Post-storm recovery assessment of urbanized versus natural sandy macro-tidal beaches and their geomorphic variability

Abstract

After an erosive event induced by storms, beach recovery occurs under fair weather conditions. The geomorphic nature and rapidity of beach recovery following such high-energy events are, however, rather poorly documented. Storm Dieter hit the Belgian coast on 14–15th January 2017 causing widespread and severe damage to the coastal environment and infrastructures. This study focuses on two representative sandy macro-tidal beaches, one characterized by environmental settings with a natural dune-beach system composed of multiple intertidal bars at Groenendijk, and another by a nourished beach with devoid of bars and backed by an urbanized coastline at Mariakerke. Airborne LiDAR survey-based morphological changes were analyzed pre, post- and 4.5 months after the storm complemented by monthly topographic profiles to investigate and compare the beach response and recovery at the study sites. Despite site-specific environmental settings, sediment was removed from the backshore in both sites at a relatively similar net rate of −0.03 m3/m2/month equivalent to −0.001 m3/m2/day, and erosion also dominated in the intertidal zone during the storm. Deposition and erosion alternated on both the backshore and intertidal zones in the 4.5 month following the storm. However, the initiation of beach rebuilding was evident from 1.1 month after the storm. The beaches were largely recovered over the 4.5 months after the storm at a rate 2 to 3 times lower than that induced by the storm. The results showed a coherence in the accretional trend in the two sites, and especially the urbanized beach retained some natural ability to rebuild without recourse to post-storm artificial nourishment. The fast return to re-equilibrium in this macro-tidal environment may be partly explained by the fact that the eroded sediment caused by the storm was still locally present in the system and returned to the beach under low-moderate onshore-wave conditions during the post-storm period. A number of distinct morphological features such as the intertidal bars, embryo dunes and a backshore berm appeared in the course of net accretion. They participated in rapid beach recovery through efficient sediment redistribution with relatively low magnitude elevation changes occurring within, and across the backshore as well as the intertidal zone.