Characteristics and implications of high-level slackwater deposits in the Fairlight Gorge, Nepean River, Australia

Abstract

The largest recorded flood on the Nepean River, with a peak discharge of 16 600 m³ s^-1, occurred in 1867. Detailed field descriptions and particle-size analyses of sediments at six sites in the Fairlight Gorge below Warragamba Dam identified high-level flood deposits. Slackwater deposits (SWDs) are typically fine-grained sand and silt, which accumulate rapidly from suspension during large floods in areas where flow velocities are locally reduced. However, the higher-level SWDs were too thin and bioturbated to be clearly differentiated from locally derived colluvium. Heavy-mineral analysisof the very fine sand fraction of these high-level SWDs identified minerals (epidote and pyroxene) that were not present in the surrounding bedrock. Epidote and pyroxene were derived from distant sources and were emplaced by at least one palaeoflood. A radiocarbon date of 3756±72 years BP was obtained by tandem accelerator mass spectrometry on small fragments of charcoal contained in the high-level SWDs. Therefore, at least one palaeoflood larger than any historic flood occurred during the late Holocene. SWDs indicated that the largest palaeoflood had a peak height at least 8.0 m higher than the 1867 flood. If such a flood had occurred before the recent raising of the dam wall, the dam would have sustained substantial damage and might have failed. Additional modifications are being discussed; it is recommended that they should be completed as soon as possible to allow Warragamba Dam to discharge safely floods of similar magnitude to the largest palaeoflood indicated by SWDs.