Dealing with coastal erosion

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Job DronkersSee the discussion page

This article describes the background and main issues related to shore protection. The article also describes on headlines which protection measures can be used to protect the coast from structural erosion and from incidental erosion (incidental shoreline retreat and recovery). The notion coastal erosion sounds simple and clear, but it is in fact a rather tricky notion. Two quite different coastal processes might cause erosion, viz.: structural erosion and incidental storm-induced beach erosion or dune erosion. When dealing with coastal protection measures one has to be aware of this distinction.

How to deal with coastal erosion problems is the main topic of this article. Selecting a proper approach for a protection scheme calls, however, for insight in the real causes of the actual erosion problem.

Background of shore protection

Dynamic sea-land transition

The transition between sea and land (shoreline) is never a fixed line along sandy coasts. Seen at a short time scale (seasonal and intra-seasonal time scale), the shoreline continuously shifts in landward and seaward direction. This is a well-known and intriguing feature of the natural behaviour of our coasts. Seen over a longer time scale, many coasts all over the world show a structural, gradual and continuous accreting or eroding tendency, although stable coasts also occur. Pure natural (autonomous) reasons, or man-induced reasons may cause this behaviour.

While accreting coasts are most of the time welcomed by coastal authorities and by people living near the sea, eroding coasts cause serious problems. Valuable land with most likely various existing interests, will be lost.

Vulnerable systems

Coastal systems are generally vulnerable systems. It is easy to harm such a vulnerable system with thoughtless surgery. This calls for sustainable, technically sound shoreline management, embedded in an appropriate social, institutional and legal framework. See the article Shoreline management for a more detailed discussion of these aspects.

In this article various coastal engineering tools to achieve the intended goals are discussed. It will be argued that these goals must be well-defined. Without well-defined goals adequate solutions to coastal engineering will probably not be found.

Time-perspective

Our earth exists for about 4.5 billion years. During most of this time pure natural developments shaped and reshaped our coasts. At some places new land was created; at other places erosion occurred. Nowadays coastal accretion is often felt 'good' and erosion 'bad'. In the past these notions did not exist. Accretion was not better than erosion or vice versa.

Since say 45,000 years (≈ 0.001 % of the age of the earth!) mankind living along coasts is faced with the caprices of the evolving coastal system (coastal behaviour). Since people had to deal with this behaviour, value judgements play a role. Coastal erosion was generally felt annoying and bad. Mankind, facing for instance a 'bad' structural eroding behaviour of the coast, showed an increasingly assertive attitude with time. The next notions might be discerned in sequence of time since mankind got in trouble with eroding coasts:

• It is a pity; we must pull down our humble hut close to the sea and rebuild the hut just a little bit farther landward; that is nature.
• It is a pity, but we notice some trends; it is clever to take past developments into account while choosing a new place for our hut.
• We don't like to replace our nice hut every moment; it would be nice if we could do something; but we realize that it is still impossible.
• Rebuilding our house every time becomes very annoying; we must find some tools to protect us; let us try to do something.
• Houses, roads and infrastructure are at stake; we can spend some money to protect us; various tools have been developed; with some kind of trial-and-error method we will see which tool might serve our goals.
• Large hotels and many tourist facilities face the consequences of a 'bad' behaviour of the coast; an integrated protection scheme based on scientific research will do the job.
• The coast must behave according our wishes and rules!

We must be careful with the latter attitude. At present, applying the best of our knowledge and experience, we are able to deal effectively with coastal systems. However, some modesty remains highly recommended.

Dealing with structural erosion

General

First of all it has to be stressed that with a proper system of shoreline management, some troublesome coastal protection issues can be avoided. If, for instance, it is not allowed (and the legal system is able to compel this!) to build too close to the brink of the mainland, a lot of (future) problems with respect to erosion due to severe storms and even due to structural erosion (for the time being), are avoided. With well-defined (and enforced) set-back lines, the risks of damage to, and losses of, properties are reduced.

Furthermore it is not excluded that as a result of a comprehensive and equitable decision making process, it is decided to renounce an intended project because of the too large adverse effects elsewhere along the coast.

In the following paragraphs we focus on alongshore sediment transport. Cross-shore sediment transport will be discussed later.

Shoreline stabilisation with hard measures

Figure 1. Sketch of the net littoral drift [math]S(x)[/math] along the shoreline for different hard interventions, such as groynes or detached breakwaters.

Structural erosion is in most cases due to gradients in the net littoral drift (longshore, mainly wave-driven net sediment transport in the surf zone). The net littoral drift [math]S(x)[/math] is usually expressed in units [math]m^3/s[/math]. With the application of 'hard' solutions to the erosion problem, the coastal authority intends to interfere with the occurring sediment transport processes. If the protection measure is properly designed, the gradient in the longshore sediment transport along the part of the coast to be protected, should vanish: [math]dS(x)/dx=0[/math]. (A more detailed discussion of littoral drift is presented in the article Littoral drift and shoreline modelling).

Consider a stretch of coast of several km including a section A – B, see Fig. 1. The longshore increase of [math]S(x)[/math] with increasing [math]x[/math] implies structural erosion without intervention: the amount of sediment leaving the section A – B is greater than the amount entering (littoral drift distribution 1). Year after year the coastline will retreat. Now suppose that the section A - B is considered a very important part of the coast, where houses, hotels and infrastructure are present. Without counter-measures sooner or later these valuable properties will be destroyed because of ongoing erosion.

One may try to solve the issue of structural erosion at section A - B with some 'hard' intervention, for example by building groynes or detached breakwaters. This may change the distribution of the net littoral drift [math]S(x)[/math], as indicated by graph (2). In section A - B the sediment transport is constant [math]dS(x)/dx=0[/math]; no erosion will occur in part A - B anymore. However, tuning of the interventions in order to achieve distribution (2) is quite difficult. Moreover, it is also clear from the sketch that the 'solution' for A - B implies an increased gradient [math]dS(x)/dx[/math] and thus increased coastal erosion at the downdrift side of B. Failure to achieve the distribution (2) may result in distribution (3) or (4). Distribution (3) reflects a situation where the intervention causes a too strong change of the net littoral drift. Section A - B will accrete, but more leeside erosion is a consequence. Distribution (4) reflects a case where the interference of the intervention was not large enough. The erosion rate in section A - B has been reduced, but still some erosion will occur in this section.

An extended description of a similar example can be found in Littoral drift and shoreline modelling and in other references, for example (Bijker and Van de Graaff, 1983) and (d’Angremond and Pluim-Van der Velden, 2001) .

Groynes and shore parallel offshore breakwaters

Series of groynes or series of shore parallel offshore breakwaters interfere with the longshore sediment transport [math]S(x)[/math]. They may 'work' as a tool in a protection scheme, requiring, as discussed above, a delicate process of fine-tuning of the design layout. However, leeside erosion is an unavoidable consequence of a well-designed protection scheme of the coast section to be protected. Downdrift of the protection scheme (where littoral drift is reduced) sediment transport is the same as before. This leads to large gradients in the longshore sediment transport (much larger than the original gradients), causing leeside erosion as mentioned.

While protecting the coast in the area of interest against the effect of a gradient in longshore transport, in fact this is achieved at the expense of the coast just outside (at the leeside of) the area of interest. The leeside erosion effect can be reduced/mitigated when several terminating groins are built as sediment permeable structures or when they have reduced lengths; the rates of permeability of these terminal groins or their reduced lengths should be determined by numerical modeling. In some cases changing the coastal orientation may reduce leeside erosion to an acceptable level.

While applying series of groynes or series of shore parallel offshore breakwaters, the coastal authority has to take these (adverse) consequences fully into account in the decision making process. Often these types of protection measures are applied too rashly.

For more information on this topic, see also the articles Groynes, Groynes as shore protection, Detached breakwaters, Detached shore parallel breakwaters and Applicability of detached breakwaters. In certain situations floating breakwaters can serve as an alternative structure for fixed breakwaters.

Shoreline stabilisation with soft measures

Structural erosion means the gradual loss of sediments from a coastal section with time (see Fig. 1). Sooner or later not only the foreshore is losing volumes of sand, but also the beaches and backshore. Finally even properties built at the mainland may be lost. If, besides incidental erosion, structural erosion (also) occurs along the considered coastal section, revetments or seawalls are not always the best protection measure (see Seawalls and revetments). Soft shoreline protection solutions might be considered a possible solution, reducing also the risk of damage during a severe storm surge.

Soft shoreline protection solutions imply that by artificially widening the beach in seaward direction, the risks may be relieved. However, this requires rather large volumes of sediments, because not only the shoreline has to be shifted in seaward direction, but the entire active coastal zone. Furthermore a 'soft' solution must in this case be applied over a rather long distance alongshore. If a 'soft' solution is applied only locally, a large maintenance effort is required because of the redistribution processes in both alongshore directions of the artificial nourishment.

Artificial nourishments are a good solution in many cases, even if they are to be repeated regularly. If the occurring losses are renourished from time to time, the average position of the coastline will be stable, seen over a number of years. Because artificial nourishments hardly interfere with occurring sediment transport processes, they do not stop the ongoing erosion process. This method thus requires permanently repeated maintenance nourishments.

The source of the fill material (borrow material), can be outside the nearshore coastal system, or inside the coastal system. In the latter case a perfect source of borrow material would be for example the accumulated sediments at the updrift side of a harbour jetty that interrupts the occurring longshore sediment transport. For this structural sand by-pass systems can be designed.

Sometimes it is felt that artificial nourishment schemes are too expensive for developing countries. However, good alternatives (if any?) are at the long run in many cases even more expensive. More details about the application of artificial nourishments can be found in the special issue on artificial nourishments of Coastal Engineering (1991) and in many handbooks, for example Dean (2002).

Structural erosion is most obviously manifested at the upper parts of the cross-shore profiles (backshore). It seems therefore logical to replenish just the upper part of the profile. This is rather expensive and not always necessary. A large scale coastal nourishment programme of the Dutch coast that has started in 1990 shows that nourishing the deeper (subaqueous) part of the cross-shore profile (shoreface nourishments in the active coastal zone) may also fulfil the requirement of shoreline stabilisation. However, shoreface nourishments hardly contribute to widening of the beach .

More detailed information on the practice of coastal nourishment can be found in the articles Beach nourishment, Artificial nourishment and Shore nourishment.

Dealing with incidental erosion; hard structures

Revetments and seawalls are so-called 'hard' structures. Such hard structures can be appropriate measures to tackle the issue of storm (surge) protection, but they are less appropriate for eroding coasts. They should be applied preferentially at a stable part of the coast (stable: seen over a number of years) or an accreting part of the coast.

Here we focus on situations where coastal erosion is mainly due to offshore-directed sediment transport. This is often occurs as result of storm surges, in which case hard coastal protection measures are often applied to reduce the risk of damage to settlements situated close to the sea. Hard protection measures are a revetment along the face of the mainland or by a seawall.

Seawalls and revetments

Revetments or seawalls hardly interfere with longshore sediment transport under average conditions if they are situated outside the reach of waves and currents which are causing the (gradient in the) longshore sediment transport. In the case of a structurally eroding coast, erosion will continue until reaching the revetment or seawall.

Figure 2. Scour in front of revetment

Sediments from the beach are transported towards deeper water during severe storms if hard protection structures are absent. Part of these sediments will return to the beach under average wave conditions (see Shoreline retreat and recovery). Full beach recovery will take place for stable or accreting coasts; structurally eroding coasts will only partially recover. Revetments or seawalls, if well-designed to withstand severe storm attack, are physically able to protect the properties built at the mainland. However, the retention of sediments behind a revetment or seawall reduces the amount of 'free' sediments that are redistributed in the active coastal zone by natural processes under storm conditions and thereafter. Wave energy dissipation is concentrated in a narrow zone in front of the revetment or seawall. These two factors will cause erosion of the beach just in front of the revetment or seawall. Deep scour holes may develop, which endanger the stability of the structure. This phenomenon has to be taken into account when defining the foundation depth of the revetment or seawall (see Fig. 2). Recovery of the beach after storm erosion may also be compromised if the beach is not wide enough. For reduction of incidental erosion (including dune erosion), regular artificial nourishment of the eroding beach in front of a hard structure can mitigate scouring at the toe of seawalls or revetments. For more information on this topic, see also the articles Seawalls and revetments and Revetments.

Mixed coast/shore protection by hard structures and beach fill

Mixed coastal protection and shore protection measures are schemes that combine hard structures and initial nourishment (called beach fill). These schemes can provide a solution for stabilising the shoreline, that combines the ability of hard structures to directly protect a section of the coast and the ability of these structures to support and maintain beach filling/nourishment. The result is protection of the beach and protection of the coast behind the beach. However, the effectiveness is questionable. Hard structures such as groynes and offshore breakwaters can generate currents that (partly) cancel out the advantage of reduced wave attack on the beach (see Groynes and Groynes as shore protection).

Related articles

• Natural causes of coastal erosion
• Human causes of coastal erosion
• Shoreline management
• Shoreline retreat and recovery
• Dune erosion
• Hard coastal protection structures
• Seawalls and revetments
• Integrated Coastal Zone Management (ICZM)
• Coastal Hydrodynamics And Transport Processes

References

The main author of this article is Jan van de Graaff Please note that others may also have edited the contents of this article. Citation: Jan van de Graaff (2024): Dealing with coastal erosion. Available from http://www.coastalwiki.org/wiki/Dealing_with_coastal_erosion [accessed on 24-11-2024] For other articles by this author see Category:Articles by Jan van de Graaff For an overview of contributions by this author see Special:Contributions/Jan van de Graaff