The port of Lagos is one of the largest and busiest in Africa and plays a key role in the Nigerian economy. Lagos, Nigeria’s largest city, with a population of 8 million, is Africa’s second largest city after Cairo in Egypt.
The harbour is protected from the Atlantic Ocean by three breakwaters known as ‘harbour moles’. The two larger breakwaters protrude into the ocean and protect the main channel leading ships in to the harbour, maintaining the flow of water through the channel and reducing the need for expensive dredging work. Without the breakwaters, an ocean surge would be more likely to flood the whole of Lagos Island and the lagoon, causing massive disruption.
The breakwaters are historic structures. Built more than a century ago in colonial times, they have been in urgent need of repair for many years. The heads and outer parts have been badly damaged by the strong tidal currents, storms and the Atlantic swell. This has created massive ‘scouring holes’, some up to 30 metres deep at the heads of the breakwaters. These undermine the breakwaters, causing the crest and side slopes to fail.
The repair work had become more urgent due to the planned construction of the Eko Atlantic City development that will be built on 8km2 of reclaimed land close to the East breakwater. The breakwater will form part of the Atlantic sea defences for the city and is currently being used as a haulage route to transport many tonnes of rock material to construct the sea revetment. When the project nears completion, two large sluices will be built into the breakwater for recreational boating, adding a Dutch element to the design.
Royal HaskoningDHV first became involved in the Lagos Moles project in 2001, when it delivered designs for the reconstruction of the breakwaters; however implementation was postponed until 2008-09. As the level of the sea bed around the breakwaters had changed significantly since 2001, the Nigerian Ports Authority recently asked Royal HaskoningDHV to act as technical adviser to review and update the original designs. A contractor had already been appointed to carry out the works, so the Royal HaskoningDHV team were under time pressure to complete the designs quickly to minimise the amount of downtime incurred by the contractor.
The work, carried out by a multi-disciplinary team from Royal HaskoningDHV’s Lagos, UK and Dutch offices, involved detailed inspection of the structures including topographic and hydrographic surveys to determine the extent of the damage. This was followed by the detailed design of repairs, requiring large quantities of rock to fill the scour holes and provide support to the structures over a length of more than a kilometre. The design and work on the smaller central breakwater, West breakwater and head of the East breakwater has been completed. The trunk of the East breakwater has been stabilised for use as a haulage road for the Eko Atlantic City project, with further work to follow.
Alec Sleigh, the Lead Technical Engineer based in Peterborough, explains some of the issues the design team faced: “The swell wave conditions have required the use of concrete units rather than rock as the primary layer of protection to improve stability. Large concrete cubes will be used at the head of the breakwater as these are less vulnerable to the re-development of scour holes and are also less susceptible to damage if settlement occurs.
“The proposed repairs to the moles have been physically modelled and tested in two and three dimensions at the Danish Hydraulics Institute in Copenhagen. This has enabled us to confirm the stability of the armour and toe protection. We have literally tested the design to destruction in several different scenarios to establish how it responds to extreme conditions.”
Royal HaskoningDHV’s designs will help to secure the breakwaters and extend their useful life for a further 50 to 100 years, protecting the port and its main shipping channel, and enhancing the lives of all who use it for decades to come.