Intranet
West and Central Africa and the challenge of recurring flooding
Different types of flooding, with various causes
There are two main types of flood. The first category involves water courses breaking their banks, unable to handle the accumulated volumes of water flowing downstream. The second corresponds to what we might call rain-induced flooding, directly attributable to torrential rain which overwhelms drainage systems.
“the increasing frequency and severity of flooding can be attributed to the combined effects of climate change and land occupation.”
Rising temperatures are driving the intensification of the hydroclimatic cycle, alternating between episodes of severe drought and extreme rainfall during monsoon season.
“The warmer the planet gets, the more water vapour is present in the atmosphere and the more violent the rainfall becomes,”
says Thierry Lebel, a hydroclimatologist IRD at the institut des géosciences de l’environnement (IGE- OSUG, CNRS / IRD / INRAE / Grenoble INP / Université Grenoble Alpes). Annual rainfall totals, however, are not necessarily the most important indicator of this change. Of greater concern is the increase in intra-seasonal variability.
Land occupation, meanwhile, is a decisive factor in the catastrophic consequences of floods, for various reasons. In the wake of the great droughts experienced between the 1970s and the 2000s, local populations began to occupy riverbanks and low-lying areas which were previously under water. However, as rainfall has intensified these areas have become particularly vulnerable to flooding, and are now regularly ravaged by waterways bursting their banks. This risk is further accentuated by the lack of town planning controls, and the failure to give due consideration to rainwater management.
“In Dakar, for example, in the absence of proper drainage networks, the water table is saturated and cannot absorb any more rainwater, which leads to major problems with run-off”
explains Ansoumana Bodian, a hydrologist at the Department of Geography at Université Gaston Berger de Saint-Louis, in Senegal.
Such run-off phenomena are rendered even more troubling by the breathless pace at which the megacities of West and Central Africa are growing, an expansion which often involves extensive use of concrete, effectively rendering the ground entirely impermeable. In rural areas, the partial impermeabilisation of soils is also spreading, not least due to the formation of a hard topsoil crust as a result of farming practices and increasingly intense droughts.
Better understanding and preparing for hydroclimatic developments
While climate research is capable of providing a fairly clear long-term vision of how temperatures will rise at the global and regional levels, there is far less clarity regarding the way in which the water cycle will respond to this warming.
“The predictive models we have for rainfall are much less reliable than those for temperatures, because the physics involved is more complex, with scale effects playing a fundamental role,”
notes Thierry Lebel.
In order to advance scientific knowledge in this domain, and to keep track of key hydroclimatic trajectories, it is now essential to establish a network of high-quality observatories for the long term, and to train competent staff capable of running them.
“Hydrology programs, particularly urban hydrology, are few and far between in West Africa. And yet, training people and collecting high-quality data are essential to finding concrete solutions for flood management,”
explains Ansoumana Bodian.
“satellites provide us with information which is interesting but indirect. That information needs to be cross-checked with data collected from a network of sensors at ground level,”
Ansoumana Bodian continues. The issue of the availability of open data is also of fundamental importance, in order to ensure that hydrological forecasting models are pertinent and up to date.
“There needs to be a dialogue between the different scientific disciplines, so that meteorological forecasts can be translated into forecasts for hydrological impact,”
adds Yves Tramblay.
Pertinent adaptation strategies
In addition to the scientific and technical challenges of producing and retrieving data, there is also a sociological dimension to be considered in order to mitigate, as far as is possible, the disastrous consequences of flooding: the installation of effective early warning systems. Awareness-raising, training of local specialists, use of existing mobile telephone networks… There are many avenues to be explored and utilised in order to avoid further loss of life. Last but not least, it is crucially important that local governments should take new hydroclimatic realities into consideration in their town planning regulations, with special focus on territorial development choices and adequate drainage systems.
All of these solutions will require close collaboration between scientists, political decision-makers and a broad swathe of civil society.
Local contact scientist
– Thierry Lebel, researcher IRD at IGE (IRD/Université Grenoble-Alpes/Inrae/CNRS/Grenoble INP-UGA)
This article, rédigé par Louise Hurel, IRD, was initially pubished by IRD le Mag’.
Updated on 22 November 2024