A regular failing of coastal practitioners is reliance on the notional concept of littoral transport, supplemented by assumption of direct relationships between environmental forcing (often solely waves) and alongshore sediment transfer. This ignores the capacity for morphology to alter pathways and rates of sediment transport. On a compartmentalised coast, such as southwest Australia, the capacity for sediment to transfer between compartments (mainly through headland bypassing) is related to the volume and distribution of sediment. Consequently, the simplified representation of wave-driven littoral transport, on its own, has limited capacity to represent regional-scale coastal sediment dynamics.
Illustration of how geomorphic frameworks affects regional sediment dynamics is available over both short and long time scales, with spatial scales around 20-30km for Cockburn Sound (Stul et al. 2008) and southern Geographe Bay (Eliot 2011), through to 300km (Searle & Semeniuk 1985). Within the Mandurah region, alongshore sediment supply has been measured as part of sand bypassing works, showing year-to-year variability that is not solely associated with wave conditions, including a substantial lag in supply following the protracted la Nina conditions from 2011-2013. Longer-term patterns of sediment dynamics are illustrated by the distribution of sedimentary coastal landforms (Gozzard 2009).
A partial explanation of observed variability in sediment supply can be developed through the concept of the beach as a sedimentary storage unit. Observed relationships between beach volume and environmental parameters have been demonstrated (Eliot & Clarke 1983; Masselink et al. 2001; Eliot & Travers 2011), including responses to sea level and anomalistic alongshore wind conditions. For other parts of Australia, relationships to wave conditions have been demonstrated (Turner et al. 2016). Variation of environmental parameters can therefore change the effective beach storage, increasing or reducing the available sediment supply.
Misunderstanding of geomorphic frameworks has been demonstrated in design and interpretation of impacts associated with coastal facilities. Near-field responses, typically involving updrift accretion and downdrift erosion, commonly provide the most rapid response. However, far-field responses, particularly those developed through changes to alongshore sediment supply, are accumulative and may involve more substantial volumes. A valuable demonstration of this process was provided by Guerin St groyne at Busselton, including its subsequent removal. Progressive shoreline erosion or accretion responses may occur along much longer sections of coast, in the order of kilometres, but are often obscured by natural coastal variability.
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