A foundation, trench or basement is filling with water and work has stopped. This guide is for the site engineer or contractor who must choose between a submersible and a surface-mounted centrifugal dewatering pump, and set it up so it keeps running.
What does a dewatering pump have to do on your excavation?
It has to move water out faster than it comes in, for as long as the work lasts. Before you call a supplier, note how far the lowest point is below the discharge point, how fast the water rises, what it carries and what power is on site.
Submersible or centrifugal pump: which one?
The rule: use a submersible when the pit is deep, access is awkward or the pump will run unattended. Use a surface-mounted pump when the water is within a few metres of the edge, you can reach it easily and you want to refuel and service it on site.
Most submersibles are centrifugal too, with the motor sealed under water. Here "centrifugal" means the surface-mounted type.
A submersible pump sits in the water and pushes it out. It is usually electric, so it needs a supply such as a generator. A surface-mounted centrifugal pump draws water up through a suction hose and must be primed first. Suction relies on atmospheric pressure, so the practical lift is only a few metres.
| Submersible | Surface-mounted centrifugal | |
|---|---|---|
| Priming | Not needed | Needed; self-priming types re-prime after the first fill |
| Suction lift limit | None; limited by delivery head instead | A few metres in practice |
| Good for | Deep, narrow pits and sumps | Shallow digs where you can reach the pump to prime, refuel and service it |
| Main risk | Cable damage, blocked intake | Losing prime, running dry |
How do you size the dewatering pump: flow, total head and solids?
- Flow. How much water must move per hour, plus the water already standing in the pit.
- Total head. Total head starts with the vertical distance from the water level in the sump up to where the hose discharges. Add the friction loss in the hose and fittings, and any pressure needed at the outlet. Longer, narrower hose means more friction.
- Solids. What the water carries: sand, silt, mud, stones or debris.
To find the flow, use the case that matches your pit:
- Pit still filling: mark the water level, time the rise over 10 to 15 minutes and scale up to an hour.
- Pump already running: if the pit level is safe, stop the pump briefly and time the rise.
- Pit full or you cannot stop pumping: you cannot measure the inflow. Give the supplier the pit size, say the inflow is unknown, and ask them to size for emptying it plus a margin.
Volume gained = pit length × width × rise. For example, a pit 10 m × 5 m whose water rises 0.1 m in an hour gains about 5 m³ an hour. The pump must move clearly more than that at your total head, with a margin for rain.
The supplier can show flow and head together on the pump's curve: more head means less flow. For gritty water ask for a solids-rated (trash or semi-trash) pump and the largest solid it passes.
Watch out. Do not size by the horsepower or kW label alone. Ask for the flow at your total head.
For example, water 3 m below the edge with a hose climbing 2 m to a drain 80 m away is not 3 m of head. It is 5 m of lift plus the friction of 80 m of hose, often the larger part.
Electric or engine-driven: how will it be powered?
Engine-driven pumps carry their own power and suit sites without a supply, but decide who refuels them and when, because a pump that stops overnight lets the pit refill. Electric motors draw a surge at start-up, so size the generator for it: see how to size a generator. The site setup checklist covers temporary power for the rest of the site.
How do you set up the sump, intake and discharge hose?
Make the lowest point a sump, a deeper hole the pump draws from.
- Dig the sump before the pit floods if you can, at the lowest corner, away from people and machines.
- If the pit is already flooded, dig the sump from the edge with an excavator (what size excavator you need). A sump in loose sand or soft ground needs its sides supported, so ask the engineer.
- Sit the intake off the muddy bottom, for example on a block, so it does not draw in silt.
- On a surface pump, use a suction hose that is reinforced so it cannot collapse, fit a strainer, and keep the hose short and straight. It should rise steadily to the pump, with no high points where air collects.
- Route the discharge hose so it does not kink or flow back into the excavation, and fix its end so it cannot whip.
- Never lift or drag a submersible by its cable. Lower and recover it from the edge on a rope or chain fixed to its lifting handle. Nobody goes down into a flooded pit to place or recover a pump.
- Start the pump and check that water is moving before you leave.
Agree the discharge point with the site owner and the relevant authority before pumping starts. Requirements differ by project, site owner and municipality, so confirm before mobilising. Do not assume a nearby drain or road is acceptable.
How do you keep the pump running and the excavation safe?
Running dry damages the seal of most pumps. Ask the supplier whether your model tolerates it, and whether it suits salty or brackish water near the coast.
- Pump loses prime. Usually an air leak in the suction hose or fittings, so check every joint. A blocked strainer or too much lift can also cause it.
- Pump runs dry. Lower the intake deeper into the sump, fit a float switch (it stops the pump at a low water level) or check more often.
- Water runs back into the pit when the pump stops. Ask the supplier for a non-return valve on the delivery side.
- Flow drops, or the pump clogs repeatedly. The intake may be blocked, the hose kinked, or the pump worn by grit. Clean, straighten, lift the intake off the bottom and ask the supplier to check.
Motor trips or overheats. Check in this order:
- Intake and impeller: blocked or jammed.
- Hose: kinked, or a closed delivery, which makes the pump work against itself.
- Water level: the pump is running dry.
- Supply: a long, thin cable or an undersized generator gives low voltage.
- Three-phase pump: ask the electrician to check the rotation, because wrong rotation gives poor flow and a hot motor.
Watch out. Water can weaken excavation walls. A competent person decides whether it is safe before anyone goes in.
Have a qualified electrician set up the supply, with earth-leakage protection and cables protected from traffic. Isolate the power before anyone handles the pump.
Watch out. Engine exhaust can kill. Never run an engine-driven pump inside a trench, basement or other enclosed space, and keep fuel and refuelling away from the pit edge.
When is a simple pump not enough?
Call a dewatering specialist when the level does not fall although the pump matches the inflow you measured, or the inflow keeps growing. Call one too if discharged water seeps back into the pit, the walls are collapsing, neighbouring structures could be affected, or the water table must be lowered over a wide area. These cases may need well points or deep wells, not one pump in a sump. For sewage or sludge, a vacuum truck may suit better.
How to do this on Maedah
Post a request once. Choose a water pump, and add the generator in the same project request if the pump is electric. Add your city, the site and the dates. Say what the pump must do: the flow, the lift, the hose length, the solids, and whether someone will attend it.
Suppliers who offer that category and serve your city send sealed quotes. Compare what is included (hose, strainer, fuel, delivery) and agree payment directly with the supplier.
