Stabilisation of severe bank erosion

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

For 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).

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

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,

Hans King, owner of Hans King SRS

so that sediment movement during floods would be reduced.

Groynes were chosen as river training structures (as opposed to riprap or other longitudinal protection), partly because this is a lot more economical, but also because the use of groynes promotes a wider and slowerflowing watercourse during floods, and that the spaces between the groynes are used for trapping sediment and the establishment of indigenous wetland vegetation. The height of the groyne structures above the riverbed was kept low so that large sections of the groynes would be overtopped during moderate floods and double the river’s flow width.

Rock-filled gabions were selected as the construction material because they are flexible enough to accommodate foundation movements, should they occur during an extreme flood. They are also easy to work with, and this facilitated the use of 70% local (unskilled) persons.

Design and construction challenges

The bend where the river enters the site is very sharp (90 degrees) and this caused concern. The stabilisation of sharp bends is problematic because they can require many groyne structures to effectively turn the water. The problem was addressed by keeping the rehabilitated channel as close to the north bank (inside of the bend) as possible. This enabled the construction of very long groyne structures; they could therefore be spaced further apart and were fewer in number than the short groyne alternative design. The economy of this approach is not so much because there are fewer groynes in number, but rather because less work is involved in constructing groyne tips. The groyne tips are excavated 2 m deeper than the rest of the structure and, because this is below the water table, it is very expensive.

During construction, it was found that several islands of indigenous wetland vegetation existed along the route of the rehabilitated channel. The design of the rehabilitated channel called for this area to be dropped by between 0.5 m and 1 m. As the Department of Agriculture had not started establishing nursery facilities for these projects at that time (and supplies of indigenous plants were hard to obtain), it was decided to maintain these islands of vegetation in their current habitat without disturbance. This caused inconvenience for the contractor, but it was believed that the impact of the vegetation islands on the structures would be negligible.

The project was one of the first large-scale river stabilisation projects in the Eden area. As a result, many people visiting the site were alarmed at the sight of the many large heaps of excavated soil in the riverbed during the construction phase. It took a lot to convince them that the disturbance was a shortterm necessity in order to achieve long-term stability. Once the project was complete, they all appreciated the work that was done.

The groynes finally installed to train the river away from the road fill and to trap sediment

A drone image of the completed project looking downstream from the top end