High flood levels along the barrage reach of the Vaal River

As development pressures rise along the Barrage reach of the Vaal River so also do the costs of flood damage. By means of computer-operated flood plain management programs developed in the Hydrological Research Unit it has been possible to determine the heights to which floods of various recurrence intervals are likely to rise, and thus to delineate areas that would be affected. The computer is programmed to plot the water surface profiles for floods of various return periods as affected by bridges and other obstructions. The printout shows chainage, water surface elevation, energy level, velocity, Manning etc. The flood marks left by the severe 1975 flood served to calibrate the flood plain model. In view of the relatively gentle gradient of the Vaal within the Barrage reach, the floods are not violent in character and most of the low-lying areas affected at high flood are inundated by slowly moving or stagnant water. Velocities even during high flood seldom exceed 2 m/s in the main channel or 0,5 m/s on the flood plains. It was found that all existing bridges and the Barrage structure can be negotiated with negligible afflux by floods of up to 200-year return period.