[Ord. 3-2016, 10/13/2016]
1. 
Method of Computation. Peak discharge and runoff gauge by computer using the following:
A. 
Applicants may select a combination of runoff control techniques which are most suitable to control stormwater runoff from the site. All controls shall be subject to approval of the Municipal Engineer. The Municipal Engineer may request specific information on design and/or operating features of the proposed stormwater controls in order to determine their suitability and adequacy in terms of the standards of this section.
B. 
The applicant shall consider the effect of the proposed stormwater management techniques on any special soil conditions or geological hazards which may exist on the development site. In the event such conditions are identified on the site, the Municipal Engineer may require in-depth studies by a competent geotechnical engineer.
C. 
Any BMP which is a dam, culvert, stream, enclosure or outfall as defined in 25 Pa. Code Chapter 105 shall be designed according to the requirements in those regulations.
D. 
Any stormwater BMP which does not constitute a dam under 25 Pa. Code Chapter 105 and is designed to store runoff and requires a berm or earthen embankment (i.e., detention basin) shall be designed to satisfy the following:
(1) 
Berms and earthen embankments shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year post-development conditions.
(2) 
Berms and earthen embankments shall be designed to provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year post-development inflow.
(3) 
Water obstructions shall convey runoff from the twenty-five-year design storm with a minimum of 1.0 foot of freeboard measured below the lowest point along the top of the roadway, without damage to the drainage structure or the roadway.
(4) 
Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm.
(5) 
Drainage conveyances must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm without surcharging.
(6) 
Adequate erosion and sediment control protection shall be provided along all pen channels and at all points of discharge.
(7) 
All BMP facilities will be sized for the two-, ten- and twenty-five-year storm and release ratings.
E. 
Stormwater runoff from all development and post-development sites shall be calculated using either the Rational Method or a Soil-Cover-Complex methodology.
F. 
Any stormwater runoff calculations involving areas greater than 200 acres, including on- and off-site areas, shall use a generally accepted calculation technique that is based on the NRSC Soil-Cover-Complex Method. Table I summarizes acceptable computation methods. All methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site. These assumptions shall be used in runoff calculations:
(1) 
Average antecedent moisture conditions.
(2) 
Type II distribution storm.
(3) 
Meadow in good condition shall be used in predevelopment runoff calculations for all areas of existing cultivation.
(4) 
All areas other than cultivation shall use the land cover conditions which existed during the past 10 consecutive years.
(5) 
All areas to be disturbed during construction and subsequently returned to open space will be assumed to be reduced one hydrologic group category level for post-development runoff.
(6) 
If the initial condition of the site is undeveloped land, the land use shall be considered as "meadow" unless the natural land cover is proven to generate lower curve numbers or Rational "C" value, such as forested lands.
G. 
All calculations consistent with this chapter using the Soil-Cover-Complex Method shall use the appropriate design rainfall depths for the various return-period storms. If a hydrologic computer model such as TR-20, PSRM or HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours.
H. 
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 from Pennsylvania Department of Transportation Design Rainfall Curves (1986). Times of concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, and TR-55 (as amended or replaced from time to time by NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's Equation.
I. 
Runoff Curve Numbers (CN) for both existing and proposed conditions are to be used in the Soil-Cover-Complex Method.
J. 
Runoff coefficient (c) for both existing and proposed conditions is to be used in the Rational Method.
K. 
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.
L. 
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.
M. 
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 through these facilities 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 municipality may approve the use of any generally accepted full hydrograph approximation technique, which shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
N. 
The municipality may require that computed existing runoff rates be reconciled with field observations and conditions. If the design professional can substantiate through actual physical calibration that more-appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendations of the Municipal Engineer. Calibration shall require detailed gauge and rainfall data for the particular site in question.
Table I
Acceptable Computation Methodologies for Stormwater Management Plans
Method
Method Developed By
Applicability
TR-20 or commercial package based on TR-20
USDA-NRCS
When use of full model is desirable or necessary
TR-55 or commercial package based on TR-55
USDA-NRCS
Applicable for plans within the model's limitations
HEC-1
U.S. Army Corps of Engineers
When full model is desirable or necessary
PSRM
Penn State University
When full model is desirable or necessary
Rational Method or commercial package based on Rational Method
Emil Kuiching (1889)
For sites less than 200 acres
Other methods
Various
As approved by the Municipal Engineer
O. 
Rainfall Frequency Data. (Available from the United States Department of Commerce, National Weather Service and Pennsylvania Department of Environmental Protection, Research Publication Number 70).
P. 
Maintenance of Natural Drainageways. All natural streams, channels, swales, drainage systems and/or areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by Canton Township. All encroachment activities shall comply with the requirements of Chapter 105 (Water Obstructions and Encroachments) of Title 25, Rules and Regulations of the Pennsylvania Department of Environmental Protection.
Q. 
Methods of Stormwater Runoff Detention and Control.
(1) 
The following is a listing of detention and control methods which may be utilized in stormwater management systems, if appropriate. The choice of control techniques is not limited to the ones appearing on this list.
(a) 
Detention basins.
(b) 
Rooftop storage.
(c) 
Parking lot and street ponding.
(d) 
Seepage pits, seepage trenches or other infiltration structures.
(e) 
Porous pavement and concrete lattice block surfaces.
(f) 
Grassed channels and vegetated strips.
(g) 
Cisterns and underground reservoirs.
(h) 
Routed flow over grass.
(i) 
Decreased impervious area coverage.
(2) 
The use of other control methods which meet the criteria in this section will be permitted when approved by the Canton Township Engineer. Various combinations of methods should be tailored to suit the particular requirements of the type of development and the topographic features of the project area.
R. 
Design. The applicant is urged to consult the publications listed in the appendix to this chapter for aid in design of control methods. The appendix is not a part of this chapter.[1]
[1]
Editor's Note: The appendix is on file in the Township offices.
[Ord. 3-2016, 10/13/2016]
1. 
In order to implement the provisions of the Washington County Stormwater Management Plan, Canton Township is hereby divided into stormwater management districts which shall be designated:
District Name
Washington County FEMA Map Panel
Brush Run, Brush Run Tributary 2
304E
Wolfdale Run
304E, 308E, 309E
Georges Run Tributary 1
306E, 308E
Georges Run Tributary 1.1
306E
Georges Run
306E, 308E, 309E
Charters Creek
309E, 317E
Log Pile Run
306E, 317E
Catfish Creek
317E
2. 
The location and boundaries of the stormwater management districts are shown on an official map which is available for inspection from the Canton Township Secretary.
3. 
When a project or land disturbance activity is located in more than one stormwater management district, stormwater may not be transferred from a district with stricter stormwater management criteria to a district with less-strict criteria, unless the need for such a transfer is identified in the Washington County Stormwater Management Plan, the Regional Water Quality Management Plan or the State Water Plan.
[Ord. 3-2016, 10/13/2016]
1. 
Stormwater Rate and Quality.
A. 
Peters Creek No. 1 Gastonville Watershed. There shall be no increase in the rate of stormwater discharge from any activity covered by this chapter than would have occurred from the land prior to the activity, using (predevelopment) as the prior condition. The peak release rate of stormwater from detention facilities shall be 95% of the predevelopment rate. The water quality discharge standards shall be the removal of 80% of the total suspended solids load (TSS) for all storage structures that have a volume greater than 1,000 cubic feet (for a twenty-five-year storm).
B. 
Peters Creek No. 2 Trax Watershed. There shall be no increase in the rate of stormwater discharge from any activity covered by this chapter that would have occurred from the land prior to the activity, using predevelopment as the prior condition. The peak post-development release rate of the stormwater from detention facilities shall be 90% of the predevelopment rate. The water quality discharge standards shall be the removal of 80% of the total suspended solids load (TSS) for all storage structures that have a volume greater than 1,000 cubic feet (for a twenty-five-year storm).
C. 
Piney Fork Watershed. There shall be no increase in the rate of stormwater discharge from any activity covered by this chapter than would have occurred from the land prior to the activity, using predevelopment as the prior condition. The peak post-development release rate of the stormwater from detention facilities shall be 100% of the predevelopment rate. The quality discharge standards shall be the removal of 80% of the total suspended solids load (TSS) for all storage structures that have a volume greater than 1,000 cubic feet (for a twenty-five-year storm).
D. 
Unnamed Watershed. There shall be no increase in the rate of stormwater discharge from any activity covered by this chapter than would have occurred from the land prior to the activity, using predevelopment as the prior condition. The peak post-development release rate of the stormwater from detention facilities shall be 100% of the predevelopment rate. The water quality discharge standards shall be the removal of 80% of the total suspended solids load (TSS) for all storage structures that have a volume greater than 1,000 cubic feet (for a twenty-five-year storm).
E. 
Peters Creek No. 3 Hacket Watershed. There shall be no increase in the rate of stormwater discharge from any activity covered by this chapter than would have occurred from the land prior to the activity, using predevelopment as the prior condition. The peak post-development release rate of the stormwater from detention facilities shall be 95% of the predevelopment rate. The water quality discharge standards shall be the removal of 80% of the total suspended solids load (TSS) for all storage structures that have a volume greater than 1,000 cubic feet (for a twenty-five-year storm).
F. 
Peters Creek No. 4 Finleyville Watershed. There shall be no increase in the rate of stormwater discharge from any activity covered by this chapter than would have occurred from the land prior to the activity, using predevelopment as the prior condition. The peak post-development release rate of the stormwater from detention facilities shall be 95% of the predevelopment rate. The water quality discharge standards shall be the removal of 80% of the total suspended solids load (TSS) for all storage structures that have a volume greater than 1,000 cubic feet (for a twenty-five-year storm).
G. 
Mingo Creek No. 2 Watershed. There shall be no increase in the rate of stormwater discharge from any activity covered by this chapter than would have occurred from the land prior to the activity, using predevelopment as the prior condition. The peak post-development release rate of the stormwater from detention facilities shall be 100% of the predevelopment rate. The water quality discharge standards shall be the removal of 80% of the total suspended solids load (TSS) for all storage structures that have a volume greater than 1,000 cubic feet (for a twenty-five-year storm).
H. 
Mingo Creek No. 1 Watershed. There shall be no increase in the rate of stormwater discharge from any activity covered by this chapter than would have occurred from the land prior to the activity, using predevelopment as the prior condition. The peak post-development release rate of the stormwater from detention facilities shall be 100% of the predevelopment rate. The water quality discharge standards shall be the removal of 80% of the total suspended solids load (TSS) for all storage structures that have a volume greater than 1,000 cubic feet (for a twenty-five-year storm).
2. 
Erosion and Sedimentation. All land disturbance activities shall be conducted in such a way as to minimize accelerated erosion and resulting sedimentation. Measures to control erosion and sedimentation shall, at a minimum, meet the standards of the Conservation District and Chapter 102 (Erosion Control) of Title 25, Rules and Regulations of the Pennsylvania Department of Environmental Protection.