Holsloot weir river project in full flow

[Article by Nane Zietsman]

A new diversion works and weir were recently completed on the Holsloot River for irrigation, replacing the weir that washed away during the 2013 flood. The Stellenbosch Hydraulics Laboratory (SHL) of the Department of Civil Engineering at Stellenbosch University (SU), was appointed by the Western Cape Department of Agriculture to do the hydraulic design and physical model study for the project. This R78 million project is located on the Holsloot River, approximately 10km south-east of Rawsonville and 11km downstream of the Stettynskloof Dam. It is set to directly impact approximately 150 farming households and 4 000ha of mainly wine grapes under irrigation.

Professor Gerrit Basson, a specialist in river hydraulics, sedimentation and the design of hydraulic structures led the SU consulting team. “During the design and construction phases of the project the Stellenbosch Hydraulics Laboratory (SHL) acted as the main consulting engineers responsible for the hydraulic design, while Ingerop consulting engineers were appointed as sub-consultants for the geotechnical and structural design, as well as site supervision.”

According to Prof. Basson, the SHL team was involved throughout the lifecycle of the project, from feasibility level to construction completion and commissioning. “It is particularly fulfilling to see a design come to life and to compare prototype river flows with those tested in the laboratory.” The Holsloot Diversion Works was required to replace a number of temporary rockfill diversion weirs as part of a more sustainable river management plan. “Diversion works are typically faced with sedimentation issues whereby sediment deposits block intakes or are abstracted while downstream erosion can threaten the structural integrity of the weir, particularly in this case where there was no rock for the weir to be founded upon.”

The hydraulic design and physical model study resulted in the construction of a 55 m long and 3,4 m high weir, with a concrete roller bucket for energy dissipation. “The intake was designed to be self-scouring during floods with a gravel trap and two sand traps at the intake structure that can be flushed during smaller floods as required.” The design used a 100-year Recommended Design Discharge (RDD) of 579 m3/s and a Safety Evaluation Discharge (SED) based on the Regional Maximum Flood (RMF) of 1136 m3/s. Prof. Basson further explains that the diversion works and roller bucket had to be constructed with safe accessibility, so the design includes a pedestrian bridge over the roller bucket which is uncommonly seen. “The piers had to be designed to resist hydraulic forces and debris accumulation in the roller bucket.”

The project has formally been handed over to the Holsloot Water User Association to function with CASIDRA acting as the implementation agent for the Western Cape Department of Agriculture. The significant work of the Stellenbosch Hydraulics Laboratory on this project once again shows the impact of the Faculty’s research and consulting services on the industry.

Photograph: Holsloot Diversion works and weir viewed from downstream (left) and upstream (right).