How geology put a South African metropolis vulnerable to landslides

Some articles have urged the catastrophes in Durban and the better eThekwini area of South Africa following latest floods are because of local weather change and maladministration. While these components play a task, the truth that landslides occurred comes as no shock, contemplating the geology of the realm.

eThekwini is a coastal metropolis characterised by hilly terrain dissected by a number of main rivers such because the Umgeni, Mlazi and Mbokodweni. The area is subtropical, however latest rains had been unprecedented relative to latest information and resulted in a number of landslides.

Some of the earliest research of landslides on this area date to the Thirties. Mitigation measures together with zoning laws are in place for some components of the area. But data of the geotechnical dangers of the realm has not prevented one other tragic catastrophe.

As a geotechnical engineer I examine the consequences of water on the soundness of sloping floor. Many pure slopes are in a fragile state of equilibrium: the downward motion of the inclined soil is simply balanced by the resisting energy between the soil grains.

When water is launched, the water stress pushes soil particles aside, lowering this energy. Also some soils, corresponding to clays, are extra slippery and current a good better landslide threat.

In eThekwini, sloping floor, water and clay mixed to go away devastation of their wake.

Greater eThekwini geology

Ground adjoining to the ocean from Durban to Mtunzini (a coastal city 140km north of Durban) is sort of completely made up of historic pink sand dunes termed the Berea formation. South of the Durban harbour these sands type a ridge referred to as the Bluff and north of the harbour they type the Berea Ridge. In some locations these sand dunes are extraordinarily steep.

The sands of the Berea formation had been topic to intense investigation within the late Nineteen Fifties following landslides alongside the Bluff. The energy of the Berea sands was not uncharacteristic for sandy materials. But the Bluff slopes had been steeper than can be anticipated based mostly on the soil’s energy. The solely motive the slopes hadn’t collapsed was that plant progress was reinforcing the soil.

The investigation confirmed the slopes’ stability was not considerably affected by rainfall. That is smart as these slopes have been battered by storms over geological time. But concentrated flows from poorly managed flood water or damaged water pipes had been discovered to be catastrophic.

The motive is that because the Berea sands have little to no clayey stickiness, or plasticity, they’re extraordinarily vulnerable to erosion. When giant torrents of water stream over the sands they merely erode to type gulleys. The video beneath clearly demonstrates the ability of water over these free sands.

Umdloti has been devastated by heavy rainfall, mudslides and landslides.

Inland shales

Inland from Durban are shales of the Pietermaritzburg formation. They dip down from their most important publicity within the inland metropolis of Pietermaritzburg in direction of the ocean at Durban.

These shales had been shaped from skinny layers of clay and silt deposited in slow-moving water our bodies through the geological previous. When uncovered on the floor, the shales break down or climate. Depending on their mineral content material, some layers climate to very slippery clay.

It solely takes a small improve in water pressures alongside these layers to set off a landslide. The shales are usually not very permeable, so it will probably take a very long time for the water stress to turn out to be excessive sufficient to trigger instability. It might ultimately occur because of seepage from leaking pipes or lengthy wet seasons.

The residential space of Clare Estate, which witnessed landslides through the latest flooding, is constructed on these shales. Tragically this isn’t the primary time landslides have occurred right here. Slides in these shales have been investigated for the reason that Thirties.

Natal group sandstones

Another a part of the geology of eThekwini are the sandstones of the Natal group. These sandstones climate right into a sandy materials however there are additionally layers containing clay. While water can simply stream into the sandy layers it will probably turn out to be trapped by the clayey layers.

During heavy rainfall occasions this may trigger extra water stress, liquefying the sand so it runs like water. A evaluate of landslides within the eThekwini area between 1971 and 1991 confirmed that when brief rainfall occasions exceeded 20% of imply annual rainfall, landslides may very well be anticipated.

During the latest floods, over 300mm fell over 4 days (Sept. 11 April 2022), equating to 30% of imply annual rainfall. This clearly exceeded the landslide threshold. The scenario was made worse by the storms coming on the finish of the wet season when the bottom was already saturated.

Sadly, the homes of the KwaNdengezi settlement close to Durban had been no match for the forces of nature unleashed by the deluge of water over these three days. Some homes had been swept away as the bottom gave means and others had been just too near flood traces to face up to the surging water.

Mitigation measures

Landslides within the eThekwini area ought to come as no shock. We know which areas are in danger and why. Zoning and early warning techniques would appear to be logical measures to take.

The eThekwini municipality does have by-laws in place for some dangerous areas just like the Bluff and it does publish maps highlighting areas of threat. The latest landslides counsel extra work is required to totally zone the area and to implement by-laws.

Landslide mitigation is all the time troublesome, although. Landslide zoning has met with fierce political resistance in Japan and the US as land devalues if marked as dangerous. Landslide prediction techniques are additionally pricey to implement and might warn residents too late.

Anywhere on the planet, the technical issues surrounding landslide mitigation are sometimes the least difficult to resolve. The actual issues sometimes lie with social and political points which require appreciable finesse and thought to resolve.The Conversation

Charles MacRobert, Senior lecturer, Stellenbosch University

This article is republished from The Conversation underneath a Creative Commons license. Read the unique article.

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