Dam Safety Series: Foundation

Between a Rock and Management Priorities – Immense Pressure on Scott Dam’s Questionable Foundation

Of the many safety concerns at Scott Dam, the first we featured in our blog series was the uncertain quality of the bedrock and redesigned dam as discussed in Part 1: Design – What’s Wrong With This Picture? In this post we will address the potential for Scott Dam to detach from its foundation.

Immense pressure behind the dam in this highly erosive landscape and “flashy” watershed threatens dam safety and reliability.

In part, the question of dam safety and reliability lies in the quality of bedrock anchoring the foundation and the amount of pressure that the foundation can withstand. The variable layers of the Franciscan Mélange make a weak anchor for Scott Dam in this erosive landscape. The weight of the body of water and trapped sediments eroded from upstream of Lake Pillsbury are the main source of the immense pressure on Scott Dam’s questionable foundation.

Regional Geology
     Click to enlarge image.

On the upstream side of Scott Dam, the box that surrounds the inlet to the needle valve has sediment piled up to its brim. If, or more likely when, more sediment falls into the inlet, the valve is likely to become irreparably clogged. There is, in fact, no plan for how to repair the needle valve if it becomes clogged.

This is why dam managers are careful to maintain water levels in the reservoir which they hope will prevent accumulation of additional sediment from either the toe of the active landslide or the underwater sediment banks. We’ll address this in further detail in future posts.

Pressure on Scott Dam requires continuous monitoring due to safety concerns that the dam may detach from its questionable foundation.

Many piezometers are installed within Scott Dam, monitoring and producing valuable data on water levels, pore pressures, and movement, such as uplift. These instruments are essential and must be replaced regularly to continuously monitor dam safety and reliability at Scott Dam. In correspondence related to regular dam safety evaluations in 2018, FERC recommended to PG&E’s Chief Dam Safety Engineer “that several new sealed piezometers be installed in the gallery in holes which are newly drilled and independent of existing drain holes”.

How does the pressure on the foundation impact dam safety, particularly in the context of the “flashy” Eel River? Is the bedrock below Scott Dam’s foundation a solid anchor for withstanding the immense weight of water and significant sedimentation in the reservoir?

Expert assessment of the original designs and the actual structure may answer these questions.

The California Division of Safety of Dams is undergoing a multi-year investigation of the safety and stability of the Scott Dam and Cape Horn Dam for the first time in some 20 years, but we have an opportunity too. Given the serious concerns about safety and reliability, and the lack of any entity capable of addressing these concerns, we must continue advocating to remove Scott Dam.