Ponds play a vital role in Liesbeeck River system

ANDREW BENNETT

In 1997 when the River Park office complex in Mowbray was first developed, a series of ponds were constructed along Liesbeek Parkway between the road and the Liesbeek River itself.

The ponds were essentially designed to reduce flooding in the area by capturing excess run-off during winter rains but also pleasingly served to create a vibrant natural green corridor for the area.

An inlet pipe positioned just upstream of the Gloucester Road bridge diverted stormwater to feed the ponds, which, in turn, filtered and released water more gently back into the main river just before it reaches the N2. Unfortunately, due to neglect and pollution, the system quickly began to clog up with silt, debris and litter. The once free-flowing wetland soon became stagnant pools obscured by dense overgrowth, in particular Cape bullrushes (Typha capensis), which choked the life out of the ponds for the last 15 years or so.

Realising the vital role that the ponds play in the health of an urban river system, local non-profit organisation The Friends of the Liesbeek put their river maintenance project team to work unblocking the area. Starting at the beginning of this year, the team did extensive rehabilitative work on the original ponds clearing bullrushes and re-opening a channel to reconnect the isolated ponds. Post-graduate students from the University of Cape Town waded in too, taking water samples and compiling research as part of the initiative.

The rehabilitation has been highly effective according to Kyran Wright, The Friends river maintenance project team manager. “The ponds are now receiving and depositing a significant amount of water from and back into the river. The water quality has improved drastically and birdlife is already starting to flourish again around the ponds – a sure sign of improved biodiversity,” Wright said.

To mark the successful reinvigoration of the ponds, The Friends of the Liesbeek is going with the flow and planting 20 indigenous trees along the system adding to both the ecological integrity and public enjoyment of the area.

Mr Wright continues: “Ponds such as these along the Liesbeek are essential for the management of our urban waterways across the Peninsula. Not only are they nature’s way of protecting us from extreme flood events triggered by climate change but they also provide critical natural habitat in our rapidly expanding built environment and, of course, they improve the quality of the water entering the Salt River, the main tributary, which which flows out to sea at Table Bay.”

The aim of The Friends of the Liesbeek is to transform the ponds alongside the Liesbeek River into what are known as “bio-retention ponds” whereby through ingenious geo- and bio-engineering water is not only strategically diverted but treated too. In this way select species and placement of plants, substrate (eg soil, sand and mulch) and natural obstacles slow, filter and leech the water of pollutants before returning it to the main river.

This is an exciting project and one in which the work can and should be done by everyone in the community.

The first step has been taken but this bio-retention pond system is the only one of its kind so far along the Liesbeek River. If it continues to be successful, the work could be used as a model for other urban rivers in the Western Cape (and across Southern Africa) where both flood water attenuation and water treatment is required.

The Friends of the Liesbeek as well as another local non-profit, Friends of the Rosebank and Mowbray Greenbelt (FROG), will be monitoring and maintaining the ponds going forward to ensure that the previous neglect is not repeated, allowing the ponds to fulfil their original purpose.

* Andrew Bennett is The Friends of the Liesbeek education officer.