Introduction
Submersion denotes a recurrent, often sustainable phase of coastal change in which sediment is transferred from the exposed beach into the submerged nearshore and later returned to rebuild the dry-beach profile. Conceptually the process separates into two morphological domains: the subaerial beach (berms, foreshore and other dry-sand features visible to users) and the subaqueous nearshore (the shoreface and breaker zone where waves and currents temporarily store material). Sediment thus moves between a visible, recreational zone and a submerged reservoir without necessarily representing permanent loss from the local system.
Offshore transport is driven by hydrodynamic forcing—ordinary wave action, wave-driven currents, tidal flows and episodic storm waves—which mobilize sand and gravel cross‑shore into the nearshore storage area. The subsequent recovery phase, accretion, involves onshore-directed transport that redeposits these sediments to rebuild berms and foreshore width, restoring the pre‑event beach profile. Viewed through a sediment‑budget lens, submersion plus accretion constitute internal redistribution within a coastal cell: when input, output and internal transfers remain in balance the cycle operates as a sustainable, reversible exchange rather than as net erosion.
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Balance can be upset, however, turning temporary submersion into progressive shoreline retreat. Persistent alongshore sediment losses, export to deep offshore sinks, storms that exceed the system’s recovery capacity, and human interventions (for example seawalls, groynes or dredging) can interrupt the return of nearshore stores. Distinguishing cyclical submersion–accretion from true sediment deficit therefore requires repeated beach‑profile surveys, nearshore bathymetric monitoring and sediment sampling to quantify cross‑shore volumes and the timing of onshore versus offshore transport. These observations are essential for diagnosing whether observed beach lowering is transient and recoverable or indicative of long‑term sediment loss.
Submersion versus erosion
Coastal sediment is redistributed continuously by wave action: high-energy storms drive large volumes of sand and gravel seaward, often building elongate offshore ridges (storm bars) parallel to the shoreline, while lower-energy intervals encourage transport of that material back onto the visible beach. This onshore–offshore cycling produces characteristic, often seasonal, adjustments in beach profile—calm conditions tend to steepen and narrow the surf zone as sediment returns, whereas storm-dominated conditions flatten and widen the shore as material is moved offshore.
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Superimposed on this vertical exchange is lateral sediment transport by longshore drift. Oblique wave incidence generates a net alongshore flux that can relocate sediment substantial distances from its source, producing spatial variability in sediment budgets and downstream changes in beach form. Many stretches of coast exist in a dynamic equilibrium in which offshore storage, alongshore transfer and onshore recovery balance each other so that storm-induced submersion is temporary and the system remains stable over time.
This balance is fragile where human activities alter sediment supply or interrupt transport pathways. Coastal defenses, shoreline armouring, dredging, river regulation and intensive development commonly reduce inputs or break connectivity between onshore, offshore and alongshore stores; material removed from the visible beach under such conditions is less likely to be returned, converting what would be reversible submersion into permanent shoreline loss. Consequently, distinguishing between transient submersion and true erosion requires assessment of the sediment budget and the integrity of transport linkages; effective management must address sediment sources, longshore drift and offshore reservoirs (including storm bars) to restore or maintain coastal stability.
Community perception
Communities commonly conflate shoreline loss with a single process, treating all retreat as harmful erosion. In reality, beach recession is a composite phenomenon comprising both transient, reversible changes and longer‑term, permanent losses. Temporary submersion—short‑term lowering or inundation of the foreshore—is often an intrinsic expression of healthy, dynamic shorelines and should not automatically be interpreted as degradation. By contrast, the irreversible component of recession, typically driven by human activities or persistent climate change, represents lasting alteration of the coast and warrants distinct management responses.
Effective local decision‑making therefore requires the capacity to decompose observed recession into its constituent parts: short‑term variability versus permanent change. This distinction has direct policy consequences, determining whether intervention is required, which measures are appropriate, and how limited resources and legal protections should be allocated. A precautionary, evidence‑based approach—based on sustained monitoring, explicit characterization of temporal scales, and careful attribution of causes—is essential for communities to differentiate sustainable foreshore dynamics from anthropogenic or climate‑driven loss and to choose proportionate, durable management actions.