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|Language of final thesis:||English|
|Title of the thesis:||Human influences on floodplain lakes sedimentation.|
|Summary:||Lateral Erosion of the Sacramento River, California: Human influences and consequences for floodplain lake habitats The human influences on changes in bank erosion became an important issue in management strategies. Along the Sacramento River, California, requirements for agricultural lands and ecosystem preservation frequently conflict, and resolving this task requires the intimate understanding of factors contributing to geomorphic changes and resulting ecological effects. It is often difficult to distinguish the relative importance of potential controlling factors influencing the bank erosion. This study is based on a spatial and temporal analysis of the active channel and associated floodplain lakes using aerial photographs spanning five decades (1942, 1962, 1985, 1999) over the entire 140 km long reach. Planimetric changes were analysed longitudinally (synchronic analysis) and temporally (diachronic) to highlight the spatial structures and their evolution through time. While some planimetric changes are clear, such as channel narrowing and increased bank erosion in the middle reach from 1962-1985 and the upstream reach from 1985-1999, the interpretation of these changes is complex because of various human pressures acting over different time scales (bank protection, flow diversion, sediment starvation, land-use changes) and various natural influences (flood sequences through out the period, geological setting). A longitudinal and temporal analysis is then used to assess the importance of each of them. In such context, we show that bank erosion rates were higher in the period 1962-1985 than before or after, probably caused by the greater floods during that period, reflecting a relationship between bank retreat and flood magnitude. Moreover the Shasta dam appears to have caused decreases in active channel width from 1942-1999 due to peak flow decreases, and downstream propagation of narrowing, presumably due to sediment starvation. Following these results open-questions and challenging issues in terms of bank erosion preservation and of ecological consequences are then discussed. Revitalisation of Ht. Rhône and First Feedbacks on Revitalised Floodplain Lakes Sedimentation along the Rhône River The revitalisation work on Rhône River has been done on three different localities: Chautagne, Belley and Brégnier-Cordon. The post-revitalisation time period of the floodplain lakes of Brégnier-Cordon and Belley is relatively short and so the comparison between pre- and post-revitalisation state is only just possible. For floodplain lakes with the longest post-revitalisation observations there are unfortunately no pre-revitalisation observations. If the processes of flowing water are the agents for fluvio-geomorphologic evolution and change, the hydrolic connexion and exchange of water and sediment fluxes are the principal control factor for floodplain lake. The morphologic form (geometry of floodplain lake) as a result of processess become consequently the control factor. Three steps were considered: i) to define the number connexions "floodplain lake-old Rhône channel" by year (the frequency of overbank connexion) ; ii) the measurement of the sedimentation rates based on the ratio between the mean sediment thickness and the time since the revitalisation works; ii) the statistical analysis of the relationships and inter-lake analysis (the characterise of connexion frequency and to define the life expectancy of floodplain lakes). The different groups were defined by nPCA analysis. Morphodynamics of the Exit of a Cut-off Meander: Experimental Findings from Field and Laboratory Studies The morphological evolution of the entrances and exits of abandoned river channels governs their hydrological connectivity. The study focusses on flow and sediment dynamics in the exit of a cut-off meander where the downstream entrance is still connected to the main channel, but the upstream entrance is closed. Two similar field and laboratory cases were investigated using innovative velocimetry techniques (acoustic Doppler profiling, image analysis). Laboratory experiments were conducted with a mobile-bed physical model of the Morava river (Slovakia). Field measurements were performed in the exit of the Port-Galland cut-off meander, Ain river (France). Both cases yielded consistent and complementary results from which a generic scheme for flow patterns and morphological evolution was derived. A simple analogy with flows in rectangular side cavities was used to explain the recirculating flow patterns which developed in the exit. A decelerating inflow deposits bedload in the downstream part of the cavity, while the upstream part is eroded by an accelerating outflow, leading to the retreat of the upstream bank. In the field, strong secondary currents were observed, especially in the inflow, which may enhance the scouring of the downstream corner of the cavity. Also, fine sediment deposits constituted a silt layer in a transitional zone, located between the mouth of the abandoned channel and the oxbow-lake within the cut-off meander. Attempts at morphological prediction should consider not only the flow and sediment conditions in the cavity, but also the dynamics of the main channel.|
|Key words:||lateral erosion, floodplain lakes, Sacramento River, Rhône River, Morava River, human influence|
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|Bylo zvoleno non-release of final thesis - iný závažný dôvod (v súlade s § 51 Zákona č. 618/2003 Z. z. o autorskom práve a právach súvisiacich s autorským právom v znení neskorších predpisov (autorský zákon)):|
praca vytvorená ako these en cotutelle na základe medziuniverzitnej zmluvy s Université Jean Moulin Lyon 3.
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