How Will Rising Sea Levels Impact Mill Valley?

The flooding we see now in the Sycamore Triangle, Miller Avenue and downtown areas is only the tip of the iceberg, so to speak, in terms of what the Southern Marin Watershed Program predicts will happen over the next century.

With sea levels in Mill Valley and throughout the San Francisco Bay expected to rise 16 inches by 2050 – and as much as 55 inches by 2100 – on Monday night the county organization brought the City Council up to speed on the regional plan to combat these changes.

“You really have to work across jurisdictions,” said Liz Lewis, principal watershed planner for the Marin County Department of Public Works.

The watershed program is a collaborative effort between the City of Mill Valley, the County of Marin, and Flood Control Zones 3 and 4 to address 14 square miles draining into Richardson Bay. It includes the watersheds of Arroyo Corte Madera del Presidio, Coyote Creek, Ryan Creek, Sutton Manor Creek and East and West Creeks in Tiburon.

Taking a page from the Stetson report, which specifically addresses flooding in Mill Valley’s Arroyo Corte Madera del Presidio watershed, Lewis said the agency is developing hydrology models to measure how much water is flowing off of Mount Tam, and hydraulic models to determine exactly where the water is going and how much the creeks draining into Richardson Bay can carry.

Factoring in rising sea levels, engineers will evaluate the information and determine measures like where to build levees or dredge channels to protect against future flooding, she said.

Specifically, it will take into consideration elements such as the inherent sponginess of wetlands, where additional water can be stored, ways to get storm water into the ground sooner and then pump it slowly into the creeks, and areas of the floodplain that can retain water and still allow for the safe passage of emergency vehicles, for instance.  

The study began in 2008, and a draft of the initial evaluations and recommendations is expected to wrap up by 2015.

“We envision some sort of a master plan at the end of this process that will really lay out the projects that are most eligible for grant funding,” Lewis said.

Funding for the $680,000 study includes $100,000 from Marin County, $240,000 from Flood Zone 3, $240,000 from Flood Zone 4, and $100,000 from Mill Valley.

Bob Burton, president of the Marin County Flood Control District Zone 3 Advisory Board, questioned whether the city was getting enough bang for its buck in terms of how the funds are being used.

“The zone has spent most of its money outside the city limits of Mill Valley,” Burton said. “In some respects I guess that’s appropriate because Tam Valley is sinking and they have more consistent and steady flooding problems. But we all pay the same taxes whether we’re inside the city limits or outside.”

He did, however, applaud the overall study.

“I think,’ he said “that it highlights the real problems we’re going to face in the years ahead because of the obvious effects of sea level rise.”