Representative Hillslope Approach for Modeling Flash Flood Generation in Ungauged Catchments

Convective rainfall extremes usually trigger due to their highly localised and intense input of mass and momentum ‘hot moments’ in water and matter cycling. Terrestrial systems then respond with strong Hortonian overland flow and erosion up to the formation of flash floods. While heavy precipitation events are characterised by multi-decadal variability, it is noteworthy that the largest observed floods in many rivers of Europe have occurred in the last three decades. Similarly, flash floods have also intensified. The recent clustering of extremes likely reflects the ongoing acceleration of the hydrological cycle, with expected increasing frequencies of intense convective rainstorms and related flash flood and erosion events due to Clausius-Clapeyron scaling. This urgently calls for an improved understanding and models that allow the design of strategies to mitigate onsite and catchment-wide offsite damages of flash floods and erosion events.

Hortonian overland flow occurs when precipitation intensity exceeds the soil’s infiltration capacity. The latter depends on the soil water content, soil hydraulic properties and the density and connectivity of vertical preferential flow paths and are often biologically mediated, as in the case of worm borrow and root channels. Whether locally generated surface runoff reaches the stream depends on the generated spatial connectivity of overland flow paths to the river network.

Here we propose that land use management and soil surface preparation bear the key to reducing the formation of Hortonian overland flow and the connectivity of its flow path, e.g., through a locally elevated infiltration capacity and roughness, thereby reducing the overland flow velocity and favouring its re-infiltration. Moreover, we demonstrate that physically based hydrological models are key to quantifying how changes in landuse and surface preparation techniques (including buffer areas, vegetation barriers, and fascines) in combination with local flood defense reservoirs reduce the formation of flood runoff during convective extremes. Specifically, we use the model CATFLOW and the representative hillslope approach to investigate flash floods observed in four ungauged headwaters catchments in the Kraichgau, Baden-Württemberg (Germany) in 2016. While each catchment drains into a regulated flood defense reservoir, we inverted the flood hydrograph/ inflow into the flood reservoirs using water level measurements and reservoir geometry equations. LULC maps are derived from LANDSAT images using spectral profiles obtained from field surveys over the region. Since flash floods are often associated with localised short-duration, high-intensity rainfall of convective origin, the model is forced using commercial radar-based precipitation products. The CATFLOW model was set up separately for the four headwaters by transferring a completed hillslope setup (soil catena, soil hydraulic properties, plant roughness parameters) from a gauged Weiherbach experimental catchment in the same landscape while deriving the representative hillslope profiles from the digital elevation data. Our results indicate that physically based models perform well in capturing the dynamics of the reconstructed hydrographs, which speaks a) for the transferability of physically based model structures within the same hydrological landscape and b) the feasibility of representative hillslope approach and c) the usefulness of the radar product.