Author(s): J. P. Britton; C. J. Nygaard; S. J. Schlenker

Linked Author(s):

Keywords: Mount St. Helens; Sediment Retention Structure; Spillway raise; Roller Compacted Concrete; Fish passage

Abstract: The Sediment Retention Structure (SRS) was constructed by the U.S. Army Corps of Engineers (USACE) from 1987 – 1989 to trap sediment eroding from the Mount St. Helens debris avalanche, for the purpose of maintaining flood risk levels downstream in the Cowlitz River. In 1998 the operation of the SRS changed and the sediment trapping efficiency decreased. The USACE began studies and an alternatives analysis to identify a long-term plan given the current conditions. The studies and alternatives analysis led to a preferred adaptive-management plan including up to three incremental SRS spillway raises to trap more sediment. The first spillway raise was constructed in 2012. The 2.1-m-high structure was constructed using 8,700 cubic meters of Roller Compacted Concrete (RCC). The RCC structure was set back from the original spillway crest to allow room for the subsequent two raises. The primary hydraulic design goals were downstream fish passage and the promotion of a separated and vegetated floodplain terrace in the flat sediment plain above the spillway. The main features of the spillway raise included the RCC structure, a plunge pool, and a channel excavated in rock connecting to the original spillway crest. The RCC structure was designed with a three-tiered crest, and angled RCC sills were constructed on the downstream face to collect and concentrate low flows and to dissipate energy at high flows. The spillway raise has increased the trapping efficiency of the SRS and maintained flood risk levels in the Cowlitz River downstream.

DOI: https://doi.org/10.15142/T3500628160853

Year: 2016