A methodology for regional flood frequency estimation based on scaling properties

There is a persisting need for extreme value assessments of discharge for ungauged catchments for design purposes, flood protection and flood risk analysis. A methodology is put forward for estimating flood quantiles in ungauged catchments, taking into account the scaling behaviour of discharge in a homogeneous region. From an assumption that the spatial distribution of specific discharge generation is a shifted exponential distribution, theoretical expressions of the maximum specific discharge values relative to catchment size are developed and a regional scaling equation is presented. The homogeneity constraint refers to climatic conditions, whereas catchment heterogeneity due to physiographic features of catchments is incorporated in the models using multiple regression analysis. The methodology is tested for a region in Southern Norway with homogeneous climatic conditions but with varying physiographic catchment characteristics. It is investigated if the extreme discharge values estimated with traditional methods are adequately reproduced with the proposed methodology. For the tested region, the agreement between flood quantiles estimated with the proposed methodology and flood quantiles estimated from observed data is good. The effective lake percentage and the river gradient proved to be significant descriptors of the deviation from the scaling procedure. Copyright © 2004 John Wiley & Sons, Ltd.