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Was One Building Responsible for 50% of the 2005 San Anselmo Flood?
On October 25th 2016, Matt Smeltzer made a presentation to the San Anselmo town council about ways to modify the San Anselmo creek in order to reduce future flooding. Mr. Smeltzer is a quick-witted goateed forty-something engineer of waterways. His company, Geomorph, was hired by the Town.
He spoke with the organized diction of an academic professional – no adjectives unless they were technical or absolutely necessary. Near the beginning of his presentation, Mr. Smeltzer called San Anselmo “ground zero” for flood control and noted that “building bridge 2 may be the most severe constriction” in San Anselmo Creek.
Building bridge 2 is located at 634 & 636 San Anselmo Avenue. It houses an Italian restaurant, a realtor, and an optometrist. It’s called “building bridge” because it’s both a building and a bridge. Its supporting structure narrows the creek down to 20 ft. wide. According to Mr. Smeltzer, for meaningful flood mitigation the creek would need to be 35 ft. wide at that location, as it was naturally, before the the building bridge was constructed.
Is San Anselmo ground zero for flood control?
A 100-year floodplain is an area, which has a 1% chance of flooding in any given year or once every 100 years. FEMA and mortgage companies require flood insurance for parcels in the 100-year floodplain because over the course of a 30-year mortgage, there is a 30% chance of flooding at least once.
According to FEMA maps, in the Ross Valley, the 100-year floodplain includes 0 parcels in Corte Madera, 8 parcels in Greenbrae, 128 parcels in Kentfield, 162 parcels in Fairfax, 224 parcels in Ross and 579 parcels in San Anselmo. Clearly, based on this alone, San Anselmo should be very interested when it comes to flooding.
But, has the town been taking this issue seriously enough? Let’s look at the FEMA maps for San Anselmo.
According to the most recent flood plain mapping:
- There is a 1% chance that 579 parcels in San Anselmo would flood in any given year (a 100-year flood).
- There is a 2% chance that 472 parcels in San Anselmo would flood in any given year (a 50-year flood).
- There is a 4% chance that 377 parcels in San Anselmo would flood in any given year (a 25-year flood).
- There is a 20% chance that 12 parcels in San Anselmo would flood in any given year (a 5-year flood). Households in this category should enroll their kids in swim team.
The flood that took place on December 31st 2005 in San Anselmo damaged government buildings, 140 businesses and 290 homes according to the San Anselmo Historical Society.
San Anselmo is indeed ground zero for flood control.
What would the 2005 flood have been like without building bridge 2?
On the morning of December 31st 2005, there were 5,980 cubic feet of water per second flowing into the town of San Anselmo. 4105 cubic feet of water per second was flowing in the creek channel and 1875 cubic feet of water per second was rushing through the streets, on the floodplain. If building bridge 2 had been removed and replaced with a naturally widened creek bed, the differences would have been dramatic, according to Mr. Smeltzer’s hydrological model. Most significantly, the model showed that only 1000 cubic feet per second would have been on the floodplain – a reduction of nearly 50%!
Mr. Smeltzer also estimates that “the resulting flood water surface elevation would have been reduced on hundreds of parcels and shallow flooding would have been avoided or eliminated on dozens of parcels." He estimated that the water level of the 100-year flood plain would have been lowered by 1.5 feet, which would have spared “dozens” of homes.
So, was one building responsible for 50% of the 2005 flood damage?
Sean Condry, San Anselmo’s Town Manager, said “Eleven years ago, after the 2005 flood, the County came up with a list of 180 in-channel measures for addressing flood control within Ross Valley.” That considered, it would be hard to say any one measure, such as removing building bridge 2, could be deemed responsible.
However, could the town San Anselmo still be legally liable for not purchasing and removing building bridge 2 and saving hundreds from flooding? At the same meeting, Sean Condry addressed the complexities of this question:
"One of the analogies I use, which is not necessarily a perfect analogy is: We have trip hazards on a sidewalk. If we don’t know about them, we have very little liability as a town, but as soon as it’s brought to our attention, it becomes a high liability. We actually have to make sure that we’re moving forward and addressing it. We now know that we flood. We’ve know it for a long time, but in 2005 we flooded and the town was sued along with other entities and I guess to some degree we prevailed. Now that we know we have an issue and we know that there are potential fixes, I believe that if we take building bridge 2 out and four or five hundred homes are out of the floodplain or flooding is reduced, but 5 or 6 more are in [the floodplain]. Is our liability higher or lower? I like to believe it’s a lot lower. “
This begs the question: Did the town of San Anselmo know the consequences of the blockage that building bridge 2 might cause? Was its removal one of the measures noted in the County list of flood control measures?
The answer on both counts is, yes. And, that information was well known.
Town Councilmember Ford Greene, who opposed the Memorial Park detention basin proposal, was quoted by the Marin IJ as saying,
“In terms of downtown San Anselmo, removing what is known as ‘building bridge No. 2’ is more important than any detention basin. That’s where the blockage is.”
Why does this story matter?
At this point, some readers may be wondering why this story of one building in one Marin town matters to the rest of us. The answers to that are many.
Climate change and the record rainfall this past winter made it clear that flood control is an increasingly important issue in Marin County and elsewhere. At the same time plans are afoot to take on major projects to address it.
The proposal to turn much of San Anselmo’s Memorial Park into a flood control basin was one such project and the hotly contested Lefty Gomez field proposal to develop the recreational space as a flood control basin in Fairfax, is another.
But, do we really need these massively disruptive and expensive projects?
We need to take a moment to ask ourselves this. If one small building in San Anselmo can have that much impact on the flooding of an entire town, wouldn't it make sense to do a much more careful analysis of every town in Marin before undertaking multi-million dollar projects like Lefty Gomez Field or Memorial Park, instead of after we spend all that money?
Considering what’s at stake, one would think we’d want to spend scarce funding as efficiently and productively as possible, right? Such analysis might even reveal that our current “sledge hammer” approach to flood control isn’t really needed at all.
Where are we now?
Unfortunately, San Anselmo is not alone in this lack of attention to major flooding risks. For example, the City of Mill Valley faced one of the most expensive lawsuits in its history in the aftermath of the 2005 flood and the majority of flood control measures that were recommended by consultant’s studies in the aftermath, have yet to be implemented.
It is highly likely this is true for other cities and unincorporated parts of the County, as well.
As for San Anselmo, on May 3rd, 2017, the town had a community meeting to specifically discuss building bridge 2. In a meeting notice, the town wrote:
This meeting will provide the public with information on the Real Estate acquisition and Relocation Assistance processes should an EIR be approved for a downtown flood mitigation project that includes the Building Bridge 2 alternative.
Better late than never?