RELATION BETWEEN IMMERSED WEIGHT AND VOLUME 

 RATES OF LONGSHORE TRANSPORT 



by 

 Cyril J. Galvin 



I . INTRODUCTION 



1 . Volume and Immersed Weight Rates of Longshore Transport . 



Two general formulas are presently (1978) in use for predicting 

 longshore transport rates from incident wave conditions. They are 

 usually identified as the energy flux method and the immersed weight 

 rate. 



The energy flux method empirically relates longshore transport 

 rate, Q, to a computed variable called the energy flux factor, P^ s 

 by an equation of the form: 



Q = K P 



is 



CD 



The equation of this form recommended for design in the Shore Protection 

 Manual (SPM) (U.S. Army, Corps of Engineers, Coastal Engineering Research 

 Center, 1977) is 



Q = 7,500 P £s (2) 



where Q is in cubic yards per year and P £s is in power per unit 

 length of shore line (foot-pounds per second per foot). The proportion- 

 ality constant, K, has units to balance the equation (cubic yards- 

 seconds per pound-year) . 



A number of investigators recommend using the immersed weight rate 

 of transport, l£, rather than Q (Bagnold, 1963; Komar and Inman, 1970; 

 Longuet-Higgins, 1972). The immersed weight rate leads to a dimension- 

 ally homogeneous equation with a dimensionless coefficient, instead of 

 the peculiar units that K has in equation (1). The immersed weight is 

 related to the volume rate by 



I £ = 27 a' y^ Q 



where 27 converts cubic feet to cubic yards, 

 sand in place, and y' is the difference 



(3) 

 a' is volume solids/volume 



. i _ 

 •s 



YA = Y Q -Y, 



between specific weight of sand grain, y s , 

 immersed specific weight of the sand grain, 

 immersed weight longshore transport rate is 



(4) 



and water, y • i.e., the 

 From equation (1), the 



I £ = (27 a' y£ K) 



Is 



(5) 



