1. Guidelines. All calculations shall be consistent with the guidelines set forth in the BMP manual, as amended herein.
2. Methods. 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 | 24-Hour Rainfall Total |
Return | 24-Hour Rainfall Total |
(Year) | (Inches) |
1 | 2.06 |
2 | 2.46 |
10 | 3.46 |
25 | 4.11 |
50 | 4.64 |
100 | 5.21 |
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 Township Engineer; and
(4) 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. Comparison. 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: use Table C-1 (Appendix 23-C).
B. NRCS Rainfall-Runoff Method: use Table C-2 (Appendix 23-C). 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-year, two-year, ten-year, 25-year, 50-year and 100-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 100-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 50-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 pre-approved by the Township Engineer:
(1) Pre-development - NRCS’s Lag Equation:
(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 post-development Tc shall never be greater that the pre-development Tc for any watershed or sub-watershed. This includes when the designer has specifically used swales to reduce flow velocities. In the event that the designer believes that the post-development -lc is greater, it will still be set by default equal to the pre-development Tc for modeling purposes.
(2) The minimum Tc for any watershed shall be five minutes.
(3) The designer may choose to assume a five-minute Tc for any post development watershed or subwatershed without providing any computations.
(4) The designer must provide computations for all pre-development Tc paths. A five-minute Tc can not be assumed for pre-development.
(5) 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. Excluded from analysis. Drainage areas tributary to sinkholes or closed depressions in areas underlain by limestone or carbonate geologic features shall be excluded from the modeled point of analysis defining pre-development flows. If left undisturbed during construction activities, areas draining to closed depressions may also be used to reduce peak runoff rates in the post-development analysis. New, additional contributing runoff should not be directed to existing sinkholes or closed depressions.
10. Manning’s equation. 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. 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.
11. Total volume. The township 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.
12. Reconciliation of rates. The township 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 township.
(Ord. 2011-04-11, passed 4-11-2011)