Abstract
AbstractThirty‐seven years post‐eruption, erosion of the debris avalanche at Mount St Helens continues to supply sediment to the Toutle–Cowlitz River system in quantities that have the potential to lower the Level of Protection (LoP) against flooding unacceptably, making this one of the most protracted gravel‐bed river disasters to date. The Portland District, US Army Corps of Engineers (USACE) recently revised its long‐term plan for sediment management (originally published in 1985), in order to maintain the LoP above the Congressionally‐authorized level, while reducing impacts on fish currently listed under the Endangered Species Act, and minimizing the overall cost of managing sediment derived from erosion at Mount St Helens. In revising the plan, the USACE drew on evidence gained from sediment monitoring, modelling and uncertainty analysis, coupled with assessment of future LoP trends under a baseline scenario (continuation of the 1985 sediment management strategy) and feasible alternatives. They applied geomorphological principles and used engineering science to develop a phased Sediment Management Plan that allows for uncertainty concerning future sediment yields by implementing sediment management actions only as, and when, necessary. The phased plan makes best use of the potential to enhance the sediment trap efficiency and storage capacity of the existing Sediment Retention Structure (SRS) by incrementally raising its spillway and using novel hydraulic structures to build islands in the North Fork Toutle River (NFTR) and steepen the gradient of the sediment plain upstream of the structure. Dredging is held in reserve, to be performed only when necessary to react to unexpectedly high sedimentation events or when the utility of other measures has been expended. The engineering‐geomorphic principles and many of the measures in the phased Sediment Management Plan are transferrable to other gravel‐bed river disasters. The overriding message is that monitoring and adaptive management are crucial components of long‐term sediment‐disaster management, especially in volcanic landscapes where future sediment yields are characterized by uncertainty and natural variability. © 2017 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.
Highlights
The catastrophic eruption of Mount St Helens (MSH) on 18 May 1980 altered the surrounding landscape both physically and ecologically (Swanson and Major, 2005), with the catchment of the North Fork Toutle River (NFTR) being the most severely affected (Lipman and Mullineaux, 1981; Janda et al, 1984)
Soon after the eruption the United States Congress authorized the Portland District, US Army Corps of Engineers (USACE) to manage sediment in the Toutle–Cowlitz system as necessary to ensure that the Level of Protection (LoP) provided by levees along the lower Cowlitz River is maintained at or above the value prescribed in the Congressional authorization throughout a 50-year period that began in 1985
This paper reports how these more recent studies led to development and adoption of a revised Sediment Management Plan (SMP) that uses phased and adaptive sediment management actions as needed to ensure that the LoP will exceed the Congressionally-authorized value up to and beyond 2035, despite persistence of annual sediment yields from upper NFTR that are elevated compared to preeruption levels, highly sensitive to the occurrence of floods, and unpredictably variable
Summary
The catastrophic eruption of Mount St Helens (MSH) on 18 May 1980 altered the surrounding landscape both physically and ecologically (Swanson and Major, 2005), with the catchment of the North Fork Toutle River (NFTR) being the most severely affected (Lipman and Mullineaux, 1981; Janda et al, 1984). The original SMP was formulated on the expectation that elevated sediment yields to the NFTR would decline through time and that sediment loads in the Toutle–Cowlitz system would be monitored and interpreted to make available the data needed to characterize sediment transport rates and trends (USACE, 1985) In this context, during the late-1990s, scientists at the US Geological Survey, Cascades Volcano Observatory (CVO) analysed post-eruption suspended sediment measurements in the Toutle River, together with USACE’s records of sediment accumulation upstream of the SRS since its completion in 1989.
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