Storm surge barriers and closure dams are hard engineered structures with a primary function of preventing coastal flooding. Their secondary role is to shorten the required length of defences behind the barrier. This reduces the risk of defence failure and reduces the cost of providing the additional defences. Surge barriers are movable or fixed barriers or gates which are closed when an extreme water level is forecast in order to prevent flooding. Closure dams are fixed structures that permanently close off a river mouth or estuary. For these and fixed barriers, water is discharged through, or pumped over the barrier (IOC, 2009).
The description of this technology originates from Linham and Nicholls (2010).
Storm surge barriers and closure dams are large-scale coastal defence projects, capable of protecting tidal inlets, rivers and estuaries from occasional storm surge events (UNFCCC, 1999). They provide a physical barrier which prevents storm surges travelling upstream. This helps to keep upstream water levels low and therefore minimises coastal flooding. The two solutions are most frequently applied at narrow tidal inlets, where the length of the structure is not required to be so great and where defences behind the barrier can be reduced in height or length. An example of the construction of a closure dam in Bangladesh is shown in Figure 1.
Storm surge barriers most commonly consist of a physical, movable barrier across the mouth of a tidal inlet or estuary. While there are no known examples in the developing world, a number of projects have been completed in developed countries, mainly in Europe. For example, the Thames Barrier, London, the Maeslantkering Barrier, Rotterdam and the St. Petersburg Flood Protection Barrier, while the MOSE project in Venice is scheduled for completion in 2012. Although each of these projects has roughly the same objective, the design of these structures varies significantly.
Fixed barriers and closure dams are a lower technology option which may be more appropriate in developing countries. These are non-movable barriers across tidal inlets or estuaries. They constructed through gradual or sudden closure of an inlet. Gradual closure can be accomplished through land-based construction which gradually narrows the inlet, or by water-based construction which builds a barrier up, layer by layer, from the seabed. Alternatively, sudden closure blocks an inlet in a single operation, using pre-installed gates or by the placement of a caisson (a retaining, watertight structure).
Examples of completed closure dams include the Feni closure dam in Bangladesh, constructed mainly to provide a freshwater reservoir for irrigation purposes, several projects in Korea to close tidal basins, mainly for land claim (van Houweninge & de Graauw, 1982) and the Afsluitdijk, in the Netherlands, which separates what is now Lake IJsselmeer from the North Sea.
Movable barriers will require the simultaneous implementation of a storm surge monitoring and forecasting system (an adaptation option in its own right). This will allow the barrier to be moved into position before a storm surge arrives. Because closure dams are fixed structures, they do not require these systems.
While there are clear differences between storm surge barriers and closure dams, the coastal defence purpose of the structures is the same; to prevent extreme water levels penetrating an estuary. The method, by which this is achieved, is illustrated in Figure 2.
Surge barriers and closure dams can be easily integrated into a larger, overall flood prevention systems. For example, barriers may be present alongside additional flood prevention works such as dikes and flood warning systems.
An important characteristic of surge barriers is that they are movable. As such, they are often partly opened during normal conditions. This will allow tides and saltwater to enter the areas behind the barrier (Hillen et al., 2010) and allows continued use of waterways for shipping and transport. Conversely, closure dams permanently close off estuarine areas. This prevents interactions between freshwater and the sea and also prevents use of the waterway for shipping and transport.
Storm surge barriers and closure dams provide a high degree of protection against coastal flooding by preventing storm surges from entering low-lying estuarine areas. Although permanently closing off the estuary mouth using a closure dam, would achieve the same outcome, the use of a movable barrier allows waterways to remain open during normal conditions. This can be beneficial to trade if the estuary also acts as a trading port and is also valuable for estuarine species reliant on brackish water conditions (a mixture of salt and fresh water – brackish water is salty but not as salty as sea water).
The two technologies effectively reduce the height of extreme water levels in the area behind the barrier, if closed in a timely fashion. Doing so may allow the strength of existing defences behind the barrier to be reduced (Hillen et al., 2010). This will reduce both construction and maintenance costs for defences on the landward side of these structures.
By reducing the height of extreme water levels inside of the barrier, the length of a coastal flood defence system may also be shortened (Hillen et al., 2010). This too, would have the effect of reducing maintenance and construction costs of defences on the landward side of the barrier.
More than one barrier may be constructed to close off narrow inlets into a tidal system, such as a lagoon. This is the case in Venice under the MOSE project where three barriers are under construction to close three of the lagoon’s narrow tidal inlets. Through the construction of multiple barriers, the scheme offers the additional benefit of enhancing the lagoon’s natural capacity to clean itself. This is achieved by independently opening and closing selected barriers, depending on wind direction. By closing barriers it enhances the ability of the wind to drive water out of the lagoon, therefore increasing the turnover of water, dispersing pollutants.
Closure dams can provide additional benefits by forming a permanent barrier between freshwater and the sea. For example, in Bangladesh, the Feni closure dam was constructed primarily to provide a reservoir of freshwater for irrigation purposes. Closure dams may also be used in conjunction with land claim and may even be used for the production of tidal energy (van Houweninge & de Graauw, 1982).
One of the key disadvantages of the storm surge barrier is the high capital and maintenance costs. Significant investment is required to construct these structures and to continually maintain them. In addition, movable barriers also require simultaneous investment in flood warning systems which provides information on when to close the barrier. This cost is avoided through the use of a closure dam, which also has lower capital and maintenance costs.
A potential disadvantage of both surge barriers and closure dams is they can cause flooding on the landward side of the barrier when river levels are high and, in the case of movable barriers, if the defence remains closed for an extended period. Landward flooding occurs as a result of water backing up on the landward side of the barrier due to the obstruction of continued river discharge by the barrier. This should not present a problem, provided closure dams are designed to cope with extreme river discharges and that studies to determine the maximum duration of closure have been undertaken in the case of movable barriers.
Both surge barriers and closure dams have the capacity to change the chemical, physical and biological properties of estuarine systems by altering the inflow and outflow of water from the estuary. This may include alterations to water salinity, temperature, suspended matter, nutrients which all have the potential to affect local communities of organisms (Elgershuizen, 1981). These changes will be more significant in the case of a closure dam as the barrier is permanent. The application of movable rather than fixed gates can mitigate these impacts (IOC, 2009).
Table 1 shows the costs for storm surge barrier construction of both completed projects and projects near completion. Since there are no known examples of movable surge barriers in the developing world, it has unfortunately, not been possible to include costs estimates for developing countries.
Storm surge barrier construction costs are highly variable, as shown in Table 1. Influential factors in the cost of these structures include the design and hydraulic head over the barrier (see Figure 2).
Table 1: Overview of storm surge barriers, types and costs
|Barrier and location||Barrier type||Hydraulic head (m)||Construction costs (2009 price level) (US$mil|
|Thames barrier, London, UK||Sector gates||7.2||2043|
|IHNC barrier, New Orleans, USA||Sector gates||4||730|
|SeaBrook barrier, New Orleans, USA||Vertical lifting gates/ Sector gates||4|
|Hartel barrier, Hartel channel, NL||Vertical lifting gates||5.5|
|Eastern Scheldt barrier, NL||Vertical lifting gates||5|
|Maeslantkering Rotterdam, NL||Floating sector gate||5|
|MOSE project, Venice, IT||Flap gates||3|
|Ramspol, near Ijssellake, NL||Below barrier||4.4|
Source: Hillen et al. 2010
Hillen et al. (2010) investigated the unit costs of storm surge barriers and found that the hydraulic head will be an important determinant for the forces on the barrier and the required construction properties and costs. They also found that there is a weak relationship between the head and the unit costs, although the factors determining unit costs still need to be investigated further. They concluded that unit costs for storm surge barrier construction range between US$0.7 and 3.5 million per unit metre width, at 2009 price levels. Maintenance costs are an ongoing expense which must also be accounted for; annual costs have been estimated at approximately 5-10% of the capital, for movable barriers (Nicholls et al., 2007b).
The costs of constructing closure dams in Bangladesh are given in Table 2. The three projects for which cost data is available, were constructed largely of traditional materials but with the guidance of experienced coastal engineering consultancies. Traditional Dutch construction methods were used in all three projects.
Table 2: Costs of completed closure dams in Bangladesh
|Project||Year completed||Barrier width x depth (m)||Construction materials||Cost (2009 value)|
1200 m width, Unknown depth
|Clay filled sacks, Bamboo, Reed rolls, Steel beams, Bricks and blocks||US$38 million|
|Chaka Maya Kal||1979||210x5.5||Bamboo, palm leaves, reed bundles, timber piles, Jute||US$1.3 million|
|Amtali Khal||1982||130x8||Reed bundles, Golpata leaves, Clay filled sacks, Timber piles||Tk 16 million|
Source: DHV Haskoning, 2007
As shown in both Table 1 and Table 2, the costs of surge barrier and closure dam construction are highly variable with project costs likely to be influenced by the factors below:
- Type of barrier
- Local soil characteristics
- Desired height of the barrier
- Required hydraulic head for the structure
- Anticipated wave loadings; higher wave loadings require more robust and expensive structures
- Single or multi stage construction; 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
It has been noted that 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, cause increased wave heights and wave loadings on the structure.
Effective implementation of storm surge barriers always requires considerable engineering studies to design and install these structures (IOC, 2009). Barrier design is likely to be technologically challenging and almost impossible to undertake at the community level. Additionally, as seen under the costs and financial requirements section, surge barriers can be highly expensive and funds may be lacking at a local level. As such, technical assistance may be sought from coastal engineering consultancies or other experienced organisations, while funding may be obtained from external organisations such as NGOs or local government and enterprises which benefit from the structure.
In addition to the hardware, effective forecast and warning systems are required when implementing a movable storm surge barrier. This may require significant institutional capacity (IOC, 2009). Implementation of a flood warning system requires some or all of the following tasks to be conducted: system design, management and forecasting of floods, operation, detection of storms and warning dissemination (Sene, 2008).
Closure dams and non-movable barriers are lower technology alternatives to movable surge barriers. A number of such projects have been successfully constructed in countries such as Bangladesh and Korea. To make these projects more feasible at a local level, construction methods may employ local materials and labour, although guidance from experienced contractors would also prove beneficial (e.g. DHV Haskoning, 2007).
The high cost of surge barrier construction (shown in Table 1) and the requirement for specialist knowledge in the design and implementation phases may prove a barrier to implementation of storm surge barriers.
Additionally, surge barriers and closure dams are not suitable for all locations. They are most appropriate in locations where a narrow river mouth or inlet can be closed. Alternatively, they are appropriate where the length behind the barrier that would otherwise require defending can be substantially reduced; in the case of a short defensive length, it may be more effective to upgrade defences than to construct a barrier.
Although barrier construction across narrow channels is cheaper, it is apparent that surge barriers can be implemented where narrow inlets are absent, provided sufficient funds for construction are available and the political will exists. For example, the St. Petersburg Flood Protection Barrier employs two movable storm surge barriers within a man-made 25.4 km long barrier, across the mouth of the Neva Bay on the Gulf of Finland.
Opportunities for the implementation of storm surge barriers are numerous. The MOSE project in Venice, Italy, has demonstrated the capacity for surge barriers to offer co-benefits alongside flood protection. For example, opening and closing specific barriers depending on the wind direction can facilitate dispersion of pollutants thus helping to improve coastal water quality. This is beneficial for both recreation and tourism.
Storm surge barriers can also provide additional services such as recreation, amenity and water supply when appropriately designed. The Marina Barrage in Singapore was completed in 2008 and provides an excellent example of the additional benefits which can be gained from a well designed surge barrier. As well as providing protection against coastal flooding, construction of the barrier has also provided a large reservoir which will help meet water demand in one of the island’s most urbanised catchments (Moh & Su, 2009). By eliminating tidal influence inside the reservoir the area is now an ideal venue for recreational activities such as boating, windsurfing and water skiing (Moh & Su, 2009). By integrating an art gallery and retail outlets into the barrier design, the defence is also now a significant tourist attraction.
Storm surge barrier projects have also been seen to act as a catalyst for development of newly protected areas. This was observed following construction of the Thames Barrier, when London’s derelict docklands were regenerated with new transport links, homes, businesses and the important financial district around Canary Wharf (Nicholls, 2006).
In future there could even be opportunities to integrate storm surge barrier or closure barrier design with the production of renewable hydroelectricity. This will provide long-term, sustainable energy as well as security of energy supply for local communities.
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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