Planform changes and large wood dynamics in two torrents during a severe flash flood in Braunsbach, Germany 2016

Abstract

This work presents a post-event survey study, addressing the geomorphic response and large wood budget of two torrents, Grimmbach and Orlacher Bach, in southwestern Germany that were affected by a flash flood on May 29, 2016. During the event, large amounts of wood clogged and damaged a bridge of a cycling path at the outlet of the Grimmbach, while the town of Braunsbach was devastated by discharge and material transported along the Orlacher Bach. The severity of the event in these two small catchments (30.0 km and 5.95 km, respectively) is remarkable in basins with a relatively low average slope (10.7 and 12.0%, respectively). In order to gain a better understanding of the driving forces during this flood event an integrated approach was applied including (i) an estimate of peak discharges, (ii) an analysis of changes in channel width by comparing available aerial photographs before the flood with a post-flood aerial surveys with an Unmanned Aerial Vehicle and validation with field observations, (iii) a detailed mapping of landslides and analysis of their connectivity with the channel network and finally (iv) an analysis of the amounts of large wood recruited and deposited in the channel. The morphological changes in the channels can be explained by hydraulic parameters, such as stream power and unit stream power, and by morphological parameters such as the valley confinement. This is similar for LW recruitment amounts and volume of exported LW since most of it comes from the erosion of the valley floor. The morphological changes and large wood recruitment and deposit are in the range of studied mountain rivers. Both factors thus need to be considered for mapping and mitigating flash flood hazards also in this kind of low range mountains.

Drivers of Low Instream Large Wood Retention and Imprints of Wood Mobility in Mountain Nonperennial Streams of a Mediterranean Semiarid Environment

“…This underlines the importance of the channel width versus wood length ratio as the main factor controlling wood stability in the channel (Braudrick & Grant, ; Gurnell et al, ). Valley confinement is another possible predictor of wood retention because of the different characteristics of LW recruitment processes based on hillslope‐channel coupling and the potential space for free channel migration, which are also connected with channel widening during high‐magnitude floods (Lucía et al, ; Martin & Benda, ; Mazzorana et al, ).…”

A Novel Hybrid Swarm Optimized Multilayer Neural Network for Spatial Prediction of Flash Floods in Tropical Areas Using Sentinel-1 SAR Imagery and Geospatial Data

“…Flash flood hazards are often triggered by heavy downpours, torrential rainfalls, or tropical rainstorms. Reports on the destructive effects of flash floods on human lives have been observed worldwide [ 4 , 5 , 6 , 7 , 8 , 9 ]. Human factors also contribute to the occurrence of flash floods i.e., deforestation and unplanned land use.…”

Impact of Scientific Scrutiny after the 2016 Braunsbach Flash Flood on Flood-Risk Management in the State of Baden-Württemberg, Germany

“…Hence, it comes as no surprise that a task force was formed shortly after the event, to assess the risk and adjust mitigation efforts accordingly. Fifth, evaluations by independent technical experts suggest that the task force above initially did not pay sufficient attention to the potential risks associated with morphological changes and the transport of sediment and large woody debris ("large wood" hereafter) in the risk assessment and planning of mitigation measures [34,[40][41][42][43]. Consequently, the municipalities' mitigation efforts were slow to take into consideration these aspects of the fluvial geomorphology.…”