The Levee and Channel System
Flood Control Channel Flood Control Channel |
Near the San Francisco Bay, portions of San Lorenzo Creek flood control channel are bordered on both sides by earthen levees to help control flooding during high storm flows. The upper portions of the channel are concrete-lined.
In the 1950s, engineers had only 10 years of stream gauge information about water flowing down from the hills and calculated that the amount of water that would flow during a major storm (a storm that has a 1% chance of occurring in any given year) would be 9,700 cubic feet per second (cfs). They designed both the concrete channel and the levees to hold significantly more water (10,500cfs) and both have provided over a half-century of protection.
Now we have 60 years of stream gauge data provided by the United State Geological Survey, combined with new original mapping techniques and flood engineering technology. Engineers constructed a new hydraulic model that simulates how water flows during different amounts of rainfall.
The new modeling shows that 16,100cfs of water could flow through the San Lorenzo Creek flood control channel during the 1% annual chance storm. That’s over 60% more water than engineers designed for in the 1950s.
For FEMA to recognize a levee as providing adequate flood protection, it must meet criteria found in Federal Regulations, which state that the top of the levee must be three feet higher than the water level that would flow during a 1% chance storm. The US Army Corps of Engineers ad the District determined that the levees do not meet that standard; they are not high enough. For that reason, FEMA will not recognize those portions of the San Lorenzo Creek levees as providing adequate flood protection and some properties will now be in FEMA’s newly designated flood-prone areas. In addition, the amount of water that the concrete-lined channel can hold was determined to be inadequate. The new FEMA maps will also show a widened flood-prone area along the channel.
