c. Model Appurtenances, Instrumentation, and Measurements . Estuary 

 models are equipped with the necessary appurtenances to reproduce and 

 measure all pertinent phenomena such as tidal elevations, hurricane surges, 

 saltwater intrusion, current velocities, freshwater inflow, waves, littoral 

 currents, mass (dye) dispersion, heat dispersion, and sedimentation. Tide 

 generators were discussed previously; other model appurtenances are de- 

 scribed below. 



(1) Water Supply Sump . The water supply sump is the reservoir 

 from which saltwater is pumped into the model; the saltwater returns to 

 the sump from the model via the gravity return line. The sump is normally 

 of sufficient size to store the entire volume of the model (at least the 

 saltwater part of the model); therefore, during model operations, it is 

 usually less than one-quarter full. Because salt must be added to the 

 sump to maintain a constant ocean salinity, the sump must be rather tur- 

 bulent and have a good circulation to achieve rapid mixing and, if nec- 

 essary, rapid dissolving of salt. Proper design of the return line, 

 supply line, and supply pump bypass line can result in sufficient mixing 

 conditions. A supplemental mixing system is often required; however, in 

 this case, two pumps are generally required, one to lift water from the 

 sump to the model, and one to circulate and mix water in the sump with 

 the water returned from the model. 



(2) Salinity Control . Normally, freshwater is introduced into 

 an estuary model in the upstream reaches, moves downstream into the model 

 ocean, and becomes mixed with saltwater. Water must be removed from the 

 model ocean at the same rate as the freshwater inflow rate in order to 

 maintain a constant volume in the model-sump system. Since the discharge 

 is contaminated with salt, it cannot be recycled and must be wasted. To 

 maintain a constant ocean salinity, the salt lost in the waste-water dis- 

 charge must be replaced. Most estuary models have a relatively small 

 salt consumption, and the salt replenishment is accomplished by merely 

 dumping finely granulated salt into a riser on the gravity return line 



or directly into the sump. Models with large freshwater inflows normally 

 have a large salt consumption, and a salt brine injection system (Lixator) 

 is used which eliminates manual handling of the salt. A Lixator is basi- 

 cally a container filled with rock salt into which water is added to dis- 

 solve the salt (Fig. 3-19). The system is designed to discharge a fully 

 saturated brine. In addition to the loss of salt from the system, the 

 freshwater inflow causes an appreciable dilution in the model ocean. 

 Since increasing the ocean salinity only with the saltwater required for 

 tide generation is a very slow process, additional circulation between 

 the sirnip and model ocean is provided. The inflow and outflow lines carry 

 considerably more flow than required for tide generation, thus increasing 

 the exchange rate between model and sump. Models with a large ocean and 

 substantial freshwater inflows usually develop a thin layer of relatively 

 freshwater on the surface of the ocean. To minimize this effect, siq)ple- 

 mental skimming weirs are often provided which remove this water from the 

 ocean surface and return it to the sump where the salinity is increased 

 to the proper value. 



86 



