All development that is subject to WES Stormwater Standards must address water quality, detention, and infiltration requirements. If infiltration is not feasible, the design engineer may submit a design modification request in accordance with Stormwater Standards, Section 1.6. The request shall include a narrative of why the infiltration standard cannot be achieved, as well as an equivalent alternative design that can accomplish the same design intent of these standards. The BMP Sizing Tool is an equivalent alternative option that is available to developers.
WES, in cooperation with other local jurisdictions, has developed a BMP Sizing Tool to assist developers in meeting flow duration matching requirements customized to Clackamas County conditions. The Tool sizes facilities so that post-development peak flow durations will match the pre-development peak flow durations ranging from 42% of the 2-year to the 10-year flows, as determined by HSPF continuous rainfall model simulation.
Note: The BMP Tool can only be used with an approved design modification request.