(3) Freshwater Inflow Control . All tributary rivers with sig"- 

 nificant freshwater inflows are equipped with flow control devices of 

 various types. The inflows of streams with minor freshwater inflows are 

 combined with those of nearby tributaries of significant inflow or with 

 those of several other minor streams, and the combined inflow is intro- 

 duced into the model at a central point. Since most models are supplied 

 by large water systems subject to pressure fluctuations, each inflow 

 device is usually equipped with a constant head tank or a pressure con- 

 troller. Large inflows are introduced through Venturi meters or over 

 sharp-crested weirs (usually V-notch) . Intermediate flows are introduced 

 through Van Leer (California pipe) weirs or rotameters. Rotameters are 

 normally used for measuring very small inflows. If it is necessary to 

 vary the inflow at a large number of locations to reproduce a long-term 

 hydrograph or to vary the inflow continuously throughout a tidal cycle, 

 programable inflow devices will be required. For example, on the Chesa- 

 peake Bay model, digitally controlled multiple-orifice valves are used. 

 The desired flow is obtained by opening the appropriate combination of 

 orifices. 



(4) Skimming Weirs . The mixed saltwater and freshwater that 

 accumulates in the model ocean must be removed to maintain a constant 

 volume and source salinity. This is accomplished by skimming weirs which 

 remove a quantity of mixed water from the surface layer equal to the fresh- 

 water inflow to the model. Either a long, fixed-elevation, horizontal, 

 sharp-crested weir or a floating weir is used, depending on whether the 

 tidal range is small or large, respectively. Precise measurement of the 

 discharge from the skimming weir is made by a V-notch weir, a Van Leer 

 weir, or a rotameter, depending on the magnitude of the flow rate. 



(5) Saltwater-Freshwater Inflow Separator . Occasionally, it is 

 necessary to locate the upstream model limit within the zone of saltwater 

 intrusion. In such a case, the artificial control of salinity conditions 

 at the model boundary is necessary. This can present a considerable 

 operational problem, since the salinity may vary with time throughout 

 the tidal cycle and with depth. If the estuary is well mixed, two inflow 

 devices can be used, one for freshwater and one for saltwater. By vary- 

 ing the ratio of flow between the two inflow meters with time, the desired 

 time variation of salinity at the model limit can be achieved. If the 

 estuary is highly stratified and exhibits a distinct saltwater wedge, a 

 flow separator is necessary to permit simultaneous freshwater inflow at 

 the surface and saltwater outflow at the bottom. A freshwater inflow pit 

 and a saltwater outflow pit are separated by a horizontal plate which is 

 hinged so that its lower elevation can be set to that of the saltwater 

 interface. An observation pit with a glass panel is located beside the 

 separator to permit visual observations and measurements of the saltwater 

 interface. Freshwater flows are introduced above the separator plate, and 

 saltwater flows out under the plate by gravity. 



(6) Hurricane Surge Generator . A surge generator must be pro- 

 vided for models used to investigate hurricane surges. Only the methods 

 of surge generation used by the Corps are described, and they include 



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