Estimating Generalised Flood Sew Coeficients for Peninsular Malaysia
Flood estimations based on itting the frequency of occurrence of annual peak discharges using the Log-Pearson Type 3 distribution are commonly used but they are sensitive to the skew coeficients of the gauging stations. The estimation accuracy can be improved by using a weighted average population skew coeficient calculated from the sample station skew and the
generalised unbiased skew. The U.S. Water Resources Council (WRC) has documented guidelines for estimating the generalised skew coeficients and published a map of generalised skew values for the United States. The map shows isolines of skew coeficient values and the average skew coeficient for each 1-degree quadrangle of latitude and longitude for the United States. Following the WRC guidelines, many of the state authorities in the US have developed the generalised skew coeficients separately on a state/regional basis. In Malaysia, the Log Pearson Type 3 distribution has been widely used for lood peakestimation but there are no guidelines available for estimating the generalised skew coeficients for use in con–unction with the distribution and as more accurate results can be obtained if these data are available, it is clear that a regional lood skew study is needed. With the regional sew data available, the peak low can be simply and easily calculated with the aid of a software such as HEC-SSP. The aim of this paper is to use the WRC guidelines to derive the generalised skew coeficients using the peakannual discharge data of Peninsular Malaysia for general use.The WRC recommended several techniques for estimating and evaluating generalised s—ew of the Log-Pearson Type 3
distribution for the annual peakdischarges. Station skews (skew coeficients computed from gauging station records) and unbiased and weighted skews derived from these station skews are to be used to develop these techniques. In this study, peak discharge records at 66 gauging stations having 16 or more annual peak discharges in Peninsular Malaysia were selected for computing station skews. Station skew values ranged from -0.831(log10 unit) to 1.475 (log10 unit).The three techniques recommended by WRC used for estimating the generalised skew of annual peak discharges were adopted for this study. These methods are: (1) An isoline map, (2) a prediction equation (3) a regional mean skew. Attempts to develop
a prediction equation were unsuccessful. An error analysis showed that the regional mean skew method has a lower MSE (mean square error) than that obtained from the state wide generalised skew coeficient contour map. As a result, the mean station skew for the selected gauging stations can be used to estimate the generalised skew for any gauging site in the peninsula.The mean skew is -0.022 (log10 unit) and the associated mean square error is 0.05 (log10 unit).