Extension of TR-55 for Microwatersheds

Concerns about protecting first-order receiving streams, improving water quality, and on-site control of storm runoff create a need to extend commonly used hydrologic design methods for application to microwatersheds. Existing design methods cannot adequately be extrapolated for use on small lot-sized watersheds. This research extends the widely used Natural Resources Conservation Service TR-55 design procedures for use on microwatersheds. Specifically, the graphical peak discharge estimation method is extended so that it can be used for times of concentration as small as 0.02 h. The kinematic-wave time of concentration estimation method is made applicable for multiple-section sheet flow, and a new pond-and-swamp adjustment procedure enables the design of small on-site bioretention ponds. The latter design method uses the actual storage depth and can be applied for both partially and fully contained storage.