Exploring the effects of catchment morphometry on overland flow response to extreme rainfall using a 2D hydraulic-hydrological model (IBER)

Abstract

The hydrological response of a catchment to heavy rainfall is determined by many environmental and climatic variables, including the catchment size, shape and morphology, land use, soil depth and type, and prevailing weather conditions. As a result of the interaction between these variables, the hydrological response of a catchment is unique and can be highly variable depending on the antecedent conditions and characteristics of the meteorological event. This complexity makes it problematic to identify the significance of individual drivers of hydrological response. Here, the role of catchment morphometry in controlling hydrological response was explored by creating abstract catchments so the impact of morphometry could be isolated from the confounding impacts of other parameters that are known to impact hydrological response and flood risk. A procedure was developed to select catchments that represent a range of global morphometries, and these were then simplified and treated as synthetic catchments. A coupled hydrological-hydraulic model (IBER) was used to model the hydrological response to 4 constructed rainfall events (one magnitude, four distribution types), consistent across all catchments, and representing torrential rainfall (60 mm h−1). Designed rainfall were equally distributed across the whole area in each catchment. Catchments were statistically partitioned into groups based on their morphometry (i.e. shape, slope, size, hypsometric slope, etc) and distinct hydrological responses were observed between groups. Catchment size was significantly related to peak flow or lag time with a power law, in line with previous studies, and relationships with other morphometric parameters were stronger when catchments were assessed in separate size groupings. Area and the average slope of catchments were key parameters in controlling peak flow magnitudes. Variation in hydrological response was much greater between catchments for the same rainfall event, than within the same catchment for different rainfall events. The results suggest that hydrological response can be broadly characterised using morphometric variables, increasingly obtainable using Earth Observation methods, providing potential benefits for flood prediction, flood alerts, and focusing where more detailed flood modelling is required.