542 INMAN AND BAGNOLD [CHAP. 21 



offsliore topograiDhy causes diflFerential wave refraction, the longshore currents 

 are de])endent on the gradient of breaker height along tlie beach, as well as on 

 the angle between the breaking wave and the beach. For such cases, the 

 ])ositions of the circulation cells are largely dependent upon the location of 

 zones of wave convergence and divergence. Also, it has been observed that 

 ])oints, breakwaters, and piers all influence the circulation pattern and alter 

 the direction of the currents flowing along the shore. In general, these obstruc- 

 tions determine the position of one side of the circulation cell. 



Field measurements of longshore currents (Inman and Quinn, 1952) show 

 that the velocity progressively increases from zero, immediately downstream 

 from a rip current, to some maximum value just preceding its seaward deflec- 

 tion at the next rip current. Also, the spacing between rip currents decreases 

 as the intensity of wave action increases, suggesting that the longshore flow 

 has some limiting value above which it breaks seaward into rip currents. The 

 maximum velocity of longshore currents appears not to exceed 2 to 3 knots. 

 The observed spacing between rif) currents ranges from about 30 to 1000 m. 

 Measurements following a period of low waves along a 10-mile section of 

 straight beach at Clatsop Spit, Oregon, gave a mean separation between ri]5 

 currents of 400 m and a standard deviation of 145 m (Shepard and Inman, 

 1951). 



Calculations indicate that rip currents along straight beaches discharge 

 seaward between 2 and 10% of the total volume which is estimated to be trans- 

 ])orted shoreward by the waves between rip currents ; whereas their seaward 

 discharge is of the same order as the estimated longshore component of the 

 shoreward transport. The implication is that the normal component of the 

 shoreward wave transport has no real existence relative to the shoreline, 

 because the general body of the water is being displaced outwards at an equal 

 rate. 



B. Magnitude of the Longshore Current 



Putnam, Munk and Traylor (1949) derived a relation between the longshore 

 current velocity and the characteristics of the oncoming waves in terms of the 

 angle a of attack and of a beach drag coefiicient Ca A\hich defines the frictional 

 resistance of the bed to the flow of the longshore current. Their relation, which 

 was based on considerations of energy and momentum and tested by field and 

 laboratory experiments, indicated that the beach drag coefficient ca varied for 

 medium sand from 0.04 in the laboratory to 0.008 in the field. 



Later observations over a Avider range of field conditions (Inman and Quinn, 

 1952) indicated that the relation of Putnam, Munk and Traylor gave values of 

 Cd which varied unaccountably by a factor exceeding 10^. These field observa- 

 tions show that rip currents, which were absent or only poorly developed in 

 the laboratory, were an important form of return flow ; also, longshore currents 

 in the field did not obtain a steady state of flow, but were continually fluctuating 

 or pulsating in response to the periodicity of the grouping of higher and lower 

 waves impinging on the surf zone, whereas in the model they were constrained 



