Quick answer: Add up the water that will collect on the dry side, which is rain falling inside the defended area plus seepage through and under the defence, and convert it to litres per minute. Choose a pump that delivers that flow at your real lift and hose length, not its headline maximum, then add spare capacity and a second pump in case one fails. For rain, the sum is simple: area (m²) × rainfall (mm per hour) ÷ 60 = litres per minute. Seepage is harder to predict, so allow generously.
We’ve explained elsewhere why flood barriers still need water pumps. This post deals with the next question: how big?
Step 1: work out the rain falling inside the defence
Once a barrier or wall is up, the area inside it becomes a bowl. Rain that lands there, and often rain from your roof, has nowhere to go if the drains are closed off by non-return valves or the ground is already saturated.
One millimetre of rain on one square metre is one litre of water. So:
Litres per minute = area (m²) × rainfall intensity (mm per hour) ÷ 60
Measure every surface that drains into the defended area:
- the garden, yard, patio or drive inside the defence; and
- any roofs whose downpipes discharge inside it, or into drains that will back up or be shut during a flood.
Example: a rear garden and patio of 150m², plus 80m² of roof draining into it, in a heavy downpour of 50mm an hour:
- 150 × 50 ÷ 60 = 125 L/min
- 80 × 50 ÷ 60 = 67 L/min
- Rain inflow ≈ 190 L/min
Very heavy rain doesn’t usually last long at its peak, and some will soak in, but for sizing a pump, it’s sensible to plan for a short, intense burst.
Step 2: allow for seepage
No wall or barrier is perfectly watertight. When floodwater sits against one side, pressure pushes water through the masonry, under the foundations and up through the ground on the dry side. How much depends on the flood depth, how long it lasts, the soil (sand lets water through far faster than clay), and how the defence was built.
There’s no simple formula a homeowner can use, but there are useful reference points.
Through building walls. Safeguard test data shows that at a flood depth of 0.6m, a single-skin Fletton brick wall can let in around 12.5 litres per square metre per minute. Repointing and treatments can reduce this a lot, but it shows how quickly water can add up behind a barrier fitted to a building.
Under a garden or boundary wall: a real example. Nick Lupton built a flood wall around his home, Pixham House, beside the River Severn, and documented the whole project in his book The Great Wall of Pixham, published on Flood Mary’s website. His pump sizing is one of the few real, published examples we know of:
- He fitted three sumps, at the lowest point inside the wall and at the front and back of the house, connected to perforated land drains laid along the foundations.
- Each sump has one pump of around 5 litres per second (300 L/min), so roughly 15 l/s installed in total.
- During one of the highest floods he’d experienced, he estimated he needed to remove about 10 litres per second (600 L/min) at the peak. That is Nick’s own calculation, and it left him with around 50% spare capacity.
- He chose large 800 litre sumps to reduce how often the pumps start and stop, extending their life. Larger chambers like these are available from us as a bespoke request: see our below-ground pump stations and tanks.
- Each sump pump is on its own electrical circuit, so one fault can’t stop all three, and he planned to keep a spare pump ready.
- He noted the wall seeped more than he expected, which the sumps dealt with easily.
His soil was a mix of sand and clay. On a different site the figure could be far lower or higher, but it shows that seepage behind a full perimeter wall can far outweigh rainfall. Nick’s book is well worth reading: The Great Wall of Pixham on floodmary.com.
Rule of thumb: a doorway barrier on a building sees modest seepage and is usually handled by a low-level puddle pump. A wall or barrier around a garden, yard or whole property, especially on permeable ground near a river, can see seepage in the hundreds of litres per minute, and needs a designed sump and pump arrangement.
Step 3: add it up, then add a margin
Design inflow = rain + seepage. Then add spare capacity, because your estimate will never be exact. Nick’s installation had around 50% more capacity than his peak estimate, and that’s a sensible place to start.
Step 4: check the flow at your real lift
A pump’s maximum flow is measured at zero lift, through no hose. Behind a wall, the pump has to lift water out of a sump and over the defence, through a length of hose. That’s the duty point you need to check.
Total head = height from the water to the top of the wall or discharge point + friction in the hose and fittings.
Then read the pump curve at that head. If the curve gives less than your design inflow, you need a bigger pump, more pumps, or a larger hose. Our guides to why max flow is not your real pump flow, how to read a pump curve and how hose length and diameter change performance explain it, and the Water Pump Performance Calculator does the sums.
Step 5: build in redundancy
A pump behind a barrier is protecting a property. If it fails, the bowl fills.
- Never rely on a single pump for a perimeter defence. Use two or more, or a twin pump station.
- Separate circuits, as Nick did, so one fault doesn’t stop everything.
- Back-up power. Floods and power cuts often go together. A petrol pump such as the Honda WB20 (up to 620 L/min) makes a good standby. See pumps you can use without mains electricity.
- Sump size matters. A bigger sump means fewer starts per hour and a longer pump life.
- Non-return valves on every discharge, so water doesn’t flow back over the wall.
Matching the pump to the defence
| Defence | Typical inflow | Pumps to consider |
|---|---|---|
| Doorway or threshold barrier | Low: seepage and a little rain | Stream SPP2-4A, APP RS32EA, EVAK Residox 400 (205 L/min, to about 1mm) |
| Garden, yard or drive behind a wall or temporary barrier | Moderate: rain on the defended area plus seepage | EGO 500 GI (around 170 L/min, 9mm solids), EVAK Residox 750 (330 L/min, 16m head) |
| Permanent perimeter wall | Moderate to high: hundreds of L/min possible | FPS Iguazu 635 or Iguazu Twin stations, several sumps, larger site pumps such as the APP HD-15 (830 L/min) for high inflows |
We’ve seen these in action in our case studies: a Residox puddle pump behind a flood barrier, the EGO 500 GI behind a garden flood wall, and an RS32EA protecting a doorway.
When sizing becomes design
For a doorway barrier, the steps above are usually enough. For a new flood wall or a whole-property perimeter, pump sizing should be part of the design, alongside the wall’s foundations, drainage and discharge route. Our Engineering Review service can check a proposed pump arrangement, and for a full site assessment and drainage design, FPS Environmental, the consultancy in our group, offers a drainage review.
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