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MAPPING AREAS EXPOSED TO EROSION AND WATER FORCES DURING EXTREME FLOODS IN STEEP TERRAIN MICHAL PAVLÍČEK, ODDBJØRN BRULAND Department of Civil and Environmental Engineering, Norwegian University of Science and Technology, Trondheim, Norway. Introduction Results Discussion and conclusions References Methods The extent and potential consequences of floods in large water courses are mostly well mapped in Norway (NVE 2018). Here, the flood risk is related to inundation which has been mapped using the hydraulic routing of the floods of different return periods. The risks in small and steep catchments are not that well mapped. The faster response and the forces due to high water velocities induce another risk dimension that is significantly more challenging to handle. The main goal of this study is to examine the critical points in the river channel during flash floods in steep rivers. In this study, critical points are spots where water could find its way out from the river channel, which could be caused by erosion and deposition of sediments. Fig. 1. Aerial view of Utvik flood event from the fjord (VG 2017), with original river thalweg (blue line) and bridges. TELEMAC – MASCARET was selected as a software package to carry out simulations of flash floods in steep rivers. Telemac’s 2D module was used to run 2D hydrodynamic simulations (i.e. rigid terrain) in the horizontal plane. The code solves Saint-Venant (shallow water) equations in non-conservative form (Hervouet 2007). Regarding the morphodynamic simulations (i.e. including sediment transport and river’s bed evolution), Sisyphe module was used. the module solves river’s bed evolution with the sediment mass conservation equation (Exner equation) (Tassi 2017). Fig. 5. Results of hydrodynamic (left) and final morphodynamic (right) simulations. Displayed variable is maximum water depth; purple lines represent flow paths during flood event. Fig. 6. Results of hydrodynamic simulation (left, maximum shear stress), final morphodynamic simulation (center, bed evolution in the end of simulation) and bed evolution measured after real flood event (right, bed evolution (Kartverket 2018)); purple lines represent flow paths during flood event. Fig. 7. Results of hydrodynamic simulation. Maximum water depth (left); maximum shear stress (right); purple lines represent flow paths during flood event. Hydrodynamic simulation The simulation described next is first applied to Utvik and the same parameters and set-up were used in Innvik afterwards. Fig. 2. Flow hydrograph of Utvik‘s flood on July 24, 2017 (Bruland 2018). By discretization, the domain was divided into triangular mesh elements: size of 20 m in the fjord, 2 m in the river channel and of 3 m in the rest of the domain. A flow hydrograph (Fig. 2) of Utvik’s flood event was assigned as the inflow open boundary condition. Constant water surface elevation 0 m a.s.l. was attached to the outflow open boundary condition. Different values of Manning roughness coefficient (n) were assigned: 0.045 for the river channel and the fjord bed, 0.025 for the roads, and 0.100 for the rest of the domain (i.e. possible inundation area). Finite element method was used for resolution of Saint-Venant equations. Constant viscosity turbulent model was assumed. 0 50 100 150 200 0 2 4 6 8 10 12 14 16 18 20 22 0 Q [m3/s] t [h] Bruland, Oddbjørn. 2018. Extreme flood in small steep cathcment case Utvik. In XXX Nordic Hydrological Conference. Bergen, Norway. Hervouet, Jean-Michel. 2007. Hydrodynamics of Free Surface Flows: Modelling with the Finite Element Method ( John Wiley & Sons: UK). Kartverket. 2018. Høydedata, Accessed 13/12/2018. https://hoydedata.no/LaserInnsyn/. NVE. 2018. Kartlegging, Accessed 13/12/18. https://www.nve.no/flaum-og-skred/kartlegging/. VG. 2017. Per Inge Verlos (58) hus sto midt i flommen: – Helt ufattelig Accessed 13/12/2018. https://www.vg.no/nyheter/innenriks/i/GVbeQ/per-inge-verlos-58-hus-sto-midt-i-flommen-helt-ufattelig Tassi, Pablo. 2017. Sisyphe User Manual Version 7.2. The results presented in this poster showed that the hydrodynamic simulation could be used to determine the capacity of the river channel and finding the critical points due to the shear stress. However, it does not take into account the creation of the new flow paths due to erosion and deposition processes. As for the morphodynamic simulation, as can be seen in Fig. 5, the inundation area in both types of simulations is not much different. The results of the current morphodynamic simulation present a non-erodible bed, thus, a negligible volume of transported sediment in the river channel. Therefore, the set-up of the simulation should be tested and improved on the simpler cases with steep slope, covering an erodible bed in the river channel. Erosion and sedimentation processes are important to determine the critical points during flash floods, hence, the bed load transport in steep terrains needs to be investigated further. Further research will focus on numerical and physical modeling of the phenomena. Numerical simulations of lab experiments and cases with field measurements will be carried out to calibrate and validate the models. More numerical models will be used (TELEMAC-MASCARET, REEF3D, HEC-RAS). Innvik Hydrodynamic simulation was carried out in Innvik so far. As it can be seen in Fig. 7, the river channel in Innvik is straighter than Utvik’s and there is no clear critical point as in Utvik. Utvik Due to the instabilities, non-erodible bed in the river channel was assumed for the final simulation set-up. Sediment diameter in the inundation area was set up 0.5 mm. Meyer-Peter and Müller formula was used for bed load transport and no suspension load was simulated. It was assumed an active layer thickness of sediment 2.5 m. The results of the final morphodynamic simulation are presented in Fig. 4 (right) and Fig. 6 (center). In the hydrodynamic simulation (Fig. 5, left), the main flow path is located in the river channel and the other paths matches quite well with flow paths from the real flood. As for the final morphodynamic simulation, results show that the water found new flow paths on the left side of the bridge 02 (Fig. 5, right). But the inundation area is similar as in the hydrodynamic simulation. As it can be seen in Fig. 6, the results of shear stress from hydrodynamic simulation are suitable to find the critical point besides bridge 02. The flooding of Utvik (western Norway, Fig. 1) in July 2017 is reconstructed using TELEMAC–MASCARET numerical simulating software. The methodology is also tested and compared in the neighboring river in Innvik. Coarse sediment, boulders and rocks are located in the river bed and the slope of the river channel in Utvik and Innvik is steep (ca. 3-17%). In the morphodynamic simulation, instabilities in the bed evolution were observed in the reaches with the steepest slope and steep river banks. The examples of the instabilities in Utvik can be seen in Fig. 3 and 4. The instabilities in the river channel were observed in all simulations with different set-ups. Fig. 3. Longitudinal profile of the river channel in Utvik. Black line represents the original river bed, red and green lines represent river bed obtained from morphodynamic simulations with different sediment diameters. Fig. 4. Cross-section of the river channel in Utvik in the reach with slope 17 %. Black line represents the original river bed, red and green lines represent river bed obtained from morphodynamic simulations with different sediment diameters. 0 20 40 60 80 200 250 300 350 400 450 500 550 600 650 700 750 800 Elevation [m a.s.l.] Distance [m] original river bed d50 = 0.01 m d50 = 0.001 - 0.1 m 17 % 3 % 12 17 22 27 32 40 50 60 70 80 90 100 110 120 130 140 Elevation [m a.s.l.] Distance [m] original river bed d50 = 0.01 m d50 = 0.001 - 0.1 m Morphodynamic simulation Several combinations of sediment parameters, bed load transport formulas, mesh size and numerical set-up were tested to find out the best match with the flow paths observed during the flood event (Fig. 1).
