There is a reluctance on many responsible authorities to invest in mobile flood barriers (also known as temporary flood barriers); this despite mounting evidence that they can work very effectively, and the increasing risk of flooding.
It is not economic to provide new multi-million dollar levees or diversion channels for all flood risk areas, but is it right to leave these areas with nothing more than a relic of world wars, with which to defend themselves from flooding? This article considers the merits of sandbags versus the technology now available.
Let’s consider first the alternative and traditional approach, the use of sandbags. These have a very low upfront cost and in some circumstances the fill material (sand or earth) may be readily available somewhere close to where they are required. If a sandbag wall is built correctly then it can perform and it can be completely removed after the event or even built into a permanent feature using additional materials for stabilising and greening.
In contrast commercially-supplied flood barriers may be expensive, have relatively short life cycles and may be prone to incremental or even catastrophic failure.
The most prevalent flood barriers are water-filled tubes. These use water as an infill material to build structure and weight to stabilise them and are therefore very cost-effective. A typical tube flood barrier will be used as flood defence up to 75% of its filled height but may become unstable beyond that level and become subject to failure through rolling, sliding and collapse.
A high-quality tube such as Rapidam Tube will include features to help stabilise the barrier and minimise the movement especially the rolling.
All tubes however maybe subject to relatively short lifespans because every time they are used they may be pressurised, and a failure may be difficult to detect until it has happened.
To mitigate this, look for a greater material weight and for high-frequency welding rather than stitches which may be subject to rot. This will also help with resistance to impacts such as a log or other floating debris.
These flood barriers are filled by the incoming flood water and use the flood water to cover an apron on the ground as the means of stabilisation, increasing friction with the ground. The ground apron is connected to the flood wall element via straps or rods to retain the floodwater on the dry side. They are typically very fast to deploy and can seal well to uneven ground. However, being made of fabric they may be prone to damage should an impact occur, albeit this is visible and normally repairable even in field.
The most likely failure to occur is sliding due to insufficient friction with the ground. This risk can be managed by increasing the apron length, increasing means of friction with the ground or by anchoring the barrier with weight or pegs or screws. These are all options in the deployment of Rapidam Flexi which was awarded a prestigious award by BBC Tomorrow’s World at the start of the Millennium.
RIGID FRAME BARRIERS
These barriers are formed of metal frames and usually also have metal panels or beams which present a 45-degree flood wall to create a self-anchoring effect through the frame. They are in themselves structurally very sound and this can be proven by engineering analysis even to the extent of their resistance to debris impact. The only significant risks are of sliding in the case of greater flood depths which could lead to a catastrophic failure if the barrier moves excessively.
The other potential failure is non-deployment because there are likely to be a much greater number of components to assemble than for tubes or flexible membranes. And in most cases, they require complete assembly before a sheet membrane is used to cover the skeleton and requiring anchoring to the ground on the flood side of the barrier. This often creates a situation where the lives of people deploying the barrier may be at risk with rising floodwater at their feet or knees.
These risks are mitigated in the design of Rapidam Rigid which has a ground apron mechanically-connected to the bottom beam of the barrier. This and the facility for anchoring through the feet, mean that sliding can be ruled out as a risk whilst the immediate seal created, means that responders can concentrate on assembling the barrier along the length to prevent outflanking.
ASSESSMENT OF THE TRUE COST
Whilst sandbags may on the face of it appear to be a good option with the low upfront cost, the true costs may be much more:
- Plant and depot costs
- Manpower costs (for deployment)
- Transportation costs
- Clean up costs
The biggest cost of this sandbag approach is likely to be that of non-deployment or failure of the flood barrier by being outflanked or over-topped due to the speed or onset or rise of the floodwater. There may simply not be either the human or physical resources available to build a flood barrier of 100metre or mile length fast enough, yet alone in all the locations where this is required. This may see the costs of failure in the millions of dollars or even risk to lives.
These are complex issues for decision-makers but when deciding to invest or not, and when putting together a technical specification, then expertise is required. This will ensure that what is procured will provide the flexibility, durability, safety and speed of deployment required for the scenarios considered. Getting this right will see a focus on the whole-life costs versus the whole-life benefits i.e. the costs of consequential damage as a result of failure or doing nothing.
Aquobex has attempted to make this simple by bringing together the best-in-class for each flood barrier type, together with a real-time information and an early warning action system. For example, combining our virtual control room with our apex mobile flood barrier Rapidam Rigid, responders can focus on building linear length of barrier quickly and efficiently. Resource such as additional beams and the requisite manpower can be deployed elsewhere until it is confirmed that the flood defence height should be increased.
Unlike sandbags they are easy to remove without a big clean-up effort and the associated costs and they may be used countless times in their 50 year plus service life.