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Mount St. Helens Future Expected Deposition Scenario (FEDS)

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This Future Expected Deposition Scenario FEDS report introduces the hydraulic and sediment transport tools that will be used to evaluate performance of alternatives for development of a future Mount St. Helens MSH Long Term Sediment Management Plan. The scenario being investigated is the best deterministic estimation of future conditions through the authorization time frame if no actions are taken in the watershed and a continuation of the existing processes and dynamics occur. The FEDS report, along with the ToutleCowlitz River Sediment Budget, May 18, 2010, and the 2009 Progress Report, June 2010, will be part of a Limited Reevaluation Report LRR. The selected technical approach utilizes a set of deterministic hydraulic and sediment transport models arranged in series extending from the toe of the debris avalanche in the upper North Fork Toutle watershed downstream to the Columbia River. This set of models is driven by a time series of flows and sediment inputs extending from present time to the end of the authorization period, Water Year WY 2035. This 28-year series of water and sediment discharges was based on 9 years of historic hydrologic record WYs 1999 to 2007. Data developed in the Sediment Budget for WYs 1999 to 2007 were used as surrogates for future forecast years through 2035. Analysis of the surrogate hydrologic period compared to the historic flow record shows that the period is reasonably representative of the historic sediment yield record. Considerations of global climate change are not addressed in this analysis due to the relatively near 2035 end-of-project authorization. The total predicted load from the MSH debris avalanche to the Sediment Retention Structure SRS sediment plain for the period from 2008 to 2035 is estimated to be 215 M Tons. Sediment transport models using both 1-D HEC-RAS and 2-D MIKE 21C software were developed to determine future trends in sediment deposition and outflow from the SRS through 2035.

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  • Geology, Geochemistry and Mineralogy
  • Hydrology, Limnology and Potamology
  • Computer Programming and Software

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