Grassing of Hout Bay denuded Sandy Bay
THE city council has created a Frankenstein’s monster by planting grass on the Hout Bay dunes to stabilise them, according to one of South Africa’s top dune scientists. After a decade of being grassed, not only have these beach dunes grown enormously, but they have “spawned” a second set of dunes behind them which are gobbling up some buildings and the parking lot.
Now the council plans to truck away 30 000m3 of sand forming the back dunes in a bid to have the sand blown down to cover Sandy Bay.
This is enough sand to cover a rugby field 4.3m deep. It will take six trucks working every weekday for two months to move the sand. It will help for a time, geomorphologist Peter Holmes, one of the country’s eminent dune scientists, said yesterday, but it is likely to build up again. What has happened with the Hout Bay dunes is an example of what can happen when humans change a dynamic, sensitive system.
“Once Hout Bay was established, ratepayers expected to have the sand kept out of their properties. They didn’t like sand in their G&Ts, so the council planted grass on the foredunes to stabilise them. This created a Frankenstein’s monster. If left alone the foredunes would have been eroded by wind and waves, but now they have grown, and you have a secondary set of dunes. These back dunes are what they want to cart away, but they will come back again.”
Until about the 1940s there was a mobile dunefield that stretched from Hout Bay across to Sandy Bay. In summer, when the sand was dry, the south-easter would blow it across from Hout Bay to Sandy Bay, in this way building up the beach at Sandy Bay. Then people began building on the Hout Bay dunes, and introduced alien rooikrans. Both of these interfered with the movement of sand in the dunefield, meaning there was less sand going to Sandy Bay. This little beach is slowly becoming denuded.
Once the council grassed the Hout Bay beach dunes, they trapped more sand, and the beach dunes began growing.
“There are thresholds in geomorphology, and when you pass these thresholds, things can suddenly behave in a very different way. We may have reached the stage that the dunes are so high they begin to alter the wind regime off the beach. The physics behind it is quite complex, and we don’t know yet what is happening there. My hypothesis is we could get more dunes forming along the road, and what we are doing is recreating a dune field.”
Gregg Oelofse, head of strategy and policy at the council’s environmental department, agreed that the dune system in Hout Bay had been extensively altered so the natural system no longer functioned.
“This is one of the implications of urban development: they could not have developed Hout Bay and retained the dune system. It’s a bit like spilt milk: what we have to do now is to look at how we can best manage it. Some say leave it and let the sand take over, but that’s neither possible nor practical.
“Also, these are lessons learned. Now we know that we must not prioritise coastal development at all costs, but we need to be much more considered,” Oelofse said.