Dewatering a Flooded IDP Site: A Pump-Selection and Deployment Playbook

Size the pump to the water, not the catalogue. To clear standing water from a flooded displacement site you need a self-priming trash pump matched to three numbers, total flow, total head and the largest solid it must pass, and then sited, primed and fuelled to run for days. A 3-inch trash pump such as the Multiquip QP3TH moves on the order of 1,500 litres a minute and passes solids around 37 mm; a 4-inch unit such as the QP4TH steps up to roughly 2,100 litres a minute and 51 mm solids. This playbook sets out how to choose between them and how to deploy.

Why does standing water on an IDP site have to be pumped at all?

Because the floodwater is both the disease vector and the thing cutting off the response, and gravity drainage rarely exists on a site that was never engineered to be a settlement. Flooding in South Sudan is not an edge case. In 2021, floods affected around 1.2 million people, displaced more than 300,000, and caused US$671 million in damages, and the country ranks seventh of 165 countries for population exposed to river floods every year. This is structural, not seasonal bad luck.

The scale has held year after year. By the end of 2021, flooding affected more than 835,000 people across 33 of 78 counties, with Jonglei, Unity and Upper Nile worst hit. By October 2022, floods had reached over 900,000 people across nine states. As of late November 2024, around 1.4 million people remained flood-affected across 44 counties and Abyei, with more than 379,000 flood-displaced. When water sits in a camp it contaminates the shallow water table, floats latrine contents into living space, and strands the very supply convoys that are meant to bring relief. Pumping it out is the precondition for everything else.

Dewatering is also what protects the water supply you are trying to keep safe. A camp is planned around a survival minimum of 15 litres per person per day for drinking, cooking and hygiene under the Sphere standard, and floodwater that mixes with that supply turns a WASH programme into a cholera risk overnight. The pump is upstream of the chlorine.

What three numbers actually decide which pump you need?

Flow, head and solids. Get these three right and the rest is logistics; get any one wrong and the pump either stalls, cavitates or never empties the site.

• Flow is the volume you must move per minute, set by the area of standing water, the depth, and how fast you need it gone. A larger pond, or a deadline driven by an incoming convoy, pushes you up from a 3-inch to a 4-inch pump.

• Head is the total vertical and friction resistance the pump works against: the lift from the water surface up to the pump (suction lift), the rise from the pump to the discharge point, and the friction loss along the hose run. A long hose to a drainage channel outside the camp perimeter adds head even on flat ground, and head is what robs you of flow. A pump rated at its maximum litres per minute delivers that only near zero head.

• Solids is the largest particle the pump must pass without clogging. Floodwater off a camp carries mud, twigs, rags, plastic and organic debris, which is exactly why a clean-water pump is the wrong tool here.

Work the three together. Estimate the volume to move, measure or pace the hose run to your discharge point, and assume the debris load is worse than it looks. Then read those numbers against a real datasheet rather than a price tag. We size each job against the published pump curve and the actual site, which is the work behind the Multiquip and Aussie Pumps equipment page.

Trash pump or submersible: which type for a flooded site?

For surface floodwater full of debris, a self-priming trash pump is the default; reserve submersibles for boreholes and deep sumps. The three types do different jobs and they are not interchangeable.

• Trash pump (self-priming, engine-driven, on the bank). This is the dewatering workhorse. It sits above the water, draws through a suction hose, and its open impeller and large clearances let it pass solids that would jam a standard pump. A 3-inch unit like the QP3TH passes around 37 mm solids at roughly 1,500 L/min; a 4-inch unit like the QP4TH passes around 51 mm at roughly 2,100 L/min. Because the engine stays dry on the bank, you can refuel and service it without wading, and you can move it from pond to pond in minutes.

• Submersible pump. Sits in the water and pushes rather than lifts, so it is excellent for a borehole or a deep, relatively clean sump, but it is the wrong instinct for sheet floodwater across a site: it needs a depth to sit in, it is harder to retrieve and service, and most units are built for cleaner water than a flooded camp produces.

• High-pressure / firefighting pump. Built for head, not volume. The twin-impeller Aussie "Mr T" petrol fire pump reaches 95 m of head but only 480 L/min; the Aussie Fire Chief delivers up to 450 L/min at heads to 75 m. That is the profile for pushing water a long way uphill or for fire response, not for shifting a large volume off flat ground. Use it when head dominates, never as a general dewatering pump.

The view is simple: a flooded IDP site is a trash-pump problem. Reach for the 3-inch first and step up to the 4-inch when the volume or the deadline demands it.

What size trash pump dewaters a typical flooded site?

Start at the 3-inch, step up to the 4-inch when flow or the debris load grows. A 3-inch unit such as the QP3TH gives on the order of 1,500 L/min and passes around 37 mm solids; a 4-inch unit such as the QP4TH steps up to roughly 2,100 L/min and around 51 mm solids, at a similar maximum head. Self-priming petrol pumps in this class draw from a suction lift of about 7.6 m, in line with the Australian Pump Industries spec sheet. Choose by answering four questions on site.

1. How much water, and by when? A larger volume or a hard deadline pushes you to the 4-inch, or to running two 3-inch pumps in parallel so a single failure does not stop the site draining.

2. How dirty is it? Heavy debris, larger chunks of organic matter or building rubble argue for the 4-inch and its 51 mm clearance, which clogs less often and costs you fewer strip-downs.

3. How far and how high to the discharge? A long hose run to a drainage line or a channel beyond the perimeter adds head and cuts delivered flow, so size up to keep real throughput where you need it.

4. How many days will it run? A multi-day pumpout favours the engine you can fuel and service easily and the spares you can get locally, which is a logistics question as much as a hydraulic one.

When the only available water for the camp is the flood itself, the same selection logic carries to supply: a camp planned on 20 litres per person per day under UNHCR planning figures needs pump capacity matched to its population, and Sphere caps a single tap at 250 people and a hand pump at 500. Size the dewatering pump for the flood and keep the supply pump sized for the standard; they are two jobs, not one.

How do I site and prime the pump so it actually keeps running?

Site it low, close and stable, prime it before you start, and protect the suction. A self-priming trash pump still needs its casing filled with water before the first start, and most clogs and dry-runs come from siting and suction errors, not from the pump.

• Keep the suction lift short. Self-priming petrol pumps in this class draw from about 7.6 m, in line with the Australian Pump Industries spec sheet, but the closer the pump sits to the water surface, the faster it primes and the more flow it delivers. Set it as low and as near the water as the bank safely allows.

• Prime the casing. Fill the pump casing with water through the priming port before starting. A "self-priming" pump primes the suction hose itself, but it will not pull a dry start from an empty casing.

• Screen the suction. Fit and keep the suction strainer in place, and lift the foot of the hose clear of the mud so it draws water, not silt. Most trash-pump stoppages are a blocked or buried suction, not a failed pump.

• Stabilise and seal. Set the pump level on firm ground so it does not creep or vibrate loose, keep all suction-side connections airtight because an air leak kills the prime, and run the discharge hose to a point that drains away from the site, never back toward it.

• Commission against the standard. Confirm the pump is actually delivering, that the discharge is going where it should, and that the operator can restart it after a stop. On a borehole-supply pump the same discipline means a yield test before you commission, the difference between a pump on paper and a pump that holds the Sphere queuing target of under 30 minutes.

Fuel, uptime and the part nobody plans for

Plan fuel and spares before the flood, because in a landlocked response the pump is only as available as its diesel and its impeller. An engine-driven trash pump on the bank is easy to refuel without wading, which is half the reason it beats a submersible on a flooded site, but the fuel still has to be there. South Sudan's 2026 fuel crisis is the live warning: fuel prices have roughly doubled since March 2026, raising transport costs and restricting access. A dewatering plan that assumes cheap, available diesel is a plan that stops when the convoy does.

That fuel exposure is also why solarising the supply side is now mainstream wherever a pump runs continuously rather than in flood bursts. By the end of 2023, UNHCR had solarised 295 boreholes, half of all boreholes in its operations, saving more than 32,000 tonnes of CO2 a year, and the World Bank finds solar pumping cost-competitive with diesel in all size ranges, cutting life-cycle cost by around 36 per cent. A diesel borehole pump typically burns over US$5,000 a year in fuel, roughly 40 per cent of its initial system cost, every year. For emergency flood dewatering the engine pump still wins on portability and burst capacity; for the borehole that supplies the same camp month after month, the fuel maths increasingly points the other way.

Either way, uptime is a spares question. Keep a service kit with the pump: spare seals, an impeller, the suction strainer, and the fuel and oil to run it. Buying ex-works from a distant port means the pump and its spares arrive late, separately, or not at all, which is why we hold inventory in-country and install, commission and support what we sell.

Who deploys the pump, and why does in-country matter?

Whoever installs it has to be in the country when the water rises, not at the end of a ten-week shipping lead time. A trash pump bought ex-works from an overseas catalogue still has to clear customs, survive the road, and reach a Jonglei or Upper Nile site before the floodwater spreads, and it arrives as a crate, not as a working, commissioned pump with an operator who can restart it. The flood does not wait for freight.

This is the case for buying from an in-country distributor that installs and commissions on site. As an authorised East and Central Africa distributor for Multiquip and Aussie Pumps, with pre-positioned inventory and 35 years of in-country supply, we size the pump to the site, install and commission it, yield-test borehole supply, and hold the spares that keep it running through a multi-week pumpout. That is the difference between owning a pump and dewatering a site, and it is the work behind our South Sudan country page and our UN agency and NGO sector page.

Frequently asked questions

What size trash pump do I need to dewater a flooded IDP site?

Start with a 3-inch self-priming trash pump and step up to a 4-inch for higher volume or heavier debris. A 3-inch unit such as the Multiquip QP3TH moves on the order of 1,500 L/min and passes around 37 mm solids; a 4-inch QP4TH steps up to roughly 2,100 L/min and around 51 mm solids. Match the choice to the volume, the debris load, the hose run and how many days it must run.

What is the difference between a trash pump, a submersible pump and a firefighting pump?

A trash pump self-primes on the bank and passes solids, which makes it the tool for debris-laden surface floodwater. A submersible sits in the water and suits boreholes or deep sumps. A firefighting pump is built for head, not volume; the Aussie "Mr T" reaches 95 m of head but only 480 L/min, so it is for pushing water far uphill or for fire response, not general dewatering.

Why does my trash pump keep losing prime or clogging?

Almost always a suction-side fault, not the pump. Keep the suction lift well within the roughly 7.6 m self-priming limit for petrol pumps in this class, fill the casing before starting, fit the strainer, lift the hose foot clear of the mud, and seal every suction connection because a single air leak kills the prime.

How much water do I need to supply a camp once the site is drained?

Plan on 20 litres per person per day under UNHCR figures and at least the Sphere survival minimum of 15 litres per person per day, with no more than 250 people per tap. Dewatering and supply are two pump jobs; size each to its own standard.

Should I run a diesel or a solar pump on a flooded site?

For emergency flood dewatering, run the portable engine-driven trash pump; it gives burst capacity and refuels on the bank. For the borehole that supplies the camp long-term, solar is increasingly the better buy: the World Bank finds it cost-competitive with diesel in all size ranges and around 36 per cent cheaper over its life, and a diesel pump burns over US$5,000 a year in fuel, a real exposure during the 2026 fuel crisis.

Talk to a supplier who installs and commissions in-country

Specialized Logistics Solutions is in Juba, not at a distant port, and we are an authorised distributor for Multiquip and Aussie Pumps, for Aquatabs (Medentech/Kersia), P&G Purifier of Water and Oxfam tanks (Butyl Products), registered on the UN Global Marketplace as vendor 380716. We hold pump inventory in-country and install, commission, yield-test and support what we sell, the part that decides whether a crate becomes a dewatered site. Tell us the standing water, the discharge point and your timeline, and request a quotation.