III. FORCING FUNCTIONS AND MECHANISMS 



A number of natural forces, geometric features, fluid properties, and 

 sediment characteristics interact to create longshore currents, nearshore 

 circulations, and rip currents. The fundamental cause-effect principles 

 were first stated by Johnson (1919). Waves attacking the coast at oblique 

 angles produced longshore currents while those arriving normal to the coast 

 created rip currents. Thirty years later Shepard and Inman (1950) demon- 

 strated how wave ray convergence and divergence patterns resulting from 

 offshore bathymetry created regions of high and low breakers along the 

 coast and explained nearshore circulation cells that result. Offshore 

 return flows and rip currents occur where low breakers are found. The 

 direction and period of approaching waves were also a factor in whether a 

 longshore current system, a nearshore circulation system, or a mixed system 

 was observed. These conclusions on forcing functions (wave characteristics, 

 hydraulic water surface gradients) and mechanisms (offshore and local bathy- 

 metry) are still valid in explaining many coastal flows. A general classi- 

 fication of nearshore current systems by Harris (1969) is shown in Figure 

 12. Oblique wave attack produces longshore currents on plain, straight 

 beaches. Normal wave attack is accompanied by some mechanism that triggers 

 rip currents and also causes water surface gradients, producing currents 

 normal and alongshore, i.e., circulation cells separated by the rips. The 

 asymmetric case corresponds to the mixed system where the offshore flows 

 (rips) are found to meander or move along the coast with a net longshore 

 current translating down the coast. 



Other forcing functions (tides, surface wind stress, atmospheric 

 pressure) and mechanisms (wave interactions, boundary interactions) have 

 been postulated to help explain the space and time variations observed in 

 the field or laboratory. Continuous recording current meters at a fixed- 

 point location in the surf indicate velocity variations due to many causes. 

 The instantaneous velocity is due to wave orbital velocities, the rollers 

 and eddies from wave breaking, bottom and rip current return flows, mass 

 transport, tidal currents, wind-generated currents, currents due to mean 

 water surface gradients (however induced), density-driven currents, bed 

 shear-generated turbulence, and currents resulting from the excess momen- 

 tum along the coast produced by wind waves breaking at an angle. All 

 these currents can have components directed alongshore or onshore-offshore 

 during some time interval. Some are random or periodic so that no net 

 current results over a standard averaging period. The forces that create 

 these currents are discussed in further detail below. 



I. Forces Causing Currents in A.longshore Direction . 



a. Wind-Generated Waves . Averaging water wave orbital motion over 

 the wave period will result in a net longshore current under certain 

 conditions. 



(1) Momentum Thrust . Longuet-Higgins and Stewart (1962) set 

 the principles by which gravity water waves, when integrated over the 

 water column and averaged over the wave period, produce a net horizontal 

 thrust (force) above the local hydrostatic force. A physical description 

 can be found in Longuet-Higgins and Stewart (1964). The term "radiation 



