A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering of the Asian Institute of Technology, Bangkok, Thailand.

Thesis (M.Eng.) - Asian Institute of Technology, 1975

The M.I,T. Transient Salinity . Intrusion Model was used for the prediction of salinity distribution in the Chao Phya Estuary. The model is formulated in finite-difference terms using the one dimensional, tidal time, variable area equations of continuity, longitudinal momentum and conservation of salt. The tidal dynamics equations were solved by explicit scheme while the salt balance equation was solved by implicit scheme. For a given set of initial and boundary conditions, the instantaneous tidal motion and salinity distribution can be obtained throughout any specified number of tidal periods. The effect of boundary conditions was discussed and the schematization of Chao Phya Estuary from river mouth up to 116 kilometers upstream was presented. The model was calibrated by adjusting the Manning roughness coefficient. The calibrated Manning roughness coefficient was then used to predict the transient salinity distribution from the mathematical model for three periods each of which having 32 tidal cycles. Records of fresh water inflows, salinity and water surface profiles at the entrance were used as the inputs to the model during the transient runs. The predicted and observed salinity distributions were in good agreement. Finally, the maximum salinity distributions along the estuary at various regulated fresh water discharges were also presented for the purpose of salt water intrusion control.