TWK/General/TBT-TWK-015

Temporary Dewatering Systems

Temporary Works › General › Temporary Dewatering Systems

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Temporary dewatering removes groundwater from excavations to allow safe construction below the water table.
Common methods include wellpoints, submersible pumps, deep wells, and sump pumping arrangements.
Dewatering is a temporary works activity requiring design, approval, and monitoring by competent persons.
Uncontrolled dewatering can cause ground settlement, foundation damage, and subsidence to nearby structures.
Discharged water must be managed to prevent pollution — an environmental permit may be required.
Electrical safety is critical as pumps operate in wet conditions, often 24 hours a day.
Noise from continuous pump operation can impact neighbours and breach planning conditions.
Loss of dewatering (pump failure or power cut) can flood excavations rapidly, endangering workers.
The dewatering design must consider the effect on groundwater levels across the wider area.
Monitoring of water levels, flow rates, and settlement must be carried out throughout the dewatering period.

Prevent flooding	Pump failure or power loss causes rapid flooding of excavations, creating drowning and collapse risk.
Protect structures	Uncontrolled drawdown can settle foundations and damage buildings, services, and infrastructure nearby.
Environmental compliance	Discharging pumped groundwater without a permit or treatment is an offence under the Environmental Permitting Regulations.
Temporary works duty	Dewatering systems must be designed, installed, and monitored as formal temporary works under CDM 2015.

Don't

Ensure the dewatering system is designed and approved as a temporary works element
Monitor water levels, pump flow rates, and nearby settlement readings daily
Install backup pumps and a standby power supply to maintain dewatering continuously
Obtain an environmental permit before discharging pumped water to any watercourse or drain
Test discharge water quality to confirm it meets permit conditions before release
Use RCD-protected electrical supplies for all dewatering pumps operating in wet areas
Maintain clear access to pumps and wellpoints for inspection and emergency maintenance
Brief all workers on the emergency procedure if dewatering is lost during a shift
Inspect all hoses, pipework, and connections regularly for leaks and deterioration
Notify the temporary works coordinator before switching off any dewatering system

DON'T start dewatering without a designed and approved temporary works plan in place
DON'T discharge pumped groundwater to drains or watercourses without a valid permit
DON'T leave dewatering systems without backup pumps or a standby generator on site
DON'T enter a flooded excavation to inspect or repair dewatering equipment
DON'T ignore settlement monitoring alarms — they may indicate excessive drawdown
DON'T switch off pumps without consulting the temporary works coordinator first
DON'T allow pump discharge hoses to create trip hazards across access routes
DON'T use damaged or unprotected electrical cables to supply dewatering pumps
DON'T assume groundwater levels will stay stable — they change with rainfall and tides
DON'T dismiss noise complaints from neighbours — check pump noise against permitted levels

See also: Temporary Works Awareness | Dewatering (Wellpoints and Sumps)

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