Flood by numbers
Today we report on two major flooding events that have occurred in recent days, and look at how the rainfall numbers compare to what we have seen in Eswatini.
In China, in Henan province earlier this week, widespread heavy rain affected several cities. At least 25 deaths have been confirmed. In some places more than half a metre of rain ( 552mm) fell in 24 hours, while 640mm fell in a three day period. One of the biggest single downpours saw 202mm in one hour on Tuesday afternoon.
To compare this to local experiences, during Cyclone Demoina in 1984, 615mm of rain fell on Piggs Peak in 24 hours, with around 900mm over a few days. Cyclone Eloise in January this year brought 300mm to Mbabane in a 24 hour period, while Siteki recorded the most at around 400mm. The most I have recorded from a single afternoon storm in Pine Valley was 120mm, at which point the rain
gauge overflowed.
One of the biggest factors in whether rainfall turns into a flooding disaster is city design, and specifically, the ground’s absorption capacity and the stormwater drainage design. If the water has somewhere to go, like sink into the ground or be channelled into streams, then the potential for flooding is greatly reduced. This is why when Eswatini experienced these high levels of rain, comparatively few people died, and those who did mostly lost their lives crossing flooded waterways.
In city areas though, with asphalt and concrete, the rainwater cannot sink into the impermeable ground. Cities rely on stormwater drains to have the capacity to channel water away. But such drains can be clogged with litter, or simply of inadequate depth to cater for sudden steep rises in water run- off levels, and this is what leads to flooding of roads and buildings.
Six hundred millimetres of rain on Piggs Peak is therefore very different to 600mm on the built- up Chinese city of Zhengzhou.
This week in various Chinese cities stormwater runoff channels were overwhelmed. In some areas, the water drained into the subway system, reaching chest height. People were trapped inside train carriages while rescue services took to cutting off the roofs of the compartments in order to free people. At least 12 deaths have been confirmed in the subways.
Meanwhile in Germany, the death toll from recent flooding has reached 171, with another 155 people still unaccounted for. Services such as electricity, drinking water and gas have been interrupted for thousands of people.
But looking at the actual rainfall numbers, and I can find not a single record from any location that surpasses what Mbabane received in January this year. The heavy rains and flooding affected 12 European countries, bringing a total death count thus far of 209. The highest rain record seems to come from Belgium, where one station marked 271mm of rain. But most rainfall records across these countries, including Germany which was the worst- hit in terms of deaths, show “only” 100mm to 150mm of rain. This is barely half what we received. A part of London reported “severe flooding” after just 47mm of rain, a typical amount Eswatini receives in a usual summer highveld storm.
Some people point to the topography of German valleys for an explanation of the high death count. Many are blaming climate change. Too few are looking at the city design and the need for green spaces to be incorporated into cities.