Restoration Of The Hex River To EquilibriumMorphological Conditions

Abstract

The Hex river near the town of Worcester, South Africa, has a braided character with wide floodplains, a steep bed slope and high bed load. In June 1996, one of the highest floods on record caused severe head cutting, scouring the river bed by two metres which caused instability problems at two bridges. A comprehensive hydraulic study has been undertaken to identify the problem causes and impacts, and to establish a long-term solution for river equilibrium and bridge safety. Man's involvement in altering the river include : extensive mining of boulders from the main river channel with the aim of reducing flood levels which was found to be the main reason for head cutting, construction of bridges with fixed bed levels, river channel alteration, the construction of groynes on the floodplain, and closing of a major part of the braided system for agricultural development. The current braided river system has been modelled with a one dimensional mathematical model, which could accurately simulate the observed bed degradation due to the mining. Restoration of the scoured river bed has been achieved by constructing two weirs on the river in order to raise the bed level to its natural long-term equilibrium state. The bridges have been modified with fixed concrete beds at the elevation of the simulated equilibrium bed profile, with the addition of energy dissipation structures to prevent local scour.