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.
The BMP Sizing Tool automates some of the required calculations to support BMP sizing and design for a specific set of stormwater management facility types (See Section 9.0 of User's Guide).
Responses to frequent BMP Sizing Tool questions can be found in Appendix A of the User's Guide, including:
- Rain gardens, stormwater planters, vegetated swales, and infiltrators are referred to as LID facilities. The BMP Sizing Tool uses different methods for sizing LID facilities (Section 9.2) and detention ponds (Section 9.3).
- If there are multiple types of post-development surface covers on the site, each of the areas should be delineated separately and entered separately in the BMP Sizing Tool under the Discharge Management Areas tab.
- Predevelopment surface conditions should reflect historical conditions (i.e., grass or forest). Even when the pre-development condition is impervious surface, it should be included in the BMP Sizing Tool as grass or forest.
- The BMP Sizing Tool sizes facilities to manage the 10-year peak flow (based on continuous simulation.) However, WES design standards require conveyance systems to be sized for the peak flow from a 25-year, 24-hour design storm.
- The BMP Sizing Tool is based on LID principles, including the concept of prioritizing small, distributed surface vegetated facilities. As a guideline, it is recommended that the area draining into a single BMP facility should not exceed approximately 10 acres.