Applying geomorphological principles and engineering science to develop a phased Sediment Management Plan for Mount St Helens, Washington

Abstract

Thirty-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 longterm sediment-disaster management, especially in volcanic landscapes where future sediment yields are characterized by uncertainty and natural variability.

Multidecadal Geomorphic Evolution of a Profoundly Disturbed Gravel Bed River System—A Complex, Nonlinear Response and Its Impact on Sediment Delivery

“…Future episodic events, especially floods, will certainly modify channel morphology, most likely through further episodes of acute widening, which may or may not be accompanied by significant degradation or aggradation. Such episodic events, shown to be important drivers of geomorphic response in other disturbed fluvial systems (e.g., Collins et al, ; James, ; Tunnicliffe et al, ), are likely to reverse the overall decline in downstream sediment delivery temporarily, a pattern observed following previous large floods (Major et al, ; Sclafani et al, ).…”

“…Horizontal line highlights daily mean streamflow of 150 m 3 /s, a streamflow that recurs about every 4 years. Shaded region shows period when streamflow was regulated by passage through culverts in SRS (Sclafani et al, ). See supporting information Data Set S1.…”

“…Initial erosion and channel development of upper NFTR delivered vast amounts of sediment downstream to the Toutle-Cowlitz River system (Dinehart, 1998;Major, 2004;Major et al, 2000;Sclafani et al, 2018). Rates and magnitudes of channel incision and widening in upper NFTR Basin diminished nonlinearly during the 1980s (Simon, 1999;Simon & Thorne, 1996), and by 1990 the river had developed a relatively stable long profile that appeared to be approaching a graded condition (Zheng et al, 2014).…”

“…Shaded region shows period when streamflow was regulated by passage through culverts in SRS (Sclafani et al, 2018). See supporting information Data Set S1.…”

Special Issue: Gravel Bed Rivers 8

“…Their study was based on the measured displacement length and the virtual velocity of painted tracers, on the thickness of the active bed layer using scour chains and morphological changes determined by repeat cross-sectional surveys. Sclafani et al (2017) summarized a recently revised longterm plan to maintain the level of protection, reducing impacts on fish, and minimizing the overall cost of managing sediment derived from erosion at Mount St Helens. Liedermann et al (2017) conducted comprehensive monitoring of bedload transport behavior on the large gravel bed Danube river.…”