Single extreme storm sequence can offset decades of shoreline retreat projected to result from sea-level rise

The study addresses how extreme storm sequences influence long-term shoreline change under sea-level rise (SLR). While SLR is projected to be 0.63–1.01 m by 2100 under SSP5–8.5, and models like the Bruun rule predict widespread sandy beach retreat, the coupling of storm impacts with SLR over long time scales remains ambiguous. Conventional approaches often treat storm-driven variability as short-term noise relative to SLR-driven change. Yet, extreme storms can transport sediment from the lower shoreface and adjacent headlands into the nearshore, potentially offsetting SLR-induced retreat. The paper investigates whether extreme storm-recovery sequences can lead to net sediment gains across the upper shoreface sufficient to mitigate projected SLR-driven shoreline retreat and evaluates implications for the reliability of Bruun rule-based projections.

The Bruun rule, despite well-documented limitations, is widely used for predicting shoreline response to SLR for local and global assessments. It assumes an upward and landward profile translation balancing volumetric losses and gains but typically does not rigorously incorporate sediment budget terms without detailed knowledge of cross-shore/longshore fluxes and sources. Geological and historical evidence indicates onshore sand transport from the lower shoreface at O(1 m³/m/yr) has driven coastal progradation under stable sea levels (e.g., Australia, Brazil, The Netherlands). Under rising seas, such sand supply can partly or fully offset SLR impacts. Mechanisms include wave-nonlinearity-driven onshore transport at depth during energetic conditions, longshore redistribution and headland bypassing, and fluvial/estuarine exchanges. Reviews emphasize limited understanding of lower shoreface morphodynamics due to sparse observations, subtle transport imbalances, and bedform/gravity flow dominance. These gaps question the appropriateness of simplified shoreline translation models and motivate comprehensive sediment budget assessments.

Study sites and monitoring: Three wave-dominated sandy coasts were examined: Narrabeen (SE Australia, 3.6 km embayment), Perranporth (SW England, 3.5 km embayment), and La Misión (NW Mexico, 2.2 km open coast). All have D50 ≈ 0.3–0.4 mm and tanβ ≈ 0.03–0.04. Tidal regimes: microtidal at Narrabeen (spring range 1.3 m) and La Misión (2.3 m), macrotidal at Perranporth (6.3 m). Each site maintains monthly subaerial beach surveys; targeted full upper-shoreface surveys were conducted pre-storm, post-storm, and ~12 months into recovery. Survey techniques: Subaerial: Airborne LiDAR (σz ~0.11–0.15 m), UAV photogrammetry (σz ~0.06–0.07 m), and RTK-GNSS (walking or ATV-mounted; σz ~0.05 m). Subaqueous: single-beam echosounder on boat/jetski (σz ~0.05–0.10 m); at Perranporth, deeper-water multibeam (σz 0.06–0.30 m). Cross-shore transects spaced ~50 m (Narrabeen, Perranporth) and 150 m (La Misión). Seamless DEMs were created via cubic interpolation bridging small gaps between subaerial and subaqueous data. Events: Narrabeen: June 2016 East Coast Low (largest subaerial erosion in >40 years; ~121 m³/m). Perranporth: 2013–2016 winter storm cluster (extreme 2013/14 winter; ~212 m³/m subaerial erosion). La Misión: 2018/19 winter storm cluster (largest on record; ~208 m³/m erosion). Depth of closure (DoC): Over storm sequences: ~−11.6 m (Narrabeen), −19.3 m (Perranporth), −9.1 m (La Misión) relative to MSL. Long-term DoC from ERA5 (1979–2020) for offset calculations: −14.3 m (Narrabeen), −20.2 m (Perranporth), −18.2 m (La Misión). Sediment budget and error analysis: DEMs of Difference (DoDs) quantified elevation changes between survey epochs. A limit of detection (LoD) per grid cell was computed as sqrt(σDEM1² + σDEM2²); only changes exceeding LoD at 95% confidence were integrated. Volume changes were separated into subaerial and subaqueous components and normalized per unit shoreline length (m³/m). Alongshore distributions and net changes over storm and recovery phases were analyzed. Equivalent SLR-offset years: Using Bruun rule framework, the annual sediment input required to offset 21st-century SLR recession was Qoffset = W*·S/Y, where W* is upper-shoreface width (berm to long-term DoC), S is median SLR for scenarios (0.44 m for SSP1–2.6; 0.77 m for SSP5–8.5, 2000–2100), and Y = 100 years. Equivalent years offset = ΔV / Qoffset. Wave orbital velocities at DoC: Using ERA5 Hs and Tp (1979–2020), seabed orbital velocity Urms at DoC was estimated (JONSWAP-based explicit formulation). A threshold of motion Ucrit ≈ 0.2 m/s for typical shoreface sand (D50 0.2–0.7 mm, T ≈ 10 s) was used to infer transport potential. Frequency and magnitude of extreme Urms at DoC were compared with observed net gains. Data/code: Survey and supporting data are available via Zenodo (10.5281/zenodo.5748645); Narrabeen program data at http://narrabeen.wrl.unsw.edu.au/. MATLAB code available on request.

- All three sites exhibited extensive subaerial beach erosion during the storm phases and storm-bar formation offshore, followed by partial recovery under modal conditions. Pivot depths separating erosion and deposition were ~−2.9 m (Narrabeen), −5.8 m (Perranporth), and −1.2 m (La Misión). - Integrated over the entire upper shoreface plus beach, sediment budgets were unbalanced and showed large net gains over the storm–recovery sequence: +91 m³/m (Narrabeen), +140 m³/m (Perranporth), +59 m³/m (La Misión). Absolute volumetric gains: ~+400,000 m³ (Narrabeen), ~+420,000 m³ (Perranporth), ~+130,000 m³ (La Misión). - Spatial patterns suggest site-specific pathways: Narrabeen gains during storm and skewed to southern embayment (consistent with counter-clockwise beach rotation and possible onshore transport from lower shoreface sand bodies). Perranporth gains mainly during recovery and concentrated in the south, consistent with headland sand bypassing inputs. La Misión gains mainly during storm without strong alongshore bias, possibly from lower shoreface onshore transport and winter fluvial inputs. - Qoffset under SSP5–8.5: ~3.7 m³/m/yr (Narrabeen; W* ≈ 480 m), ~8.4 m³/m/yr (Perranporth; W* ≈ 1090 m), ~6.2 m³/m/yr (La Misión; W* ≈ 810 m). Observed gains translate to theoretical offsets of Bruun-rule retreat of: Narrabeen 25 years (SSP5–8.5) or 43 years (SSP1–2.6); Perranporth 18 years (SSP5–8.5) or 31 years (SSP1–2.6); La Misión 10 years (SSP5–8.5) or 17 years (SSP1–2.6). - At DoC, extreme Urms reached up to ~1.0 m/s (Narrabeen), 1.2 m/s (Perranporth), and 0.6 m/s (La Misión) during the sequences, well above Ucrit (~0.2 m/s), indicating strong transport potential across DoC. Historically, such extreme Urms were rare at Narrabeen (91 h >1.0 m/s over 41 years) and Perranporth (9 h >1.2 m/s), but more frequent at La Misión (636 h >0.6 m/s). A potential inverse frequency–magnitude relation is suggested: rarer, more extreme events may drive larger net sediment influx.

Findings demonstrate that extreme storm–recovery sequences can produce substantial net positive sediment budgets over the upper shoreface, contradicting the common assumption in simplified models that storm losses are exactly balanced locally offshore. Because these net gains occur largely between MSL and DoC, they are less visible than dune/beach erosion but volumetrically comparable, akin to nourishment-scale inputs, and can theoretically offset decades of Bruun-rule shoreline retreat even under SSP5–8.5. This underscores the importance of explicitly representing sediment exchanges across the DoC and alongshore sources (e.g., headland bypassing) in long-term projections. The observed relationships between extreme seabed orbital velocities at DoC and net gains suggest that infrequent, high-energy conditions can mobilize lower-shoreface sand reserves and deliver them shoreward. Consequently, projection methods that omit these processes risk systematic bias and error propagation, particularly given projections of increased extreme wave activity along a large fraction of global coasts. Nonetheless, the study acknowledges that storm sequences could also yield net losses where sediment pathways export material (e.g., to canyons, estuaries, adjacent compartments), highlighting site-specific controls and the need for comprehensive sediment budgets.

The study provides empirical evidence from three continents that single extreme storm or storm-cluster sequences, followed by recovery, can generate large net sediment gains across the upper shoreface (59–140 m³/m), potentially offsetting up to multiple decades of Bruun-rule shoreline retreat under 21st-century SLR scenarios. These results question the reliability of simplified shoreline translation approaches that neglect storm-driven cross-DoC and alongshore exchanges. To improve long-term coastal projections, the authors recommend: (1) substantial expansion of seabed mapping to quantify lower-shoreface sediment reservoirs; (2) routine, system-scale monitoring of entire nearshore compartments to capture sediment fluxes and connectivity; and (3) leveraging, but not relying solely on, emerging remote sensing (e.g., satellite-derived bathymetry) given current vertical accuracy limitations. Future research should target process-based measurements on the lower shoreface, tracer studies to resolve sediment sources/sinks, and integration of storm-driven sediment budgets into predictive shoreline models.

- Source attribution remains uncertain without process-based measurements and tracers; multiple pathways (onshore transport across DoC, headland bypassing, estuarine inputs) may contribute. - Lower shoreface processes are poorly constrained due to sparse in situ data beyond the surf zone, dominance of bedload/gravity flows at depth, and difficulty scaling short-term observations. - Seabed composition and morphology on the lower shoreface are largely unmapped globally, introducing uncertainty in reservoir estimates (a large fraction between MSL and −200 m remains uncharted). - Remote sensing vertical accuracies (e.g., SDB σz > 0.4 m) limit detection of subtle changes offshore; hence reliance on in situ methods is necessary. - Generalizability: While all three cases showed net gains, other settings may experience net losses depending on sediment compartment connectivity (e.g., export to canyons/estuaries) and storm characteristics. - Bruun rule comparison is theoretical; offsets are calculated within a simplified framework and do not imply guaranteed mitigation of observed shoreline retreat at each site.