Building a flood wall is not as simple as getting some quotes for a wall and picking one. First and foremost there needs to be a full Flood Risk Assessment, determining the flood mechanisms, depths, drainage routes, soil type – and the displacement of water to ensure it will not impact on flow paths, or exacerbate risk elsewhere.
Whilst cost is no guarantee of performance, a properly designed and constructed flood wall is not a low-cost undertaking, and unusually cheap quotations should be treated with caution. However, when compared with the potential cost of flood damage or the impact on property value, a flood wall can be highly cost-effective and is often a preferred solution, particularly where it can deliver reliable and long-term protection.
Flood walls can be used to create dry areas around a property, thus increasing the likelihood of an effective flood defence, subject to suitable drainage backflow and pumping.
The design and style of the wall is primarily determined by the risk and thus height required, and the soil type impacting on the foundation design. Of course, the proposals also need to be buildable, and therefore a construction specialist understanding the site constraints, before determining any designs is important, and something FPS can assist with.
There are also other considerations such as the surrounding area and planning constraints, particularly in conservation areas or for listed buildings.
For low level walls, a standard double skin wall with suitable footing may prove suitable, however even in these scenarios, the mortar should include a waterproof additive, such as Additive No.2 by Safeguard, or SBR (Styrene Butadiene Rubber), to increase the bonding and reduce water penetration. The bricks should also be suitable, and as a minimum be F2 rated, with engineering bricks often preferable. The choice would form part of the site specific design.
F2 rated bricks are frost-resistant bricks designed for severe exposure.
A popular style of wall includes using “Stepoc” blocks which are hollow and easy to lay, rebar should extend from the foundation into the hollow, before concrete is poured. The wall can then be faced with bricks or rendered.
Stepoc allows a simple and easy construction method effectively using the blockwork as formwork, allowing for concrete to be poured into it. Traditional walls may have used falsework to create a concrete stem, which was later faced with bricks.
Falsework refers to temporary shuttering removed after the concrete has cured, whereas formwork remains in place.
Stepoc has made the construction of flood walls and retaining walls much easier.
In certain instances, a double skin wall with ties between each skin, and a cavity can be used with the cavity filled with concrete.
A common assumption is that if floodwater rises from 300 mm to 600 mm, the force on a flood wall simply doubles.
In reality, it increases far more than that.
Water pressure at any point does increase in direct proportion to depth. However, a wall doesn’t experience pressure at a single point, it carries the entire pressure distribution from the water surface down to the base.
Because pressure starts at zero at the surface and increases linearly with depth, the overall load on the wall forms a triangular distribution. When you add up that triangular load, the total horizontal force on the wall is proportional to the square of the water depth.
This means:
• At 300 mm depth, the lateral load has a certain magnitude
• At 600 mm depth, the lateral load is four times larger, not double
The overturning effect grows even faster, because the resultant force also acts higher up the wall.
This is why modest increases in flood depth can rapidly move a wall from stable to unstable if it has not been designed for the correct load case.
When floods threaten, ensuring your property remains safe from water damage is crucial. FPS Flood Barriers are manufactured in the UK with full quality assurance meeting tough PAS testing. Designed to provide a reliable, removable and easy-to-use flood protection solutions for both residential and commercial properties.
When considering a flood wall, the gaps in the wall also need to be considered.
Higher flood walls may require structural returns or pillars at openings. These act as vertical structural elements that receive the lateral load transferred through the barrier system and deliver it safely into the foundations.
Water pumps are required behind a flood wall due to the hydraulic gradient, where water level will try to equalise. For infrequent or flash flooding, a puddle pump may suffice, however for longer term flooding you may wish to consider a Packaged Pump Station which would activate automatically and pump water out of the dry side.
Once installed, they are fully automatic, requiring minimal intervention, ensuring reliable performance with simple, straightforward operation.
Learn more about the Stepoc flood wall pictured above by visiting our case study below.
Flood walls are often constructed by asking a local builder to install a wall, with little consideration given to design, drainage, durability or structural capacity. A wall built in this way may look substantial, but can perform poorly when it is actually needed.
Common failure points include:
Flood walls fail in several ways. They can crack, slide, rotate or suffer bearing capacity failure. Importantly, failure does not only occur as water levels rise. Walls can also fail as water levels fall.
We have seen walls collapse under flood conditions due to inadequate design and construction, resulting in sudden and catastrophic failure. A flood wall that is not properly engineered can create a false sense of security and increase risk to property and occupants.
To ensure your flood wall is structurally suitable and to avoid failure, you can read further in our blog: How Do Flood Walls Fail and How Can You Prevent It?
We pride ourselves on our ability to provide customers with the best products for their requirements. If you need further advice, please feel free to call us on 0115 9870358.