A GUIDE FOR ESTIMATING LONGSHORE TRANSPORT RATE USING FOUR SPM METHODS 



by 

 Philip Vitale 



I. INTRODUCTION 



This report presents guidelines for calculating estimates of longshore 

 transport rates which are important in most coastal engineering projects. The 

 step-by-step procedures described herein are condensed from Sections 4.53 and 

 4.831 of the Shore Protection Manual (SPM) (U.S. Army, Corps of Engineers, 

 Coastal Engineering Research Center, 1977). 



II. DEFINITION OF TERMS 



The longshore transport rate, Q, is the volumetric rate of movement of 

 sand parallel to the shoreline. Under most conditions, longshore transport 

 takes place in or near the surf zone and is caused by the approach of waves at 

 an angle to the shoreline. Q is expressed in terms of sand volume per unit 

 time (such as cubic yards per year or cubic meters per year). Longshore trans- 

 port per unit time to the right of an observer onshore looking seaward is Q^^- 

 Transport to the observer's left is Q^^- The gross longshore transport rate is 



The net longshore transport rate is 



On = Qrt - Q£t (2) 



Note that a negative Q^ means net transport to the left. 



The quantities Qp^, Q^f %i> ^^^ Qa ^^^^ engineering uses. For example, 

 Q^ is used to predict shoaling rates in inlets; Q^ is used for design of 

 jetties and for predicting beach erosion on an open coast; Q^^^ and Q^^ are 

 used for design of jetties and impoundment basins behind weir jetties. In 

 addition, Q^ provides an upper limit on Qj^, ^lt> ^i^d ^t- 



Another representation of longshore transport rate (referenced in literature 

 sources) is the immersed weight rate, l^, which is given in units of force per 

 unit time (such as pounds per second or newtons per second). The conversion 

 from Q to Ig, is 



I^ = (Ps - p3 ga'Q (3) 



where 



Pg = mass density of sand 



p = mass density of water 



g = acceleration of gravity 



a' = volume solids/total volume (accounts for the sand porosity) 



