COMPUTATION OF LONGSHORE ENERGY FLUX 

 USING LEO CURRENT OBSERVATIONS 



by 

 Todd L. Walton, Jr. 



I. INTRODUCTION 



Prediction of sand transport rates along beaches is necessary to determine 

 dredging quantities at inlets, effective life of various coastal structures 

 such as jetties, and magnitude of erosion-accretion on beaches adjacent to 

 inlets. Most computations of sand transport rate have previously been deter- 

 mined by computing a wave parameter dependent quantity termed the longshore 

 energy flux factor Pj^g . Chapter 4 of the Shore Protection Manual (SPM) (U.S. 

 Army, Corps of Engineers, Coastal Engineering Research Center, 1977) gives 

 various equations for Viq as a function of wave height, wave period, and 

 wave angle with the shoreline at breaking. As wave angle is a difficult param- 

 eter to measure, an alternate approach is to use the longshore current as an 

 independent quantity with which to determine ^is> since the wave angle with 

 the shoreline is explicitly contained within the most acceptable formulas for 

 longshore currents due to breaking waves (e.g., Longuet-Higgins, 1970). The 

 present report incorporates the longshore current model (due to breaking waves) 

 of Longuet-Higgins to determine the longshore energy flux factor, which in 

 turn, can be used to estimate longshore sand transport rates. 



II. DATA SOURCE 



The computational technique in this report uses current observations from 

 the Littoral Environmental Observation (LEO) program. The LEO program was 

 developed by the Coastal Engineering Research Center (CERC) and is discussed 

 by various investigators (Berg, 1969; Szuwalski, 1970; Bruno and Hiipakka, 

 1973; and Balsillie, 1975a). In the LEO program nearly simultaneous visual 

 observations of breaker conditions (height, period, angle of approach, and 

 type), local winds, longshore currents, rip currents, and beach geometry are 

 made daily for a year or more. The selection of observation sites is not 

 generally hindered by lack of access to the beach which often limits the use 

 of instrumentation. Thus, depending on availability of trained observers, many 

 sites along a considerable segment of shoreline may be established using LEO 

 techniques . 



The longshore current is estimated by measuring the shore-parallel distance 

 and observing the direction that a sodium- fluoroscein dye packet injected into 

 the surf (between the breakers and shore) travels in 1 minute. Observation of 

 longshore current movement from the dye injections is representative of surface 

 movement at the injection site, but may not always reflect the movement of 

 water at depth or represent the average speed across the surf zone. As LEO 

 measurements include the width of the surf zone as well as the distance from 

 shore to the injection point of the dye, the longshore current can be treated 

 as a point measurement on a spatially variable (across the surf zone) long- 

 shore current, the longshore current chosen in accordance with a theoretical 

 profile having an assumed mixing constant. Balsillie (1975b) has shown that 

 the LEO measurements of longshore currents (across surf zone) correlate very 

 well with longshore currents calculated by the theoretical formula of Longuet- 

 Higgins (1970). 



