Cofferdam Dewatering — A Contractor's Guide

Cofferdam Dewatering — A Contractor's Guide

Cofferdam construction is one of the most technically demanding dewatering applications in civil construction. Whether you're building a bridge pier, installing underwater infrastructure, or repairing a dam, cofferdams create a temporary dry work environment in otherwise wet conditions. Managing water inside and around a cofferdam is critical to the success of the entire operation.

Here's what contractors need to know about cofferdam dewatering.

What is a Cofferdam?

A cofferdam is a temporary watertight enclosure built in a body of water or wet ground to allow construction or repair work to be performed in a dry environment. Once the cofferdam is constructed the enclosed water is pumped out creating a dry work area inside.

Cofferdams are used for:

• Bridge pier and abutment construction
• Dam repair and rehabilitation
• Underwater pipeline and utility crossings
• Port and marine infrastructure construction
• River and waterway structure installation
• Culvert installation and replacement

Types of Cofferdams

Sheet pile cofferdams — interlocking steel sheet piles driven into the riverbed or lakebed to form a watertight wall around the work area. The most common type for construction dewatering applications. Can be single wall for shallow water or double wall cellular for deeper water and higher pressures.

Earthen cofferdams — compacted earth or sandbag barriers used in shallow water applications. Simple and inexpensive but limited to low water depth and velocity situations.

Inflatable cofferdams — flexible inflatable dams that can be deployed quickly in shallow water. Used for temporary stream diversions and shallow water work.

Rock filled cofferdams — rock or gravel fill contained by geotextile fabric. Used in stream diversion applications where water can flow through the cofferdam while it's being constructed.

The Cofferdam Dewatering Process

Dewatering a cofferdam follows a specific sequence:

Step 1 — Construct the cofferdam Install sheet piles, earthen berms, or other cofferdam type around the work area. The cofferdam must be structurally sound and as watertight as possible before dewatering begins.

Step 2 — Initial dewatering Begin pumping the enclosed water volume out of the cofferdam. This initial dewatering removes the standing water inside the enclosure. High volume pumps are required to lower the water level quickly before significant inflow occurs.

Step 3 — Maintain dewatering against inflow Once the standing water is removed continuous pumping maintains a dry work environment against ongoing inflow. Inflow sources include:

• Leakage through sheet pile interlocks
• Seepage through the bottom of the cofferdam
• Surface water runoff into the enclosure

Step 4 — Manage throughout construction Dewatering continues throughout the construction or repair operation. Water inflow rates change as conditions change — tidal fluctuations, river stage changes, and seasonal groundwater variations all affect inflow.

Step 5 — Controlled flooding for removal When construction is complete the cofferdam is flooded in a controlled manner and the cofferdam structure is removed.

Sizing Dewatering Equipment for Cofferdams

Cofferdam dewatering equipment sizing requires careful analysis of inflow sources:

Sheet pile leakage — even properly installed sheet pile cofferdams leak at the interlocks. Leakage rates depend on sheet pile condition, interlock quality, water pressure differential, and soil conditions. An experienced dewatering engineer can estimate leakage rates for design purposes.

Bottom seepage — water seeps upward through the bottom of the cofferdam from the surrounding water body. The rate depends on the permeability of the bottom sediments and the head differential.

Tidal and river stage variation — in tidal areas and rivers the water level outside the cofferdam changes continuously. Dewatering equipment must handle peak inflow conditions at maximum external water level.

Safety factor — always design for significantly more pump capacity than your calculated inflow. A cofferdam that floods during construction is a serious safety and schedule event. Most cofferdam dewatering designs include 100% redundant pump capacity.

Equipment for Cofferdam Dewatering

High volume submersible pumps — the primary dewatering tool for most cofferdams. Lowered to the bottom of the cofferdam to remove water as it accumulates. Multiple pumps are typically installed for redundancy.

Diesel surface pumps — used for high volume initial dewatering and as backup capacity. Diesel independence is valuable in marine and remote locations where power supply may be unreliable.

Wellpoint systems — for cofferdams in permeable soils wellpoints installed around or within the cofferdam perimeter can lower the groundwater table and reduce upward seepage through the bottom.

Automatic controls — float switches and level controls automate pump operation to maintain water level within acceptable limits without continuous manual monitoring.

Monitoring equipment — water level monitors inside and outside the cofferdam allow continuous tracking of inflow rates and pump performance.

Critical Safety Considerations

Cofferdam dewatering failures are serious safety events. Water inflow that exceeds pumping capacity can flood a cofferdam rapidly trapping workers inside.

Never reduce pump capacity during construction — maintain full dewatering capacity throughout the work. The temptation to reduce operating costs by running fewer pumps during low inflow periods creates unacceptable risk.

Maintain backup pumps — always have backup pumping capacity staged and ready for immediate deployment. For critical cofferdams backup pumps should be pre-connected and able to start within minutes.

Monitor continuously — water level inside the cofferdam should be monitored continuously. Rising water level is an early warning of pump failure or increased inflow.

Establish emergency procedures — all workers inside a cofferdam must know the emergency exit procedures and evacuation routes in case of rapid flooding.

Environmental Considerations

Cofferdam dewatering in waterways has environmental implications:

Turbid discharge — pumped water from a cofferdam is often turbid with disturbed sediment. Discharge permits typically require turbidity controls before discharge to the waterway.

Fish and wildlife — construction in waterways may require seasonal restrictions to protect fish spawning or other sensitive periods.

Permits — cofferdam construction and dewatering in navigable waters requires permits from the Army Corps of Engineers and state environmental agencies.

How Flowcor Equipment Supports Cofferdam Projects

Flowcor Equipment sources high volume submersible pumps, diesel dewatering equipment, and complete dewatering solutions for cofferdam and marine construction projects across the U.S.

Tell us your cofferdam dimensions, estimated inflow, and project timeline and we'll source the right equipment and get you a quote within 1 business hour.

Submit a quote request at flowcorequipment.com or call us at 610-241-6770.