peak
discharge rates will follow the applicable approved release rate maps.
For any areas not shown on the release rate maps, the post-development
discharge rates shall not exceed the predevelopment discharge rates.
[Ord. No. 509, 6/18/2014]
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 Part.
B. For development and redevelopment sites, 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 26-125.1 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 Borough of Hatfield 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 Part 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. 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 26-125.1
|
---|
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
|
United States 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 Borough
of Hatfield and/or Municipal Engineer
|
Other methods
|
Varies
|
Other computation methodologies approved by the Borough of Hatfield
and/or Municipal Engineer
|
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.
5. Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be based on Table
B-4 in Appendix B.
6. Runoff coefficients (C) for both existing and proposed conditions
for use in the Rational Method shall be consistent with Table B-7
in Appendix B.
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 topsoiling or seeding. The proposed
condition "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 Borough
of Hatfield is necessary.
9. Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Part using the generally
accepted hydraulic analysis technique or method of the Borough of
Hatfield.
10. The design of any stormwater detention facilities intended to meet
the performance standards of this Part 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 Borough of Hatfield 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. No. 509, 6/18/2014]
1. Hotspots.
A. The use of infiltration BMPs is prohibited on hotspot land use areas.
Examples of hotspots are listed in Appendix G.
B. Stormwater runoff from hotspot land uses shall be pretreated. In
no case may the same BMP be employed consecutively to meet this requirement.
Guidance regarding acceptable methods of pretreatment is located in
Appendix G.
2. West Nile Guidance Requirements. All wet basin designs shall incorporate
biologic controls consistent with the West Nile Guidance found in
Appendix H.