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No end to flooding in sight

When some people see cars stuck in rushing water at the intersection of Highways 121 and 12 in Schellville after a heavy rain, they simply roll their eyes.

Schellville has been the site of disastrous flooding for more than 100 years, ever since farmers “reclaimed” the natural wetlands at the urging of the federal government back in the late 1880s. The very nexus of that flooding is that intersection, which seems now to wash out with every downpour.

The first efforts to address flooding concerns date back to 1917 when a system of ditches and pumping plants were constructed in the former tidal marshes. Following a major flood and levee breaches in 1955, the U.S. Army Corps of Engineers did $20,000-worth of emergency work. Various other work has been done to stanch the flow over the ensuing decades, but as flooding resulting from this winter’s first major storm systems shows, the area remains vulnerable.

Caltrans completed work at these crossroads this summer, dramatically altering the intersection to address traffic safety concerns. The project, budgeted at $2.4 million, reconfigured the angle of the intersection and added a stop light in response to high accident rates. But nothing was done to address the flooding.

“That was never intended to be any kind of flood control project,” said Caltrans spokesperson Robert Haus. “This particular project was purely a traffic-safety improvement project.”

However, the Southern Sonoma Resource Conservation District, in coordination with the Sonoma County Water Agency and the U.S. Army Corps of Engineers and others, this fall released the results of an in-depth study of the flooding problem, though Caltrans had no part in the study, and did not consult the many hydraulic models created for the years-long effort.

“Studying the hydrology and the flooding possibilities at the same time you’re working on the road are two things that really should be done together, because roads become dams in floods, too,” said Brett Sanders, Ph.D., professor and chair of the department of Civil and Environmental Engineering, University of California at Irvine, who did not have any part in the Schellville study, but has extensive experience in hydraulic flood simulations and has done ground-breaking work in urban flood risk management.

Roads, said Sanders are like levees. “With any roadwork, you’re grading the land,” he said. “You’re going to change the way water flows and it would make sense to have a look at how flooding’s going to impact the project and also how the projects’ going to impact flooding throughout the floodplain.”

The intersection at Highway 121 is on a floodplain that had been tidal marsh. Hydraulic modeling done over the past few years by the firm ESA PWA for the Lower Sonoma Creek Flood Management and Ecosystem study, found that the Schellville floodplain was heavily affected by up watershed activity. Flooding below the highway is heavily affected by tides and San Pablo Bay water levels. Flooding above the highway is controlled by a combination of creek flows and high water levels downstream. During high tides and periods of heavy rainfall, these factors converge to wash out roads, flood properties and close businesses.

While Caltrans surely could have done more to consider the flooding there, Sanders is not quick to blame the agency’s intersection work for the severity of this year’s flood at that site. “It’s unlikely that paving a road over a few miles is going to effect the flooding so severely. Unless for some reason all that concrete funneled water into a low spot and that pooled up and there wasn’t proper drainage from that,” he said.

Sanders explained that several factors play into the worsening of flooding over time. One common factor is up-watershed development, which sends more water rushing faster across the floodplain. Since much of the floodplain is no longer in its natural tidal wetland state, it does a much poorer job of retaining water and sediment rushing into it. Water speeds up over the flat expanses and concrete, and strips soil away instead of depositing it. In fact, many areas of Schellville have lowered over time and some are now below sea level.

Climate change and sea-level rise are also factors, and ones taken into account in the district’s study. These are factors that could be significant over time and certainly might also explain greater frequency and ferocity of storms. But, said Sanders, “Development is the strongest driver of change. The ways we change the land surface has a stronger effect in most cases than climate change.”

PWA’s models analyzed scenarios for up to 5.5 feet of sea-level rise, and accounted for many different management strategies. These types of models can be used to calculate, according to Sanders,“basically, damages avoided.”

In the end, though, the current thinking among planners and engineers is not to mitigate or control flooding at all costs, but to manage the risks relative to the rewards. The district’s study came to the conclusion that a “watershed-scale” approach was necessary, one that takes into account the entirety of the 122-square-mile watershed. The study was limited to the lower Sonoma Creek watershed and did not address opportunities in the upper watershed, though the Sonoma County Water Agency began exploring such a study on its own in November 2010.

According to the report, “No distinct flood reduction alternative emerged” from the years of study, saying that most methods showed “minor benefits for significant costs.”

A new system of levees has been deemed unfeasible. “With sea level rise, the exposure to hazards from San Pablo Bay will increase over time,” the report found. One of the key strategies it advised is taking action up stream from Sonoma Creek, reducing the amount of water coming downstream through “attenuation or diversion into storage,” to reduce peak flows reaching Schellville. This has the added benefit of providing valuable water sources to the district during the summer.

The flooding “problem” arises, of course, not so much because water suddenly rushes the land where it hadn’t previously been, but because for more than a century, the land has been developed where riparian growth once slowed and spread the water upstream, and tidal marsh absorbed the water downstream. As such, one of the recommendations of the recent study done by the conservation district is to return lands, wherever possible, to natural tidal wetlands. But given that much of the land is now used for vineyard cultivation, this might be unrealistic.

Restoring wetlands provides wildlife habitats, creates buffer zones to absorb water, and is simply, “flood compatible.” And, at some point, this area could be converted into an area not unlike Sacramento’s Yolo and Sutter Bypasses, where the highway is elevated over wetlands that accommodate both agriculture and wildlife habitat.