Temporary Dams and Cofferdams

Cofferdams are temporary watertight enclosures used to exclude water from an area for construction work.
They are classified as temporary works and must be designed, checked, and approved before construction.
Common types include sheet pile cofferdams, sandbag dams, earth bunds, and inflatable water-filled barriers.
Cofferdam failure can cause sudden flooding of the work area, trapping and drowning workers inside.
Dewatering pumps must be adequately sized with standby capacity in case the primary pump fails.
Water pressure on cofferdam walls increases with depth — deeper cofferdams need stronger designs.
The temporary works coordinator must manage the design, construction, and eventual removal of the cofferdam.
Seepage monitoring, pump capacity tracking, and structural inspections must continue throughout use.
Environmental permits are typically required for works that divert or impound watercourses.
An emergency evacuation plan for cofferdam flooding must be briefed to all workers before entry.

Drowning risk	Cofferdam failure causes rapid flooding of the enclosed work area — workers can be trapped and drowned within minutes.
Structural failure	Water pressure can overwhelm cofferdam walls if the design is inadequate or seepage undermines the structure.
Legal requirement	CDM 2015 and BS 5975 require cofferdams to be designed, managed, and inspected as formal temporary works.

Do

Don't

Ensure the cofferdam is designed and approved through the temporary works procedure
Install dewatering pumps with standby backup capacity before entering the enclosure
Monitor seepage, water levels, and pump performance continuously during occupation
Brief all workers on the emergency evacuation plan for cofferdam flooding
Inspect the cofferdam structure at the start of each shift and after heavy rain
Obtain environmental permits for any works impounding or diverting watercourses
Maintain clear escape routes from inside the cofferdam at all times
Record all inspections and monitoring data in the temporary works register
Ensure the TWC manages the full lifecycle including design, use, and removal
Position rescue equipment including lifebuoys and throw lines at the cofferdam entry

DON'T enter a cofferdam until pumps are running and water levels are controlled
DON'T ignore increasing seepage rates — report them to the TWC immediately
DON'T occupy a cofferdam during extreme rainfall or flood warning conditions
DON'T remove structural bracing or waling frames inside the cofferdam
DON'T allow pumps to run without standby capacity available on site
DON'T block escape routes with plant, materials, or construction debris
DON'T continue working if the cofferdam shows signs of movement or distress
DON'T construct or modify the cofferdam without the approved temporary works design
DON'T work inside the cofferdam without personal flotation devices accessible
DON'T dismantle the cofferdam without a specific removal method statement

See also: Temporary Works Awareness | Working Near Open Water

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