Southeast Asia’s coasts are among the world’s most physically and ecologically sensitive environments, facing escalating threats from climate change, rising seas, extreme events and pervasive land subsidence. Understanding the magnitude, rate and trajectories of coastal transformation is critical for adaptation planning and hazard mitigation. This review synthesises current knowledge of coastal landform change across Southeast Asia (SEA), emphasising spatial and temporal variability driven by geological, climatic and anthropogenic boundary conditions. While globally sea levels are rising, tectonic deformation and vertical land motion dominate local trajectories, resulting in large spatial variability in relative sea-level rise (RSLR), offsetting SLR in uplifting areas and amplifying it in subsiding areas, where rates exceed 8 mm/yr, rivalling those of the Holocene marine transgression and surpassing global averages. The evolution of coastal landform types in response to boundary controls is examined. Paleo-reconstructions reveal that the Holocene transgression inundated ~2.3 million km2 of the Sunda Shelf, fundamentally resetting the land–sea interface of SEA countries. Sea-level fall from the mid-Holocene highstand drove extensive coastal deposition, generating >100,000 km2 of lowlands over the past 6,000 years, with progradation rates of 101–102 m/yr. These geologically young landscapes now face rapid, multidirectional change under contemporary forcing. Coastal change in SEA is complex, and rather than uniform erosion and inundation, the region exhibits a mosaic of responses, from stability to rapid progradation and island migration. Contemporary rates of change are comparable to those documented across the Holocene (102 m/yr), but are responding to a broader suite of drivers, including tectonic deformation, variable RSLR, extreme events and intense human activity. Attribution remains elusive, constrained by sparse spatial coverage, uneven representation of landform types and limited temporal resolution of data. Addressing these gaps requires comprehensive, site-specific studies of both geological and contemporary dynamics across all coastal typologies, supported by high-resolution monitoring and integrated modelling. Such efforts will provide the empirical foundation needed for informed coastal management and adaptation strategies in one of the world’s most vulnerable regions.

Tropical cyclones (TC) can produce waves and water levels that markedly reshape sand cay shorelines. TC Jasper (December 2023) passed near Low Island (Low Isles, Northern Great Barrier Reef [GBF]) as a category 2 storm. Using a combination of remote sensing and ground surveys, we compare detailed, high-resolution digital terrain models created before and after TC Jasper to quantify sediment redistribution around the cay during and after the event. During TC Jasper, net transport of 8,870 m3 occurred to elongate the spits at the eastern and western ends of the cay, but the sediment volume of the cay did not significantly change. Following TC Jasper, the shoreline at Low Island returned to its modal seasonal state within six months. This accords with historical accounts of seasonal shifts in shoreline configuration driven by prevailing wind and wave regimes, as well as the relatively rapid readjustment to a modal form following episodic extreme events. Overall, the documented changes to Low Island following cyclonic events highlight the complex interplay between episodic disturbances and longer-term geomorphic stability, emphasising the importance of ongoing research into these interactions as higher-intensity cyclones increase in frequency due to climate change.

Wave-dominated sandy beaches are highly valued by societies and are amongst the world’s most energetic and dynamic environments. On wave-dominated beaches with unlimited sand supply and limited influence of tide and geology, beach change has long been conceptualised in the morphodynamic framework of Wright and Short (1984). Such framework describes the occurrence of beach types based on wave conditions and sediment characteristics across the complete reflective–dissipative spectrum. Building on theoretical work, field/laboratory measurements and monitoring programmes, the physical mechanisms underpinning this morphodynamic framework have been progressively unravelled. Cross-shore morphological changes are primarily controlled by equilibrium and beach memory principles with below (above) average wave conditions driving down-state (up-state) transitions associated with onshore (offshore) sediment transport. Such cross-shore behaviour mostly reflects the imbalance between the onshore-directed sediment transport driven by wave nonlinearities and the offshore-directed sediment transport driven by the undertow. Self-organised morphological instabilities resulting from different positive feedback mechanisms are primarily responsible for alongshore morphological variability and the generation of rhythmic morphological features, such as crescentic bars, rip channels and beach cusps. Critically, wave climate and changes in wave regimes are key in driving the coupled cross-shore and longshore behaviour that ultimately explains modal beach state and frequency-response characteristics of beach morphological time series.

Constructional sedimentary history of coastal barriers can provide insights regarding meteorological and oceanographic processes, and relative sea-level changes. We investigated the Holocene evolution of a segment of the Rio Grande do Sul Coastal Plain in southernmost Brazil. Data were obtained from ground-penetrating radargrams, optically stimulated luminescence dating, altimetry measurements, Google Earth imagery, and aerial photographs. These data allowed a qualitative and quantitative analysis of the beach profiles recorded in the radargrams. From which eolian, backshore/foreshore, breaker bars, and upper and lower shoreface radar facies were identified. The beach-related radar facies are recorded in twenty-eight progradational sand units (sigmoidal bodies). These units record increments of relatively steady deposition bounded by erosional surfaces produced by anomalous, high-magnitude storm events taking place about every 250 years. The upper shoreface strata include two to three breaker bars. Several pieces of evidence (number of breaker bars, upper shoreface gradient and progradation rate) suggest an alternation between dissipative and intermediate stages of barrier morphodynamics and a decrease of wave energy from 2 ka onwards. The barrier prograded during the last 7.1 ka, and initially, barrier progradation occurred because of a normal regression during a relative sea-level rise followed by a stillstand. Later, barrier progradation took place as a forced regression driven by a relative sea-level fall.

This paper reveals deglaciation palaeodynamics (Marine Oxygen Isotope Stage 2 [MIS 2]) in Poland and the southern Baltic Sea (SBS) development during marine transgression/regression phases (MIS 1) determined by a numerical modelling method. The introduced approach uses a high-level polynomial regression followed by the integral calculus of successive functions and an application of formulae. As a result, palaeogeographic relations from primary matrix transform instantly into palaeodynamics within a nested matrix. Accordingly, within 9 ka of the late Pleistocene, glacial recession dynamics increased by two orders of magnitude, from −8.5 m/yr between Leszno (L, 24 ka BP) and Poznań (Poz, 20–19 ka BP) phases, through several dozen (−37.2 m/yr, −60.6 m/yr, −90.7 m/yr) to the maximum average equalling −427.3 m/yr (max. −861.4 m/yr) between the Pomeranian (Pom, 17–16 ka BP) and the Gardno (G, 16.8–16.6 ka BP) phases. In turn, SBS coastline transgression and regression dynamics varied by three orders of magnitude. Since the Baltic Ice Lake (BIL, 10.5–10.3 ka BP) up to the Yoldia Sea (YS, 10–9.9 ka BP) regression was intense and equalled −56.8 m/yr (max. −128.7 m/yr), followed by marine transgression towards the Ancylus Lake (AL, 8.7–8.5 ka BP) at 21.43 m/yr through 9.30–2.20 m/yr during the Littorina Sea 1 and Littorina Sea 2 stages (LS1 and LS2, since 7.7 ka BP), eventually 0.51 m/yr in the last 6.05 ka. The 2 m sea-level rise scenario projections indicate approx. 3400 km2 of land and 684,000 inhabitants face flood risk around 2150–2240 CE, with marine transgression dynamics expected to range from 23.9–38.2 m/yr.

Tidal channel networks, estuaries and ebb deltas are usually formed over a period longer than observations cover. Much is known about their characteristics and formation from linear stability analyses, numerical modelling and field observations. However, experiments are rare whilst these can provide data-rich descriptions of morphological evolution in fully controlled boundary and initial conditions. Our objective is to ascertain whether tidal basins can be formed in experiments, what the possible scale effects are, and whether morphological equilibrium of such systems exists.

We experimentally created tidal basins with simple channel networks and ebb deltas in a 1.2 by 1.2 m square basin with either a fixed or self-formed tidal inlet and initially flat sediment bed in the tidal basin raised above the bed of the sea. Rather than create tides by varying water level, we tilted the entire basin over the diagonal. The advantage of this novel method is that the bed surface slopes in downstream direction both during flood and ebb phases, resulting in significant transport and morphological change in the flood phase as well as the ebb phase. This overcomes the major problem of earlier experiments which were entirely ebb-dominated, and reduces the experiment time by an order of magnitude.

Ebb deltas formed in sand were entirely bedload dominated whereas the lightweight plastic sediment was intermittently suspended. Channels bifurcated during channel deepening and backward erosion to form a network of up to four orders. For initially dry tidal plains, the tidal prism increased as more sediment eroded from basin to ebb delta, so that evolution accelerated initially. The rate of change, the size of the channels and the final length of channels and delta were very sensitive to the tidal amplitude, tidal period and initial water depth in the basin. Most experiments with sand terminated with all sediment below the threshold for motion, whilst lightweight sediment remained mobile in the inlet region and firstorder channels, suggesting that sustained morphodynamics are feasible in experiments. We discuss how this novel experimental setup can be extended to produce tidal deltas, estuaries and other tidal systems and study their dynamics as a function of their forcing.

The availability of nearly 100 years of bathymetric measurements allows the analysis of the morphodynamic evolution of the Dutch Wadden Sea under rising sea level and increasing human constraint. The historically observed roll-over mechanisms of landward barrier and coastline retreat cannot be sustained naturally due to numerous erosion control measures that have fixed the tidal basin and barrier dimensions. Nevertheless, the large continuous sedimentation in the tidal basins (nearly 600 million m3), the retained inlets and the similar channel-shoal characteristics of the basins during the observation period indicate that the Wadden Sea is resilient to anthropogenic influence, and can import sediment volumes even larger than those needed to compensate the present rate of sea-level rise. The largest sedimentation occurs in the Western Wadden Sea, where the influence of human intervention is dominant. The large infilling rates in closed-off channels, and along the basin shoreline, rather than a gradual increase in channel flat heights, render it likely that this sedimentation is primarily a response to the closure of the Zuiderzee and not an adaptation to sea-level rise. Most of the sediments were supplied by the ebb-tidal deltas. It is, however, unlikely that the sediment volume needed to reach a new equilibrium morphology in the Western Wadden Sea can be delivered by the remaining ebb-tidal deltas alone.