Building the Coffer Dam (and a Siphon)

To rebuild the headgate and intake, we need to work below the waterline — without the water. That's what August was for.

The coffer dam went up this month: a temporary barrier of one-ton sandbags and liner upstream of the headgate, creating a dry, safe work area downstream of it. With the pond already drawn down, the coffer dam holds back what water remains, so crews can work at the base of structures that are normally submerged.

Alongside it we built a siphon — a large-diameter pipe running over the dam that keeps water moving downstream at all times. The river doesn't stop because we're working, and neither do our obligations to it: the reach below the dam needs continuous flow, drawdown or not. The siphon delivers that automatically, no pumps required, powered by nothing but physics.

With the work area dry, the old trash racks came out in late August. Seeing them on dry land for the first time confirmed what we suspected: decades of service had left them ready for full replacement rather than repair. The new trash rack system is being designed now — modern steel, engineered supports, and this time with maintenance access in mind.

A coffer dam is a temporary structure doing a serious job, and we treat it that way — it gets inspected constantly, and everything downstream of it is planned around the assumption that water always finds a way. Hold that thought.