jetty and offshore to about the 60-foot depth. The Fire Island Inlet 

 problem was caused primarily by littoral drift trapped in the inlet, 

 thus starving the downdrift beaches and shoaling the navigation channel. 

 Therefore, it was necessary that sand movement along the beaches be simu- 

 lated in the model. The Fire Island Inlet model was first constructed as 

 a fixed-bed model and all proposed improvement alternatives were tested 

 quickly and economically to determine their effects on hydraulic condi- 

 tions in the inlet. After completion of hydraulic tests, the problem 

 area of the model was converted to a movable bed, and the most promising 

 alternatives from the results of the fixed-bed studies were investigated 

 further. In the fixed-bed studies, the entire model bed was molded of 

 concrete; in the movable-bed studies, the part of the model outlined by 

 a dashline in Figure 7-11 was molded of sand with a mean grain diameter 

 of 0.25 millimeter and a specific gravity of about 2.65. 



The model was constructed to linear scale relations, model-to- 

 prototype, of 1:500 horizontally and 1:100 vertically with a resultant 

 slope scale of 5:1. One prototype semidiurnal tidal cycle of 12 hours 

 25 minutes was reproduced in the model in 14.9 minutes. The computed 

 time scale of 1:50 was applied only to reproduction of prototype hydrau- 

 lic forces in the fixed-bed model and had no relation to time required 

 in the movable-bed model to reproduce observed changes in prototype hy- 

 drographic conditions. The results of the movable-bed verification 

 indicated that, using the operation schedule derived empirically, the 

 time scale for bed movement in the model was approximately 36 tidal 

 cycles (or 9.0 hours of model operation) to 1 year in the prototype. 

 In the final verification test, a total of 1 million cubic yards of 

 movable-bed material was introduced at the extreme east end of the 

 model beach to replace the quantity of sand moved in a westerly direc- 

 tion by the model waves. This rate of movement, applied to the empir- 

 ically determined time scale for bed movement, is in close agreement 

 with the computed rate of prototype littoral drift along this reach of 

 the south shore of Long Island. 



(8) Test Procedures . Eleven different plans were investigated 

 during the fixed-bed phase of the model study to determine plan effects 

 on tides, current velocities, tidal discharges, current patterns, and 

 flow distribution in the problem area. These plans involved (a) devel- 

 opment of the best location and dimensions for the littoral reservoir 

 and the rehandling basin (plans 2, 3, and 3A) ; (b) groins along Oak Beach 

 to divert the stronger ebb currents away from the beach and into the navi- 

 gation channel (plans 4 and 4B) ; (c) extension of the Federal jetty (plans 

 6 and 7); (d) dikes to partially or completely close the secondary channel 

 off the end of the existing sand dike and thus divert more flow into the 

 navigation channel (plans 8, 9, and 10); and (e) a deflector dike located 

 on the west side of the navigation channel to deflect some ebb flow from 

 the secondary channel into the navigation channel (plan 5). 



In the movable-bed tests, the effects of plans on movement and deposi- 

 tion of bed material were determined by direct comparison of test results 

 for existing conditions with those incorporating proposed improvements. 



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