Herrera partnered with the Washington Department of Transportation (WSDOT) and subconsultant NHC to develop a list of fifth order stream reaches to exempt from flow control requirements in the Ecology 2001 Stormwater Management Manual for Western Washington. The Department of Ecology was also an active partner in the project.

Herrera conducted this complex and innovative study using a three-phase scientifically-based approach. During the first phase, the team synthesized the relevant literature and developed a methodology to exempt river reaches from stormwater flow control based on stream order, cumulative changes in watershed vegetative cover, percent impervious area, stream gradient, and tidal influence. As a test case for the flow control exemptions, Herrera conducted HEC-RAS modeling on the mainstem of the Nooksak River to determine flow thresholds that potentially cause channel deformation and channel changes in these streams. In the final phase, the methodology was applied to western Washington to define stream reaches that are exempt from stormwater flow control regulations. Learn more about the study.