Middle Fork Snoqualmie River - Hydraulic Modeling and Analysis

King County intends to prepare a Corridor Management Plan for the Middle Fork Snoqualmie River from approximately Snoqualmie River Mile (RM) 45 near the Three Forks Natural Area (upstream of its confluence with the South Fork Snoqualmie River) to RM 49 near the community of Tanner. This plan will be used by King County to develop a prioritized implementation strategy for near-term and long-term actions for the lower Middle Fork Snoqualmie River with the following objectives:

• Reduce risks to life and property from flood and channel migration/avulsion hazards;
• Avoid or minimize environmental impacts of flood hazard management on fish and wildlife habitat and other environmental resources;
• Protect and restore fish and wildlife habitat through restoration of riverine processes;
• Reduce the long-term cost of public flood hazard management

Preparation of the Corridor Management Plan requires characterization of baseline geomorphic and hydraulic conditions, flood and erosion hazards, and ecological resources in the study reach. One element of this work is the development and calibration of a detailed two-dimensional (2D) hydraulic model of the area. WATERSHED Science & Engineering (WSE) was retained to perform this task and analyze the results to provide King County with valuable information for the preparation of the Corridor Management Plan.

Role / Services: WSE developed and calibrated a detailed RiverFLO-2D hydraulic model to investigate and map current flood and erosion hazards of the study area. WSE simulated several flood events with the model and analyzed key model outputs, including water surface elevation, depth, velocity, and shear stress to provide King County with an existing conditions flood and erosion hazard assessment. This model is a tool that can be used in the future to quantify hydraulic conditions associated with potential floodplain management actions by allowing comparisons of these results to baseline conditions.

Outcome: King County gained an understanding of the complex geomorphic and hydraulic conditions related to flood and erosion hazards that currently exist in the Middle Fork Snoqualmie River and on the surrounding floodplain. The 2D model was provided to King County for use in analyzing impacts of potential future floodplain management actions.