Erosion hotspots
Definition of Erosion hotspot:
An erosion hotspot is a coastal section where erosion or beach narrowing is significantly greater than along neighboring sections over a specified period and spatial scale[1].
This is the common definition for Erosion hotspot, other definitions can be discussed in the article
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A hotspot may be temporary and reversible, may migrate alongshore, or may persist over the considered time scale because the processes causing a local sediment deficit continue to operate. Persistent erosion does not imply indefinite shoreline retreat: morphological adjustment may eventually reduce the sediment imbalance and establish a new, more stable shoreline configuration.
An illustration of an erosion hotspot along the Polish coast is shown in Fig. 1. The local sand balance is disrupted by excessive sand loss and/or deficient sand supply. Erosion hotspots can be generated by natural variations in geology, bathymetry, waves, currents and sediment supply, but they are also frequently created or intensified by coastal structures, dredging, sediment extraction and interruption of river or littoral sand supply. Persistent natural hotspots can result from geological controls, land subsidence or uplift, river-mouth and inlet migration, delta-lobe abandonment, and persistent spatial variations in wave exposure and sediment supply. Human-induced hotspots deserve particular attention because they can often be prevented or mitigated through better design and sediment management.
The table below presents an overview of situations where erosion hotspots can be expected. The table was inspired by an earlier overview of erosion hotspot characteristics published by Kraus and Galgano (2001[3]). Several recommendations for possible preventive and remediation measures from this publication have been adopted in the table.
For explanations about the processes generating erosion hotspots, the reader is referred to coastal wiki articles and references indicated in the table. General overviews of processes involved in coastal erosion are given in the articles Natural causes of coastal erosion, Human causes of coastal erosion, Accretion and erosion for different coastal types and Dune erosion.
A hotspot does not necessarily indicate a net loss of sand from the wider coastal system. Sand may merely have been redistributed to an adjacent beach, bar or shoal. A persistent hotspot develops when the local sediment deficit is repeatedly renewed or when sediment is permanently removed from the active coastal zone. Establishing the relevant sediment budget is therefore essential before selecting a remedy.
| Hotspot type | Possible cause of erosion | Preventive and remediation measures to be investigated |
|---|---|---|
| Beach facing natural offshore bathymetric structures, e.g. rocky outcrops, reefs, islands, shoreface-connected ridges [4]. | - Erosion at beach spots where incident waves are focused due to refraction/diffraction over offshore bathymetry[5]. - Rip currents that transport sand offshore beyond the nearshore circulation system, resulting from alongshore-varying wave breaking across complex bathymetry[6]. |
- Compensate with shore nourishment or accept shoreline retreat until a new stable shoreline configuration develops. |
| Beach facing natural nearshore bathymetric structures, e.g. crescentic bars, oblique bars (see Rhythmic shoreline features) | Local erosion due to: - wave focusing over nearshore bathymetry (see Shallow-water wave theory#Bathymetry effects on Refraction). - rip currents that transport sand offshore beyond the nearshore circulation system[7]. |
- Monitor the position and migration of bars, rips and shoreline embayments before intervening; temporary hotspots often recover or move alongshore[8]. |
| Beach facing offshore manmade structures, e.g. sand borrow pit, dredge disposal site, stranded ship[1]. | - Local erosion due to wave focusing on beach spots by refraction/diffraction on offshore structures. | - This type of erosion hotspot is often accompanied by accretion of an adjacent area (e.g. formation of a salient). - Redistribute the sand along the beach. - Remove or adapt the structures to mitigate the erosion hotspots. . |
| Beach facing nearshore manmade structures, e.g. detached emerged or submerged breakwater | Local erosion due to: - wave focusing by refraction/diffraction on offshore structures, - rip-cell circulation and offshore directed rip currents[7]. |
- This type of erosion hotspot is often associated with accretion in the sheltered area or elsewhere nearby (e.g. formation of a salient). - Redistribute sand along the beach. - Remove or adapt the structures to mitigate the erosion hotspots. |
| Beach facing an offshore canyon or other relict bathymetric depressions | - A canyon head intersecting or approaching the littoral transport zone can intercept longshore-moving sand and transfer it to depths from which it does not readily return. - A bathymetric depression may create an erosion hotspot by wave focusing. |
- Compensate with shore nourishment or accept shoreline retreat until a new stable shoreline configuration develops. |
| Beach with sand borrow pit within the active coastal profile | - The pit will collect beach sand and may also alter wave transformation and longshore transport. | - Relocate the sand borrow pit farther offshore (if feasible and less expensive than beach nourishment). |
| Beach situated downdrift* of abrupt change in shoreline orientation (e.g. at a natural or artificial headland, or at the tip of a sand spit) | - Interruption of longshore sand transport that may be directed away from the downstream coast. | - Headland: investigate beach fill protected with a downdrift structure (e.g. an incurved groyne to minimize downdrift erosion). - Sand spit: Assess the natural evolution (extension or breach) before intervening. |
| Beach subject to persistent high-angle wave incidence | Erosion hotspots related to shoreline instability (see Rhythmic shoreline features): - alternating erosion-accretion by alongshore sand waves (Fig. 1)[2]. - shore-oblique bars with rhythmic wave focusing. |
- Shoreline and beach profile monitoring. - Early assessment. - Nourishment of the alongshore moving sand wave trough. |
| Beach adjacent to a tidal inlet, river mouth or ebb-tidal delta | Alternating erosion-accretion by alongshore propagating sand impulses from: - shore attachment of ebb shoals or bars, - high fluvial sediment discharge events. |
- Shoreline and beach profile monitoring. - Early assessment. - Preventive beach nourishment. |
| Beach situated downdrift* of shore-perpendicular structures such as groynes and jetties | - Disruption of morphodynamic equilibrium by blocking the littoral drift. | - Remove groynes or adapt (shortening, lowering). - Preventive beach nourishment at the structure's leeside - Apply sand bypass |
| Artificially compartmented beach | - Persistent rip currents formed next to shore-perpendicular structures (e.g. groynes, jetties) transport sediment offshore[7] | - Reduce rips by structure adaptation (e.g. a spur to deflect current laterally) - Adaptation of groyne length and height. - Groyne burial under beach nourishment. |
| Beach with hardened backshore (e.g. protruding buildings, seawall, berm revetment) | Enhanced beach lowering and toe scour under storm conditions due to - frequent wave attack on the structure (see Seawalls and revetments), - deficient sand supply from the backshore berm for the formation of a protective nearshore bar[9], - after-storm recovery by aeolian transport impeded by frequent wetting of lowered beach[10]. |
- Remove buildings. - Sand nourishment for widening the beach. - Accept beach loss in front of the seawall or revetment. |
| Beach with inadequately designed sand nourishment | - Rapid local erosion can result from sediment that is too fine, insufficient fill volume, strong spreading at the nourishment ends, profile equilibration, or placement in a reach with continuing sediment loss. | - Thorough nourishment design study prior to implementation. |
| Beach close to the inlet of an engineered estuary | - Sand loss to siltation areas in harbor docks and inlet channels[3]. - Reclamation or closure reduces the tidal prism and forces the inlet channels and tidal deltas to adjust to a smaller equilibrium size. The resulting redistribution of large sediment volumes can temporarily increase or reduce sand supply to adjacent beaches, depending on where erosion and deposition occur[11]. |
- Beach nourishment with suitable sand dredged from docks and channels. |
| Beach downdrift* of the mouth of a river with reduced sand supply due to upstream dams and reservoirs | - Persistent sediment deficit, which may be concentrated near the river mouth and propagate downdrift. | - Shore nourishment. |
Note: *Downdrift, updrift with respect to dominant longshore sand transport direction
Hotspot identification and diagnosis
A hotspot should be identified from repeated observations covering a period long enough to distinguish persistent erosion from seasonal shoreline fluctuations and storm recovery. Shoreline position alone can be misleading because water-level variation and beach rotation may produce large apparent changes without sediment loss from the active coastal zone. Beach profiles, bathymetry and sediment volumes should therefore be considered where possible. The analysis should establish whether the eroded sand accumulates nearby or leaves the active coastal system. Monitoring should also establish whether the hotspot is stationary, migrating or reversing, because these behaviors require different management responses. Comparison with historical changes in structures, dredging, river supply, wave exposure and inlet morphology can then help identify the cause.
Related articles
- Natural causes of coastal erosion
- Human causes of coastal erosion
- Accretion and erosion for different coastal types
- Dune erosion
- Shallow-water wave theory
- Rhythmic shoreline features
References
- ↑ 1.0 1.1 Dean, R. G., Liotta, R. and Simon, G. 1999. Erosional hot spots. UFL/COEL-99/021, Coastal & Oceanographic Eng. Program, Univ. of Florida, Gainesville, FL, 60 pp.
- ↑ 2.0 2.1 Uscinowicz, G., Uscinowicz, S., Szarafin, T., Maszloch, E. and Wirkus, K. 2023. Rapid coastal erosion, its dynamics and cause — an erosional hot spot on the southern Baltic Sea coast. Oceanologia 66: 250—266
- ↑ 3.0 3.1 Kraus, N.C. and Galgano, F.A. 2001. Beach Erosional Hot Spots: Types, Causes, and Solutions. USACE report ERDC/CHL CHETN-II-44
- ↑ Schupp, C.A., McNinch, J.E. and List, J.H. 2006. Nearshore shore-oblique bars gravel outcrops, and their correlation to shoreline change. Mar. Geol. 233: 63–79
- ↑ Healy, T.R. 1987. The importance of wave focusing in the coastal erosion and sedimentation process. Proceed. Coastal Sediments ’87. American Society of Civil Engineering, New York, pp. 1472–1485
- ↑ Szczyrba, L., Mulligan, R. P., Pufahl, P., Humberston, J. and McNinch, J. 2024. Nearshore flow dynamics over shore-oblique bathymetric features during storm wave conditions. Journal of Geophysical Research: Oceans 129, e2023JC020630
- ↑ 7.0 7.1 7.2 Castelle, B., Scott, T., Brander, R.W. and McCarroll, R.J. 2016. Rip current types, circulation and hazard. Earth Science Reviews 163: 1–21
- ↑ Castelle, B., Marieu, V., Bujan, S., Splinter, K.D., Robinet, A., Sénéchal, N.and Ferreira, S. 2015. Impact of the winter 2013–2014 series of severe Western Europe storms on a double-barred sandy coast: Beach and dune erosion and megacusp embayments. Geomorphology 238: 135–148
- ↑ Pontiki, M., Puleo, J. A., Bond, H., Wengrove, M., Feagin, R. A., Hsu, T.-J. and Huff, T. 2023. Geomorphic response of a coastal berm to storm surge and the importance of sheet flow dynamics. Journal of Geophysical Research: Earth Surface 128, e2022JF006948
- ↑ Houser, C., Hapke, C. and Hamilton, S. 2008. Controls on coastal dune morphology, shoreline erosion and barrier island response to extreme storms. Geomorphology 100: 223–240
- ↑ Elias, E. and Van der Spek, A. 2017. Dynamic preservation of Texel Inlet, the Netherlands: Understanding the interaction of an ebb-tidal delta with its adjacent coast. Netherlands Journal of Geosciences 96: 293-317
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