A new look at how global changes in river flood are affecting different climates

JBA scientists recently contributed to research by a University of Oxford-led international team, investigating the effects of climate and land cover change on river (fluvial) flooding.

The key finding is that, since the 1970s, 20-year and 50-year extreme river floods have mostly increased in temperate climates but decreased in arid, tropical, polar, and cold climates.

The authors argue that flood hazard assessments should be updated regularly to take into account changes in world climate. More accurate information will help business planning in the re/insurance and other industries.

As lead author Dr Louise Slater comments in an accompanying press release, ‘Our work shows the magnitude and frequency of floods are changing substantially in different climates of the globe. Flood maps need to be regularly updated to keep up with these changes and protect people and their livelihoods.’

Data and methods

The research team obtained 10,093 river gauge records from a combination of global and national streamflow archives extending from before 1980 to after 2007. Comparisons are focused on the 1970s as this decade provides a good compromise for observed streamflow record length and availability worldwide.

Changes in 20-, 50- and 100-year return periods were investigated, using a minimum of 30, 50 and 70 complete years of data, respectively; these record lengths offer a workable trade-off between statistical requirements for robustness and global data availability.

Estimates of flood frequency are most commonly expressed using a return period – for example, a 100-year flood event, which has, on average, a 1% chance of occurring or being exceeded in any given year. However, there is growing acceptance that stationary concepts like a fixed 100-year flood (constant parameters) are too easily misinterpreted, and that flood frequency and magnitude changes dynamically due to climate and land cover changes.

Therefore, the research used an alternative, non-stationary approach (where one or more parameters are dependent on time, rather than being constant). The method was:

  1. Quantify how the magnitudes of flows of a given exceedance probability (e.g. 20-, 50-, and 100-year return period) have changed over the historical record in each stream gauge
  2. Estimate how the return periods of flows of given magnitude (e.g. 20-, 50-, and 100-year flood as assessed in the 1970s) have changed over time, from the 1970s to today
  3. Assess corresponding changes in flood probabilities.

An example in the article demonstrates that at the Kanakee River at Momence, Illinois (USA) a 50-year flood in 1970 had, by 2019, become an 8.5-year flood.

Results

When all the sites analysed are grouped by the five broad Köppen-Geiger climate regions, estimates of change vary notably.

The median change in the 20-year flood magnitude is +21.9% at 1,243 sites in temperate regions, with increases particularly apparent in Atlantic Europe, south-eastern Brazil and south-eastern China.
In contrast, decreasing flood magnitudes are found in arid (median -33.1%), tropical (-25.8%), polar (-23.1%) and cold (-12.5%) regions. Decreases are most visible in north-eastern Brazil, eastern Europe, parts of western USA, and parts of northern China.

The African continent has few publicly accessible streamflow data records, but recent research suggests that floods have mostly been increasing in western and southern Africa since the 1980s.
The spatial patterns of change in different climate zones described in the report are distinct, but with differing flood magnitudes within each area. Causes may vary from decreasing soil moisture or negative rainfall trends to groundwater depletion.

Figure 1: The percentage change in magnitude of 20-, 50- and 100-year floods since the 1970s. Background colours on the map indicate the 5 Köppen-Geiger climate regions (Beck et al., 2018): tropical, arid, temperate, cold, and polar.

The paper moves on from looking at peak river flow magnitudes to a discussion of return periods, derived from nonstationary analysis, which has not previously been common practice, to help describe how extreme river flows are occurring more or less frequently relative to any chosen point in the past. Using the time-varying distribution parameters at each site, the authors estimate the changing return period of the 1970s floods in each year.

Some distinct regional patterns can be observed: decreasing return periods are found in temperate zones (the 20-/50-year flood, as estimated in the 1970s, is now an 8-year to 21-year flood in these areas), while decreasing flood hazard (increasing return periods) is found in the other four climate regions. Caution should be observed here, as lack of availability of data in certain regions means that return periods should be seen as indicative of the general direction of change rather than precise estimates.

Conclusions of the research

For floods with moderate recurrence intervals (e.g. between 20 and 50 years), the paper shows that the majority of increased flood hazard can be found in temperate climate zones, with a decrease in flood hazard in arid, tropical, polar and cold zones. Results for more extreme recurrence intervals, (e.g. 100 years) are slightly different, with decreases in arid and temperate zones, mixed trends in cold zones, and increases at a small sample of tropical sites.

The differences serve as a reminder that regional results depend inherently on site selection and different return periods of interest, as well as on the availability of up-to-date local flood hazard information.

Regular updates of flood mapping and flood assessments are vital not only for research like this into changes in hazard and risk, but also for managing this risk effectively to protect people, property, and livelihoods.

The research also underlines the complex nature of climate change. Although it supports the notion that climate change may already be impacting flooding, and is likely to increase the risk of flood in many areas of the world, it highlights that these changes will not be uniform and will need detailed, local response to the hazard.

Latest global data

At JBA we regularly update our flood data at high resolution to support effective, localised flood risk assessment. We provide annual updates to our UK flood data and follow a comprehensive update plan for our data more globally, with recent updates to our Canada and Australia data.

Our global probabilistic models and event sets can help clients understand the geographical and temporal variations in flood risk, supported by our newly released Climate Change Analytics, which can demonstrate this geographical variation with estimates of areas that may experience more or less flooding under future scenarios.

Read the original research letter here in Geophysical Research Letters: Global Changes in 20‐Year, 50‐Year, and 100‐Year River Floods - Slater - 2021 - Geophysical Research Letters - Wiley Online Library

For more information on the research or our work in flood risk modelling, get in touch using the form below.

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