Severe climate, rain and flooding are on the forefront of the minds of many South Africans, particularly these in KwaZulu-Natal. Early final week (11-12 April 2022), the province’s coast obtained heavy rain, with some areas recording over 300mm in 24 hours. This is a couple of third of the annual rainfall in KwaZulu-Natal.
The rain was brought on by a robust cut-off low climate system off the east coast of southern Africa. Cut-off lows incessantly happen off this coast in the course of the autumn months. These
programs could cause localised flooding in addition to massive wave occasions.
The port metropolis of Durban (within the municipality of eThekwini) has skilled flooding occasions virtually yearly since 2016. Storms liable for these floods usually dropped 100mm-150mm of rain in 24 hours, in keeping with the eThekwini knowledge portal.
Between 11-12 April 2022, a storm system dropped much more rain than that. It precipitated devastating floods, landslides, and lack of life. It is pure to ask whether or not the current floods within the space will happen extra typically inside the context of local weather change or whether or not they had been merely freak occasions. It just isn’t attainable to state conclusively that this storm was a results of local weather change. However, scientists consider a majority of these intense climate programs will happen extra incessantly within the future.
Perhaps extra pertinent is the query: did anybody see this coming? There isn’t a easy reply. It’s one thing that has been difficult scientists and engineers around the globe for many years. For instance scientists like myself have been making an attempt to use the rules of fluid mechanics to handle a majority of these questions.
This is vital as a result of if we are able to predict the incidence of an occasion, we are able to arm catastrophe administration groups with life-saving info.
Predicting flooding just isn’t new and consists of two strategies. The first depends on historic rainfall and flood data that come from climate stations and river gauges. South Africa has many of those stations all through the nation. It is significant that these stations are correctly maintained.
The second methodology entails pc modelling. It is troublesome to foretell the place rain will fall, how a lot will fall and whether or not this may trigger flooding. These processes depend upon spatial gradients that aren’t resolved in regional local weather fashions. For instance wind strikes from excessive stress to low stress, typically bringing with it rain. However the wind velocity (and the way rapidly the rain arrives) relies on the distinction between the excessive and low stress. This is usually very troublesome to mannequin precisely. Advances in pc energy will possible have the ability to tackle this sooner or later.
Predicting the place the water will movement as soon as it reaches the bottom can also be difficult. Some water infiltrates the soil and flows as groundwater, whereas some water runs alongside the floor (referred to as floor runoff). Both the floor and groundwater runoff contribute to the water flowing in rivers. If the bottom is saturated, there may be much less infiltration and extra floor water will movement into rivers, inflicting extra flooding. Increased floor runoff additionally contributes to landslides and erosion. Hardened impermeable surfaces in cities and residential areas additionally trigger rising floor runoff. Storm length is one other issue that may affect flooding.
All these components can mix to drive important flood occasions.
How do engineers and metropolis managers develop plans to reply in real-time to those occasions? A attainable reply lies in growing forecast early warning programs. There are good examples within the Netherlands.
Forecast early warning system
The eThekwini coastal, stormwater and catchment administration division has developed a forecast early warning system along side the University of KwaZulu-Natal and the Dutch engineering analysis institute Deltares. The design incorporates climate forecasting, flood prediction and coastal modelling. The aim of the system is to alert authorities to extreme climate to offer them with temporal and spatial info to information determination making. An instance of a system like that is the Global Storm Surge Information System.
The division’s engineers and managers have deployed a whole lot of rain, climate, water stage and coastal monitoring gauges all through the area that present authorities with real-time info. The system continues to be in a developmental stage and requires useful resource funding from nationwide authorities, South African universities and native authorities.
The early warning system works by downscaling output from international modelling centres such because the European Centre for Medium-Range Weather Forecasts and National Centres for Environmental Prediction. This knowledge is fed into numerical fashions that predict flooding and coastal processes at regional and native scales.
The European Centre mannequin did predict extreme rain, about three days earlier than the flood. The National Centres mannequin didn’t. This highlights the difficulties in predicting climate and any determination making that follows.
Observations from the current flood occasion recommend there may be nonetheless a great distance to enhance and additional develop the system. This will depend on improved knowledge sharing between authorities departments, universities and communities. Currently this isn’t simple in South Africa, for coverage causes. Furthermore, authorities and native authorities should put money into metropolis engineering workers and technical decision-makers.
Lastly, even when the forecast system works, African cities of the long run want important “on the ground” assist within the type of catastrophe administration groups akin to police, rescue employees, paramedics and locations of shelter.