RESPONSE AND RECOVERY OF A SAND-BED STREAM TO A CATASTROPHIC FLOOD

Between 17 and 18 June 1949 up to 508 mm of rainfall occurred over the 2000 km(2) Wollombi Brook drainage basin in south-eastern Australia. The resultant flood peak was almost 27 times greater than the mean ann ual flood but had a return period of only 87 years on the annual maxim um series. Wollombi Brook is characterised by one of the steepest annu al series flood frequency curves in the World and this results in rela tively low return periods for large events. Furthermore, the catastrop hic flood was followed by a series of large events over the succeeding 5 years. River response to the flood varied in direct proportion to t he degree of lateral confinement by materials of limited erodibility. At the storm centre on the upper reaches of the river, the channel is unconfined and did not respond to the flood. In the middle reaches, th e channel is closely confined by bedrock and responded to the flood by widening up to 100% and by up to 4m of bed aggradation. In the lower reaches, the channel is partially confined by bedrock and terrace sedi ments and responded in the same way but to a slightly lesser degree th an the middle reaches. Since the late 1950s partial channel recovery h as occurred by bed degradation excavating the flood-deposited sand and by in-channel bench formation contracting the channel. Nevertheless, the recovery time is likely to be greater than the return period of th e 1949 flood. Therefore, the channel is likely to be unstable in the l ong term because the frequency of perturbing events is greater than th e channel recovery time.