| dc.description.abstract | Before human interference the Mississippi River system transported on average approximately 400 MT/yr of suspended sediment to the Gulf of Mexico. In the last decades this transport has declined to only 110 MT/yr. The primary cause for this decline is the construction and implementation of human modifications to the river system. These human modifications include: the removal of obstacles, channel straightening, dikes, revetments, levees, floodways, dams and reservoirs. The decline in suspended sediment transport has resulted in the degradation of wetlands and coastal zones, extensive land loss and increased flood risks in the delta, which will only become more severe in the future as a result of climate change, subsidence and sea-level rise. This research focuses on the construction of dams and reservoirs as this is the primary source of suspended sediment reduction.
In this thesis the effect of the construction of reservoirs on the suspended sediment transport is analyzed by developing a quantitative model of the suspended sediment flux using the hydrological model PCR-GLOBWB. PCR-GLOBWB incorporates the hydrologic system of the Mississippi River Basin (MRB) and the reservoirs, for this research the model was extended to include suspended sediment deposition, transportation and trapping. The model was run for two end-member scenarios that included the 'initial' conditions without any reservoirs and the present conditions with all reservoirs. The results of the scenario with reservoirs were compared to previous research to validate the performance of the model. Furthermore, the temporal and spatial variability between the model output with and without reservoirs were analyzed to identify the effect of reservoir construction on the suspended sediment flux. The model was also run for three specific scenarios that included the removal of selected reservoirs in the tributaries and the main channels. These scenarios included the removal of nine main stem Missouri reservoirs, 34 large reservoirs (with a surface area of more than 100 $km^2$) and nine main stem Ohio reservoirs. These scenarios were run to identify the effect of the removal of selected dams of the suspended sediment flux and provide possible management options.
The relative error between the literature and the model output was primarily caused by the assumption that the sediment production of an entire subcatchment was constant in space and time. Furthermore, the model did not make a distinction between different types of erosion and therefore neglects the effect of local differences and extreme conditions. The year-to-year variability and seasonal variability in suspended sediment transport declined significantly if the reservoirs were introduced due to the storage capacity of the reservoirs, which resulted in a more constant discharge by lowering the peak discharges and increasing the minimum discharges. The greatest decline in sediment transport could be seen along the main channel of the Missouri and Lower-Mississippi, while the decline in the tributaries was less apparent. The sediment production in the Great Plains is the main source of suspended sediment which is independent of reservoir construction, however less of the available sediment is routed downstream if the reservoirs were introduced. The removal of the nine main stem Missouri reservoirs generated the largest increase in suspended sediment transport in comparison to the scenario that includes all reservoirs. | |
