162 | INTERPRAEVENT 2016 – Extended Abstracts IP_2016_EA163
INTRODUCTION, SUBJECT OF INVESTIGATION
Bed load transport in rigid torrent channels means a crucial process for torrential hazard management on alluvial fans. Small changes of topography and constructional design of the channel have a major influence on the transport behavior. Knowledge of transport capacities in the rigid torrent channel at specific discharges and for the expected grain size spectrum is important for the design of sediment retention structures, such as filtering or dosing structures in the upper catchment. When focusing on protection of settlement areas against floods, main tasks at torrent channels generally are to best possibly avoid overbank sedimentation and provide the in-channel movement of sediments into the receiving water course (Gems et al., 2014). Specific focus has to be put on narrow points along the torrent channel, such as bridges, and changes in cross section geometry and channel gradient.
The research presented within this paper deals with the analysis of bed load transport processes in a rigid torrent channel. Effects of changing gradients along the channel on the appearance of deposition processes were systematically analyzed for a specific set of loading conditions (discharge, bed load fraction, grain size distribution). A physical scale model of the Schnannerbach torrent channel was therefore set up at a scale of 1:30. The Schnanner- bach torrent is situated in the Tyrolean Limestone Alps and represents an orografically left tributary of the Rosanna River. At the catchment‘s alluvial fan, the torrent channel features gradients between 6 and 12 %. The gradient is significantly decreasing towards the confluence with the receiving water.
According to the Austrian Service for Torrent and Avalanche Control, the 150 years-design-flood features a peak discharge of 24 m³/s and 35,000 m³ of bed load volume, which is potentially mobilized under torrential hazard conditions (Rudolf-Miklau et al., 2006). With regard to the protection against
harm-bringing flood events, sediment retention barriers were built in the middle reach of the Schnannerbach catchment. Deposition processes in the torrent channel are strongly influenced from the confluence with the Rosanna River, but also suffer from the significant decrease in gradient before entering the receiving water (Gems et al., 2014).
EXPERIMENTAL MODELLING (1:30)
Steady-state experiments with specific discharges, bed load rates and grain characteristics were ex- ecuted with regard to the transport and sedimenta- tion processes in the rigid torrent channel. Thereby, the confluence zone and as well the channel bed of the receiving stream were not considered in order to (i) avoid regressive sedimentation processes and (ii) fully focus on the capacities of the torrent channel (Gems et al., 2014). Figure 1,a provides a sketch of the physical scale model. The investiga- tions primarily focus on the determination of critical loading conditions for overbanking, each for different gradients in the torrent channel or rather gradient changes in the longitudinal profile from steep to flatter conditions. Further investigations on changes in the longitudinal profile at bridges were made and adaptations of the bridge structure in order to decrease sediment deposition were tested.
The location of the bridge structure compared to the change in gradient is thereby analyzed in detail.
Additionally, focusing on the characteristics of the channel bed and the flow resistance in the channel, two specific channel bed forms and its influence on bed load transport were analyzed: Firstly, the channel bed was characterized by a sequence of artificial steps and pools (Fig. 1,a (i)). Further, experiments were also made with a plane channel bed (Fig. 1,a (ii)), implying lower flow resistance and form roughness and, consequently, higher transport rates. All experiments significantly support a better understanding of deposition
Experimental investigations on bed load transport and deposition processes in a rigid torrent channel
Michael Sturm, DI1; Bernhard Gems, DI Dr.1; Markus Moser, DI2; Markus Aufleger, Univ.-Prof. Dr.-Ing. habil.1
DATA ACQUISITION AND MODELLING (MONITORING, PROCESSES, TECHNOLOGIES, MODELS)
INTERPRAEVENT 2016 – Extended Abstracts | 163
processes in rigid torrent channels, of effects of small changes in the boundary conditions (Fig. 1,b) and, with it, planning tasks for protection against torrential hazards or rather floods intense sediment loads.
The work presented within this paper mainly focuses on model set-up, the testing programme and on main findings and results of the investiga- tions. Fig. 1,b gives an impression on experimental modelling, the situations showed are each based on a constantly impacting discharge and a bed load ratio of roughly 12 %. As observed in every experi- ment, the transport capacity of the rigid torrent channel is obviously always limited to its flattest part. The screenshots in Fig. 1,b at a specific time during the experiments point out the impact of small changes in gradient. The entire sediment loads are transported through the channel with a gradient of 9 %. First flooding appears for the experiment with a lower gradient of 7.5 % at that
time. Further, fully leakage of the flow mixture appears with a lower gradient of 6 %.
The experimental analysis was preceded by com- prehensive tests of regressive deposition in the torrent, considering also the entry to the Rosanna River and the transport behavior in the confluence zone (Gems et al., 2014).
ACKNOWLEGEMENTS
The authors thank the Austrian Service for Torrent and Avalanche Control for their support and the consideration of the present work in the project
„SedAlp“, which is funded by the European Union within the Alpine Space Programme.
REFERENCES
- Gems B., Sturm M., Vogl A., Weber C., Aufleger M. (2014). Analysis of damage causing hazard processes on a torrent fan - scale model tests of the Schnannerbach Torrent channel and its entry to the receiving water. In: Fujita, M. (ed.): Extended Abstracts of the Interpraevent 2014 in the Pacific Rim. November 25-28 in Nara, Japan. Klagenfurt:
Internationale Forschungsgesellschaft Interpraevent / International Research Society Interpraevent, 64-65.
- Rudolf-Miklau F., Ellmer A., Gruber H., Hübl J., Kleemayr K., Lang E., Markart G., Scheuringer E., Schmid F., Schnetzer I., Weber C., Wöhrer-Alge M.
(2006). Hochwasser 2005 - Ereignisdokumentation, Teilbericht der Wildbach- und Lawinenverbauung, Vienna (in German).
inlet reservoir - discharge hydrograph
continuous bed load supply (manually)
285 m
45 m
weighting of accumulated bed load moveable bridge with gradient change (optional)
(iii) (i),(ii)
(iii)
(i) (ii) different
layouts of the channel bed
gradient change
T = 30min 12%
9%
12%
7,5%
12%
6%
T = 30min
T = 30min
Fig. 1, a Fig. 1, b
(i) (i) (i)
Figure 1. Sketch of the physical scale model (1:30) (a) and selected experimental results (b)
KEYWORDS
torrent; Bed Load Transport; Rigid Torrent Channel; Physical Scale Model
1 Unit of Hydraulic Engineering, University of Innsbruck, Innsbruck, AUSTRIA, michael.sturm@uibk.ac.at 2 Austrian Service for Torrent and Avalanche Control, Tamsweg, AUSTRIA