b. Model Verification for Tidal Constituents . Installation and op- 

 eration of an automated model data acquisition and control system (ABACS) 

 will increase accuracy and reduce time required for verification of tidal 

 inlet (or estuary) hydraulic models, either fixed-bed, undistorted-scale 

 or fixed-bed, distorted-scale. The flexible sampling rate (usually about 

 200 samples per model tidal cycle for each gage) and digital recording of 

 the data by an ABACS allow harmonic analysis and comparison with prototype 

 data defining the coefficients and phase for each tidal constituent at 

 various key locations within a tidal lagoon and at an open-ocean station 

 removed from the immediate influence of the tidal inlet. 



The ABACS can also rapidly and efficiently operate the model and 

 collect required data. Its principal functions are to provide automated 

 acquisition of wave and tide data in a format compatible for digital re- 

 duction and analysis, and automated control of model sensor calibration 

 and of wave and tide generators. 



The concept used is to force the model with the M2 tidal constituent 

 with the amplitude being correct at the ocean tide gage. A harmonic anal- 

 ysis is performed at all other gage locations corresponding to the proto- 

 type measurements, and the amplitude and phase (relative to the ocean tide 

 gage) are calculated and compared with the prototype data. Investigation 

 of the relative phases between various gages shows the areas that require 

 either more or less model roughness. All phases for the M2 constituent 

 are expected to be verified within 1°. In most cases, tidal elevations 

 are expected to be verified to within a maximum of ±0.1 foot (prototype) 

 in both tidal height and mean tide level. After verification of the M2 

 constituent, to ensure that the proper channel roughness is obtained, a 

 progressive tide can be constructed; an attempt should then be made to 

 verify a 14.765-day (synoptic period for M2 and S2 components) pro- 

 gressive tide (at U.S. east coast locations) using the prototype measure- 

 ments of tidal velocities for the final verification data. If additional 

 roughness is necessary, it will mostly be on mudflats or marsh areas. 

 Computations have shown that energy transfer occurs from the M2 con- 

 stituent to higher order harmonics as the wave propagates from the ocean 

 to the back of the estuary, and that this energy transfer is, at worst, 

 the same order of magnitude in both the model and prototype. 



The principal information required in the verification process is 

 tidal elevation data. Various types of tidal height sensors are used; 

 one particular type is a "bubbler system" or gage which measures small 

 hydrostatic pressure changes associated with changes in model tidal ele- 

 vations. The bubbler system consists of a high precision pressure trans- 

 ducer, a scani-valve. device for sequencing input ports, and multiple 

 pressure inputs. Wave gages can also be installed in the model but may 

 not be essential for the verification process. Velocities of tidal cur- 

 rents may be measured with miniature Price-type current meters or with 

 electromagnetic velocity meters. Electromagnetic current meters appear 

 quite promising for future model use. 



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