THE ANATOMY OF A FLASH-FLOOD IN THE HARTBEESPOORT CATCHMENT

As in many other semi-arid regions, rainfall in northern South Africa is erratic in both space and time. In response to such rainfall runoff in the Hartbeespoort catchment tends to be flashy, particularly runof f arising from rain falling on paved surfaces in the northern suburbs of Johannesburg. A specific flash flood was monitored from the origina ting storm down to the dissipation of the underflow in the lake. The s torm was fairly typical of the region. Owing to low pondage in the cha nnel the velocities of the surge and current were almost equal and the hydrograph at the lower end of the channel could be derived by a simp le model from those higher up. Two distinct silt loads were generated by the flood, the first probably containing larger particles eroded fr om around the channel and the second containing smaller particles erod ed from the ground upon which the rain had fallen. The second silt loa d appeared to aggregate a large proportion of the chemicals dissolved in the river and to lead to a density current which produced anomalies in the normal current and temperature profiles. The underflowing floo d-water displaced the water resident in the drowned river channel well down into the lake, but entrained sufficient upper-level water during its passage to warm it to a temperature which prevented it from reach ing the deepest parts of the lake.