11/06/2013

Riverside Drive, (May 2005)

Assessment of Estuarine Tidal and Water Levels in South West WA

Simon Rodgers [email protected]

A history

• ~300 km of coast • Cyclone Alby (April 1978) • Major population centres • Damage (ie, Jetty) • Perth • Inundation (Bunbury, Busselton) • Mandurah • Erosion (Coast between Perth and Dunsborough) • Bunbury • Busselton • ~80 % WA population

Cyclone Alby - Bunbury Cyclone Alby - Busselton

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A history A history

• Cyclone Alby (April 1978) • Cyclone Alby (April 1978) • Flood Studies (circa1980’s) • Flood Studies (circa1980’s) – Based on extreme event analysis of Data – River flooding • State Coastal Planning Policy (2003) » 1 dimensional hydraulic modelling – Consideration of sea level rise within planning » Water balance modelling (Inflow/outflow Volume) » “bathtub” mapping » Additional freeboard for local variations within the estuary and wind/waves

Issues

• Need for more of a risk based approach • Incorporating sea level rise • Changes to estuary/ocean outlets • Dawesville Cut, tide gates, dredging, etc • Technological and model advancement • Varying levels within the water body • Wind effects • Joint probability

Typical methodology Results

• Extreme event analysis on the tide gauge data • Calibrated 2D hydrodynamic modelling – Inputs • Bathymetry – bathymetric surveys (DoT) and Lidar (DoW) • Tide gauge/near shore water levels • River flows • Wind • Mean sea level rise – Estimated water levels for a range of scenarios with several combinations of marine surges, river flood flows and sustained wind • Overtopping Analysis

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Results Results

• Design Water Levels – The marine surge is partially attenuated and reduced by the narrow and shallow channel between coast and main estuary body

Results Results

• Design Water Levels – The marine surge is partially attenuated and reduced by the narrow and shallow channel between coast and main estuary body

– Design flood water levels in the main estuary body are based on storm surge and not river flooding.

Results Results

• Design Water Levels – The marine surge is partially attenuated and reduced by the narrow and shallow channel between coast and main estuary body

– Design flood water levels in the main estuary body are generated from storm surge and not river flooding.

– Joint Probability (river flows, rainfall/stormwater, marine surge, tide)

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Results Results

• Wind and waves • Changes in Mean Sea Level – Water levels in the estuary are expected to rise directly – Wind forcing on the models resulted in ~0.1 to 0.2 metre proportional to a rise in mean sea level. increase in levels – A rise in mean sea level affects normal water levels well – Localised wave modelling for overtopping analysis – upstream (~20 - 50 km inland) differences between beaches, walls – River flooding still dominant mechanism outside of the main – Overtopping analysis identified that areas are currently estuarine water body. vulnerable to wave overtopping at present and provides approximate rates/volumes of water to be managed. – The frequency of flooding from overtopping can be expected to increase and the risk of inundation is also increased significantly as mean sea level rises

QUESTIONS ???

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