Sedimentological and geomorphological effects of the 1997 and 1998 flood sequence on the Lower Snowy River, Victoria

Abstract

Three floods with flood peak ratios (peak discharge/mean annual flood) ranging between 0.65 and 5.35 occurred on the lower Snowy River in Victoria between June 1997 and August 1998. The June 1998 flood was the largest event downstream of the Rodger River junction where the flood peak ratio was >4. Pre- and post-flood investigations were carried out at the three Snowy River benchmarking sites in Victoria (McKillops Bridge, Sandy Point and Bete Bolong) to determine the impact of the floods on channel-boundary sediments and morphology. Few significant changes in graphic grain-size statistics for channel-boundary sediments were found at McKillops Bridge and Sandy Point. At Bete Bolong, there were many significant changes in the grain-size statistics of both the bed material and bank sediment. The variance and mean of a number of benchfull and bankfull channel morphologic parameters (width, area, mean depth, maximum depth, width‒maximum depth ratio) did not change significantly at McKillops Bridge and Sandy Point. At Bete Bolong, benchfull mean depth and area increased significantly due to bed degradation. Floods with a flood peak ratio of at least 4 are important for mobilising channel-boundary sediments and hence modifying channel morphology on the lower Snowy River.