reproduction of average tidal voliimes and discharges, and little, if any, 

 attention is paid to current velocities. If the control tide gage is 

 located inside the estuary, at least limited tidal information is required 

 for the ocean near the estuary mouth in order to properly adjust the model 

 roughness between the ocean and the control gage. If available, data from 

 temporary National Ocean Survey (NOS) tide gages within the study area are 

 often useful. This process is usually repeated for various conditions of 

 tidal range and freshwater inflow to ensure that the gross model roughness 

 distribution is valid for a wide range of known conditions. 



In estuaries with extensive marshes which are inundated at high tide, 

 the alternate waterflow into and out of the areas comprises a considerable 

 part of the tidal prism. In such cases it may be necessary to accomplish 

 the tidal adjustment in two phases, since different types of model rough- 

 ness are used in the deep channel and tidal marsh areas. First, a neap 

 tide is reproduced in the model and the channel roughness is adjusted 

 until tidal elevations in the channels are reproduced as accurately as 

 possible. A spring tide is then reproduced, and the marsh roughness is 

 adjusted until proper tidal elevations are obtained throughout the model. 



Although a redistribution of the model roughness is made during the 

 current (and possibly salinity) adjustment, tidal elevations are checked 

 to ensure that the tidal reproduction is still accurate. Examples of the 

 tidal verification achieved in various estuary models are presented in 

 Figures 3-23 and 3-24. 



b. Currents. During a substantial part of the current adjustment, 

 the introduction of saltwater into the model is still unnecessary. After 

 it has been determined that average tidal volumes and discharges are being 

 reproduced with reasonable accuracy (tidal adjustment), the depth-average 

 prototype velocity is determined at hourly or 1/2-hour intervals through- 

 out a tidal cycle at several points on ranges across the estuary. The 

 lateral distribution of velocity (prototype) as a function of time during 

 the tidal cycle can then be determined for each velocity range and com- 

 pared to measurements in the model. Extensive alterations in the lateral 

 distribution of model roughness are usually necessary to bring the lateral 

 velocity distribution in the model into agreement with that of the proto- 

 type. The total amount of roughness between tide gages cannot be altered 

 significantly, because of the necessity of maintaining an accurate tidal 

 adjustment. During this stage of the model adjustment, it is unnecessary 

 to achieve an accurate reproduction of the absolute magnitude of veloc- 

 ities; only the lateral distribution is needed. Therefore, the prototype 

 velocity data need not be corrected for moderate differences in tidal 

 range between the tide which occurred when the prototype velocities were 

 measured and the tide reproduced in the model. However, the prototype 

 velocity observations for each range must all be made during a single tide 

 or corrected to represent conditions for a single tide. As for the tidal 

 adjustment, the process is repeated for various conditions of tidal range 

 and freshwater inflow to ensure that the lateral roughness distribution is 

 valid for a wide variety of known conditions. 



98 



