SA Affordable Housing November - December 2018 // Issue: 73 - Page 6

NEWS stormwater surveys undertaken by Graham McGill of Graham McGill Consulting. The channel forms part of a system which has been designed to ensure that stormwater flows are managed and dispensed in a controlled manner. Several tributaries and stormwater pipes which previously flowed into several evaporation ponds, situated on the land earmarked for the new housing estate, now drain into the main channel, Channel 1 and into two tributary channels 2 and 3. The unlined section of the main channel was designed to accommodate a 1:100 year flood (50.2m³/s) whereas the lined section can accommodate a 1:5 (18m³/s) year flood. The deployment of the blocks and mini gabion dams, which were built into the downstream section of Channel 1, reduce flow-rates and allow the SUDS system to better filtrate any runoff. The other major component of the system is a large attenuation pond which was excavated at the bottom end of Channel 1. 1 The pond further reduces the flow rate and prevents surges by releasing water gradually through a 450mm outlet pipe at the far end of the pond. However, when water volumes are such that the pond fills to capacity, the overflow pushes back into the main channel at the entrance to the pond. In addition, a secondary and smaller attenuation pond has been excavated immediately above some culverts which have been installed under a main tarred road adjacent to the estate. Grass has been planted on the floors of both ponds to minimize dust during dry periods and encourage water infiltration. The main channel is composed of four material layers: the sub-soil which was compacted with rollers to 93% MOD AASHTO; a layer of Grade A2 geofabric, which prevent the blocks from settling too much and allows filtration while preventing the washout of locally sourced bedding sand (20-30mm); and the grass blocks. The blocks are being filled with a bio-charcoal material and mushroom spores, the former to absorb pollutants in the water and the latter to recharge the charcoal and keep it fit for purpose. Specific plants are currently being grown in a nursery to be planted in and around the blocks, these are chosen for their water purification properties, as the storm water from this channel ultimately ends in the Berg river. The channel banks above the blocks are planted with grass. The main channel, the bulk earthworks, including the attenuation ponds and other channels, were constructed by JVZ Construction. 1. The mini gabion dams also allow the system to better filtrate any runoff. 2. The completed channel was finished off by planting specific plants that contribute to water purification. 2 4 NOVEMBER - DECEMBER 2018