INNOVATIVE REHABILITATION OF A DEEP GULLEY IN DISPERSIVE SOIL
Vergelegen Wines in Somerset West are very proud of their farm’s heritage and take great care of the environment. Amongst other things, at their own cost they have cleared more than 2000 hectares of alien vegetation to create a fynbos reserve.
THE DEVELOPMENT OF THE GULLEY
After the 2013 storms which flooded the Vergelegen Mediclinic Hospital, the appearance of a small gulley on the farm caused them concern. From 2017 on this gulley suddenly accelerated in size as the dispersive weathered Cape Granite soils below were exposed and eroded rapidly. Between 2017 and 2018 the gulley increased from 30 m long to 400 m long, and from 2m to 6m deep. Downstream from the gulley an area of Lourens River fynbos was smothered by the sediment.
LEGISLATIVE CHALLENGES FACED BY THE PROJECT
Plans for rehabilitation using cascades of weirs and other measures were prepared in early 2018 and the lengthy environmental approval process was started. An attempt was made to recognized the gulley as a disaster and shorten the approval process, but because the damage began in 2013 this was not allowed. The approval process took close on a year and in this time the gulley grew even longer and deeper.
TECHNICAL CHALLENGES TO THE PROJECT
The first challenge to the design was the wetland specialist’s requirement to ensure that the natural water table in the area had to be restored as close to it’s original level as possible, so that natural vegetation in the areas adjacent to the gulley would not be negatively impacted on by the drying out of the soil. The implication of this was that the floor of the rehabilitated gulley had to be as close to the original ground level as possible, and this necessitated the construction of weirs 5-7 m high, a potentially very costly exercise
A serious challenge to the stabilization of the gulley was the nature of the sub-soil. Being highly dispersive, water flowing over it 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 soil. 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.
At the design stage there was an understanding that the depth of the gulley may well increase 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 IMPLEMENTED
A cascade of 7 gabion weirs was planned to reduce the slope of the gulley from 6% to around 1,5% and so reduce the flow velocity. Between the weirs the gulley was widened to 4 m wide as a primary measure to reduce the flow velocity in the channel. To further increase the hydraulic surface roughness (and reduce flow velocities), it was planned to cover the channel with an assortment of indigenous wetland plants. These plants would have added benefit of binding 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 weirs was first filled with cement stabilized soil, so that most of the weir spillways only had to be 1m 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 un-affordable. This cement stabilized 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 make sure that the topsoil layer spread over the weathered granite base was not washed away before the vegetation could be properly established, a 150 mm thick Kaytech Multicell was placed over the formed channel and the topsoil was placed into the Multicell. A trial on site conducted prior to the finalization of the design proved that the Multicell retained more than 90% of the soil placed in it when subjected to flow velocities of 2m/s.
To achieve a maximum support to the water table, the level of the weirs were 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 and 2 m below the natural ground level.
Client: Vergelegen Wines (Pty) Ltd
Project design and management: Hans King SRS (Pty) Ltd
Construction: Martin and East
Year of construction : 2021
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