The quick answer
Specifying a temporary cofferdam comes down to five numbers and one judgement call: the water depth you must hold back, the freeboard margin above it, the flow velocity of the watercourse, the length and shape of the isolation you need, and the bed conditions the barrier will sit on. Get those right and you can isolate a culvert, bridge pier, pipeline crossing or foreshore work area, pump it dry, and work in the dry safely.
Apex Envirocare is the exclusive Australian representative of the Aqua-Barrier™ water-inflated cofferdam — a reusable barrier that sits standalone on the bed with no piling, no bed penetration and no anchorage, and fills with the site's own water (fresh or salt). It's held in stock in 1.8 m and 2.4 m barrier heights, and Apex Envirocare self-performs the installation with its own crews and plant, mobilising nationally including to remote and mine sites.
Step 1 — Size the barrier to the water depth (the 25% freeboard rule)
The single most important specification is freeboard — the height of barrier standing above the water line. Aqua-Barrier™ requires a minimum 25% freeboard, meaning the water you hold back should be no more than about 75% of the barrier height.
In practice:
- A 2.4 m barrier holds back up to ~1.8 m of water (and sediment) combined.
- A 1.8 m barrier holds back proportionally less — roughly ~1.35 m.
When you write the spec, state the design water level you're isolating against, not just the average depth. Watercourses rise. If a rain event or tidal push could lift the level, size up so the freeboard margin holds at the peak you're willing to design for. If your controlling water depth exceeds ~1.8 m, you're beyond the stock 2.4 m unit and into a taller/custom barrier or an engineered solution — flag that early rather than under-specifying.
Why water-inflated, not sheet pile or earth bund
A driven sheet-pile cofferdam or earth bund penetrates and disturbs the bed, needs plant to install and remove, and leaves a footprint. A water-inflated cofferdam sits on the bed and fills from the watercourse itself. That makes it faster to deploy, reusable many times, and far gentler on sensitive beds — which matters when your isolation area is exactly the area you're trying to protect.
Step 2 — Check the flow velocity
A cofferdam holds still water well; moving water changes the engineering. Specify the current velocity at the work location.
- Below ~0.9 m/s (3 ft/s), the standard freeboard rules apply.
- At or above ~0.9 m/s, the barrier needs extra freeboard and engineering sign-off — faster flow loads the barrier and can overtop or shift it if under-specified.
Don't guess flow from the annual average. Specify the velocity for the conditions you'll actually work in, and consider whether upstream diversion (piping flow past the work zone) reduces velocity across the barrier. On flowing creeks, rivers and drains, the usual method is to isolate the work area and route the live flow around it — Aqua-Barrier™ units overlap at any angle, so you can build a U or L shape that both isolates the site and channels the bypass.
Step 3 — Assess the bed and the loads
Aqua-Barrier™ relies on its own weight and footprint for stability, so the ground it sits on matters. Specify the bed material and geometry, because these conditions need a site assessment and possibly external support:
- Slick, weak or unstable soils — soft silts or slurries that won't provide friction.
- Steep or excessive slope, or very uneven ground — the barrier needs a reasonably level, continuous contact line.
- Wave or dynamic loads — open-water fetch, boat wash or tidal action.
Where any of these apply, don't just pick a height off the chart — get the layout assessed. Apex Envirocare's crews install and supervise Aqua-Barrier™ deployments and can advise on where a site's conditions push it toward extra measures.
Step 4 — Specify length, shape and layout
Because the units overlap at any angle, you specify the cofferdam as a plan layout, not a single straight run. Map the perimeter you need to enclose — around a bridge pier, along both banks of a culvert, boxing in a pipeline trench, or fencing off a stretch of foreshore for sediment retention. Total the run length, note every change of direction, and allow overlap at each joint.
This flexibility is what makes water-inflated cofferdams suit awkward, real-world sites: irregular banks, structures in the water, and shapes a rigid system can't follow.
Step 5 — Confirm the fill source and the dewatering plan
Aqua-Barrier™ fills from any water source, including saltwater, so on most sites you inflate it from the watercourse you're damming — no trucked-in water. Once the barrier is up and the area is isolated, you pump the enclosed zone dry to work. Specify:
- the fill source and pump for inflation;
- the dewatering pump capacity to empty the isolated area and manage seepage;
- where the pumped water goes — and whether it needs sediment control or treatment before discharge.
What a temporary cofferdam is NOT for
Specify honestly. Aqua-Barrier™ is a temporary system and is the wrong tool when:
- the water is too deep — more than ~75% of barrier height, or beyond ~1.8 m for the tallest stock unit, without a taller/custom barrier or engineering;
- the flow is too fast — at/above ~0.9 m/s without added freeboard and sign-off;
- the bed or loads are marginal — slick/weak soils, steep or very uneven ground, or wave/dynamic loading, without assessment;
- you need permanent retention — it is not a substitute for an engineered permanent dam.
Writing these exclusions into the spec protects the project. Under-specifying a cofferdam is how work areas flood.
Where this gets used
Temporary cofferdams built this way suit bridge, dam and culvert repairs; canals, diversion pipes and intake/outfall structures; pipeline, utility and services crossings; concrete pours and structural repair in a watercourse; boat ramps, jetties and shoreline restoration; isolating an area for dredging or sediment control; pond and pool maintenance; and temporary flood protection to keep floodwater off a work area.
Because it sits standalone with no bed penetration and can fill with the site's own water, it's particularly suited to sensitive foreshores. Apex Envirocare used this approach for sediment retention on the Swan River foreshore for the Camfield Drive Extension with Laing O'Rourke at Burswood, building its methods to align with WA DWER requirements on a sensitive urban watercourse.
Why specify Apex Envirocare
Apex Envirocare doesn't just supply the barrier — it is the exclusive Australian representative of the Aqua-Barrier™ system and a self-performing contractor that runs its own crews and plant. That means you can spec the cofferdam, buy or lease the units with spare parts, get on-site installation supervision and technical support, or have Apex Envirocare deliver the whole water-diversion works. With 20+ years in business and clients including John Holland, Water Corporation, Mainroads WA and major miners, it works Australia-wide — from urban foreshores to remote and mine sites.
