When designing stormwater facilities in western Washington, the PEO must use an Ecology-approved continuous-simulation hydrologic model to meet the requirements of the most current version of the Highway Runoff Manual. A continuous-simulationhydrologic model captures the back-to-back effects of storm events that are more common in western Washington. These events are associated with high volumes of flow from sequential winter storms rather than high peak flow from short-duration events, as is characteristic in eastern Washington.
WSDOT uses MGSFlood (see Highway Runoff Manual), which uses the HSPF routines for computing runoff from rainfall on pervious and impervious land areas. In addition, MGSFlood has the BMP design criteria built into the software and will help the sizing of the stormwater facility to meet the Highway Runoff Manual–required runoff treatment and flow control flow rates and volumes. WSDOT also uses MGSFlood to estimate seasonal flows for temporary stream diversion designs.
MGSFlood does have limitations that the PEO should understand before using the program, regarding the project location, conveyance design, and basin size. MGSFlood is for projects in western Washington with elevations below 1,500 feet. The program does not include routines for simulating the accumulation and melting of snow, and its use should be limited to areas where snowmelt is not usually a major contributor to floods or to the annual runoff volume. MGSFlood is not used for conveyance design but is capable for conveyance design when a small time step, such as 5 or 15 minutes, is used. For projects located in western Washington that fall outside the modeling guidelines described in this paragraph, contact the RHE or State Hydraulics Office staff for assistance.
