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Protection against coastal erosion

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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 dune erosion. The article Shore protection, coast protection and sea defence methods also describes how the coast can be protected from erosion, but mainly describes different types of coastal protection methods.

Background of coastal protection

Dynamic sea-land transition

The transition between sea and land (waterline / coastline) is never a fixed line along our (sandy) coasts. Seen at a short time scale, e.g. scale of tidal variations, the waterline continuously shifts in landward and seaward direction. This is a well-known example of the natural behaviour of our coasts and is often considered as an intriguing feature 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 also stable coasts do occur). Pure natural (autonomous) reasons, or man-induced reasons might cause this behaviour.

While accreting coasts are welcomed in most cases by Coastal Zone Managers and by people living near the sea, eroding coasts bother the people. Valuable land with most likely various existing interests, will be lost.

Coastal erosion

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

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. In the article Types and background of coastal erosion this distinction is discussed into more detail. In dealing with wanted / required coastal protection measures one has to be aware of this distinction.

Other coastal engineering problems

Although important, eroding coasts form as a matter of course only a restricted class of coastal engineering problems. Also problems like coastline stabilization (e.g. near tidal inlets), widening of (recreation) beaches and reduction of erosion during a severe storm surge are typical examples of problems to be resolved with the help of coastal engineering knowledge and coastal engineering tools.

Vulnerable systems

Coastal systems are generally vulnerable systems. It is very easy to harm such a vulnerable system with thoughtless surgery. This calls for a skilled and powerful, and, that is very important, a legally backed system of Coastal Zone Management.

In the discussions in the present contribution also the various coastal engineering tools to achieve the aims are discussed. It will be argued that these aims must be well-defined. Without well-defined aims never adequate solutions to coastal engineering problems can be found.

Time-perspective

It is good to realize that 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 mankind has to do with this behaviour, value judgements play a role. Often e.g. coastal erosion is felt annoying and bad. Mankind, facing for instance a 'bad' structural eroding behaviour of the coast, shows an increasing 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 somewhat to do.
  • Houses, roads and infrastructure are at stake; we can spend some money to protect us; various tools have been developed; with some kind of a 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 very careful with the last 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.

Coastal protection measures

General

First of all it has to be stressed that with a proper system of Coastal Zone 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 maintained) 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, but most important: a honest decision making process, it is decided to renounce an intended project because of the too large adverse effects elsewhere along the coast.

Protection against dune erosion; 'hard' method

If it is felt that e.g. an existing house or an existing hotel is situated too close to the sea, and one likes to reduce the chance of damage due to a severe storm surge (see also dune erosion). In fact the only possibility to reduce the risks of an existing building is to protect the site by a revetment along the face of the mainland or by a seawall. See also the articles chances and risks, hard shoreline protection structures and seawalls and revetments.

In the unprotected situation sediments from the mainland are transported towards deeper water during the storm. Revetments or seawalls, if well-designed to withstand the storm attack, are physically able to protect the properties built at the mainland. If by the revetment or seawall sediments from the mainland are denied to join the physical processes, erosion of the beaches just in front of the revetment or seawall will occur, causing deep scour holes. With the choice of the foundation depth of the revetment or seawall, one has to take this phenomenon into account (see Figure 1).

Figure 1 Scour in front of revetment

Revetments and seawalls are so-called 'hard' structures. They might be applied in this special case, provided that the storm (surge) protection problem is the only issue for this stretch of coast. So it refers in fact to a stable part of the coast (stable: seen over a number of years) or an accreting part of the coast.

Protection against structural erosion

Structural erosion means in fact the gradual loss of sediments with time out of the control volume (See e.g. Figures 1 and 2 in structural erosion.) Sooner or later not only the foreshore is loosing volumes of sand, but also the beaches and mainland. At the end, even properties built at the mainland will be lost. If, besides dunes erosion, structural erosion (also) occurs along a stretch of coast under consideration, revetments or seawalls can in no way be selected as the only protection measure. In this case, soft shoreline protection solutions might in principle be selected as a possible solution (reducing the chances at damage during a severe storm surge).

Soft shoreline protection solutions imply that by artificially widening of the mainland in seaward direction, the risks may be relieved. However, this calls for rather large volumes of sediments, because not only the face of the mainland has to be shifted in seaward direction, but also at least the active part of the cross-shore profile. Furthermore a 'soft' solution must in this case be applied over a rather long distance alongshore. Applying a 'soft' solution only locally, calls for a large maintenance effort because of the redistribution processes in both alongshore directions of the artificial nourishments. A possible disadvantage of 'soft' solutions are that widening (and perhaps heightening) of the mainland and/or dunes in seaward direction may cause loss of undisturbed sea views of the most seaward row of houses and hotels. Although the widening measure was primarily meant for the owners of these buildings, they might be unhappy with this solution.

Artificial nourishments are certainly to be repeated at a regular time basis and to be continued for ever. Nevertheless they are a perfect solution in many cases. The occurring losses are to be refilled from time to time. Seen over a number of years the average position of the coastline will be stable. Because artificial nourishments, to a first approximation, do not interfere in the occurring sediment transport processes, this method has to be applied for ever indeed.

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 new port which interrupts the occurring longshore sediment transport (applying a sand by-pass system).

Sometimes it is felt that artificial nourishment schemes are too expensive for developing countries. Good alternatives (if any?) are at the long run in many cases, however, 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 many handbooks.

The structural erosion of the upper parts of the cross-shore profiles (beach; mainland) is the most visible, and striking phenomenon. To replenish just the upper part of the profile seems consequently logical. However, also nourishing the deeper part of the cross-shore profile (shoreface nourishments) fulfils the requirements. By redistributing cross-shore sediment transport processes, also the upper part of the profile is fed at the end of the day.

If the structural erosion problem is due to a gradient in the longshore sediment transport (dS/dx ≠ 0; S: annual sediment transport rate; x: alongshore coordinate), with the application of 'hard' solutions to the erosion problem, the Coastal Zone Manager intends to interfere into 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, just vanishes (dS/dx = 0).

In the article Hard structures and structural erosion some very basic notions regarding the application of 'hard' solutions in case of a structural erosion problem are outlined.

See also

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.