displacement by horizontal plunger, displacement by vertical plunger, 

 and gravity inflow. Hurricane surges could be generated in the model 

 ocean with an independent pumped inflow and gravity outflow system sim- 

 ilar to that used for tide generation, except that the large amplitude 

 of the surges would require a very large pump and sump, pipes, and valves, 

 Since construction of the system would be costly, a displacement surge 

 generator is normally used. A horizontal displacement surge generator 

 consists of a reservoir (or basin), adjacent to and integral with the 

 model ocean, that contains a volxjme of water somewhat larger than that 

 of the largest surge to be studied. The surge is reproduced by program- 

 ing the forward and backward movement of a motorized, relatively water- 

 tight bulkhead located in the basin. The bulkhead is operated in such 

 a manner that its forward motion displaces water from the surge basin 

 into the model ocean at any desired rate, thus reproducing the rising 

 phase of the selected surge; its backward motion permits water to flow 

 from the model ocean into the basin, thus producing the falling phase of 

 the surge hydrograph. The bulkhead drive motor is a three-phase type to 

 permit the necessary reversal in direction, and a positive, infinitely 

 variable (PIV) speed control unit is installed in the drive mechanism to 

 permit a highly accurate control of the bulkhead speed. A vertical dis- 

 placement hurricane surge generator is similar in basic concept, except 

 that a large caisson is driven down into the reservoir to displace water 

 into the model or driven up out of the basin to allow water to flow out 

 of the model. The drive mechanism on the caisson consists of a variable- 

 speed power supply connected to hydraulic jacks on the caisson. For very 

 large models, the use of a displacement surge generator may be infeasible. 

 In this case, either pumped or gravity inflow can be used. In pumped in- 

 flow, a large pumping system and a large sump are required; in gravity 

 inflow, a large elevated supply sump is required in addition to a return 

 sump. For the Chesapeake Bay model, the gravity inflow system was se- 

 lected (shown schematically in Fig. 3-18). During operation for normal 

 tides, the supply sump is only partly full, but just before generation 

 of a hurricane surge the sump is filled with the volume required for the 

 surge to be studied. During the falling phase of the surge hydrograph, 

 water flows from the model by gravity into the return siomp. 



(7) Wave Generators . If sedimentation studies are required in 

 the entrance to the estuary, the model ocean is usually equipped with one 

 or more wave generators to reproduce the effects of ocean waves on the 

 transportation and deposition of sediments. The wave generators are nor- 

 mally a vertical plunger-type and can be quickly adjusted to produce the 

 desired wave height and period so that the model waves will move the model 

 bed material (sediment) in the same manner as prototype waves produce bed 

 movement in the prototype. The wave generators are mounted on wheels for 

 ease in moving from place to place in the model ocean to generate waves 

 from various directions. 



(8) Littoral Current Generator . During entrance area shoaling 

 studies (particularly movable-bed studies) it is often necessary to arti- 

 ficially generate littoral currents. The littoral current system consists 

 of an intake-outflow header with ports at regular intervals at each end of 



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