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casing. A float- type gage monitors the water level. A strip chart and pen 

 displacement gage, mounted on a foundation that "rides" the ground surface, 

 records the subsidence as the pipes appear to protrude from the ground. 

 Several of these gages located in the same area, but penetrating to 

 different strata, provide information about the vertical distribution of 

 compaction. A single gage which penetrates to bed-rock will record the 

 total subsidence. 



3.3 IMPLICATIONS OF COMPACTION 



Compaction enters the discussion of sea level rise in two distinct 

 places. First is the obvious effect that relative sea level will rise as 

 ground or bed level subsides, resulting in deeper water in rivers and 

 estuaries, and increasing the likelihood of erosion and flooding in coastal 

 conununities . This will occur even without global sea level changes and 

 seismic activity. Second is the possible contamination of estimates of 

 eustatic rise due to compacting of regions where tide gages are located. 

 Gornitz (personal communication) indicated that 23 tide gage stations 

 located on compactable coastal plain sediments along the southern East 

 coast of the U.S. indicate a sea level rise of 2.96 + 0.78 mm/yr as 

 compared to 2.57 + 0.71 mm/yr for 16 stations in the north located on 

 crystalline rock. Although the means do not differ significantly, the 

 large variance in each may well be due to compaction and tectonic effects. 

 Compaction rates comparable to estimates of eustatic sea level rise (~ 1 

 mm/yr) are not obvious without detailed measurements using devices such as 

 that described. Because tide gages are usually located near coastal cities 

 where both loading by structures and groundwater extraction/depletion are 

 to be expected, the potential for compaction contamination of the 

 measurements exists. 



3.4 REMEDIAL MEASURES 



Of all types of subsidence, only that which is man- induced can be 

 prevented, arrested, and perhaps partially reversed. Extraction of oil and 

 gas can be accompanied by recharge of the soil stratum with water, as was 

 the case at Terminal Island, California to be discussed. Protection of the 



