Clear Creek Solutions developed a WWHM-SWMM and HEC-RAS model representing the Hydrology and Hydraulics of the Lake Stevens outlet channel.

The purpose of the Lake Stevens Hydrologic and Hydraulic modeling is to evaluate alternative approaches for managing the outlet from Lake Stevens. The modeling has been conducted for the existing land use condition. Additional information provided by the modeling includes detailed flood elevations at select cross-section locations in the outlet channel. These were for multiple outlet configurations and management options.

A hydrologic and hydraulic (H&H) model of the Lake Stevens Watershed and outlet was completed using WWHM-SWMM stormwater modeling software developed by Clear Creek Solutions. WWHM-SWMM was used to simulate the existing conditions of the Lake Stevens outlet and evaluate multiple proposed alternative outlet configurations. WWHM-SWMM is a stormwater modeling tool that combines the Western Washington Hydrology Model (WWHM) as a continuous rainfall hydrologic computer model, with the open-source version of the Storm Water Management Model (SWMM 5) for hydraulic modeling.

The hydraulic component in the model represents the infrastructure including manholes, sewers, catch basins, stream cross-sections and boundary conditions. The best available GIS data was processed and imported into the model with respective basin boundaries pulled from previous Lakes Stevens HSPF models. The outlet configuration was pulled from available cross-section data provided by Davido Consulting Group. This data was derived from updated survey data and was first imported into HEC-RAS. The HEC-RAS model was used to process the raw cross-section data and was then used as a secondary check on model performance. The cross-section data from the HEC-RAS model was then imported into WWHM-SWMM and combined with the Basin information for Lake Stevens.

In this way, the WWHM portion of the model calculates the runoff and routes the runoff to either Lake Stevens or to the appropriate location in the outlet channel. Rainfall data was taken from the Sunnyside and Soper Hill precipitation gage for water years 1992 to 2021, with a precipitation factor of 1.0 applied by WWHM-SWMM for the project location.