EXTREME SEDIMENTATION PLANNING- PROACTIVE DESIGN FOR LONG TERM PERFORMANCE OF THE SRS - NORTH FORK OF THE TOUTLE RIVER, WASHINGTON

摘要

The May 18, 1980 eruption of Mount St. Helens dramatically altered the hydrologic, hydraulic and sedimentation regimes of the Cowlitz and Toutle rivers. This event resulted in persistent high sedimentation rates in the sand bed Cowlitz River and increased probability of flooding for communities along the reach. The USACE's Sediment Retention Structure (SRS) on the North Fork Toutle River is a primary component of the long term sediment management plan. The SRS traps sediment and reduces sedimentation in the Toutle and lower Cowlitz River. Completed in 1989, the SRS was expected to operate in three stages over time: 1) nearly all sand-sized and larger particles are trapped; 2) sands begin passing the SRS in significant quantities; 3) gravels begin to pass the SRS. Currently in phase two, the SRS is passing sand in sufficient quantities to aggrade the lower Cowlitz River and affect flood protection. As the valley slope upstream of the SRS increases due to aggradation in this phase, the trapping efficiency of the project decreases and higher quantities of larger-grained sand particles pass the structure. Understanding performance of the SRS during phase two is a key to developing a responsive long term sediment management strategy. A fully coupled two-dimensional hydrodynamic model was developed from available LIDAR surveys of the Toutle River and associated sediment plain. The model was supplied with 28-year projected water and sediment inflow hydrographs to simulate future sedimentation of the SRS and to quantify the expected SRS trapping efficiency through the current design horizon (present to 2035). The timing, magnitude, and predicted deposition patterns upstream from the SRS were also studied for two project alternatives that retain sediment high in the system, by either modifying the SRS or developing sediment retention projects within the sediment plain upstream of the SRS. The USACE, Portland District is using a 2-D sediment model to demonstrate the effectiveness of proposed sediment management alternatives. Lessons learned from the application of this 2-D hydrodynamic sedimentation model can be applied to the predictive management of sediment in reservoirs and dams over long-term time horizons and in all types of hydrologic and sedimentation conditions.