Highway 121 Flood Prevention—Upstream Runoff Reduction is Key

North Bay drivers know it doesn't take much to flood the intersection of highways 121 and 12 in Schellville, south of Sonoma. The area is flat, and nearby Sonoma Creek has accumulated silt over the years, turning it into a slowly meandering stream. When the San Pablo Bay tide comes in during heavy rain, Sonoma Creek gets slammed from both tidal waters and its upstream runoff, which rushes toward the Bay, causing damage to roads, pastures, crops, front yards and sometimes buildings.

Lack of funding for a sustainable fix, the expense of solutions such as levies, and the high value of vineyard properties that could otherwise be used for flood plains are some of the issues that have stood in the way of downstream solutions for decades. So agencies are looking upstream to prevent a key source of the problem—increased stormwater runoff getting into the creeks.

"We call it 'Slow it, spread it, sink it,' Mike Thompson, assistant general manager of Sonoma County Water Agency told Patch this week. "Our work is upstream, to see if we can hold back flows. A lot of areas in the watershed used to be more swampy but now there's a lot of water going into the creek," he said. "We're working to identify projects to hold back stormwater so it can infiltrate and go back into the groundwater, to reduce runoff."

State Fish and Game environmental scientist Adam McKannay, based in Napa, echoed the concept. Several agencies like these are working together to study the problem and come up with solutions.

"As upland development increases, the volume of water rushing down Sonoma Creek increases. Instead of taking a day to get from Kenwood to the end of Sonoma Creek like it used to, it now takes a lot less time. It's because of the hardscape up creek and the conversion from forest to vineyards."

'Hardscape' is the term used for urban surfaces such as asphalt and concrete that don't allow runoff to penetrate back into the ground.

The flooded highways brought a costly and inconvenient twist into the lives of several motorists last week, when they had to abandon their cars in the water at the highway merge at about 5 a.m. and wait up to 7 hours until tow trucks could go safely in, as shown on this Patch video.

"It's very frustrating for us," Tom O'Kane, interim director of Sonoma County Public Works, told Patch. "We can’t protect the public, which is our job. How many people have to have their cars ruined? This has been going on 20 years. It's amazing (flood prevention) has been held up."

He said public safety should be the primary concern.  

In some respects, Schellville residents may have made decisions decades ago that contributed to the lack of funding for flood prevention.

"We can do flood protection in the county where residents develop a mechanism to collects fees to do works in their streams," Thompson of Sonoma County Water Agency said. "Sonoma Creek end is in an area where in the 1950s and '60s, that area chose not to form a flood protection assessment area, so our organization hasn’t been involved there," he said.

Various agencies and stakeholders have come together to study the area and work on solutions. They are being gathered together by Southern Sonoma County Resource Conservation District, which is managing a study focusing on the southern end of Sonoma Creek Watershed. The area is between Sonoma Creek and Schell Creek, from immediately upstream of Highway 121 to San Pablo Bay. The study was funded by the Army Corps of Engineers, California Coastal Conservancy and Sonoma County Water Agency. It's called Lower Sonoma Creek Flood Management and Ecosystem Enhancement study. A copy of the report summary, and the full report, are attached to this story.

The study was completed last month and the RCD is disseminating the report to stakeholders through its website and presentations at local meetings.

These are the recommendations from the study:

A watershed‐wide approach is recommended, consisting of three main elements:

1) stormwater detention/retention in the upper watershed;

2) facilitation of a shift to flood‐compatible land uses along Lower Sonoma Creek, such as through the acquisition of easements on flood‐prone lands for seasonal flooding;

3) acquisition of flood‐prone lands for restoration to tidal wetlands.

Future implementation
1) Upper watershed stormwater management to reduce flood peak flows and reduce overbank flow into Schell Creek and across Highway 121 through attenuation or diversion into storage;

2) Elevating or flood-proofing some buildings or constructing small lengths of levee to protect remaining developments within the floodplain from most hazards;

3) Restore valuable tidal wetlands in Schellville area for endangered species habitat, managed retreat to protect some inland uses, carbon sequestration, promotion of tidal scour, and potential reduction in levee maintenance costs. There is significant potential to create valuable wetlands and expand areas on San Pablo Bay with good opportunity to create valuable mid-elevation, or middle, marsh habitat. Timing of such creation of middle marsh is important to
initiate marsh accretion and thereby attain marshes that are sustainable in the mid‐term, given sea level rise;

4) Initiate and continue landowner outreach to conduct appraisals, solicit funds, and negotiate purchase agreements and/or flood easements;

5) Leverage infrastructure investments (highway and rail) and initiate climate change adaptation planning to benefit climate change resilience and reduce flood risks in the Schellville area;

6) Collaborate and seek funding for watershed‐wide effort to establish parameters of a multiple benefit project involving elements of: water quality improvement, surface and groundwater storage, rainwater harvesting, use of recycled water, wetland restoration, and seasonal flood easements to allow continued agriculture. For more information and assistance, contact 707.794.1242, ext.

TELL US: We'd like to hear your experiences and opinions on the flooding. What do you think of these recommendations?

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