high discharge period, salinities throughout the bays increase gradually, 

 usually reaching a maximum in about September or October (also the period 

 of high use of irrigation water). These data also indicate that the pri- 

 mary source of saltwater into Vermilion Bay is through Southwest Pass, 

 with a secondary source through Atchafalaya Bay and East and West Cote 

 Blanche Bays. Therefore, the possibility was considered that closure 

 of Southwest Pass would reduce the rate of influx of saltwater into 

 Vermilion Bay to such an extent that pumping from the Vermilion River 

 could be continued for an appreciably longer period of time than is now 

 possible. 



(6) Purpose of Model Study . The principal objectives of the 

 model study were: (a) to determine whether the closure of Southwest 

 Pass would reduce salinities in Vermilion Bay to concentrations permis- 

 sible in irrigation; (b) if so, to determine the effects on the reduced 

 salinities of withdrawing 10,000 cubic feet per second for irrigation 

 and industrial use from the north and west parts of the bay; and (c) to 

 obtain data for use, if needed, in evaluating the effects of the South- 

 west Pass closure on fish and wildlife. 



(7) The Model . The Vermilion Bay model (Fig. 3-83) reproduced 

 a part of the Gulf of Mexico, all of Atchafalaya Bay, East and West Cote 

 Blanche Bays, Vermilion Bay, the lower reaches of the Atchafalaya and 

 Vermilion Rivers, and the lower reaches of other streams which contrib- 

 ute freshwater to the bay complex. The model was constructed to linear 

 scales of 1:2,000 horizontally and 1:100 vertically. 



(8) Test Procedures . Tides were reproduced by a tide generator 

 located in the Gulf of Mexico part of the model, which also contained 

 provisions for reproducing littoral or alongshore currents from either 

 direction. All freshwater tributaries were equipped with weirs for 

 metering freshwater inflow, and the model was operated with the Gulf of 

 Mexico part filled with saltwater to the salinity scale of 1:1. Because 

 the entire bay complex is quite shallow, usually less than 12 feet in 

 depth, surface wind waves play a significant role in the vertical mixing 

 of saltwater and freshwater, and this effect had to be reproduced in the 

 model to reproduce accurately the salinity regimen of the prototype. 

 Since the reproduction of the wind waves in the distorted-scale model 

 was infeasible, the mixing effects were simulated by oscillating fans 

 positioned to blow in a random pattern on the model water surface. 



Two conditions of freshwater inflow were selected for test purposes: 

 the first used prototype freshwater inflow data for 1954 (which repre- 

 sented a year of low inflow) , and the second used inflow data for 1955 

 (which represented a year of normal flow). For both conditions, the 

 model was first operated with Southwest Pass open to establish the 

 salinity regimen for existing conditions, and then the pass was closed 

 and the model test was repeated to establish the regimen following clo- 

 sure of the pass. 



180 



