@phdthesis{Leiser2021,
  author    = {Leiser, Rico},
  title     = {Biogeochemical processes governing microplastic transport in freshwater reservoirs},
  doi       = {10.25932/publishup-52024},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-520240},
  school      = {Universit{\"a}t Potsdam},
  pages     = {ix, 143},
  year      = {2021},
  abstract  = {The presented study investigated the influence of microbial and biogeochemical processes on the physical transport related properties and the fate of microplastics in freshwater reservoirs. The overarching goal was to elucidate the mechanisms leading to sedimentation and deposition of microplastics in such environments. This is of importance, as large amounts of initially buoyant microplastics are found in reservoir sediments worldwide. However, the transport processes which lead to microplastics accumulation in sediments, were up to now understudied. The impact of biofilm formation on the density and subsequent sedimentation of microplastics was investigated in the eutrophic Bautzen reservoirs (Chapter 2). Biofilms are complex microbial communities fixed to submerged surfaces through a slimy organic film. The mineral calcite was detected in the biofilms, which led to the sinking of the overgrown microplastic particles. The calcite was of biogenic origin, most likely precipitated by sessile cyanobacteria within the biofilms. Biofilm formation was also studied in the mesotrophic Malter reservoir. Unlike in Bautzen reservoir, biofilm formation did not govern the sedimentation of different microplastics in Malter reservoir (Chapter 3). Instead autumnal lake mixing led to the formation of sinking aggregates of microplastics and iron colloids. Such colloids form when anoxic, iron-rich water from the hypolimnion mixes with the oxygenated epilimnetic waters. The colloids bind organic material from the lake water, which leads to the formation of large and sinking iron-organo flocs. Hence, iron-organo floc formation and their influence on the buoyancy or burial of microplastics into sediments of Bautzen reservoir was studied in laboratory experiments (Chapter 4). Microplastics of different shapes (fiber, fragment, sphere) and sizes were readily incorporated into sinking iron-organo flocs. By this initially buoyant polyethylene microplastics were transported on top of sediments from Bautzen reservoir. Shortly after deposition, the microplastic bearing flocs started to subside and transported the pollutants into deeper sediment layers. The microplastics were not released from the sediments within two months of laboratory incubation. The stability of floc microplastic deposition was further investigated employing experiments with the iron reducing model organism Shewanella oneidensis (Chapter 5). It was shown, that reduction or re-mineralization of the iron minerals did not affect the integrity of the iron-organo flocs. The organic matrix was stable under iron reducing conditions. Hence, no incorporated microplastics were released from the flocs. As similar processes are likely to take place in natural sediments, this might explain the previous described low microplastic release from the sediments. This thesis introduced different mechanisms leading to the sedimentation of initially buoyant microplastics and to their subsequent deposition in freshwater reservoirs. Novel processes such as the aggregation with iron-organo flocs were identified and the understudied issue of biofilm densification through biogenic mineral formation was further investigated. The findings might have implications for the fate of microplastics within the river-reservoir system and outline the role of freshwater reservoirs as important accumulation zone for microplastics. Microplastics deposited in the sediments of reservoirs might not be transported further by through flowing river. Hence the study might contribute to better risk assessment and transport balances of these anthropogenic contaminants.},
  language  = {en}
}
@phdthesis{Nguyen2014,
  author    = {Nguyen, Van Manh},
  title     = {Large-scale floodplain sediment dynamics in the Mekong Delta : present state and future prospects},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-72512},
  school      = {Universit{\"a}t Potsdam},
  pages     = {ix, 95},
  year      = {2014},
  abstract  = {The Mekong Delta (MD) sustains the livelihood and food security of millions of people in Vietnam and Cambodia. It is known as the "rice bowl" of South East Asia and has one of the world's most productive fisheries. Sediment dynamics play a major role for the high productivity of agriculture and fishery in the delta. However, the MD is threatened by climate change, sea level rise and unsustainable development activities in the Mekong Basin. But despite its importance and the expected threats, the understanding of the present and future sediment dynamics in the MD is very limited. This is a consequence of its large extent, the intricate system of rivers, channels and floodplains and the scarcity of observations. Thus this thesis aimed at (1) the quantification of suspended sediment dynamics and associated sediment-nutrient deposition in floodplains of the MD, and (2) assessed the impacts of likely future boundary changes on the sediment dynamics in the MD. The applied methodology combines field experiments and numerical simulation to quantify and predict the sediment dynamics in the entire delta in a spatially explicit manner. The experimental part consists of a comprehensive procedure to monitor quantity and spatial variability of sediment and associated nutrient deposition for large and complex river floodplains, including an uncertainty analysis. The measurement campaign applied 450 sediment mat traps in 19 floodplains over the MD for a complete flood season. The data also supports quantification of nutrient deposition in floodplains based on laboratory analysis of nutrient fractions of trapped sedimentation.The main findings are that the distribution of grain size and nutrient fractions of suspended sediment are homogeneous over the Vietnamese floodplains. But the sediment deposition within and between ring dike floodplains shows very high spatial variability due to a high level of human inference. The experimental findings provide the essential data for setting up and calibration of a large-scale sediment transport model for the MD. For the simulation studies a large scale hydrodynamic model was developed in order to quantify large-scale floodplain sediment dynamics. The complex river-channel-floodplain system of the MD is described by a quasi-2D model linking a hydrodynamic and a cohesive sediment transport model. The floodplains are described as quasi-2D presentations linked to rivers and channels modeled in 1D by using control structures. The model setup, based on the experimental findings, ignored erosion and re-suspension processes due to a very high degree of human interference during the flood season. A two-stage calibration with six objective functions was developed in order to calibrate both the hydrodynamic and sediment transport modules. The objective functions include hydraulic and sediment transport parameters in main rivers, channels and floodplains. The model results show, for the first time, the tempo-spatial distribution of sediment and associated nutrient deposition rates in the whole MD. The patterns of sediment transport and deposition are quantified for different sub-systems. The main factors influencing spatial sediment dynamics are the network of rivers, channels and dike-rings, sluice gate operations, magnitude of the floods and tidal influences. The superposition of these factors leads to high spatial variability of the sediment transport and deposition, in particular in the Vietnamese floodplains. Depending on the flood magnitude, annual sediment loads reaching the coast vary from 48\% to 60\% of the sediment load at Kratie, the upper boundary of the MD. Deposited sediment varies from 19\% to 23\% of the annual load at Kratie in Cambodian floodplains, and from 1\% to 6\% in the compartmented and diked floodplains in Vietnam. Annual deposited nutrients (N, P, K), which are associated to the sediment deposition, provide on average more than 50\% of mineral fertilizers typically applied for rice crops in non-flooded ring dike compartments in Vietnam. This large-scale quantification provides a basis for estimating the benefits of the annual Mekong floods for agriculture and fishery, for assessing the impacts of future changes on the delta system, and further studies on coastal deposition/erosion. For the estimation of future prospects a sensitivity-based approach is applied to assess the response of floodplain hydraulics and sediment dynamics to the changes in the delta boundaries including hydropower development, climate change in the Mekong River Basin and effective sea level rise. The developed sediment model is used to simulate the mean sediment transport and sediment deposition in the whole delta system for the baseline (2000-2010) and future (2050-2060) periods. For each driver we derive a plausible range of future changes and discretize it into five levels, resulting in altogether 216 possible factor combinations. Our results thus cover all plausible future pathways of sediment dynamics in the delta based on current knowledge. The uncertainty of the range of the resulting impacts can be decreased in case more information on these drivers becomes available. Our results indicate that the hydropower development dominates the changes in sediment dynamics of the Mekong Delta, while sea level rise has the smallest effect. The floodplains of Vietnamese Mekong Delta are much more sensitive to the changes compared to the other subsystems of the delta. In terms of median changes of the three combined drivers, the inundation extent is predicted to increase slightly, but the overall floodplain sedimentation would be reduced by approximately 40\%, while the sediment load to the Sea would diminish to half of the current rates. These findings provide new and valuable information on the possible impacts of future development on the delta, and indicate the most vulnerable areas. Thus, the presented results are a significant contribution to the ongoing international discussion on the hydropower development in the Mekong basin and its impact on the Mekong delta.},
  language  = {en}
}
@phdthesis{Mamede2008,
  author    = {Mamede, George Leite},
  title     = {Reservoir sedimentation in dryland catchments : modelling and management},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-17047},
  school      = {Universit{\"a}t Potsdam},
  year      = {2008},
  abstract  = {Semi-arid environments are mainly characterized by scarce water resources and are usually subject to risks of water stress. In these regions, water supply for drinking and irrigation purposes depends strongly on storage in surface reservoirs and sediment deposition in these reservoirs affects adversely the water storage. In order to reproduce the complex behaviour of sediment deposition in reservoirs located in semi-arid environments and the effects of using sediment management techniques, a reservoir sedimentation model is developed and coupled within the WASA-SED model, which simulates rainfall-runoff processes and sediment transport at the hillslope and river network. The reservoir sedimentation model consists of two modelling approaches, which may be applied according to reservoir size and data availability. For reservoirs with information about their geometric features (reservoir topography, stage-area and stage-volume curves) and physical properties of sediment deposits, such as deposition thickness, grain size distribution of sediment deposits and sediment densities, a detailed modelling approach of reservoir sedimentation may be applied. For reservoirs without those characteristics, a simplified modelling approach is used. The detailed modelling approach of reservoir sedimentation enables the assessment of sediment deposition pattern in reservoirs and the evaluation of sediment release efficiency of sediment management techniques. It simulates sediment transport along the longitudinal profile of a reservoir. The reservoir is divided into cross sections to elaborate the sediment budget. The sediment transport component is calculated using a non-uniform sediment transport approach based on the concept of sediment carrying capacity. Four different sediment-transport equations can be selected for the simulations. The simplified modelling approach of reservoir sedimentation is suitable to simulate water and sediment transfer in dense reservoirs network. Nevertheless, it allows simulating neither sediment management techniques, nor spatial distribution of sedimentation. In this approach, the reservoirs are classified into small and strategic reservoirs according to their location and size. Strategic reservoirs are medium and large-sized reservoirs located on main rivers at the sub-basin's outlet or reservoirs of particular interest. The small reservoirs are located at tributary streams and represented in the model in an aggregate manner by grouping them into size classes according to their storage capacity. A cascade routing scheme is used to describe the upstream-downstream position of the reservoir classes. The water and sediment balances of small reservoirs are computed for one hypothetical representative reservoir of mean characteristics. Sediment trapping efficiency and effluent grain size distribution are estimated using the overflow rate concept. Three model applications are carried out within this research, as follows: • The detailed modelling approach of reservoir sedimentation is applied to the 92.2 Mm³ Barasona Reservoir, located in the foothills of the Central Pyrenees (Aragon, Spain). A two-stage calibration was performed to account for changes on the sediment deposition pattern caused by sediment management. The reservoir sedimentation model is then validated for another simulation period which confirms that the processes related to reservoir sedimentation are well represented by the model. • An application is carried out to the 933-km² Bengu{\^e} catchment, located in the semi-arid region of Northeast Brazil. The catchment is characterized by a dense reservoir network, covering almost 45\% of the catchment area, with a significant lack of data. Water and sediment balances of those reservoirs are computed using the simplified modelling approach. Three spatial configurations describing the cascade routing scheme are tested. • The reservoir sedimentation model is applied again to the Barasona reservoir to evaluate the sediment release efficiency of sediment management strategies. Cost analysis is presented to help in the choice of the most promising sediment management technique for that situation. Thus, the model enables the assessment of technical features of the sediment management strategies. Overall, simulation results are characterized by large uncertainties, partly due to low data availability and also due to uncertainties of the model structure to adequately represent the processes related to reservoir sedimentation.},
  language  = {de}
}