Overwash occurs when the combined elevation of storm surge, wave runup and astronomical tide exceeds the crest of a barrier’s frontal dunes, sending pulses of water and sand surging over the top. The sand carried by these flows is deposited on the landward side as lobate features called washover fans, which can merge into terraces or spread out as sheetwash where dunes are absent. Geologists regard this landward transfer of sediment as the engine of barrier ‘rollover’, the process by which a retreating barrier climbs across its own back-barrier environment instead of simply eroding away. It is one of nature’s strangest survival strategies: the barrier keeps its shape only by abandoning its old footprint and rolling inland.

The research team, led by Edvan Casagrande dos Santos of the Federal University of Rio Grande do Sul, took advantage of the PlanetScope constellation of small imaging satellites to watch this process unfold almost in real time. Over eighty-four months, the team visually inspected 1,240 true-color images at spatial resolutions of three to five metres, tracking the appearance of fresh washover deposits by their shape and by the telltale tonal contrast between wet and dry sand. They subdivided the sand spit barrier into five sectors based on dune heights and the recurrence of overwash, then cross-checked each candidate event against tide gauge measurements from Balneário Rincón, synoptic weather charts from the Brazilian Navy, and hourly ocean and atmosphere data from the ERA5 reanalysis.

The screening protocol was deliberately conservative. Any observed tide of 1.2 metres or more above local mean sea level flagged a potential event, yielding 129 candidates. Each one was then validated against the storm impact scale developed by Sallenger and later refined by Stockdon and colleagues, which compares the maximum water level to the elevation of the dune base and crest. Only when the hydrodynamic threshold, the modeled overwash regime and a visually confirmed washover fan all converged within a window of a few days was the event counted. The result: ninety-three confirmed overwashes, concentrated overwhelmingly between April and July, the southern hemisphere’s autumn and winter.

The meteorological fingerprint of these events is remarkably consistent. Virtually every overwash coincided with the passage of low-pressure systems, typically extratropical cyclones spawned near Patagonia, the Río de la Plata or the coasts of Uruguay and Rio Grande do Sul, often trailing cold fronts across southern Brazil. Winds swung from the northeast ahead of the front to the south and southwest behind it, piling coastal water against the shore through wind shear and Ekman transport. Storm surge emerged as the dominant driver of extreme water levels, contributing an average of forty-one percent of the observed tide during events, and in the record April 2020 storm a staggering seventy-seven percent of the elevation above mean sea level. On that day, a surge of 1.314 metres combined with six-metre significant waves and maximum individual waves of 11.2 metres to drown the barrier entirely.

Not every stretch of the spit suffered equally, and the differences map directly onto dune geometry. Sector 2, protected by foredunes averaging more than four metres high and reaching six to seven metres at their peaks, remained essentially untouched throughout the study, with washover fans appearing only once, in 2023, and even then only after human activity had disturbed adjacent Sector 1. Sector 3, where segmented dunes average just 2.2 metres, responded to nearly every moderate storm, hosting continuous washover fans, terraces and narrow overwash channels since 2017. Sectors 4 and 5, near the migrating river mouth, experienced repeated complete overtopping and inundation. The message is blunt: dune height and continuity are the barrier’s first line of defense, and even modest waves can breach it where the dunes are low or fragmented.

Human engineering turns out to be a co-conspirator in the barrier’s retreat. Since the late nineteenth century, the Araranguá River mouth has been artificially opened at least five times, and machinery was dispatched again in May 2022 and October 2023 to cut drainage channels through the spit during river floods. Each artificial breach lowers and segments the foredune field, creating depressions that invite subsequent overwash. The study draws parallels with the Ria Formosa barrier system in Portugal, where human interventions accounted for roughly twelve percent of washover occurrences, and with Masonboro Island in North Carolina, where hurricane-eroded dunes left the barrier prone to years of follow-on overwash. In Araranguá, the engineered inlets transformed Sector 1 into a persistent vulnerability that later amplified the ocean’s reach into the river channel.

Perhaps the most consequential finding lies in the eighty-three-year historical record. By orthorectifying aerial photographs from 1938, 1957 and 1978 using Structure-from-Motion photogrammetry, and combining them with later orthophotos and satellite scenes, the team computed four morphometric indicators along transects spaced every hundred metres: shoreline position, the outer and inner margins of the river channel, and channel width. Three evolutionary phases emerged. From 1938 to 1978 the system prograded seaward by an average of 207 metres, a textbook regressive barrier fed by ample sediment supply. From 1978 to 2005 a milder transgression set in, with the shoreline retreating an average of 24 metres between key transects. Then, after 2005, the retreat accelerated sharply, averaging 65 metres in just sixteen years, with erosion extending deep into the barrier interior, including a 94-metre landward migration of the inner channel margin.

The synchrony is hard to ignore: the sectors retreating fastest on the decadal scale are precisely those where washover fans recur most often on the monthly scale. The authors conclude that the Araranguá barrier has entered a rollover regime driven by the sheer frequency of overwash, reversing the regressive behavior that dominated the Holocene evolution of the broader coastal system. They also note a hydrodynamic compensation mechanism: no single parameter triggers overwash on its own, but when a surge nears a threshold, even modest increases in wave height or period push the total water level over the dune crest. This explains why overwash can occur with significant wave heights as low as two metres in the low sectors, while tall dunes shrug off far larger storms elsewhere along the spit.

The implications stretch well beyond one Brazilian estuary. Coastal barriers line up to fifteen percent of the world’s shorelines and ten percent of open-ocean coasts, sheltering lagoons, wetlands and communities from the sea. As sea level rises and storm climates intensify, the balance between sediment supply and overwash frequency will determine which barriers roll landward and survive, and which narrow into breaching and collapse. Washover deposits in Araranguá proved ephemeral, buried by wind-blown sand within days and reworked by the river, a caution for anyone hunting these features in the geological record. But the real-time satellite record offers something the rock archive cannot: a front-row seat to a barrier deciding, one storm at a time, to move inland.

Subject of Research: Overwash processes and the interdecadal evolution of the Araranguá sand spit barrier in southern Santa Catarina, Brazil

Article Title: Overwash processes: spatio-temporal scales and dynamics of the coastal system in southern Santa Catarina, Brazil

Article References: dos Santos, E. C., Medina, V. V., da Costa Cristiano, S., & da Camara Rosa, M. L. C. (2026). Overwash processes: spatio-temporal scales and dynamics of the coastal system in southern Santa Catarina, Brazil. Natural Hazards, 122(19), Article 630. https://doi.org/10.1007/s11069-026-08399-w

Image Credits: AI Generated

DOI: 10.1007/s11069-026-08399-w

Keywords: overwash, washover fans, coastal barrier, sand spit, storm surge, extratropical cyclones, shoreline retreat, barrier rollover, remote sensing, Santa Catarina, Brazil, coastal geomorphology