- Case study
- Dunkirk New
- France
Berth Scour Protection for a Piled RORO Ferry Berth, Dunkirk, France
A new Dover-route berth taking twin-propeller ferries up to every 45 minutes, on a bed of fine sand. The protection had to go in beneath a completed piled deck, 50 m from a live ferry lane, and keep working as the bed around it dropped.
Location
Dunkirk Ferry Terminal, France
Client
Port of Dunkerque (GPMD)
Designer
Eiffage Génie Civil
System
Concrete mattress, hinged edge
Period
2023
Results at a glance
Concrete placed
In place of 9,000 to 12,000 t of rock armour
Placed in 15 weeks
Finished a week ahead of programme, ferry service uninterrupted
From a live ferry lane
Installed beneath the completed deck with conventional plant
Contents
- Twin propellers berthing up to every 45 minutes drive rapid, localised erosion of the fine sand bed.
- The apron had to go in beneath a completed piled deck, take cyclic scour loading, and carry no propeller-strike risk.
- A hinged edge anchored in a 4 m deep trench lets the apron settle with the bed as scour develops, rather than resisting it and being undercut.
- 72 mattresses and about 6,100 m³ of concrete placed over 15 weeks, 50 m from a live ferry lane, finishing a week ahead of programme.
The constraints
RORO6 was built at Dunkirk Ferry Terminal to take increased traffic on the Dover route. It is the operating pattern of the berth, not any single event, that sets the scour problem.
frequency
propellers
complete
operations
lane
depth
The solution
An insitu concrete mattress answers every line of that table. It gives a continuous protective layer around the piles and the berth structure, and it goes in as flexible rolled formwork that can be positioned, restrained and pump-filled in the restricted space beneath a completed deck.
Incomat formwork holds its dimensions as it fills, which is what made mattresses up to 55 m long placeable and fillable to thickness on variable gradients under the deck.
Hinged edge design
The hinged edge lets the mattress flex and settle gradually as scour develops beneath it. Rather than resisting the natural tendency of the seabed to erode, the hinged edge accommodates it, so the apron stays in contact with the bed and keeps protecting it even as local levels drop.
The trench is 4 m deep, enough for embedment and to take the early scour. Beyond that depth the design allows targeted addition in defined locations rather than protection sized for the worst case from the outset.
That removes the undercutting and toe failure that rigid or shallow-anchored protection is prone to, and it installs faster, with no complex shaping or deep backfill.
Pile seals and sliding restraint
Sealing around the piles was essential to stop vertical scour propagating down around each pile shaft, which is a vulnerability in many traditional scour protection systems and a recurring problem on piled revetments and jetties. The mattress incorporated tight pile seals, using tailored collars and infill fabric to wrap around the piles and tie back into the main mattress body.
Those seals prevent water jetting down the pile line, which would otherwise initiate focused scour holes; maintain the integrity of the apron so it stays continuous at every pile penetration; and reduce prop-wash interaction at the pile–mattress interface, which could otherwise lead to early failure. They were installed with close diver supervision and filled as part of the main pour sequence, giving both visual inspection and structural integrity during placement.
The central compartment of each mattress provides sliding restraint, preventing longitudinal movement in the deep toe trench. That mattered here because of the sloping profile and the potential for the fabric to shift under hydraulic loads before the concrete cured. The general principles are set out in our article on preventing under-scour from vessel action. Filling that internal restraint zone first anchored the mattress before the hinged edge and upper fills were completed.
Constructability and installation
Installation was carried out by Eiffage Génie Civil, just 50 m from an active ferry lane, with no disruption to service. The team ran shore-based training and dry runs to refine techniques and mix design before live placement, and the mattress works finished a week ahead of programme.
Ballast the empty mattress
Divers carefully ballasted each empty mattress before filling, so it held its position on the bed.
Fill the sliding restraint first
Compartment 1, the central sliding restraint, is filled first, anchoring the mattress in the deep toe trench against longitudinal movement.
Then the hinged edge compartments
Compartment 2, the hinged edge, follows for stability before the upper fills go in.
Always start at the lowest point
Filling begins at the lowest point of each section, so concrete distributes evenly up the gradient.
Pour with the pile seals in sequence
Pile seals were filled as part of the main pour under close diver supervision, giving both visual inspection and structural continuity at each penetration.
volume
Outcome
The berth carries a continuous sealed apron designed for the loading it actually sees: twin-propeller wash up to every 45 minutes, on fine sand that had already proved it would move. Because the edge follows the bed rather than resisting it, early scour is absorbed instead of undercutting the toe.
The works were completed in 2023, one week ahead of programme, with no disruption to ferry service throughout. Rock armour remains available to the port authority as a later, targeted addition if scour eventually exceeds the trench depth, and only in isolated areas.
Why it suited this berth
Full-bed protection at reduced thickness. An equivalent rock solution could require 9,000 to 12,000 t of material depending on placement density.
Installed with conventional plant and dive teams beneath the completed deck.
The trenched, hinged design pre-empts early scour, reducing ongoing dredging in the high-energy zone.
Unlike loose rock, the mattress gives a stable surface with reduced risk of propeller strike or movement under jet wash.
The hinged edge supports targeted rock addition by the port authority if scour eventually exceeds trench depth, in isolated areas and long after construction.
Conclusion
RORO6 shows what berth scour protection by insitu concrete mattress can do in a high-frequency, high-scour environment. With tailored trenching, sliding restraint and a hinged edge, the solution delivers long-term protection under demanding operational conditions, with logistical, cost and maintenance advantages over traditional rock armour.
Construction of the scour protection works was completed in 2023, giving a resilient and future-proof berth to support continued growth of the Dunkirk–Dover ferry service.
Who did what on this project
Owner of the ferry terminal and the new RORO6 berth.
Carried out installation of the scour protection, 50 m from an active ferry lane.
Concrete mattress system, hinged edge and trench detail, pile seals and the filling sequence.
Continue
Port of Szczecin, berth deepening
The same hinged edge detail used for the opposite problem: 11,000 m² of sealed apron letting an existing quay be dredged 1.9 m deeper without undermining the wall.
Read the case study →Concrete mattress
How the form is filled, the variants, and the hinged edge and trench detail that lets a thin apron follow a scouring bed.
Read →Piled revetments
RORO6 is a piled deck over a slope. This is the same problem set out generally: protecting the bed beneath a deck against propeller wash.
Read →Designing scour protection for a RORO berth? Send us the berth geometry and vessel data.
Send us the berth geometry, the vessel classes and propeller arrangement, and the berthing frequency, and we will come back with an outline approach: apron extent, toe trench depth and the edge detail. If rock armour is the better call on your berth, we will say so.
- Structure type and as-built section
- Vessel classes, propulsion and thrusters
- Berthing frequency and any scour survey
- Bed material and available trench depth