[Ord. 2011-01, 4/7/2011, § 601]
1. 
General Design Guidelines.
A. 
Stormwater shall not be transferred from one watershed to another, unless (1) the watersheds are sub-watersheds of a common watershed which join together within the perimeter of the property; (2) the effect of the transfer does not alter the peak rate discharge onto adjacent lands; or (3) easements from the affected landowner(s) are provided.
B. 
Consideration shall be given to the relationship of the subject property to the drainage pattern of the watershed. A concentrated discharge of stormwater to an adjacent property shall be within an existing watercourse or confined in an easement or returned to a pre-development flow type condition.
C. 
Stormwater BMPs and recharge facilities are encouraged (e.g., rooftop storage, drywells, cisterns, recreation area ponding, diversion structures, porous pavements, holding tanks, infiltration systems, in-line storage in storm sewers, and grading patterns). They shall be located, designed, and constructed in accordance with the latest technical guidance published by PADEP, provided they are accompanied by detailed engineering plans and performance capabilities and supporting site specific soils, geology, runoff and groundwater and infiltration rate data to verify proposed designs. Additional guidance from other sources may be accepted at the discretion of the Municipal Engineer (a pre-application meeting is suggested).
D. 
All existing and natural watercourses, channels, drainage systems and areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by the appropriate regulatory agency.
E. 
The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The municipality shall reserve the right to disapprove any design that would result in the continuation or exacerbation of a documented adverse hydrologic or hydraulic condition within the watershed, as identified in the plan.
F. 
The design and construction of multiple use stormwater detention facilities are strongly encouraged. In addition to stormwater management, facilities should, where appropriate, allow for recreational uses including ball fields, play areas, picnic grounds, etc. Consultation with the municipality, and prior approval are required before design. Provision for permanent wet ponds with stormwater management capabilities may also be appropriate.
(1) 
Multiple use basins should be constructed so that potentially dangerous conditions are not created.
(2) 
Water quality basins or recharge basins that are designed for a slow release of water or other extended detention ponds are not permitted for recreational uses, unless the ponded areas are clearly separated and secure.
G. 
Should any stormwater management facility require a dam safety permit under PADEP [25 Pa. Code,] Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety.
2. 
Stormwater Management Facility Design Considerations. All stormwater management facilities shall meet the requirements contained in the Crawford County Stormwater Management Facility Design Criteria.
[Ord. 2011-01, 4/7/2011, § 602]
1. 
All calculations shall be consistent with the guidelines set forth in the BMP manual, as amended herein.
2. 
Stormwater runoff from all development sites shall be calculated using either the rational method or the NRCS rainfall-runoff methodology. Methods shall be selected by the design professional based on the individual limitations and suitability of each method for a particular site.
3. 
Rainfall Values.
A. 
Rational Method. The Pennsylvania Department of Transportation Drainage Manual, Intensity-Duration-Frequency Curves, Publication 584, Chapter 7A, latest edition, shall be used in conjunction with the appropriate time of concentration and return period.
B. 
NRCS Rainfall-Runoff Method. The Soil Conservation Service Type II, 24-hour rainfall distribution shall be used in conjunction with rainfall depths from NOAA Atlas 14 or be consistent with the following table:
Return Interval
(year)
24-Hour Rainfall Total
(inches)
1
2.08
2
2.49
10
3.50
25
3.60
50
4.67
100
5.23
4. 
Runoff Volume.
A. 
Rational Method. Not to be used to calculate runoff volume.
B. 
NRCS Rainfall-Runoff Method. This method shall be used to estimate the change in volume due to regulated activities, combining curve numbers for land areas proposed for development with curve numbers for areas unaffected by the proposed development into a single weighted curve number is NOT acceptable.
5. 
Peak Flow Rates.
A. 
Rational Method. This method may be used for design of conveyance facilities only. Extreme caution should be used by the design professional if the watershed has more then one main drainage channel, if the watershed is divided so that hydrologic properties are significantly different in one versus the other, if the time of concentration exceeds 60 minutes, or if stormwater runoff volume is an important factor. The combination of rational method hydrographs based on timing shall be prohibited.
B. 
NRCS Rainfall-Runoff Method. This method is recommended for design of stormwater management facilities and where stormwater runoff volume must be taken into consideration. The following provides guidance on the model applicability:
(1) 
NRCS's TR-55: limited to 100 acres in size.
(2) 
NRCS's TR-20 or HEC-HMS: no size limitations.
(3) 
Other models as pre-approved by the Municipal Engineer.
The NRCS antecedent runoff condition II (ARC II, previously AMC II) must be used for all simulations. The use of continuous simulation models that vary the ARC are not permitted for stormwater management purposes.
C. 
For comparison of peak flow rates, flows shall be rounded to a tenth of a cubic foot per second (cfs).
6. 
Runoff Coefficients.
A. 
Rational Method. Appropriate coefficients to represent existing and proposed conditions subject to approval by the Township Engineer.
B. 
NRCS Rainfall-Runoff Method. Appropriate coefficients to represent existing and proposed conditions subject to approval by the Township Engineer. Curve numbers (CN) should be rounded to tenths for use in hydrologic models as they are a design tool with statistical variability. For large sites, CN's should realistically be rounded to the nearest whole number.
C. 
For the purposes of pre-development peak flow rate and volume determination, existing non-forested pervious areas conditions shall be considered as meadow (good condition).
D. 
For the purposes of pre-development peak flow rate and volume determination, 20% of existing impervious area, when present, shall be considered meadow (good condition).
7. 
Design Storm.
A. 
All stormwater management facilities shall be verified by routing the proposed one-, two-, ten-, twenty-five-, fifty-, and one-hundred-year hydrographs through the facility using the storage indication method or modified puls method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.
B. 
The stormwater management and drainage system shall be designed to safely convey the post development one-hundred-year storm event to stormwater detention facilities, for the purpose of meeting peak rate control.
C. 
All structures (culvert or bridges) proposed to convey runoff under a municipal road shall be designed to pass the fifty-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway.
8. 
Time of Concentration.
A. 
The time of concentration is to represent the average condition that best reflects the hydrologic response of the area. The following time of concentration (Tc) computational methodologies shall be used unless another method is preapproved by the Municipal Engineer:
(1) 
Pre-development — NRCS's Lag Equation:
Time of Concentration = Tc = [(Tlag/.6) * 60] (minutes)
026 Section 242 Fraction 1.tif
Where:
Tlag
=
Lag time (hours)
L
=
Hydraulic length of watershed (feet)
Y
=
Average overland slope of watershed (percent)
S
=
Maximum retention in watershed as defined by:
026 Section 242 Fraction 2.tif
CN
=
NRCS Curve Number for watershed
(2) 
Post-development; commercial, industrial, or other areas with large impervious areas (>20% impervious area) — NRCS Segmental Method. The length of sheet flow shall be limited to 100 feet. Tc for channel and pipe flow shall be computed using Manning's equation.
(3) 
Post-development; residential, cluster, or other low impact designs less than or equal to 20% impervious area — NRCS Lag Equation or NRCS Segmental Method.
B. 
Additionally, the following provisions shall apply to calculations for time of concentration:
(1) 
The designer must provide computations for all pre-development and post-development Tc paths.
(2) 
Undetained fringe areas (areas that are not tributary to a stormwater facility but where a reasonable effort has been made to convey runoff from all new impervious coverage to best management practices) may be assumed to represent the pre-development conditions for purpose of Tc calculation.
9. 
Where uniform flow is anticipated, the Manning's equation shall be used for hydraulic computations and to determine the capacity of open channels, pipes, and storm sewers. The Manning's equation should not be used for analysis of pipes under pressure flow or for analysis of culverts. Manning's "n" values shall be obtained from PENNDOT's Drainage Manual, Publication 584 or other appropriate publications as approved by the Township Engineer. Inlet control shall be checked at all inlet boxes to ensure the headwater depth during the ten-year design event is contained below the top of grate for each inlet box.
10. 
The municipality may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
11. 
The municipality has the authority to require that computed existing runoff rates be reconciled with field observations, conditions and site history. If the designer 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 recommendation of the municipality.