A sea dike is a manmade structure designed to protect low-lying areas from flooding from the sea or ocean. They typically are designed with several components including a sand core, a watertight outer protective layer, toe protection and a drainage channel. Sea dikes are intended to withstand and resist water and wave action. They are widely used in countries with low lying geographies such as Vietnam, Bangladesh, Thailand, the Netherlands and parts of the United States.
The construction of a dike has three functional parts – a submerged base, the middle area that experiences constant loading from waves, the upward middle zone that can experience wave pressure during storms and finally the above area or ‘dike crest’ that mainly absolves wave run-up. A number of zones can be distinguished on the seaward slope of a sea dike.
Advantages of the technology
Dikes are hard measures that provide a high-degree of protection against flooding. Compared to other hard measures that require civil engineering, they are comparatively less expense. Dikes provide a high degree of protection against flooding in low-lying coastal areas.
Dikes designed with a slope are more effective than vertical dikes. The sloped dike forces the wave to break when the water becomes shallow, and therefore reduces the energy of the wave. Dissipation of wave energy or wave loadings reduces the damage that the wave could cause in relation to erosion of the shoreline.
When compared to vertical structures, dikes also have reduced toe scour. This is because the wave downrush is directed away from the base of the structure. This is beneficial for structural stability and helps to reduce the risk of undermining.
Disadvantages of the technology
As a hard measure, dikes are usually a big construction undertaking and require large columes of building materials including sand, clay and asphalt. While they are comparatively less costly than some other hard measures, the undertaking of such extensive construction is still expensive and require continual maintenance costs.
Sloped dikes described above are particularly important for dissipating wave energy, but they also require more land on which to be constructed. This increases their cost and the overall intrusion of a built structure on a natural environment. Moreover, the construction of dikes often prevents the areas from being used for other activities such as tourism creating conflicts of interest and competition for land. One option is to extend dikes seaward, rather than landward, however this significantly increases the costs.
Permanent structures involving significant construction have a direct impact on the natural dynamic processes of a coastline. They can create new problems related to natural responses to sea level changes, beach and dune interaction and sediment input from coastal erosion. Interfering and preventing natural coastline dynamics can create problems for the area in question as well as the adjacent ecosystems.
Financial requirements and costs
In a country comparative study by Hillen et al. (2010), the costs of building a dike are compared. As may be expected, the costs in some developing countries are significantly less than in the United States and the Netherlands. Moreover, there are considerable discrepancies in costs dependent on whether the dike is built in an urban or rural area. In the United States, the cost of dikes are estimated to range significantly from US.09 to 29.2 million per metre rise in height per km in length. In Vietnam, dike construction costs were shown to vary from US$0.9 to 1.6 million per metre rise in height, per km length (Hillen et al., 2010).
Other variables that determine cost are summarized by Hillen et al (2010).
- Land availability and cost. As shown in Figure 4.13, dike construction needs significant land input. Accurate cost studies often draw a distinction between rural and urban construction costs to reflect differential land values
- Selected dike design and in-built margin for safety. This can affect the volume of the structure and the required materials
- Anticipated wave loadings; higher wave loadings require more robust and expensive structures. Wave loading is affected by wave breaker types, cleanness of the breaking wave, seabed shape and individual storm characteristics such as storm duration, wind strength and storm orientation in relation to the structure
- Single or multi stage construction; aggregate costs are lower for single stage construction
- Proximity to and availability of raw construction materials
- Availability and cost of human resources including expertise
Institutional and organisational requirements
Dikes vary in scale and design. Construction of sea dikes is possible at a local scale, however, in such cases it is important that the science and technology behind the decision to construct a dike is sound. In some cases, poor dike design brings about lower levels of protection and wasted funds. Dikes that are built ‘ad hoc’ dike designs often pay less attention to planning, water heights, wave heights and do not accurately meet the needs of an actual extreme event mainly because information about the type and scale of the event are difficult to predict. Dikes built ad-hoc generally provide less protection.
Extreme caution should be exercised if ad-hoc, community implementation of sea dikes goes ahead. Because dikes are often designed to protect extensive areas of low-lying land, catastrophic failure caused by poor design is likely to be associated with a threat to the lives of significant numbers of people.
The most effective dikes are those designed in accordance with good quality, long-term environmental data, such as wave height and extreme sea level information.
Barriers to implementation
A major barrier to dike construction is that building requires significant coastal land. The use of coastal land for dikes raises several problems that include conflicts of interest between users, especially since dikes usually reduce accessibility and use by communities of the shore. They also require land which may be desired for other purposes or privately owned.
One of the main barriers to the building of an effective dike which accounts for local conditions is the availability of long-term datasets. Collecting such data can be expensive especially as an up-front cost, however careful planning typically makes dikes more effective.
Opportunities for implementation
Dikes have become essential to areas below sea level that have high value coastal land that cannot be surrendered to the sea (i.e. urban developments (e.g. Amsterdam)). In such cases, dikes designed correctly are extremely effective in providing high levels of protection against coastal flooding. This can enable significant development to take place behind them, even if land is low-lying. This is demonstrated by Schiphol Airport, Amsterdam, in the Netherlands – the area is enclosed by dikes but lies 4.5 m below sea level (Pilarczyk, 2000).
Dikes are a tried-and-tested method of coastal protection. Although specialised dikes, designed with local conditions in mind pose the most effective defences, it is also possible to implement more generic or lower quality designs at a lower cost.
Dikes can also be constructed to compliment other erosion and flood protection works such as beach nourishment and managed realignment. This has the potential to address the negative impacts associated with the technology and also means the benefits associated with each technology can be realised.