HYDROLOGICAL ENGINEERING A drone image of the completed groynes upstream of the old road to Brandwacht River
BRANDWACHT RIVER STABILISATION PROJECT Alien vegetation, riverbank erosion and large quantities of sediment washing downstream prompted the Brandwacht River stabilisation project. By Hans King, owner of Hans King SRS
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few kilometres from Hartenbos, near George in the Eden District of the Western Cape, lies the Brandwacht River. It flows southwards from the Outeniqua Mountains, and discharges into the Atlantic Sea via the Klein Brak River. The problem Over the past 15 years, there has been a marked increase in the presence of black wattle as well as other species of alien vegetation such as the Spanish reed. The riverbank erosion at sites opposite where the black wattle is growing has increased in severity to the point where the right bank has moved completely to where the left bank was 10 years ago. Many small farmers along the river produce a variety of crops. Not only was agricultural soil being lost on the bank of the river, but large quantities of sediment were washing downstream as a result of the impact of the alien vegetation.
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On-site inspections and historical satellite imagery revealed that the river became ever more degraded as one progressed down its course, largely due to bulldozing and the flourishing of invasive alien vegetation. Due to limited funds, a 3 km stretch was selected for rehabilitation where there was intense alien vegetation growth and serious soil loss and sediment load mobilisation were taking place. It was believed that by reducing the sediment flow down the river from the target area, the whole riparian community downstream would benefit from the work. Objectives The primary objective of the rehabilitation work was to prevent the abnormal movement of sediment down the river (in other words sediment that was mobilised by the ingress of invasive alien vegetation), and therefore protect the agricultural production potential of land downstream of the site.
Rehabilitation work would also: • protect agricultural production potential on the site itself (by preventing the washing away of the riverbanks) • provide work opportunities for unemployed locals during the project, and for the maintenance of the river after the project • protect the natural environment by the removal of alien vegetation and the partial restoration of the natural flora at the worksite • protect local road and bridge infrastructure, which impacts on the local agricultural economy. Interventions A detailed topographical survey of the 3 km long stretch of river was carried out, and a hydrological study of the catchment was done. This information was used to determine flood levels, to design a rehabilitated river channel (along the route that existed about
