Flood Defense Walls / Embankments /Dykes (Dikes)

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Flood Defense Walls / Embankments /Dykes (Dikes)

General Overview

Several different designs and techniques can be used in the construction of a flood defense wall/embankment/dike. The construction has the function of preventing waterlevel rise to affect an area of protection. It could be a costly measure that also becomes a very visible feature in the cityscape and also require frequent maintenance. The constructions need great consideration concerning stability and subsidence and well thought through and advanced measures for the foundation.

Movable flood barrier

A moveable barrier can e.g. be located at the mouth of a river which is adjacent to an ocean, bigger lake or watercourse. The barrier can protect a society against a more temporary water level rise in the sea, lake or watercourse. There may be several different designs of a movable barrier. The Maeslant barrier at Rotterdam consists of two floating curved steel gates that automatically are moved to the river channel when the water is on the rise (Keringhuis, 2010). In river Thames (London) there are nine concrete piers with ten mobile ports located in depressions in the riverbed. With a forecast of upcoming high water levels the gates can be rotated 90 ° and form a barrier (Environmental Agency, 2010).

Foto: Worldsteel

Pro
The barrier can protect a city from occasional floods without permanently changing the conditions surrounding the river. Ecosystem, shipping and possibly drinking water supply doesn’t need not be affected in a significant way compared to permanent barriers. It is also one measure that will protect the whole city.

Contra
However, the measure will lead to some change in the ecosystem. It can also be problems when smaller rivers transport much water into the main river and make it impossible to keep the barrier closed without flooding the city from inside.

Foto: Storm Surge Barrier New Waterway, Delta Marine Consultants, The Netherlands

Non-movable flood barrier

A solid barrier is designed so that water inside and outside will be separated. It means that the water that possibly enters the bay through rivers regularly needs to be drained outside the barrier. In could possibly be done through the locks used to enable ships to get in and out. The barrier can also be built wide so the top side of it can be used for other purposes. Example of a solid barrier is Afsluitdijk in the Netherlands where a bay has been separated from the North Sea. On top of the barrier, which is 32 km long, is a European highway (Deltaverken, 2010; Wikipedia, 2010).

Foto: NedWater

Pro
A solid barrier may work well for a bay where the river flows are low. You can also use the top side of the barrier for transportation, thereby using the barrier for additional advantage. It is a solid and robust design that you know will be in place in case of rising water levels. The ability to build houses close to water becomes greater when you know you have the protection of a solid barrier.

Contra
Ecosystems can be influenced to a large extent when the exchange of water becomes difficult or impossible and the water quality may get worse. The basin inside may also be filled if there is high flows in connecting rivers. Pumping systems may then be needed, which could be a weakness in the system if there is power failure.

Soil embankment

Soil embankment is a permanent measure that provides protection against a certain water level rise. A soil embankment can be built with geo textile fabrics, fillers and a very coarse material that prevents erosion (Trelleborg AB, 2010). The water pipes whit a outflow on the outside of the embankment must be able to be closed manually or be equipped with check valves so that no leakage occurs inside the barrier (MSB, 2010a).When putting up a barrier it is desirable to have pumps, so water flowing in from behind the barrier (e.g. through rain, sewage systems) can be pumped outside. Pumping systems requires maintenance and depending on the volume of water that needs to be pumped the life length of the pumps varies.

Pro
It is a relatively inexpensive measure that can relatively easily be removed or built higher. Depending on the design it can be a positive element as walking path and a positive feeling through the greenery.

Contra
Soil embankment possibly does not suit in any urban environment and there can be major consequences if an embankment fails. If pillars or piles are necessary to prevent settlement, it may be more costly.

Foto: Trelleborg Municipality, Sweden

Wall of concrete or stone

One way to keep water out is to set up a traditional wall which can consist of concrete or stone. In connection with the construction the water pipes which flow outside the embankment need to be closed manually or be equipped with check valves so that no leakage occurs inside the barrier (MSB, 2010a).

Pro
It is a stable solution that one known is in place and provides a secure protection. The risk is less that human or technical failure should occur in the acute phase. If it is a broad concrete/stone wall, it can also serve as walking paths and can aesthetically fit in an urban environment.

Contra
A concrete/stone wall can depending on the design create a barrier between people and water and make an area less attractive.

Foto: USACE, US Army Corps of Engineers

Height adjustable wall

An alternative to permanent walls, which also do not need people's immediate response, is a structure that raises automatically at high tide, creating a protection. There may be several different designs that work with the same basic principle, i.e. the embankment is raised automatically by the rising water (Terra Firma, 2011). In connection with the construction the water pipes which has outflows outside the embankment need to be closed manually or be equipped with check valves so that no leakage occurs inside the barrier (MSB, 2010a).

Pro
It is a temporary solution that does not need human intervention in the acute phase. It also works without electricity. At low tide the constructiondoes not need to be visible and can also be aesthetically appropriate to have in an urban environment.

Contra
The design requires continuous monitoring and maintenance to maintain the function. A vertically adjustable wall may be assembled in sections and because of that there may be leaks in the joints. It is also uncertain how well it works in a cold climate and with icing. Electrical wires might be useful to de-ice the construction.

Foto: Flood Barrier

Retention Structures

Optimizing the Operation of Retention Structures

Flood Safety Standards