Conceptual Model of Salinity Intrusion by Tidal Trapping

Abstract

Shallow bays along the fringe of an estuarine channel impact salinity distribution. The primary mechanism of impact has been termed “tidal trapping,” and is a result of the phase difference between the filling and emptying of the bay and the flow in the channel. This mechanism has been proposed as a major contributor to salinity intrusion in some locations. This paper creates a conceptual model to explain the increase or decrease in salinity intrusion based upon the channel tidal waveform and the character of the adjacent bay. Two idealized scenarios are used for this explanation: (1) Standing wave in the channel with a small phase lag in the bay tide; and (2) Progressive wave in the channel again with a small phase lag between the channel tide and the bay. This conceptual approach is useful for inspecting and understanding salinity intrusion processes in a complex estuary bay setting since the form of the wave can often be determined by generally available water surface gages along the estuary. A constructed numerical model of an estuary supports the conceptual model.