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State of the Coast: KZN
State of the Coast: KZN

State of the Coast: KwaZulu-Natal Chapter 1: Introduction & Background Chapter 2: Overarching Drivers Chapter 3: Climate Variability
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28 Jul 2016

A river ran through it

Colin Eriksen (South Coast Sun)

Photo Credit: Emanuel Maria

“It is all very well adding on toilets, shopping centres and roads, but the hidden infrastructure needs serious attention“

The innocuous little Umbogavango stream became a raging torrent on Tuesday evening, 26 July and caused extensive damage to the main railway line close to Toti station.

The stream has two tributaries, one originating in the marshlands near Kissin Lane above Amanzimtoti Bird Park, and the other in Umbogavango Nature Reserve between the Southgate and AECI complex and Galleria Mall.

The 235mm of rainfall experienced in Toti over the 48 hours leading up to the 26 July flooding needs some historical perspective.

Photo Credit: Emanuel Maria

 

The Natal floods of September 1987, where 327 people were killed, were proclaimed as one of the worst natural disasters to hit South Africa. Some areas of the Upper South Coast received as much as 900mm of rain in four days.

During cyclone Domoina in 1984, up to 600mm of rain fell over four days in the Toti area.

In June 2008 the area experienced about 250mm of rain in 24 hours.

None of the three extreme historical storm events alluded to above resulted in the Toti railway station platform being covered in two feet of water. So what has changed?

Vast tracts of natural bush and grassland have made way for Southgate and Galleria. Rainwater that previously infiltrated the soil now runs off impermeable surfaces into stormwater drains. Water from a storm event, which would have moved underground through soil and rock, taking many days to reach a river channel, now reaches a stream within minutes.

It seems that rapid surface run-off from Galleria, Southgate and associated infrastructure has now exceeded the capacity of the canals and tunnels built under the CBD to safely carry Umbogavango stream water into the Amanzimtoti River.

The Umbogavango stream enters the river via the enclosed canal under the taxi rank. 

Above: Toti CBD. Photo Credit: Iain Pike 

 

Galleria and Southgate may have been the final straw in a problem that has been brewing for some time.

In the early 1970s, half of the large tract of wetland between what is now the Adams Road intersection in the south and Amanzimtoti Bird Park in the north was drained to make way for Kuswag and Kingsway high schools.

A canal was built alongside to speed up the movement of water away from both school properties. Speeding up water flow is a recipe for a flood.

A swamp acts like a sponge, trapping floodwaters that enter it and releasing that water slowly. It is for this reason that streams with healthy wetlands rarely flood.

Only a small portion of the original swampland now still exists between Kuswag Skool and the N2 at Adams Road. This part of the wetland through which the Umbogavango stream flows is now heavily silted by fine sand eroded during construction of industrial and commercial properties to the north.

Above: Baggies Beach. Photo Credit: Carol Lane 

 

As the Umbogavango stream approaches the tunnel built to channel it under the N2, it meanders within three metres of the railway line. Deposition of sediment has raised the bed of the Umbogavango stream at this point so that it is only a metre or so below the level of the tracks.

On Tuesday evening the tunnel built to accommodate the Umbogavango stream under the N2 proved far too small, the water chose the larger two railway tunnels instead and followed the walled-in section of railway line directly through the station into Toti River.

If the water had not found this alternate route, it almost certainly would have undermined the N2 embankment. Damage to the embankment is evident on the south side of the river tunnel under the N2.

Traditionally the opening of the main Amanzimtoti Lagoon has mitigated flooding of the CBD area, as it reduced the volume of water backing up in the drainage canals. This event, however was different as there is evidence of violent flow in the Umbogavango stream north of the N2 more than three metres above mean sea level.

One shudders to think what another 1987 storm system would do. It is all very well adding on toilets, shopping centres and roads, but the hidden infrastructure needs serious attention.

 

Online Article