FLUSHING CHARACTERISTICS OF A SEASONALLY-OPEN TIDAL INLET - A NUMERICAL STUDY

The seasonal closure of tidal inlets is a common feature in microtidal , wave dominated regions where streamflows and/or longshore sediment t ransport are highly seasonal. This closure of the inlet obstructs navi gation and deteriorates the flushing mechanism of the estuary/lagoon c onnected to the inlet, which could result in a degradation of the wate r quality. This paper presents results of a numerical study undertaken at Wilson Inlet, which is a seasonally open tidal inlet situated alon g the south western coast of Australia. The objectives of the study we re to determine the dominant processes that govern the flushing rate ( i.e. tidal exchange, wind, streamflow etc.), to test the response of t he flushing characteristics of the estuary to different management opt ions and to obtain a quantitative and qualitative estimate of the flus hing rate of the estuary under different conditions. The results of th e study indicate that streamflow is the major influence governing the flushing of the estuary while tidal exchange, wind and entrance channe l location have a minimal effect. Regions of high residence times (low flushing rates) exist along the eastern and southern boundaries of th e estuary. Under high streamflow conditions sea water does not propaga te into the deeper parts of the estuary as the intrusion that forces i nto the estuary during flood periods is flushed out by the following e bb period. However, when the streamflow is less than the mean annual s treamflow seawater propagates into the deeper parts of the estuary and is not flushed out by the ebbing tides.