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.