Rhode Island Code of Regulations › Title 250 - Department of Environmental Management › Chapter 150 - Water Resources › Subchapter 10 - Wastewater And Storm Water › Part 8 - Stormwater Management, Design and Installation Rules › Section 250-RICR-150-10-8.37 - Pollutant Loading Analyses - Overview of the Simple Method

Current through June 14, 2024
A. 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.