MMWD seeks water storage gains by altering dam spillways

The Marin Municipal Water District is exploring options to increase its storage capacity by modifying spillways on reservoir dams.

The district board recently authorized spending up to $2.2 million to design proposals for potential projects at Nicasio Reservoir, Soulajule Reservoir, Kent Lake and Alpine Lake.

Spillways are the parts of a dam that provide controlled release of water downstream.

Work has already been done to determine that it could be possible to alter Seeger Dam at the Nicasio Reservoir with floodgates to add about 3,000 acre-feet of storage capacity. Early estimations indicate it would be around a $5 million construction project.

“This project is what’s kind of called the spin-off of the bigger water storage projects that staff are working on,” Alex Anaya, director of engineering, said during a project update on June 21.

Anaya said that compared to other $290 million-plus water storage proposals, the prospect of a spillway modification is “a unique opportunity to advance the district’s water supply in a cost-effective way … which will increase the district’s resiliency in short order.”

The district serves 191,000 residents in central and southern Marin. Its seven reservoirs make up about 75% of the district’s water supply. The reservoirs can hold approximately 80,000 acre-feet of water, about a two-year supply. An acre-foot is about 326,000 gallons.

The spillway project is an offshoot of capacity projects discussed in the water supply roadmap that was approved last year. That planning document set the stage for the district to significantly increase water supply for the first time since the 1980s. The plan seeks to add another 12,000 to 20,000 acre-feet of annual supply by 2035.

The big-ticket, albeit controversial, proposals that have dominated recent conversations include reservoir expansion projects ranging from $290 million to $1.29 billion, and potentially more. The projects have potential to increase capacity for billions of gallons of additional water to defend against the drought.

In April, the board narrowed its focus to study the potential expansion of only Soulajule Reservoir, Kent Lake and the area of upper Nicasio. At the same time, the board directed staff to separately pursue options for spillway gate modifications because it is a relatively inexpensive project with a quick turnaround compared to the other massive undertakings.

“The spillway modifications alternative selection and design project is really the district’s next step forward in enhancing its reliability, flexibility and resiliency of the district’s water system to help improve and service the Marin water customers,” Anaya said.

The spillway project will also build on findings of a yearlong study approved in May to examine the impact of climate change on the district’s seven dams, Anaya said.

District staff expect that consultants will have a proposal ready for approval and construction within about 18 months, Anaya said.

The board authorized staff at its meeting on July 2 to hire consultants Black and Veatch to work on the spillway project proposals.

“I think we’ve been looking at options for decades and now you guys are moving forward,” board member Jed Smith said. “I am thrilled about this opportunity to get 3,000 acre-feet, maybe a little bit more, in the most cost-effective way possible. I think this would be a historic increase to our dam capacity at Nicasio.”

Larry Minikes, a member of the Marin Conservation League, said that when looking at project design, every inch counts toward extra water storage, “so it’s going to be critical to determine what the maximum height these spillway gates can be.”

“It’s an exciting project, you know, a lot of us have been following for quite a while,” Minikes said.