Generation of Flood Dominance in a Mixed Tidal System: a Numerical Investigation of the Murray Mouth

Abstract

Flows in the River Murray were severely reduced between 2000 and early 2010, with the result that freshwater discharges did not reach the sea. During this period, the morphology of the Murray Mouth was strongly controlled by flood dominant tidal asymmetry, coupled with a high-energy offshore wave climate. Constant dredging of incoming marine sediments was required to prevent closure of the mouth and thus ensure the continuation of tidal flushing of the sensitive Coorong coastal lagoon. The mechanisms for the generation of tidal asymmetry in a mixed, mainly diurnal, tidal regime have not previously been well understood. This paper identifies the key drivers and unique factors responsible for the tidal asymmetry and morphodynamics of the Murray Mouth in the absence of freshwater outflows. Results of numerical hydrodynamic and morphodynamic modelling of the Murray Mouth are presented in conjunction with findings from analytical tidal studies. Tidal asymmetry in the Murray Mouth and Coorong coastal lagoon is found to be caused by both cyclic and persistent distortion of the tidal signal. The use of standard relationships between the M2 and M4 tides as asymmetry indicators is shown to be invalid for a mixed tidal regime. Cyclic offshore tidal asymmetry is attributed to relationships between the frequencies of major astronomic driving tides and may be a significant contributing factor to seasonal inlet closure in regions experiencing diurnal or mixed tides. Within the shallow, microtidal lagoon, bathymetry-induced tidal transformations are profound, and inlet and channel configurations have been identified as important controlling factors.