Subsidence of Peat in California and Florida

Abstract

Subsidence of reclaimed peaty sediments in warm climate is mostly caused by biochemical oxidation and is relatively little known to engineering geologists. Two examples of such subsidence located in areas with major peat deposits, one in the Sacramento-San Joaquin Delta in California and the second one in Florida9s Everglades, described in the paper are good illustrations of the potentially enormous economic impacts of such subsidence. In the 2,835 km 2 Delta, subsidence of original freshwater tidal marsh peat locally exceeded 10–15 m. Initially flat Delta islands became saucer like depressions with surface submerged below the sea level and protected from flooding by some 1,125 km of man-made levees. Levee breaks and inundations of islands already caused multimillion dollar damages and costly repairs and costly pumplift dewatering of flooded islands. With progressing subsidence unavoidable future collapses of levees built frequently with poor materials and on poor foundations in a highly seismic area will cause irreversible flooding of the Delta. Both “soaking” of salt water from the San Francisco Bay and increased evaporation from flooded islands will increase water salinity in the Delta and jeopardize existing giant water conveyance from the Delta to the southern portion of the state. This will endanger the major agroindustry in the arid San Joaquin Valley which depends on importation of surface irrigation water. The annual crop value of the valley is about 7.5 billion dollars. In the Bell Glade area well studied subsidence caused by agricultural drainage of peat locally exceeds 3 m. Subsidence destroys highly valuable agricultural land and damages buildings, roads, and utilities. Data collected in Bell Glade indicates that such subsidence can be minimized by restriction of drainage, i.e., by properly designed agricultural technique. Engineering geologists should be aware of possibility of biochemical subsidence in organic soil and suggest 1) its proper monitoring using compaction recorders and common bench marks, and 2) its control and/or arresting by proper design of drainage or restriction of oxidation by blanketing of organic material.