ENVIRONMENTAL ENGINEERING
The erosion of part of the west arm of the Duiwenhoks Riverbank (north of Heidelberg, Eden District, Western Cape) during the 2013 flooding was so severe that an adjacent, unsurfaced road connecting the R322 to Heidelberg via the Duiwenhoks River valley collapsed. By Hans King, owner of Hans King SRS
The erosion of the riverbank and fill as a result of the 2013 flood
Stabilisation of severe bank erosion
F
or about 300 m, the unsurfaced road was about 10 m higher than the river itself, and was about 50 m from the edge of the river. The Duiwenhoks River started to meander with unnatural vigour due to upstream erosion supplementing the sediment load in the river. The unnatural meandering of the river was due to: • severe run-off events of 2008 and 2013 • rapid growth of alien invasive vegetation (black wattle – Acacia mearnsii) on the inside of the bend at the site, deflecting the flow of water into the opposite bank • massive amount of sediment washed out of a palmiet reed wetland at a site about 1 km upstream over a short period of time (about 250 000 m3 of sediment was washed out during three major floods from the late 1970s to 2013).
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IMIESA June 2022
Objectives of the project The main objective of the R10.5 million project was to help ensure the sustainability of agricultural land of the whole community downstream of the site (as the funding was sourced via the Conservation of Agricultural Resources Act [No. 43 of 1983]). To do this, it would be necessary to limit the amount of sediment being washed away from the site unnaturally. Other objectives included the: • creation of job opportunities for the local community during the construction phase, and later during river maintenance • protection of the river environment by removing alien vegetation, stabilising the movement of sediment, and the reintroduction of indigenous wetland vegetation • protection of the public road from
Hans King, owner of Hans King SRS
undermining, so as to support the access of the local farming community to their markets. After the site was surveyed, a hydrological study of the catchment was performed, and historical aerial imagery studied; a new alignment for the river was planned more or less along the route it had been 10 years previously. The width and slope of the rehabilitated channel was designed to keep the flow velocity during floods low, and to mimic that which probably existed historically,
