Dam Safety Series: Design

What is wrong with this picture? 

Looking below the surface at Scott Dam’s design

Scott Dam | by PG&E

Did you know Scott Dam was originally designed to run straight across the Eel River Canyon? So why is there an angle in Scott Dam? How does this design impact dam safety? Scott Dam, as we see it today, is a result of the lack of information about the surrounding geology at the time it was designed.

During construction in the winter of 1920, seasonal rains and high river flows resulted in a very large boulder, which was assumed to be bedrock, falling into the river channel. In response, dam engineers redesigned Scott Dam to go around the rock known as “The Knocker”.

And this is why Scott Dam has a sharp angle.

The Redesigned Dam
When dam engineers redesigned Scott Dam to accommodate for the lack of stable bedrock, construction workers built around the “Knocker” that fell into the river, creating the angle that we see today. The shrink-swell clays and crumbling shales are still beneath the dam. The easily eroded bedrock, greywacke and siltstone, make a questionable foundation for a project of this scale.

When Scott Dam was originally designed, dam engineers were unaware of the dynamic landscape at the project site, in the landslide-prone slopes along the Eel River. According to PG& E’s geologic report, the foundation of the dam sits atop variable layers of shale, siltstone, and greywacke of the Franciscan Mélange Complex and Great Valley Sequence. This series of rocks is intermingled with loosely compacted zones of clay. These intermingled layers are easily eroded and relatively unstable.

In future blogs we will discuss findings from a recent study of these landslide-prone slopes, and how this ever-changing landscape impacts dam safety.

If Scott Dam were proposed to be built today, preliminary reviews would stop immediate approval of this project. Unfortunately, that is not our current reality, and we must now address the legacy hazards left on our landscape.

Past issues of dam safety were apparent to Michael O’Shaughnessy, developer of the Hetch-Hetchy water system. In a memo dated February 21, 1921, to the California Railroad Commission (the predecessor to today’s Division of Dam Safety), O’Shaughnessy was quoted “in essence, that he had been at the site and was alarmed by what he had seen of the method and character of the work done in the construction of the dam and that he seriously questioned the safety of the structure.

Memo from Chief Engineer to Hydraulic Engineer re: Michael O’Shaughnessy’s concerns about Scott Dam construction, February 21, 1921. Click image to enlarge.

Gravelly Valley and the Eel River are not geologically suitable for any dam.

A hundred years ago, dam engineers and workers thought they were doing the right thing. However, we are living with the fallout of their lack of information. We have a chance and it is our responsibility to shape the future of this watershed.