With summer in full swing, flooding may be the last thing on most people’s mind during the hot days.
But, right now is exactly when Flood Zone 9, which covers the Ross Valley, is preparing a wealth of capital improvement projects designed to turn the tide of floods during the long, wet months.
In May, the flood zone presented a list of improvements that would address the feared 100-year flood, as well as help with smaller flooding events.
“Right now, there’s no level of flood protection,” said Jack Curley, the engineer and project manager for Flood Zone 9. The provides a good point for the engineers to study and start with, said Curley. Though, statistically, that flood was a 100-year event, meaning it has just a one percent chance of happening any year, because of a lack of flood protection the probability of that level of flooding has grown to three to five percent in any year, he said.
Read about the history of the watershed and flooding .
Though there are almost 200 proposed measures in the report, coming out of an extensive hydraulic model from Stetson Engineering, the mitigation steps are primarily in three categories: increasing creek capacity, managing sediment and marshland, and building detention basins.
A full list of all proposed projects and the study report can be found on the Marin watersheds website here.
The Friends of Corte Madera Creek also outlines some of the key proposals in their Creek Chronicles here.
Curley said the hope is to start with expanding the creek capacity downstream before moving upstream, or else steps taken upstream could create flooding in lower-lying areas that don’t have the capacity to deal with the increase volume.
Expanding creek capacity comes in the form of simple steps like replacing bridges and culverts, shoring up banks, building bigger flood walls, and removing dams. Ideally, these projects would not only expand capacity but also deal with environmental issues like fish passage. But, said Curley, the flood district will work with each town to determine what works for them.
“We’ll basically say ‘here’s what you need in order to reach this level [of protection], what do you think?’ and then we’ll work on reaching a consensus,” said Curley.
The majority of the proposed measures are capacity improvement measures – with 46 steps pinpointed in the Fairfax Creek, 33 in the Sleepy Hollow Creek, 82 in San Anselmo, and six in the Corte Madera Creek, along with the Army Corp of Engineering project in Ross.
According to the Friends of Corte Madera Creek here are the key capacity improvement projects proposed (starting downstream):
Corte Madera Creek
· Remove timber fish ladder up-stream of the concrete channel.
· Construct a gradual transition from the natural channel to the concrete channel.
· Construct floodwalls upstream and downstream of the ladder.
San Anselmo Creek
· Enlarge the channel downstream of 634-636 San Anselmo Ave., by measures including replacing the Winship Avenue bridge, increasing channel capacity beneath the bridge at Tunstead Avenue, and modifying the channel beneath the creek-spanning building at 241-255 Sir Francis Drake Blvd.
· Either modify or remove the structure at 634-636 San Anselmo Ave., a critical constriction, and remove sediment from beneath the neighboring up-stream structures at 638-730 San Anselmo Ave.
· Modify or replace bridges at intersection of Center Boulevard, Sycamore and San Anselmo Avenues.
· Replace Madrone Avenue bridge.
· Replace the Sir Francis Drake Boulevard bridge downstream of Winship Avenue.
· Modify or replace the Sir Francis Drake bridge at Tunstead Avenue.
· Raise the building at 638-730 San Anselmo Ave.
· Replace Bridge Avenue bridge.
· Replace Nokomis Avenue bridge
Sleepy Hollow Creek
· Replace Taylor Avenue bridge.
· Replace Mountain View Avenue bridge.
· Replace Morningside Avenue bridge.
· Replace Broadmoor Avenue bridge.
· Evaluate potential conveyance capacity improvements at other bridges.
· Replace Sherman Avenue culvert.
· Replace Merwin Aveue bridge.
· Evaluate status of Spruce Road bridge.
· Replace Azalea Avenue bridge.
· Replace Scenic Road bridge.
The number of these projects that happen, however, will depend on funds and support from the towns and community.
After creek capacity steps are taken, said Curley, the next key step in the capital improvement plan are the .
Detention basins would allow engineers to redirect excess water into the basins during high rainfall events, easing the pressure on the creek.
The key spots being considered for detention basins are , Lefty Gomez Field, and Loma Alta Open Space. Red Hill Park is also a possibility – but only in the future when the new facilities need replacement.
The flood district is working with the owners of each of these public lands to determine how a detention basin would work.
“Everything is in the concept stage,” said Curley.
The . That $15 million proposal would retrofit the lake to hold more storm run-off, strengthen the dam, as well as increase storage, dredge the lake, fix some of the trails and improve the water quality. MMWD and Flood Zone 9 will find out about the grant award in September.
The flood district is also working with on a proposal for Lefty Gomez field by. The key to the proposal would be to make the field better for usage year-round by creating better drainage and turf and then to use it as a flood basin only during high-rain period.
“Our mantra is win-win,” said Curley.
In the next two years, Curley hopes to have plans for the detention basins in the works with the respective public agencies and to have construction underway in five years.
The “last hurdle” in the proposed projects is dealing with sediment management and tidal marshes.
The county has a complicated arrangement with the Army Corp of Engineers, who built the concrete channel in Ross. The Corp requires that the creek be dredged to a 250-year level, which costs between $12 and $15 million, but at the same time the Corp was never able to finish the project in Ross that would have provided 250-year level protection because they ran out of back in the 1970s. The hope is for the Flood Zone to decrease the requirements for dredging to a 100-year level of flood protection at a cost of just $2 to $5 million every seven years, and for the Corp to be able to finish the Ross project if Congress allocates the money for it.
In addition, the tidal marshlands in the lower creek flush out much of the sediment in that area as water comes in and goes back out. If the tidal prism could be expanded, then dredging might be stretched to only every nine years.
There are countless variables, though, to implementing the extensive list of projects into a natural environment that is constantly changing.
To do everything one of the things proposed by the flood zone would cost $123 million – money that isn’t available right now.
Flood Zone 9 collects an average of $125 fee per parcel throughout the district – Larkspur, Corte Madera, San Anselmo, Fairfax, Greenbrae, Kentfield, and Ross – following a controversial 2007 vote on the flood fee. The balloting process of the mail-in vote, which narrowly passed, was challenged in court by San Anselmo Mayor Ford Greene and was upheld last year by the California Supreme Court.
The fee raises just over $2 million per year for the flood zone and has collected about $8 million so far, said Curley. The study from Stetson Engineering cost the flood zone $640,000 and nearly $1 million in total has been spent to date on that hydraulic model and study, staff time, and some on the lawsuit.
Only about $40 million is expected to be collected in total from the flood fee – well short of the $123 million bill. That means the flood zone will work to leverage money to get matching grant funds from a variety of sources and will prioritize projects as money comes in.
“In 10 to 20 years, we should be able to make a significant dent in this,” said Curley.