Modeling approaches for floods in different spatio-temporal scales – Do smaller catchments need smarter models?

Over the past decade, the focus of flood risk assessment and management has been on floods along the major rivers. The methods and modelling approaches to identify the potential floodplains along the large river basins are getting gradually standardized. As part of the second round of the European Flood Risk Assessment (EU Floods Directive), the picture of the procedure for identifying flooded areas and the subsequent risk management has become clearer. Due to some hazardous events in small river basins and flooded urban areas by heavy convective rainfall events, interest also increases for the consideration of flood risk and management in small catchment areas. The awareness of floods in small (about 10 -200 km²) and urban catchment areas is getting greater. This is also reflected in the establishment of the European Flood Protection System (EFAS) and the development of national early warning systems for flash floods. But in addition to the development of early warning systems, the (small catchments’) flood risks and risk management has to be better understood and communicated to the local authorities. Apart of the short warning times, the interaction between high (convective) precipitation, surface flow and channel flow in small watersheds is much faster than in the slow-responding large river basins. The flooding is not only caused by rivers passing over their banks (fluvial flooding), but also by flooded surface area (pluvial flooding) and reached sewer capacity (urban flooding). This presentation deals with the topic, to what extent the different flood characteristics in small catchments make an adaptation of hydrological and hydrodynamic model approaches necessary. In addition to a short theoretical classification of modelling techniques, applications and results of various hydrological (HEC-HMS, OpenLISEM) and hydraulic models (HEC-RAS, FloodArea) in a typical central German low mountain range are going to be shown. The aim of these different model applications is to show that hydrological-hydraulic processes are strongly linked, especially in small catchment areas. The project area, the "Fischbach"-catchment area (about 37 km²), has recently been well monitored through our Department monitoring campaign and there are long-term observation time series of the Hessian Agency for Nature Conservation, Environment and Geology (HLNUG) and German Weather Service (DWD) available. In this presentation different hydrological and hydraulic modelling approaches are shown