Seawalls are hard engineered structures with a primary function to prevent further erosion of the shoreline. They are built parallel to the shore and aim to hold or prevent sliding of the soil, while providing protection from wave action (UNFCCC, 1999). Although their primary function is erosion reduction, they have a secondary function as coastal flood defences.
The physical form of these structures is highly variable; seawalls can be vertical or sloping and constructed from a wide variety of materials. They may also be referred to as revetments.
The description of this technology originates from Linham and Nicholls (2010).
Seawalls are very widespread around the world’s coasts and many ad-hoc seawalls are found in developing countries. Here, we emphasise best practice guidance, although these principles could be used for more ad-hoc structures.
Seawalls form a defining line between sea and land. They are frequently used in locations where further shore erosion will result in excessive damage, e.g. when roads and buildings are about to fall into the sea. However, while they prevent further shoreline erosion, they do not deal with the causes of erosion (French, 2001). Seawalls range in type and may include steel sheetpile walls, monolithic concrete barriers, rubble mound structures, brick or block walls or gabions (wire baskets filled with rocks) (Kamphuis, 2000). Some typical seawall designs are shown in Figure 1. Seawalls are typically, heavily engineered, inflexible structures and are generally expensive to construct and require proper design and construction supervision (UNFCCC, 1999).
The shape of the seaward face is important in the deflection of incoming wave energy; smooth surfaces reflect wave energy while irregular surfaces scatter the direction of wave reflection (French, 2001). Waves are likely to impact the structure with high forces and are also likely to move sand off- and along-shore, away from the structure (Kamphuis, 2000). Since seawalls are often built as a last resort, most are continually under severe wave stress.
Seawalls usually have a deep foundation for stability. Also, to overcome the earth pressure on the landward side of the structure, ‘deadmen’ or earth anchors can be buried upland and connected to the wall by rods (Dean & Dalrymple, 2002).
The main advantage of a seawall is that it provides a high degree of protection against coastal flooding and erosion. A well maintained and appropriately designed seawall will also fix the boundary between the sea and land to ensure no further erosion will occur – this is beneficial if the shoreline is home to important infrastructure or other buildings of importance.
As well as fixing the boundary between land and sea, seawalls also provide coastal flood protection against extreme water levels. Provided they are appropriately designed to withstand the additional forces, seawalls will provide protection against water levels up to the seawall design height. In the past the design height of many seawalls was based on the highest known flood level (van der Meer, 1998).
Seawalls also have a much lower space requirement than other coastal defences such as dikes, especially if vertical seawall designs are selected. In many areas land in the coastal zone is highly sought-after; by reducing the space requirements for coastal defence the overall costs of construction may fall. The increased security provided by seawall construction also maintains hinterland values and may promote investment and development of the area (Nicholls et al., 2007b). Moreover, if appropriately designed, seawalls have a high amenity value – in many countries, seawalls incorporate promenades which encourage recreation and tourism.
When considering adaptation to climate change, another advantage of seawalls is that it is possible to progressively upgrade these structures by increasing the structure height in response to SLR. It is important however, that seawall upgrade does not compromise the integrity of the structure. Upgrading defences will leave a ‘construction joint’ between the new section and the pre-existing seawall. Upgrades need to account for this weakened section and reinforce it appropriately.
Provided they are adequately maintained, seawalls are potentially long-lived structures. The seawall in Galveston, Texas was constructed in 1903 and continues to provide coastal flood and erosion protection to the city to this day (Dean & Dalrymple, 2002).
Seawalls are subjected to significant loadings, as a result of wave impact. These loadings increase with water depth in front of the structure because this enables larger waves close to the shoreline. Seawalls are designed to dissipate or reflect incoming wave energy and as such, must be designed to remain stable under extreme wave loadings. The effects of SLR, increased wave heights and increased storminess caused by climate change must all be taken into account.
Smooth, vertical seawalls are the least effective at dissipating wave energy; instead, the structures reflect wave energy seawards. Reflection creates turbulence, capable of suspending sediments (Bush et al., 2004), thus making them more susceptible to erosion. In a worst-case scenario, reflected energy can interact with incoming waves to set up a standing wave which causes intense scouring of the shoreline (French, 2001).
Scour at the foot of a seawall is a particular problem with vertical seawall designs. This phenomenon is caused by the process shown in Figure 2. Incoming waves impact the structure, causing water to shoot upwards. When the water falls back down, the force on the seabed causes a scour hole to develop in front of the structure. This can cause structural instability and is an important factor leading to the failure of many seawalls. As a result, seawall maintenance costs can be high (Pilarczyk, 1990a). A similar process occurs on inclined seawalls but in this case scour will occur away from the foot of the structure.
The problems of wave reflection and scour can be reduced to some degree by incorporating slopes and irregular surfaces into the structure design. Slopes encourage wave breaking and therefore energy dissipation while irregular surfaces scatter the direction of wave reflection (French, 2001). Pilarczyk (1990a) recommends the use of maximum seawall slopes of 1:3 to minimise scour due to wave reflection.
Sediment availability is also affected by seawall construction. The problem is caused by replacing soft, erodible shorelines with hard, non-erodible ones. While this protects the valuable hinterland, it causes problems in terms of sediment starvation; erosion in front of the seawall will continue at historic or faster rates but the sediment is not replaced through the erosion of the hinterland (French, 2001). This can cause beach lowering, which reduces beach amenity value and increases wave loadings on the seawall by allowing larger waves close to the shore.
In the absence of a seawall, natural shoreline erosion would supply adjacent stretches of coastline with sediment, through a process known as longshore drift. Once a seawall is constructed however, the shoreline is protected from erosion and the supply of sediment is halted. This causes sediment starvation at sites located alongshore, in the direction of longshore drift and this has the capacity to induce erosion at these sites.
Although seawalls prevent erosion of protected shorelines, where the seawall ends, the coast remains free to respond to natural conditions. This means that undefended areas adjacent to the wall could move inland causing a stepped appearance to the coast (French, 2001). The downdrift end of the seawall is also typically subjected to increased erosion as a result of natural processes (see Figure 3). This flanking effect can cause undermining and instability of the wall in extreme cases.
Because seawalls are immovable defences, they can also interfere with natural processes such as habitat migration which is naturally induced by sea level change. Seawalls obstruct the natural inland migration of coastal systems in response to SLR, therefore causing coastal squeeze. This process causes a reduction in the area of intertidal habitats such as sandy beaches and saltmarshes because these environments are trapped between a rising sea level and unmoving, hard defences.
In estuaries, seawalls also cause changes to the area inundated by the tides thus, reducing the available area for occupation by water on a high tide. With the same volume of water flowing into the estuary, the level of the water after seawall construction will be higher. This may mean areas in front of the defence remain submerged longer and by greater depths. In turn, this is likely to affect the distribution of vegetation and could increase tidal range upstream of the defence (French, 2001).
Another potential problem is overtopping. This occurs when water levels exceed the height of the seawall, resulting in water flow into areas behind the structure. Overtopping is not a continuous process but usually occurs when individual high waves attack the seawall, causing a temporary increase in water level which exceeds the structure height (Goda, 2000). If the structure is too low, excessive overtopping can remove considerable amounts of soil or sand from behind the wall, thus weakening it. Further, overtopping water saturates and weakens the soil, increasing pressures from the landward side, which can cause the foot of the structure to ‘kick out’ and collapse (Dean & Dalrymple, 2002). Overtopping will become increasingly problematic with SLR, increased wave heights and increased storminess.
As mentioned in the advantages section, seawalls increase security by reducing the risk of flooding and erosion. However, the coastal zone remains a high risk location not least due to the presence of residual risk. To combat unwise development of the coastal zone, future developments need to be carefully planned.
Additionally, by encouraging development, hard defences necessitate continued investment in maintenance and upgrades, effectively limiting future coastal management options. Although authorities may not have a responsibility to continue providing protection, the removal of defences is likely to be both costly and politically controversial (Nicholls et al., 2007b).
Seawalls also reduce beach access for handicapped people and for emergency services. This can be problematic if the beach fronting such structures is to be used for recreation. The appearance of seawalls can be aesthetically displeasing which can further negatively affect beaches dependent upon a tourist economy.
A study by Linham et al. (2010) indicates that the unit cost of constructing 1 km of vertical seawall is in the range of US$0.4 to 27.5 million. The study found seawall costs for around ten countries. Most were developed country examples, although a number of newly developed and developing countries, such as Egypt, Singapore and South Africa were also found. Problems arise in the reporting of unit costs for vertical seawalls as the effect of height on unit costs is rarely considered. As such, these costs are likely to relate to seawalls of various heights; this explains some of the significant variation in costs between projects.
Some of the best unit cost information is given by the English Environment Agency (2007), for unit costs relevant to the UK. This source gives an average construction cost for seawalls of US$2.65 million (at 2009 price levels). This cost includes direct construction costs, direct overheads, costs of associated construction works, minor associated work, temporary works, compensation events and delay costs. This does not include Value Added Tax (VAT) or external costs such as consultants, land and compensation payments.
Variation in costs between projects is a result of numerous factors, such as:
- Design height is a major factor affecting costs per unit length of seawall. Height affects the volume of materials required for construction and the build time
- Anticipated wave loadings will affect how resilient the structure needs to be; deeper waters and exposed coasts cause higher wave loadings which will mean the structure needs to be more robust, thus higher costs
- Single or multi stage construction; costs are lower for single stage (Nicholls & Leatherman, 1995)
- Selected seawall design and the standard of protection desired. Certain design features will increase costs and more robust seawalls will be more costly
- Construction materials (e.g. rubble blocks, pre-cast concrete elements, metal, soil, etc.)
- Proximity to and availability of raw construction materials
- Availability and cost of human resources including expertise
Maintenance costs are another significant and ongoing expense when a hard defence is selected. These costs are ongoing for the life of the structure and are therefore likely to result in significant levels of investment through a project’s lifetime. Continued investment in maintenance is highly recommended to ensure defences continue to provide design levels of protection (Linham et al., 2010).
It has been noted that construction and maintenance costs are likely to increase into the future in response to SLR (Burgess & Townend, 2004; Townend & Burgess, 2004). This is caused by increases in water depth in front of the structure which, in turn cause increased wave heights and wave loadings on the structure.
Maintenance costs are also likely to be higher when seawalls are poorly designed or constructed of inappropriate materials. In many cases, design can be of secondary importance to the availability of raw materials, especially in locations where appropriate construction materials are scare. This was found to be the case in a study of shoreline protection in rural Fiji by Mimura and Nunn (1998). Their study highlights the problem that inappropriate design often leads to unfavourable effects, such as wave reflection and toe scour. In the absence of improper design, it is not unusual for designs from one location to be blindly copied at another. Such an approach is likely to result in exaggerated socio-economic and environmental costs (UNFCCC, 1999). The provision of even, basic design guidance would improve project performance in many cases.
Seawall construction is possible on a community scale. There are many examples of ad-hoc construction to protect individual properties and communities. However, ad-hoc seawalls are likely to give much less consideration to the water levels, wave heights and wave loadings during an extreme event. This is largely because these events are hard to foresee without a well-developed science and technology base. For example, traditional seawall construction methods in Fiji involved poking sticks into the ground to create a fence, behind which logs, sand and refuse would be piled to pose a barrier to the sea. This type of traditional construction has shown to have low effectiveness against significant events, however, and in many cases, these defences are washed away during extreme events (Mimura & Nunn, 1998).
A degree of technical guidance would be of benefit in the design and construction of effective seawalls. This would improve their effectiveness during extreme events and would also help to reduce adverse impacts on adjacent coastlines.
Although it is clearly possible to construct ad-hoc, or traditional, low technology seawalls at a community level, these structures have been shown to afford lower levels of protection against extreme events than designs with a solid science and technology base. They have also been known to exacerbate existing problems.
At present, the advice given in developing countries for modern seawall construction appears to be informal, if given at all. If effective design and construction is to occur, local communities must be given at least basic design guidance. This may come from government or voluntary organisations.
Seawall maintenance is likely to be possible at a community level when given appropriate training. This may include educating maintenance engineers on the likely failure mechanisms, how often to survey the structure, what to look for and how to identify weaknesses in the design. If major weaknesses are found, it may be necessary to employ a professional organisation to repair the structure in the most effective manner.
One of the main barriers to the implementation of a well designed seawall is cost. The design of an effective seawall requires good quality, long-term environmental data such as wave heights and extreme sea levels. This is frequently unavailable in developing countries and can be costly to collect. Secondly, because seawalls are frequently exposed to high wave loadings, their design must be highly robust, requiring good design, significant quantities of raw materials and potentially complicated construction methods. In locations of high energy waves, additional cost must be expended on protective measures such as rip-rap (Wide-graded quarry stone normally used as a protective layer to prevent erosion (Coastal Research, 2010)) to protect the structure’s toe.
A case study from the Pacific island of Fiji (Mimura & Nunn, 1998) shows seawall construction to be very costly even when local materials were utilised in conjunction with other materials supplied by the government. Seawall construction in Fiji consumed the villagers’ time and also required significant time and money to be spent on the provision of catering services for workers.
The availability of experience, materials, labour and specialised machinery for the construction of seawalls may also pose a barrier to the implementation of this technology.
French (2001) recommends proactive construction of seawalls at some distance inland. This reduces interference with coastal processes and creates a buffer zone to protect against coastal flooding and erosion. A key barrier to this type of approach lies in convincing and educating landowners of the necessity for, and benefits of, these measures (Mimura & Nunn, 1998).
Seawall construction is one of several options available when high value land cannot be protected in other ways. The approach provides a high level of protection to valuable coastal areas although the long-term sustainability of the approach should also be taken into account.
Less technologically advanced designs can be implemented at local levels, utilising local knowledge and craftsmanship. This requires less investment and a reduced need for involvement of large organisational bodies such as national or sub-national government or non-governmental organisations (NGOs). While ad-hoc implementation is possible, technological guidance from expert organisations is desirable to ensure sufficient levels of protection.
Seawalls can also be implemented as part of a wider coastal zone management plan which employs other technologies such as beach nourishment and managed realignment. Placement of seawalls inland, following managed retreat, reduces interference with coastal zone processes and creates a buffer zone to protect against coastal flooding and erosion (French, 2001). The seawall therefore acts as a last line of defence. Use of seawalls in conjunction with beach nourishment can also address some of the negative impacts of seawall construction, such as beach lowering and downdrift erosion.
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Matthew M. Linham, School of Civil Engineering and the Environment, University of Southampton, UK
Robert J. Nicholls, School of Civil Engineering and the Environment and Tyndall Centre for Climate Change Research, University of Southampton, UK