Adapting coastal areas to sea level rise
Defining strategies and implementing solutions
Sea levels are rising at an average rate of 4 to 5 mm a year. This is caused by thermal expansion of the ocean and melting glaciers – one of the consequences of global warming. Between now and the end of the century, scientists estimate that sea levels will rise by at least 40 cm and potentially more than a metre, if certain processes accelerate. This change is happening and set to continue, with a degree of inertia, even if humanity succeeds in reining in its greenhouse gas emissions, which are the main driver of the current climate disruption.
Adapting to sea level rise is now a very real concern for all coastal stakeholders (States, local and regional authorities, ports, industrial firms, farmers, inhabitants, etc.), as it is likely to cause substantial harm to people and property. Mitigating these impacts means defining new strategies drawing on updated assessments, and implementing development programmes tailored to each individual case. Robin Sigwald, project director in Artelia’s Coastal Resilience team, tells us about his experience in this field.
What are the expected impacts for coastal regions and their inhabitants?
Sea level rise is having widespread consequences. It is leading to increased erosion, and aggravating coastal flooding and inundation. It is exacerbating various climate risks (storms, hurricanes) and affecting river basins, altering the conditions at their mouths into the sea. Coastline retreat can also result in naturally low-lying land becoming permanently submerged, and increase the salinity of land and freshwater resources.
With around 1.4 billion people worldwide living in coastal areas, sea level rise poses a threat to homes, agricultural facilities, tourist attractions, and vital port, logistics and industrial infrastructure. Recent assessments (UN Habitat, World Cites report, 2024) based on 136 of the world’s largest coastal cities estimate that annual losses could exceed a thousand billion dollars by 2050 if no adaptation strategies are implemented.
Even though uncertainties remain as to the level that will be reached in 30, 50 or 80 years’ time (between 40 cm and 3 or 4 m in the worst-case scenarios), sea level rise is underway, and being confirmed by measurements. An increase of 40 cm is enough to transform a “conventional” storm – one we are used to handling – into a catastrophic event. There is now a broad consensus that sea level rise will reach at least 1 m between 2080 and 2150, forcing coastal stakeholders and inhabitants to acknowledge that they cannot carry on as before.
How are adaptation strategies drawn up – and at what scale?
One of our strengths is that we can act at any spatial scale. We support States, regions and local authorities with their integrated coastal management strategies, but our assessments also cover port infrastructure, businesses and homes.
We start by drawing up an initial appraisal using existing data on the area concerned along with scientific research on climate change, including that carried out by the IPCC. Next, we assess the potential transformations to the coastline in order to make projections and pinpoint vulnerabilities, according to various climate change scenarios. This initial appraisal includes a detailed analysis of existing developments and structures, because many of them, designed several decades ago for a relatively stable sea level, are no longer suited to the current changes and those to come.
Carrying out these assessments is a complex but fascinating task, requiring solid technical expertise in a wide range of disciplines. For these projects we call not just on our experts in the climate, the coastal environment, and coastal and maritime engineering, but also on our urban and regional development and construction specialists, because it’s vital to factor roads, transport systems, drainage networks, energy facilities and so on into the discussions. We also include architects, urban planners and landscape designers, because in many cases an entire sea front has to be redesigned. In parallel, our multi-disciplinary teams are engaged in numerous internal R&D activities and the supervision of PhD theses, with current topics including the use of nature-based solutions and planning in the context of spatial reconfiguration.
Which authorities have you supported recently?
In France, we are working with a large number of municipal and local authorities, because the French ‘Climate and Resilience’ act is prompting them to study changes taking place on their coastline and implement adaptation strategies. We are currently assisting the inter-municipal authority for the Seine-Maritime coast, whose territory covers nearly 140 km of coastline and about 270,000 inhabitants. In this case, the strategic approach is being cascaded right down to the municipal level, and covers a wide range of concerns. It’s an area where local ports such as Le Tréport, Fécamp, and Dieppe play a major economic role (the port of Le Havre is covered within other complementary approaches), but it also has some highly valued natural areas which the authorities want to preserve.
In Ivory Coast, as part of a study for the World Bank, we are contributing to the development of coastal and maritime spatial planning for the municipality of Assinie. This region is severely affected by coastal erosion and sea-level rise. The adaptation strategy being drawn up puts a strong emphasis on nature-based solutions.
We are also working in Denmark, a country that has focused on building resilience to surface water flooding in recent years, but where coastal flooding is also becoming an increasingly pressing issue. On these low-lying coasts with small tidal ranges, sea-level rise can also have a major impact on stormwater management. In Copenhagen, for example, predictions show that damage from the sea could exceed that caused by extreme inland flooding by 2050.
When we help local stakeholders draw up their adaptation strategies, consultation and co-creation often lie at the heart of our approach. With the Seine-Maritime and Assinie projects, the discussions brought together elected officials, technical officers from regional authorities, inhabitants, economic stakeholders, and more. These exchanges, between stakeholders with different levels of knowledge, points of view and interests, foster the emergence of a project supported by all and make certain choices much easier to accept.
What solutions do you recommend in the context of adaptation strategies like these?
A proportionate approach is the key, to ensure that the responses are geared to the actual vulnerability of a given region or site. The outcomes from our risk assessment are used to perform cost-benefit analyses. By ‘cost’, I mean the amount of investments needed to implement a solution, and by ‘benefit’ I mean an estimate of the losses thus avoided.
The range of solutions we propose is a subtle blend that depends on the area concerned and the time scale being considered. For large urban areas, hard structures such as protective embankments or breakwaters are required, as the other options may not be feasible in the short term. The design of these defences includes urban planning measures to make the area less vulnerable and give it room to adapt in the future. In rural or more natural areas, with less dense housing, we give preference to ‘softer’ solutions, which we nowadays call ‘nature-based solutions’. These involve working with ecosystems (harnessing the protection provided by dune ridges, or the ability of coastal marshes to absorb water) by giving them back the space they need to fulfil their functions. These solutions are tied in with regional projects which might also include changes to agricultural practices, land uses, or types of housing.
These various solutions can also be staggered over time. The height of a breakwater can be increased to protect buildings and infrastructure for 20 or 30 years, which is long enough for them to pay for themselves, after which it is more feasible to consider dismantling them or relocating them once the exposed assets behind them have also been reconfigured or adapted.
Can you give us some examples of projects that are underway, or have been completed?
In France, we were involved in restoring the Vieux Salins dune ridge in Hyères. This is an excellent example of how nature-based solutions can be used to combat coastal erosion. The study carried out in 2017 demonstrated that rewilding the coastline and allowing it to regain its balance naturally was a pertinent scenario. It included removing a rip-rap revetment that was disrupting sediment transport processes across the site. Today, following completion of the works (2018-2021), the natural sedimentary dynamics have been restored and the site morphology is changing in the way we hoped it would.
We have also completed another protection scheme at the border between Togo and Benin in the Gulf of Guinea, another part of the world which is severely affected by coastal erosion. Artelia was involved in drawing up a concerted, sustainable cross-border strategy as part of the WACA programme initiated by the World Bank. The studies performed, which included modelling sediment transport over an 80 km segment of coastline spanning the two countries, showed that these coastal zones are interdependent and helped pinpoint the best solutions to protect them. In Togo, they involved building and rehabilitating short groynes and replenishing the sand on the beaches between them to reduce interception of longshore drift. In Benin, a few short groynes were built upstream of a huge sand replenishment operation over a length of 4 km and a width of about 200 m (6.4 Mm3 in total). Nearly 200,000 people living along this coast are set to benefit.
In another vein, we helped the GPMDLR (Port Authority of Reunion Island, France) draw up its climate change adaptation strategy (2020-2022). We are now entering the implementation phase, which involves striking the right balance between the need to both protect the port (by rebuilding breakwaters or increasing their heights) and expand its facilities, which are very hemmed in by their surroundings.
Published on June 6, 2025