Flood risk underestimated, particularly in the tropics

Published: 29 June 2021

Deltares has applied a new dataset and found that coastal zones in much of the world are lower than has been realised. Flood risks are therefore higher, particularly given sea level rise and land subsidence.

“To assess flood risks, we need to know the exact land elevation,” explains Al Hooijer, a researcher at Deltares and the National University of Singapore, and first author of the paper ‘Global LiDAR land elevation data reveal greatest sea-level rise vulnerability in the tropics‘ published online on 29 June in Nature Communications.

In places like the Netherlands or Singapore, the land surface is mapped very well and we know the elevation relative to mean sea level (MSL) for any location. However, the exact land elevation is not known for most regions around the world. Accurate data are particularly scarce in the tropics and it has now emerged that two-thirds of the world’s lowest coastlines are there.

Flooded land on the coast of Bangladesh

Flooded land on the coast of Bangladesh, Adobe Photostock

New, better elevation measurements with the LiDAR satellite

The measurement methods that have been used until now are inaccurate, especially in less accessible areas with dense vegetation, as is common in the tropics. Coastlines that were thought to be well above sea level have now been found to be only two metres, or even just one metre, above MSL. And that is precariously close to current water levels during high tides, or MSL in a few decades. “We have seen the frequent inundation of millions of hectares in tropical Asia over the years,” says Ronald Vernimmen of Data for Sustainability, co-author of the paper. “With the new data this is easier to understand”.

According to the new data, a sea level rise of 1 metre, which some studies have indicated is possible by 2100, would put more than 500,000 square kilometres of land below MSL, the equivalent of 50 times the area of the Netherlands, which is now famously below MSL.

The researchers used elevation data obtained through LiDAR laser technology. Until recently, such data were collected from airplanes, an expensive approach that only richer nations can afford. “But since NASA launched the ICESat-2 satellite in 2018, very accurate land elevation data are becoming available globally,” explains Deltares researcher Maarten Pronk.

Tropical regions in particular

“Now that we have these accurate data, experts and policymakers will be better informed about the locations of the most vulnerable areas and they can focus on those areas,” says Al Hooijer. “Three-quarters of the populations most at risk are in the tropics, especially Southeast Asia”. And the problem is exacerbated there by land subsidence resulting from drainage and groundwater pumping. The researchers therefore advise strengthening support for countries like Indonesia and Bangladesh to prepare for a sea level rise while reducing land subsidence.

Land elevation near sea level in Southeast Asia: left in 2020 and right after 1 metre sea level rise (Figure from Nature Communications paper)

Land elevation near sea level in Southeast Asia: left in 2020 and right after 1 metre sea level rise (Figure from Nature Communications paper)