A.
Storm drainage systems shall be provided in order
to permit unimpeded flow of natural watercourses except as modified
by stormwater detention facilities or open channels consistent with
this chapter.
B.
The existing points of concentrated drainage discharge
onto adjacent property shall not be altered without written approval
of the affected property owner(s).
C.
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 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.
D.
Where the development site is traversed by or contains a watercourse, there shall be provided a drainage easement conforming substantially with the line of such watercourse. The width of the easement shall be adequate to provide for and preserve the unimpeded flow of storm runoff based on calculations made in conformance with § 138-10 for the one-hundred-year return period runoff (assuming the upstream drainage basin to be fully developed) and to provide a freeboard allowance of 1/2 foot above the design water surface level. A minimum width of 10 feet from the edge of each side of the watercourse shall be provided, but the Township may require a greater easement when necessary. 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 municipal floodplain regulations.
E.
Any drainage facilities required by this chapter that
are located within state highway rights-of-way shall be subject to
approval by the Pennsylvania Department of Transportation. A letter
from the Department indicating such approval shall be submitted to
the Township.
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.
Storm drainage facilities and appurtenances shall
be so designed and provided as to minimize erosion in swales, watercourse
channels and at all points of discharge.
H.
Consideration shall be given to the design and use
of volume controls for stormwater management, where geology and soils
permit. Areas of suitable geology for volume controls shall be determined
by the Township. Documentation of the suitability of the soil for
volume controls shall be provided by the applicant. Volume controls
shall be acceptable in areas of suitable geology where the soils are
designated as well drained in the County Soil Survey. Other soils
may be acceptable for use of volume controls based on site-specific
soils evaluations provided by the applicant.
[1]
Editor's Note: Ordinance No. 2015-14, adopted 3-8-2016, provided
that the requirements contained in Subsections A through H do not
apply to the PRRC Zoning District.
A.
Mapping of stormwater management districts. In order
to implement the provisions of the Hokendauqua Creek Watershed and
Trout/Bertsch Creeks Watershed and Lehigh River Sub-basin 1 Stormwater
Management Plan, Lehigh Township is hereby divided into stormwater
management districts consistent with the release rate map presented
in those plans. The boundaries of the stormwater management districts
within Lehigh Township are shown on official maps which are available
for inspection at the municipal office. Copies of these official maps
at a reduced scale are included in the appendixes for general reference.[2] See Appendix A for the Hokendauqua Creek Watershed (within
Lehigh Township only) and Appendix B for the Trout/Bertsch Creek Watershed
(within Lehigh Township only).
[2]
Editor's Note: Appendixes A, B and C are included
at the end of this chapter.
B.
Description of stormwater management districts. Three
types of stormwater management districts shall be applicable to Lehigh
Township; namely Conditional No Detention I Districts, Conditional
No Detention II Districts, and Dual Release Rate Districts, as described
herein.
(1)
Conditional No Detention I 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 postdevelopment 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 100% release rate control or provide increased capacity of downstream elements to convey increased peak flows consistent with § 138-9M. 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 § 138-9C. By definition, a storm drainage problem area associated with the "local" runoff conveyance facilities indicates that adequate capacity does not exist.
(2)
Conditional No Detention II Districts. Within these districts, the capacity of the "local" runoff conveyance facilities must be calculated in the same manner as the Conditional No Detention I Districts. In this case, however, adequate capacity must be demonstrated from the site to the Lehigh River. After determining if adequate capacity exists, the developer shall use either no detention, a 100% release rate or provide capacity improvements as detailed in Subsection B(1), Conditional No Detention I Districts.
(3)
Dual release rate districts. Within this district,
the two-year postdevelopment runoff must be controlled to 30% of the
predevelopment two-year runoff peak. Further the ten-year, twenty-five-year,
and one-hundred-year postdevelopment runoff must be controlled to
the stated percentage of the predevelopment peak.
[1]
Editor's Note: Ordinance No. 2015-14, adopted 3-8-2016, provided
that the requirements contained in Subsections A and B do not apply
to the PRRC Zoning District.
A.
Any stormwater management controls required by this chapter and subject to a dual release rate criteria shall meet the applicable release rate criteria for each of the two-, ten-, twenty-five- and one-hundred-year return period runoff events consistent with the calculation methodology specified in § 138-10.
B.
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
provided as part of the drainage plan. The district boundaries as
originally drawn coincide with topographic divides or, in certain
instances, are drawn from the intersection of the watercourse and
a physical feature such as the confluence with another watercourse
or a potential flow obstruction (e.g., road, culvert, bridge, etc.).
The physical feature is the downstream limit of the subarea and the
subarea boundary is drawn from that point upslope to each topographic
divide along the path perpendicular to the contour lines.
C.
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. Acceptable velocities shall be based
upon criteria included in the DEP Soil Erosion and Sediment Pollution
Control Program Manual (March 2000). Permissible velocities from the
DEP Manual for selected channels are presented in Appendix C of this
chapter.[2]
[2]
Editor's Note: Appendix C is included at the end of this chapter.
(2)
Natural or man-made channels or swales must be able
to convey the increased twenty-five-year return period runoff peak
within their banks or otherwise not create 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 have
sufficient capacity to pass or convey the increased flows associated
with the twenty-five-year return period runoff event, except for facilities
located within a designated floodplain area which must be capable
of passing or conveying the one-hundred-year return period runoff.
Any facilities which constitute stream enclosures per DEP's Chapter
105 regulations shall be designed to convey the one-hundred-year return
period runoff.
D.
For a proposed development site located within only
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 100% release rate so long as
the total runoff from the site is controlled to the applicable release
rate.
E.
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 100% release rate provided
that the overall site discharge meets the weighted average release
rate.
F.
For proposed development sites located partially within
a release rate category area and partially within a provisional or
conditional no detention area, in no event shall a significant portion
of the site area subject to the release rate control be drained to
the discharge point(s) located in the no detention area.
G.
Within a release rate category area, for a proposed
development site which has significant areas which drain to a closed
depression(s), the design release from the site will be the lesser
of:
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, and
provide easements for this flow.
I.
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.
J.
Development proposals which, through groundwater recharge
or other means, do not increase the rate or volume of runoff discharged
from the site are not subject to the release rate provisions of this
chapter.
K.
"No harm" option.
(1)
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 "no harm" would be caused by discharging at a higher runoff rate than that specified by the plan. 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 studied creek with the Lehigh River. Proof of "no harm" must be shown using the capacity criteria specified in Subsection C if downstream capacity analysis is a part of the "no harm" justification.
(2)
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 Penn State
Runoff Models for the applicable watershed for the Conditional No
Detention I or Dual Release Rate. For the Conditional No Detention
II areas, the PSU-IV peak flow values will be used. These flow values
will be supplied to the developer by the Municipal Engineer 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 M.
L.
Regional detention alternatives. For certain areas
within the study areas, 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 or subregional 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 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 version of the
Penn State Runoff Model as developed for the stormwater management
plan.
M.
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 are to 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 C. 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.
N.
Waiver of runoff control based on minimum additional
impervious cover.
(1)
Any proposed regulated activity, except those defined in §§ 138-4C(5) and (6), which would create 10,000 square feet or less of additional impervious cover shall be exempt from meeting the runoff control provisions of this chapter. For developments which are to take place in stages, the entire development plan must be used in determining conformance to this criteria. Predevelopment impervious cover is that which is in place as of the effective date of this chapter. Additional impervious cover shall include, but not be limited to, any roof, parking or driveway areas and any new streets and sidewalks constructed as part of or for the proposed development. Any postdevelopment areas which may be designed to initially be semipervious (e.g., gravel, crushed stone, porous pavement, etc.) shall be considered impervious areas for the purpose of waiver evaluation. Any semipervious predevelopment areas shall be considered pervious areas for purposes of waiver evaluation unless demonstrated otherwise by the applicant.
O.
Allocation of 10,000 square feet or less of additional impervious
coverage for a subdivision plan: Any subdivision plan shall allocate
the 10,000 square feet of impervious coverage exempt from runoff control
provisions on a pro rata basis for each proposed subdivision lot.
When the total impervious coverage exceeds the amount allotted to
that lot, the stormwater runoff control provisions shall apply to
that lot.
[Added 1-7-2013 by Ord. No. 2013-1[3]]
[3]
Editor's Note: This ordinance also redesignated former Subsection
O as Subsection P.
P.
Compatibility with NPDES requirements. Any proposed
regulated activity for which a permanent stormwater quality control
detention basin is required under the NPDES regulations shall use
the more stringent runoff control criteria between this chapter and
the NPDES requirements.
[1]
Editor's Note: Ordinance No. 2015-14, adopted 3-8-2016, provided
that the requirements contained in Subsections A through N do not
apply to the PRRC Zoning District.
A.
Stormwater runoff from all development sites shall
be calculated using either the rational method or a soil-cover-complex
methodology.
B.
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. For basins designed
using the rational method technique, the detention volume shall, at
minimum, equal the volume derived from the approximate routing process
as contained in SCS Technical Release Number 55 (TR55, most recent
edition).
C.
All stormwater detention facilities shall provide
a minimum 1.0 foot freeboard, measured to the invert of the emergency
spillway, above the maximum pool elevation associated with the two-
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 two- 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 entire one-hundred-year
return frequency storm peak flow rate with a minimum 0.5 foot freeboard
measured to the top of basin. The spillway should function to control
overflows in the event of a complete blockage of the basin outlet
system. 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.
D.
The minimum circular orifice diameter for controlling
discharge rates from detention facilities shall be three inches provided
that as much of the site runoff as practical is directed to the detention
facility.
E.
All calculations using the soil-cover-complex method
shall use the National 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 are
taken from the PennDOT Intensity-Duration-Frequency Field Manual (May
1986) for Region 4:
|
Return Period
(years)
|
Twenty-Four-Hour
Rainfall Depth
(inches)
| |
|---|---|---|
|
1
|
2.40
| |
|
2
|
3.00
| |
|
5
|
3.60
| |
|
10
|
4.56
| |
|
25
|
5.52
| |
|
50
|
6.48
| |
|
100
|
7.44
| |
|
NOTE: A graphical and tabular presentation of
the Type II-24 hour distribution is included in Appendix C.[1]
| ||
[1]
Editor's Note: Appendix C is included at the end of this chapter.
F.
All 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 C.
G.
Runoff curve numbers (CN's) to be used in the soil-cover-complex
method shall be based upon the matrix presented in Appendix C.
H.
Runoff coefficients for use in the Rational Method
shall be based upon the table presented in Appendix "C."
I.
Proposed volume controls shall be designed with sufficient
storage volume for a one-hundred-year return period event. The storage
volume shall equal or exceed the volume of the twenty-four-hour storm
for the drainage area to the volume control.
J.
All predevelopment calculations for a given discharge
direction shall be used on a common time of concentration considering
both on-site and any off-site drainage areas. All postdevelopment
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.
K.
The Manning Equation shall be used to calculate the
capacity of regular engineered watercourses. Standard engineering
practice shall be used to calculate the capacity or irregular channels
or watercourses. Manning "n" values used in the calculations shall
be consistent with the table presented in Appendix C. Pipe capacities
shall be determined by methods acceptable to the Municipal Engineer.
L.
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. The designer shall meet the requirements of
this chapter and Chapter 105.
