Difference between revisions of "Natural causes of coastal erosion"

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Natural coastal erosion is caused by many factors which are discussed in detail in this article.
 
  
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==Transport gradient==
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Natural coastal [[erosion]] of sandy coasts is caused by many factors which are shortly reviewed in this article. Coastal erosion may also result from man-induced activities. Erosion due to human interventions is described in e.g. [[Human causes of coastal erosion]]. Most of the content of this article is drawn from Mangor et al. 2017 <ref> Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines</ref>.
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One cause of natural coastal erosion is an increasing gradient in transport rate in the direction of the net transport. This can be due to gradients in the wave conditions at certain stretches, a curved coastline, or special bathymetric conditions. An example of this kind of coastal condition is the West Coast of Skaw Spit, the northernmost tip of Denmark. The presence of the headland and the port at Hirtshals, combined with the shadow effect of southern Norway, results in increasing transport along the section of coastline some kilometres east of Hirtshals to Gammel Skagen. For this reason the entire NW-oriented section of the Skaw spit is exposed to erosion.
 
  
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[[Image:increasing littoral drift rate_b.jpg|center|500px| Increasing littoral drift rate along Skaw Spit, Denmark. This causes coastal erosion along the entire 25 km long section.]]
 
  
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:<small>Fig. 1. Increasing littoral drift rate along Skaw Spit, Denmark. This causes coastal erosion along the entire 25 km long section.</small>
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==Temporal erosion and ongoing erosion==
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An important notion for understanding coastal erosion refers to the concept of [[Coastal cell|coastal sediment cell]]. A coastal sediment cell (also called ''littoral cell'' or ''sediment cell'') is a coastal compartment that contains a complete cycle of sedimentation including sources, transport paths, and sinks. Erosion at one place in a coastal sediment cell implies accretion at another location within the same cell; the sediment distribution within the cell changes without affecting other coastal regions. A coastal sediment cell is in morphodynamic equilibrium if changes in the sediment distribution under the influence of fluctuating forcing (fluctuations in water levels, wave climate, including storms) have a temporal, quasi-cyclic character. Ongoing trends of erosion or accretion are excluded; ongoing erosion or accretion will finally lead to destruction or basic alteration of the coastal sediment cell. In practice there is always a net leakage of sediment from or to other coastal regions, but this can be a very slow process. So even though the coastal sediment cell is a theoretical concept, it can be very useful in practice for analysing and managing coastal erosion processes.
  
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==Loss of sand==
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One example of a coastal sediment cell is a pocket beach enclosed between headlands, assuming absence of net offshore or onshore sand transport. The orientation of the beach can change in response to fluctuations in the dominant direction of incident waves. However, the resulting erosion and accretion have just a temporal character. 
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Causes of coastal erosion due to loss of sand may come under the following circumstances:
 
  
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Another example of a coastal sediment cell refers to the concept of [[active coastal zone]]. The active coastal zone (sometimes also called ''active coastal profile'') is the beach zone over which sand is exchanged in cross-shore direction by natural processes. The seaward limit corresponds to the [[Closure depth|closure depth]] and the landward limit to a hard boundary (seawall, cliff, ..).  In the case of a dune coast the active zone comprises part of the front dune that can be eroded by storm waves (see [[Dune erosion]]). In the absence of net offshore or onshore sand transport and in the absence of gradients in [[littoral drift]] the active coastal zone is a one-dimensional sediment cell; the sediment volume in the active zone will be constant in time. Shoreline erosion and accretion in response to fluctuations in water level and wave climate (including storms) are temporal quasi-cyclic phenomena in this case, see [[Shoreline retreat and recovery]].
  
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An obvious example of ongoing erosion is cliff erosion. A coastal cliff eroded by wave attack will never be rebuilt by natural processes. Cliff erosion can be a fast process in case of soft cliffs (till, clay) and very slow in case of hard-rock cliffs. Other examples of ongoing natural erosion are given below.
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 +
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==Longshore sand loss due to transport gradients==
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[[Image:increasing littoral drift rate_b.jpg|left|thumb|550px|Fig. 1. Increasing littoral drift rate along Skaw Spit, Denmark. This causes coastal erosion along the entire 25 km long section.]]
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 +
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One cause of ongoing natural coastal [[erosion]] is a longshore increase of sand transport: in this case more sand is leaving a coastal section than entering. As longshore sand transport (also called [[littoral drift]]) depends primarily on the direction and height of breaking waves, a gradient in longshore transport can be due to longshore varying wave conditions, coastline curvature, or nearshore bathymetric features. An example of this kind of coastal condition is the West Coast of Skaw Spit, the northernmost tip of Denmark. The presence of the headland and the port at Hirtshals, combined with the shadow effect of southern Norway, results in increasing transport along the section of coastline some kilometres east of Hirtshals to Gammel Skagen. For this reason the entire NW-oriented section of the Skaw spit is exposed to erosion. <br clear=all>
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 +
==Cross-shore sand loss==
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Sand loss from the [[active coastal zone]] in cross-shore direction can occur by different processes.
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[[Image:overwash.jpg|300px|right|thumb|Fig. 2. Over-wash fans at Skallingen, S part of the Danish North Sea coast.]]
  
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[[Image:overwash.jpg|thumb|Fig. 2. Over-wash fans at Skallingen, S part of the Danish North Sea coast.]]
 
 
===Breaching and over-wash===
 
===Breaching and over-wash===
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The loss of sand inland due to breaching and over-wash of a barrier island and wind transport. This kind of sand loss takes place along the exposed coast of Skallingen barrier island on the southern part of the Danish North Sea coast, see Fig. 2.
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The loss of sand inland due to [[breaching]] and over-wash of a barrier island. This kind of sand loss takes place along the exposed coast of Skallingen barrier island on the southern part of the Danish North Sea coast, see Fig. 2.
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===Aeolian transport to the dunes===
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[[Image:EgmondCoastalRetreat.jpg|300px|left|thumb|Fig. 3. The old village Egmond at the North-Holland coast was swallowed by the sea due to fast shoreline retreat in the 17th-19th century. The new village has been reconstructed behind the present shoreline.]]
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A wide vegetated dune area can trap fine sands carried inland from the beach by onshore winds. When the dunes along the Dutch coast were fixed by vegetation from the 16th century they started growing by capturing large amounts of sand. This contributed to fast shoreline retreat, illustrated in Fig. 3. The import of beach sand to the dune area at the Dutch is estimated at 5-35 <math>m^3/m/year</math> <ref>De  Vries,  S.,  de Schipper,  M., Stive,  M.,  Ranasinghe,  R. 2011.  Sediment  exchange  between  the sub-aqueous  and  sub-aerial  coastal  zones.  Proceedings  of  the  International  Conference  on  Coastal Engineering 1 (32).</ref>  Assuming that this sand volume is withdrawn from the active zone (width of the order of 2 km, average slope of the order of 1/100), the resulting shoreline retreat can be estimated at 0.2-2 m/year. <br clear=all>
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===Offshore sand loss under extreme wave and storm surge conditions===
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High energetic waves cause seaward migration of [[Nearshore sandbars|breaker bars]] and high storm surges further cause an offshore movement of sand due to non-equilibrium in the profile during the high surge. Sand that is transported sufficiently far offshore will not return to the coast by wave-induced onshore transport under a milder wave climate.
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===Offshore sand loss to canyons===
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If there is a deep canyon close to the shore, sand may be lost into the canyon by [[littoral drift]].
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===Offshore transport at the tip of a [[sand spit]]===
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[[Image:skaw spit.jpg|400px|right|thumb|Fig. 4. Sand deposition at the north beach at Skaw Spit and corresponding loss of sand from the downstream beach.]]
  
  
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Sand can get lost by littoral drift into deep water at the tip of a sand spit forming the end point of  a [[Definitions of coastal terms#Sediment cell, Littoral cell or Coastal cell|littoral cell]]. Sand lost in this way causes accumulative shore and shoal  features in the deposition areas, at the expense of the downstream coast. An example of this is the Skaw Spit, see Fig.4.
  
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===Erosion downstream of accumulative forms===
  
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[[Image:Fehnam island.jpg|250px|left|thumb|Fig. 5. Grünerevle at the north coast of Fehmarn Island (grey colour), Germany. Littoral drift is from west to east (left to right in the figure). Sand spit formation at Grünerevle, parallel to the coast, starves the downstream coastline. Sand capture by the spit also explains the absence of sand accumulation west of the downstream port.]]
  
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A similar process of natural coastal erosion occurs downstream of accumulative forms at coastlines with very oblique wave approach, [[Classification of sandy coastlines|coast types]] 4M, 4E, 5M and 5E. Along such coastlines there is a tendency for the natural formation of spits parallel to the coast. They accumulate the sand and shift the sand supply offshore, which means that the downstream coastline is starved and begins to erode, see Fig. 5.  <br clear=all>
  
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===Sand loss at coastal protrusions===
  
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[[Image:NWNZ.jpg|500px|right|thumb|Fig. 6. The NW coast of North Zealand, Denmark, an example of a simplificated coast. The moraine landscape (red) has been cut back to a nearly straight line; the marine platform (yellow) has been formed in between.]]
  
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===Extreme wave and storm surge conditions===
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The loss of material from a protruding area to one or two sides is a natural cause of coastal erosion. This typically happens at till/sandstone headlands, where fine eroded material is washed away by currents and coarse material is transported alongshore or offshore away from the headland. More generally, any semi-hard seaward-concave section of a coastline will suffer erosion in case of insufficient supply of sand from rivers. The natural state of such a coastline is erosion and straightening; the straightened coastline is referred to as a simplificated coast, see Fig. 6.
  
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Offshore loss during extreme wave and storm surge conditions. The high waves cause the bars to move seawards and the high storm surges also cause an offshore movement of sand due to non-equilibrium in the profile during the high surge.
 
  
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===Canyons===
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Marine deposit shorelines suspended between eroding headlands (till or sandstone, for example) will retreat similarly. The headlands have historically provided material for building up the sedimentary shorelines and the suspended shoreline is consequently dependent on the presence of the headlands. However, as the headlands continue to erode, the sedimentary shorelines will follow suit despite the fact that they were originally accumulative forms. This development is part of the ''simplificated coast'' (see previous paragraph).
  
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The loss of sand into canyons. If there is a deep canyon close to a littoral transport coastline, sand may be lost into the canyon.
 
  
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===Accumulative beaches===
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Erosion also occurs at deltas coasts when the natural fluvial sand supply is reduced, depriving the delta protrusion from fluvial sand supply for its maintenance. This is generally caused by human interventions, see [[Human causes of coastal erosion]], but it can also have natural causes. Droughts in large river basins can result in long periods with almost no sand supply to the shoreline, leading to shore erosion. The historic large variations in the shorelines of the Nile delta were partly due to this situation, whereas the more recent erosion is mainly the result of human interventions along the Nile. Natural shifts between distributary channels are another cause of delta erosion.
  
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The loss of sand to an accumulative beach at the tip of a sand spit and into
 
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deep water at the leeward of the tip of a sand spit at the termination point of
 
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a littoral cell. Sand lost in this way causes accumulative shore and shoal
 
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features in the deposition areas, but the upstream coastline has lost the
 
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sand. An example of this is the Skaw Spit, see Fig. 3. below.
 
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[[Image:skaw spit.jpg|450px|center|Sand deposition at the north beach at Skaw Spit and corresponding loss of sand from the downstream beach.]]
 
  
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::<small>Fig. 3. Sand deposition at the north beach at Skaw Spit and corresponding loss of sand from the downstream beach.</small>
 
  
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==Protuding areas==
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==Climate change and sea-level rise==
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The loss of material from a protruding area to one or two sides is a natural cause of coastal erosion. This typically happens at till/sandstone headlands and at the tip of deltas, which do not receive sufficient material from the river due to natural shifting of the river alignment. Delta erosion can also be caused by human impact, which will be discussed later. It also occurs along semi-hard concave-shaped sections of coastline, which have no or only a small supply of sand from rivers. The natural state of such a coastline is erosion and straightening; the straightened coastline is referred to as a simplificated coast, see Fig. 4.
 
  
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[[Image:NWNZ.jpg|500px|centre|The NW coast of North Zealand, Denmark, an example of a simplificated coast; the moraine landscape (red) has been cut back to a nearly straight line, marine platform (yellow) has been formed in between.]]
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===Climate change impacts===
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::<small>Fig. 4. The NW coast of North Zealand, Denmark, an example of a simplificated coast; the moraine landscape (red) has been cut back to a nearly straight line, marine platform (yellow) has been formed in between.</small>
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Climate change will impact on coastal erosion in different ways. Here the focus is on sea-level rise; other potential impacts are related to changes in meteorological conditions – wind, temperature and precipitation. Changes in the precipitation regime will affect the sediment discharge of rivers and the resulting sand supply to the coast. Extreme conditions of strong precipitation and long periods of drought are expected to become more frequent. Temperature may play a role too, by its impact on soil erosion. The influence of variations of fluvial sand supply to the coast were shortly discussed in the previous section. Change in temperature will affect all life forms in the coastal zone. Coastal erosion is particularly sensitive to changes in coastal vegetation, dune vegetation for example. Mangrove coasts are sensitive to temperature change, but also to sea-level rise, see [[Potential Impacts of Sea Level Rise on Mangroves]]. Change in the wind regime and wave climate will modify the alongshore and cross-shore sand distribution. The alongshore sand distribution is very sensitive to the littoral drift, which strongly depends on wave direction. The shape of the cross-shore coastal profile is strongly influenced by [[wave run-up]], with an important role for storm events with high waves and water levels. Great uncertainty still exists regarding predictions for local changes in wind regime and wave climate caused by climate change.  
  
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==Marine deposit shorelines==
 
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Erosion can occur of the marine deposit shorelines suspended between sections of protruding semi-hard sections of the coastline, such as till or sandstone. The hard sections have historically provided material for building up the sedimentary shorelines. The shape of these shorelines is consequently dependent on the presence of the semi-hard sections and the wave climate. However, as the semi-hard sections continue to erode, the sedimentary shorelines will follow suit despite the fact that they were originally accumulative forms. This development is part of the ''simplificated coast'' (see ''Protruding Areas'' section, above).
 
  
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==Downstream erosion of accumulative forms==
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===Relative sea-level rise===
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[[Image:Fehnam island.jpg|thumb|Fig. 5. Grünerevle at the north coast of Fehmarn Island, Germany.]]
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The sea level will rise globally as a consequence of global warming, but regional differences are considerable. This holds in particular for relative sea-level rise, i.e. the change of sea level with respect to the local land level. Some coasts experience uplift (especially in previously glaciated regions) while others are subject to subsidence. Uplift can always be considered "natural", whereas subsidence often has an important human-induced component (groundwater, oil, gas extraction). According to the so-called "[[Bruun rule]]", an increasing relative sea level will cause a shoreline setback, which is approximately equal to the sea level rise divided by the average slope of the active coastal profile, when considering equilibrium profiles. Consider, for example, a sea level rise of 0.5 m and an equilibrium coastal profile with a slope of the shoreface and the shore of 1/100. The setback caused by such a sea level rise will be 50 m. Littoral coasts consisting of fine sediments will be exposed to higher setbacks than coasts consisting of coarser sediments.
  
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Another cause of natural coastal erosion is the erosion downstream of accumulative forms at coastlines with very oblique wave approach, [[coast types]] 4M, 4E, 5M and 5E. Along such coastlines there is a tendency for the natural formation of spit formations parallel to the coast. They accumulate the sand and shift the sand supply offshore, which means that the downstream coastline is starved and begins to erode, see Figure, Grünerevle at the north coast of Fehmarn Island, Germany, an example of the formation of a sand spit formation parallel to the coast, which starves the downstream coastline. Note that there is no sand accumulation west of the port because the sand is trapped in the sand spit further upstream..
 
  
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==Sea level rise==
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==Related articles==
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A world-wide sea level rise is a phenomenon, which has been discussed for decades. A global sea level rise of 0.1 to 0.25 m was recorded over the last century. The forecast for the global sea level rise for the next century varies considerably; however, with a central estimate of 0.2 m and 0.5 m at the middle and end of the 21st century, respectively, according to International Panel on Climate Change, IPCC<ref>IPCC, 2004[http://www.ipcc.ch/]</ref>. An increasing sea level will cause a shoreline setback, which is approximately equal to the sea level rise divided by the slope of the active coastal profile, when considering equilibrium profiles. Consider, for example, a sea level rise of 0.5 m and an equilibrium coastal profile with a slope of the shoreface and the shore of 1/100. The setback caused by such a sea level rise will be 50 m. Littoral coasts consisting of fine sediments will be exposed to higher setbacks than coasts consisting of coarser sediments.
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* [[Dealing with coastal erosion]]
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* [[Coastal Hydrodynamics And Transport Processes]]
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* [[Littoral drift and shoreline modelling]]
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* [[Shoreline retreat and recovery]]
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* [[Active coastal zone]]
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* [[Shoreface profile]]
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* [[Sea level rise]]
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* [[Bruun rule]]
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* [[Closure depth]]
  
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==Subsidence==
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Coastal erosion due to human intervention:
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Subsidence lowers the surface in a specific region. Subsidence is a local/regional phenomenon in contrast to the sea level rise, which is global. Subsidence can be caused by many different phenomena, natural as well as human. Natural causes can be the settling of soft sediments, tectonic activity and different kinds of rebound processes, whereas human causes can be the extraction of groundwater, oil or gas in the coastal area. Subsidence acts in the same way as sea level rise in relation to shore erosion apart from the fact that a sea level rise will always be a gradual and slow process, whereas subsidence may occur rapidly depending on the cause of the subsidence.
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* [[Human causes of coastal erosion]]: an article on human-related factors causing coastal erosion.
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* [[Accretion and erosion for different coastal types]]: Erosion caused by a large port for different types of coasts
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* [[Port breakwaters and coastal erosion]]: Effects of breakwaters from different types of ports (isolated, river mouth in the sea, mouth of a large estuary) on coastal erosion
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* [[Hard coastal protection structures]]: Explains the possible impacts of hard structures on the structural erosion of a stretch of coast
  
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==Natural variation==
 
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The natural variation in the supply of sand to a coastline from a river can contribute to erosion. Droughts in large river basins can result in long periods with decreasing supplies of sand to the shoreline, leading to shore erosion. The historic large variations in the shorelines of the Nile delta were partly due to this situation, whereas the more recent erosion is mainly the result of human interventions along the Nile.
 
  
 
==References==
 
==References==
 
<references/>
 
<references/>
  
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==See also==
 
  
  
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{{author
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|AuthorID=13331
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|AuthorFullName=Mangor, Karsten
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|AuthorName=Karsten}}
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{{Review
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|name=Job Dronkers
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|AuthorID=120
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}}
  
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''Author: Karsten Mangor, DHI, 2004. km@dhi.dk.''
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[[Category:Coastal protection]]
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''All content is written by Karsten Mangor unless referenced otherwise.''
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[[Category:Physical coastal and marine processes]]
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[[Category:Beaches]]
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[[Category:Sediment]]
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[[Category:Climate change, impacts and adaptation]]
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[[Category:Sea level rise]]

Latest revision as of 16:21, 26 August 2022

Natural coastal erosion of sandy coasts is caused by many factors which are shortly reviewed in this article. Coastal erosion may also result from man-induced activities. Erosion due to human interventions is described in e.g. Human causes of coastal erosion. Most of the content of this article is drawn from Mangor et al. 2017 [1].


Temporal erosion and ongoing erosion

An important notion for understanding coastal erosion refers to the concept of coastal sediment cell. A coastal sediment cell (also called littoral cell or sediment cell) is a coastal compartment that contains a complete cycle of sedimentation including sources, transport paths, and sinks. Erosion at one place in a coastal sediment cell implies accretion at another location within the same cell; the sediment distribution within the cell changes without affecting other coastal regions. A coastal sediment cell is in morphodynamic equilibrium if changes in the sediment distribution under the influence of fluctuating forcing (fluctuations in water levels, wave climate, including storms) have a temporal, quasi-cyclic character. Ongoing trends of erosion or accretion are excluded; ongoing erosion or accretion will finally lead to destruction or basic alteration of the coastal sediment cell. In practice there is always a net leakage of sediment from or to other coastal regions, but this can be a very slow process. So even though the coastal sediment cell is a theoretical concept, it can be very useful in practice for analysing and managing coastal erosion processes.

One example of a coastal sediment cell is a pocket beach enclosed between headlands, assuming absence of net offshore or onshore sand transport. The orientation of the beach can change in response to fluctuations in the dominant direction of incident waves. However, the resulting erosion and accretion have just a temporal character.

Another example of a coastal sediment cell refers to the concept of active coastal zone. The active coastal zone (sometimes also called active coastal profile) is the beach zone over which sand is exchanged in cross-shore direction by natural processes. The seaward limit corresponds to the closure depth and the landward limit to a hard boundary (seawall, cliff, ..). In the case of a dune coast the active zone comprises part of the front dune that can be eroded by storm waves (see Dune erosion). In the absence of net offshore or onshore sand transport and in the absence of gradients in littoral drift the active coastal zone is a one-dimensional sediment cell; the sediment volume in the active zone will be constant in time. Shoreline erosion and accretion in response to fluctuations in water level and wave climate (including storms) are temporal quasi-cyclic phenomena in this case, see Shoreline retreat and recovery.

An obvious example of ongoing erosion is cliff erosion. A coastal cliff eroded by wave attack will never be rebuilt by natural processes. Cliff erosion can be a fast process in case of soft cliffs (till, clay) and very slow in case of hard-rock cliffs. Other examples of ongoing natural erosion are given below.


Longshore sand loss due to transport gradients

Fig. 1. Increasing littoral drift rate along Skaw Spit, Denmark. This causes coastal erosion along the entire 25 km long section.


One cause of ongoing natural coastal erosion is a longshore increase of sand transport: in this case more sand is leaving a coastal section than entering. As longshore sand transport (also called littoral drift) depends primarily on the direction and height of breaking waves, a gradient in longshore transport can be due to longshore varying wave conditions, coastline curvature, or nearshore bathymetric features. An example of this kind of coastal condition is the West Coast of Skaw Spit, the northernmost tip of Denmark. The presence of the headland and the port at Hirtshals, combined with the shadow effect of southern Norway, results in increasing transport along the section of coastline some kilometres east of Hirtshals to Gammel Skagen. For this reason the entire NW-oriented section of the Skaw spit is exposed to erosion.

Cross-shore sand loss

Sand loss from the active coastal zone in cross-shore direction can occur by different processes.

Fig. 2. Over-wash fans at Skallingen, S part of the Danish North Sea coast.

Breaching and over-wash

The loss of sand inland due to breaching and over-wash of a barrier island. This kind of sand loss takes place along the exposed coast of Skallingen barrier island on the southern part of the Danish North Sea coast, see Fig. 2.

Aeolian transport to the dunes

Fig. 3. The old village Egmond at the North-Holland coast was swallowed by the sea due to fast shoreline retreat in the 17th-19th century. The new village has been reconstructed behind the present shoreline.

A wide vegetated dune area can trap fine sands carried inland from the beach by onshore winds. When the dunes along the Dutch coast were fixed by vegetation from the 16th century they started growing by capturing large amounts of sand. This contributed to fast shoreline retreat, illustrated in Fig. 3. The import of beach sand to the dune area at the Dutch is estimated at 5-35 [math]m^3/m/year[/math] [2] Assuming that this sand volume is withdrawn from the active zone (width of the order of 2 km, average slope of the order of 1/100), the resulting shoreline retreat can be estimated at 0.2-2 m/year.

Offshore sand loss under extreme wave and storm surge conditions

High energetic waves cause seaward migration of breaker bars and high storm surges further cause an offshore movement of sand due to non-equilibrium in the profile during the high surge. Sand that is transported sufficiently far offshore will not return to the coast by wave-induced onshore transport under a milder wave climate.

Offshore sand loss to canyons

If there is a deep canyon close to the shore, sand may be lost into the canyon by littoral drift.

Offshore transport at the tip of a sand spit

Fig. 4. Sand deposition at the north beach at Skaw Spit and corresponding loss of sand from the downstream beach.


Sand can get lost by littoral drift into deep water at the tip of a sand spit forming the end point of a littoral cell. Sand lost in this way causes accumulative shore and shoal features in the deposition areas, at the expense of the downstream coast. An example of this is the Skaw Spit, see Fig.4.

Erosion downstream of accumulative forms

Fig. 5. Grünerevle at the north coast of Fehmarn Island (grey colour), Germany. Littoral drift is from west to east (left to right in the figure). Sand spit formation at Grünerevle, parallel to the coast, starves the downstream coastline. Sand capture by the spit also explains the absence of sand accumulation west of the downstream port.

A similar process of natural coastal erosion occurs downstream of accumulative forms at coastlines with very oblique wave approach, coast types 4M, 4E, 5M and 5E. Along such coastlines there is a tendency for the natural formation of spits parallel to the coast. They accumulate the sand and shift the sand supply offshore, which means that the downstream coastline is starved and begins to erode, see Fig. 5.

Sand loss at coastal protrusions

Fig. 6. The NW coast of North Zealand, Denmark, an example of a simplificated coast. The moraine landscape (red) has been cut back to a nearly straight line; the marine platform (yellow) has been formed in between.

The loss of material from a protruding area to one or two sides is a natural cause of coastal erosion. This typically happens at till/sandstone headlands, where fine eroded material is washed away by currents and coarse material is transported alongshore or offshore away from the headland. More generally, any semi-hard seaward-concave section of a coastline will suffer erosion in case of insufficient supply of sand from rivers. The natural state of such a coastline is erosion and straightening; the straightened coastline is referred to as a simplificated coast, see Fig. 6.


Marine deposit shorelines suspended between eroding headlands (till or sandstone, for example) will retreat similarly. The headlands have historically provided material for building up the sedimentary shorelines and the suspended shoreline is consequently dependent on the presence of the headlands. However, as the headlands continue to erode, the sedimentary shorelines will follow suit despite the fact that they were originally accumulative forms. This development is part of the simplificated coast (see previous paragraph).


Erosion also occurs at deltas coasts when the natural fluvial sand supply is reduced, depriving the delta protrusion from fluvial sand supply for its maintenance. This is generally caused by human interventions, see Human causes of coastal erosion, but it can also have natural causes. Droughts in large river basins can result in long periods with almost no sand supply to the shoreline, leading to shore erosion. The historic large variations in the shorelines of the Nile delta were partly due to this situation, whereas the more recent erosion is mainly the result of human interventions along the Nile. Natural shifts between distributary channels are another cause of delta erosion.


Climate change and sea-level rise

Climate change impacts

Climate change will impact on coastal erosion in different ways. Here the focus is on sea-level rise; other potential impacts are related to changes in meteorological conditions – wind, temperature and precipitation. Changes in the precipitation regime will affect the sediment discharge of rivers and the resulting sand supply to the coast. Extreme conditions of strong precipitation and long periods of drought are expected to become more frequent. Temperature may play a role too, by its impact on soil erosion. The influence of variations of fluvial sand supply to the coast were shortly discussed in the previous section. Change in temperature will affect all life forms in the coastal zone. Coastal erosion is particularly sensitive to changes in coastal vegetation, dune vegetation for example. Mangrove coasts are sensitive to temperature change, but also to sea-level rise, see Potential Impacts of Sea Level Rise on Mangroves. Change in the wind regime and wave climate will modify the alongshore and cross-shore sand distribution. The alongshore sand distribution is very sensitive to the littoral drift, which strongly depends on wave direction. The shape of the cross-shore coastal profile is strongly influenced by wave run-up, with an important role for storm events with high waves and water levels. Great uncertainty still exists regarding predictions for local changes in wind regime and wave climate caused by climate change.


Relative sea-level rise

The sea level will rise globally as a consequence of global warming, but regional differences are considerable. This holds in particular for relative sea-level rise, i.e. the change of sea level with respect to the local land level. Some coasts experience uplift (especially in previously glaciated regions) while others are subject to subsidence. Uplift can always be considered "natural", whereas subsidence often has an important human-induced component (groundwater, oil, gas extraction). According to the so-called "Bruun rule", an increasing relative sea level will cause a shoreline setback, which is approximately equal to the sea level rise divided by the average slope of the active coastal profile, when considering equilibrium profiles. Consider, for example, a sea level rise of 0.5 m and an equilibrium coastal profile with a slope of the shoreface and the shore of 1/100. The setback caused by such a sea level rise will be 50 m. Littoral coasts consisting of fine sediments will be exposed to higher setbacks than coasts consisting of coarser sediments.


Related articles

Coastal erosion due to human intervention:


References

  1. ↑ Mangor, K., Drønen, N. K., Kaergaard, K.H. and Kristensen, N.E. 2017. Shoreline management guidelines. DHI https://www.dhigroup.com/marine-water/ebook-shoreline-management-guidelines
  2. ↑ De Vries, S., de Schipper, M., Stive, M., Ranasinghe, R. 2011. Sediment exchange between the sub-aqueous and sub-aerial coastal zones. Proceedings of the International Conference on Coastal Engineering 1 (32).


The main author of this article is Mangor, Karsten
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Citation: Mangor, Karsten (2022): Natural causes of coastal erosion. Available from http://www.coastalwiki.org/wiki/Natural_causes_of_coastal_erosion [accessed on 21-11-2024]




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