Alice Springs flood study.
Huq, M. N.
E-Publications; E-Books; PublicationNT; Report no. 3/1980
Northern Territory Government
Report no. 3/1980
Attribution International 4.0 (CC BY 4.0)
Northern Territory Government
Technical Report WRD80003 Viewed at 14:07:38 on 29/07/2010 Page 17 of 40. I I I I I I I I I I I I I I I I I I I I RETURN 96 Hrs PERIOD MAX YEARS nun/hr P .M.P. 4.00 100 2.30 50 1.91 20 1.53 10 1.28 5 1.07 2 0.73 15. TABLE 7.2 ADJUSTED RAINFALL INTENSITY OVER THE TODD RIVER CATCHMENT AFTER USING ... CORRECTION FACTOR OF 0.85 72 Hrs 48 Hrs 24 Hrs 12 Hrs 8 Hrs MAX HAX M.J<X MAX MAX mm/hr mm/hr mrn/hr mm/hr mm/hr 4.93 6.46 10 .03 15.13 18.96 2.80 3.51 5.44 8.16 10.20 2.30 2.98 4.68 6.97 9.67 1.87 2.47 3.83 5.78 7.23 1.59 2.13 3.32 4.93 6.21 1.32 1. 74 2.72 4.08 5.10 0.89 1.19 1.87 2.82 3.61 ! P.M.P. Probable Maximum Precipitation 7 .2 UNITGRAPH 6 Hrs 4 Hrs MAX MAX mrn/hr mm/hr 22.36 28.48 11.90 14.62 10.20 12.58 8.33 10.46 7.23 9.18 5.95 7.65 4.25 5.61 The conventional method of deriving unitgraphs was not used due to limited pluviograph data in the catchment. The method used was developed by Ash (1973) to enable derivation of unitgraphs using only streamflow data. The method involves plotting several hydrographs as percentage ordinates of the peak flow of each hydrograph with all hydrograph peaks being plotted at a common point. The unitgraph for the station is defined as the envelope curve which has the sharpest rising and falling limbs. The volume of runoff under the curve is then adjusted to unity. In accordance with the recommendation of Ash (1973) the unitgraph time to peak and the total time base of ~~e unitgraph define the unitgraph as being a one hour unitgraph~ The adopted unitgraph is shown in figure 7.6. 7.3 TEMPORAL PATTERN Studies of temporal pattern of Australian storms have been reported by Pilgrim, French and Cordery (1969) and by Hall and Kneen (1973). For this study the temporal pattern adopted are those recommended by Australian Rainfall and Runoff (1958 & 1977). Figure 7.7 indicates that 6 hour storm periods are the most significant~ 7.4 ESTI~~TTON OF LOSS COEFFICIENTS Rainfall hydrographs and runoff hydrographs of selected flood events in the Todd River catchment have been used to estimate the initial loss and continuing loss rate. Continuing loss rates were found to vary between 4 and 11 millimetres per hour. Considering the nature of the Todd River catchment an initial loss of 8 nun and a continuing loss rate of 6 rom/hour were adopted for use in all unitgraph analyses. 7.5 FLOOD DERIVATION AND PROBABILITY CURVE By using the storm intensities and temporal patterns for durations of four hours, six hours, eight hours and the derived one hour unitgraph it was possible to estimate the peak flows for 2, 5, 10, 20, 50 and 100 year
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