A.
Stormwater drainage systems shall be provided in order
to permit unimpeded flow along natural watercourses, except as modified
by stormwater management facilities or open channels consistent with
this chapter.
B.
The existing points of concentrated drainage that
discharge onto adjacent property shall not be relocated and shall
be subject to any applicable release rate criteria specified in this
chapter.
C.
Diffused drainage.
(1)
Areas of existing diffused drainage shall be subject
to any applicable release rate criteria in the general direction of
existing discharge, whether proposed to be concentrated or maintained
as diffused drainage areas.
(2)
If diffused flow is proposed to be concentrated and
discharged onto adjacent property, the developer must document that
adequate conveyance facilities exist to safely transport the concentrated
discharge, or otherwise prove that no erosion, sedimentation, flooding,
or other harm will result from the concentrated discharge.
D.
Where a development site is traversed by watercourses
other than permanent streams, a drainage easement shall be provided
conforming substantially to the line of such watercourses. The terms
of the easement shall prohibit excavation, the placing of fill or
structures, and any alterations that may affect adversely the flow
of stormwater within any portion of the easement. Also, maintenance
and moving of vegetation within the easement shall be required.
E.
Any stormwater management facilities regulated by
this chapter that would be located on state highway rights-of-way
shall be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
F.
Any stormwater management facilities regulated by
this chapter that would be located in or adjacent to waters of the
commonwealth or potential wetlands shall be subject to approval by
PADEP through the joint permit application process, or, where deemed
appropriate by PADEP, the general permit process. When there is a
question whether wetlands may be involved, it is the responsibility
of the developer to show that the land in question cannot be classified
as wetlands, otherwise approval to work in the area must be obtained
from PADEP.
G.
When it can be shown that, due to topographic conditions,
natural drainageways on the site cannot adequately provide for drainage,
open channels may be constructed conforming substantially to the line
and grade of such natural drainageways. Work within natural drainageways
shall be subject to approval by PADEP, through the general permit
process.
H.
Sinkholes shall be protected as follows:
(1)
Stormwater from roadways, parking lots, storm sewers,
roof drains, or other concentrated runoff paths shall not be discharged
directly into sinkholes.
(2)
If increased or concentrated runoff is to be discharged
into a sinkhole, including filtered discharge, a geologic assessment
of the effects of such runoff on increased land subsidence and groundwater
quality shall be prepared and the results submitted with the drainage
plan. Such discharge shall be prohibited if the Borough of Waynesboro
determines that the discharge poses a hazard to life, property, or
groundwater resources.
[Amended 3-19-2008 by Ord. No. 1071]
Predevelopment peak runoff rates shall be computed for the two-,
ten-, twenty-five-, and one-hundred-year storm events. The allowable
postdevelopment peak runoff rate for each storm event shall be equal
to the arithmetic product of the predevelopment peak runoff rate and
respective release rate percentage shown on Plate A.[1] If the postdevelopment peak runoff rates are less than
equal to the allowable release rates, no controls are required. Where
site conditions are acceptable, infiltration practices shall be employed
to control the postdevelopment peak runoff to the appropriate rate.
Where the exclusive use of infiltration practices are not practical
or feasible, detention facilities shall be provided solely or in combination
with the infiltration facilities to achieve the required peak runoff
rate.
[1]
Editor's Note: Plate A is included as an attachment
to this chapter.
A.
Any stormwater management facilities required or regulated
by this chapter shall be designed to meet the performance standards
enumerated above. Compensatory stormwater management facilities shall
not be permitted.
B.
Any stormwater management facilities required or regulated
by this chapter shall be designed to provide a minimum 1.0 foot of
freeboard above the maximum twenty-five-year water surface elevation
for postdevelopment conditions. Where storage ponding is proposed,
all ponds shall include in their design an emergency spillway capable
of passing the one-hundred-year postdevelopment storm event. Should
any stormwater management facilities qualify as a dam under PADEP
Chapter 105, the facility shall be designed in accordance with Chapter
105 and meet the regulations of Chapter 105 concerning dam safety.
C.
Any hydraulic capacity analysis conducted in accordance
with this chapter shall use the following criteria to determine if
adequate hydraulic capacity exists:
(1)
Open channels must be able to convey postdevelopment
runoff from a ten-year design storm within their banks at velocities
that would not erode the channel bed or banks. Acceptable velocities
shall be based on criteria included in the PADEP Erosion and Sediment
Pollution Control Program Manual (as amended or replaced from time
to time by PADEP).
(2)
Open channels conveying a postdevelopment flow of
more than 19 cfs shall have a minimum 1.0 foot of freeboard and not
create a hazard to any persons or property.
(3)
Any regulated activity under PADEP Chapter 105 (e.g.,
culverts, bridges, outfalls, channel modifications or stream enclosures),
and any work involving wetlands as described in PADEP Chapter 105
regulations (as amended or replaced from time to time by PADEP), shall
be designed in accordance with Chapter 105 and will require a permit
from PADEP. Any other drainage conveyance facility that doesn't fall
under Chapter 105 regulations must be able to convey, without damage
to the drainage structure or roadway, runoff from the twenty-five-year
design storm with a minimum 1.0 foot of freeboard measured below the
lowest point along the top of the roadway. Any facility that requires
a dam safety permit must be designed in accordance with PADEP Chapter
105 Regulations. Any facility located within a PennDOT right-of-way
must meet PennDOT minimum design standards and permit submission requirements.
(4)
Storm sewers must be able to convey post development
runoff from a twenty-five-year design storm without surcharging inlets.
(5)
Storm sewer inlet spacing and road cross-section design
must ensure that postdevelopment runoff resulting from a ten-year
design storm does not flood more than one-half of a driving lane.
Capacity reduction factors, suggested for use when a possibility of
clogging of grates, side openings or combination inlets exists, are
shown in Table IV. The reduction factors are to be applied to the
theoretical capacity of the inlet. For inlets on continuous grades,
the minimum amount of water that should be bypassed on to the next
downstream inlet is 10%. Allowable flow of water across street intersections
by street type are shown on Table III.
|
Table III
| |
|---|---|
|
Allowable Flow of Water Across Intersection
By Street Type
| |
|
Type of Street at Intersection
|
Allowable Flow
|
|
Collector
|
No flow allowed
|
|
Subcollector
|
Flow less than 3 cfs allowed; greater flow must
be collected
|
 |
|
Table IV
| ||
|---|---|---|
|
Inlet Capacity Reduction Factors Assuming
Partial Clogging
| ||
|
Condition
|
Inlet Type
|
Reduction Factor*
|
|
Sump
|
Side opening
|
0.80
|
|
Sump
|
Grate
|
0.50
|
|
Sump
|
Combination
|
0.65
|
|
Continuous grade
|
Side opening
|
0.80
|
|
Continuous grade
|
Side opening with deflector
|
0.75
|
|
Continuous grade
|
Longitudinal bars
|
0.60
|
|
Continuous grade
|
Transverse bars
|
0.50
|
|
Continuous grade
|
Combination
|
0.60
|
|
* Percentage of theoretical capacity
|
(6)
The maximum allowable spacing between structures to
be used for inspecting and cleaning storm sewers (manholes, on-line
inlets, risers and terminal cleanouts) shall be based on the pipe
size and spacing shown in Table VI.
|
Table VI
| |
|---|---|
|
Spacing of Inspection and Cleanout Structures
for Storm Sewers
| |
|
Size of Pipe
(inches)
|
Maximum Allowable Spacing
(feet)
|
|
15
|
400
|
|
18 to 36
|
500
|
|
42 to 60
|
700
|
|
66 or larger
|
Unlimited
|
D.
Easements along open channels shall be provided. The
minimum width of the required easement shall be equal to the width
of the one-hundred-year water surface (for postdevelopment conditions),
including a minimum 1.0 foot of freeboard.
E.
In subareas where individual stormwater management
facilities would be provided for each development site, the individual
stormwater management facilities shall be designed to ensure that
the postdevelopment peak discharge at the mouth of the subarea does
not exceed the arithmetic product of the applicable release rate,
specified on Plate "A" of this chapter,[1] and the predevelopment peak discharge at the mouth of
the subarea.
[1]
Editor's Note: Plate A is included as an attachment
to this chapter.
F.
For development sites that would be located in two
or more subareas, the applicable release rate for the portions of
the site located in different subareas shall be based on natural subarea
drainage boundaries. The natural drainage boundaries between subareas
shall not be modified, nor shall drainage from a development site
be diverted or otherwise conveyed from one subarea to another subarea,
except where runoff naturally crosses subarea drainage boundaries.
G.
Regional or subregional stormwater management facilities.
For certain areas within the watershed, it may be more cost effective
to provide one stormwater management facility for an entire subarea,
group of subareas, or portions of a subarea incorporating more than
one development site than to provide an individual stormwater management
facility for each development site. The initiative and funding for
any regional or subregional stormwater management alternatives are
the responsibility of prospective developer(s). The design of any
regional stormwater management facilities must assume development
of the entire area that would drain to the regional facility. The
type and amount of development that the developer(s) must consider
shall be either based on current zoning or on projections established
by the Franklin County Planning Commission, whichever results in a
greater amount of imperviousness. The peak outflow from a regional
stormwater management facility would be determined on a case-by-case
basis using the Penn State Runoff Method (Ps Rm), as developed for
the Antietam Creek Watershed Act 167 Stormwater Management Plan. When
regional or subregional stormwater management facilities are utilized,
the effect of phased growth on stormwater runoff flows must be corrected.
At no time from the initial phase through ultimate development shall
the peak runoff flows exceed the predevelopment peak multiplied by
the applicable release rate.
H.
Capacity improvements. If the developer could prove
that it would be feasible to provide capacity improvements to relieve
the capacity deficiency in the existing drainage network, then adequate
capacity improvements could be provided by the developer in lieu of
stormwater management facilities on the development site. Any capacity
improvements would be designed based on development of all areas tributary
to the improvements and the capacity criteria specified in this chapter.
The type and amount of development that the developer must consider
shall be either based on current zoning or on projections established
by the Franklin County Planning Commission, whichever results in a
greater amount of imperviousness. It shall be assumed that all new
development upstream of a proposed capacity improvement would implement
applicable stormwater management techniques, consistent with this
chapter.
I.
Adequate erosion protection shall be provided along
all open channels, and at all points of discharge.
J.
Ponds and other similar water features that are not
designed as stormwater management facilities shall be designed in
accordance with United States Department of Agriculture, National
Resource Conservation Service (NRCS), Ponds — Planning, Design,
Construction (as amended or replaced from time to time by NRCS), and
shall be treated as impervious surfaces for stormwater runoff computations.
K.
The design of all stormwater management facilities
shall incorporate sound engineering principles and practices. The
Borough of Waynesboro shall reserve the right to disapprove any design
that would result in the occurrence or perpetuation of an adverse
hydrologic or hydraulic condition within the watershed.
A.
Any stormwater runoff calculations involving drainage
areas greater than 20 acres, including on- and-off site areas, shall
use any generally accepted calculation technique that is based on
the NRCS Soil Cover Complex Method. Table I below summarizes acceptable
computation methods. All methods shall be selected by the design professional
based on the individual limitations and suitability of each method
for a particular site. The Director of Engineering for the Borough
of Waynesboro may approve the use of the Rational Method to estimate
peak discharges from drainage areas that contain less than 20 acres.
|
Table I
| ||
|---|---|---|
|
Acceptable Computation Methodologies for
Stormwater Management Plan
| ||
|
Method
|
Developer
|
Applicability
|
|
TR-20 (or commercial 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
|
United States Army Corps of Engineers
|
Applicable where use of full hydrologic computer
model is desirable or necessary
|
|
PSRM
|
Penn State Unv.
|
Applicable where use of a hydrologic computer
model is desirable or necessary; simpler than TR-20 or HEC-1
|
|
Rational Method (or commercial computer package
based on the Rational Method)
|
Emil Kuichling (1889)
|
For sites less than 20 acres, or as approved
by the Director of Engineering
|
|
Other methods
|
Varies
|
Other computation methodologies approved by
the Director of Engineering
|
B.
The design of any stormwater detention facilities
intended to meet the performance standards of this chapter shall be
verified by routing the design storm hydrograph using the Storage-Indication
Method. For drainage areas greater than 20 acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. The Director of Engineering may approve
the use of any generally accepted full hydrograph approximation technique
which uses a total volume that is consistent with the volume from
a method that produces a full hydrograph.
C.
All calculations consistent with this chapter using
the soil cover complex methods shall use the appropriate design rainfall
depths in inches for the various return period storms presented in
Table II of this chapter. If a hydrologic computer model such as PSRM
or HEC-1 is used for stormwater runoff calculations, then the duration
of rainfall shall be 24 hours.
|
Table II
| ||||||
|---|---|---|---|---|---|---|
|
Antietam Creek Watershed Rainfall Intensity/Duration
— Frequency
| ||||||
|
Duration
(hours)
| ||||||
|
Storm Event
(years)
|
1
|
2
|
3
|
6
|
12
|
24
|
|
2
|
1.18
|
1.35
|
1.55
|
1.75
|
2.00
|
2.38
|
|
10
|
2.11
|
2.55
|
2.80
|
3.25
|
3.75
|
4.45
|
|
25
|
2.49
|
2.95
|
3.25
|
3.85
|
4.60
|
5.20
|
|
50
|
2.79
|
3.50
|
3.75
|
4.40
|
5.15
|
5.78
|
|
100
|
3.08
|
3.70
|
4.10
|
4.75
|
5.55
|
6.32
|
D.
All calculations using the Rational Method shall use
rainfall intensities consistent with appropriate times of concentration
for overland flow and return periods from the design storm curves
in Figure 1 of this chapter.[1] Times of concentration for overland flow shall be calculated
using the methodology presented in Chapter 3 of Urban Hydrology for
Small Watershed, NRCS, TR-55 (as amended or replaced from PennDOT
Design Manual No. 2, Region 4 for watershed tributary to the East
Branch and Region 3 for watershed tributary to the West Branch). Times
of concentration for channel and pipe flow shall be computed using
Manning's Equation.
[1]
Editor's Note: Figure 1 is included as an attachment to this
chapter.
E.
Runoff curve numbers (CN) for both existing and proposed
conditions to be used in the soil cover method shall be obtained from
Table 2-2 of the USDA NRCS TR-55, except that all agricultural predevelopment
conditions shall be considered good with the best conservation practices
assumed to be employed.
F.
Runoff coefficients (c) for both existing and proposed
conditions for use in the Rational Method shall only be obtained from
Table VII, Rational Runoff Coefficients,[2] and shall be subject to the approval of the head of Borough
Engineering Services or his representative.
[Amended 3-19-2008 by Ord. No. 1071]
[2]
Editor's Note: Said Table VII is included as an attachment
to this chapter.
G.
Where uniform flow is anticipated, the Manning Equation
shall be used for hydraulic computations, and to determine the capacity
of open channels, pipes, and storm sewers. Values of roughness coefficient
"n" for use in the Manning Formula are shown in Table V. Where nonuniform
flow is anticipated, the hydraulic effects of backwater caused by
hydraulic obstructions (e.g., culverts, bridges, dams, reservoirs,
etc.) shall be evaluated using the standard step method for determining
water surface profiles.
|
Table V
| |
|---|---|
|
"n" Values For Manning Formula
| |
|
Type of Pipe
|
"n" Value*
|
|
Asbestos cement
|
0.013
|
|
Concrete culvert pipe
|
0.013
|
|
Concrete sewer pipe
|
0.013
|
|
Cast iron
|
0.013
|
|
Corrugated metal (plain)
|
0.024
|
|
Corrugated metal (coated)
|
0.021
|
|
Plastic
|
0.011
|
|
Vitrified clay
|
0.013
|
|
*
|
Adjustments for specific conditions shall be
based on engineering experience and judgment and submitted to the
Borough Engineer for approval.
|
H.
Outlet structures for stormwater management facilities
shall be designed to meet the performance standards of this chapter
using any generally accepted hydraulic analysis technique or method
approved by the Director of Engineering.
