Dam Safety Series: Sediment

A Much Bigger Problem – More ways poor design impacts dam safety at Scott Dam

Restorative Vision for the Upper Eel River
Needle valve releases draw down the reservoir in preparation for the rainy season and to provide flows for fish.

Decreased water storage, high temperatures, and evapotranspiration cause the sediment that has piled up in steep banks behind Scott Dam to dry out and slough off, threatening to irreparably clog the ‘needle valve’ which is the only way to release water when the Lake Pillsbury reservoir is lower than completely full. These eroded sediments have built up in the reservoir at the base of the dam, as discussed in the previous article: Part 2 – Foundation.

Thus, though the accumulated sediments have only displaced a relatively small portion of the reservoir’s storage capacity, they still threaten the safety of Scott Dam and the functionality of the entire Potter Valley Project. If the needle valve is blocked, it would be impossible to release water essential to support Eel River fisheries, much less for irrigation diversions to Potter Valley. Further, PG&E has acknowledged that if the needle valve becomes clogged, there is no plan to repair it.

The water storage forecast from PG&E illustrates this year’s storage requirements to prevent dam failure and maintain flow requirements, see Figure 1. Bank instability and sloughing occurs when the reservoir levels reach less than 12 thousand acre feet (TAF). In fact, as PG&E noted in its presentation to the Potter Valley Drought Working Group in January 2021, “If reservoir reaches 5 TAF (1849’), the needle valve must be shut down, ceasing reservoir releases, as a dam safety precaution to prevent catastrophic damage to the reservoir low level outlet infrastructure.”

Figure 1. Water Storage Forecast Water Year 2021. Click image to enlarge.

Lake Pillsbury water storage is currently around 27,306 AF, balancing on a precipice of management priorities and safety requirements. How long do we have before the water levels in the reservoir get so low that the risk of dam failure becomes a reality?

Significant sedimentation in Lake Pillsbury can have other consequences for dam safety. The added load of sediment can apply additional pressure on Scott Dam itself and increase vulnerability to seismic events, which we will discuss in a future post. Similarly, the consequences of dam failure for a reservoir with accumulated sediment may be more destructive than a flood alone. Most importantly, if this infrastructure becomes clogged or damaged with no plan to fix it, no one will benefit from the flows of the Eel River.

Engineering analysis and updated bathymetry studying the accumulated sediment and the background rate of erosion in Lake Pillsbury could help dam managers understand ways to safely release water for conveyance to Potter Valley and for downstream flows in the Eel River. Better yet, dam removal would provide “run of the river” seasonal flows, and, over time, transport sediment downstream rather than impounding and accumulating behind a poorly designed and questionable dam.