Mount and Twiss Report to ISB

Executive Summary Based on the 2000 Record of Decision, an explicit goal of the California Bay-Delta Program is to stabilize and maintain the current configuration of channels and islands in the Delta. This effort is based on the conclusion that hydraulic integrity of the Delta is necessary for meeting water supply, water quality and ecosystem restoration objectives. Landscape level simulations of historic and projected subsidence of Delta islands indicate that the Delta landscape, including both channels and islands, is undergoing considerable change. These simulations indicate that it will become increasingly difficult and expensive to maintain the Delta levee system. Additionally, change in Delta landscape is not confined to gradual shifts. Major floods and earthquakes are capable of creating rapid changes in the Delta, with significant impacts to all CALFED Bay-Delta programs. Anthropogenic accommodation space, or that space in the Delta that lies below sea level and is filled neither with sediment nor water, serves as a useful measure of the regional consequences of Delta subsidence and sea level rise. Microbial oxidation and compaction of organic-rich soils due to farming activity is the primary cause of Delta subsidence. During the period 1900-2000, subsidence created approximately 2.5 billion cubic meters of anthropogenic accommodation space in the Delta. From 2000-2050, subsidence rates will slow due to depletion of organic material and better land use practices. However, by 2050 the Delta will contain more than 3 billion cubic meters of anthropogenic accommodation space due to continued subsidence and sea level rise. An Accommodation Space Index, which relates subaqueous accommodation space to anthropogenic accommodation space, provides an indicator of past and projected Delta conditions. While subsidence and sea level rise create increasing anthropogenic accommodation space in the Delta, they also lead to a regional increase in the forces that cause levee failure. Although these forces take many forms, a Levee Force Index can be calculated that is a proxy for the cumulative forces acting on levees. The Levee Force Index increases significantly over the next 50 years reflecting regional increases in the potential for island flooding. Based on continuing increases in the Levee Force Index and the Accommodation Space Index, and limited support for Delta levee upgrades, there will be a tendency for increases in and impacts of island flooding, with escalating costs for repairs and increasing threat to CALFED program elements. Additionally, there is a two-in-three chance that 100-year recurrence interval floods or earthquakes …