
%0 Journal Article
%A Saynor, MJ 
%A Erskine, WD 
%T Characteristics and implications of high-level slackwater deposits in the Fairlight Gorge, Nepean River, Australia
%J Marine and Freshwater Research
%D 1993/10/01
%V 44
%N 5
%P 735-747
%K palaeofloods, Warragamba Dam, heavy minerals, dam spillway capacity
%R https://doi.org/10.1071/MF9930735
%U https://www.publish.csiro.au/paper/MF9930735 
%X 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. 
