Seawalls and revetments

This article introduces seawalls and revetments as hard coastal protection structures. Soft coastal protection structures are discussed in the article Nature-based shore protection. The article Ecological enhancement of coastal protection structures describes how seawalls and revetments can be adapted and to provide ecological habitats. The design and stability of seawalls and revetments is discussed in the article Stability of rubble mound breakwaters and shore revetments.

This article focuses on how and why the application of a seawall or revetment can be used to solve coastal engineering problems in practice. It also discusses why they should not be used as a solution to problems related to structural erosion.

Introduction

Seawalls or revetments are shore-parallel structures at the transition between the (sandy) beach and the (higher) mainland or dune. The height of a seawall often covers the total height difference between the beach and the surface level of the land just behind. In many cases, there is a horizontal stone-covered part adjacent to the top of a seawall (e.g. boulevard, road or parking lot). At the time of construction, a seawall is often located on the upper dry beach. In the current article, a seawall is considered to be a nearly vertical structure (often a bulkhead). The seaward side of the seawall has a relatively smooth surface.

A revetment, like a seawall, is a structure parallel to the shore. The main difference is that it is more inclined than a seawall. The slope of a revetment is often 1:2-1:4 and the surface of a revetment can be smooth or rough (rubble). The height of a revetment does not necessarily cover the total height difference between beach and mainland.

Slightly different definitions are given in the definition pages of seawall and revetment. For more information on different types, characteristics and application of revetments, see also the article Revetments. For some general information on seawalls, see also Seawall.

Seawalls and revetments have different morphological effects. A smooth vertical seawall produces stronger wave reflection and local turbulence than a rough, permeable sloping revetment. The sediment-budget effect, however, is similar: both structures fix the landward boundary and reduce the supply of sediment from dunes, bluffs or the upper beach to the active coastal profile.

Seawalls and revetments are increasingly combined with beach nourishment, dune reinforcement, buried revetments or ecological enhancement. Such hybrid solutions can reduce some adverse effects, but they do not remove the need for a sediment budget and long-term maintenance strategy.

Solving coastal engineering problems

Beach-mainland transition

Figure 1 Seawall and boulevard

Especially in sandy coastal areas with many human (recreational) activities, a clear and fixed distinction between beach and mainland can be desirable. A seawall will serve that purpose. On the sea side of the seawall a more or less normal beach is assumed to be present and on the landward side a road or a boulevard. Staircases facilitate the access to the beach.

A normal beach is assumed to be present in front of the seawall that can be used for recreational purposes. This function is only sustainable if the beach in front of the seawall is stable or actively maintained. Otherwise, the recreational beach will narrow and the structure will increasingly be exposed to direct wave attack. See Figure 1.

While even a moderate storm (surge) will attack and erode the mainland in a situation without a seawall, this is prevented in the situation with a seawall. Scour in front of the seawall during a storm surge must be taken into account in the design. A part of the 'denied' erosion volume from the mainland is now eroded from the beach just in front of the seawall. The scour hole may undermine the seawall. An estimate of the expected scour depth can be made with numerical models.

Figure 2 Seawalls with measures to reduce overtopping

The design conditions for the defense structure must be chosen carefully. The more severe the design conditions, the heavier the structure must be built and especially the required foundation depth will increase accordingly. Building a seawall which will be safe under 'all' conditions may be an unrealistic option. The selected design conditions determine the degree of protection provided for infrastructure on the mainland (see Stability of rubble mound breakwaters and shore revetments).

The crest height of a seawall largely determines the rate of overtopping. The design height depends on water level, wave height and period at the toe, foreshore slope, wall slope, roughness, berms, recurved parapets and permeability. Rough sloping revetments generally reduce run-up and overtopping compared with smooth steep seawalls, whereas vertical or recurved walls may reduce mean discharge but can generate high splash and impulsive loads. Water reaches the mainland by wave run-up, breaking waves and splash water transported by landward directed wind. These phenomena are discussed in the article Wave overtopping. Quantitative estimates of mean overtopping discharge and individual overtopping volumes are normally based on empirical guidance such as EurOtop, supported where necessary by physical or numerical modelling. With an additional wall and/or a slightly curved front, rates of overtopping may be reduced; see Figure 2.

Protection of coastal assets

Infrastructure and buildings situated close to the edge of mainland or dunes have a probability of being destroyed during a severe storm surge.

The risk (risk = probability x consequence) can be deemed too high in a given situation. An increased risk may arise, for example, when urban infrastructure on the seafront is expanded. An acceptable risk level can be recovered by reducing the failure probability. With a robust seawall or revetment the desired goal can be achieved. Aspects like proper design conditions and scour holes are in this case as discussed before.

A natural unprotected coast can suffer major erosion during an extreme storm surge. The following scenario may occur if a seawall is constructed. The seawall will prevent erosion of the mainland, but a scour hole will be formed in front of the seawall. If the seawall collapses at some location, a local breach is formed that allows seawater to pass behind the seawall. The collapsed seawall fills the initial scour hole, but scouring continues by the channeled flow through the breach. A large volume of sediment from the mainland can disappear through the gap and the ultimate mainland erosion can be larger than in the unprotected case. This illustrates that hard coastal protection structures to prevent coastal erosion must be designed according to high standards.

Similar phenomena occur at the transitions between the seawall and the adjacent, unprotected parts of the coast. Adjacent to an abrupt end of a seawall, strong erosion can occur during a severe storm surge. The ends of hard structures and transitions to softer or lower defences require special attention, because local wave concentration, return flows and differences in erosion resistance can create erosion hotspots.

In some cases the hard structure can be buried within an artificial dune. The dune then provides the normal visible and functional coastal buffer, while the buried revetment acts as a last-resort erosion stop during extreme storm conditions. This solution can reduce the visual and recreational impact of a continuously exposed hard structure, but it still fixes the landward limit of profile retreat.

Scour at the toe of the seawall/revetment

Figure 3 Scouring in front of (smooth) revetment after a test in the Delta Flume of Delft Hydraulics. Most of the water was already released from the flume when the picture was taken; the still water level during the test was much higher.

Figure 4 Scouring in front of a revetment.

The development of a scour hole in front of a seawall was investigated in the Delta Flume of WL|Delft Hydraulics, see Figure 3. For rather smooth revetments it was shown that the depth of the scour hole depends on the slope characteristics. With a slope of 1:3.6 a deeper scour hole was found than with a slope of 1:1.8 (other test conditions the same). The deepest point of a scour hole is not always found at the intersection point between revetment and cross-shore profile (see Figure 4 for a sketch). It is probable that the depth of a scour hole is less for a rough slope than for a smooth slope.

Different pieces of evidence on the development of scour holes in front of a seawall or revetment are provided by field observations, laboratory experiments and numerical modeling:

• Maximum wave impact is observed when the seawall is positioned on the beach at the still water line. Consequently, the strongest scour at the seawall toe is observed for this location. Locating the wall away from this position leads to a decrease in wave impact and consequently a decrease in the scour depth at seawall toe. However, tides and storm surges move the still water line.
• Seawalls located around or above the storm surge high water line only have a minor influence on the beach profile.
• The scour depth [math]S[/math] decreases with an increasing water depth at the seawall toe.
• The scour depth increases with incident wave height [math]H_0[/math]. Reflection of high waves on the seawall leads to the development of a standing wave pattern, which creates a second scour hole seawards.
• The seawall scour increases with increasing seabed slope steepness. For a mild slope, most of the wave energy is dissipated before hitting the seawall. This results in a reduced wave impact and consequently a smaller scour at the seawall toe. However, for steep seabed slopes, the incident waves break on the wall. A stronger wave impact and a greater scour depth are therefore observed at the seawall toe.
• The scour at the seawall toe increases with increasing wave steepness. This effect is more significant if the wave steepness exceeds 0.032 .
• The seawall scour depth depends on the surf similarity parameter [math]\xi=m / \sqrt{H_0/L}[/math], where [math]m[/math] is the seabed slope in front of the seawall and [math]L[/math] the deep water wavelength. The ratio [math]S /H_0[/math] increases almost linearly from about 0.35 to about 0.8 as [math]\xi[/math] increases from 0.5 to 1.3, but does not increase further for larger values of [math]\xi[/math] .

Toe scour is one of the most important failure mechanisms of seawalls and revetments. The structural toe is often buried and should not be confused with the visible beach–structure contact line. Progressive beach lowering or storm scour can expose the toe, increase water depth at the structure, increase overtopping and wave loading, and reduce the geotechnical support needed to prevent sliding, settlement or overturning. Good practice therefore requires monitoring of both beach levels and structural condition. Maximum scour may occur during storm high water and may partly infill within hours. Routine beach surveys can therefore underestimate the maximum scour experienced by the structure. Post-storm inspection and, where necessary, more frequent or dedicated scour monitoring may be required at vulnerable sites. For managed structures, critical beach levels and higher trigger or alert levels should be defined, so that maintenance or strengthening can be planned before failure risk becomes unacceptable. Mitigation can either aim to restore beach levels, for example by nourishment or recycling, or to adapt the structure to lower beach levels, for example by adding a rock toe, sheet-piled toe, concrete apron or flexible mattress.

Accelerated beach erosion

Figure 5 A seawall may stop landward retreat of the protected land, but on an eroding coast the beach can be lost because it can no longer migrate landward. Without a seawall, the land retreats and the beach may migrate landward, provided accommodation space and sediment supply are available.. Photo credit Chip Fletcher (Hawaii).

Construction of a seawall or revetment on an eroding sandy coast fixes the landward boundary of the beach. It prevents further retreat of the protected land, but it does not remove the sediment deficit in the active coastal profile. If the coast would otherwise retreat, the beach in front of the structure becomes progressively narrower; this is often called passive erosion or coastal squeeze (see Figure 5).

Local effects, such as wave reflection, storm scour, delayed post-storm recovery and interruption of sediment exchange with dunes or bluffs, can further contribute to beach lowering when waves frequently reach the structure. Beach erosion can be accelerated or a previously stable beach can become erosional. Active erosion due to the construction of a seawall or revetment can be promoted by the following mechanisms:

• A seawall cuts off the beach from inshore sand sources (e.g. dune, bluff).
• When storm waves erode the upper beach, the lack of inshore sand supply accelerates and amplifies beach lowering and retards the development of a nearshore bar.
• Profile lowering and lowering of the nearshore sand bar expose the beach to strong wave attack.
• If the scour trench is filled with sand from updrift longshore transport, the downdrift beach will erode due to insufficient sand supply.
• Shoreface steepening concentrates the wave attack on a smaller part of the profile and hampers beach recovery after storm.
• The width of the dry part of the lowered beach after storm erosion is not sufficient to allow the rebuilding of the upper beach by aeolian transport.

Field evidence of this active role of seawalls in accelerating beach erosion is controversial, however.

Longshore sediment transport

A seawall can stop erosion of the land behind it, but it cannot replace the sediment that is being removed from the active profile by a longshore transport gradient. Structural erosion caused by a gradient in alongshore sediment transport (spatial increase in the rate of sediment transport) means that volumes of sediment are lost from the active cross-shore profile (for a more detailed article, see Dealing with coastal erosion). This loss process mainly occurs under average conditions; the contribution of storm conditions to the total sand loss is often relatively small. The sand losses mainly occur in the subaqueous part of a cross-shore profile where alongshore sediment transport takes place. The 'dry' parts of a cross-shore profile are not involved in alongshore sediment transport; this gives the impression that the 'dry' parts are not an integral part of the cross-shore profile under normal conditions.

Figure 6 Seawall and gradient in longshore transport

During high tides and/or moderate storms (storm surges) all parts of a cross-shore profile are subject to sediment transport processes. By offshore directed cross-shore sediment transports, sediments are transported from the higher parts of the profile (the 'dry' beach, and even the mainland under severe conditions) to deeper water, where they are removed by downdrift sediment transport (see Figure 6). This sequence of processes causes a permanent loss of material out of the upper parts of a cross-shore profile.

By protecting the mainland in this case with a seawall, one indeed prevents sediments from the mainland from being transported in a seaward direction. However, the losses continue; the 'dry' beach disappears and it becomes increasingly deeper in front of the seawall. After the construction of the seawall, a more or less normal dry beach was initially present. The beach did protect the seawall to some extent; only moderate storms could reach the seawall. After the beach has disappeared, waves more often directly attack the seawall. Continuous reinforcements of the seawall are necessary to prevent damage, undermining and collapse. The foregoing shows that seawalls and revetments do not solve erosion problems caused by gradients in longshore sediment transport. In coastal engineering practice, this still frequently applied solution does not address the cause of the erosion. Many bad examples can be found all over the world.

Coastal squeeze and sea level rise

Natural sedimentary coasts are characterized by the presence of a land-sea transition zone that buffers the impact of extreme storms. Besides protecting the hinterland against coastal hazards, this transition zone also provides many other services, so-called ecosystem services, that benefit marine life and society. The land-sea transition zone, also called land-sea interaction zone, is typically highly dynamic; it moves landward or seaward depending on the balance of eroding and accreting sediment transport processes.

When erosion prevails, the landward migration of the land-sea transition zone conflicts with the interests of landowners, either public or private. The example below shows how such conflicts are usually settled.

Local landowners on the coast, whose houses are at stake, have noticed that in the past every storm surge has taken some square meters of their gardens. The edge of the mainland is coming closer and closer to their homes. Local pressure often forces coastal managers to implement visible protective measures. Locally building a seawall or revetment in front of the properties which are situated closest to the sea, indeed seems to resolve the problem. During the next storm surge, the just 'protected' parts of the coast show no further erosion; in the unprotected parts the erosion of the mainland continues. Landowners are happy to see that this solution 'works'. The coastal zone manager is forced to build seawalls along the other parts of the coast as well. However, when time elapses, it will become clear that a wrong solution has been chosen. The buffer zone between land and sea has disappeared. Safety against coastal hazards can be maintained only at high and increasing costs.

Relative sea-level rise inevitably promotes landward transgression of sedimentary coasts. In some cases the coastal profile may adjust by landward migration and sediment redistribution. However, where hard structures prevent this migration, beaches, salt marshes and mangroves are squeezed between the rising water level and the fixed landward boundary. The seawall solution should therefore be limited to coastal zones where the long-term costs of retreat exceed the long-term costs of hard protection and loss of transition zones.

Related articles

• Structural erosion
• Seawall
• Revetments
• Stability of rubble mound breakwaters and shore revetments
• Hard coastal protection structures
• Bulkhead
• Dealing with coastal erosion
• Erosion hotspots
• Wave overtopping
• Wave run-up
• Nature-based shore protection
• Ecological enhancement of coastal protection structures
• Beach nourishment
• Coastal squeeze

References

The main authors of this article are Jan van de Graaff and Job Dronkers Please note that others may also have edited the contents of this article. Citation: Jan van de Graaff; Job Dronkers; (2026): Seawalls and revetments. Available from http://www.coastalwiki.org/wiki/Seawalls_and_revetments [accessed on 11-07-2026] For other articles by this author see Category:Articles by Jan van de Graaff For other articles by this author see Category:Articles by Job Dronkers For an overview of contributions by this author see Special:Contributions/Jan van de Graaff For an overview of contributions by this author see Special:Contributions/Dronkers J