Influences on flood frequency distributions in Irish river catchments

摘要

This study explores influences on flood frequency distributions in Irishrivers. A Generalised Extreme Value (GEV) type I distribution is recommendedin Ireland for estimating flood quantiles in a single site flood frequencyanalysis. This paper presents the findings of an investigation thatidentified the GEV statistical distributions that best fit the annual maximum(AM) data series extracted from 172 gauging stations of 126 rivers inIreland. Analysis of these data was undertaken to explore hydraulic andhydro-geological factors that influence flood frequency distributions. Ahierarchical approach of increasing statistical power that used probabilityplots, moment and L-moment diagrams, the Hosking goodness of fit algorithmand a modified Anderson-Darling (A-D) statistical test was followed todetermine whether a type I, type II or type III distribution was valid.Results of the Hosking et al. method indicated that of the 143 stations withflow records exceeding 25 yr, data for 95 (67%) was best represented byGEV type I distributions and a further 9 (6%) and 39 (27%) stationsfollowed type II and type III distributions respectively. Type I, type II andtype III distributions were determined for 83 (58%), 16 (11%) and 34(24%) stations respectively using the modified A-D method (data from 10stations was not represented by GEV family distributions). The influence ofkarst terrain on these flood frequency distributions was assessed byincorporating results on an Arc-GIS platform showing karst features and usingMonte Carlo simulations to assess the significance of the number andclustering of the observed distributions. Floodplain effects were identifiedby using two-sample t-tests to identify statistical correlations between thedistributions and catchment properties that are indicative of strongfloodplain activity. The data reveals that type I distributions are spatiallywell represented throughout the country. While also well representedthroughout the country, the majority of type III distributions appear inareas where attenuation influences from floodplains are likely. The majorityof type II distributions appear in a single cluster in a region in the westof the country that is underlain by karst but importantly, is characterisedby shallow of glacial drift with frequent exposures of rock outcrops. Thepresence of karst in river catchments would be expected to provide additionalsubsurface storage and in this regard, type III distributions might beexpected. The prevalence of type II distributions in this area reflects thefinite nature of this storage. For prolonged periods of rainfall, risinggroundwater levels will fill karst voids, remove subsurface storage andcontribute to recharge related sinkhole flooding. Situations where rainfallintensities exceed karst percolation rates also produce high levels ofsurface runoff (discharge related flooding) that can promote type IIdistributions in nearby river catchments. Results therefore indicate that insome instances, assuming type I distributions is incorrect and may result inerroneous estimates of flood quantiles at these locations. Where actual datafollows a type II distribution, flood quantiles may be underestimated by inexcess of 35% and for type III distributions, overestimates by over 25% can occur.