Dealing with coastal erosion

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 (also called dune erosion for sandy barrier coasts). 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 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.

What is coastal erosion?

Figure 1. Control volume

Consider a (still) more or less natural stretch of a sandy coast (so without any hard structures). A cross-shore profile of that stretch of coast, seen in seaward direction, consists out of the mainland or dunes at a level well above mean sea level (MSL), an often rather steep mainland or dune face, a beach and a shoreface. See Figure 1 for a plan view picture and a schematic cross-shore profile.

In Figure 1 also two vertical lines and a horizontal line have been indicatively sketched in the cross-shore profile; these lines and the position of the actual cross-shore profile enclose a so-called control volume area. The developments of the volume V (in m³/m) with time of this control volume area, is further used to explain some typical features which might occur along a coast. (Mainland or dunes is further written in this contribution as mainland only.)

Let us look at the erosion of the mainland (including dunes). This is often the most valuable part of a cross-shore profile to mankind. The very seaward part of the mainland sometimes carries valuable properties like houses and hotels or infrastructure like roads and car park areas. Also coastal erosion forms an important element in the process of coastal squeeze ^[1].

In fact two entirely different processes might cause loss of mainland:

• Dune erosion during a severe storm (surge)
• Structural erosion

The basic principles of both types of erosion are discussed in the sections below. For more information on the background of coastal erosion, see also the article Coastal Hydrodynamics And Transport Processes.

Dune erosion

Dune erosion is a typical example of cross-shore sediment transport process and results from sediment transport during a severe storm. Often a storm is accompanied with much higher water levels at sea than usual (storm surge) and also much higher waves do occur. Depending on the characteristics of the storm, erosion rates of several metres per event (say per day) have to be considered. These episodic events do occur along all types of coasts (along structural eroding, stable and even along accreting coasts). After the storm generally a recovery towards the original situation will occur due to the occurring processes under normal conditions.

Morphological background

Figure 2. Dune erosion due to storm surge

While under ordinary conditions the shape of a cross-shore profile might be considered to be in accordance with these normal conditions ('dynamic equilibrium' shape), under storm (surge) conditions the initial shape before the storm can be considered to be far out of the equilibrium shape which belongs to the severe storm conditions. Profile re-shaping processes will occur, causing erosion of sediments from the mainland and the settlement of these sediments at deeper water again (see Figure 2). Notice in Figure 2 that the control volume does not change by the storm surge processes.

Although this dune erosion process will cause loss of mainland, to a first approximation no loss of sediments out of the cross-section occurs; only a redistribution of sediments over the cross-shore profile takes place during the storm.

For further information see the articles Dune erosion and Shoreface profile.

Structural erosion

Many coasts all over the world suffer from structural erosion. Seen over a number of years one might observe that the position of e.g. the waterline is shifting in landward direction. Often a gradient in the (natural) occurring longshore sediment transports is the reason of structural erosion. Structural erosion is quite different from dune erosion. This implies that resolving a structural erosion 'problem' in coastal engineering practice calls for a quite different approach compared to the solution to a dune erosion 'problem'. For people living along the coast the distinction between both causes for erosion is often not so clear. In both cases it looks like that the storm is the malefactor. During a storm actual damage at the mainland will occur. In the structural erosion problem the storm is only a link in the various coastal processes.

Figure 3 Structural erosion

Structural erosion rates are often in the order of magnitude of a few metres per year. To overcome erosion problems related to either dune erosion or structural erosion, calls for quite different counter-measures. A coastal zone manager must be aware of this.

Morphological background

Structural, long-term erosion yields a gradual loss of sediments out of a cross-shore profile. Looking at the volume of sediments in the control volume area as a function of time, we will see a diminishing tendency with time.

A gradient in the longshore sediment transport is often the reason of structural erosion. See Figure 3 for the development with time of an eroding cross-shore profile.

Figure 4 Volume loss out of control volume due to structural erosion

Notice the change with time of the control volume. Notice also that the mainland is eroding as well, although the longshore sediment transports, and so the gradients in the longshore sediment transport, do not occur at the mainland level. Events with some dune erosion redistribute sediments from the dunes to the beach and foreshore. In this case the recovery of the dunes is only partly; at the end of the day also the mainland retreats.

Figure 4 shows the loss of volume out of a control volume as a function of time.

The box Rate of recession versus volume rate of erosion in the appendix explains the relationship between the annual loss of volume out of a cross-section and the annual rate of recession of the waterline.

Possible causes of structural erosion

In order to select a proper scheme to protect a structural eroding stretch of coast, it is necessary to understand the cause of the erosion problem. Principally pure natural and man-induced causes are to be distinguished. A pure natural morphological development of a part of the coast is often called autonomous behaviour. What looks like an autonomous behaviour at present, is in some cases in fact (the tail of) a man-induced development started a long time ago. So the distinction between both is not very clear in many cases.

A convex stretch of coast under wave action is a typical example of a natural, structural eroding coast. See the box Erosion because of a convex coast in the appendix.

For further information on structural erosion see the articles Accretion and erosion for different coastal types, Natural causes of coastal erosion, Littoral drift and shoreline modelling. Some examples of structural erosion are given in the article Typical examples of structural erosion.

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.

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) ^[2] and (d’Angremond and Pluim-Van der Velden, 2001) ^[3].

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, 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 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. 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. Soft shoreline protection solutions have another disadvantage. Widening (and perhaps heightening) of the beach (and/or dunes) in seaward direction may cause loss of undisturbed sea views of the most seaward row of houses and hotels. Although the protection measure was primarily meant for the owners of these buildings, they might be unhappy with this solution.

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 a structural sand by-pass system 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)^[4] and in many handbooks, for example Dean (2002)^[5].

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 ^[6].

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

Other soft measures for shoreline stabilisation are beach and dune vegetation (see Dune stabilisation and Shore protection vegetation) and beach drainage.

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.

Hard coastal protection measures are often applied to reduce the risk of damage due to a severe storm surge when settlements are situated too close to the sea. The coast is protected by 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. They are 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

During severe storms sediments from the beach are transported towards deeper water if hard protection structures are absent. Part of these sediments will return to the beach under average wave conditions. 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). For more information on this topic, see also the articles Hard coastal protection structures, Seawalls and revetments, Seawall 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. The advantage of this combination is that it minimises the requirements for regular recharging of the fill or nourishment. These schemes make use of littoral processes, either the longshore littoral drift and/or the cross-shore transport. 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.

Appendix

Figure 1 Convex piece of coast Erosion because of a convex coast Consider a part of a sandy coast, a few kilometres long. In plan view it refers to a convex coast (see Figure 1). In the figure only the waterline is indicated, but it is assumed that the depth contours are more or less parallel to the waterline. Waves approach the coast as indicated in the figure. Along the part of the coast as has been plotted, the angle between the wave crests and the orientation of the coast is ever changing. Longshore currents and longshore sediment transports [[math]S[/math]: e.g. in [math]m^3/year[/math]] are generated. In Figure 1 the magnitude and direction of the longshore sediment transport rates are schematically indicated. Gradients in the longshore sediment transport rates seem to occur [[math]dS/dx \ne 0[/math]]. Due to the gradients loss of sediments out of the control volume area occur; volume [math]V [m^3/m][/math] is diminishing [[math]dV/dt = dS/dx[/math]].

Figure 1 Structural erosion Figure 2 Volume loss out of control volume due to structural erosion Rate of recession versus volume rate of erosion Figure 1 sketches a typical structural erosion case. Two cross-shore profiles have been indicated, representing the position of the profiles at two different moments within time. To a first approximation it can be assumed that the shape of the profiles are identical (the boundary conditions; e.g. wave climate and tides are the same indeed). The one profile can be found from the other by a horizontal shift. It is assumed that not only for example the waterline shifts in landward direction with a certain speed, but that this holds in fact for all depth contours. If on an average the waterline shows a recession of [math]r[/math] m/year, the annual loss of volume out of the control volume area [math]dV[/math] (m3/m.year) is approximately given by [math]r[/math] times [math](h + d)[/math]; see Figure 1 for the definition of [math]h[/math] and [math]d[/math]. The dune height [math]d[/math] with respect to MSL is easy to estimate. The under water part of the cross-shore profile [math]h[/math] which has to be taken into account, is more difficult to determine. Often the depth of the so-called active part of the profile is taken as representative. The closure depth (seaward boundary of the active part of the profile) is related to the annual wave climate. A first approximation for depth [math]h[/math] can be found by multiplying the significant wave height which is exceeded for one day a year, with 2 - 3. With actual profile measurements often a more accurate estimate of [math]h[/math] can be determined. Figure 2 gives the development of the volume [math]V[/math] of the control volume area with time of cross-shore profile. The slope [math]m = dV/dt[/math] (m3/m.year) is in this case a measure of the gravity of the erosion problem A similar figure like Figure 2 can be made of the recession of the position of the waterline (in m with respect to a reference point) with time of an eroding profile in a structural eroding part of the coast. The slope [math]r[/math] (m/year) is a measure for the erosion rate. From a morphological point of view representing erosion rates according to volumes is preferred above a representation according to distances. While the associated longshore sediment transports take place in the 'wet' part of the cross-shore profile, at the end of the day, also the mainland will permanently lose sediments. This can be understood by taking the erosion processes during a(n even moderate) storm into account. In a (seen over a number of years) stable condition the erosion of mainland is in fact only a temporary loss. In a structural erosion case this is (partly) a permanent loss. Sediments eroded from the mainland during the storm and settled at deeper water are eroded (by the gradient in the longshore sediment transport), before they have the chance and time to be transported back to the mainland. Although it looks like that the storm is the reason of the (permanent) erosion problem of the mainland, in this case the actual reason is the gradient in the longshore sediment transport (see Littoral drift and shoreline modelling). The storm is only to be considered as a necessary link in a chain of processes.

Related articles

Shoreline management

Shoreface profile

Active coastal zone

Dune erosion

Bruun rule

Littoral drift and shoreline modelling

Coastal Hydrodynamics And Transport Processes

Littoral drift and shoreline modelling

Hard coastal protection structures

Human causes of coastal erosion

Natural causes of coastal erosion

Perched beaches

Integrated Coastal Zone Management (ICZM)

References