30 



surface recharge areas of aquifers, and water use management to avoid 

 extreme draw- down of the water table also can prevent or reduce compaction. 



3 . 5 EXAMPLES 



Mississippi River Delta - A striking example of subsidence due to 

 natural compaction is the delta of the Mississippi River. According to May 

 et al. (1983), the Louisiana coast is retreating at an average rate of 

 4.2 m/yr, most of which is attributed to erosion and inundation in response 

 to relative sea level rise induced by natural compaction. The levees built 

 along the river have cut off the source of sediment to the mud flats, and 

 their natural rate of compaction is causing some areas to sink at rates of 

 1 cm/yr or more (see also Table 10.1). Only in a small area of delta 

 formation is the rate of deposition greater than the rate of compaction. 

 This high rate of rise in relative sea level is drowning salt marshes and 

 causing existing small sandy barriers to migrate over the backbarrier muds, 

 further exacerbating the compaction. Penland et al . (1985) predict that at 

 present rates of sea level rise, the Chandelieur Islands and Isles 

 Dernieres will be lost during the next 100 years. Because the loading in 

 this region is naturally induced and the affected area so large , the only 

 functional remedial measure would be to remove the levees in the delta 

 region in hopes of restoring the sediment supply and deposition rate. 

 Although proven successful on a local scale, this is not a cost-effective 

 nor practical solution on a regional basis. 



Terminal Island. California - This classic example of the increase in 

 relative sea level due to man- induced subsidence demonstrates many of the 

 possible consequences of natural sea level rise. Due to withdrawal of oil 

 and gas from the Wilmington Oil Field, an area 5 km wide and 6 . 5 km long 

 subsided an average of about 1.5 m, and encompassed Terminal Island and a 

 portion of Long Beach, California. In some areas the overall subsidence 

 reached 7 m and resulted in considerable damage to harbor facilities as 

 relative sea level rose. This damage required substantial remedial efforts 

 including diking in areas of extreme subsidence, reconstruction of damaged 

 facilities, and bridge repair. The compaction was arrested by injecting 

 water into several of the existing wells in order to maintain pore pressure 



