Winter rains a mudslide threat following fire

Winter rains a mudslide threat following fire

2 May 2009

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South Africa —

The authorities are watching the denuded slopes of Table Mountain and Devil’s Peak carefully for any signs of a mudslide like those that damaged homes in the Clifton and Vredehoek areas in the 1990s, following devastating mountain fires.

In March a fire raged across the slopes of Table Mountain and Devil’s Peak, consuming large tracts of veld and leaving the mountain devoid of soil-holding vegetation.

Now the bare mountain poses another threat as winter sets in. There is a possibility that heavy rains could cause slides of debris, rocks and branches which could destroy roads and property from the Rhodes Memorial to parts of the CBD.

With winter fast approaching, the SA Weather Service says the Western Cape can expect cold fronts over the next few months.

Mid-term predictions indicate normal rainfall patterns until July but from mid-July to August we could see higher than normal rainfall. This could lead to local flooding.

The last time the city experienced major mudslides was in the winter of 1994 after devastating fires on Devil’s Peak. Two years later similar slides were experienced in Clifton. A less serious mudslide above Muizenberg four years ago closed off Boyes Drive.

In Vredehoek in 1994 timber beams, laid horizontally across river beds and secured with iron stakes, had started to rot and as the rain came down the soil loosened. Then, in the middle of the night tons of sand, mud, branches and timber were swept through properties in the Derry Street area of Vredehoek.

The slide eventually stopped at the Castle and the resulting mess took months to clear.

Gabions, wire mesh baskets filled with stones, are used to control the flow of rivers on the mountain. The gabions are placed in a stepped fashion so instead of water rushing down the slopes, gathering earth-shattering energy, it cascades, lessening the effects if it gets to the urban edge.

The Table Mountain National Park, SANParks and the City of Cape Town are working together to ensure the stability of the mountain slopes.

The city will do the lion’s share of the work on the mountain because city properties will be most affected.

It will spend R400 000 constructing gabions and gabion weirs (where the gabions form a pond which traps the debris).

The city’s regional manager for roads and storm water, Peter Arton-Powell, said they could only “mitigate” against mudslides, not guarantee that mudslides would not happen.

The city and SANparks have a four-step mitigation plan.

They will ensure intake pipes, which channel mountain streams into the sea, remain clear of debris; install the gabion basket weirs across stream beds; put down silt berms using hessian; com-municate with residents on what they can do to safeguard their properties; and ensure that drainage ditches on access roads are cleaned out so that water goes through the underground pipes.

The silt screen berms will ensure that top soil is not washed away, allowing vegetation to take root.

If the intake pipes are not kept clear mud, debris and rocks will back up and end up flowing on to roads, including De Waal Drive, an important access road into and out of the CBD.

Arton-Powell said they were watching the weather carefully and hoping that vegetation would take root in the coming weeks as this would reduce the likelihood of a landslide.

Gabions had proved effective at other places on the mountain. Those above Vredehoek had been installed 14 years ago and were still effective, as were the eight gabion weir ponds scattered on the mountain.

But Arton-Powell warned that the weather was unpredictable. “There are no guarantees, we do not know how hard it will rain.”

Even though it rained heavily last weekend, Arton-Powell said the ground had been so dry it had just soaked up the rain. “We are at the mercy of the weather. If we have mild, steady rain the vegetation will regrow, reducing the threat.

“If there is exceptionally long rain, followed by intense storms, the threat (of a landslide) goes up.”

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