system brought about by weir-jetty construction and, to a limited extent, 

 changes in shoaling and deposition patterns. Hydraulic factors that can be 

 quantified are the effects of jetty construction on the tidal prism, the tidal 

 range in the bay, and the current patterns in the vicinity of the inlet. 

 These factors are not peculiar to weir jetties but rather pertain to all jetty 

 projects. If undistorted, with vertical and horizontal scales the same, a 

 fixed-bed model may be used to study the effect of waves on circulation pat- 

 terns, to estimate the level of wave protection afforded vessels navigating 

 the channel, and to a dredge in the deposition basin. Additionally, any 

 tendency for currents to move up against the jetty or weir structures can be 

 observed. Design modifications may then be made and evaluated in the model 

 that will preclude undermining the prototype structures. For relatively 

 simple weir geometries such as sheet-pile weirs, the part of the tidal prism 

 entering the inlet across the weir can be estimated from the model. How flow 

 over the weir varies with time can be studied along with the relative magni- 

 tude of ebb and flood current velocities in the navigation channel. 



2. Scour, Deposition, and Sediment Transport . 



Scour, deposition, and sediment transport in proposed weir-jetty systems 

 can be studied using either movable-bed models or fixed-bed models with tracer 

 materials such as coal, plastic beads, etc., placed on the bottom. Movable- 

 bed modeling technology has not developed to the point where reliable informa- 

 tion on bottom changes can be obtained. The models are difficult to calibrate 

 and verify, requiring large quantities of prototype data for this purpose. 

 They are also expensive to operate and the cost of obtaining data for verifi- 

 cation can be prohibitive. In some cases, even after careful calibration, the 

 results may be at best qualitative. 



The validity of results obtained from fixed-bed models with tracer mate- 

 rials used to simulate scour and deposition is quite limited. They can not 

 model changes in bathymetry but can be used to identify areas where scour and 

 deposition will probably occur. Any large-scale changes in bathymetry and 

 their subsequent effect on additional scour and deposition are not considered. 

 Scour and deposition patterns are detected by placing a uniform thin veneer of 

 a tracer material on the fixed bed of the model. The model is then run and 

 the redistribution of tracer observed. A tendency to scour is indicated by 

 removal of tracer while accumulation of tracer is interpreted as a tendency to 

 accrete. Some information can be obtained by putting a grid system on the 

 model floor and quantifying the relative amounts of tracer accumulated or 

 removed from each grid square and comparing this with the amount originally 

 present in the square. For example, scour near the base of proposed jetties 

 may indicate a potential problem with undermining of the structure. Currents 

 moving up against the structure could lead to hazardous navigation conditions 

 and currents moving through the deposition basin might reduce its effective- 

 ness in trapping sand. 



The efficiency of a weir in trapping sediment in longshore motion can be 

 estimated by tracer in a fixed-bed model. Tracer material is injected into 

 the model along the updrift beach and waves are allowed to move it alongshore 

 to the structure. Generally, a part of the tracer is trapped in a fillet 

 along the updrift beach; a part moves over the weir into the deposition area 

 and some may move along the weir and updrift jetty into the navigation chan- 

 nel. Various jetty layouts and weir configurations may be evaluated and an 

 optimum layout selected from a sand bypassing point of view. Such tests are 

 only semiquantitative but may still provide information on relative effective- 

 ness of various jetty layouts. 



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