MORPHODYNAMICS OF A WAVE-DOMINATED RIVER MOUTH

L.D. Wright

Abstract


The mouth of the Shoalhaven River on the southeast coast of Australia is subject to direct attack by high energy waves and offers a general model of wave-dominated river-mouth deposition. During river floods seawater is completely flushed from the lower reaches of the channel and significant quantities of sandy bed load and suspended silts are debouched into the Tasman Sea. However, breaking waves cause intense mixing between the effluent and ambient waters while wave-induced mass transport and setup oppose and partially impound outflow. Unusually rapid deceleration and lateral effluent expansion result. Sediments accumulate in the form of a broad creseentic river-mouth bar with its crest situated about 2 channel widths seaward of the outlet and as broad shallow subaqueous levees capped by swash bars. Post-depositional shoreward return of sands by shoaling waves produces a constricted outlet. During low river stage wave setup enhances flood tidal currents and partially inhibits ebb tide outflow. This leads to a gradual shoreward migration of the river-mouth bar, a narrowing of the constricted outlet and to upstream migration of river-mouth sands into the lower reaches of the channel.

Keywords


river mouth; wave domination; morphodynamics

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