The salinity regime and exchange characteristics of a shallow coastal bay system

Abstract

A shallow coastal bay system, consisting of several bay units each of which is characterized by quasi-homogeneous salinity and by relatively restricted interconnecting tidal passes, has been investigated with respect to its salinity regime and exchange characteristics. Particular emphasis is on the temporal features of the salinity and their predictability from known controlling factors. Data from prototype and hydraulic model observations for the Lake Pontchartrain-Lake Borgne system have been used for the analysis. The salinity prediction model is formulated on the basis of the equations of continuity and of salt conservation. The derivation of prediction equations based on this model was made by the method of regression analysis and the derived predictor coefficients were examined in terms of the effective exchange efficiency through the passes. The prediction equations for the model were tested by reconstructing the salinity regime for two separate years as an initial value problem using only the inflow data and the control salinity of the outer bay as input. The stability and inherent errors of the forecasting relations is discussed. Spectral analysis applied to 12 years of prototype data (salinity and inflow) demonstrates the effectiveness of correlation functions, power spectra and cross-spectra of the variables involved in the description and interpretation of the salinity regime with respect to the frequency response characteristics of the bay system. The cross-spectra are examined in relation to the possibility of applying optimum linear prediction. It was found that better optimum prediction could be made by transforming the input and/or output variables. It is demonstrated that the prediction of the major features of the salinity regime can be made and the merits of the various approaches are discussed.