SEAWALLS - A Background
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).
Description:
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).
Advantages of the technology:
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).
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