This Buffer Equivalency method is adapted from Chesapeake Bay Local Assistance (CBLAD) Draft Information Bulletin #3. and should be used for RPA buffer compensation calculations:
Step 1: Determine the pollutant load (L) generated by the drainage area of
the buffer.
(Assume nutrients means phosphorous.)
L = 0.000047 (lbs/in-ft) x annual rainfall (in) x lot width (ft)
This equation is derived by the following method:
Assume a maximum of 200 feet of overland sheet flow can be handled by the buffer. [The 200-foot maximum is established by the new state Erosion and Sediment Control regulations (VR 625-02-00), and is suggested as policy by both VDOT and ASCS-SCS.] Multiply the average lot width (see attached graphic) in feet by the 200-foot overland sheet flow factor and divide by 43,560 (sqft/ac) to determine the “drainage area” in acres. Let the average land cover condition be 16 percent impervious and use the Simple Method to calculate the load generated by the drainage area. Since the drainage area always includes dividing 200 by 43,560 and other constants from the Simple Method, the only variables are the lot width and annual rainfall. Therefore, this procedure and be reduced to the above equation, where L is in pounds.
Step 2: Determine the maximum load (RMAX) capable of being removed by the full buffer.
RMAX = L x 0.4
Multiply the load generated (from Step 1) by 0.40 (the removal rate dictated by the Regulations for a full 100-foot buffer).
Step 3: Determine the actual load removed (RACT) by the remaining, undisturbed buffer.
RACT = L x EFF
Multiply the load generated (from Step 1) by the appropriate removal efficiency shown on the next page
Total Buffer Width (BW) Removal Efficiency (EFF)
100 (no encroachment) .40
90 (10’ encroachment) .37
80 (20’ encroachment) .35
70 (30’ encroachment) .32
60 (40’ encroachment) .30
50 (max encroachment) .25
Step 4: Determine the load removal requirement (RR) of an “equivalent” BMP.
RR = RMAX - RACT
The load removal requirement is the difference between the maximum load removal (Step 2) and the load removal provided by the remaining buffer (from Step 3).
Step 5: Determine available BMP options.
Refer to Table 2 in Appendix C of the Chesapeake Bay Local Assistance Manual for pollutant removal efficiencies of BMPs. Sometimes an additional (relocated)e buffer width may be appropriate.
Step 6: Provide adequate BMP selection and design
Site conditions may determine ultimate selection. Refer to Controlling Urban Runoff: A Practical Manual for Planning and Designing Urban BMPs by Thomas Schuler, (Washington, DC: Metropolitan Washington Council of Governments, 1987)