Return Periods – Are They Still Useful For Floods?
By Yuanchao Xu 18 August, 2020
CWR's Xu breaks down how flood return periods are calculated & shares why they are definitely still useful
A return period refers to the repeat interval between extreme weather events such as earthquakes, floods etc. It is also used to measure the intensity of such events (e.g., the water level when a flood occurs): the longer the return period, the more intensive it is. A flood with a 50-year return period means the flood occurs every 50 years. We show how return periods are calculated below. However, in reality such events are usually not on time and in fact are happening at a faster and more intense rate due to climate change. So, are they still useful?
First, let’s look at how to calculate a return period. According to the China Meteorological Administration, a return period is calculated based on the following equation:
For example: Assuming we have a sample of flood intensity of five years, respectively 5, 6, 8, 3, 2. Here is how we calculate the return period for an event with a flood intensity 6.
As the result shows, the return period is 3 years. However, as time passes by more records will be added to the sample. Assuming the flood intensity for the next two years is 9 and 7 respectively. The new calculation for an event with a flood intensity 6 is:
For the same event, the return period is shortened as more intensive events are recorded. In other words, for the same return period, the event intensity becomes higher. In reality, we usually have decades of records, but the pattern is the same. Unfortunately, recent facts show that we are moving to the more intensive direction.
Recent facts show that floods are more frequent & more intense…
In 2018, the floods in upper reaches of Yellow river and Yangtze river broke 13 rivers’ record water level. Typhoon Mangkhut in the same year also broke the intensity record of typhoons in Hong Kong. And most recently, in June 2020, Southeast China has suffered again serious floods. 433 rivers are above dangerous levels and already more than 33.85 million people have been affected across 27 provinces & regions. Global water gurus, Biswas & Tortajada look at China’s floods here.
It is likely that more events will return earlier than expected if the climate emergency continues as is. However, this is not a simple “see you earlier”. It brings new challenges to existing infrastructures. As more intensive events are recorded, the intensity of the same return period becomes higher. A dam/seawall designed for floods with a 50-year return period today may not be able to stop floods with the same return period in 20 years.
…still, return periods are useful as it’s more than just a time
So, while floods are happening more frequently and more intensely, return periods are still useful. Actually, they are very important for engineering. For example, a flood with a 50-year return period will result in the water level to be 5 meters high and thus dams or other infrastructure will be built to withstand a 5m rise. A return period is so much more than just time.
Climate change will make this worse & bring more water
Apart from these unexpected climate fluctuations, the baseline projection is also showing that climate change will bring us more water. According to CWR’s latest report Yangtze Water Risks, Hotspots & Growth, projections of the entire Yangtze river basin indicate a hotter future with more rain. The fluctuations based on this trend will become even more intensive and more likely to break current records.
In recent years many actions have been taken to deal with climate change as the world tries to achieve a less than two degree target by the end of this century. However, while these actions may be a solution for the rising temperature, they may not be that effective for extreme flood events. The black swan theory basically tells us that we are exposed to extreme outliers all the time and there is no way to prevent them from happening. But holistic adaptation plans and actions can help us be more prepared when it occurs.
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