Existing hydrologic studies have been developed for many rivers in Washington State. FEMA has developed most of these reports. USACE and local agencies have developed other reports.
Many small and medium streams within urbanizing areas have had some modeling by local government. These can be useful and appropriate to adopt for WSDOT use, following examination of model assumptions and drainage basin delineation.
These reports are a good source of flow information because they were developed to analyze the flows during flooding conditions of a particular river or stream. The types of calculations used by the agency conducting the analysis are more complex than the Rational Method or USGS regression equations and are therefore more accurate. The increased time required to perform these complex calculations is not justified for the structure that WSDOT is designing; however, if the analysis has already been performed by another agency, then it is in WSDOT’s best interest to use this information.
FEMA reports and USACE existing hydrologic studies are available on the FEMA map service center website. The State Hydraulics Office should be contacted for local agency reports. The State Hydraulics Office may also have basin planning documents or action plans that could contain flow rate information. These studies should be used with caution as they may have been developed for a different purpose and therefore may not be transferable/applicable for the design of transportation infrastructure.
