IV. ANALYSIS OF BEACH PROFILE DATA 



1. Excursion Distance Technique. 



If successive aerial photos of a beach face are compared with each 

 other and a change in location of the beach is noted, then this change 

 is indicative of either a period of erosion or accretion. Horizontal 

 displacement of the planform position of any one point on the beach, 

 from one survey to another, is the excursion distance for that point for 

 the survey period. On an accreting beach, the excursion distance of a 

 point relative to its initial position is positive, and on an eroding 

 beach, it is negative. The rate of change of the excursion distance 

 with time is the excursion rate. 



If successive beach profiles are reduced to a common base line, the 

 excursion distance of each point on the profile indicates the magnitude 

 of the onshore-offshore movement. The relative magnitude of the excur- 

 sion distances between two or more points on the same profile identifies 

 and quantifies the change in beach slope between those points. Beach 

 excursions can be converted to volumetric changes for the entire active 

 profile by applying to the excursion distances a volumetric equivalent 

 factor. This factor was developed from measured changes at two piers 

 located along Wrightsville Beach (U.S. Army Engineer District, Wil- 

 mington, 1977), which showed that for a closure depth of approximately 

 8.23 meters, each meter of excursion was equivalent to 8.23 cubic meters 

 of change for the entire active profile per meter of beach front. 

 Equivalently in English units, for a closure depth of 27 feet, each foot 

 of excursion was equivalent to 1 cubic yard of change for the entire 

 active profile per foot of beach front. Consequently, excursion 

 distance analysis is a simple but powerful technique which is used to 

 identify and quantify both long-term beach changes and the response of a 

 beach to short-term impacts resulting from storm activity, beach fills, 

 and other man-induced changes. 



2 . Historical Events Affecting Excursion Distance Analysis . 



Meaningful interpretation of excursion distance plots can only be 

 performed if known short-term or sudden impact events are identified and 

 accounted for within the analysis. In order to do this, all major 

 erosion-causing storms and all man-related activities which cause 

 erosion-accretion during the study period must be abstracted from the 

 historical records. 



Table 4 lists all beach-fill changes reported along the study area 

 beaches from 1965 to 1974. The initial fill excursion distances in the 

 table were estimated by applying the volumetric equivalent factor of 

 8.23 cubic meters of change for each meter excursion per meter beach 



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