Reso 2016-063 - Laserfiche WebLink

Home

5.11 Levee Failure The Sacramento-San Joaquin River Delta and Suisun Marsh are vitally important to the Bay Area economy and environment and contain many levees. The region contains highly fertile agricultural land and provides a unique habitat to many estuarine animals. The Delta region contains critical infrastructure including pipelines, highways, and power and communication lines. The Delta is the hub of the California water system, providing water to 25 million people in the State and 3 million acres of farmland.55 The probability of levee failure is increasing over time due to sea level rise, increased flooding potential due to early winter snow melts, and the likelihood of an earthquake. An earthquake is the single biggest risk the Delta Region faces. If an earthquake occurs, levees may fail and as many as 20 or more islands could be flooded instantaneously. This would result in an economic impact of $15 billion or more. Some researchers have estimated the likelihood of a multiple levee failure disaster at about two percent per year. Little is known about the local faults in the Delta. These have only exhibited a low-level pattern of scattered small earthquakes since 1966, but are still believed to be capable of moderate to strong earthquakes (M>6.0). While local Delta faults contribute most significantly to the hazard at longer return periods, and will produce stronger shaking due to their proximity to the levees, the major Bay Area faults pose a greater risk to the Delta levees. While they are farther away and will produce smaller ground motions at Delta sites, earthquakes occur much more frequently on these faults. The Hayward fault, in particular, is the greatest concern for the Bay Area. It is capable of producing large earthquakes that will be devastating to the Bay Area and is close enough to the Delta to damage levees. Other Bay Area faults, such as the Concord and Green Valley, are also likely to produce earthquakes that will damage Delta levees. Additionally, the soils in the western delta are extremely weak and liquefaction will trigger at even low levels of shaking. Much of the land in the Delta Region is below sea level and is protected by approximately 1,115 miles of levees in the Delta and 230 miles of levees in the Suisun Marsh. The majority of these levees were constructed at heights of three to five feet high and were maintained by local landowners in the last 130 years to protect farm land from flooding inundation. As a result of land subsidence, sea level rise and increased demand for land in the delta, these levees have been raised and increased in length over the years. Today, most of these levees retain water 365 days a year, and carry additional loads during flood events. While levees of Delta islands fail frequently, these occurrences typically are not on islands within the nine-county San Francisco Bay Area. If one were to fail, lives and property could 55 ABAG, (2010)