TenCate Geosynthetics is involved in a United States Army Corps of Engineers (USACE) pilot evaluation at Mount Saint Helens (Washington,US). USACE is here looking for an economical, long-term method to prevent volcanic debris (mainly ash) from the Mount Saint Helens eruption from flowing down the Toutle and lower Cowlitz river systems and eventually into the Columbia river.
The ash will settle out in the larger rivers, causing the river beds to rise, resulting in major flooding. Structures using TenCate Geotube® containers with an impact-resistant geotextile were evaluated to see their effectiveness in generating ash build-up and retainage.
TenCate Geotube® unit being filled. In the background Mount St. Helens
Mount Saint Helens was once 2,950 metres high. As a result of the huge eruption of this volcano in Skamania County on 18 May 1980, part of it was lost, so that its height is now just 2,550 metres. When it erupted, more than two billion cubic metres of volcanic ash and debris was blasted some 20 kilometres into the sky, which then thundered down the mountain in an immense landslide of mud and rock. The Corps of Engineers managed to offset the impact of the flow of debris into the Toutle, Cowlitz and Columbia rivers. The Spirit Lake tunnel, completed in 1985, helped stabilise the water levels of the lake. And a sediment retention strucure, completed in 1989, helps to keep immense quantities of sediment from rushing down the Toutle river, preventing significant flooding and navigation problems. Nevertheless, sediment from the avalanche continues to cause flooding concerns to the more than 50,000 inhabitants of Castle Rock, Kelso, Lexington and Longview, which lie downstream on the Cowlitz river.
TenCate Geotube® unit in the horseshoe-shaped structures using piles, planks and tree roots. This created eddies, where sediment would be deposited, thus generating islands of ash
The Corps recently evaluated two sediment retention structures to see the effectiveness of generating ash build-up and retainage. One system was a cross-valley structure (CVS), created by driving piles into the riverbed and then joined by heavy planks, making a labyrinth checker board system of different levels. By directing the river flow through this, the sediment is deposited into the system. The second system consisted of a combination of horseshoe-shaped structures, with piles, planks and tree roots. This created eddies, where sediment would be deposited, thus generating islands of ash. For this containers based on TenCate Geotube® technology were used. With local volcanic ash as fill material, they block and divert the Toutle river around an island, generating a containment area for the diverted river, forcing the Toutle river to go through the CVS, and not allow it to scour between the CVS and the island, while taking the full force of the river with debris.
The sediment retention structure in the North Fork Toutle River is 575 m long and 56 m high. The aim was to prevent flooding downstream by containing the sediment from Mount Saint Helens (in the background)
This is the first time that volcanic ash has been used to fill geotextile tube containers and to make a structure. The containers must be tight to both the CVS and the island, while having the full height and mass to ensure stability. This required that they have connections that are flat and tight so that they will seal completely. They are, after all, subject to a continuously flowing, strong river current carrying debris, from full-size trees to volcanic boulders, as well as abrasive volcanic ash.
The CVS structure
Mount St Helens and its vicinity
The continuous monitoring and measurements have to date shown positive results. The structures with TenCate Geotube® are doing what they were intended to do. The added benefit of using volcanic ash at the site has resulted in a solid, sturdy system.