4 minute read
Dangote Sandbar Breakwater
Lekki, Lagos State, Nigeria
Preventing erosion using nature-based solutions at Dangote’s marine terminal. The coastal system of Nigeria is unique. Regular swells arriving at the coast at a relatively constant angle combined with a uniform steep and sandy coast result in a large and unidirectional longshore sediment transport. However, the coastal system of West Africa is highly affected by human interventions, such as maritime infrastructure, and is aggravated by climate change. Such infrastructure interrupts the steady eastward transport of sand, leading to coastal erosion and loss of habitat. The Dangote Oil Refining Company’s new greenfield refinery, polypropylene, and fertilizer plant’s own infrastructure required protection against the heavy wave energy. A conventional rubblemound breakwater would cause a rapid advance of the coastline on the updrift side, ultimately burying the expensive armor rock. Instead, in 2016, CDR International and Svašek Hydraulics, both coastal engineering specialists, developed and designed a sandbar breakwater concept to be constructed mainly out of sand. The use of natural processes and the natural availability of construction materials made it a cost-effective design. In 2018 in less than five months, the dredging contractor Boskalis constructed the breakwater. Since, natural wave action has been shaping the sandbar breakwater and adjacent updrift coastline as designed.
Article Cover: Aerial view of the completed breakwater and marine terminal entrance. (Photo by Boskalis and CDR)
Producing Efficiencies
Following a thorough investigation of the environmental systems and processes of the Lagos State coastline, the project team used state-of-the-art mathematical modeling tools to calculate the optimum placement, angle, and sand volume for the sandbar breakwater. This ensured that the structure will remain dynamically stable and that the steady eastward sand transport will continue to strengthen the breakwater. Additionally, the use of offshore sand eliminated the need to use rocks, stones, or concrete. This significantly reduced construction time, materials, and costs.
Using Natural Processes
Any obstruction of the alongshore sediment transport, whether a conventional breakwater, groyne, or a sandbar breakwater, will inherently lead to downdrift erosion and potentially significant loss of land. The most natural way to counteract this effect is by restoring the natural sand balance of the system. Therefore, a small-scale, nature-driven sand engine has been implemented in the design to mitigate the coastal retreat downdrift. As erosion continues, this sand engine will provide local distribution of the sand to stabilize the coastline over the coming decades.
Broadening Benefits
The natural shoreline growth at the west of the sandbar breakwater is already being used to enlarge the Dangote quay infrastructure. And, because sand can be moved and shaped more easily than traditional quarried rock, this new concept is flexible and allows for relatively easy future port expansion. Furthermore, the soft nature of the sandbar makes it more adaptable to climate change than conventional breakwaters are. All of these features work together so that the new breakwater keeps the Dangote loading facilities calm and safe.
Promoting Collaboration
The collaborative design efforts of the three Dutch companies, CDR International, Svašek, and Boskalis, optimized construction time, materials, and costs while retaining the natural characteristics of the coastline and design. This unique coastal zone benefits from resilient and sustainable maritime infrastructure solutions. By considering both the needs of the system and the function of the harbor, the team was able to maintain the natural aesthetics and habitats without hindering community access. The combined efforts resulted in an innovative and sustainable naturebased solution.
