A.
All regulated activities in the Borough shall be subject to the stormwater
management requirements of this chapter.
B.
Storm drainage systems shall be provided to permit unimpeded flow
in natural watercourses except as modified by stormwater detention
facilities, pipe systems or open channels consistent with this chapter.
C.
The existing locations of concentrated drainage discharge onto adjacent
property shall not be altered without written approval of the affected
property owner(s).
D.
Areas of existing diffused drainage discharge onto adjacent property
shall be managed such that, at minimum, the peak diffused flow does
not increase in the general direction of discharge, except as otherwise
provided in this chapter. If diffused flow is proposed to be concentrated
and discharged onto adjacent property, the developer must document
that there are adequate downstream conveyance facilities to safely
transport the concentrated discharge to the point of predevelopment
flow concentration, to the stream reach, or otherwise prove that no
harm will result from the concentrated discharge. Areas of existing
diffused drainage discharge shall be subject to any applicable release
rate criteria in the general direction of existing discharge whether
they are proposed to be concentrated or maintained as diffused drainage
areas.
E.
Where a site is traversed by watercourses other than those for which a one-hundred-year floodplain is defined by the Borough, there shall be provided drainage easements conforming substantially with the line of such watercourses. The width of any easement shall be adequate to provide for unimpeded flow of storm runoff based on calculations made in conformance with § 378-17 for the one-hundred-year return period runoff and to provide a freeboard allowance of 1/2 foot above the design water surface level. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations which may adversely affect the flow of stormwater within any portion of the easement. Also, periodic maintenance of the easement to ensure proper runoff conveyance shall be required. Watercourses for which the one-hundred-year floodplain is formally defined are subject to the applicable Borough floodplain regulations.[1]
F.
When it can be shown that, due to topographic conditions, natural
drainage swales on the site cannot adequately provide for drainage,
open channels may be constructed conforming substantially to the line
and grade of such natural drainage swales. Capacities of open channels
shall be calculated using the Manning Equation.
G.
Post-construction BMPs shall be designed, installed, operated and
maintained to meet the requirements of the Clean Streams Law[2] and implementing regulations, including the established
practices in 25 Pa. Code Chapter 102 and the specifications of this
chapter, to prevent accelerated erosion in watercourse channels and
at all points of discharge.
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
H.
No earth disturbance activities associated with any regulated activities
shall commence until approval by the Borough of a plan which demonstrates
compliance with the requirements of this chapter.*
I.
Techniques described in Appendix D (Low-Impact Development)[3] of this chapter are encouraged because they reduce the
costs of complying with the requirements of this chapter and the state
water quality requirements.*
[3]
Editor's Note: Appendix D is included at the end of this chapter.
J.
Infiltration for stormwater management is encouraged where soils
and geology permit, consistent with the provisions of this chapter
and, where appropriate, the Recommendation Chart for Infiltration
Stormwater Management BMPs in Carbonate Bedrock in Appendix B.[4] Infiltration is encouraged for capturing and treating the water quality volume (as calculated in § 378-14), any part of the water quality volume, or for otherwise meeting the purposes of this chapter.
[4]
Editor's Note: Appendix B is included at the end of this chapter.
The following permit requirements apply to certain regulated
and earth disturbance activities and must be met prior to commencement
of regulated and earth disturbance activities, as applicable:
A.
All regulated and earth disturbance activities subject to permit
requirements by DEP under regulations at 25 Pa. Code Chapter 102.
B.
Work within natural drainageways subject to permit by DEP under 25
Pa. Code Chapter 102.
C.
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.
D.
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).
E.
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.
A.
No regulated earth disturbance activities within the Borough shall
commence until approval by the Borough Engineer of an erosion and
sediment control plan for construction activities. Written approval
by DEP or a delegated county conservation district shall satisfy this
requirement.
B.
An erosion and sediment control plan is required by DEP regulations
for any earth disturbance activity of 5,000 square feet or more under
25 Pa. Code § 102.4(b).
C.
A DEP NPDES Stormwater Discharges Associated with Construction Activities
Permit is required for regulated earth disturbance activities under
25 Pa. Code Chapter 92.
D.
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 before the commencement of
an earth disturbance activity.
E.
A copy of the erosion and sediment control plan and any permit, as
required by DEP regulations, shall be available at the project site
at all times.
A.
No regulated earth disturbance activities within the Borough shall
commence until approval by the Borough Engineer of a plan which demonstrates
compliance with this chapter. DEP has determined that this chapter
meets state water quality requirements. Therefore, any approvals under
this chapter would satisfy the post-construction stormwater management
requirements associated with an NPDES permit for stormwater discharges
associated with construction activities.*
B.
The water quality volume (WQv) shall be captured
and treated. The WQv shall be calculated two
ways.
(1)
First, WQv shall be calculated using the following
formula:
|
WQv = (c)(P)(A)/12
| ||||
|---|---|---|---|---|
|
Where
| ||||
|
WQv
|
=
|
Water quality volume in acre-feet
| ||
|
c
|
=
|
Rational Method post-development runoff coefficient for the
two-year storm
| ||
|
P
|
=
|
1.25 inches
| ||
|
A
|
=
|
Area in acres of proposed regulated activity
| ||
(2)
Second, the WQv shall be calculated as the
difference in runoff volume from predevelopment to post development
for the two-year return period storm. The effect of closed depressions
on the site shall be considered in this calculation. The larger of
these two calculated volumes shall be used as the WQv to be captured and treated, except that in no case shall the WQv be permitted to exceed 1.25 inches of runoff over the
site area.
C.
The WQv shall be calculated for each post-development
drainage direction on a site for sizing BMPs. Site areas having no
impervious cover and no proposed disturbance during development may
be excluded from the WQv calculations and do
not require treatment.
D.
If an applicant is proposing to use a wet pond, constructed wetland
or other BMP that ponds water on the land surface and may receive
direct sunlight, the discharge from that BMP must be treated by infiltration,
a vegetated buffer, filter strip, bioretention, vegetated swale or
other BMP that provides a thermal benefit to protect the high quality
waters of the Saucon Creek from thermal impacts.
E.
Any stormwater runoff from the site as a result of the regulated activities must either be treated with infiltration or two acceptable BMPs, such as those listed in Subsection N.
F.
Infiltration BMPs shall not be constructed on fill.
G.
The applicant shall document the bedrock type(s) present on the site
from published sources. Any apparent boundaries between carbonate
and noncarbonate bedrock shall be verified through more detailed site
evaluations by a qualified geotechnical professional.
H.
For each proposed regulated activity in the watershed, the applicant
shall conduct a preliminary site investigation on the portion of the
site that is judged to be the best candidate hydrogeologically for
possible infiltration, including gathering data from published sources,
a field inspection of the site, a minimum of one test pit and a minimum
of two percolation tests, as outlined in Appendix E.[1] This investigation will determine depth to bedrock, depth
to the seasonal high-water table, soil permeability and location of
special geologic features, if applicable. The location(s) of special
geologic features shall be verified by a qualified geotechnical professional.
[1]
Editor's Note: Appendix E is included at the end of this chapter.
I.
For entirely noncarbonate sites, the WQv shall
be infiltrated unless the applicant demonstrates that it is infeasible
to infiltrate the WQv for reasons of seasonal high-water table, permeability rate, soil depth or isolation distances; or except as provided in Subsection T. The preliminary site investigation described in Subsection H shall continue on different areas of the site until a suitable infiltration location is found or the entire site is determined to be infeasible for infiltration. For proposed infiltration areas, the additional site investigation and testing as outlined in Appendix E [2] shall be completed. The Borough may determine infiltration
to be infeasible if there are known existing conditions or problems
that may be worsened by the use of infiltration. The following conditions
are suitable for infiltration in noncarbonate areas:
(1)
Depth to bedrock below the invert of the BMP greater than or equal
to two feet.
(2)
Depth to seasonal high-water table below the invert of the BMP greater
than or equal to three feet. (If the depth to bedrock is between two
and three feet and the evidence of the seasonal high-water table is
not found in the soil, no further testing to locate the depth to seasonal
high-water table is required.)
(3)
Soil permeability greater than or equal to 0.5 inches/hour and less
than or equal to 12 inches per hour.
(4)
Setback distances or buffers as follows:
(a)
One hundred feet from water supply wells.
(b)
Ten feet downgradient or 100 feet upgradient from building foundations.
(c)
Fifty feet from septic system drainfields.
(d)
Fifty feet from a geologic contact with carbonate bedrock unless
a preliminary site investigation is done in the carbonate bedrock
to show the absence of special geologic features within 50 feet of
the proposed infiltration area.
(e)
One hundred feet from the property line unless documentation
is provided to show that all setbacks from wells, foundations and
drainfields on neighboring properties will be met.
(5)
If it is not feasible to infiltrate the full WQv, the applicant shall infiltrate that portion of the WQv that is feasible based on the site characteristics.
[2]
Editor's Note: Appendix E is included at the end of this chapter.
J.
In entirely carbonate areas, in addition to the testing required in Subsection H, the preliminary site investigation shall include an assessment of the remainder of the site for possible infiltration based on required isolation distances from special geologic features and the likely soil depth and permeability based on published data or other site data available. Where infiltration BMPs are proposed, the applicant shall conduct the additional site investigation and testing as outlined in Appendix E.[3] The soil depth, percolation rate and proposed loading rate, each weighted as described in § 378-17, along with the buffer from special geologic features shall be compared to the Recommendation Chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock in Appendix B[4] to determine if the site is recommended for infiltration.
If at any point in the preliminary site investigation the data (e.g.,
location of Karst features on the site or the published soils data
for the site) indicates that the entire site will not be recommended
for infiltration based on the chapter standards, then no further investigation
is required. In addition to the recommendation from Appendix B, the
following conditions are required for infiltration in carbonate areas:
(1)
Depth to bedrock below the invert of the BMP greater than or equal
to two feet.
(2)
Depth to seasonal high-water table below the invert of the BMP greater
than or equal to three feet. (If the depth to bedrock is between two
and three feet and the evidence of the seasonal high-water table is
not found in the soil, no further testing to locate the depth to seasonal
high-water table is required.)
(3)
Soil permeability greater than or equal to 0.5 inches/hour and less
than or equal to 12 inches per hour.
(4)
Setback distances or buffers as follows:
(a)
One hundred feet from water supply wells.
(b)
Ten feet downgradient or 100 feet upgradient from building foundations.
(c)
Fifty feet from septic system drainfields.
(d)
One hundred feet from the property line, unless documentation
is provided to show that all setbacks from wells, foundations and
drainfields on neighboring properties will be met.
(5)
Applicants are not required to use infiltration BMPs on a carbonate
site even if the site falls in the "Recommended" range on the chart
in Appendix B.[5] If infiltration is not proposed, the WQv shall be treated by two acceptable BMPs, as specified in Subsection O.
[5]
Editor's Note: Appendix B is included at the end of this chapter.
[3]
Editor's Note: Appendix E is included at the end of this chapter.
[4]
Editor's Note: Appendix B is included at the end of this chapter.
K.
If a site has both carbonate and noncarbonate areas, the applicant
shall investigate the ability of the noncarbonate portion of the site
to fully meet this chapter to control runoff for the whole site through
infiltration. If that proves infeasible, the applicant shall perform
the preliminary site investigation for the carbonate area to determine
the appropriate design strategy. No infiltration structure in the
noncarbonate area shall be located within 50 feet of a boundary with
carbonate bedrock, except when a preliminary site investigation has
been done showing the absence of special geologic features within
50 feet of the proposed infiltration area.
L.
If infiltration BMPs are proposed in carbonate areas, the post-development
two-year runoff volume leaving the site shall be 80% or more of the
predevelopment runoff volume for the carbonate portion of the site
to prevent infiltration of volumes far in excess of the predevelopment
infiltration volume.
M.
Site areas proposed for infiltration shall be protected from disturbance
and compaction except as necessary for construction of infiltration
BMPs.
N.
If infiltration of the entire WQv is not proposed,
the remainder of the WQv shall be treated by
two acceptable BMPs in series for each discharge location. Sheet flow
draining across a pervious area can be considered as one BMP. Sheet
flow across impervious areas and concentrated flow shall flow through
two BMPs. If sheet flow from an impervious area is to be drained across
a pervious area as one BMP, the length of the pervious area must be
equal to or greater than the length of impervious area. In no case
may the same BMP be employed consecutively to meet this requirement.
Acceptable BMPs are listed below along with the recommended reference
for design.
|
Best Management Practice
|
Design Reference
| |
|---|---|---|
|
Bioretention
|
Low Impact Development Design Strategies, Prince George's County,
Md., June 19992
| |
|
Capture reuse
|
Texas Guide to Rainwater Harvesting, 2nd Edition. Texas Water
Development Board, Center for Maximum Potential Building Systems,
19973
| |
|
Constructed wetlands
|
2000 Maryland Stormwater Design Manual, Maryland Department
of the Environment4
| |
|
Minimum disturbance/minimum maintenance practices
|
Conservation Design for Stormwater Management Delaware Dept.
of Natural Resources and Brandywine Conservancy, September 19975
| |
|
Oil/water separators
|
Georgia Stormwater Management Manual Volume 2 Technical Handbook,
August 20016
| |
|
Sediment traps/catch basin sumps
|
U.S. Environmental Protection Agency's Post-Construction Stormwater
Management in New Development & Redevelopment BMP Fact Sheet for
"Catch Basins/Catch Basin Insert"7
| |
|
Significant reduction of existing impervious cover
|
N/A
| |
|
Stormwater filters (sand, peat, compost, etc.)
|
Design of Stormwater Filtering Systems. Claytor, R. and Schueler,
T., Center for Watershed Protection, December 19968
| |
|
Trash/debris collectors in catch basins
|
Pennsylvania Handbook of BMPs for Developing Areas9 or latest PA Department of Environmental Protection
Manual
| |
|
Vegetated buffers/filter strips
|
Pennsylvania Handbook of BMPs for Developing Areas9 or latest PA Department of Environmental Protection
Manual
| |
|
Vegetated roofs
|
Roof Gardens: History, Design, and Construction. Osmundson,
T., W. W. Norton & Co., 199810
| |
|
Vegetated swales/filter strips
|
2000 Maryland Stormwater Design Manual, Maryland Department
of the Environment4
| |
|
Water quality inserts for inlets
|
Pennsylvania Handbook of BMPs for Developing Areas9 or latest PA Department of Environmental Protection
Manual
| |
|
Wet detention ponds
|
Pennsylvania Handbook of BMPs for Developing Areas9 or latest PA Department of Environmental Protection
Manual
|
|
NOTES:
| ||
|---|---|---|
|
1
|
If this BMP is used to treat the entire WQv, then only one BMP is required because of this BMP's superior water
quality performance.
| |
|
2
|
Available at www.co.pg.md.us/Government/AgencyIndex/DER/PPD/LID/LiDNatl.pdf
as of January 2004
| |
|
3
|
Available at www.twdb.state.tx.us/publications/reports/RainHarv.pdf
as of January 2004
| |
|
4
|
Available at www.mde.state.md.us as of January 2004
| |
|
5
|
Available at www.dnrec.state.de.us/dnrec2000/Divisions/Soil/Stormwater/Apps/
DesignManualRequest.htm as of January 2004
| |
|
6
|
Available at www.georgiastormwater.com as of January 2004
| |
|
7
|
Available at http://cfpub.epa.gov/npdes/stormwater/menuofbmps/post_7.cfm
as of January 2004
| |
|
8
|
Available from the Center for Watershed Protection (www.cwp.org)
as of January 2004
| |
|
9
|
Available at www.dep.state.pa.us (keyword "stormwater") as of
January 2004
| |
|
10
|
Available at www.wwnorton.com as of January 2004
| |
O.
Stormwater runoff from hot spot land uses shall be pretreated. In no case, may the same BMP be employed consecutively to meet this requirement and the requirement in Subsection N.
(1)
Acceptable methods of pretreatment are listed below.
|
Hot Spot Land Use
|
Pretreatment Method(s)
| ||
|---|---|---|---|
|
Vehicle maintenance and repair facilities including auto parts
stores
|
•
•
•
•
|
Oil/water separators
Use of drip pans and/or dry sweep material under vehicles/equipment
Use of absorbent devices to reduce liquid releases
Spill prevention and response program
| |
|
Vehicle fueling stations
|
•
•
•
|
Oil/water separators
Water quality inserts for inlets
Spill prevention and response program
| |
|
Storage areas for public works
|
•
•
•
•
•
•
•
|
Oil/water separators
Sediment traps/catch basin sumps
Water quality inserts for inlets
Use of drip pans and/or dry sweep material under vehicles/equipment
Use of absorbent devices to reduce liquid releases
Spill prevention and response program
Diversion of stormwater away from potential contamination areas
| |
|
Outdoor storage of liquids
|
•
|
Spill prevention and response program
| |
|
Commercial nursery operations
|
•
•
•
|
Vegetated swales/filter strips
Constructed wetlands
Stormwater collection and reuse
| |
|
Salvage yards and recycling facilities*
|
•
|
BMPs that are a part of a stormwater pollution prevention plan
under an NPDES Permit
| |
|
Fleet storage yards and vehicle cleaning facilities*
|
•
|
BMPs that are a part of a stormwater pollution prevention plan
under an NPDES Permit
| |
|
Facilities that store or generate regulated substances*
|
•
|
BMPs that are a part of a stormwater pollution prevention plan
under an NPDES Permit
| |
|
Marinas*
|
•
|
BMPs that are a part of a stormwater pollution prevention plan
under an NPDES Permit
| |
|
Certain industrial uses (listed under NPDES)*
|
•
|
BMPs that are a part of a stormwater pollution prevention plan
under an NPDES Permit
| |
|
NOTES:
| ||
|---|---|---|
|
*
|
Regulated under the NPDES Stormwater Program
| |
(2)
Design references for the pretreatment methods, as necessary, are
listed below. The applicant may demonstrate that, due to the site
characteristics, the land use is not a hot spot land use.
|
Pretreatment Method
|
Design Reference
| |
|---|---|---|
|
Constructed wetlands
|
2000 Maryland Stormwater Design Manual, Maryland Department
of the Environment1
| |
|
Diversion of stormwater away from potential contamination areas
|
Pennsylvania Handbook of BMPs for Developing Areas2 or latest PA Department of Environmental Protection
Manual
| |
|
Oil/water separators
|
Georgia Stormwater Management Manual Volume 2 Technical Handbook,
August 20013
| |
|
Sediment traps/catch basin sumps
|
U.S. Environmental Protection Agency's Post-Construction Stormwater
Management in New Development and Redevelopment BMP Fact Sheet for
"Catch Basins/Catch Basin Insert"4
| |
|
Stormwater collection and reuse (especially for irrigation)
|
Texas Guide to Rainwater Harvesting, 2nd Edition, Texas Water
Development Board, Center for Maximum Potential Building Systems,
19975
| |
|
Stormwater filters (sand, peat, compost, etc.)
|
Design of Stormwater Filtering Systems. Claytor, R. and Schueler,
T., Center for Watershed Protection, December 19966
| |
|
Trash/debris collectors in catch basins
|
Pennsylvania Handbook of BMPs for Developing Areas2 or latest PA Department of Environmental Protection
Manual
| |
|
Vegetated swales/filter strips
|
2000 Maryland Stormwater Design Manual, Maryland Department
of the Environment1
| |
|
Water quality inserts for inlets
|
Pennsylvania Handbook of BMPs for Developing Areas2 or latest PA Department of Environmental Protection
Manual
|
|
NOTES:
| ||
|---|---|---|
|
1
|
Available at www.mde.state.md.us as of January 2004
| |
|
2
|
Available at www.dep.state.pa.us (keyword Stormwater) as of
January 2004
| |
|
3
|
Available at www.georgiastormwater.com as of January 2004
| |
|
4
|
Available at http://cfpub.epa.gov/npdes/stormwater/menuofbmps/post_7.cfm
as of January 2004
| |
|
5
|
Available at www.twdb.state.tx.us/publications/reports/RainHarv.pdf
as of January 2004
| |
|
6
|
Available from the Center for Watershed Protection (www.cwp.org)
as of January 2004
| |
P.
The use of infiltration BMPs is prohibited on hot spot land use areas.
Q.
Stormwater infiltration BMPs shall not be placed in or on a special
geologic feature(s). Additionally, stormwater runoff shall not be
discharged into existing on-site sinkholes.
R.
Applicants shall request, in writing, public water suppliers to provide the Zone I wellhead protection radius, as calculated by the method outlined in the Pennsylvania Department of Environmental Protection Wellhead Protection Regulations, for any public water supply well within 400 feet of the site. In addition to the setback distances specified in Subsections I and J, infiltration is prohibited in the Zone I radius as defined and substantiated by the public water supplier in writing. If the applicant does not receive a response from the public water supplier, the Zone I radius is assumed to be 100 feet.
S.
The volume and rate of the net increase in stormwater runoff from
the regulated activities must be managed to prevent the physical degradation
of receiving waters from such effects as scour and stream bank destabilization,
to satisfy state water quality requirements.
T.
The Borough may, after consultation with DEP, approve alternative
methods for meeting the state water quality requirements other than
those in this section, provided that they meet the minimum requirements
of and do not conflict with state law, including but not limited to
the Clean Streams Law.[6]
[6]
Editor's Note: See 35 P.S. § 691.1 et seq.
A.
To implement the provisions of the Saucon Creek Watershed Stormwater
Management Plan, the Borough is hereby determined to be a "conditional
no-detention district."
B.
Conditional no-detention districts. Within these districts, the capacity of the "local" runoff conveyance facilities (as defined in Article II) must be calculated to determine if adequate capacity exists. For this determination, the developer must calculate peak flows assuming that the site is developed as proposed and that the remainder of the local watershed is in the existing condition. The developer must also calculate peak flows assuming that the entire local watershed is developed per current zoning and that all new development would use the runoff controls specified by this chapter. The larger of the two peak flows calculated will be used in determining if adequate capacity exists. If adequate capacity exists to safely transport runoff from the site to the main channel (as defined in Article II), these watershed areas may discharge post-development peak runoff without detention facilities. If the capacity calculations show that the "local" runoff conveyance facilities lack adequate capacity, the developer shall either use a one-hundred-percent release rate control or provide increased capacity of downstream elements to convey increased peak flows consistent with § 378-16P. Any capacity improvements must be designed to convey runoff from development of all areas tributary to the improvement consistent with the capacity criteria specified in § 378-16D. By definition, a storm drainage problem area associated with the "local" runoff conveyance facilities indicates that adequate capacity does not exist.
A.
Applicants shall provide a comparative preconstruction and post-construction
stormwater management hydrograph analysis for each direction of discharge
and for the site overall to demonstrate compliance with the provisions
of this chapter.
B.
Any downstream capacity analysis conducted in accordance with this
chapter shall use the following criteria for determining adequacy
for accepting increased peak flow rates:
(1)
Natural or man-made channels or swales must be able to convey the
increased runoff associated with a two-year return period event within
their banks at velocities consistent with protection of the channels
from erosion.
(2)
Natural or man-made channels or swales must be able to convey the
increased twenty-five-year return period runoff without creating any
hazard to persons or property.
(3)
Culverts, bridges, storm sewers or any other facilities which must
pass or convey flows from the tributary area must be designed in accordance
with DEP Chapter 105 regulations (if applicable) and, at minimum,
pass the increased twenty-five-year return period runoff.
C.
For a proposed development site located within one release rate category
subarea, the total runoff from the site shall meet the applicable
release rate criteria. For development sites with multiple directions
of runoff discharge, individual drainage directions may be designed
for up to a one-hundred-percent release rate so long as the total
runoff from the site is controlled to the applicable release rate.
D.
For a proposed development site located within two or more release
category subareas, the peak discharge rate from any subarea shall
be the predevelopment peak discharge for that subarea multiplied by
the applicable release rate. The calculated peak discharges shall
apply regardless of whether the grading plan changes the drainage
area by subarea. An exception to the above may be granted if discharges
from multiple subareas recombine in proximity to the site. In this
case, peak discharge in any direction may be a one-hundred-percent
release rate, provided that the overall site discharge meets the weighted
average release rate.
E.
For a proposed development site located partially within a release
rate category subarea and partially within a conditional no-detention
subarea, a significant portion of the site area subject to the release
rate control may not be drained to the discharge point(s) located
in the no-detention subarea except as part of a no-harm or hardship
waiver procedure.
F.
No portion of a site may be regraded between the Saucon Creek Watershed
and any adjacent watershed except as part of a no-harm or hardship
waiver procedure.
G.
Within a release rate category area, for a proposed development site which has areas which drain to a closed depression(s), the design release from the site will be the lesser of: (a) the applicable release rate flow assuming no closed depression(s) or (b) the existing peak flow actually leaving the site. In cases where (b) would result in an unreasonably small design release, the design discharge of less than or equal to the release rate will be determined by the available downstream conveyance capacity to the main channel calculated using Subsection D and the minimum orifice criteria.
H.
Off-site areas which drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site using the capacity criteria in Subsection D and the detention criteria in § 378-17.
I.
For development sites proposed to take place in phases, all detention
ponds shall be designed to meet the applicable release rate(s) applied
to all site areas tributary to the proposed pond discharge direction.
All site tributary areas will be assumed as developed, regardless
of whether all site tributary acres are proposed for development at
that time. An exception shall be sites with multiple detention ponds
in series where only the downstream pond must be designed to the stated
release rate.
J.
Where the site area to be impacted by a proposed development activity
differs significantly from the total site area, only the proposed
impact area shall be subject to the release rate criteria. The impact
area includes any proposed cover or grading changes.
K.
Development proposals which, through groundwater recharge or other
means, do not increase either the rate or volume of runoff discharged
from the site compared to predevelopment are not subject to the release
rate provisions of this chapter.
L.
No-harm water quantity option. For any proposed development site not located in a conditional no-detention district, the developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove that special circumstances exist for the proposed development site and that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. Special circumstances are defined as any hydrologic or hydraulic aspects of the development itself not specifically considered in the development of the plan runoff control strategy. Proof of no harm would have to be shown from the development site through the remainder of the downstream drainage network to the confluence of the creek with the Lehigh River. Proof of no harm must be shown using the capacity criteria specified in Subsection D if downstream capacity analysis is a part of the no-harm justification.
(1)
Attempts to prove no harm based upon downstream peak flow versus
capacity analysis shall be governed by the following provisions:
(a)
The peak flow values to be used for downstream areas for the
design return period storms (two-, ten-, twenty-five-, and one-hundred-year)
shall be the values from the calibrated Watershed Model for the Little
Lehigh Creek or as calculated by an applicant using an alternate method
acceptable to the Borough. The flow values from the watershed model
would be supplied to the developer by the Borough upon request.
(b)
Any available capacity in the downstream conveyance system as
documented by a developer may be used by the developer only in proportion
to his development site acreage relative to the total upstream undeveloped
acreage from the identified capacity (i.e., if his site is 10% of
the upstream undeveloped acreage, he may use up to 10% of the documented
downstream available capacity).
(c)
Developer-proposed runoff controls which would generate increased
peak flow rates at storm drainage problem areas would, by definition,
be precluded from successful attempts to prove no harm, except in
conjunction with proposed capacity improvements for the problem areas
consistent with Subsection O.
M.
Regional detention alternatives. For certain areas within the Borough,
it may be more cost-effective to provide one control facility for
more than one development site than to provide an individual control
facility for each development site. The initiative and funding for
any regional runoff control alternatives are the responsibility of
prospective developers. The design of any regional control basins
must incorporate reasonable development of the entire upstream watershed.
The peak outflow of a regional basin would be determined based on
the required release rate at the point of discharge.
N.
Capacity improvements.
(1)
In certain instances, primarily within the conditional no-detention areas, local drainage conditions may dictate more stringent levels of runoff control than those based upon protection of the entire watershed. In these instances, if the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, then the capacity improvements could be provided by the developer in lieu of runoff controls on the development site. Peak flow calculations shall be done assuming that the local watershed is in the existing condition and then assuming that the local watershed is developed per current zoning and using the specified runoff controls. Any capacity improvements would be designed using the larger of the above peak flows and the capacity criteria specified in Subsection D. All new development in the entire subarea(s) within which the proposed development site is located shall be assumed to implement the developer's proposed discharge control, if any.
(2)
Capacity improvements may also be provided as necessary to implement
any regional detention alternatives or to implement a modified no-harm
option which proposes specific capacity improvements to provide that
a less stringent discharge control would not create any harm downstream.
A.
Stormwater runoff from all development sites shall be calculated
using either the Rational Method or the Soil-Cover-Complex Methodology.
B.
Infiltration BMP loading rate percentages in the Recommendation Chart
for Infiltration Stormwater Management BMPs in Carbonate Bedrock in
Appendix B[1] shall be calculated as follows:
|
(Area tributary to infiltration BMP/base area of infiltration
BMP) *100%
|
|
The area tributary to the infiltration BMP shall be weighted
as follows:
| ||
|
All disturbed areas to be made impervious:
|
Weight at 100%
| |
|
All disturbed areas to be made pervious:
|
Weight at 50%
| |
|
All undisturbed pervious areas:
|
Weight at 0%
| |
|
All existing impervious areas:
|
Weight at 100%
| |
[1]
Editor's Note: Appendix B is included at the end of this chapter.
C.
Soil thickness is to be measured from the bottom of any proposed
infiltration system. The effective soil thickness in the Recommendation
Chart for Infiltration Stormwater Management BMPs in Carbonate Bedrock
in Appendix D is the measured soil thickness multiplied by the thickness
factor based on soil permeability, as follows:
|
Permeability Range*
(inches/hour)
|
Thickness Factor
| |
|---|---|---|
|
6.0 to 12.0
|
0.8
| |
|
2.0 to 6.0
|
1.0
| |
|
1.0 to 2.0
|
1.4
| |
|
0.75 to 1.0
|
1.2
| |
|
0.5 to 0.75
|
1.0
|
|
NOTES:
| ||
|---|---|---|
|
*
|
If the permeability rate falls on a break between two thickness
factors, the smaller thickness factor shall be used.
| |
|
Sites with soil permeability greater than 12.0 inches/hour or
less than 0.5 inch/hour are not recommended for infiltration.
| ||
D.
The design of any detention basin intended to meet the requirements
of this chapter shall be verified by routing the design storm hydrograph
through the proposed basin using the Storage Indication Method or
other methodology demonstrated to be more appropriate. For basins
designed using the Rational Method technique, the design hydrograph
for routing shall be either the Universal Rational Hydrograph or the
modified rational method trapezoidal hydrograph which maximizes detention
volume. Use of the modified rational hydrograph shall be consistent
with the procedure described in Section "PIPE.RAT" of the Users' Manual
for the Penn State Urban Hydrograph Method (1987).
E.
BMPs designed to store or infiltrate runoff and discharge to surface
runoff or pipe flow shall be routed using the Storage Indication Method.
F.
BMPs designed to store or infiltrate runoff and discharge to surface
runoff or pipe flow shall provide storage volume for the full WQv below the lowest outlet invert.
G.
Wet detention ponds designed to have a permanent pool for the WQv shall assume that the permanent pool volume below the
primary outlet is full at the beginning of design event routing for
the purposes of evaluating peak outflows.
H.
All stormwater detention facilities shall provide a minimum one foot
freeboard above the maximum pool elevation associated with the two-year
through twenty-five-year runoff events. A 0.5 foot freeboard shall
be provided above the maximum pool elevation of the one-hundred-year
runoff event. The freeboard shall be measured from the maximum pool
elevation to the invert of the emergency spillway. The two-year through
one-hundred-year storm events shall be controlled by the primary outlet
structure. An emergency spillway for each basin shall be designed
to pass the one-hundred-year return frequency storm peak basin inflow
rate with a minimum 0.5 foot freeboard measured to the top of basin.
The freeboard criteria shall be met considering any off-site areas
tributary to the basin as developed, as applicable. If this detention
facility is considered to be a dam as per DEP Chapter 105, the design
of the facility must be consistent with the Chapter 105 regulations,
and may be required to pass a storm greater than the one-hundred-year
event.
I.
The minimum circular orifice diameter for controlling discharge rates
from detention facilities shall be three inches. Designs where a lesser-size
orifice would be required to fully meet release rates shall be acceptable,
provided that as much of the site runoff as practical is directed
to the detention facilities.
J.
Runoff calculations using the Soil-Cover-Complex Method shall use
the Natural Resources Conservation Service Type II twenty-four-hour
rainfall distribution. The twenty-four-hour rainfall depths for the
various return periods to be used consistent with this chapter may
be taken from NOAA Atlas 14, Volume 2 or the PennDOT Intensity —
Duration — Frequency Field Manual ("PDT-IDF") (May 1986) for
Region 4.
(1)
The following values are taken from the PDT-IDF Field Manual:
|
Return Period
|
24-Hour Rainfall Depth
(inches)
| |
|---|---|---|
|
1-year
|
2.40
| |
|
2-year
|
3.00
| |
|
5-year
|
3.60
| |
|
10-year
|
4.56
| |
|
25-year
|
5.52
| |
|
50-year
|
6.48
| |
|
100-year
|
7.44
|
(2)
A graphical and tabular presentation of the Type II twenty-four-hour
distribution is included in Appendix A.[2]
[2]
Editor's Note: Appendix A is included at the end of this chapter.
K.
Runoff calculations using the Rational Method shall use rainfall
intensities consistent with appropriate times of concentration and
return periods and the intensity-duration-frequency curves as presented
in Appendix A.[3]
[3]
Editor's Note: Appendix A is included at the end of this chapter.
L.
Runoff Curve Numbers (CNs) to be used in the Soil-Cover-Complex Method
shall be based upon the matrix presented in Appendix A.
M.
Runoff coefficients for use in the Rational Method shall be based
upon the table presented in Appendix A.[4]
[4]
Editor's Note: Appendix A is included at the end of this chapter.
N.
All time of concentration calculations shall use a segmental approach
which may include one or all of the flow types below:
(1)
Sheet flow (overland flow) calculations shall use either the NRCS
average velocity chart (Figure 3-1, Technical Release-55, 1975) or
the modified kinematic wave travel time equation (Equation 3-3, NRCS
TR-55, June 1986). If using the modified kinematic wave travel time
equation, the sheet flow length shall be limited to 50 feet for designs
using the Rational Method and limited to 150 feet for designs using
the Soil-Cover-Complex Method.
(2)
Shallow concentrated flow travel times shall be determined from the
watercourse slope, type of surface and the velocity from Figure 3-1
of TR-55, June 1986.
(3)
Open channel flow travel times shall be determined from velocities
calculated by the Manning Equation. Bankfull flows shall be used for
determining velocities. Manning "n" values shall be based on the table
presented in Appendix A.[5]
[5]
Editor's Note: Appendix A is included at the end of this chapter.
(4)
Pipe flow travel times shall be determined from velocities calculated
using the Manning Equation assuming full flow and the Manning "n"
values from Appendix A.
O.
If using the Rational Method, all predevelopment calculations for
a given discharge direction shall be based on a common time of concentration
considering both on-site and any off-site drainage areas. If using
the Rational Method, all post-development calculations for a given
discharge direction shall be based on a common time of concentration
considering both on-site and any off-site drainage areas.
P.
The Manning Equation shall be used to calculate the capacity of watercourses.
Manning "n" values used in the calculations shall be consistent with
the table presented in Appendix A[6] or other appropriate standard engineering 'n' value resources.
Pipe capacities shall be determined by methods acceptable to the Borough.
[6]
Editor's Note: Appendix A is included at the end of this chapter.
Q.
The Pennsylvania DEP Chapter 105 Rules and Regulations apply to the
construction, modification, operation or maintenance of both existing
and proposed dams, water obstructions and encroachments throughout
the watershed. Criteria for design and construction of stormwater
management facilities according to this chapter may not be the same
criteria that are used in the permitting of dams under the Dam Safety
Program.
