Climate change projections indicate a significant global sea-level rise (SLR) by 2100, potentially leading to substantial shoreline retreat and loss of sandy beaches. While increased storminess is generally expected to exacerbate coastal erosion, the long-term impact of extreme storms on coastal recession and its interaction with SLR remains ambiguous. The Bruun rule, a widely used method for predicting shoreline change due to SLR, simplifies the process and does not adequately account for the complex sediment budget dynamics, particularly those influenced by extreme storm events. These storms can redistribute sediment within the nearshore zone, potentially moderating long-term erosion. This research investigates the impact of extreme storm sequences on the nearshore sediment budget and its implications for long-term shoreline projections, particularly in relation to the limitations of the Bruun rule.

The Bruun rule, despite its limitations, is the most commonly used method to predict shoreline retreat due to SLR. It assumes a simple upward and backward translation of the shoreface profile, neglecting complexities such as sediment gains and losses. While the rule has been adapted to include sediment fluxes, accurate assessment requires a detailed understanding of the nearshore sediment budget. This budget encompasses cross-shore and longshore sediment transport, sediment production, and anthropogenic contributions. Extreme storms significantly influence this sediment budget, maximizing sediment fluxes and potentially impacting long-term shoreline change. Studies have shown that lower magnitude onshore sediment transport from the lower shoreface to the beach can contribute to coastal progradation under stable sea levels, and this process may also offset, or reverse, the effects of SLR. Previous research has also pointed to the role of energetic wave conditions in driving onshore sediment transport from depths beyond the depth of closure (DoC).

This study analyzed three high-resolution morphological datasets from Australia, the UK, and Mexico. Each dataset encompassed an extreme storm or storm cluster followed by a recovery period. The datasets included continuous subaerial beach measurements and detailed three-dimensional surveys of the upper shoreface before, immediately after, and approximately 12 months after the extreme storm events. Subaerial surveys used a combination of methods such as Airborne Lidar, UAVs, and RTK-GNSS, while subaqueous surveys employed single-beam and multibeam echosounders. Seamless digital elevation models (DEMs) were created from the combined data, and DEMs of Difference (DoD) were calculated to assess changes in beach elevation. Sediment budget error analyses were conducted considering survey errors, focusing on statistically significant morphological changes. The long-term DoC was determined using 41 years of wave reanalysis data, and wave orbital velocities at the DoC were estimated to assess sediment transport potential. The observed sediment gains were then related to equivalent years of shoreline retreat predicted by the Bruun rule under different emissions scenarios (SSP1-2.6 and SSP5-8.5).

The three study sites (Narrabeen, Australia; Perranporth, UK; La Misión, Mexico) all showed patterns of extensive subaerial beach erosion during the storm sequences, coupled with sediment deposition in the shallow subaqueous zone. Analysis revealed significant net sediment gains (59–140 m³/m) over the storm-recovery sequence at all three locations. These gains were comparable in magnitude to the observed subaerial beach erosion. The alongshore variability of the net gains offered insights into potential sediment pathways, including beach rotation, headland sand bypassing, and onshore transport from the lower shoreface. Conversion of the sediment gains to equivalent years of shoreline retreat predicted by the Bruun rule indicated that the observed sediment gains could theoretically offset decades of predicted shoreline retreat under both the SSP1-2.6 and SSP5-8.5 emission scenarios. Estimates of wave orbital velocities at the depth of closure suggested sufficient sediment transport potential during the storm events to account for these sediment gains, particularly at Narrabeen and Perranporth. The long-term subaerial beach volume data at Narrabeen showed no significant erosion or accretion trend, hinting at the potential for rare, significant short-term sediment gains.

The findings challenge the commonly used Bruun rule for long-term coastal projections by demonstrating that extreme storm events can result in substantial net sediment gains over the entire upper shoreface. The magnitude of these gains, comparable to typical artificial beach nourishment projects, highlights the potential for significant errors in shoreline predictions using simplified models that do not account for storm-driven sediment fluxes. The study emphasizes the need for more sophisticated models incorporating the dynamic interplay between extreme storm events and sediment redistribution within the nearshore system. This is particularly important given projections of increased extreme wave activity.

This study demonstrates that extreme storm sequences can cause substantial net sediment gains in the nearshore zone, potentially offsetting decades of shoreline retreat predicted by the Bruun rule under various sea-level rise scenarios. The results underscore the crucial need for improved understanding of nearshore sediment budget dynamics and highlight the limitations of simplified models like the Bruun rule for long-term coastal management. Future research should focus on improving our understanding of lower shoreface morphodynamics, upscaling seabed mapping efforts, and increasing the frequency of comprehensive nearshore monitoring to better predict coastal evolution under climate change.

The study's findings are based on three specific locations, and the generalizability of these results to other coastal environments requires further investigation. While wave orbital velocities provide an indication of sediment transport potential, it is acknowledged that other factors such as longshore sediment transport, headland bypassing, and fluvial inputs also influence the sediment budget. Moreover, the study focuses on short-term storm-recovery sequences and cannot fully capture all complexities of long-term coastal evolution. The limited understanding of the composition and dynamics of the lower shoreface poses a significant challenge in comprehensively modeling sediment budgets.