9.10 Temporary and demountable defences
A temporary flood barrier is one that is only installed when the need arises (that is, when high flood levels are forecast).
A demountable flood defence is a particular form of temporary defence that requires built-in parts and therefore can only be deployed in one specific location. The removable stoplog defence shown in Figure 9.8 is a particular form of demountable defence applicable only for small openings in a permanent defence. The more commonly adopted gate option for closing off a gap in a floodwall is neither temporary nor demountable, as it is part of the permanent defence and is left in place all the time (albeit normally in an open position).
Both temporary and demountable defences require a considerable amount of pre-planning to ensure that they can provide an effective defence. It is essential that the operational resources, storage facilities and the logistics of deployment are fully appreciated by anyone planning to rely on these types of defence.
9.10.2 Temporary flood barriers
There are many forms of temporary flood barrier, ranging from sandbags to the so-called pallet barrier. These are suitable for deployment in response to a developing flood in situations where there are no permanent defences or where the existing defences are not high enough.
Sandbags can be used to provide a degree of protection at the entrances to individual properties. They have also been used successfully to raise the crest level of significant lengths of flood embankment. However, sandbags do not provide a watertight defence and are practical only up to a height of about 0.3m, perhaps 0.5m in ideal circumstances. They require considerable effort to deploy and often prove ineffective.
The pallet barrier is a more sophisticated system which is deployable over a length of several hundred metres. It consists of a lightweight structural system which supports an impermeable membrane. The system relies on a reasonably firm and level foundation. The effective use of such systems also depends on the ability to mobilise them fairly quickly in response to a growing threat of flooding. Storage of the necessary equipment reasonably close to the site is therefore essential, otherwise there is a risk that the defence will not be delivered to site and erected in time to prevent flooding (see also the guidance below on demountable defences in this regard).
Whereas temporary barriers may seem to offer a viable solution in situations where there is sufficient time to mobilise and erect them, such systems have other potential disadvantages. It is essential to ensure that no significant drains or other flow pathways are present that would compromise the effectiveness of the defence. There are also likely to be issues with parked cars and road closures that may delay the deployment of the defence to the point where deployment is too late.
Temporary barriers therefore require a considerable amount of pre-planning to ensure that they can provide an effective defence. For any type of temporary defence, it is essential that the operational resources (labour, plant and equipment), storage facilities (access to and from, security, proximity to point of deployment), and the logistics of deployment are fully explored before the commitment to rely upon this approach is made.
9.10.3 Demountable flood defences
Demountable flood defences are used where a permanent defence is unacceptable, usually because of the visual intrusion and loss of amenity that a permanent defence would entail. These are relatively new to the UK, but have recently been successfully installed and operated in Shrewsbury and Bewdley on the River Severn (see Figures 9.15 and 9.16), and in parts of north Wales.
Time is the key factor concerning the adoption of demountable defences. There must be sufficient advance warning of a flood to allow the defence to be deployed. In addition, the advance warning must be reasonably reliable so as to avoid excessive precautionary deployment of the defences. They are therefore suitable on the middle and lower reaches of the River Severn, for example, where flood conditions can be predicted days in advance, then confirmed with plenty of time to mobilise the erection team.
More details of demountable defences can be found in Environment Agency R&D Publication 130 (Ogunyoye and van Heereveld, 2002).
Figure 9.15 Frankwell flood alleviation scheme, Shrewsbury (River Severn)
First deployment of the defences in the flood of February 2004, at the peak of which 1.9m depth of floodwater was held back, preventing the flooding of up to 74 properties.
The key feature of a demountable defence is the provision of built-in sockets or similar foundations that the demountable supports slot into. There may also be built-in provision for struts to lend additional support to the defence.
Figure 9.16 A demountable defence being erected at Bewdley (River Severn)
It is easy to see why a permanent floodwall in this location would not be acceptable to the local residents – its height would obscure the view of the river from the waterfront and the residences, and would restrict access.
The use of a demountable defence means that the residents can enjoy the view and have flood defence, but such win–win solutions are only suitable in certain situations.
Demountable defences may make use of standard panels, but they often have to be custom-designed to suit a particular location. They rely on built-in foundations for stability and ease of erection.
The components of the demountable defence must be stored relatively near to the site to reduce the risk of delays once the decision to mobilise has been made. A skilled erection team is required to ensure that the defence can be safely and securely erected in the limited time available. For practical reasons, the length of a demountable defence is unlikely to exceed a few hundred metres.
Whereas demountable defences may seem to offer the ideal solution where visual impact is a key factor, this form of defence tends to be expensive and there will always be a risk that the defence elements are not deployed in time to avert flooding. Theft of defence units, damage to built-in parts and delays in transporting the units to site can all conspire to make deployment problematic.
In July 2007, the delivery of the components of a demountable defence at Upton-on-Severn was delayed due to the severe disruption to the transport infrastructure caused by surface water flooding. The defence was not deployed in time and there was considerable flood damage as a result. In fact, had these demountable defences been deployed, they would have been overtopped due to the severity of the flood, but this example illustrates just how important it is to make sure that these defences can be successfully deployed in a flood event.