[Ord. 2011-12, 6/20/2011]
1.
The standards contained in this Part shall apply as minimum design
standards; however, federal, state and other Doylestown Borough regulations
may impose additional standards subject to their jurisdiction. Approvals
issued and actions taken under this Part 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.
2.
Retention of Existing Watercourses and Natural Drainage Features.
A.
Whenever a watercourse, stream or intermittent stream is located
within a development site, it shall remain open in its natural state
and location and shall not be piped, unless required or approved by
the Borough.
B.
No stormwater runoff or natural drainage shall be so diverted as
to overload existing drainage systems (including existing stormwater
management facilities) or create flooding.
C.
The Borough Council may require a developer to provide a permanent
easement along any watercourse located within or along the boundary
of any property being subdivided or developed. The purpose of any
such easement shall be for the maintenance of the channel of any watercourse
and their structures and any alterations which may adversely affect
the watercourse. The required width of any such easement shall be
the greater width as determined by the Borough Engineer, U.S. Army
Corps of Engineers, Pennsylvania Department of Environmental Protection
or other public agency having jurisdiction, but, in no case, shall
such easement be less than 50 feet in width. The developer will retain
the easement until such time as one of the following is accomplished:
(1)
The easement is offered for dedication by the developer and accepted
by Doylestown Borough.
(2)
If an easement acceptable to the Borough is established, the maintenance
shall then be the responsibility of the individual lot owners over
whose property the easement passes. For land developments, the maintenance
shall then be the responsibility of the owner of the affected property.
The maintenance responsibility shall be clearly established and recorded
with the deed of easement.
(3)
A homeowners' association or other legal entity, approved by Doylestown
Borough, assumes responsibility for the maintenance of the development,
including the retention and maintenance of the watercourse easement.
3.
Developers shall construct and/or install stormwater management facilities,
and BMPs on site and off site, as necessary to:
A.
Prevent erosion damage and to satisfactorily carry off, detain or
retain and control the rate of release of stormwater.
B.
Manage the anticipated peak discharge from property being subdivided
or developed and existing runoff being contributed from all land at
a higher elevation in the same watershed.
C.
Convey stormwater along or through the property to a natural outfall.
If a developer concentrates dispersed stormwater flow or redirects
stormwater flow to exit at another location on the property, the developer
is responsible for constructing an adequate channel on the adjacent
property and on all downstream properties until a natural outfall
is reached. A natural outfall shall have sufficient capacity to receive
stormwater without deterioration of the facility and without adversely
affecting property in the watershed. This natural outfall may be a
river, creek or other drainage facility so designated by Doylestown
Borough for the proposed system.
D.
Protect and maintain existing levels of water quality in all streams.
4.
Where applicable, stormwater management facilities or programs shall
comply with the requirements of Chapter 102 (Erosion Control), Chapter
105 (Dam Safety and Waterway Management) and Chapter 106 (Floodplain
Management) of Title 25, Rules and Regulations of the DEP.
5.
Stormwater management facilities which involve a state highway shall
be subject to the approval of PennDOT.
6.
Stormwater management facilities located within or affecting the
floodplain of any watercourse shall also be subject to the requirements
of any Doylestown Borough ordinance that regulates construction and
development within areas which are subject to flooding.
7.
Access to facilities shall be provided for maintenance and operation.
This access shall be a cleared access that is, when possible, approximately
20 feet wide. Proximity of facilities to public rights-of-way shall
be encouraged in order to minimize the length of accessways. Multiple
accesses shall be encouraged for major facilities.
8.
Additional studies and higher levels of control than the minimum
provided in the requirements and criteria of this Part may be required
by the Borough Council to ensure adequate protection to life and property.
9.
In addition to runoff control, all stormwater management plans shall
also incorporate BMPs in conjunction with the permitting, implementation
and operation and maintenance of the selected stormwater runoff controls.
10.
Applicants proposing regulated activities shall submit a stormwater
management (SWM) site plan to the Doylestown Borough for review, unless
exempt by provisions of this Part. 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 Doylestown Borough issues written approval of a SWM site
plan which demonstrates compliance with the requirements of this Part.
11.
SWM site plans approved by Doylestown Borough shall be on site throughout
the duration of the regulated activity.
12.
Doylestown Borough 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.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
13.
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.
14.
For all regulated activities, implementation of the volume controls
is required as regulated by this Part.
15.
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.
16.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written notification
of the adjacent property owner(s). Such stormwater flows shall be
subject to the requirements of this Part.
17.
All regulated activities shall include such measures as are necessary
to:
A.
Protect health, safety, and property.
B.
Meet the water quality goals of this Part 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), as amended and updated. See Appendix
VIII for a summary description.[3]
[3]
Editor's Note: Appendix VIII is on file in the Borough offices.
18.
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.
19.
The design of all facilities over karst shall include an evaluation
of measures to minimize the risk of adverse effects.
20.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
hours and not more than 72 hours from the end of the design storm.
21.
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, U.S. Department
of Commerce, National Oceanic and Atmospheric Administration (NOAA),
National Weather Service, Hydrometeorological Design Studies Center,
Silver Spring, Maryland, as amended. NOAA's Atlas 14 can be accessed
at http://hdsc.nws.noaa.gov/hdsc/pfds/.
22.
23.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (PA BMP Manual).
[Ord. 2011-12, 6/20/2011]
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.
The green infrastructure and low impact development practices provided
in the BMP Manual shall be utilized for all regulated activities wherever
possible. Volume controls shall be implemented using the Design Storm
Method in Subsection A or the Simplified Method in Subsection 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.
[Amended by Ord. No. 2022-7, 9/19/2022]
A.
Design Storm Method (any regulated activity): This method requires detailed modeling based on site conditions. For modeling assumptions refer to Section 8-124, Subsection 1.
(1)
Post-development total runoff should not be increased from pre-development
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:
(a)
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 below. Table B-3 in Appendix VII[1] is available to guide a qualified design 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:
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|
Step 1: Runoff (in) = Q = (P — 0.2S)2/(P + 0.8S)
| ||||
|
Where
| ||||
|
P
|
=
|
2-year rainfall (in.)
| ||
|
S
|
=
|
(1000/CN) — 10, the potential maximum retention (including
initial abstraction, 1A)
| ||
|
Step 2: Runoff Volume (Cubic Feet) = Q x Area x 1/12
| ||||
|
Where
| ||||
|
Q
|
=
|
Runoff (in.)
| ||
|
Area
|
=
|
SWM Area (square feet)
| ||
[1]
Editor's Note: Appendix VII is on file in the Borough offices.
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" runoff/12 inches) x impervious surface
(square feet)
|
(2)
At least the first one 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" runoff/12 inches) x impervious surface
(square feet)
|
(3)
Infiltration facilities should be designed to accommodate the first
0.5 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, per the Doylestown Borough Zoning Ordinance § 27-513, the applicant is required to submit a SWM site plan. The SWM site plan 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: Appendix VI is on file in the Borough offices.
(2)
Calculate the volume controls provided through nonstructural BMPs.
Table B-5 in Appendix VI is recommended as guidance.
(3)
Volume controls provided through nonstructural BMPs should be subtracted
from the required volume to determine the necessary structural BMPs.
|
Required Volume Control (feet3)
|
—
|
Nonstructural Volume Control (feet3)
|
=
|
Structural Volume Requirement (feet3)
|
(4)
Calculate the volume controls provided through structural BMPs. Table B-6 in Appendix VI 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 VI[3] 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 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.
[3]
Editor's Note: Appendix VI is on file in the Borough offices.
(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 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 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.
[Ord. 2011-12, 6/20/2011]
Peak rate controls for large storms, up to the one-hundred-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
one-hundred-year design storms are shown in Table 8-123.1. 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 8-123.1.
|
Table 8-123.1
| ||
|---|---|---|
|
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 and in this section of this Part. Doylestown Borough
is only in Stormwater Management Districts A and B listed above.
C.
District Boundaries. The boundaries of the stormwater management
districts are shown on official maps and are available for inspection
at the county planning offices. A copy of the map for the Borough
is included in Appendix III.[1] 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.
[1]
Editor's Note: Appendix III is on file in the Borough offices.
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 Doylestown Borough.
[Ord. 2011-12, 6/20/2011]
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 Tables B-4 or B-7
in Appendix VI of this Part.[1]
[1]
Editor's Note: Appendix VI is on file in the Borough offices.
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 8-124.1 summarizes acceptable
computation methods. The method selected by the qualified design professional
shall be based on the individual limitations and suitability of each
method for a particular site. The Borough 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. See Table B-1 in Appendix VI. This data may be directly
retrieved from the NOAA Atlas 14 website. If a hydrologic computer
model such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff
calculations, then the duration of the rainfall shall be 24 hours.
|
Table 8-124.1
| ||
|---|---|---|
|
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 or 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 Doylestown
Borough and/or the Doylestown Borough Engineer
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
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.
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 VI.
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 municipality
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 municipality.
10.
The design of any stormwater management facility intended to meet
the performance standards of this Part shall be verified by routing
the design storm hydrograph through these facilities. 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. 2011-12, 6/20/2011]
1.
Design Storms. Any stormwater management controls, required by this Part and subject to the volume requirement (§ 8-122) and the stormwater runoff peak rate requirements (§ 8-123), shall meet the applicable requirements consistent with the calculation methodology specified in § 8-124. Provisions must also be made for safely passing the runoff greater than that occurring from the largest design storm.
2.
Runoff Control Measures or Best Management Practices (BMPs).
A.
Increased stormwater runoff, which may result from regulated activities,
shall be controlled by permanent stormwater runoff control measures.
Any selected BMP must meet the required volume and runoff peak rate
requirements of this Part.
B.
As a minimum, first flush detention basin BMPs must be used to meet the volume requirement, and dual purpose detention basin BMPs must be used to meet the water quality requirement and the stormwater runoff peak rate requirements where appropriate as specified in § 8-123.
C.
Infiltration Best Management Practices (BMPs). Infiltration BMPs
shall be designed in accordance with the design criteria and specifications
in the PA BMP Manual and shall meet the following minimum requirements:
(1)
Maximizing the groundwater recharge capacity of the area being leveled
is encouraged. Design of the infiltration facilities shall consider
groundwater recharge to compensate for the reduction in the recharge
that occurs when the ground surface is disturbed or impervious surface
is created. It is recommended that roof runoff be directed to infiltration
BMPs which may be designed to compensate for the runoff from parking
areas. These measures are required to take advantage of utilizing
any existing recharge areas.
(2)
Minimum Requirements. Infiltration BMPs shall meet the following
minimum requirements:
(a)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(3)
Extreme caution should be exercised where infiltration is proposed
in source water protection areas as defined by the Borough, county,
DEP, or Water Authority.
(4)
Infiltration facilities shall be used in conjunction with other innovative
or traditional BMPs, stormwater control facilities and nonstructural
stormwater management alternatives.
(5)
Extreme caution shall be exercised where salt or chloride (municipal
salt storage) would be a pollutant since soils do little to filter
this pollutant and it may contaminate the groundwater. The qualified
design professional shall evaluate the possibility of groundwater
contamination from the proposed infiltration facility and perform
a hydrogeologic justification study, if necessary.
(6)
The infiltration requirement in high quality or exceptional value
waters shall be subject to the PADEP's Chapter 93, Antidegradation
Regulations.
(7)
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.
(8)
The Borough shall require the applicant to provide safeguards against
groundwater contamination for land uses that may cause groundwater
contamination should there be a mishap or spill.
(9)
General note for all groundwater recharge/infiltration subsurface
recharge system. The following note shall be placed on all plans which
propose a groundwater recharge/infiltration system: "The infiltration
facilities shown on these plans are a basic and perpetual part of
the stormwater management system of the proposed site located in Doylestown
Borough, Bucks County, Commonwealth of Pennsylvania and, as such,
are to be protected and preserved in accordance with the approved
final plans by the owners, their successors and assigns of these lands.
Doylestown Borough and/or its agents reserve the right and privilege
to enter upon these lands from time to time for the inspection of
said facilities in order to determine that proper operation and maintenance
and the structural and design integrity are being maintained by the
owner."
D.
Wet Pond and Artificial Wetland Best Management Practices (BMPs).
Wet pond and artificial wetland BMPs shall be designed in accordance
with the design criteria and specifications in the PA BMP Manual and
shall meet the following minimum requirement:
(1)
Wet pond and artificial wetland BMPs shall be constructed on hydric
or wet soils and/or soils which have an infiltration rate and percolation
rate of less than 0.2 inch/hour.
E.
Regional or Subregional Control Alternatives. The initiative and funding for any regional or subregional runoff control alternatives are the responsibility of a prospective developer(s). The design of any regional control facility must incorporate reasonable development of the entire upstream watershed. The peak outflow of a regional facility would be determined on a case-by-case basis using the hydrologic model of the watershed consistent with protection of the downstream watershed areas. "Hydrologic model" refers to the calibrated Neshaminy Creek model as developed for the Stormwater Management Act 167 Plan. Nevertheless, the volume requirement of § 8-122 must be met by the regional facility design.
F.
All stormwater control facility designs shall conform to the applicable
standards and specifications of the following governmental and institutional
agencies:
(1)
American Society of Testing and Materials (ASTM).
(2)
Asphalt Institute (AI).
(3)
Bucks County Conservation District.
(4)
Federal Highway Administration (FHWA).
(5)
National Crushed Stone Association (NCSA).
(6)
National Sand and Gravel Association (NSGA).
(7)
Pennsylvania Department of Environmental Protection (DEP).
(8)
Pennsylvania Department of Transportation (PennDOT).
(9)
United States Department of Agriculture, Soil Conservation Service,
Pennsylvania (USDA, SCS, PA).
G.
If special geological hazards or soil conditions, such as carbonate-derived
soils, are identified on the site, the developer's engineer shall
consider the effect of proposed stormwater management measures on
these conditions. In such cases, the Borough may require an in-depth
report by a competent soils engineer.
3.
Engineering Design Standards.
A.
General.
(1)
Storm drainage required. Overland or sheet flow is encouraged over
concentrated flow. A storm sewer system and all appurtenances shall
be required to be constructed by the developer or subdivider in any
area from which the surface or subsurface drainage could impair public
safety or cause physical damage to adjacent lands or public property.
The system shall be designed to collect water at any point where three
to five cubic feet per second is accumulated, at the bottom of all
vertical grades, and immediately upgrade from all street intersections.
The system shall discharge to the nearest practical natural drainage
channel or storm sewer.
(2)
Off-site improvements. Any increase in runoff may require necessary
corrective measures, including deeds of easement, deemed appropriate
by the Borough to alleviate any off-site drainage problem affected
by the earth disturbance activity. The costs for such measures will
be borne entirely by the developer or subdivider.
(3)
Total watershed evaluation. The future use of undeveloped areas upstream,
as shown on the approved Borough Comprehensive Plan, shall be taken
into account in the design of all storm sewer systems. The capacity
and maximum anticipated present flow of the body or system receiving
the proposed system(s) discharge shall be calculated to verify its
capability of receiving any additional flow caused by the development
or subdivision. The runoff from any proposed development shall be
subject to an evaluation, which includes the anticipated runoff from
other existing or proposed developments within the same watershed.
Stormwater management facilities designed to serve more than one property
or development in the same watershed are encouraged. Consultation
with the Borough is required prior to the design of such facilities.
B.
Underdrains.
(1)
All building foundations, grade slabs, and cellar floors located
in soils that have a community development limitation degree of moderate
to severe seasonal high-water table, as defined by an on-site soils
test witnessed by the Borough Engineer, shall be provided with an
underdrain system. This system shall provide for drainage of the enclosed
volume above the slab and relief of subsurface water to a depth of
not less than six inches below the slab or foundation bottom. The
system shall consist of a perforated pipe field of the herringbone
or gridiron configuration in coarse, gravel-filled trenches that are
in direct contact with the slab or foundation sub base. The excavation
shall provide a minimum of 0.005-foot/foot slope to the gravel-filled
trenches.
(2)
All proposed roadways shall be constructed with an underdrain system
consisting of six-inch perforated pipe wrapped in a Class 1 geotextile
material. The underdrain shall be bedded with a minimum of six inches
of 2B stone. The bedding material shall extend to the bottom of the
base course. The underdrain shall discharge to an inlet.
C.
Storm drain pipes and inlets.
(1)
All pipe and inlets shall meet material capacity and construction
specifications as outlined in the latest revisions of the PennDOT
Publication 408, as amended and the PennDOT Design Manual, as amended.
[Amended by Ord. No. 2022-7, 9/19/2022]
(2)
Design flow rate. All stormwater collection pipes and inlets which
flow to the detention basin's stormwater rate management facilities
shall be designed to handle the peak flow rate for the twenty-five-year
storm. All other stormwater system facilities shall be designed to
handle the peak flow rate for the one-hundred-year storm. The design
flow rate shall be determined by the rational formula, as follows:
|
Q = CiA
| ||||
|
Where
| ||||
|
Q
|
=
|
Peak runoff rate in cubic feet per second (cfs).
| ||
|
C
|
=
|
Runoff coefficient equal to the ratio of the peak runoff rate
to the average rate of rainfall over a time period equal to the time
of concentration.
| ||
|
i
|
=
|
Average rainfall intensity in inches per hour for a time equal
to the time of concentration.
| ||
|
A
|
=
|
Drainage area in acres.
| ||
|
NOTE: Appropriate values for the runoff coefficient and rainfall
intensity can be found in the Commonwealth of Pennsylvania Department
of Transportation Design Manual, Part 2, as amended and updated.
|
(3)
Inlet capacity. The capacity of all C-type, M-type or S-type inlets
shall be determined from the Commonwealth of Pennsylvania Department
of Transportation Design Manual, Part 2, as amended and updated.
(4)
Straight pipe sections. Wherever possible, all storm drain pipes
shall be designed to follow straight courses. No angular deflections
of storm sewer pipe sections in excess of 5° shall be permitted.
No vertical curves shall be permitted in the storm drain pipe system.
(5)
Minimum grade and size.
(a)
All storm drain pipes shall be designed to maintain a minimum
grade of 1/2%. All storm pipes shall have a minimum inside diameter
of 18 inches or a cross-sectional area of 254 square inches, except
that pipes under a fill of 25 feet or greater shall not be less than
24 inches or have a cross-sectional area of less than 453 square inches.
(b)
Reinforced concrete pipe shall be used in all public rights-of-way
and when the pipe is subject to vehicular or excessive overburden
loading conditions. High-density polyethylene pipe (HDPE) may be permitted
only within field or lawn areas if approved by the Borough Engineer.
(6)
Pipe capacity. The capacity of all pipe culverts shall, as a minimum,
provide the required carrying capacity as determined by the following
sources:
(7)
Pipe arches. Where headroom is restricted, equivalent pipe arches
may be used in lieu of circular pipe.
(8)
Allowable headwater depth. At all inlets or manholes, the maximum
allowable headwater depth shall be one foot below the top of the inlet
grate or the manhole cover.
(9)
Horizontal pipe deflections. A manhole or inlet shall be provided
at all horizontal deflections in the storm pipe system exceeding 5°.
(10)
Minimum and maximum cover. A minimum of three feet of cover
shall be maintained over all storm drain pipes. The top of storm drain
pipes shall be at least 1/2 foot below subgrade elevation. The maximum
cover over storm drain pipes shall be 10 feet unless special structural
design calculations are submitted or approved.
(11)
Match crowns. The crowns of all pipes tying into an inlet or
manhole shall be set at equal elevations.
(12)
Diversions of runoff. All storm drain pipes shall be designed
to carry the runoff into a detention basin or similar facility utilized
to control the rate of runoff.
(13)
Structural backfill. All storm drainage structures shall be
backfilled in twelve-inch layers with 2A stone to a dimension at the
base of the structure 12 inches outside all faces of the structure
tapering to a width of 24 inches outside all faces of the structure
at the top of the structure.
D.
Detention basins/stormwater rate management facilities.
(1)
Design of detention basins. All detention basins shall be designed
as per the procedures developed by the United States Department of
Agriculture, NRCS, as outlined in its Technical Release No. 55, Urban
Hydrology for Small Watersheds, Soil-Cover Complex Method.
(2)
Basin design criteria. All detention basins and/or other stormwater management facilities in the Borough shall be designed in accordance with the stormwater runoff peak rate requirements and districts contained in § 8-123 herein and the runoff calculation methodology specified in § 8-124 herein. All flows in excess of the basin design standards shall flow over an emergency spillway.
(3)
Maximum depth of detention basins. The maximum depth of water in
a detention basin shall not exceed five feet.
(4)
Emergency spillway. Whenever possible, the emergency spillway for
detention basins shall be constructed on undisturbed ground. Emergency
spillways shall be constructed of reinforced concrete, vegetated earth
or other approved material. All emergency spillways shall be constructed
so that the detention basin berm is protected against erosion. The
minimum capacity of all emergency spillways shall be equal to the
peak flow rate from the post-development one-hundred-year design storm.
Emergency spillways shall extend along the upstream and downstream
berm embankment slopes. The upstream edge of the emergency spillway
shall be a minimum of three feet below the spillway crest elevation.
The downstream slope of the spillway shall, as a minimum, extend to
the top of the berm embankment. The emergency spillway shall not discharge
over earthen fill and/or easily erodible material.
(5)
Anti-seep collars. Anti-seep collars shall be poured-in-place concrete
and installed around the principal pipe barrel within the normal saturation
zone of the detention basin berms. The collar shall be poured against
the pipe bell. The anti-seep collars and their connections to the
pipe barrel shall be watertight. The anti-seep collars shall extend
a minimum of two feet beyond the outside of the principal pipe barrel.
The spacing between collars shall be 10 feet. The use of an O-ring
pipe shall be required for all detention discharge pipes.
(6)
Freeboard. Freeboard is the difference between the design flow elevation
in the emergency spillway and the top of the settled detention basin
embankment. The minimum freeboard shall be one foot, and the maximum
freeboard shall be two feet.
(7)
Slope of detention basin embankment. The maximum slope of earthen
detention basin embankments shall be four to one. The top or toe of
any slope shall be located a minimum of five feet from any property
line. Whenever possible, the side slopes and basin shape shall be
amenable to the natural topography. Straight side slopes and rectangular
basins shall be avoided whenever possible.
(8)
Width of berm. The minimum top width of detention basin berms shall
be 10 feet.
(9)
Slope of basin bottom. In order to ensure proper drainage of the
detention basin, a minimum grade of 2% shall be maintained for all
sheet flow.
(10)
Energy dissipaters. Energy-dissipating devices (riprap, level
spreaders, end sills, etc.) shall be placed at all basin outlets.
Forebays or split flow controls are encouraged at the inlet to all
basins. When a forebay or split flow control is not used, an energy-dissipating
device shall be used and extend to the bottom of the basin.
(11)
Key trench. All basin berms in fill areas are to have a key
trench. The minimum depth shall be one foot and increased to a depth
of three feet at the minimum point of fill. The minimum width of the
key trench shall be eight feet.
E.
Culverts and drainage channels.
(1)
Design flow standard. All culverts and drainage channels tributary
to detention basins/stormwater rate management facilities shall be
designed to carry a flow rate equal to a twenty-five-year, twenty-four-hour
storm. All other culverts and drainage channels shall be designed
to carry a flow rate equal to a one-hundred-year, twenty-four-hour
storm (Soil-Cover Complex Method by the NRCS, Technical Release No.
55).
(2)
Erosion prevention. All drainage channels shall be designed to prevent
the erosion of the bed and bank slopes. The flow velocity in all vegetated
drainage channels shall not exceed the maximum permissible velocity
to prevent erosion. Suitable bank stabilization shall be provided,
where required, to prevent erosion of the drainage channels. Where
storm sewers discharge into existing drainage channels at an angle
greater than 30° from parallel to the downstream channel flow,
the far-side bank shall be stabilized by the use of riprap or masonry
and/or concrete walls. The stabilization shall be designed to prevent
erosion and frost heave under and behind the stabilizing media.
(3)
Maximum side slope. Any vegetated drainage channel requiring mowing
of the vegetation shall have a minimum grade of four horizontal to
one vertical.
(4)
Design standard. Because of the critical nature of vegetated drainage
channels, the design of all vegetated channels shall, as a minimum,
conform to the design procedures outlined in the Erosion and Sediment
Control Program Manual. Several acceptable sources outline procedures
for nonvegetated drainage channels, including the following:
[Ord. 2011-12, 6/20/2011]
1.
No regulated earth disturbance activities within the Borough shall
commence until approval of an erosion and sediment control plan for
construction activities. The Bucks County Conservation District requires
an Erosion and Sediment Control Plan for any earth disturbance activity
of 1,000 square feet or more.
2.
In addition, under 25 Pa. Code Chapter 102.5, a DEP NPDES permit
for stormwater discharges associated with construction activities
is required for any earth disturbance activity that involves equal
to or greater than one acre of earth disturbance prior to commencing
the earth disturbance activity. This includes earth disturbance on
any portion, part, or during any stage of, a larger common plan of
development or sale.
3.
Detailed construction schedules shall be included in all erosion
and sedimentation plans and stormwater management plans reviewed by
the Bucks County Conservation District.
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. The issuance of an NPDES construction permit or permit coverage under the statewide general permit (PAG-2) satisfies the requirements of Subsection 1. 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.
[Added by Ord. No. 2022-7, 9/19/2022]
1.
The landowner or the owner's designee (including Doylestown
Borough for dedicated and owned facilities) shall inspect SWM BMPs,
facilities and/or structures installed under an NPDES permit issued
by the County Conservation District or the PADEP according to the
following frequencies, at a minimum, to ensure the BMPs, facilities
and/or structures continue to function as intended:
2.
Inspections should be conducted during or immediately following precipitation
events. A written inspection report shall be created to document each
inspection. The inspection report shall contain the date and time
of the inspection, the individual(s) who completed the inspection,
the location of the BMP, facility or structure inspected, observations
on performance, and recommendations for improving performance, if
applicable. Inspection reports shall be submitted to Doylestown Borough
within 30 days following completion of the inspection.
