Between 1980 and 2016, flooding caused an estimated $1 trillion in damage and 215,000 deaths worldwide, with populations centred around river deltas and estuaries suffering particularly heavy losses. Now a team has found that in some areas raised sea-level and increased river discharge, which together can produce severe floods, are more strongly dependent on each other than previously thought.
“Many of the large and disastrous floods in deltas and estuaries are the result of a combination of coastal floods – for example, caused by storm surges – and floods from rivers and rainfall,” says Philip Ward at Vrije University Amsterdam in The Netherlands. “When they interact, the floods can be more severe than when they occur in isolation. These are examples of so-called ‘compound events’.”
With climate change set to bring higher sea levels and heavier rainfall, the threat of flooding is likely to increase in the coming decades. “Deltas and estuaries are home to a large proportion of the global population, and the number of people is expected to grow rapidly in the 21st century,” says Ward. “At the same time, they are located in areas that are prone to flooding from the sea, rivers, and rainfall. It is therefore essential that we understand how these phenomena interact, in order to improve planning of flood risk reduction and mitigation measures.”
Ward believes this will require a greater knowledge of the combinations of natural events that cause the most severe seaside floods. He and colleagues used statistical analysis of flooding events in deltas and estuaries, aiming to ascertain a relationship between coastal and river flooding for the first time.
The calculations were based on sea-level data from the Global Extreme Sea Level Analysis (GESLA-2) project, combined with river discharge data from the Global Runoff Data Centre, gathered at 187 coastal locations around the world.
“Our paper shows that there is a strong statistical dependence between river and coastal floods in many locations all around the globe,” Ward says. High sea-levels and high river discharge showed significant dependence at more than half of the measurement station combinations.
The researchers argue that their results reveal a need for an update of flooding risk assessments so that they account correctly for compound events. “By ignoring this dependence, the chance of a really large flood occurring in a delta or estuary can be underestimated, which means that flood defence infrastructure could be under-designed,” says Ward. “By getting a better understanding of the dependence between coastal and river floods, we can improve policies and measures to reduce the impacts of floods.”
Interventions such as river channelling, reforestation, and adapting agricultural practices to increase water retention in soils could greatly reduce the risk of flooding.
Ward and colleagues reported their findings in Environmental Research Letters (ERL).