Erosion hotspots
Definition of Erosion hotspot:
A limited area which erodes more rapidly and/or equilibrates with a significantly narrower beach width compared to adjacent beaches[1].
This is the common definition for Erosion hotspot, other definitions can be discussed in the article
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Erosion hotspots are sites along the beach subject to excessive erosion compared to adjacent stretches, see illustration Fig. 1. The local sand balance is disrupted either by excessive sand loss or deficient sand supply. Erosion hotspots can be related to bathymetric features in the alongshore or cross-shore surroundings that affect wave and current patterns or retain sediment. These features are in most cases related to human interventions. Natural processes may also play a role, especially time-dependent processes related to shoreline instability. Long-term persistent hotspots can arise from natural contemporary events, such as tectonic activity, extreme meteorological conditions, river mouth shift and delta abandonment. Events in the geological past are seldom relevant for the formation of erosion hotspots because of the shorter time scale of morphodynamic adaptation of the beach system. The formation of erosion hotspots are most often a consequence of coastal development policies or inadequate coastal protection practices. The formation of these hotspots can more easily be avoided or mitigated than hotspots resulting from natural processes.
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.
Hotspot type | Cause of erosion | Possible preventive and remediation measures |
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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]. - Offshore directed rip currents resulting from alongshore-varying wave breaking across complex bathymetry[6]. |
- Compensate with shore nourishment or accept the loss. |
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[7]. |
- Temporary situation with natural recovery if the beach is close to long-term morphodynamic equilibrium[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. |
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 always accompanied by accretion of an adjacent area (e.g. formation of a salient). - Redistribute sand along the beach. - Remove or adapt the structures. - Long linear shore-parallel structures (no gaps) can solve the problem (but may create others). |
Beach facing an offshore canyon or other relict bathymetric depressions | - Canyon acting as a conduct for irreversible offshore sand loss. - Wave focusing over bathymetric depressions. |
- Compensate with shore nourishment or accept the loss. |
Beach with sand borrow pit within the active coastal profile | - The pit will collect beach sand. | - Move sand borrow location 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) | - Longshore sand transport away from the coast. | - Headland: Protect beach fill with a downdrift structure (e.g. incurved groyne). - 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. - Preventive beach nourishment. |
Beach situated downdrift* of a tidal inlet 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, coastal headlands | - 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. headlands, piers, groins, 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 erosion under storm conditions due to - wave breaking on the toe of the constructions (see Seawalls and revetments), - deficient sand supply from the backshore berm for the formation of a protective nearshore bar[9], - after-storm recovery impeded by moisture[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 (e.g. inappropriate slope, inappropriate sediment grainsize) | - Sand loss due to non respect of longshore or cross-shore morphodynamic equilibrium. | - Thorough nourishment design study (lab or numerical) prior to implementation. |
Beach close to the inlet of an engineered estuary | - Sand loss to siltation areas in harbor docks and inlet channels[3]. - Sand loss due to land reclamation in an adjacent estuary or tidal lagoon where tidal prism reduction has created accommodation space for sedimentation[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 | - Beach erosion due to disruption of the equilibrium coastal sand balance resulting from the reduction of fluvial sand supply. | - Shore nourishment. |
Note: *Downdrift, updrift with respect to dominant longshore sand transport direction
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
- ↑ Jump up to: 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.
- ↑ Jump up to: 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
- ↑ Jump up to: 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
- Jump up ↑ 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
- Jump up ↑ 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
- Jump up ↑ 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
- ↑ Jump up to: 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
- Jump up ↑ 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
- Jump up ↑ 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
- Jump up ↑ 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
- Jump up ↑ 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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