The velocity generated by the propellers of the vessels in the new Harbour of Tadjoura exceeded the critical velocities of the granular materials, resulting in scour.
Scour Protection of the Port Of Tadjoura
The original scour protection measure consisted of a 2.0m thick rip-rap layer, to be positioned at a depth of –15.00m (CD), so as to leave a freeboard of –13.00m (CD). The total area of the seabed to be protected was some 15,000m2, which would have required an excavation volume of 30,000 m3, and the same volume of rip-rap. The protection was to extend 22m from the quay.
An alternative solution using rock-filled Reno® Mattresses was investigated. It was found that 300mm thick mattresses, positioned over a suitable non-woven geotextile, would satisfy the design stability requirements. To comply with the long-term requirements and the aggressive environmental conditions, the steel wire of the mattresses was required to be Galfan coated first, and then extruded with a high abrasion resistance polymer.
The mattresses were pre-filled in the yard, with the geotextile pre-attached, and fitted with steel loops, so that they could be lifted by using a suitable lifting and installation frame.
The flexible lining provided by the geotextile filter and the Reno® Mattresses offered a number of benefits in comparison with the original design:
- Reduction of the imported rock volumes to 15%
- Reduction of the excavation volumes to 15%
- Quicker installation time than rip-rap
- Better thickness and gap control
- Lower environmental impact