midflow speed, material rolled and saltated along the bed. The use of high 

 flow speeds was found to be limited by the depletion of sand in the test 

 section because of the resultant high transport rate. The practical upper 

 limit on flow speed was found to be 72 cm/sec, and the mode of sand movement 

 at this high flow condition was full sheet flow. 



67. The Reynolds number is the dimensionless ratio of the inertia force 

 to friction force; two flows are dynamically similar if the Reynolds number is 

 equal for both. Use of the Reynolds number to model flow conditions presumes 

 that gravitational force is neglected, implying that the free surface boundary 

 is not of significance and that buoyancy balances the force of gravity in the 

 interior of the fluid. Reynolds numbers Re were calculated and compared for 

 both laboratory and typical field conditions to verify that the field flow 

 regimes were replicated in the tank as follows: 



Re = ™ (9) 



where 



V = midflow speed (laboratory) or longshore current 

 speed (field) 



D = characteristic depth, taken as the water depth in the tank 

 (laboratory) or average water depth in the surf zone (field) 



v = kinematic viscosity of water at 20° C, equal to 0.0100 cm 2 /sec 

 (fresh water) or 0.0105 cm 2 /sec (salt water) 



68. Reynolds numbers determined for the tank tests for the 42 and 

 72 cm/sec midflow speeds ranged from approximately 80,000 "transition to 

 turbulence," to 140,000 "turbulent," according to White (1979). Mean long- 

 shore current speeds measured during trap deployments at the DUCK85 field data 

 collection project varied from 11 to 31 cm/sec (Kraus and Dean 1987), with a 

 representative surf zone depth equal to 1.0 m, resulting in field Reynolds 

 numbers ranging from 105,000 to 295,000 "turbulent" (White 1979). Therefore, 

 typical nonstorm field conditions can evidently produce turbulent flow even if 

 the dissipation of wave energy due to breaking in the surf zone is not taken 

 into account. Comparison of the Reynolds numbers indicates that the labora- 

 tory conditions were comparable to those in the surf zone. However, it is 



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