Established in 1879, the U.S. Geological Survey (USGS) is an organization that monitors and analyzes the health of the environment of the United States.
The Project
After the eruption of Mount St. Helens in 1980, the upper section of the North Fork Toutle River in southwestern Washington was clogged with debris and sediment. The U.S. Army Corps of Engineers created a Sediment Retention Structure (SRS) in 1989 to block sediment from traveling down the Toutle River and into the Cowlitz River. The SRS worked effectively for about ten years, but gradually the level of sediment surpassed the spillway base and a higher percentage of sediment went over the SRS, raising concerns of an increased risk of flood in the Cowlitz River.
Challenge
To help mitigate flood risks and provide useful data for geologists to identify the erosion and deposition impacts found as rivers and streams change over long periods of time, the USGS team needed to find an effective regression model to accurately estimate the suspended sediment concentration being transported through the Toutle River. An increase in sediment load can be an indicator that a lahar (a volcanic mudflow consisting of a mixture of water and volcanic debris) is occurring, putting the Cowlitz and Toutle rivers at risk of flood.
Solution
The USGS team chose to use turbidity as a surrogate for suspended sediment, and created a regression model to predict the SSC levels. Suspended-sediment-surrogate monitoring has been shown to be an effective method of obtaining affordable time-series data, while minimizing the risk to the field personnel responsible for collecting the data. The USGS team selected the FTS DTS-12 turbidity sensor because of its SDI-12 interface, large optical face, as well as a minimal need for maintenance and calibrations. The sensors were installed in three sites: two in the North Fork and one in the main stem of the Toutle River, with an FTS DigiTemp sensor also installed in the North Fork gaging station. The team found the DTS-12 units to be robust, reliable, and required maintenance only once per year.
