Figure 30. Yearly mean sea level changes at Galveston, TX, 

 from 1908-1986. Subsidence of the land around 

 Galveston may be caused by groundwater withdrawal 

 and compaction 71 



Figure 3 1 . Late Quaternary sea level curves inferred from 



radiocarbon-dated samples along the U.S. coastlines 



(modified from Dillon and O'Doyle (1978) 72 



Figure 32. Net shoreline movement of a portion of the South 

 Carolina coast based on historical maps surveyed 

 between 1857 and 1983 79 



Figure 33. Morphologic indicators of littoral drift along natural 

 and modified shorelines. Natural features such as 

 rock headlands show accretion on the updrift side 

 and erosion on the downdrift side (A), tidal inlets 

 and spits show extension in a downdrift direction (B- 

 C), and beach ridge headlands show successive 

 growth on the updrift end influenced by the 

 development of coastal cells, which form shoreline 

 irregularities (D). Coastal engineering structures 

 including groin fields, jetties, seawalls, attached 

 breakwaters, and detached breakwaters (E-I) 

 generally show accumulation of sediment on the 

 updrift side, and reduced sediment supply on the 

 downdrift side 80 



Figure 34. Principles of obtaining subbottom seismic data 83 



