Civil Engineering August 2021 | Vol 29 No 7

Civil Engineering August 2021 11 5 m to 7 m high weirs – a potentially very costly exercise. Next, the nature of the sub-soil posed a challenge to the stabilisation of the gulley. Being highly dispersive, water flowing over the soil erodes it very quickly. To address this, steps had to be taken to make sure that water did not flow over the exposed decomposed granite. In addition, the flow velocity of water had to be kept low so that the topsoil placed on top of the weathered granite was unlikely to be washed away. Yet another challenge was that, at the design stage, there was an understanding that the depth of the gulley may well in- crease by the time the necessary approvals had been obtained and a contractor was finally on site. The design had to be adaptable so that it could accommodate changes to the gulley shape. THE SOLUTION A cascade of seven gabion weirs was planned to reduce the slope of the gulley from 6% to around 1.5%, thereby reducing the flow velocity. Between the weirs, the gulley was widened to 4 m as a primary measure to reduce the flow velocity in the channel. To further increase the hydraulic surface roughness (and reduce flow velocities), the team planned to cover the channel with an assortment of indigenous wetland plants which would bind the soil with their roots. Some novel ideas were implemented with the design of this project. To drastically lower the construction cost of the gabion weirs, the gulley under the footprint of the weirs was first filled with well-compacted cement-stabilised soil so that most of the weir spillways only had to be 1 m high. Traditionally, the weirs would have been constructed from the base of the gulley floor and the cost of such structures would have made the project unaffordable. This cement-stabilised soil had the added advantage of being able to adapt to the shape of the gulley, which a built structure could not. Trials were done using the decomposed granite and varying cement concentrations to determine a mix suitable for the project. To help stabilise the topsoil spread over the decomposed granite until it could be covered with vegetation, a 150 mm thick Kaytech Multi-Cell was placed over the formed channel and the topsoil was placed into the Multi-Cell. This deci- sion was supported by a trial section of Multi-Cell on site conducted prior to the finalisation of the design. To meet the requirement for the design to provide maximum support to the water table, the level of the weirs was planned such that there was just enough clearance between the spillways and the natural ground level for the expected floods to remain in the depression. This resulted in the depression between the weirs remaining between 1 m and 2 m below the natural ground level. Since the completion of construction in April 2021, severe storms have been en- dured with limited ongoing maintenance. Where the vegetation has established well, no further soil erosion has been experienced. This project highlights how engineered structures in conjunction with the soil stabilising properties of natural vegetation can be used to provide solu- tions for environmental problems.  Placing topsoil over the Multi-Cell Weir 5 after heavy rain, two months after the vegetation was planted The rehabilitated gulley