§ 52.039 CALCULATION METHODOLOGY.

   Stormwater runoff from all development sites shall be calculated using either the rational method (stormwater conveyance systems only) or a soil cover complex methodology as follows.

   (A)   Major and minor conveyance systems up to 20 acres may be designed using the rational method. The rational method may also be used in sizing the minor conveyance systems for larger sites. Any stormwater runoff calculations involving drainage areas greater than 20 acres, including on-site and off-site areas, shall use a generally accepted runoff hydrograph technique that is based on the NRCS soil cover complex method. Runoff hydrograph methods must be used for major drainage system designs for all systems with greater than 20 acres of drainage area and for design of all stormwater storage facilities.

   (B)   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 presented § 52.035(L). If a hydrologic computer model is used for stormwater runoff calculations, the duration of rainfall shall be 24 hours. The SCS Rainfall Type II curve shall be used for the rainfall distribution.

   (C)   For the purposes of existing pre-development runoff rate determination, undeveloped non-forested pervious areas including disturbed areas must be considered meadow in good condition, or its equivalent in good condition. Those areas that have existing impervious areas within the planned development area may be included in the determination of the predevelopment flow rate.

   (D)   All calculations using the rational method shall use rainfall intensities from the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland. NOAA’s Atlas 14 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.

   (E)   Times-of-concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55. Travel time for channel and pipe flow shall be computed using velocities determined using Manning’s equation.

   (F)    Runoff curve numbers (CN) for both existing (pre-development) and proposed (post-development) conditions to be used in the soil cover complex method shall be obtained from Chapter 2 of Urban Hydrology for Small Watersheds, NRCS, TR-55.

   (G)   (1)   Runoff coefficients (C) for both existing (pre-development) and proposed (post-development) conditions for use in the rational method shall be:

Runoff Factors for the Rational Equation
Type of Drainage Area or Surface	Runoff Factor “C”
	Minimum	Maximum
City business sections	0.60	0.80
Clay soil, bare or light growth	0.35	0.75
Clay soil, woods or heavy growth	0.25	0.60
Dense residential sections	0.50	0.70
Gravel, bare or light growth	0.20	0.40
Gravel, woods or heavy brush	0.15	0.35
Pavement, bituminous macadam or surface-treated gravel	0.65	0.80
Pavement, concrete or bituminous concrete	0.75	0.95
Pavement, gravel, macadam and the like	0.25	0.60
Rural areas, parks, golf courses	0.15	0.30
Sandy soil, cultivated or light growth	0.15	0.30
Sandy soil, woods or heavy brush	0.15	0.30
Suburban, normal residential areas	0.35	0.60

 

Runoff Coefficient for Rural Watersheds
	Slope
	Extreme	High	Normal	Low
Relief (Cr)	0.28—0.35 steep, rugged terrain with average slopes above 30%	0.20—0.28 hilly, with average slopes of 10-30%	0.14—0.20 rolling, with average slopes of 5—10%	0.08—0.14 relatively flat land, with average slopes of 0-5%
Soil infiltration (Ci)	0.12—0.16 no effective soil cover either rock or thin soil mangle of negligible infiltration capacity	0.08—0.12 slow to take up water, clay or shallow loam soils of low infiltration capacity or poorly drained	0.06—0.08 normal; well drained light or medium textured soils, sandy loams	0.04—0.06 deep sand or other soil that takes up water readily, very light well drained soils
Vegetative cover (Cv)	0.12—0.16 no effective plant cover, bare or very sparse cover	0.08—0.12 poor to fair; clean cultivation, crops or poor natural cover, less than 20% of drainage area over good cover	0.06—0.08 fair to good; about 50% of area in good grassland or woodland, not more than 50% of area in cultivated crops	0.04—0.06 good to excellent; about 90% of drainage area in good grassland, woodland, or equivalent cover
Surface (Cs)	0.10—0.12 negligible; surface depression few and shallow; drainageways steep and small, no marshes	0.08—0.10 well defined system of small drainageways; no ponds or marshes	0.06—0.08 normal; considerable surface depression storage lakes and ponds and marshes	0.04—0.06 much surface storage, drainage system not sharply defined; large floodplain storage or large number of ponds or marshes
The total runoff coefficient based on the four runoff components is: C = Cr + Ci + Cv + Cs
As obtained from Pub. 584, PennDOT Drainage Manual, §§ 7.5.c through 7.5.g.

 

      (2)   Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations such as the capacity of open channels, pipes and storm sewers. Values for Manning’s roughness coefficient (n) shall be consistent with Pub. 584, PennDOT Drainage Manual § 7.3.

   (H)   Outlet structures for stormwater storage facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method. Orifices smaller than three inches diameter are not recommended; however, if the qualified professional can provide proof that the smaller orifices are protected from clogging by use of anti-clogging devices, then smaller orifices may be permitted.

   (I)   The design of any stormwater storage facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities.

(1980 Code, Ch. 24, Part 3, § 305) (Ord. 1403, passed 7-18-2011, § 305)