Tidal Inlet or Stream 



Spit 



Offshore Breakwater 



Figure 41 . 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 offshore breakwaters (E-l) generally 

 show accumulation of sediment on the updrift side, and reduced sediment 

 supply on the downdrift side 



Many possible datums can be used to monitor historical changes of the 

 shoreline. In many situations, the high water line (HWL) has been found to 

 be the best indicator of the land-water interface (Crowell, Leatherman, and 

 Buckley 1991). The HWL is easily recognizable in the field and can usually 

 be approximated from aerial photographs by a change in color or shade of the 

 beach sand. The datum printed on the NOS T-sheets is listed as "Mean High 

 Water." Fortunately, the early NOS topographers approximated HWL during 

 their survey procedures. Therefore, direct comparisons between historical 

 T-sheets and modern aerial photographs are possible. In order to calculate the 

 genuine long-term shoreline change, seasonal beach width variations and other 



106 



Chapter 5 Analysis and Interpretation of Coastal Data 



