a. Waterline . 



(1) Identification . The line indicating the exposed beach-water 

 intercept. A sharp gray (landward) to black (seaward) tonal change 

 and the presence of white foam often help to define this boundary. 



(2) Potential Problems . The boundary position varies with 

 changes in wave conditions and with tidal elevation. For example, 

 the average beach slope in southern New Jersey is 0.03 and tidal 

 range is 1.2 meters; the horizontal position of the waterline 

 will vary 40 meters over a tidal cycle. The correction for tide 

 effect is made using tide data, beach slope, and the time period 

 the photos were obtained. 



b. Wetted Bound . 



(1) Identification . This is the line forming the boundary be- 

 tween sand saturated at the time of high tide and drier sand land- 

 ward of that limit (Stafford, 1971). Dry sand is light gray on 

 aerial photos. Dolan, et al. (1980) found that the wetted bound 

 (high waterline) moved an average of only 1 to 2 meters over a 

 tidal cycle. 



(2) Potential Problems . The wetter bound position is dependent 

 upon changing wave conditions, tidal elevation, and water table 

 fluctuations which cause variations from an "average" location. 

 Sometimes a debris line obscures this boundary. In a test at Ludlam 

 Beach, New Jersey, the wetted bound was identifiable on 80 percent 

 of aerial photos analyzed (Everts, DeWall, and Czerniak, 1980). 



c. Dune Line Position . 



(1) Identification . Where the frontal dune is relatively con- 

 tinuous, the seaward edge of vegetation may be used. 



(2) Potential Problem . If the coast is retreating, the loss of 

 dunes and vegetation may preclude using the dune line to determine 

 the shoreline retreat rate. 



2. Shore Structures . 



Shore-normal structures, such as groins and jetties, and shore-parallel 

 structures such as breakwaters, seawalls, revetments, and bulkheads, may be 

 traced on base maps using the different time-sequence sets of aerial photography. 

 This provides an approximation of when the structures were constructed, with the 

 accuracy of the approximation depending on the time interval between the aerial 

 photo sets. Plan view dimensions of the structures may also be measured. Prob- 

 ably the most important use of determining shore structures on the base maps 

 is in evaluating the effectiveness of the structures when the structure posi- 

 tion is coupled with shoreline change data. 



