In a port the bed takes a beating from ship propellers and bow thrusters, often much harder than from natural currents. Wash from a manoeuvring vessel scours the toe of quay walls and berths, and over time it can undermine the structure.
Rock bags are used to armour quay toes, berth pockets and the foot of breakwaters. They can be placed from the quay or from a barge, in the wet, between vessel movements.
Where scour hits a port
- Quay wall toes, where propeller wash is directed at the wall during berthing and departure
- Berth pockets, where repeated thruster use deepens the bed
- Ro-ro and ferry ramps, where vessels use high power close to the structure
- Breakwater and revetment toes, where waves and currents remove bed material
- Around piles of jetties and dolphins
Why rock bags suit port work
Port operators need repairs that do not close a berth for long. A rock bag is filled ahead of time on land, then lifted and placed in a single movement, so placement can be scheduled between ship calls.
The bags work in salt water, and the rock inside dissipates flow energy in the gaps between stones. Grouped bags interlock into a continuous apron that follows the shape of the bed at the toe of the wall.
Design checks to make
Propeller wash velocity at the bed depends on the design vessel, its engine power, propeller size and the depth under the keel. The port's design vessel and berthing method should be used to set the design velocity, which is then compared with the bag ratings in the size guide.
Where wave action is significant, as at an exposed breakwater, stability has to be checked for waves as well as currents. Our current ratings do not cover wave loading; send the wave conditions and we will advise.
Questions
- Can rock bags be placed while the port stays open?
- Usually, yes. Bags are pre-filled and each one is a single lift, so work can be scheduled around vessel movements at a berth.
- Do rock bags work in salt water?
- Yes. EVO Rock Bags are used in both fresh and saline water.
- What decides the bag size at a quay wall?
- The bed velocity from propeller wash or currents at the toe. Compare it with the single and grouped ratings in the size guide, and check wave loading separately if the site is exposed.
Published by EVOSNT Trading FZCO, manufacturer of EVO Rock Bags. Product figures are from our technical specifications. General guidance only; the project engineer is responsible for the final design.
