IV. POTENTIAL APPLICATIONS 



1 . Coastal Engineering . 



A primary reason for coastal engineering interest in suspended- sediment 

 concentrations in the surf zone is to estimate the contribution of suspended 

 sediment to littoral transport. There has long been an interest in using 

 such concentrations, along with longshore current velocity and surf zone 

 area, to predict longshore transport (Watts, 1953). The measurements in this 

 report indicate a difficulty in characterizing average suspended- sediment 

 values in the surf zone since concentration rises rapidly near the bottom 

 (Figs. 19 and 20), and is influenced by many variables. In addition, con- 

 centration values are low, averaging less than 0.2 part per thousand in 

 the measurements reported here, and only occasionally rising above 1.0 

 part per thousand. 



Fairchild (1973) suggested that suspended-sediment concentrations in 

 the surf zone increase with wave height in the same trend that suspended 

 concentrations increase with depth in unidirectional open channel flows. 



The data also show that suspended sediment has a smaller median size 

 than the contemporaneous bottom sediment. This is expected since it is 

 easier for the turbulence to maintain smaller sizes in suspension. Such 

 a size differential provides the mechanism for sediment sorting, both in 

 the longshore and onshore-offshore direction. Data on this size differen- 

 tial should assist in the design of beach fills and in a better understand- 

 ing of longshore transport. 



2. Longshore Transport Example . 



Order of magnitude estimates of the contribution of suspended sediment 

 to the total longshore transport rate have been made by Watts (1953), Calvin 

 (1973), and Fairchild (1973). This section presents a modification of 

 previous examples. From the data presented, it appears that most of the 

 suspended sand is within an elevation E^ = 0.4 foot of the bottom, and that 

 C = 1.0 parts per thousand is a characteristic concentration within that 

 zone very near the bottom. A continuity equation for longshore transport 

 rate, Q, based on the amount of sand transported through the near-bottom 

 surf zone is: 



Q = 0.65 CE^WV^ , (3) 



where 0.65 is the conversion factor between concentration by weight and 

 effective volumetric concentration (Calvin, 1973, p. 965), W is the width 

 of the surf zone, and V^^ is the longshore current velocity. For the surf 

 zones in this study, W is 300 feet or less under ordinary conditions. 

 Usually, V£ is less than 1 foot per second (U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center, 1975, p. 4-47). Evaluating for Q 

 with C = 1 part per thousand, E^, = 0.4 foot, W = 300 feet, V^ = 1 foot per 

 second gives Q = 91,000 cubic yards per year. Evaluations of Q, thus 

 computed, are about 10 to 20 percent of long-term estimated transport rates 



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