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Marin County Flood Control
Reasons to question Flood Control District's claims of future flood benefits
In 2015, consultants sold the Marin County Supervisors on a plan to build a detention basin in Fairfax that was supposed to provide storage for 88 acre-feet of flood water. As built, that basin only holds 13.5 acre-feet of flood water. What effect does having 84.7% less storage capacity have on flood benefits claimed for 10, 25, and 100-year flood events in Environmental Impact Reports (EIR) and studies that predate the completion of this diminished basin?
Outdated studies should not be a basis for current and undocumented County claims of flood benefits from proposed projects.
At present, construction bids on San Anselmo Flood Risk Reduction (SAFRR) have been deferred while design concepts and hydraulic models undergo additional review by FEMA: if the models are flawed, the plan is flawed. Meanwhile, the County persists in unfounded claims that SAFRR benefits over 500 properties.
Where are the site surveys that prove claimed flood benefits in 10, 25, and 100-year events? Individual surveys providing elevations of at-risk individual properties and structures must be compared with project-caused increased floodwater surface elevations to define flood impacts. Recent attempts to dismiss flood impacts as ‘wet dirt’ do not adequately describe flood impacts.
CEQA prohibits ‘piecemeal' assessment of the environmental impact of plans that limit the public’s understanding of the cumulative environmental impacts of a project. That the San Anselmo’s Flood Risk Reduction project ‘stops' at the Sir Francis Drake Bridge is a clear example of piecemealing. Where are the surveys for downstream properties in San Anselmo, Ross, Kentfield, Greenbrae, and Corte Madera at risk for increased flooding from the both SAFRR and Corte Madera Creek Flood Risk Management Project (CMC FRMP)anticipated increased water surface elevations during 10, 25, and 100-year storms?
There is no question that the proposed projects increase channel water surface elevations. However, the County claims these increased flood waters will remain “in-channel,” which enables evasion of adherence to FEMA’s ‘no-rise’ guidelines for all properties downstream of SAFRR.
But is this assumption correct?
The 2021 Corte Madera Creek Environmental Impact Report (EIR) lists over 2400 feet of new flood walls. Why build costly flood walls if no out-of-channel flows are anticipated? By using piecemealing to discuss only SAFRR, does the County hope to push SAFRR through before they have to quantify and remediate impacts of both projects on downstream properties?
Residents from Ross to Corte Madera need site surveys of their individual properties, too. Downstream property owners are equally deserving of the right to know whether they will benefit or be placed at risk from the flood projects' anticipated increased water surface elevations. Surveys by licensed surveyors can enable informed comparisons of future impacts by documenting land and structure elevations as compared with new water surface elevations in 10, 25 and, 100-year events.
And where is the mitigation documentation for impacted properties to enable a valid comparison of the benefits received versus the costs due for 10, 25 and, 100-year storms? And, finally, what are the fiscal consequences to Marin’s National Flood Insurance Program and annual Community Service Rating costs if SAFRR continues to disregard FEMA guidelines?
The Army Corps of Engineers (USACE) Design Storm concept for the concrete channel was for a 250-year flood, but the Corte Madera Creek channel overtops when storm flows exceed 6-year events. This example of flawed oversight, diminished results, and mismanaged tax dollars is history that should not be repeated, but SAFRR incorporates design errors of the dysfunctional Corps project.
The 2021 EIR for Corte Madera Creek's projected 2400 feet of additional flood walls will add new weight and footings along the Corps 1971 concrete channel walls. These are walls the County describes as "seismically unsafe” in Grant Agreement # 4600012423. The County has purchased rights of way (ROW) for these walls.
Because flood walls from Ross to Corte Madera are in Unit 4,3,2,1 of the USACE Corte Madera Creek project, residents of these areas have a right to know whether the County will receive a variance from the Army Corps of Engineer’s prohibition of trees within 15 feet of either side of flood walls. If all trees are prohibited in that 30-foot-wide vegetation prohibition zone along flood walls, how many trees will be cut for the construction of those walls? And how high are these new walls?
Because I believe it is time to replace current dysfunctional flood management with an effective informed, more inclusive process able to deliver real benefits and to have a new Supervisor able to distinguish between flood realities and political aspirations, I urge a YES vote on “Measure F” and for Ryan O’Neil for District 2 Supervisor.