Tahoma Creek Bridge focused condition assessment
Jost R, Kenyon T, Beason S . 2024. Tahoma Creek Bridge focused condition assessment. Science Report. NPS/SR—2024/103. National Park Service. Fort Collins, Colorado. https://doi.org/10.36967/2303321
The Tahoma Creek Bridge on the southwest side of Mount Rainier is an essential crossing for year-round vehicular access to Mount Rainier National Park (MORA). This site is also exposed to significant hydrologic variability, which the current structure was not designed to withstand. Locally enhanced vertical increases to the riverbed elevation, known as aggradation, threaten the structure’s long-term viability. The purpose of this report is to (1) clarify the details of channel maintenance operations related to the bridge; (2) synthesize the natural and human influences of sediment deposition in the lower watershed; and (3) discuss the potential solutions for sustainable bridge maintenance. Information was collected through prior structural inspection reports, raw hydrologic data stored by the National Park Service, and both published and unpublished reports focused on Tahoma Creek. It was found that the height and width of the Tahoma Creek Bridge are severely underfit for relatively common flooding events on Tahoma Creek. Additionally, dredging and sediment storage practices employed to maintain the bridge are ineffective and even enhance aggradation in nominal flow conditions. Further research in the Tahoma Creek Watershed suggests the river is generally efficient at exporting produced sediments and maintains a relatively stable vertical profile in its lower reaches, where the bridge is located. The singular variation from this stable vertical trend at the bridge site suggests locally enhanced aggradation at the bridge is entirely due to the presence of the bridge itself and the channel modification actions taken to support the structure. All told, neither the overall size of the Tahoma Creek Bridge nor the channel modification actions taken to maintain the crossing, are viable for long-term access into MORA. With no changes to the existing bridge profile, the status-quo for operations to manage the Tahoma Creek Bridge will eventually fail to maintain access.
- Type
- Published Report
- Authors
- Jost, Robert; Kenyon, Taylor; Beason, Scott
- Date of Issue
- 2024-04
- Publisher
- National Park Service
- DOI
- 10.36967/2303321
- Units
- MORA , NRSS
- Keywords
- Geomorphology, Hydrology, Infrastructure, Mount Rainier National Park, Planning, Sediment