Updrift morphological impacts of a coastal protection strategy. How far and for how long?

By examining a coastal zone with a long-term history of management strategies, the focus of this paper is on the evolution of a beach system on the updrift of a sequence of coastal interventions. Here, we review previous assessments of the natural morphodynamic adjustments of the Letitia Spit (New South Wales, Australia) in response to the construction of the Tweed River training walls (c. 1964) and a subsequent permanent artificial sand bypassing system (c. 2001) downdrift of our study site. In addition, the temporal and spatial scale of the changes on the sediment deposits is investigated for a period of 53 years (1967 to 2020). The results showed that while the immediate updrift sector (~1 km long) responds rapidly (from the first few months to two-three years) to the introduction of the coastal management structures, the extension of those impacts further updrift along the beach (~3 km) is gradual and takes decades. About two to three decades were necessary to reach a new equilibrium state after the accretionary period caused by the training wall obstruction of the littoral drift, whereas the erosional state derived from the commencement of the artificial bypassing continued for about one decade before the new equilibrium was reached. Overall, this paper contributes to the understanding of the evolution of an updrift beach in response to coastal protection works and highlights the importance of long-term morphological monitoring of the beach – from the dune to the depth of closure – for informing the planning of future coastal interventions and improving morphodynamic conceptual models focused on coastal structures and adjacent coastlines.