Search

The seasonal closure of tidal inlets is a common feature in microtidal, wave dominated regions where streamflows and/or longshore sediment transport are highly seasonal. This closure of the inlet obstructs navigation and deteriorates the flushing mechanism of the estuary/lagoon connected to the inlet, which could result in a degradation of the water quality. This paper presents results of a numerical study undertaken at Wilson Inlet, which is a seasonally open tidal inlet situated along the south western coast of Australia. The objectives of thestudy were to determine the dominant processes that govern the flushing rate (i.e. tidal exchange, wind, streamflow etc.), to test the response of the flushing characteristics of the estuary to different management options and to obtain a quantitative and qualitative estimate of the flushing rate of the estuary under different conditions. Theresults of the study indicate that streamflow is the major influencegoverning the flushing of the estuary while tidal exchange, wind andentrance channel location have a minimal effect. Regions of high residence times (low flushing rates) exist along the eastern and southern boundaries of the estuary. Under high streamflow conditions sea water does not propagate into the deeper parts of the estuary as the intrusion that forces into the estuary during flood periods is flushed out by the following ebb period. However, when the streamflow is less than the mean annual streamflow seawater propagates into the deeper parts of the estuary and is not flushed out by the ebbing tides. [ABSTRACT FROM AUTHOR]

Beach morphology and nearshore hydrodynamics were monitored over a number of sea breeze cycles on a beach with pronounced beach cusp morphology in southwestern Australia. The action of the sea breeze resulted in consistent changes to the incident wave field and beach cusp morphology, and induced a diurnal cycle of beach change. The morphological changes were accomplished without an apparent sediment gain or loss, but involved a redistribution of sediment within the cusp morphological system. During the sea breeze, the addition of locally-generated, short-period wind waves to the background swell resulted in an increase in wave height, a decrease in wave period and an intensification of the nearshore currents. The cusp morphology became increasinglysubdued due to accretion in the embayment and, to a lesser extent, erosion of the horns. After the cessation of the sea breeze, the wind-wave energy level gradually decreased and the associated windwave period increased. Accretion on the cusp horns was accompanied by minor erosion of the embayment and resulted in an accentuation of the cusp morphology. The build-up of the cusp morphology was a consequence of the morphodynamic feedback between the antecedent cusp morphology and the wave runup characteristics. The wave runup was diverted from the horn into the embayment, resulting in decreased backwash volumes and hence build-up of the horn. In the cusp embayment, increased backwashvolumes resulted in scouring and the suppression of potential swash events at the base of the beachface. Consequently, the proportion of infragravity-wave energy in the runup record was larger in the embayment than on the horn. Ten hours after the sea breeze had stopped blowing, significant amounts of wind-wave energy were still present. It is suggested that these wind waves were generated a distance of almost200 km south of the study area, implying that the presence of the local sea breeze may have regional implications for coastal processes. [ABSTRACT FROM AUTHOR]