these side boundaries do not affect flow conditions in the entrance. As 

 a general rule, the ocean part of an estuary model should have about the 

 same area as the estuary part of the model to avoid operational diffi- 

 culties with the tide generator. 



Finally, the upper (vertical) model limit must be established. The 

 bank line must be constructed to a somewhat higher elevation than the 

 highest tide level to be reproduced in order to provide some degree of 

 freeboard. This freeboard is generally taken as about 5 feet (prototype). 

 If hurricane surges are generated in the model, the model overbank area 

 must include all prototype areas subject to inundation by the highest 

 surge to be investigated. Again, a 5-foot freeboard should be provided. 



e. Field Data Required . Because of various scale effects in estuary 

 models and the attendant need for adjusting the model roughness, a large 

 amount of field data must be obtained to ensure that the model is capable 

 of reproducing prototype phenomena. This requirement is even more strin- 

 gent for those phenomena which are simulated rather than directly modeled. 

 For example, rather than reproducing a scaled wind field in the models, 

 the effects of wind action on the mixing of freshwater and saltwater 

 are simulated by fans which blow down on the water surface in a random 

 pattern or by bubbling air through the water column. Similarly, sedimen- 

 tation is simulated by developing an operating technique by trial and 

 error which will duplicate known shoaling patterns; however, no attempt 

 is made to actually scale the sediment or to determine the sedimentation 

 time scale by analytical means. 



Prototype surveys required vary widely with the characteristics of 

 the estuary and the problems to be investigated. Data required on most 

 estuary models include hydrographic and topographic surveys, tidal eleva- 

 tions, current velocities (magnitude and direction), salinities, fresh- 

 water inflows, and shoaling rates and patterns. In addition, data on 

 wave climate and dye and heat dispersion are often required. 



To ensure that the model results are valid over the range of tidal 

 and freshwater inflow conditions that can normally be expected to occur, 

 hydraulic and salinity field surveys are necessary for various tidal con- 

 ditions or freshwater inflows. Typically, two or three such surveys are 

 required. For example, if there is a wide variation of tidal range, sur- 

 veys may be made for neap and spring tides with normal freshwater inflow. 

 If tidal variations are small, surveys are made for various inflow condi- 

 tions without regard to tidal range. A long-term salinity survey may 

 also be useful where salinities are observed periodically at a limited 

 number of locations to determine seasonal fluctuations. 



(1) Hydrographic Surveys . In order for the model to be an 

 accurate geometric replication of the prototype, detailed hydrographic 

 surveys are required of the entire area to be included in the model. 

 National Ocean Survey (NOS) boat sheets may be used for this purpose; 

 however, the sheets have limited value because they frequently do not 

 show recent conditions. Project (condition) surveys by the U.S. Army 



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