Most armor systems need a dry working area. That requirement is what makes them planned-works products: you cannot coffer dam a river at flood stage.
Bagged armor can be filled and placed in flowing water or fully submerged, which is why it appears on emergencies. But “can be placed” and “should be placed now” are different questions, and the second one is a judgement.
What makes placement in flowing water possible
The unit is the element resisting the flow, not the individual stone. A filled bag lowered into moving water arrives as a single mass with its own submerged weight, rather than as loose material that the current sorts and carries away as it falls.
Compare with dumping loose stone into fast water: the fines go downstream, the gradation you specified is not what arrives, and placement accuracy is poor.
Rising limb or falling limb
The most useful question during an event, and the one most often skipped.
On the rising limb, conditions are getting worse. Velocity is increasing, the scour hole is still developing, and material placed now sits in a bed that is still being excavated. Work placed into a rising event may end up in a hole that deepens beneath it, and the crew is working in conditions that are deteriorating.
Intervening on the rising limb is justified when the alternative is losing the asset — an active undermining that will not survive the peak. It is not justified for damage that is stable.
On the falling limb, conditions are improving. Velocity is dropping, the hole has been cut and is starting to refill with loose material, and the site is becoming safer by the hour. This is when most emergency placement should happen.
At the peak, the honest answer is usually to wait unless the asset is being lost. Nobody makes good decisions at the peak, and the site is at its most dangerous.
What actually governs placement
Reach. The excavator or crane has to reach the work from a stable position. On a flooded site the stable positions are limited and they are not always where you want them.
Bank stability. The ground the plant is standing on may be the ground that is failing. This is the most common serious hazard on this work.
Visibility. You cannot see the bed. Placement is by feel, by measurement from a known point, and by counting. Accuracy is lower than on a dry site and the layout should assume that.
Water depth and velocity at the placement point, which determine whether a unit lands where it was aimed.
Debris. Floating debris during an event is a hazard to plant, to rigging, and to people.
Working blind
Because you cannot see the bed, placement in flowing water is a measured operation rather than a visual one:
- Work from a survey reference, not from what looks right
- Count units and record positions as you go
- Probe to confirm where the previous unit landed before placing the next
- Build from a known edge inward, so each unit has a reference
- Accept that the layout will be coarser than a dry placement, and specify accordingly
The answer bank covers filling, lifting, and placement generally, and how coverage is confirmed after placement.
Sequence during an event
Start at the toe. The deepest, most critical, and the part that will be hardest to reach later if conditions worsen. Also the part that arrests the mechanism.
Establish the edges early where you can. Flow works at unprotected boundaries, and a partially complete installation with a defined edge is more stable than one with a ragged perimeter.
Extend past the visible damage. Under pressure the temptation is to fill the hole and stop. The affected footprint is larger than the hole, and the new edge becomes the next problem. See why scour protection fails.
Leave the site in a defensible state at the end of each shift. Emergency work stops for weather, darkness, and crew hours. What is in place when you stop should be stable on its own.
The limits worth respecting
Being straightforward about where this stops:
Velocity has a limit. Placement into extremely fast water is not safe for the crew or accurate for the work, whatever the product will tolerate once in position.
Depth has a practical limit for excavator work, beyond which you need different plant.
Crew safety governs everything. No armor placement is worth an injury, and the pressure during an emergency to keep working past the point of good judgement is real.
Some sites should be left alone until the event passes. A structure that is already closed and an event that is receding is not an emergency that needs a crew in the water at night.
When not to use rock bags sets out the wider limits of our own data.
Where to go next
- Safety during emergency placement
- Emergency scour repair: the first 48 hours
- Before the storm vs. after
- Installation and site operations for the planned-works version
- Emergency response for how a job is scoped and shipped
- The emergency hub for the rest
For an active emergency, call. The number is on the contact page.