Risk of coastal flooding has increased by 50% in last 20 years

Ground Report | New Delhi: Risk of coastal flooding has increased; Between 1993 and 2015, the risk of marine floods increased by almost 50% on a global scale, according to an international study coordinated by the French Institute of Research for Development (IRD) with researchers from the National Center for Space Studies (CNES) and the Mercator Ocean ocean analysis center, recently published in the journal Nature Communications .

Risk of coastal flooding has increased

By combining satellite data and numerical models, we show that these flood risks are going to accelerate, especially in the intertropical zone.

This situation is explained by a combination of factors. These include the global rise in sea level, but also the breaking of waves against the coasts, an important phenomenon that until now has been little taken into account in climate forecasts due to its complexity.

Currently, the sea rises an average of about 3 millimeters a year, according to the adopted greenhouse gas emission scenarios. This level could reach 80 cm by the end of the 21st century.

The role of climate change

Low-lying coastal regions - such as the sandy coasts of West Africa and large deltas, such as the Ganges-Brahmaputra region - are home to almost 10% of the world's population .

These areas with unique and sensitive ecosystems, such as the coastal lagoons with their rich biodiversity or the fertile alluvial soils used for agriculture in the Mekong Delta (Vietnam), are suffering from coastal erosion due to human activities: the deficit of sand in the coastline due to the dams in the rivers that block the inflow, the uncontrolled extraction of sand, the subsidence of the soil related to the pumping of groundwater and urbanization… They are also exposed to the rise in sea level.

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And they are subjected to devastating dangers, be it submersion and / or flooding. Remember the storms Katrina and Xynthia, which hit the United States in 2005 and Europe in 2010; or Typhoon Haiyan, the largest tropical cyclone ever recorded, which hit Asia in 2013.

Identify "hot spots"

In our study, we combined the use of a novel global numerical model of coastal sea level, which includes the transitory effect of surface elevation waves, with a new estimate of the extreme levels reached. To do this, radar satellite altimetry data were used to monitor sea level rise, taking into account tides, wave analysis, and natural and man-made measures to protect the coastline.

We have quantified the global increase in marine floods in the period 1993-2015. To do this, we specify two key parameters of the coastal topography using satellite data: the slope and the maximum subaerial elevation of these zones.

The extreme coastal water level was calculated with an hourly resolution to identify the potential number of hours of breakdown of coastal protection in each zone on an annual basis. The result is that, in 23 years, the number of annual hours of aggregate marine floods globally has increased by almost 50%. We have gone from 10,000 hours a year to more than 15,000.

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The combination of tides and large waves (up to 10 meters) is the main factor contributing to this increase.

Several “hot spots” have been identified: the Gulf of Mexico, the southern Mediterranean, West Africa, Madagascar and the Baltic Sea. Here, the increased risk of marine submersion is greater due to low and unprotected coasts (naturally or artificially).

The number of hours of potential flooding could increase considerably by the end of the century, at a faster rate than the average rise in sea level: that is, the weight of each millimeter of rise is not constant and increases, and the risk coastal protections are broken grows.

This acceleration of the marine flood is exponential and will be clearly perceptible from 2050, whatever the climate scenario.

At the end of the century, the intensity of the acceleration will depend on the trajectories of greenhouse gas emissions and, therefore, on the rise in sea level. In case of high emissions, the number of hours of marine flooding could be multiplied by 50 compared to today.

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