Hydrodynamic simulation of the effects of stable in-channel large wood on the flood hydrographs of a low mountain range creek, Ore Mountains, Germany
Authors: Rasche Daniel , Reinhardt-Imjela Christian , Schulte Achim , Wenzel Robert 
Published: October 2019

Abstract. Large wood (LW) can alter the hydromorphological and
hydraulic characteristics of rivers and streams and may act positively on a
river's ecology by i.e. leading to increased habitat availability. On the
contrary, floating as well as stable LW is a potential threat for
anthropogenic goods and infrastructure during flood events. Concerning the
contradiction of potential risks and positive ecological impacts,
addressing the physical effects of stable large wood is highly important.
Hydrodynamic models offer the possibility of investigating the hydraulic
effects of anchored large wood. However, the work and time involved
varies between approaches that incorporate large wood in hydrodynamic
models. In this study, a two-dimensional hydraulic model is set up for a
mountain creek to simulate the hydraulic effects of stable LW and to compare
multiple methods of accounting for LW-induced roughness. LW is
implemented by changing in-channel roughness coefficients and by adding
topographic elements to the model; this is carried out in order to determine which method most
accurately simulates observed hydrographs and to provide guidance for future
hydrodynamic modelling of stable large wood with two-dimensional models. The study area comprises a 282 m long reach of the Ullersdorfer
Teichbächel, a creek in the Ore Mountains (south-eastern Germany).
Discharge time series from field experiments allow for a validation of the model
outputs with field observations with and without stable LW. We iterate
in-channel roughness coefficients to best fit the mean simulated and
observed flood hydrographs with and without LW at the downstream reach
outlet. As an alternative approach for modelling LW-induced effects, we use
simplified discrete topographic elements representing individual LW elements
in the channel. In general, the simulations reveal a high goodness of fit between the
observed flood hydrographs and the model results without and with stable
in-channel LW. The best fit of the simulation and mean observed
hydrograph with in-channel LW can be obtained when increasing in-channel
roughness coefficients throughout the reach instead of an increase at LW
positions only. The best fit in terms of the hydrograph's general shape can
be achieved by integrating discrete elements into the calculation mesh. The
results illustrate that the mean observed hydrograph can be satisfactorily
modelled using an adjustment of roughness coefficients. In conclusion, a time-consuming and work-intensive mesh manipulation is
suitable for analysing the more detailed effects of stable LW on a small
spatio-temporal scale where high precision is required. In contrast, the
reach-wise adjustment of in-channel roughness coefficients seems to
provide similarly accurate results on the reach scale and, thus, could be
helpful for practical applications of model-based impact assessments of
stable LW on flood hydrographs of small streams and rivers.