Step changes in the flood frequency curve: Process controls

Abstract

[1] Empirical distribution functions of flood peaks in small catchments sometimes show discontinuities in the slope; that is, the largest flood peaks are significantly larger than the rest of the record. The aim of this paper is to understand whether these discontinuities, or step changes, can be a consistent effect of hydrological processes. We conducted field surveys in two Austrian alpine catchments 73 km 2 in size to map the spatial patterns of surface runoff generation and hydrogeologic storage. On the basis of this information, we selected the parameters of a distributed continuous runoff model, which is designed to simulate well the point when the storage capacity of the catchment is exhausted. Then we calibrated a stochastic rainfall model and performed Monte Carlo simulations of runoff to generate flood frequency curves for the two catchments. The curves exhibit a step change around a return period of 30 years. An analysis of the storage capacities suggests that this step change is due to a threshold of storage capacity being exceeded, which causes fast surface runoff in large parts of the catchments. The threshold occurs when the storage within the catchment is spatially rather uniform. To identify step changes, reliable estimates of the catchment storage capacity are needed on the basis of detailed hydrogeological information. The occurrence of a step change is of importance for estimating low-probability floods since the flood estimates with the step change accounted for can be significantly different from those based on commonly used distribution functions. We therefore suggest that step changes in the flood frequency curve of small catchments can be real and their possible presence should be taken into account in design flood estimation.

Convection-permitting regional climate simulations for representing floods in small and medium sized catchments in the Eastern Alps

“…10) and underestimations (see Fig. 7) of the simulated rainfall, which is due to the non-linearity in the 5 rainfall-runoff process (e.g., Komma et al, 2007;Rogger et al, 2012). This threshold is consistent with usual concepts in hydrology, such as the concept of the GRADEX method (e.g., Merz et al, 1999).…”

Regional parent flood frequency distributions in Europe – Part 1: Is the GEV model suitable as a pan-European parent?

“…The investigation of the GEV as a single model for all European catchments was performed based on a very basic inspection of the flood database, but there are many examples that show that one single analytic expression across large scales and more important, across different processes, is not valid for describing all possible local characteristics at once. Rogger et al (2012) proved that step changes appear in the flood frequency curve when local runoff generation mechanisms are influenced by threshold processes, especially for small mountainous catchments, and these are not captured by any traditional statistical model so far. The case of several Mediterranean regions which are characterized by two distinct flood populations is also very common.…”

Multi-scale hydrometeorological observation and modelling for flash flood understanding

“…Wood et al, 1988;Flügel, 1995;Reggiani et al, 1998;. These "homogeneous" units should reflect the hydrological behaviour: production of infiltration excess runoff, saturation excess (Schmocker-Fackel et al, 2007), storage, transfer or accumulation zones (Lin et al, 2006a, b), surface, sub-surface or groundwater flow (Latron and Gallart, 2007;Rogger et al, 2012) and their connectivity (SchmockerFackel et al, 2007;Lin, 2010). The approach used in this study is built on those papers and it combines image analysis and field work to derive such functional units.…”

“…As a consequence, the predictability of such events remains low. In addition, this predictability is lowered by a high non-linearity in the hydrological response related to threshold effects (Rogger et al, 2012) and structured heterogeneity on all scales (Blöschl and Zehe, 2005).…”