Preliminary Hydrodynamic Investigation of Water and Sediment Fluxes in the Maggia River
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TiRiLab Report No. 1 Preliminary hydrodynamic investigation of water and sediment fluxes in the Maggia River Christos Argyrakis Emmanouil Skourtis March 2018 TiRiLab is part of the project HyMoCARES (HydroMorphological assessment and management at basin scale for the Conservation of Alpine Rivers and re- lated Ecosystem Services). Partial funding is provided under the EU Alpine Space Programme through the New Regional Policy of the Swiss Confederation and by the Republic and Canton of Ticino. Disclaimer: This is a student report produced as part of the education pro- gramme in the Master of Environmental Engineering at ETH Zürich. The results reported here are not scientifically reviewed, do not necessarily represent the views of the supervisors, and should be used with caution. For more information contact: Peter Molnar ([email protected]) Abstract The riverine corridor in the Maggia Valley, extending from Bignasco to Ponte Brolla with a length of about 23 km, is the examined river reach in this project. The river bed is characterized by strong dynam- ics and high sediment transport rates. As a result, a braided river system with gravel bars and vegetated islands is noticed over a length of around 7.5 km near the village of Someo in the central part of the riverine corridor. The dynamic morphology of the area makes it attractive for hydrodynamic investigation of water and sediment fluxes. The software system BASEMENT was used for a preliminary simulation of sediment transport in the Maggia riverine corridor in order to set the basis for the quantification of the erosion and deposition rates in the river bed as well as the required water discharge for a significant geomorphic work to take place. In the pre-processing phase the historical erosion and deposition patterns in the river reach were deter- mined based on cross-sectional data taken by BAFU and covering the period 1978-2015. In addition, a comparison of the elevations of the cross-sectional points with the values of the used Digital Terrain Model (DTM) was performed. This resulted in a mean absolute deviation of 0.50 m as well as in a histogram which indicates that the majority of the DTM values are higher than the observed cross-sec- tional elevations. This can be explained by the fact that points below water have not been surveyed for the production of the DTM. The morphodynamic simulations with BASEMENT were performed on a smaller river reach, extending from the Riveo settlement until before the Maggia settlement and having approximately a length of 8.3 km. This river section contains a braided part of the river with gravel bars and vegetated islands. In the grid generation phase two computational meshes (one coarser and one finer) were produced using different maximum area constraints with the finer mesh having a mean cell size in the main channel of 95 m2, while in the coarser mesh this value is 126 m2. Two different friction zones were also defined at each mesh (one zone for gravel/cobbles and one zone for vegetation). The calibration of the hydraulic model was implemented by comparing the simulated water depths with the observed ones at the Lodano station and the selected calibration parameter was the friction coeffi- cient of the gravel introduced with the Strickler value (kStr). Hydrodynamic simulations were run with BASEMENT using three kStr values, which define the possible range of kStr of gravel, and then the mod- elled water depths were plotted against the measured ones. The resulted calibration was satisfactory. For the identification of the threshold discharge for which we start having a large increase of the erosion rate and/or the eroded area for the two meshes, uniform sediment morphodynamic simulations were performed for two grain sizes and for different discharges. To evaluate the erosion, three different ap- proaches were used, and a threshold discharge of 220 m3/s was defined. To create the inflow hydro- graph for the main simulation in our project, only discharges above the defined threshold were used from the time series of hourly discharges from the Lodano gauge, this was done to reduce the duration of the simulation and only take into account the larger discharges. ~ i ~ It was found that the simulated Δz values have a similar range of values as the observed ones in the study reach which indicates that the model reproduces successfully the order of magnitude of erosion and deposition in the study domain. It was also found that the observed volumes of eroded and depos- ited sediment are larger than the simulated ones. A main reason for this deviation is the usage of a digital terrain model (DTM) that does not give the true river bed elevation in the case of water cover, another important reason are the assumptions that were made during the calculation of the observed sediment budget due to the lack of a dense network of measured cross-sections. ~ ii ~ Acknowledgement The authors wish to thank Prof. Dr. Peter Molnar for giving us the opportunity to work on this very inter- esting topic in the context of our master project as well as for his support by always having an open door for project related questions and scientific advice. Special thanks also go to Maria Magdali for the coordination of the data acquisition. But also for her help and support in many aspects during this project. We would also like to express our thanks to Francesco Caponi for his BASEMENT related tips on the mesh generation and the simulation. ~ iii ~ Table of contents 1 Introduction ________________________________________________________________ 1 1.1 Objectives __________________________________________________________________ 1 2 Study area _________________________________________________________________ 2 2.1 Location and general information ________________________________________________ 2 2.2 Data acquisition and origin _____________________________________________________ 3 2.3 Morphology and topography ____________________________________________________ 4 2.3.1 Historical erosion and deposition patterns _________________________________________ 4 2.3.2 Comparison of cross-sectional points with DTM _____________________________________ 5 2.4 Hydrology __________________________________________________________________ 7 3 Methodology ______________________________________________________________ 10 3.1 Model _____________________________________________________________________ 10 3.2 Pre-processing _____________________________________________________________ 11 3.2.1 Model domain ______________________________________________________________ 11 3.2.2 Grid generation _____________________________________________________________ 12 3.3 Simulation process __________________________________________________________ 15 3.3.1 Simulation _________________________________________________________________ 15 3.3.2 Simulation input files and parameters ____________________________________________ 16 3.3.3 Calibration of hydraulic model __________________________________________________ 17 3.3.4 Threshold identification for sediment transport _____________________________________ 19 3.3.5 Non-stationary flood wave morphodynamic simulation _______________________________ 21 3.4 Calculation of observed sediment budget _________________________________________ 22 4 Results and discussion _____________________________________________________ 24 4.1 Thresholds for sediment transport ______________________________________________ 24 4.2 Simulation results ___________________________________________________________ 27 4.3 Observed vs Simulated data ___________________________________________________ 29 4.3.1 Reasons for deviation ________________________________________________________ 30 5 Conclusions _______________________________________________________________ 31 5.1 Morphology and hydrology ____________________________________________________ 31 5.2 Modelling procedure _________________________________________________________ 31 5.3 Final remarks _______________________________________________________________ 32 6 Bibliography ______________________________________________________________ 33 Appendix _______________________________________________________________________ a A. Morphology and topography ____________________________________________________ a B. Hydrology __________________________________________________________________ e C. Command file of BASEMENT ____________________________________________________ f ~ iv ~ List of Figures Figure 1: Location of the Maggia valley (from Ruf, 2007). --------------------------------------------------------- 2 Figure 2: Overview map of the Maggia valley (Map by Swisstopo). --------------------------------------------- 2 Figure 3: Study site and common cross-sections for all the measurement years. --------------------------- 4 Figure 4: Bed level change Δz along the study reach for the period 1978-2015. ---------------------------- 5 Figure 5: Distribution of the 92 cross-sections based on their aggradation and degradation rates for the period 1978-2015. ------------------------------------------------------------------------------------------------------------ 5 Figure 6: Study site and bed cross-sectional points of the year 2015. ------------------------------------------ 6 Figure 7: Distribution of the bed cross-sectional points based on their elevation deviation Δz between the DTM and the measured values (2015).----------------------------------------------------------------------------- 6 Figure 8: Bed cross-sectional