<p>After extreme flash floods events 2016 in Bavaria, the cooperation project HiOS (reference map for surface runoff and flash floods) was started aiming at the detailed analysis of risk generated by flash floods using GIS methods as well as hydrological and hydrodynamic models. Part of the risk analysis is done using hydrodynamic rainfall-runoff modeling (HDRRM). HDRRM gets more and more popular since hydrodynamic models are able to accept rainfall as input. But most of the known hydrodynamic models have no integrated precipitation modules and therefore cannot be used uniquely for rainfall-runoff modeling. In this study, TELEMAC-2D is used for HDRRM because it already contains the SCS-CN-method and offers the possibility to implement new precipitation modules due to its open source license. An additional advantage of TELEMAC-2D is the good scaling on HPC cluster systems.</p><p>In this study, two different approaches for runoff creation will be compared. (1) The well-proven SCS-CN method calculates effective rain. Due to its simple structure, the process of runoff generation is completely decoupled from runoff concentration. Consequently, SCS-CN cannot account for re-infiltration due to surface runoff. (2) However, the Green-Ampt infiltration (GAI) is coupled to surface runoff as long as the water depth is non-zero. GAI is implemented recently and thus will be described in more detail. Both approaches are first tested using a simple model setup. The general model performance of the enhanced hydrodynamic rainfall-runoff modeling (EHDRRM) is verified using the case study Simbach/Triftern in Bavaria. This extreme flash flood event from 1<sup>st</sup> June 2016 hit the townships Simbach am Inn and Triftern. It is well documented and all necessary data is available in good quality. The main setup for the catchment area of 47 km² resp. 90 km² is built on a 1x1 m DEM, land use data, hydrological soil group data and 5 min-RADOLAN precipitation data. The calculated catchment outflow can be verified by measured data at the gauging stations in Simbach am Inn resp. Triftern. All comparisons include as reference results for precipitation without losses by infiltration.</p><p>The hydrodynamic precipitation runoff modeling HDRRM has proven to be a useful method for calculating flow paths, depths and velocities with a high spatial resolution during flash flood events. If the process of runoff generation is performed by the hydrodynamic model EHDRRM then the quality of results is improved significantly while keeping the modeling procedure simple. Concerning infiltration, EHDRRM allows for a physically correct representation taking the actual local water depth into consideration.</p>

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<p>In recent years, heavy precipitation and flash flood events frequently occurred in Germany. The project HiOS (reference map for surface runoff and flash floods) focusses on the analysis of these events using conceptual lumped precipitation runoff models, distributed raster-based water balance models (LARSIM and WaSiM), as well as a hydrodynamic model internally coupled with infiltration routines (TELEMAC-2D). The objective of our research is to analyze which factors and processes foster flash floods, and how they may be represented in models. We show a comprehensive methodological comparison using simulation results of some events in Bavaria. These do not include erosion and log jam scenarios.</p><p>The catchments distributed across whole Bavaria considering a variety of catchment characteristics and varying in size between 1.2 and 164km². All models are driven by 5 minute pseudo-calibrated radar precipitation data of the German Weather Service (YW product), which are available for entire Germany in a 1km² raster. The distributed water balance models are available using high-resolution cell grids. WaSiM uses a regular grid size of 50m, whereas LARSIM is run using 100m cells and an embedded hydrological response unit scheme. All TELEMAC-2D meshes are built with a standard mesh size of 5m in the catchment and 2m in the settled area of interest, while important hydrodynamic structures are resolved more in detail.</p><p>We want to highlight the variety of applied hydrological and hydrodynamic model approaches of runoff generation and concentration, whereby both, simple conceptual and complex physical methods are included. Runoff generation processes are represented using the SCS-CN method, a modified Lutz-Südbayern approach, a Xinjiang-bucket model combined with a Green&Ampt infiltration routine, as well as a layer-resolving Richards model. Beyond that, some of these consider silting up and soil crack formation. Runoff concentration processes are assessed by constant translation, Strickler flow time index method, a combination of Williams and Kalinin-Miljukov method, as well as finally with two-dimensionally resolved shallow water equations.</p><p>As expected, runoff generation is influenced by land use and soil parametrization. However, the amount of created runoff differs a lot changing the method of simulation. Furthermore, the runoff volume reacts quite sensitive to small changes in the preceding saturation conditions. Runoff concentration is influenced by slope, retention capacity of the flood plain, the network of drainages, as well as the formation of polders by water-crossing structures such as traffic infrastructure. Our results therefore clearly show the individual characteristics of extreme events depending on the catchment properties, which are reflected by the demands concerning the modelling techniques. The findings of this study illustrate the importance of improved radar-derived precipitation observations as well as the need for a spatially distributed and layered soil moisture product to enhance flash flood modelling using hydrological models.</p>

<p>Heavy rainfall and resulting flash flood events have been in the focus of research and the public in recent years. The relevance of the topic will become more prominent with increasing temperatures due to climate change. Extreme rainfall events in Germany like 2014 in Münster (North Rhine-Westphalia) or 2016 in Simbach am Inn (Bavaria) and Braunsbach (Baden-Wurttemberg) have also raised public awareness.</p><p>Hydrodynamic models for the simulation of fluvial events have been developed for a long time and are often used. However, the question arises to what extent these methods can be used for pluvial events. Hydrodynamic models allowing precipitation input are therefore well suited for the simulation of pluvial events, as they can display flow paths, depths, and velocities in high resolution. Nevertheless, defining precipitation without infiltration leads to an overestimation of the surface runoff. For this problem, an improved event simulation can be achieved by nesting hydrological processes into the hydrodynamic simulation procedure. In this study, we are using TELEMAC-2D as a hydrodynamic model because it uses precipitation in a spatially and temporally distributed manner and can be used very well by high-performance computing. LARSIM (Large Area Runoff Simulation Model) and WaSiM (Water Flow and Balance Simulation Model) are used as hydrological models.</p><p>The methodology for simulating flash floods can be divided into two important processes: runoff generation and runoff concentration. These are divided according to the strength of the respective model types:</p><ul><li>Runoff generation: SCS-CN value method (TELEMAC-2D), Green Ampt method (LARSIM), layer-resolving Richards method (WaSiM)</li> <li>Runoff concentration: Strickler roughness approach (TELEMAC-2D), Kalinin-Miljukov method (LARSIM), flow time index method (WaSiM)</li> </ul><p>In this study, we examine three different types of couplings:</p><ul><li>(1) The runoff concentration is calculated using the hydrodynamic model, the runoff generation is carried out using the CN value method.</li> <li>(2) The runoff generation in the entire catchment is calculated using the hydrological processes (LARSIM/WaSiM). The runoff concentration is still generated by the hydrodynamic model.</li> <li>(3) The runoff concentration in the upper catchment area is also calculated using hydrological methods, only the urban area is calculated hydrodynamically.</li> </ul><p>We compare the different coupling types with each other using some real flash flood events. The results are presented with the aim to identify which approach is necessary for a good representation of the flash flood event. This depends mainly on the local conditions in the catchment area (e.g.  culverts, land use) and the rainfall event (e.g. rainfall intensity and duration). The findings from this study will be transferred to unobserved catchments in the further course.</p>

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