Marine piles, mooring and berthing dolphins, fender piles, and jetty supports all scour, and for two quite different reasons.
The natural one behaves like a bridge pier. The other is caused by the vessels the structure exists to serve, and on a modern berth it usually governs.
The natural regime
A pile in a current develops the same flow structures as a bridge pier: downflow at the upstream face, a horseshoe vortex at the base, wake vortices behind. The mechanism is described in bridge pier scour.
Marine conditions add a few things:
Reversing flow. Tidal currents run both ways, so a pile is attacked from both sides across a cycle and the scour signature is more symmetrical.
Waves. Wave-induced orbital velocities near the bed combine with current, generally increasing sediment mobility.
Pile groups. Piles in a group interfere with each other. Depending on spacing and alignment to the flow, a group can produce a single merged scour hole substantially larger than any individual pile would.
Bracing and pile caps. Structural elements near the bed disturb flow and can create local mechanisms of their own.
The propeller regime
This is the one that produces the deep, fast, awkwardly located holes.
A vessel manoeuvring on or off a berth uses main propulsion, bow thrusters, and stern thrusters. Each produces a directed, high-velocity jet. When the vessel is close to the quay and close to the bed, that jet is aimed at the seabed and at the structure.
The differences from natural scour matter:
Velocity. A propeller jet delivers velocities well beyond what tidal current produces at the same location.
Direction. It is aimed. Thruster jets go sideways into the quay face and the bed beneath it; main propulsion goes astern into the bed behind.
Duration. Short bursts rather than continuous flow, but intense while they run.
Location. Highly repeatable. The same vessel class berths the same way, so the jets hit the same places, call after call.
Trend. Vessels have grown, installed power has grown, and thruster use has increased. A berth designed decades ago is frequently now serving vessels applying far more energy to the bed than the original design contemplated. This is a common reason a berth that was stable for thirty years starts scouring.
The answer bank covers which propeller-wash data a port should provide before berth protection is designed, and it is the input that most determines whether a design is right.
What it does to the structure
Piles lose embedment and lateral support. A pile with several feet of newly unsupported length has different behaviour from the designed element, particularly under berthing impact and mooring loads.
Quay walls lose toe support. Where the berth includes a vertical face, propeller wash accelerates the mechanism described in scour at bulkhead and sheet pile toes.
Fender and berthing structures move. These take impact load and depend on their foundations to resist it.
Declared depths become unreliable. A scoured berth pocket has a bed that is not where the chart says, which is an operational issue as much as a structural one, and the deposited material has to go somewhere — often a shoal at the edge of the pocket.
Where to look
Scour at a berth is not uniform, and inspection should target:
- Directly beneath and outboard of the berthing line, where thruster jets land
- At the ends of the berth, where vessels swing
- Around individual piles and dolphins, and between piles in a group
- At the quay toe, along its length
- Where the bed changes from the berth pocket to the approach
- Around any structure in the manoeuvring area — mooring dolphins, fender piles, navigation aids
Repeat surveys against a baseline are the method. Because propeller scour is repeatable in location, comparing surveys over time shows a clear pattern once you have two or three.
What is done about it
Armor the affected bed. The direct response, and it needs to cover where the jets actually land rather than only the structure footprint.
Operational measures. Restricting thruster use, tug assistance, or changing the approach can reduce the loading. Cheaper than construction and unpopular with operations, but worth pricing against the alternative.
Deeper foundations, on new build.
Periodic dredging and replacement of bed material, which is maintenance rather than a fix and gets expensive as a permanent regime.
Why bagged armor appears on berth work
The conditions are specific and they suit unit placement:
The work is always wet. A berth cannot be dewatered.
The berth stays operational. Work often has to happen between vessel calls, in windows, without closing the facility. Placement that can stop and resume suits that.
Access is from the water or from the quay edge. Barge or crane placement from above, working down.
The geometry is irregular. Around piles, under fenders, against a quay face, in an existing hole.
High local loading in a defined area. Discrete units placed where the jets land, rather than a continuous layer across the whole pocket.
Vessel impact is foreseeable. The answer bank covers what happens under vessel impact at a berth, and repair of individual units is a different proposition from repairing a continuous system.
Where to go next
- Bridge pier scour for the underlying vortex mechanism
- Scour at bulkhead and sheet pile toes for the quay face
- Why scour protection fails for what defeats armor
- Marine and ports for the application
- Post-flood scour inspection for inspection method, much of which transfers
- The scour protection hub for the rest
Send berth surveys against a baseline, vessel classes and installed thruster power, and the berth geometry through the quote form. Without the propeller data a berth design is guesswork, and it is the item most often missing.