Abstract
To estimate and understand recent subsidence, we collected elevation and soils data on Bacon and Sherman islands in 2006 at locations of previous elevation measurements. Measured subsidence rates on Sherman Island from 1988 to 2006 averaged 1.23 cm/year (0.5 in/yr) and ranged from 0.7 to 1.7 cm/year (0.3 to 0.7 in/year). Subsidence rates on Bacon Island from 1978 to 2006 averaged 2.2 cm/year (0.9 in/yr) and ranged from 1.5 to 3.7 cm/year (0.6 to 1.5 in/yr). Changing land-management practices and decreasing soil organic matter content have resulted in decreasing subsidence rates. On Sherman Island, rates from 1988 to 2006 were about 35% of 1910 to 1988 rates. For Bacon Island, rates from 1978 to 2006 were about 40% less than the 1926-1958 rates. To help understand causes and estimate future subsidence, we developed a subsidence model, SUBCALC, that simulates oxidation and carbon losses, consolidation, wind erosion, and burning and changing soil organic matter content. SUBCALC results agreed well with measured land-surface elevation changes. We predicted elevation decreases from 2007 to 2050 will range from a few centimeters to over 1.3 m (4.3 ft). The largest elevation declines will occur in the central Sacramento-San Joaquin Delta. From 2007 to 2050, the most probable estimated increase in volume below sea level is 349,956,000 million cubic meters (281,300 acre-feet). Consequences of this continuing subsidence include increased drainage loads of water quality constituents of concern, seepage onto islands, and decreased arability.
Highlights
Subsidence of organic and highly organic mineral soils in the Sacramento–San Joaquin Delta is a primary landscape altering process that threatens delta infrastructure and water supply for over 23 million Californians
We evaluated the relation of soil organic matter content and subsidence rates from
Based on the model runs that included variation in parameters described in Appendix B and evaluation of the range in subsidence rates for Bacon Island data, we developed three model input data sets for each soil series that represent the likely range of future subsidence-rate estimates
Summary
Subsidence of organic and highly organic mineral soils in the Sacramento–San Joaquin Delta is a primary landscape altering process that threatens delta infrastructure and water supply for over 23 million Californians. Drainage and cultivation of delta soils since 1850 resulted in subsidence on over 60 islands from one to over eight m (3.3 to 26.2 ft) san francisco estuary & watershed science below sea level (Thompson 1957). By reducing the landmass and resistance to hydraulic pressure from adjacent channels, subsidence has contributed to levee failure and island inundation. From 1930 to the early 1980s, over 50 delta islands or tracts flooded due primarily to levee foundation instability (Prokopovitch 1985). Flooding of Andrus and Brannan islands in June 1972 resulted in water-quality deterioration in the delta that temporarily prevented exports (Cook and Coleman 1973). Subsidence and levee failure cause local infrastructural damage, which historically has cost hundreds of millions of dollars (Prokopovitch 1985)
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