Ask what separates two proposals for the same in-water repair and the honest answer is frequently not the armor. It is whether one of them has to build a dry hole first.
This is what that involves, so it can be priced as a project rather than carried as a provisional sum.
The package
Design
Temporary works design is engineering. A coffer dam retains water and often soil, it carries loads, and it fails badly if it is wrong. It needs designing, checking, and usually certifying.
Installation
Depending on the method — sheet pile, sandbag or barrier systems, earth bunds, water-filled barriers, or a diversion channel — this is plant, material, and crew days before any permanent work begins.
A diversion has its own scale: excavating a channel, managing the transition at both ends, and reinstating afterwards.
Pumping
Continuous for the duration:
- Initial dewatering of the enclosed volume
- Ongoing pumping against seepage, which continues for the whole period
- Standby pumps, because a failure floods the works
- Fuel or power, and refuelling
- Attendance and monitoring, often out of hours
Seepage rate is the variable that most often exceeds the estimate. A coffer dam in permeable ground can take substantially more pumping than one in clay.
Discharge management
The water pumped out has to go somewhere, and it is usually turbid.
- Treatment — settlement tanks, bags, flocculation
- Discharge authorisation, frequently separate from every other permit
- Monitoring against turbidity limits
- Disposal of accumulated sediment
Environmental measures
- Fish salvage and exclusion before dewatering, often required and often needing a qualified person
- Turbidity control during installation and removal, which are the two most disturbing phases
- Monitoring for the duration
Removal and reinstatement
Removing the temporary works, restoring the bed and banks disturbed by it, and demobilising. This is where the second turbidity event occurs, and it is often the phase with the least attention.
The schedule cost
Separate from the money, and on window-constrained work it can be decisive.
The full sequence — install, dewater, salvage, work, re-water, remove — has to fit inside the in-water work window. On a short window it may not fit at all, which effectively rules out any method requiring it. See in-water work windows.
On a tidal site with a few workable hours per day, it very often does not fit.
The permitting cost
Dewatering and diversion are regulated activities in their own right, not details of the main permit:
- Authorisation for the temporary works itself
- Discharge authorisation for the pumped water
- Fish salvage authorisation and protocol
- Additional conditions in the Section 401 certification
- Additional review time for all of the above
Each is elapsed time as well as cost, and each is an opportunity for a condition that changes the method.
It also weighs against the project on the minimisation test a reviewer applies. Dewatering is a large temporary impact, and avoiding it is a genuine minimisation argument worth making explicitly in a submittal. See what an agency reviewer wants to see and Section 404.
The risk
Dewatering carries risk that lands on somebody:
- Seepage exceeds the estimate, and pumping cost rises
- The coffer dam is overtopped by an event during the works, flooding the excavation
- Ground conditions differ, and the temporary works has to be redesigned
- The window closes before the sequence completes
Ask any proposal that includes dewatering who carries that risk. If it is the client, the price is not the price.
Why this is the comparison that matters
A method that places armor in flowing water avoids every item above. Not reduces — avoids.
That is why, on an in-water job, the largest single difference between two proposals is often not the armor at all. It is whether one of them needs a dry hole.
It is also why comparing permanent works alone produces the wrong ranking. Put temporary works on the comparison sheet as its own line for every bidder. See how to compare unlike armor bids.
When dewatering is genuinely necessary
Not an argument that nothing should ever be dewatered:
- Cast-in-place concrete generally requires it
- Precise geometry that cannot be achieved wet
- Foundation work requiring inspection and preparation
- Where the specification requires a verified subgrade
- Where the alternative cannot deliver the required performance
Where the permanent works genuinely requires a dry environment, the temporary works is not avoidable, and the right move is to price it properly rather than hope.