Consideration of uncertainty in sea level rise in Australia's most exposed estuary: A discussion on allowances under different epistemic uncertainties

Abstract

Water surface level exceedances and extreme flooding for Australia's most exposed estuary, Swansea Channel, was estimated using sea level rise epistemic uncertainty with symmetric and asymmetric shapes. Flood estimates were obtained using a simple hydraulic model what was applied within a statistical simulation, included atmospheric and oceanic forcing and their aleatory uncertainties. These predictions are then used to discuss vertical allowances for coastal planning. There are different allowance approaches to include sea level rise epistemic uncertainty ranging from asset independent (allowance includes a particular degree of sea level rise uncertainty) to asset specific approaches (allowance that ensures frequency of inundation does not increase). Regardless of the allowance approach, the sea level rise uncertainty distribution and its shape are expected to influence these allowances. The impact on flood estimates and derived allowances from symmetric and asymmetric uncertainties includes expected features of increasing water level variations and allowances in both time and in space (Swansea Channel becomes more hydraulically efficient as its depth increases). Using asymmetrical shaped uncertainty for coastal planning constrained by low risk tolerance would, for example, increase the 1% annual exceedance probability flood elevation that includes 99% of sea level rise uncertainty by ca 0.8 m along Swansea Channel when compared to symmetric uncertainties. If the future sea level rise uncertainty is indeed asymmetric then application of allowances based on the symmetrically shaped distributions underestimate possibilities of future extreme water levels and may be exceeded earlier than anticipated. Annual exceedance duration estimates indicate that in 2120, epistemic (sea level rise) uncertainty is greater than aleatory (weather related flooding) variational along Swansea Channel.