Report No. : GEO Report No. 261

Report Title : Frequency Analysis of Extreme Rainfall Values (2011), 209 p.

Author : C.S.C. Tang & S.P.Y. Cheung

Abstract

The Geotechnical Manual for Slopes (GCO, 1984) adopts the extreme rainfall intensities derived by Peterson & Kwong (1981) based on frequency analysis of the rainfall data recorded at the principal raingauge of the Hong Kong Observatory (HKO) in Tsim Sha Tsui (1947 - 1980) and the raingauge at King's Park Rise (1952 - 1980). GCO (1984) presents the extreme rainfall intensities by means of intensity-duration-frequency (IDF) curves for use in the design of slope surface drainage, etc.

However, it is well recognised that the spatial distribution of rainfall is highly variable in Hong Kong. GCO (1984) recommends that for the design of major drainage structures, the validity of the IDF curves at different locations should be confirmed by undertaking an independent analysis of rainfall data obtained from the nearest raingauge.

In this study, statistical modelling of annual maximum rainfall was carried out using both the Gumbel distribution and the Generalized Extreme Value (GEV) distribution in order to estimate the extreme rainfall intensities under different combinations of return periods and durations for the 43 GEO automatic raingauges.

The IDF curves based on the GEV distribution are found to be unrepresentative for the majority of the raingauges, as rainfall data from the GEO raingauges are insufficient for fitting a GEV distribution. Therefore, they have not been adopted for further diagnosis in this study. The IDF curves based on Gumbel distribution have been compared with the IDF curves given in GCO (1984).

New IDF curves are proposed for use in slope drainage design in Hong Kong.

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