They conducted a theoretical analysis using radiation stress theory (Ch. 3) 

 and the various types of boundary conditions for these categories. Although 

 their theory neglected some of the terms in the equations of motion, their 

 results did show that the mean circulation in the basin is strongly affected 

 by the basin geometry. However, in the surf zone near the center of the 

 basin, all three basin types gave similar theoretical results. Dalrymple, 

 Eubanks, and Birkemeier (1977) therefore concluded that if a working 

 recirculation procedure was devised, the type (a) basin would reduce the 

 amount of return flow in the offshore region. The longshore current would 

 be closer to that found on an infinite beach, since wave basin recirculating 

 currents would not be included. 



The aim of recent research efforts at the Delft Technical University 

 (Fluid Mechanics Laboratory), The Netherlands, is to develop such a test 

 basin (Visser, 1980). The criteria for proper longshore current flows in 

 a laboratory basin are (a) a uniform current profile along the beach and 

 (b) a zero slope of the MWL in the longshore direction. The width of the 

 longshore current openings in both upstream and downstream waveguides and 

 the recirculation flow rate must be adjusted empirically to determine an 

 optimum combination. However, in a laboratory basin, MWL variation along- 

 shore is difficult to measure. Consequently, the recirculation flow rate 

 offshore and between the waveguides, Q , is used as an alternate criterion. 

 In this method, the wave basin geometry (waveguide openings) and the 

 pumped, recirculating flow rate, Qj-, are varied and a minimum Q found. 

 It is then hypothesized that this Qr gives the correct longshore current 

 flow rate, Q, for a uniform current profile. Lower Q value will cause 

 the excess Q to return offshore and raise Q^. Conversely, higher Q 

 values will generate a surplus wave circulation flow and also increase Q . 



The method was verified by a series of well-planned meticulous experi- 

 ments in a 16.6- by 34-meter wave basin with 20.9 meters between the wave- 

 guides. Regular waves are generated on a smooth concrete floor and 10:1 

 plain beach in 0.4-meter still water. The recirculation is through an 

 0.8-meter-diameter pipe beneath the beach to a pump on the updrift side. 

 All tests were conducted with waves of 2-second period, 10.5 centimeters 

 in height, and an incidence angle of about 21° at breaking. The depth- 

 averaged mean velocity at a measuring point was calculated as the mean of 

 the surface, bottom, plus twice the middepth velocity. The traveltime of 

 a dye cloud over 0.8 meter was used to calculate velocity in the surf 

 zone. To increase accuracy, two people made independent observations and 

 a minimum of 20 independent readings were averaged to give one surf zone 

 measurement at each depth. Also, the dye was injected at different wave 

 phases during these readings to eliminate the influence of the orbital 

 velocity on the measurement. These stringent procedures were relaxed in 

 regions with slower currents and less turbulence. Flush-mounted piezo- 

 meters on the beach and resistance wave probes measured MLW. . The 



38 



