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