addressing climate change adaptation in regional transportation plans

The islands in the lower Bay-Delta are defined as those areas below mean sealevel (see Figure 8). These areas hold back water on a continuous basis and crestheights target the peak water conditions due to tidal fluctuations from the sea,peak flows from the rivers, or the combination of the two. Levee failures andsubsequent island flooding regularly occur; over 160 failures occurred in the lastcentury (DWR, 2009). Levee failures and flooding occur due to peak water levelconditions, but they can also take the form of what are called “sunny day” failuresduring which there are no adverse loading conditions.The Delta has yet to experience a substantial earthquake in its currentconfiguration. The seismic behavior of the levees in the Delta is a concern,however, because the levees have not been designed or tested for such loadingconditions and may fail via several different mechanisms (e.g., seismic liquefactionof the foundation or embankment soil, co-seismic deformation of the foundationor embankment soil, or post-seismic reconsolidation of the foundation soil). Thescenario that threatens disruption of the state’s water supply is an earthquakethat can result in multiple levee failures, flooding the freshwater into the belowsea level islands, and allowing saltwater intrusion to degrade water quality therebyshutting down water exports to the south (DWR, 2009).Because the levees in the lower Delta currently hold back water on a continuousbasis (in some places upwards of 8 meters) incremental increases in sea level orincrease in peak-flow heights will not have an appreciable impact on the seismicvulnerability. The concern is earthquake loading of the vulnerable levees, notrelatively small increases in the static loading from increased water level heights.This also holds true for any other asset or community in the lower Delta residingbelow mean sea level. Seismic levee integrity and static levee integrity are notnecessarily addressing the same failure mechanisms.The lower Delta islands are continuously dropping in elevation, below sealevel, due to a number of factors. One main factor is the loss of topsoil fromagricultural activities. An increase in average temperatures accelerating the dryingof peaty organic soils and an increase in wind storm severity could exacerbatethis process, resulting in lower island elevations, increased static levee loading,and higher levee vulnerability. Also of concern are the high water conditions anderosion that are associated with winter storms. While they might not cause thewidespread failure that may result from a seismic event, storm events have thepotential to result in a notable increase in levee failure.Changes in precipitation can have an influence on maintaining the saltwaterfront below the intake pumps for the water delivery to the south. Currently thesaltwater front is maintained primarily by controlling the release from Shasta Dam,APG: UNDERSTANDING REGIONAL CHARACTERISTICS PAGE 54