Gabion stream diversion ENABLES ASH DUMP EXTENSION

A 3 km stream diversion completed by a joint venture between Concor and Lubocon Civils prepares the way for an expansion to the ash disposal facility (ADF) at Eskom’s Kendal power station. The diversion forms part of the JV’s contract to add 65 hectares to the ADF footprint.

The new stream is about 30 m wide and 3.5 m deep, channelling clean water around the ADF area. “It was a massive undertaking, with the installation of over 4 100 gabion baskets and almost 2 000 Reno mattresses,” explains Pierre van Vuuren, contracts manager, Concor.

At 300 mm deep, the wider dimensions of the mattresses make them ideal for creating a stable floor for the stream. In turn, the more square-shaped gabion baskets were used mainly for the stream slopes, especially where faster water flow was expected to cause erosion.

The completed rock-filled structures took approximately 120 workers over 19 months to complete. In the process, more than 42 000 t of rock was required, sourced from local quarries and crushers.

Wetland sustainability

A key concern in terms of the design was to ensure the necessary robustness of the structure, so that it can withstand a 1-in-100-year flood. Furthermore, sustainability was an equally important component to ensure the new stream will be conducive to the natural vegetation of the wetland that it replaces.

“The base of the stream comprises wetland material, which we took from the wetland itself, while the sides are layered with topsoil. This is allowing the rapid revegetation of the stream with reeds and other aquatic life,” says Van Vuuren.

The addition of biodegradable hydromulch blankets on the sides will help to prevent erosion for the first six months after completion, with a seed spray fostering the growth of grass as a ground covering.

Four key sections

To accommodate the topography of the landscape and predicted flow rates of the stream, the diversion comprises four different ‘sections’, each with their own design characteristics. The Type A section is lined with gabion baskets and a biodegradable blanket, prioritising the stability of topsoil to prevent erosion while facilitating the rapid growth of plants. Type B is the flatter portion of the stream, while Type C comprises 17 steps where the stream descends more steeply and water flow accelerates. Type D, similar to Type A, has wetland material at the bottom and topsoil on the sides.

The diversion of the stream has been a central aspect of the ADF expansion at Kendal, requiring the work to be carefully scheduled in accordance with environmental regulations. These rules specified that certain activities on the new ADF area could only proceed once the stream diversion was complete.

“Every aspect of this project is conducted with priority being given to the environmental aspects,” concludes Van Vuuren.