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Jennifer Roberts

Coastal erosion – Natural Causes

Coastal erosion is a slow, long term phenomenon occurring in virtually all coastal areas. Coastal erosion is caused by natural factors (known as natural coastal erosion) or by human factors (anthropological coastal erosion). This article will discuss the natural coastal erosion phenomenon, its causes, and its effects.

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Temporal erosion and continuous erosion

To better understand how natural coastal undermining erosion works, it's important to know what a coastal sediment cell is. Also known as a sediment cell or a littoral cell, this geographical unit is defined as a coastal compartment that includes the complete cycle of sedimentation. Essentially, this cell includes the sources of sediments, the transportation paths, and the sinks. When erosion is active in one place inside the cell, accretion will occur in a different location. The sediment distribution is thus in constant equilibrium. If ongoing periods of erosion or accretion occur, the sediment cell and its equilibrium will suffer irreparable damages. In fact, this type of net leakage between areas in a single cell occurs, but the process is very slow. Simply put, the coastal sediment cell is a theoretical concept and has exceptions, but it is an important tool when trying to understand erosion as a whole.

A good example of a coastal sediment cell is a pocket beach (a small beach enclosed by headlands), with minimal net offshore or onshore sand transport. The orientation of this beach can change, depending on the fluctuations of the waves. The resulting erosion or accretion only have a temporal character.

Another important example of a coastal sediment cell is the active coastal zone. This zone is essentially a beach zone, where sand is exchanged in a cross-shore direction, as a result of natural processes. The closure depth corresponds to the seaward limit, and the landward limit to a physical boundary, like a cliff or a seawall. If the coast has sand dunes, the active zone is the zone commonly affected by erosion, close to the undermining shoreline. The erosion, in this case, is responsive to water levels and wave activity and is temporal or quasi-cyclic.

Cliff erosion is an obvious example of ongoing erosion. Cliffs affected by erosion will never recover naturally. Cliff erosion can be fast, if the cliff is made of till or clay, or very slow if the cliff is rocky.

Long-shore sand loss caused by transport gradients

This type of ongoing sand loss is caused by irregular sand transport. Simply put, this undermining erosion occurs when more sand is leaving an area than entering. Transport depends primarily on the direction and strength of the waves. Another factor is the geometry of the shore, the curvature of the ground and the bathymetric features. This type of erosion is long-term and won't stop naturally.

Cross-shore sand loss

This type of coastal erosion can be caused by different processes. Let's take a closer look:

Breaching and over-wash

This type of sand loss is caused by breaching and an over-wash during high tides. In these cases, the beaches are very sensitive and the water washes away the sand constantly. Exposed coastal areas are more prone to this type of erosion.

 

Aeolian transport of sand

Sand can be collected in vegetated areas near the coastline. These vegetated dunes collect sand and grow, causing erosion along the undermining shoreline. A major issue is fast shoreline retreat, often estimated at 1 to 6 feet per year.

 

Sand loss caused by extreme waves and powerful storms

High energy waves cause a seaward migration of the breaker bars. Powerful storms further cause the offshore movement of the sand. The sand transported far away from the shore will not be able to naturally return to the coastal area, even if milder weather occurs.

 

Sand loss in canyon areas

Deep canyons that are located close to the undermining shoreline have a negative effect on soil erosion. Sand will collect into the canyon through the natural process of littoral drift.

 

Sandspit offshore transport

Sand can be transported by a littoral drift towards deep water, especially at the tip of a sand spit. The sand that is lost through this process will accumulate far away from the shore, forming shoals and deposition areas. This sand will not return naturally to the beach, thus creating more erosion.

Downstream erosion for accumulative forms

Massive undermining erosion occurs in coastal areas where the waves have a very oblique approach. These coastlines have a natural tendency of forming spits parallel to the coasts, in order to offer protection against erosion. The sand is accumulated far away from the shore, depleting the sand near the beach area. As a result, the shoreline is more prone to erosion.

 

Sand loss for coastal protrusions

Protruding areas are very susceptible to coastal erosion, especially if they are made of soft materials, like clay or till. The fine eroded material is constantly washed away by wind and waves. The material is then transported either along the coast or offshore. Virtually any semi-hard seaward concave section of coastline will suffer this type of erosion, particularly if there is no steady supply of sand from rivers or other natural sources. The natural state of the coastline is constant erosion and straightening; the result of this ongoing phenomenon is a simplified, straight coast.

Marine deposit shorelines will erode constantly and retreat in time. The headlands will be unrecognizable and the whole shore will be straight. As the headlines continuously erode, the sedimentary shorelines will follow suit, losing their accumulative forms.

 

The impact of climate change on coastal erosion

Climate change is expected to impact coastal erosion in various ways. The main focus is sea-level rise, which is a problem in many areas of the world. The changes will also focus on meteorological conditions – temperature, wind, and precipitation. Extreme conditions are of particular importance, as they have a massive negative effect on shorelines. For instance, long periods of drought, coupled with powerful storms, have an immense impact on erosion. The variation of river and fluvial sand is also of concern, as beaches will not be able to replenish the sand.