ALISE

Coastal areas in West Africa are vulnerable to rising sea levels, threatening people, biodiversity, infrastructure and the economy. The ALISE project is developing a multi-hazard vulnerability index based on satellite data and a Web-GIS platform that will serve as a decision-making tool for coastal risk management in Senegal.

[fr]

Senegal's coastline, here in Saint Louis, is increasingly vulnerable to a range of factors exacerbated by climate change. © OSS Saint Louis

Vulnerability analysis of the Senegalese coastal zone

Overview

Issues

Often densely populated and with essential infrastructure, coastal areas are increasingly vulnerable due to a number of environmental and human factors. The main problem is rising sea levels, which, compounded by climate change, are increasing the risk of erosion and flooding.

In West Africa, around a third of the population lives along the coast, particularly in cities such as Dakar, Abidjan and Lomé, which are experiencing strong demographic growth. These coastal areas generate 56% of the region's GDP, with fishing a very important activity.

In this context, Senegal's coasts, which are rich in biodiversity and economically vital, are particularly under threat. Coastal plains and deltas, such as those of Saint-Louis and Casamance, are highly sensitive to oceanic changes, putting local communities and infrastructures at risk. Rapid, unplanned urbanization exacerbates the situation by disrupting fragile ecosystems, exacerbating beach erosion and the degradation of marine habitats.

Protecting these coastlines against natural hazards requires a holistic and integrated approach, often referred to as 'multi-hazard'. This approach aims to anticipate, prevent and manage different types of risk to which coastal zones may be exposed, such as storms, flooding, sea-level rise and climate change.

Objective

The aim of this project is to construct a multi-hazard vulnerability index for the Senegalese coastline (and that of the Gambia), with a view to developing a dedicated digital platform as a decision-making tool based on a Web-GIS. Aimed primarily at managers, this platform will serve as an interface and aid for managing the vulnerability of Senegal's coasts to coastal risks.

Project organization

Using satellite products to map coastal indices and hazards at the scale of the Senegalese coastline, the ALISE project is structured around 4 distinct but interconnected tasks:

WP1: Hazard identification: dynamics of the Senegalese coastline. We are going to analyze the evolution of the Senegalese coastline and the retreat of the coast using aerial photos and satellite data. This study will enable us to monitor the dynamics of the coastline over time. To validate our methods, we will use data from the Saint-Louis site, where we have already carried out this type of analysis.
WP2: Identifying the issues. This work package focuses on the challenges facing the Senegalese coastline, based on:

categorization of surface areas using the images described in WP1, in particular to determine the surface areas of urban areas and mangrove swamps (using optical and multispectral images) as well as their temporal evolution in order to identify the importance of issues (such as a significant loss of mangrove area, densely populated areas, etc.).
field survey data to assess current socio-economic issues (e.g. tourism, critical infrastructure, fishing zones, historical population censuses, etc.).

WP3: Construction of vulnerability indicators. The aim is to propose indices for mapping risk hotspots on the Senegalese coast. We will focus on the calculation of spatialized issues and the development of the classification of multi-hazard vulnerability indices for the coast. The aim here is to calculate the indices for the current period that have already been proposed in the literature concerning submersion (SCO St-Louis project, Mendoza et al., 2022) and to construct an indicator corresponding to coastal erosion. At the same time, the project will map the issues at stake, whether human (demography), economic (agriculture/fishing/tourism), structural or in terms of the sustainability of mangroves. Finally, all of these hazards and issues will have to be categorized to obtain a multi-hazard map of Senegal's coastlines for each grid cell.
WP4: Co-construction and dissemination of the Web-GIS digital platform that will be made available to managers. This GIS will provide an indicator of the combined issues, hazards and risks for each grid. A historical view of changes in ground cover (urban areas, mangrove swamps, retreating coastline, etc.) will also be available to users, to raise awareness of the hotspot areas of the Senegalese coastline. Each mesh will be around 50-100m in size. The width of the stretch of coastline considered will depend on a threshold altitude corresponding to the zone that could be subject to submersion/flooding in the future (estuaries and delta will thus be taken into account). In order to help the authorities get to grips with this Web-GIS, we would like to set up face-to-face and remote training to help train future users and disseminate this tool.

Synergies

ALISE will benefit from the achievements of the SCO OSS Saint Louis project, which is the validation site for the methodologies used during this project, and the online platform will complement the WACA programme (Coastal Management in West Africa), in which the SCO WAVA-VAR project is participating, by helping to identify areas at risk.

Application site(s)

Coastal strip between Saint Louis du Sénégal in the north and Cap Skiring in the south

Data

Satellite

Sentinel-1A and 1B: SAR (radar) imagery with 23 m resolution
Sentinel-2A and 2B: Multispectral imaging with 10 m resolution
Landsat series: Multispectral imagery with 30 m resolution
Pleiades: Multispectral stereoscopic imaging with 2 m and 0.7 m resolution
SWOT: Wide swath altimetry (SWOT_L2_HR_Raster and SWOT_L2_HR_PIXC products)

Other

Reanalysis:
- Tide data from the FES2014 model (Carrere et al., 2016).
- ERA 5 global wave data.
- Dynamic atmospheric correction data used to obtain storm surge values (http://www.aviso.altimetry.fr/).

Calibration/validation in the St-Louis area:
- DTM acquisition using UAVs.
- Numerical model of flooding/submergence at the St-Louis site.
- DGPS coastline data.
- Measurement of water levels using pressure sensors.

Results – Final product(s)

This project will provide Senegalese stakeholders with:

Analysis of the variability of hydro-meteo-marine conditions along the Senegalese coast.
Any topo-bathymetric product generated as part of the project.
Mapping of issues along the Senegalese coast and classification methodology (Mendoza et al., 2022).
Hazard mapping of the Senegalese coastline and classification methodology:
Methodology for calculating risk indices from hazards and issues.
The final product, i.e. the Web-GIS, which will present the risks through a spatial classification of the vulnerability of Senegal's coasts based on multi-risk indices: cross-referencing of coastal hazards and issues associated with the coastal area.