3. Submarine Bars and Shoal Position . 



Underwater bars are sometimes visible beneath the water surface, but most 

 often their position may be located by waves breaking over the bar. The 

 breaking waves in aerial photos appear as light areas of definite shape (long 

 continuous lines) in the dark gray water. Submarine bars are usually elongated 

 features, with their long axis paralleling the coast. An analysis of the time- 

 sequence photo sets will determine the location, orientation, and distance from 

 the waterline of these bars. 



4. Wave Approach Angle . 



Wave crests before and at breaking, when traced on the base maps, provide 

 the plan view geometry of the waves and the effects of structures and natural 

 features on the wave patterns. Although waves are discernible on most photos, 

 underexposed photos may cause some difficulties. Breaking waves are easily 

 recognized as lines of white water and foam in the normal dark gray water. 

 Unbroken wave crests appear as lighter line tones. 



Wave approach angle data are not sufficient to determine the longshore 

 transport rate. A large number of aerial photo sets are required for the 

 extraction of net wave approach direction data. However, an aerial photo 

 analysis can, in some cases, provide the location of longshore transport nodal 

 points (reaches), using the following procedures: (a) by plotting the direc- 

 tion of wave approach (left or right of shore-normal) relative to shore at 

 closely spaced alongshore intervals (transects) on each base map, (b) by cal- 

 culating for each transect, using all the base maps, the number of time sets 

 when waves approach from left or right of shore-normal, and (c) by plotting 

 a ratio of left-to-right approaching waves on a master base map to determine 

 whether the preferred approach direction changes along the shore. The ap- 

 proach direction on many barrier islands changes near inlets, indicating a 

 possibility that the net direction of longshore sediment transport also 

 changes . 



5. Overwash Deposits . 



The base map procedure allows the tracing of any planform feature in an 

 exact scale from an aerial photo to a map. One example is overwash deposits 

 which are of particular interest in coastal engineering because these deposits 

 often represent large volumes of sand removed from the beach. An overwash 

 deposit is the material moved inland from the beach and deposited in a delta- 

 like form during storm events. The boundaries of the deposit are usually easy 

 to identify on aerial photos, and when the aerial extent of the overwash deposit 

 is planimetered, the volume of the deposit may be estimated by obtaining ground 

 measurement of the deposit thickness. 



IV. EVALUATION OF TECHNIQUE 



The value of any aerial photo analysis depends on the accuracy and reli- 

 ability of the procedure used; therefore, it is necessary to know the types and 

 magnitudes of errors that have been incorporated in the results. Two categories 

 of error exist. One is the error inherent in selecting features in aerial photos 

 such that changes in the position of the features allow something else to be 

 quantified. An example is the use of changes in the wetted bound position to 



II 



