Status of Levees and Channels
Effects of Urbanization
New Data Available
What This Means for Potential Flooding
Implications for Flood Insurance
Flood Protection Plans
The San Lorenzo flood control channel and earthen levees have been found not to have sufficient water capacity during a major storm (defined as a storm having a 1% chance of occurring during any given year).
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| US Army Corps of Engineers Flood Control Channel. |
The District has been working with the Federal Emergency Management Agency (FEMA) to re-examine flood protection during major rainstorms. As part of a nationwide modernization initiative, FEMA is updating its Flood Insurance Rate Maps (FIRMs), which, you will be able to view online. Based on new information and analysis, these maps will show newly designated flood-prone areas and identify properties that will be required to have flood insurance.
The flood control channel and earthen levees were designed and constructed during the 1950s. The standard has always been to provide flood protection for water flows that might occur during the 1% annual chance storm. Engineers designed both the concrete channel and the levees to hold significantly more water than was expected to flow through them during a storm of that magnitude at that time, and the flood control channel has provided over a half century of protection.
What’s changed is both the intensity of rainfall and how water now flows from the hills through our urban areas to the Bay.
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| Houses near channel. |
Effects of Urbanization
Since the 1950s, water flow and drainage patterns in the urbanized lower watershed have changed due to more development and paved areas. Urbanization in 1965 was less than half the density of today (see related maps). However, the upper watershed remains only 10% developed. Urban areas have much more paved "impermeable" surfaces that don’t let water percolate down into the ground. As a result, there is now significantly more rainwater running off lower watershed urban areas into the San Lorenzo Creek flood control channel, rather than being absorbed into the ground as it is in the upper watershed.
New Data Available
The District has much more detailed information than it had in the 1950s, such as new stream flow data provided by the US Geological Survey (covering 60 years of storms in various creeks in the watershed), new digital mapping techniques and new hydraulic analysis of how water flows from the hills to the Bay during very heavy rains.
Based on this current data, the amount of water that would flow through the channel and levees during a major storm (the 1% annual chance storm) is about 60% more than was calculated during the 1950s.
What This Means for Potential Flooding
It’s important to note that both the earthen levees and concrete channel are well designed and maintained—engineers confirm that the flood control infrastructure already in place will continue to function as originally designed. The flood risk is from water over topping the levees or channel during very heavy rains (the storm that has a 1% chance of occurring during any given year).
Implications for Flood Insurance
FEMA has issued new Flood Insurance Rate Maps (FIRMs) showing exactly where the existing and new flood-prone areas are. Lenders will require those properties that are in the flood-prone areas to have flood insurance. The National Flood Insurance Program (NFIP), administered by FEMA, offers federal flood insurance through local insurance companies (you’ll find more information at www.floodsmart.gov).
After FEMA issues the new FIRMs, there will be a 90-day public appeal period and the District will hold community meetings. FEMA and NFIP representatives will be there to discuss the maps and flood insurance. The maps become final 12 months after that, likely in early 2009. Flood insurance will not be required until the maps are final, however you might consider purchasing insurance for this year’s rainy season. If your property is in a newly designated flood-prone area and you purchase flood insurance before the new maps are final, your insurance will be grandfathered in at current rates of a non-flood prone area, as long as you maintain continuous coverage.
Flood Protection Plans
Engineers are already doing preliminary analysis on several feasible solutions that could hold more water up in the hills during heavy rains, potentially in Don Castro and Cull Creek Reservoirs, and release water slowly when the rains subside (see Healthy Creeks and Flood Protection). The goal is to lower the elevation of water flowing through the flood control channel so that levees can be re-accredited, the flood-prone designation removed and flood insurance no longer mandatory.
