Goal: Evaluation of foreshore drivers and responses in the Peel-Harvey Estuarine System, to provide scientific knowledge supporting foreshore management.
Foreshore dynamics within Peel-Harvey Estuarine System were substantially modified by construction of Dawesville Channel in 1994, which substantially increased the water level range within the estuary, and modified the foreshore dynamics.
South Yunderup approach channel was excavated across a broad intertidal flat in 1971-72, and has been subject to ongoing accretion and intermittent dredging. The rate at which accumulation occurs has apparently increased since the last dredging exercise in 1999, particularly over the period 2002-2004. Concern has been expressed that the increased siltation rate may be caused by changes to the hydrodynamics of Peel Inlet subsequent to the opening of Dawesville Channel in April 1994. Changes brought about by Dawesville Channel include increased tidal range, a reduced seasonal water level range and a dramatic change in water quality within the Peel Inlet.
This report has been prepared to support design of replacement foreshore walling on the eastern side of Manjar Bay, along the Gibson St promenade. It provides an understanding of the design implications of different levels, assessing possible time frames over which adaptation may be required due to sea level rise, and describing potential management of inundation hazards over the proposed life of the facility.
This presentation provides initial outcomes from evaluation of foreshore drivers and morphology along the western side of Peel-Harvey Estuarine System.
A secular change in flooding within Peel-Harvey Estuary occurred from 1994 when Dawesville Channel was opened, to substantially increase tidal flushing and help alleviate water quality issues. Detailed evaluation of water level records within the estuary has been undertaken, including analyses for extreme events, mean sea level fluctuations, tidal harmonics and spectral variation of tidal residuals. These demonstrated changes to both tidal and non-tidal water level phenomena. Observation of tidal change has confirmed predictions based on numerical modelling. However, previously predicted changes to non-tidal fluctuations were largely qualitative, based on parallels from small interventions in channel hydraulics. Observed changes therefore provide a valuable quantification of theoretical behaviour. This includes demonstration of a relationship between non-tidal spectral frequency and signal amplification, confirming the estuary behaves as a spectral filter. Despite 24 years of data, conclusive description of changes to flood occurrence caused by Dawesville Channel remains elusive, as this period has witnessed unprecedented mean sea level variability, obscuring definition of extreme event likelihoods or submergence occurrence. Nevertheless, the water level record demonstrates tidal enhancement by up to 400%, alteration of the seasonal mean water level cycle and amplification of surges, with brief 1-2 day surges enhanced by about 300% and smaller change for longer period fluctuations. These changes indicate increased flood occurrence, but also demonstrate the estuary's most significant water level response is now due to a different type of flood event. Detailed understanding of changes to water levels within Peel-Harvey Estuary provides improved scientific explanation of altered estuary pressures.
Coastal hazards affecting the Shire of Murray foreshores have been assessed, to support development of a Coastal Hazard Risk Management and Adaptation Plan (CHRMAP). The Shire is located on the eastern sides of Peel Inlet and Harvey Estuary, with foreshore along the banks of the lower Murray and Serpentine Rivers. The Shire’s foreshore is substantially undeveloped, with urban development at Yunderup along the lower Murray, including South Yunderup Canal Estate, and a low-density semi-rural development at Birchmont, adjacent to Harvey Estuary. Evaluation has focused on the coastal hazards of erosion and inundation, which are the two principal hazards requiring assessment under the State Coastal Planning Policy SPP 2.6. General methods used for coastal assessment have been modified to account for the estuarine setting, including consideration of extremely low-lying land present in the lower Murray River and southern parts of Harvey Estuary. A further complication has been brought about due to opening of Dawesville Channel in 1994, which caused an abrupt change in estuarine water levels and consequently modified foreshore evolution. It is noted that the method is intended to provide ‘best-estimate’ hazard lines suitable for management triggers (i.e. CHRMAP application) rather than a conservative estimate of the coastal hazard zone, corresponding to Schedule One of SPP 2.6.
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