Topographic and hydrologic insight for the Westside Road problem: Mount Rainier National Park

Thomas, Matthew; Kennard, Paul

The braided fluvial systems that flank Mount Rainier in Washington State facilitate landscape change that is appreciable on the human timescale. In Mount Rainier National Park, the debris flows and floods transmitted by these hydrogeomorphic systems have routinely challenged visitor safety, infrastructure, and the preservation of natural / cultural resources. This study focuses on the intersection of the Tahoma Creek valley and an approximately 1.5 km section of the Westside Road in the southwestern corner of the Park. Average cross-valley slopes, a relative elevation map, and one-dimensional free surface flow simulations are employed to quantitatively identify challenges associated with maintaining the problematic road section. Eighty-seven percent of the simple cross-valley slopes calculated favor the routing of water toward the road. An elevation relative to thalweg map, which highlights subtle changes in topography, reveals that the road bench is, on average, three meters above a proximal water course. One-dimensional free surface flow simulations characterize flooding risk as unevenly distributed along the road, an observation linked to local channel depth. Collectively, the topographic and hydrologic insight generated for this study indicates that it is unlikely the Westside Road, in its current form, will ever function as a low-maintenance route that accommodates public vehicular traffic. Importantly, the work reported here provides a starting point to consider along-road protection measures and a foundation to more rigorously simulate water and sediment transport for Tahoma Creek from a physics-based perspective.

Topographic and hydrologic insight for the Westside Road problem: Mount Rainier National Park

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