Hydrologic Variability of Eastern Australia

The Australian hydrologic environment and stream flow rates in particular are note- worthy for the high degree of inter-annual variability they experience. Finlayson and McMahon (1988) reviewed a number of statistical properties of Australian stream flow in relation to records from around the world and concluded that whilst Australian area specific discharge rates were generally quite low, mean annual flow variability and flood variability were both high and the latter increased with catchment area.

The degree of variability of Australian flood discharges was illustrated by Pickup (1984). He showed that the one in 100 year discharge of the Fly and Purari Rivers of Papua New Guinea is roughly twice the 2 year flood. By contrast, for the Nepean Region even the 10 year flood was at least eight times the 2 year event. Erskine (1996) noted the June 1949 flood along Wollombi Brook that caused such dramatic channel widening was 27 times larger than the mean annual flood and had an estimate average recurrence interval of only 87 years. Floods 13 to 18 times the mean annual flood were noted elsewhere in the Hunter Valley (Erskine, 1994b). The argument that high flood variability is of relevance to studies of channel morphology essentially states that because rare floods, say a 1 in 100 year event, are so much greater in magnitude relative to more frequently occurring events, such as a 1 in 5 or 10 year event, they

have the potential to be much more destructive and hence to have a more dominant role in controlling channel form.

It is important to bear in mind that directly relating channel morphology to flood variability is not necessarily justified. Flood variability is a statistical property derived from of a sequence of floods, but it is each of these floods acting individually though within the pre-existing landscape morphology that determines what pattern of erosion or deposition will occur in response to its passing. However, flood variability as a statistical property may more logically be used to examined or interpret degrees of morphologic disequilibrium that could be expected in a landscape for a given hydro- logic regime.

Baker (1977) proposed that the standard deviation of the base 10 logarithms of the annual maximum flood series be used as a measure of flood variability and this mea- sure has consequently been adopted in numerous Australian studies (Erskine, 1986; 1993; 1996; Rutherfurd, 2000). The logarithmic transformation of the annual maxi- mum flood series arguably leads to an over-emphasis on the significance of low dis- charge events, which do little to shape the channel. However it is has been adopted here as an indicative measure of flood variability in order to provide a hydrologic context for the two catchments studied in this thesis. Figure 2.9 shows this flood vari- ability index for the Macdonald and Tuross Rivers in relation to values for a selection of stations east of Australia’s Great Dividing Range (station locations are plotted in Figure 2.10). The two gauges on the Macdonald have flood variability index values of about 0.5 which is in the middle of the eastern Australian Range but are low relative to the Hawkesbury-Nepean basin. By contrast, flood variability appears to be higher along the Tuross River, with the two most downstream gauges experiencing some of the highest flood variability for the south eastern corner of the continent.

The relative magnitude of the floods that these two catchments have experienced is also of importance. Figure 2.11 shows the maximum peak instantaneous discharge

The Nepean basin is number 212 and the two Macdonald River gauging stations (212021 at Howes Valley and 212010 at St Albans) are indicated with downwards pointing triangles. The Tuross drainage basin is number 218 and the stations 218008 (Tuross at Eurobodalla) and 218005 (Tuross at Wadbilliga Junction) are indicated with upwards triangles. Only stations with a minimum of 20 years of data and which were judged to be have been minimally influenced by flow regulation structures such as dams were included.

data for the selection of east coast gauging stations shown in Figure 2.10, with stations from the Macdonald and Tuross Rivers and Wollombi Brook highlighted. Also shown are the 0.25 (first quartile), 0.5 (median) and 0.75 (third quartile) regression quantiles for the regional data. These data are from stations with varied record lengths and period of operation, but serve to illustrate the hydrologic behaviour of the region. The 891m3/s event in 1978 along the the Macdonald (which was the highest measured discharge) plots within the first quartile of the regional data. By contrast, the area specific flood magnitude observed along the Tuross River falls in the third quartile of regional observations. Gauging records for the Macdonald at St Albans commenced in 1955 and hence missed a major flood of 1949 (Henry, 1977). An indication of the magnitude of this event is provided by records from the Warkworth gauge (210004) on the adjacent Wollombi Brook. Erskine (1996) estimates the 1949 flood to be 27 times the mean annual flood for this river, and accordingly it appears to lie within the top quarter of observations from eastern Australia.

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Figure 2.11: Maximum of the peak instantaneous discharge rates, normalised against catchment area

for gauging stations east of Australia’s Great Dividing Range. The Macdonald stations are 212010 and 218008 whilst for the Tuross, station 218008 and 218005 are plotted. Also shown is an observation of the Wollombi Brook at Warkworth (210004) which recorded the June 1949 event.