salinity at the model limit can be varied with time throughout a tidal 

 cycle (and perhaps seasonally) and with depth (if there is a vertical 

 salinity gradient at that point). Such a control system is not only 

 costly, but it also complicates operation of the model. This presents 

 two serious drawbacks to the model testing capabilities: (a) the effects 

 of any plan under investigation on the upstream extent of saltwater in- 

 trusion cannot be determined, and (b) any condition in the model where 

 salinity conditions at the model limit are not known from prototype 

 observations cannot be reproduced. These are rather severe restrictions 

 to the use of the model in defining the environmental impact of projects 

 being investigated and defining the existing salinity regimen for condi- 

 tions for which no prototype data are available. 



Since the proper tidal conditions at the upstream end of the model 

 must be reproduced, it is necessary either to extend the model to the 

 head of tide or to provide a tide-generating mechanism at the upstream 

 model limit. This decision is usually made on the basis of the cost of 

 constructing that part of the channel upstream from the problem areas to 

 be investigated compared to the cost of a secondary tide generator. The 

 part of the channel that is well beyond the areas of investigation can be 

 reproduced as a labyrinth (Fig. 3-1). The space available for the model 

 site may determine if the use of a secondary tide generator is necessary. 

 If a secondary tide generator is used at the upstream model limit, the 

 change in tidal elevations, phases, or prism at that point which may be 

 caused by a plan under investigation cannot be determined. This is not 

 usually a serious limitation to the model as long as the study area is 

 well downstream from the model limit. 



To conserve shelter space or to keep the model entirely inside the 

 shelter, it is often necessary to introduce artificial bends in confined 

 channels. This procedure does not adversely affect the hydraulic or 

 salinity conditions of the model, although detailed investigations of 

 flow patterns cannot be made in the immediate vicinity of an artificial 

 bend. Examples of bends introduced into two models are shown in Figures 

 3-2 and 3-3. A labyrinth at the upstream end of a model is an extreme 

 example of folding the model to conserve space. In this case, the natural 

 form roughness may be greatly altered and extensive model roughness adjust- 

 ment may be required in the labyrinth. No model measurements other than 

 tidal elevations should be made in the labyrinth. 



The limits of the model ocean are somewhat nebulous to define. To 

 properly reproduce saltwater intrusion into the estuary, the model ocean 

 should extend seaward to a prototype depth contour at least 10 to 20 feet 

 deeper than any entrance channel which is investigated in the model. If 

 the entrance area is investigated, the seaward model limit should be far 

 enough offshore that the ocean boundary does not significantly affect 

 current patterns in the immediate vicinity of the entrance. If entrance 

 jetties are investigated, the offshore boundary should be far enough off- 

 shore that currents between the ends of the jetties and the model limits 

 are not adversely affected by the model configuration. The model limits 

 should be located far enough upcoast and downcoast of the entrance that 



