Section 250-RICR-150-10-8.37 - Pollutant Loading Analyses - Overview of the Simple MethodA. Stormwater pollutant export load (L, in pounds or billion colonies) from a development site can be determined by solving the following equation: L = [(P)(Pj)(Rv)/12](C)(A)(2.72)
Where:
P = rainfall depth (inches)
Pj = rainfall correction factor
Rv = runoff coefficient expressing the fraction of rainfall converted to runoff
C = flow-weighted mean concentration of the pollutant in urban runoff (milligrams/liter)
A = contributing drainage area of development site (acres)
12, and 2.72 are unit conversion factors
B. For bacteria, the conversion factor is modified, so the loading equation is: L = 1.03( 10-3)[(P)(Pj)(Rv)](C')(A)
Where:
P = rainfall depth (inches)
Pj = rainfall correction factor
Rv = runoff coefficient expressing the fraction of rainfall converted to runoff
C = flow-weighted mean bacteria concentration (#col/100 ml)
A = contributing drainage area of development site (acres)
1.03 is a unit conversion factor
C. P (depth of rainfall). The value of P selected depends on the time interval over which loading estimates are necessary (usually annual rainfall - see figure in § 8.37(H) of this Part. Appropriate annual rainfall values for a site specific location can be interpolated from the figure in § 8.37(H) of this Part or obtained from the Northeast Regional Climate Center. If a load estimate is desired for a specific design storm, the user can supply the relevant value of P derived from § 8.6(E) of this Part. Caution is required as event mean concentration values vary as a function of rainfall amount and intensity and those presented in the table in § 8.36(D) of this Part are median values from a range of storms more representative of long-term loading. If a load is desired from a larger storm, applicants shall provide appropriate documentation of the source of the event mean concentration used. All rainfall data used in the analysis must be applicable to site location and referenced for review.D. Pj (correction factor). Use a value of 0.9 for Pj. This represents the percentage of annual rainfall that produces runoff. When solving the equation for individual storms, a value of 1.0 should be used for Pj.E. Rv (runoff coefficient). 1. Rv is the measure of site response to rainfall events and is calculated as: Rv = r/p Where:
r = storm runoff (inches)
p = storm rainfall (inches)
2. The Rv for a site depends on soil type, topography, and vegetative cover. However, for annual pollutant loading assessments, the primary influence on Rv is the degree of watershed imperviousness. The following equation has been empirically derived from the Nationwide Urban Runoff Program studies (USEPA, 1983) and is used to establish a value for Rv. Rv = 0.05 + 0.009(%I)
Where:
%I = the percent of site impervious
3. A value for I can be calculated by summing the areas of all impervious surfaces and dividing this area by the total contributing drainage area. If more than one land use is present at the site, divide the impervious portion of each land use by its respective total area.F. A (drainage area). The total contributing drainage area (acres) can be obtained from site plans.G. C (pollutant concentration). Choose the appropriate value of C from the table in § 8.36(D) of this Part.250 R.I. Code R. 250-RICR-150-10-8.37
Amended effective 11/13/2018