MMWD to study effects of climate change on dam system

The Marin Municipal Water District is embarking on a yearlong study to examine the impact of frequent, severe storms on the utility’s seven dams.

The district board authorized spending up to $1.06 million to evaluate the capacity of the dam spillways, and to use climate change projections to assess potential hazards.

The study is a response to a critical Marin County Civil Grand Jury report published last summer. The watchdog panel said dam safety plans for the Marin Municipal Water District and the North Marin Water District are failing to account for more regular “atmospheric river” storms brought on by climate change.

The grand jury recommended, among other actions, that the water districts update their dam hazard mitigation plans with the latest science on climate change effects on storms.

Elysha Irish, engineering manager for the Marin Municipal Water District, said it has a thorough dam safety program and that the state Division of Safety of Dams has determined the agency’s dams to be safe for continued use.

“This effort of confirming our spillway capacity is a proactive and precautionary review given projections of future atmospheric rivers,” Irish said. “The spillway capacity assessment is being performed to ensure the spillways are appropriately sized for our changing climate conditions.”

MMWD operates seven of the eight large dams in Marin. Six are earthen dams, and Alpine Dam is concrete. The reservoirs can hold up to about 80,000 acre-feet of water, about a two-year supply. An acre-foot is about 326,000 gallons.

The agency said its hazard plan was updated in 2022 and contains information on dam failure in relation to climate change.

In addition to the spillway capacity analysis, the study includes a subsurface assessment of the Bon Tempe spillway that will involve surveying cracks in the system.

The study would potentially do similar work at Peters dam at Kent Lake, Seeger Dam at the Nicasio Reservoir and the Soulajule spillway.

“This is really great work, and I appreciate the dedication to safety and being responsive to the grand jury report,” board member Monty Schmitt said. “I know that it’s the intention is to draw attention to dam safety as an issue that affects, frankly, all dams, especially the older they get, and especially given changing climate.”

Schmitt said considering the district is also looking at potentially increasing water storage, it would be good to know if one dam might be a candidate for a safety retrofit, because “that might significantly shift the cost-benefit.”

The North Marin Water District’s dam safety plan is incorporated into the countywide hazard mitigation plan. The plan was last updated in 2018.

Tony Williams, general manager of NMWD, said the district is in the middle of a study in partnership with the county flood district. The analysis of the Novato Creek watershed, where Stafford Lake and its dam are located, includes an analyses of the dam’s structure, incoming flows to the lake and other details.

A draft of the study is expected to be completed this summer, Williams said.

An engineering analysis of a proposed adjustable spillway gate at Stafford dam, which could help increase the reservoir’s storage capacity, is also expected to be presented in the coming months.

The grand jury report also recommended that Marin water suppliers form a committee to plan strategies to prevent potential dam failures.

Earlier this year, the two water districts and the county joined the Center for Western Weather and Water Extremes Water Affiliates Group in an effort to understand extreme weather better.

The group researches atmospheric rivers and other severe weather to improve water management, mitigate flood risk and increase water supply reliability.

The National Oceanic and Atmospheric Administration says atmospheric rivers are storms that move most of the water vapor out of the tropics. According to the Water Affiliates Group, heavy rainfall from these flows of condensed water is responsible for almost 85% of floods on the West Coast.

At last week’s meeting, Ranjiv Khush, the MMWD board chair, said he is interested in learning if there is agreement between the near-term projections by the Water Affiliates Group and the longer-term projections that will be revealed as part of the new study.

“Keep us updated on the results,” he told staff.