Gulley stabilization with gabion weirs to mitigate river erosion
The Tesselaarsdal area is located between Caledon and Hermanus. The location of the catchment is shown below. The Department of Agriculture was requested to attend in 2003 a gulley that was rapidly developing. Due to absence of funding no erosion control structures were planned.
The Department was requested again in 2010 to assist; the gulley had now progressed some 500m upstream and was 13 m deep. Currently the gulley head has moved 300m upstream of where it was in 2010. The hydraulic erosion of the gulley is very aggressive.
A large tract of pristine wetland will disappear within the next few years if remedial measures are not implemented. The Elandkloof Stream is relatively steep, for the soil on which it flows. The peat deposit is more than 13 meters deep, and the upstream wetland is at least 2 kilometers long. The Elandskloof River gradient is around 1 to 2%. When the vegetation becomes washed out,extensive erosion takes place. The peat soil does not wash away because it is very cohesive, but it usually overlays thin layers of coarse sand and stone. When these wash out of the soil profile, the peat above breaks off in blocks.
The objective of the structural intervention is to enable the water to drop 20 meter from the wetland into the gulley, with a minimum soil loss. The structure type selected were vertical weirs with counter weirs. The risk of failure is coupled to the drop height of the spillway: a useful strategy is to split the structural intervention into a series of lower steps where the energy of the falling water can be controlled more effectively. 5 small weirs were then considered.
The preferred construction material was rock filled gabion baskets. A series of rock filled gabion weirs were designed, dropping the water vertically 20 meters over a distance of 500 meters. The advantages of using gabions are many. Structurally, the double twisted hexagonal mesh gabion is very flexible, and should the foundation material settle for any reason, this can easily and effectively be accommodated without problems by the gabion structures.
Gabions also have the advantages of being able to “green” with vegetation and are aesthetically more pleasing. Gabions also lend themselves to construction by less sophisticated teams and facilitate job-creation projects.
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