Sea Dikes

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.