Monthly to decadal evolution of sandy beaches in the Pittwater estuary

Even if the total length of shorelines in low-wave environments like bays, sounds, lagoons and estuaries greatly exceed that of open-ocean coasts, the morphodynamics of such systems are poorly understood. Only few studies developed models to predict changes in these environments and these systems are often managed with the same policies as open ocean beaches. The Pittwater estuary is located in the north of Sydney, it is a drowned valley estuary with a well developed ood-tidal delta. The present study investigates the morphodynamics of four beaches in the northern part of this estuary, at two dierent timescales. The long term stability of these beaches have been examined using historical aerial photographs that allowto determine the position of the shoreline for the last decades. At the monthly to seasonal timescale, the morphological changes have been studied from beach surveys that are perform every one or two months since May 2016. Given that the survey campaign began just before a major storm that occurred in June 2016, this data set allow to study the post-storm recovery of these sandy estuarine beaches. The results of the present study showthat the beaches within the Pittwater estuary are evolving dierently according to parameters like distance from the inlet, exposure to ocean generated waves or beach shape. Examination of the aerial photographs suggest that two out of the four studied beaches are experiencing changes visible over several decades. Concerning the monthly to seasonal changes, detailed analysis of beach surveys indicated that sandy estuarine beaches can be strongly eroded during a storm event and that post-storm recovery can take several years up to decades. The comparison with two beaches located along the open coast in the Sydney area conclude that estuarine beaches have a much slower recovery rate.