The primary function of sea dikes is to protect low-lying, coastal areas from inundation by the sea under extreme conditions (Pilarczyk, 1998a). Dikes are not intended to preserve beaches which may occur in front of the structure or any adjoining, unprotected beaches.
These structures have a high volume which helps to resist water pressure, sloping sides to reduce wave loadings and crest heights sufficient to prevent overtopping by flood waters. They may also be referred to as dykes, embankments, levees, floodbanks and stopbanks.
The description of this technology originates from Linham and Nicholls (2010).
Dikes are widely used to protect low-lying areas against inundation. As such, they have been widely applied in countries such as Vietnam, Bangladesh, Thailand, the Netherlands and the USA. Figure 1 shows a typical dike cross-section. It is a predominantly earth structure consisting of a sand core, a watertight outer protection layer, toe protection and a drainage channel. These structures are designed to resist wave action and prevent or minimise overtopping.
Dikes have been extensively utilised as flood defences in the Netherlands over the past several hundred years. As such, the Dutch have extensive experience in their design. As a result, many countries apply Dutch design practice in dike construction.
Typical Dutch practice employs the following design guidelines:
- Sloped seaward face at a gradient of between 1:3 to 1:6 – this can reduce wave loadings
- Sloped landward face at a gradient of between 1:2 to 1:3 – this minimises land take and maximises stability
- Impermeable cover layer – this is usually composed of clay but is sometimes supplemented by asphalt. It serves to protect the sand core (Barends, 2003)
- Toe protection – used as supplemental armour for the beach and prevents waves from scouring and undercutting the structure (Pilarczyk, 1998b)
- Dike core usually composed of sand to ensure that water that does enter can drain away. The core provides support for the cover layer and gives the structure sufficient volume and weight to resist high water pressures (Barends, 2003)
- Drainage channel – allows any water which does enter the structure to drain away, therefore ensuring the structure is not weakened by water saturation (Barends, 2003)
A number of zones can be distinguished on the seaward slope of a sea dike. The base of the dike, up to MHW will be regularly submerged and will experience constant, low-level loadings. The zone above MHW can be heavily attacked by waves, but the frequency of this occurrence reduces as you move further up the slope. Toward the dike crest, above the design water level, the structure should only be subjected to wave run-up.
Dikes provide a high degree of protection against flooding in low-lying coastal areas. They often form the cheapest hard defence when the value of coastal land is low (Brampton, 2002).
The sloped seaward edge of a dike leads to greater wave energy dissipation and reduced wave loadings on the structure compared to vertical structures. This is achieved because the seaward slope forces waves to break as the water becomes shallower. Wave breaking causes energy dissipation and is beneficial because the process causes waves to lose a significant portion of their energy. Because the waves have lost energy, they are less capable of causing negative effects such as erosion of the shoreline. By reducing wave loadings, the probability of catastrophic failure or damage during extreme events is also reduced.
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, as shown in Figure 2. This is beneficial for structural stability and helps to reduce the risk of undermining.
Dikes require high volumes in order to resist high water pressures on their seaward faces (Barends, 2003). As a result, their construction uses large volumes of building materials, including sand, clay and asphalt, which can be costly.
Another disadvantage of applying dikes is that the shallow slopes applied to facilitate wave energy dissipation cause dikes to have large footprints; i.e. their construction requires significant areas of land. This can increase dike construction costs where coastal land is valuable.
Raising dikes in response to SLR can cause the area of land required for dike construction to grow if slope gradients are maintained (see Figure 3). The area of land take can be problematic as coastal areas often have high associated land values. Further, construction of dikes prevents use of the coastal area for other development, hence, leading to competition for land. Extending dikes seaward may overcome this problem, but it raises costs significantly.
As with all hard defences, dikes can create a false sense of security on the landward side of defences, promoting further development landward of the dike. Hence, once protected, it is difficult to change the management policy.
The construction of hard defences permanently fixes the position of the coastline. This can have detrimental impacts because the coast is a naturally dynamic system. Fixing the position of the coastline can prevent natural coastal processes, such as responses to sea level changes, beach/dune interactions and sediment input from coastal erosion (French, 2001). Stopping these processes not only impacts the immediate environment, but because the coastal system shares sediments within a coastal cell, knock-on impacts can also be felt elsewhere along the coast.
Significant shoreline hardening measures can be aesthetically displeasing, especially in areas which are dependent on a tourist economy where natural shorelines are valuable (IOC, 2009).
The best available cost information for sea dikes is compiled by Hillen et al. (2010) in a review of Vietnam, the Netherlands and New Orleans. The information is presented as the cost of dike heightening in millions of US dollars per linear km of defence. Heightening of dikes is reported to cost from US$0.9 to 29.2 million per metre rise in height, per km length (in 2009 US dollars) (Hillen et al., 2010).
Vietnamese costs of dike construction, reported in Hillen at al. (2010) are perhaps most relevant to developing countries. 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 – significantly less costly than construction in either the Netherlands or New Orleans (Hillen et al., 2010). Costs were variable due to varying costs of material, land-use and applied inner/outer protection of the dike’s slopes. When comparing completed projects within Vietnam, labour costs were observed to be highly variable even within the country.
Dike construction costs are shown by Hillen et al. (2010) to vary considerably between rural and urban areas with dike construction in rural areas shown to be consistently less costly. This is the case worldwide. Costs are also influenced by a number of other factors, such as:
- 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 (Nicholls & Leatherman, 1995)
- Proximity to and availability of raw construction materials
- Availability and cost of human resources including expertise
Maintenance costs are an ongoing requirement for sea dikes, to ensure the structure continues to provide design levels of protection. Information on maintenance costs is limited, although annual dike maintenance costs per linear km of dikes are reported to range from US$0.03 million in Vietnam (Hillen, 2008) to US$0.14 million in the Netherlands (AFPM, 2006). These costs are presented in 2009 US dollars. The variability in these costs is largely due to the fact that while dike maintenance in the Netherlands is well organised and given high priority, in many other locations, maintenance programmes are less rigorous. To a lesser extent, local factors such as labour and material costs, and the presence of different types of dikes/coastal defence measures will also influence costs (Hillen, pers comm.).
The 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, causes increased wave heights and wave loadings on the structure.
Construction of sea dikes is possible on a local scale. However, the improved science and technology base that the involvement of larger organisations brings can significantly improve a structure’s effectiveness. This is seen in Vietnam (a detailed case study is shown at the end of this section) where poor dike design and insufficient funding resulted in dikes providing lower levels of protection than initially intended (Mai et al., 2008).
Ad-hoc construction of coastal defences is likely to give much less consideration to the water levels, wave heights and wave loadings occurring during an extreme event. This is largely because these events are hard to foresee without a well-developed knowledge. As such, ad-hoc defences typically offer lower levels of protection.
Dikes designed and constructed by local communities are likely to employ local materials and traditional methods. This may not necessarily constitute the most effective approach, although it may be the only available option. Provision of design and construction guidance, even for small details such as recommended slopes and materials, is likely to improve the performance of defence structures.
As shown below, dikes can be expensive measures to employ with costs ranging between US$1 and 7.6 million per km length of dike depending on the global location (Linham et al., 2010) and with additional annual maintenance costs. As such, external funding may be required before a successful dike construction project can proceed.
If community level implementation goes ahead, it is essential that the wider impacts of hard defences on the coastal zone are not overlooked. When implementing projects at a local level, it is easy to focus on local benefits and neglect the bigger picture. As stated under the disadvantages of dikes, some impacts of dike construction may be felt considerable distances from the implementation site. Dike implementation at a local level may pay little attention to reduce these impacts.
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 high space requirement for sea dikes is one barrier to implementation. This factor will be especially important in areas where the value of the coastline plays an important role in deciding adaptation technologies. The availability of materials, labour and specialised machinery for the construction of dikes may also pose a barrier to the implementation of this technology.
The cost of implementing an effective dike system can prove a barrier in some cases. This will especially be the case in high wave-energy areas where additional protective elements such as rip-rap will be required (IOC, 2009).
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. One of the main barriers to the building of an effective dike which accounts for local conditions is therefore the availability of long-term datasets. The cost of collecting such data can be expensive. However, by accounting for these local conditions, dike design is typically more effective. The additional costs of data collection and exclusive design may for a barrier to implementation in some circumstances.
Where large areas of high value coastal land, which cannot be surrendered to the sea under a managed realignment policy, exist at elevations close to, or below sea level, there are often few other choices available than the construction of dikes.
Dikes are capable of providing very high levels of protection against coastal flooding if designed appropriately. 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 MSL (Pilarczyk, 2000). Long-term sustainability considerations should be borne in mind if this technology is adopted, however.
Dikes are a tried-and-tested method of coastal protection. Construction methods and design principles for these structures are well known and publicised. 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. This makes diking more affordable but does compromise safety and protection levels.
Dikes can be implemented in conjunction with 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.
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Mai, C.V., van Gelder, P.H.A.J.M., Vrijling, J.K. and Mai, T.C. (2008) Risk Analysis of Coastal Flood Defences - A Vietnam Case in Simonovic, S.P., Bourget, P.G. and Blanchard, S.F. (eds.). The 4th international symposium on flood defence; managing flood risk, reliability & vulnerability, Toronto, 6-8 May, 2008. Toronto: Institute for catastrophic loss reduction.
Pilarczyk, K.W. (1998a) Design philosophy and methodology in Pilarczyk, K.W. (ed.). Dikes and Revetments: Design, Maintenance and Safety Assessment. Rotterdam: A.A. Balkema, 11-40.
Pilarczyk, K.W. (2000) Design of dikes and revetments – Dutch practice in Herbich, J.B. (ed.). Handbook of Coastal Engineering. New York: McGraw-Hill, Chapter 3.
Townend, I. and Burgess, K. (2004) Methodology for assessing the impact of climate change upon coastal defence structures in McKee Smith, J. International Coastal Engineering Conference 2004, Lisbon, 19-24 Sept 2004. London: World Scientific.
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