Levees and Earthquakes

Posted by cdj On November - 10 - 2011
In California, the Sacramento – San Joaquin – Bay Delta includes 1,115 miles of “levees,” many that function essentially as simple, year-round earth embankment dams providing valuable agricultural, recreational, and development land, an essential source of irrigation and drinking water for more than 23 million Californians, and ecological habitat for over 500 species of fauna and flora. (California Department of Water Resources). Because of the delta’s proximity to earthquake faults, seismic risk assessment of the levee system that considers possible strong motion, earthquake-induced breaches, flooding, soil liquefaction, soil subsidence, pipeline rupture, failure of organic peat foundations, etc. confounds an already complex ecological, economic, engineering, transportation, and political risk assessment. (For a sense of the complexity see: Navigating the Delta or USGS Bay-Delta Fact Sheet). Competing policy goals – improving delta levee system reliability and repair costs, reviving threatened delta ecosystems, and providing system redundancy that ensures reliable or alternative, high-quality water supply – aggravate earthquake adaptive resilience preparedness and response planning.

Map of Sacramento - Bay Area Delta - USGS

The scale and costs of responsible seismic upgrade of larger, more complicated and critical embankment dams are illustrated in the San Pablo Dam (located in El Sobrante, California not far from the Sacramento Delta, owned by the East Bay Municipal Utility District -EBMUD), a 145 ft high hydraulic fill embankment (sand, silt, clay) dam constructed in 1920 to provide a 12.6 billion gallon reservoir for drinking water in the Bay Area. After risk assessment, compacted buttresses downstream and upstream were added for seismic stability in 1967 and 1979. But the embankment and foundation soils were judged to be very susceptible to liquefaction until recent material characterizations using both CPT and laboratory analysis to complement existing soil borings suggested that shell materials of the dam are unlikely to liquefy but portions of the foundations could experience liquefaction in a large (Mw=7.5) earthquake on the nearby Hayward Fault. These analytical findings were incorporated into a revised remedial plan that used ‘Cement Deep Soil Mixing’ and enlarged downstream buttresses for seismic retrofit. Nevertheless, even this reduced seismic retrofit completed in 2010, cost more than $80 million dollars.

Deep Soil Cement Mixing, San Pablo Dam Reservoir - EBMUD

While business and economic recovery after natural disaster can seemingly be managed in wealthy countries, the social recovery process may be less reliable. (For example: the Kamaishi, Japan sea wall barrier and its reconstruction after the March 11, 2011 earthquake and tsunami damage – or the failures of the flooding protection system in New Orleans and redevelopment after the August, 2005 Hurricane Katrina.) The prudent course, i.e. choosing a preferred future state-of-affairs, is deciding before the natural disaster, what remedial actions must be performed now and what recovery actions will be realized after the event. In the USA, FEMA provides a levee system information page for those directly at risk of flooding from levee failures.
Postscript: In May, 2013, total cost of implementing the massive twin-tunnel system that would carry water from the Sacramento-San Joaquin River Delta and its accompanying delta habitat restoration plan over the 50-year permit term is now estimated at US $24.54 billion.

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