As Durban and KwaZulu-Natal salvage through the destruction of last week’s devastating floods, many have been left wondering what actually causes such unprecedented flooding and loss of life? I will try to explain some of the basics of what flooding is.
The US National Oceanic & Atmospheric Administration explains that severe flooding is caused by atmospheric conditions that lead to heavy rain or the rapid melting of snow and ice. The type of geography can also make an area more likely to flood. For example, areas near rivers and cities are often at risk of flooding.
In basic terms, flooding is an overflowing of water onto land that is usually dry. Floods can happen during heavy rains, when ocean waves come on shore, when snow melts quickly or when dams or levees break.
Damaging flooding can occur with only a few centimetres of water or it may cover a house to the rooftop. Floods can occur within minutes or over a prolonged period such as with the KwaZulu-Natal floods, and may last for days, weeks or longer. Floods are the most common and widespread of all weather-related natural disasters.
Although rare in tropical and subtropical regions, flash floods are the most dangerous kind of floods, because they combine the destructive power of a flood with incredible speed. Flash floods occur when heavy rainfall exceeds the ability of the ground to absorb it.
They also occur when water fills normally dry streams or non-perennial rivers or enough water accumulates for streams to burst their banks, causing rapid rises of water in a short space of time. They can happen within minutes of the causative rainfall, limiting the time available to warn and protect the public.
A few instances of flash flooding occurred in KZN when the heavy deluge caused bloated rivers to burst, and eroded away the river banks. This caused low-lying areas to be almost instantaneously flooded, affecting many communities living along river banks and on floodplains.
Densely populated areas are at a high risk for floods. Buildings, highways, driveways, and parking lots increase surface run-off by reducing the amount of rain absorbed by the ground. This run-off increases the potential for flash flooding.
Sometimes, rivers through cities and towns are routed underground into storm drains. During heavy rain, storm drains can become overwhelmed or blocked by debris and flood nearby roads and buildings.
Needless to say, areas close to rivers are at high risk and almost definitely will be impacted by flooding. Embankments, known as levees or canals are often built along rivers and used to prevent high water from flooding bordering land.
The Mondi/Sapref Canal, which burst during the 1987 floods, was built in the early 1950s to divert the uMlazi river away from the site on which the then Louise Botha International Airport was to be built.
Mountains and hilly terrain, a distinct characteristic of Durban and KZN, produce rapid run-off, which causes rivers to rise quickly. Rocks and shallow clay soils do not allow much water to infiltrate into the ground.
Saturated soils can also lead to rapid flash flooding, with the South African Weather Service warning that even 15mm of rain over the weekend can bring about flood-causing conditions.
Sometimes the thunderstorms that produce the heavy rainfall can occur well upstream from the impacted area, making it difficult to recognise a dangerous situation.