A common misconception is that installing a flood barrier solves the problem. In reality, a barrier only reduces or delays water entering through a specific opening. It does not remove the water that still finds its way inside. In longer duration flooding, that residual water is often what causes the most damage.
A flood barrier is not a complete solution on its own. It is one component in a wider flood mitigation system.
In almost every project we work on, from a simple doorway barrier to full perimeter wall schemes, we explain the same principle:
Barriers reduce incoming water. Pumps remove the water that still arrives. You need both.
Even with a well-installed barrier, water can still appear on the "dry" side. That is not a failure. It is simply how water behaves under pressure.
Where does the residual water come from?
1. Rainfall inside the defended area. If you defend a driveway, garden, patio or yard, any rain falling inside the protected zone has nowhere to drain during a flood event. The defended area effectively becomes a bowl, and water accumulates surprisingly quickly.
2. Seepage and hydraulic pressure. When floodwater builds up outside a wall or barrier, the external water level rises, and so does the hydraulic pressure. Water naturally moves from high pressure to low pressure. It only needs tiny imperfections in soil, masonry or service penetrations to find a pathway. This creates seepage beneath or through structures and can raise groundwater levels inside the defended area. This occurs even with reinforced walls, tanking systems or deep foundations.
3. Backflow through drainage. During flooding, sewers and surface water systems can surcharge. Water can rise back through drains unless non-return valves (NRVs) are fitted. NRVs stop reverse flow, but they do not remove the water that still collects internally.
4. Ingress through building fabric. In property-level protection schemes, barriers may seal doors and openings, but water can still enter through walls, floors and service routes. Measures like repointing or tanking reduce this, but rarely eliminate it entirely.
Flood walls and perimeter schemes: why no wall is watertight
A flood wall may look impressive, but no wall is completely watertight. When floodwater builds up on one side, it creates pressure in the ground, known as a hydraulic gradient. That pressure pushes water underneath the wall, where it can rise on the dry side as groundwater.
If that groundwater is not managed, it will eventually equalise with the floodwater outside, meaning your protected area could still flood, filling up from below, or from rainwater that the surface water network cannot drain away while it is falling inside the defence.
The rate of seepage depends on soil type. Sandy soils allow water to pass through faster than clay, but both can allow significant water movement over time.
How pumps manage seepage behind a wall
To counter this, pumps are used to collect and remove the seepage water from the dry side of the wall, typically through sumps or collection chambers. These sumps capture groundwater, and the pumps transfer it safely back to the wet side, maintaining a lower water level behind the defences. For the detail on how those chambers are built, see our guide to what a sump pump is and how to install one.
Pump capacity is usually based on experience, previous site data, or basic hydraulic calculations. While soil seepage surveys can provide more precision, practical experience often guides the decision-making process.
The engineer's view: where the water goes and where the sump sits
Flood defence designers do not expect a wall to be watertight. Most walls and embankments sit on ground that is permeable to some degree, so when the river is up against one side, water moves under it and appears on the other, and the design guidance says plainly that the defence should cater for this. A cutoff under the wall lengthens the path the water has to take and slows it down, but even with a cutoff there is some seepage, and the normal answer to a modest flow is simply to pump it. Many urban defences leak through their foundations in every flood and cause no trouble beyond the need for temporary pumping while the water is up.
Two details decide whether that pumping is easy or a panic. The first is a collector drain along the dry side of the defence, running to sumps placed where a pump can be dropped in: where that is planned from the start, deploying pumps and managing the seepage is routine. The second is a small sump at every gate, stoplog or removable panel, because those openings never seal perfectly and the leakage needs somewhere to collect. Any defence also traps the rain that falls behind it. Flap valves through the wall are the usual outlet, but they can wedge open during a flood, so pumping the runoff is the safer option, and then the sizing of the pumps and their reliability become the critical questions. A second pump covers a failure; it does not cover a power cut, which is why a petrol or diesel pump from our engine-driven range belongs in the plan alongside the electric ones.
Two warnings go with this. If the water coming through is carrying sand or silt, that is not seepage but piping: the flow is removing the ground from under the defence, and it needs an engineer, not a bigger pump. And if a wall is overtopped, it will hold floodwater on the protected side after the river has dropped, so the pumps that handled seepage on the way up are the same pumps that empty the defended area afterwards.
Why pumps turn a defence into a system
A barrier holds water back. A pump actively removes water.
Without pumping provision, the protected area can simply fill from the inside.
Pumps are installed on the dry side of the barrier so that any water entering can be removed faster than it accumulates. A real-world example of this is in our case study on using an EVAK Residox puddle pump behind a flood barrier, and the same thinking underpins sump pumps and property flood resilience.
Why we always recommend pumps alongside barriers
Across hundreds of flood risk surveys and resilience schemes, the same pattern appears:
- Small volumes of water become major damage if left unmanaged
- Groundwater levels rise inside defended zones during prolonged flooding
- Rainfall inside protected areas accumulates faster than expected
- Some seepage through walls and ground is almost always present
This is why reports often state:
"Regardless of barrier type, pumping provision is recommended."
Because resilience is not about stopping every drop, it is about controlling what happens when water inevitably appears.
Safeguard test data illustrates this clearly. At a floodwater depth of 0.6 m, a single-skin Fletton brick wall can permit approximately 12.5 litres per square metre per minute of water ingress. While repointing and masonry treatments can significantly reduce this, it demonstrates why pumping provision remains essential even where walls appear solid.
Choosing the right water pump
Pump selection depends on the site and how flooding occurs.
- Puddle pumps are popular for temporary deployment. They require no sump and can be stored ready for use.
- Automatic sump pumps in packaged pump stations are better where flooding is frequent or occurs with little warning. For unattended spaces, see our guide to pumping water away automatically at floor level.
A key principle in all flood defence schemes is redundancy: installing more pumping capacity than the minimum required. This ensures protection if inflow is higher than expected or if a pump fails.
A real example of that redundancy is shown below.
Flooding like this typically occurs via groundwater flooding, or other ingress routes in river or surface water flooding. The photo shows a property with a small sump that houses an AMA Drainer 301 Automatic Submersible Water Pump 230v. This handles the day-to-day ingress of water that can be normal in a cellar, alongside a much more powerful APP HD-15 pump that activates should the cellar become inundated and additional capacity be required during a flood event.
Where barriers are installed on properties, some internal water ingress should always be assumed. Buildings should be designed to recover quickly (for example, tiled floors instead of carpets) and pumps used to keep water levels low and damage limited.
Barriers and pumps together equal resilience.
Looking for an automatic puddle pump?
Behind a barrier, an automatic puddle pump switches itself on when water appears and off again once it has cleared, so it keeps working when nobody is there. The Stream SPP2-4A is the compact choice, and the SPK530A is the heavy-duty version for larger volumes of water. See our guide to the two new automatic Stream puddle pumps for how they compare.
Working with temporary flood barriers
Water pumps can also be used in conjunction with temporary or roll-out flood barriers, where pumps offer an inexpensive and effective line of defence against floodwaters. Temporary barriers do not require prior installation like flood doors, can be deployed in a matter of minutes, and freestanding temporary barriers such as the Water-Gate will always allow a small amount of water to seep through underneath, as they do not have a formal foundation.
For further information on flood barriers please visit: Aluminium Flood Barrier – FPS Barrier®
Getting the right advice
While we cannot give site-specific recommendations over the phone, we can help you understand the different options available. Our team has extensive experience with flood defence systems, sump and submersible pumps, and can point you towards the most suitable setup for your needs. For a formal review, our Engineering Review Service is also available.
