Hydrodynamics of the river Hiitolanjoki in Finland under dam removal process

The re-naturalization of rivers and the removal of dams is at the center of world’s attention: in fact, it is a practice that is becoming more and more important in the United States, and Europe is following the American example. The most important project present in our continent is "Dam Removal Europe": about 101 dams were removed in 2020, and it is estimated that by 2030 about 25,000 km of rivers will be restored. Obviously, only dams that are no longer used for energy production and that over time have become obsolete are considered, because in these cases the maintenance costs are exceeding the earnings. This thesis focuses on the removal of the three dams along the Hiitolanjoki River in Finland. This river is an important fish route: it is the main breeding river for Ladoga Salmons. The removal of the three dams began in 2021, with the disruption of the one in correspondence of the Kangaskoski rapid; the removal of the other two dams is planned for 2022 and 2023. In this work, the flow velocity data that were sampled in the area of the Kangaskoski rapid were analyzed. The data were collected in two different periods: during Spring 2021, before the removal of the dam, and during Autumn 2021, immediately after the removal of the dam. The purpose of the work is to analyze the short-term impact of dam removal, in particular the hydrodynamic variations, differences in the erosive potential and river depth. The data were collected using an Acoustic Doppler Current Profiler coupled with a GPS in such a way as to have precise and well localized data; surface velocity, near bed velocity and mean speed, were interpolated using the Inverse Distance Weighted and Kriging methods. Using the velocity interpolations, it is possible to see how the velocity, after the removal of the dam, has generally decreased, causing the sediment to be easily settled. This could also be the one of the reasons why the river depth has decreased after the dam removal. The changes in flow velocities and water depth could represent an obstacle for the re-naturalization of the river, because they can affect the water quality and the fish nesting. While the velocities have decreased on average, changes have been heterogeneous in space and increases have been recorded in different locations. The differences in the erosion potential were established using a hotspot analysis, with which the speed clusters (both high and low velocity) were highlighted. The analysis shows that some areas, in particular the stretches of river close to the infrastructures (Houses and Bed and Breakfasts), after the removal of the dam, begin to be potentially affected by erosion. For this reason, it is necessary to proceed with the research in order to plan mitigation interventions for the protection of the surroundings.