Flood generation: process patterns from the raindrop to the ocean

Abstract

This article reviews river flood generation processes and flow paths across space scales. The scale steps include the pore, profile, hillslope, catchment, regional and continental scales, representing a scale range of a total of 10 orders of magnitude. Although the processes differ between the scales, there are notable similarities. At all scales, there are media patterns that control the flow of water and are themselves influenced by the flow of water. The processes are therefore not spatially random (as in thermodynamics) but organized, and preferential flow is the rule rather than the exception. Hydrological connectivity, i.e. the presence of coherent flow paths, is an essential characteristic at all scales. There are similar controls on water flow and thus on flood generation at all scales but with different relative magnitudes. Processes at lower scales affect flood generation at larger scales, not simply as a multiple repetition of pore-scale processes but through interactions which cause emergent behaviour of process patterns. For this reason, when modelling these processes, the scale transitions need to be simplified in a way that reflects the relevant structures (e.g. connectivity) and boundary conditions (e.g. groundwater table) at each scale. In conclusion, it is argued that upscaling as the mere multiple application of small-scale process descriptions will not capture the larger-scale patterns of flood generation. Instead, there is a need to learn from observed patterns of flood generation processes at all spatial scales.