world, including physical displacement, pneumatic displacement, pumped 

 inflow and gravity outflow, and gravity inflow and gravity outflow (with 

 pumping between the return and supply sumps). Only the latter two systems 

 (used by the Corps of Engineers) are discussed in this section. 



The type used most extensively by the Corps is the pumped inflow and 

 gravity outflow system, shown schematically in Figure 3-15. This system 

 is programed (mechanically, pneumatically, or digitally) to maintain a 

 differential between a constant-pimped inflow of saltwater to the model 

 and a variable gravity return flow to the supply sump as required to 

 reproduce the desired tidal characteristics at a control station located 

 in the ocean or in the lower part of the estuary. In models with a very 

 large tidal volume, it is often desirable to provide a supplemental con- 

 trol on the pumped inflow by installing a programed valve on the inflow 

 line. The pumped inflow is reduced during the falling tide and increased 

 during the rising tide to reduce the fluctuation in discharge through the 

 gravity return line. With mechanical or pneumatic programing of the out- 

 flow, the system is generally limited to repetitive reproduction of a 

 single tide. The control unit must be reprogramed to reproduce any other 

 tide. 



Since computer or digital control offers a high degree of flexibility, 

 any desired sequence of tides can be reproduced in the model. A computer 

 control system can also be used to convert analog signals received from 

 model sensors (water levels, salinity, velocity, and temperature) to 

 digital signals and store the data on magnetic tapes or disks for later 

 analysis. A schematic diagram of an Automatic Data Acquisition and Con- 

 trol System (ABACS) is shown in Figure 3-16. A supplemental control on 

 the pumped inflow, if required, can also be programed on the computer 

 control system. 



Another concept using the pumped inflow and gravity outflow principle 

 requires the xise of a movable overflow weir (Fig. 3-17). Saltwater is 

 pumped into a headbay between the weir and the model. The weir is pro- 

 gramed to move up and down as required to generate the desired tide, and 

 the overflow is returned to the sump. Alternatively, either a flap gate 

 or radial gate can be used as the weir. 



Gravity inflow and gravity outflow (with puirping between return and 

 supply sumps) is used in the Chesapeake Bay model. A schematic diagram 

 of the system is shown in Figure 3-18. The reason for using this scheme 

 is to provide a sufficient inflow capacity to generate hurricane surges 

 in addition to tides. 



Often, it is necessary or desirable to terminate an estuary model 

 downstream from the head of tide (i.e., upstream boundary within effects 

 of tide) . If the upstream tidal prism cannot be simulated by using a 

 labyrinth or simple storage basin, a secondary tide generator should be 

 provided. A simplified pumped inflow and gravity outflow or pumped out- 

 flow and gravity inflow system is generally used for this purpose. In 

 either case, water is removed from the upstream end of the model during 

 the rising tide and returned to the model during the falling tide. 



82 



