Dealing with coastal erosion

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This article discusses how to deal with coastal erosion. The emphasis is not on the detailed physical processes, nor on the detailed design of coastal structures. These subjects are treated in other Coastal Wiki articles. The purpose of this article is to clarify the coastal-management question: when is coastal erosion a problem, what kind of problem is it, and what type of response can be considered?

The coastal erosion problem

Coastal erosion is often presented as if the sea is attacking the land and the land has to be defended. This is only partly true. A sandy coast is not a fixed boundary. The shoreline moves seaward and landward under the influence of waves, tides, storms, sediment supply and sea-level change. In many places this movement is part of the natural behavior of the coast.

Suppose there is a humble hut close to the edge of a retreating dune. If nobody uses the hut and no value is attached to it, the retreat of the dune is not necessarily a coastal-management problem. The coast simply behaves as a coast. If the hut is someone’s home, or if there is a road, a hotel, a village or a nature reserve behind it, the same physical process becomes a problem. The problem is therefore not erosion alone, but erosion in relation to human use, safety, property, infrastructure, nature values and future choices.

This simple example is important. Before asking how erosion should be stopped, one should first ask what has to be protected, why it has to be protected, for how long, at what cost, and with what consequences for neighboring coasts. Dealing with coastal erosion requires Integrated Coastal Zone Management (ICZM).

What kind of erosion?

Not all shoreline retreat is the same. A beach can become narrow after a storm and recover later. A dune can lose sand during an exceptional surge and rebuild partly by aeolian transport. In such cases the erosion is incidental. It may be serious, but it does not necessarily mean that the coast has a persistent sediment deficit. A more detailed explanation is given in Shoreline retreat and recovery and Dune erosion.

A different situation exists when, averaged over several years or decades, more sediment leaves a coastal section than enters it. Then the active coastal profile loses sediment and the shoreline retreats structurally. This is called structural erosion. It can be caused by gradients in longshore sediment transport, interruption of sediment supply, sediment extraction, subsidence, sea-level rise or human interference with the sediment budget. The underlying processes are discussed in Littoral drift and shoreline modelling, Natural causes of coastal erosion and Human causes of coastal erosion.

The distinction between incidental erosion and structural erosion is essential. A measure that is suitable for storm damage is not necessarily suitable for a coast with a long-term sediment deficit. Conversely, a long-term protection scheme should not be based only on the beach position observed just after a storm.

The active coastal profile

A sandy coast should not be judged only by the position of the dry shoreline. Sand moves over the whole active coastal profile: from dune and dry beach to the surf zone and nearshore bed. A temporary retreat of the shoreline can therefore occur without permanent loss of sand from the coastal system. In contrast, structural erosion means that sediment is lost from the active profile as a whole.

This is why coastal management should be based, where possible, on sediment volumes and sediment budgets, not only on the visible shoreline. A beach may look wide after a calm period but still be part of an eroding profile. Another beach may look narrow after a storm but recover if the sediment remains in the nearshore zone.

For more detailed explanation see Active coastal zone and Coastline.

Can erosion be stopped?

It is often said that coastal erosion has to be stopped. This phrase can be misleading. A coastal manager can sometimes protect a particular piece of land. It is much more difficult to stop the coastal system from adjusting to sediment deficits, storms and sea-level rise.

If a seawall is built in front of the humble hut, the hut may be saved for some time. But if the coast is structurally eroding, the beach in front of the wall may become lower and narrower. The erosion problem has then not been solved; the landward boundary has been fixed. The sea will reach the wall more often, toe scour may increase, overtopping may become more frequent, and the beach function may be lost. This is discussed in Seawalls and revetments.

If groynes are built, the beach may widen on the updrift side, where the hut is situated. But the sand stored there is no longer available downdrift. If the longshore sediment budget is not compensated, erosion may appear elsewhere. This is discussed in Groynes.

If beach nourishment is applied, sediment is added to the active coastal profile. This can maintain the beach and dune buffer without fixing the shoreline by a hard boundary. However, if the cause of erosion continues, nourishment has to be repeated. This is discussed in Beach nourishment, Artificial nourishment and Shore nourishment.

Thus the question is not simply whether erosion can be stopped. The question is which part of the coastal system is to be controlled, what side effects are acceptable, and whether the chosen solution can be maintained in the long term.

Possible management responses

There are several ways to deal with coastal erosion. They are not all suitable everywhere.

Do nothing

Doing nothing can be a deliberate management choice. If no important assets are threatened, natural coastal change may be acceptable. This option should not be confused with neglect. It requires knowledge of the expected coastal development and of the values that may be affected.

Avoid new risk

The most effective way to avoid future erosion problems is not to build too close to an eroding or mobile coast. Setback lines, zoning rules and restrictions on rebuilding after storm damage can prevent future conflicts between natural shoreline movement and human occupation, see Setback area. This is especially important under sea-level rise, because many sedimentary coasts will need space for landward migration.

Retreat or relocate

If existing buildings or infrastructure are threatened, relocation may be considered. This is often difficult socially and politically, but it can be more sustainable than maintaining hard protection indefinitely, see Climate adaptation policies for the coastal zone. Retreat is most relevant where the value of the threatened assets is small compared with the long-term costs and impacts of protection.

Nourish the coast

Nourishment adds sediment to the coastal system. It can compensate sediment losses, widen the beach and strengthen the dune buffer. It is especially useful where the objective is to maintain a beach for safety, recreation or nature. Nourishment does not remove the cause of structural erosion; it manages the consequence by repeated sediment supply.

Protect the landward boundary

Hard structures such as seawalls, revetments and bulkheads can protect landward assets against wave attack and storm erosion. They are often necessary in urban areas, harbors and other places where retreat is not acceptable. They fix the landward boundary but do not stop erosion. They lead to coastal squeeze because the beach or intertidal zone cannot migrate landward.

Influence longshore sediment transport

Groynes and detached breakwaters can influence the distribution of sediment along the coast. They may be useful where the management objective is to retain sand locally. However, they can cause or aggravate erosion elsewhere. To evaluate the consequences, the sediment budget should be considered at the scale of the coastal cell. See Detached breakwaters, Groynes and Erosion hotspots.

Use combined solutions

In practice, combined solutions are often used: a seawall with a nourished beach, a buried revetment in an artificial dune, groynes combined with beach fill, or dune reinforcement combined with periodic nourishment. Such combinations can reduce disadvantages of individual measures, locally and temporally. They do not remove the need for a comprehensive diagnosis of the erosion problem.

Work with natural processes

Where sufficient space and suitable physical conditions exist, nature-based measures can help reduce erosion and improve coastal resilience. Examples include dune restoration, salt-marsh restoration, mangrove restoration and sediment-based measures that support natural coastal dynamics. These measures are discussed in Climate adaptation measures for the coastal zone, Nature-based shore protection, Dynamics, threats and management of salt marshes, Mangroves and Shore protection vegetation. Ecological improvement of hard structures is discussed in Ecological enhancement of coastal protection structures.

Choosing a measure

Choosing a coastal protection measure is not only a technical issue. A number of questions should be answered first:

What is the cause of the erosion?
Can the cause be eliminated?
Is the erosion incidental or structural?
Which assets or values are threatened?
Is it necessary to protect the asset at its present location?
What is the expected development under sea-level rise?
Will the measure affect neighboring coasts?
Does the measure preserve or destroy beach, dune, marsh or mangrove functions?
Who pays for construction, maintenance and future adaptation?
Who benefits from the measure?
What happens if the measure fails or is no longer maintained?

A measure that solves the local problem today can create a larger problem tomorrow. For example, a seawall can protect the hut but remove the possibility of beach migration. A groyne can widen one beach but starve another. A nourishment can maintain a beach but requires a continuing sediment source and budget. A retreat policy can be physically sensible but socially difficult.

For this reason, coastal erosion management should consider alternatives before selecting a measure. It should also include monitoring and the possibility of adjustment.

Monitoring and maintenance

No coastal protection measure should be considered finished at the time of construction. Beaches and dunes change. Structures can degrade and not perform as intended. Storms can lower the beach in front of a wall or revetment. A nourishment can move alongshore or offshore. A groyne field can create erosion hotspots. A dune can recover after one storm but not after a sequence of storms.

Monitoring should therefore include shoreline position, beach and dune volumes, nearshore profiles, structure condition and storm impacts. The results should be used to decide whether maintenance, additional nourishment, strengthening, adaptation or retreat is needed.

General lessons

Several general lessons follow from experience with coastal erosion:

Coastal erosion is a problem only when something valuable is at risk.
The shoreline is not a fixed boundary; it is part of a moving coastal profile.
Structural erosion should be distinguished from temporary storm erosion.
Hard structures can protect land but do not create sediment.
Measures that trap sand locally may deprive other coastal sections of sand.
Nourishment can maintain the sediment budget but requires repetition if losses continue.
Sea-level rise increases the need for space, sediment and long-term planning.
The best solution is often not the most visible structure, but the measure that fits the coastal system and the management objective.

The humble hut at the edge of the dune illustrates the central point. The sea does not know whether the hut is valuable. The coastal manager has to decide whether the hut should be moved, protected, sacrificed, or used as a warning not to build the next hut in the same place.

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