Hydrodynamic simulation of the effects of in-channel large woody debris on the flood hydrographs of a low mountain range creek, Ore Mountains, Germany

Abstract. Fifteen years after introducing the European Union's water framework directive (WFD), most of the German surface water bodies are still far away from having the targeted good ecological status or potential. One reason are insufficient hydromorphological diversities such as riverbed structure including the absence of natural woody debris in the channels. The presence of large woody debris (LWD) in river channels can improve the hydromorphological and hydraulic characteristics of rivers and streams and therefore act positively on a river’s ecology. On the contrary, floating LWD is a potential threat for anthropogenic goods and infrastructure during flood events. Concerning the contradiction of potential risks as well as positive ecological impacts, addressing the physical effects of large woody debris is highly important, for example to identify river sections in which large woody debris can remain or can be reintroduced. Hydrodynamic models offer the possibility of investigating the hydraulic effects of fastened large woody debris. In such models roughness coefficients are commonly used to implement LWD, however, because of the complexity of the shape of LWD elements this approach seems to be too simple and not appropriate to simulate its diverse effects especially on flood hydrographs. Against this background a two-dimensional hydraulic model is set up for a mountain creek to simulate the hydraulic effects of LWD and to test different methods of LWD implementation. The study area comprises a 282 m long reach of the Ullersdorfer Teichbächel, a creek in the Ore Mountains (South-eastern Germany). In previous studies, field experiments with artificially generated flood events have been performed with and without LWD in the channel. Discharge time series from the experiments allow a validation of the model outputs with field observations. Methodically, in-channel roughness coefficients are changed iteratively for retrieving the best fit between mean simulated and observed flood hydrographs with and without LWD at the downstream reach outlet. In addition, roughness values are modified at LWD positions only and, simplified discrete elements representing LWD were incorporated into the calculation mesh. In general, the model results reveal a good simulation of the observed flood hydrographs of the field experiments without in-channel large woody debris. This indicates the applicability of the model used in the studied reach of a creek in low mountain ranges. The best fit of simulation and mean observed hydrograph with in-channel LWD can be obtained when increasing in-channel roughness through decreasing Strickler coefficients by 30 % in the entire reach or 55 % at LWD positions only. However, the increase of roughness in the entire reach shows a better simulation of the observed hydrograph, indicating that LWD elements affect sections beyond their own dimensions i.e. by forming downstream wake fields. The best fit in terms of the hydrograph's general shape can be achieved by integrating discrete elements into the calculation mesh. The emerging temporal shift between simulation and observation can be attributed to mesh impermeability and element dimensions causing too intense water retention and flow alteration. The results illustrate that the mean observed hydrograph can be satisfactorily modelled using roughness coefficients. Nevertheless, discrete elements result in a better fitting shape of the simulated hydrograph. In conclusion, a time-consuming and work-intensive mesh manipulation is suitable for analysing detailed flow conditions using computational fluid dynamics (CFD) on small spatio-temporal scale. Here, a close-to-nature design of discrete LWD objects is essential to retrieve accurate results. In contrast, the reach-wise adjustment of in-channel roughness coefficients is useful in larger scale model applications such as 1D-hydrodynamic or rainfall-runoff simulations on catchment scale.