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CRC PUBLICATIONS |
Derivation of Areal Reduction Factors For Design Rainfalls (18 - 120 hours) in Victoria
Lionel Siriwardena,
Erwin Weinmann
Publication Type:
Technical Report
This is a publication of the initial CRC for Catchment Hydrology
CRC Program:
Flood Hydrology (Previous CRC)
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Publication Keywords:
Precipitation (Atmospheric)
Design Data
Flood and Flooding
Catchment Areas
Areal Reduction Factor
Mathematical Analysis
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Abstract / Summary:
Derivation of Areal Reduction Factors For Design Rainfalls in Victoria - For Rainfall Durations 18-120 hours
Abstract
Design rainfall information for flood estimation is generally made available to designers in the form of point rainfall intensities . However, most flood estimates are required for catchments of significant size and will thus require a design estimate of the areal average rainfall intensity over the catchment. The ratio between the design values of areal average rainfll and point rainfall, computed for the same duration and annual exceedance probability (AEP), is called the areal reduction factor (ARF). It allows for the fact that larger catchments are less likely than smaller catchments to experience high intensity storms over the whole of the catchment area.
ARF values for a catchment of a given size can be determined from separate frequency analyses of extreme values of point rainfall and areal rainfall for selected durations. The determination of average ARF values for a whole region requires the repetition of this procedure for many different "sample catchments" of that size. The ARF values presented in this report are based on a detailed study using daily rainfall data from over 2000 rain gauges in Victoria. The methodology used is a modified version of Bell's method (Bell, 1976) and was selected on the basis of an extensive literature survey (Srikanthan, 1995). The adoptation of the final method described in this report followed extensive evaluation of variations in procedural steps, as detailed in Siriwardena and Weinmann (1996).
Individual ARF values were computed for a large number of circular "sample catchments" distributed over those parts of Victoria with a relatively high rain gauge density. Sets of ARF values were derived for rainfall durations of 1, 2 and 3 days, catchment areas of 125, 250, 500, 1000, 2000, 4000 and 8000 kilometres squared, and for AEPs of 1 in 2, 5, 10, 20, 50 and 100. A sample mean value of ARF was then determined for each combination of catchment area, rainfall duration and AEP, and a single equation was fitted to these mean values to represent the dependence of ARFvalues on these characteristics.
The study identified small but statistically significant differences in ARF values for different parts of Victoria, probably reflecting differences in hydrometeorological factors such as dominant storm types. However, there is at present insufficient information to allow differentiation of design values within Victoria based on catchment location. The application of a single set of design ARF values over the whole of Victoria is therefore recommended at this stage.
The design relationship established in this study allows determination of areal reduction factors in Victoria for a range of catchment areas from 1 to 10,000 kilometres squared, rainfall durations from 18 to 120 hours and AEPs from 1 in 2 to 1 in 2000.
It was shown that the ARFs derived in this study are significantly lower than the values given in Australian Rainfall and Runoff (IEAust, 1987). This study also confirmed a tendency for ARF values to decrease with AEP. It is recommended that the ARF values derived in this study replace the values in Australian Rainfall and Runoff for design flood studies of catchments in Victoria and regions with similar hydrometeorological characteristics.
This report is not currently available for downloading. Printed copies can be purchased from the Centre Office
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Centre Office:
CRC for Catchment Hydrology
Dept of Civil Engineering
Building 60
Monash University Vic 3800
Tel: +61 3 9905 2704
Fax: +61 3 9905 5033
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