We were approached to assist in the design of water attenuation structures for a river course in a new golf estate to be built near Alma in the Limpopo province. The site is to be developed into an 18 hole golf course and estate with game and lodges. Gabions and mattresses had already been used on-site, but due to poor design and implementation, excessive soil erosion occurred due to heavy of rainfall earlier year.
We visited the site with Reg Laskey from Littlecreek Civils in February and undertook the preliminary design proposals and pricing to complete the project. We were also asked to work with Mr. Helmut Keller, a hydraulic design engineer with SKC Engineers in Pretoria, who’d done the design water flows and had been of technical assistance to the client previously. We met with Helmut and he recommended some modifications to our designs to allow for further protection of the watercourse. The on-site gabions amounted to 1000m³ of galvanised materials, with extra mattress works for repairing and repacking existing mattresses not laid correctly.
Contractors often try to level off the mattress tops with smaller flat rock to achieve a flat level surface on the top of the mattresses. This allows strong water flows to remove fine rock fragments up through the mesh openings and the partial emptying of the mattresses. We found that some oversized rock placed in the mattresses had also been used and this had to be removed and replaced. Normally rock of 100mm min. size to 2/3 of mattress depth maximum size is required for this purpose.
14 Trained and 9 untrained labourers were on site for the duration of the works. Reg used the unskilled labour mainly for the collecting, passing and filling of the baskets. The skilled installers were used to erect the baskets, lace, brace and close them and also for the correct packing on the external front faces – this being the most important task to ensure great looking gabions. On-site rock was mostly used to fill the baskets to allow for reduced costs and the blending in of the rock color with the local soil on-site.
In many cases, no cut-offs were installed on the upstream ends of the mattress protections. We designed gabion weirs at specific points along the river to reduce water velocities and encourage accretion of finer river bed materials upstream of each structure. The water hydraulic energy was now focused at specific points where adequate mattress protection was designed and placed. The mattresses were placed at identified soil erosion weak spots.
A 20-30Mpa concrete 100mm thick capping was placed on all weir crests and the first two metres of the stilling basins downstream, to prevent debris damaging mesh lids during flash flood situations.
The Matkovitch Group, a landscaping and golf course erection company, was very happy with the works and like using gabions on their golf course projects due to the environmental benefits of the system. They were also working on the Nondela golfcourse in the Natal Drakensberg where we’ve supplied WBHO Construction with many gabion materials up until now.