1. All calculations shall be consistent with the guidelines set forth in the BMP Manual.
2. Stormwater runoff from all development sites shall be calculated using either the Rational Method or a Soil Cover Complex methodology. Methods shall be selected by the qualified 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. Soil Cover Complex Method. The Soil Conservation Service Type II, 24- hour rainfall distribution shall be used in conjunction with rainfall depths from NOAA Atlas 14 or consistent with the following table.
Return Interval | 24-hour Rainfall Total |
(Year) | (Inches) |
1 | 2.40 |
2 | 2.90 |
10 | 4.36 |
25 | 5.43 |
50 | 6.38 |
100 | 7.48 |
4. Peak Flow Rates:
A. Rational Method. May be used for drainage areas up to 20 acres. Extreme caution should be used by the qualified 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 verses 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.
(1) The use of the Modified Rational Method to design stormwater management facilities must be approved by the Municipal Engineer.
B. Soil Cover Complex Method. May be used for drainage areas greater then 20 acres. This method is recommended for design of stormwater management facilities and where stormwater runoff volume must be taken into consideration.
C. For comparison of peak flow rates, flows shall be rounded to no more than the nearest tenth of a cubic foot per second (cfs).
5. Runoff Coefficients:
A. Rational Method. Use Table 23-C-1 (Appendix 23-C).
B. Soil Cover Complex Method. Use Table 23-C-2 (Appendix 23-C).
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 percent of existing impervious area, when present on the project site, and contained within the new proposed limit of disturbance, shall be considered meadow (good condition) for pre-development hydrologic calculations for re-development.
6. Design Storm.
A. All drainage facilities (inlets, pipes, and swales) shall be designed to safely convey the 10-year storm.
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 stormwater management facilities shall be verified by routing the proposed 1-year, 2-year, 10-year, 25-year, 50-year, and 100-year hydrographs through the facility using the storage indication (Modified Puls) method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.
D. 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 1 foot of freeboard measured below the lowest point along the top of the roadway.
E. All design within State or Federal right-of-ways or that falls under the design criteria of any higher authority must meet the requirements of that agency in addition to meeting the minimum requirements of this Chapter.
7. Time of Concentration.
A. Time of concentration shall be computed using the NRCS Segmental Method as described in TR-55 (SCS 1986 or most current update). The length of sheet flow shall be limited to 100-feet. The Manning's “n” Roughness Coefficient for TR-55 sheet flow can be found in Table 23-C-4 (Appendix 23-C). Time of concentration for channel and pipe flow shall be computed using Manning's equation.
B For sites with insignificant channelized flow and less than 20 percent imperviousness coverage, the time of concentration may be computed using the NRCS equation for lag time:
Time of Concentration = Tc = [(Tlag/.6) * 60] (minutes)
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Where:
Tlag = Lag time (hours)
L = Hydraulic length of watershed (feet)
[IMAGE]Y = Average overland slope of watershed (percent)
S = Maximum retention in watershed as defined by:
CN = NRCS Curve Number for watershed as defined by the NRCS Loss Method
C. Additionally, the following provisions shall apply to calculations for time of concentration:
(1) The post-development time of concentration shall never be greater than the pre-development time of concentration for any watershed or subwatershed.
(2) The minimum time of concentration for any watershed shall be 5 minutes.
(3) The designer may choose to assume a 5-minute time of concentration for any post-development watershed or subwatershed without providing any computations.
(4) The designer must provide computations for all pre-development time of concentration paths. A 5-minute time of concentration cannot 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 time of concentration calculations.
8. 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 removed from peak runoff rates in the post-development analysis. New, additional contributing runoff shall not be directed to existing sinkholes or closed depressions.
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 Table 23-C-3 (Appendix 23-C). Inlet control shall be checked at all inlet boxes to ensure the headwater depth during the 10-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 approval of the Municipality.
(Ord. 11-01, 2/8/2011, §702)