Author(s): Julia Beck; Gerrit Basson
Linked Author(s): Gerrit Basson
Keywords: Estuary; Mouth breaching; Flushing efficiency; Laboratory experiments
Abstract: Many estuaries in South Africa are only open to the sea intermittently. The environmental quality of these estuaries is determined largely by the frequency, duration and timing of open mouth conditions. Unfortunately they are at present often closed more frequently and for longer periods than in the past and their environments have deteriorated. The ever-increasing reports of sedimentation problems in South African estuaries lead to calls for increased mouth breachings and flushing of these estuaries. However, breachings have occurred at water levels in the estuary that are too low, which have a negative effect on the flushing efficiency. As part of a wider research project, involving field investigations and hydraulic modelling, laboratory experiments were carried out to investigate the mouth breaching process at temporarily closed estuaries. Especially the effect of initial water levels at breaching and the changes in mouth geometry during breaching were investigated. It was found that the erosion potential is significant at the start. The channel first adjusts its depth rapidly and the hydraulic gradient gradually decreases. As the channel deepens the banks cave in occasionally. However, the greatest width changes take place after the depth has reached some form of equilibrium. The equilibrium mouth width and depth are determined mainly by the maximum discharge during breaching, with the hydraulic gradient playing a less significant role. The same is true for the flushing efficiency. With increasing discharge the cross-sectional inlet area and the rate of erosion increases. The discharge in turn is determined mainly by the water level in the estuary when breaching starts. Therefore, the higher the water level in the estuary at the start of breaching, the more efficient the breaching process will be.