Mud Mountain Dam SBT invert liner history and performance

Mud Mountain Dam (MMD) is located on the White River in Washington State, USA, where it receives river sediment derived from Mt. Rainier, a 4,392-m-tall, glaciated, active stratovolcano. MMD is a run-of-river flood control project, where large volumes of silt to boulder-sized material pass through the sediment bypass tunnel (SBT). The resultant mechanical abrasion damages the tunnel invert liner, requiring frequent and expensive maintenance. MMD’s SBT invert liner was constructed in 1942 with steel rails embedded in concrete that required frequent, costly repairs. In 1992, the rail-in-concrete design was replaced with a continuous 2.5-cm thick welded steel plate liner. However, through-going abrasion holes developed in the steel liner within ten years, necessitating annual to bi-annual concrete patching until, in 2018, a 25.4-cm thick granite block invert liner was installed. Granite blocks were placed in 536 rows overlying a porous concrete layer placed atop the older steel liner, and the tunnel was returned to service in November 2018. Between the initial granite liner operation and March 2024, the granite liner experienced flow 47% of the time, or 900 days. Annual inspections tracked the development of concentrated, asymmetrical erosion like that observed in the previous steel liner. Specifically, erosion is concentrated at the inside of the upstream and downstream tunnel bends and the middle of the straight tunnel section. This paper includes a brief history of the MMD SBT, the basis of the granite liner design, recent visual/manual granite liner erosion observations, and a summary LiDAR comparison of as-built (November 2018) and post-operation (October 2023) data.