Impacts of vegetation-generated turbulence on sediment transport
Name
1087506641-MIT.pdf
Description
Full printable version
Size
20.22 MB
Format
Adobe PDF
Checksum (MD5)
e120a0581f83f2177e23e57c4997bd1e
Author(s)
Yang, Qingjun (Judy Qingjun)
Advisor(s)
Heidi M. Nepf.
Date Issued
2018
Publisher
Massachusetts Institute of Technology
Abstract
Aquatic vegetated habitats, including wetlands and mangroves, are disappearing at an annual rate of 1 to 7%. These ecosystems provide habitats important to fisheries, enhance water quality by filtering nutrients from run-off, and also protect coastal regions from storm surges and waves. To mitigate the loss of these habitats, restoration projects import sediment to eroded areas. The success of the restoration depends on its ability to retain sediment; therefore restoration design requires a good understanding of sediment transport within vegetated landscapes. However, there is currently no quantitative model for sediment transport in vegetated regions, and many restoration projects have failed due to unanticipated erosion from the restored regions. The goal of this thesis is to develop a predictive model for sediment transport in regions with vegetation. First, the affect of vegetation on the critical condition when sediment start to move was explored. To identify the critical condition, an imaging system was designed to track the trajectories of individual moving grain through running water. The critical flow velocity (U[subscript crit]) above which sediment starts to move was identified from the tracked sediment trajectories for both bare (non-vegetated) and vegetated regions. The experimental results showed that for the same type of sediment, U[subscript crit] decreased with increasing vegetation solid volume fraction. This was attributed to the vegetation-generated turbulence, which induced a local, vertical, adverse pressure, or a lift force on the sediment grain, facilitating sediment transport. In contrast, the turbulent kinetic energy (k[subscript t]) was found to be roughly a constant at the critical condition for different vegetation volume fractions, suggesting that k[subscript t] is a more universal metric than T for predicting the critical condition of the sediment transport. A k[subscript t]-based model was developed to predict U[subscript crit] for channels with different vegetation solid volume fractions. The turbulence-based model successfully predicted U[subscript crit] for both bare and vegetated channels, providing a useful tool for ecologists to predict whether a vegetated landscape will erode or not. Second, the impact of vegetation on the bed load transport rate was explored. A system that allows sediment to be bypassed, a cart to distribute sediment, a method that measures the dry weight of wet sand without drying the sediment, a topography system, and an sediment trajectory imaging system were designed. The bed load transport rate (Q[subscript s],) was measured for both bare channels and channels with different vegetation solid volume fractions ([phi]) under different flow rates. At the same [tau], the measured Q[subscript s], increased with increasing [phi], suggesting that vegetation-generated turbulence, which also increased with increasing ]phi], was augmenting the bed load transport. At the same near-bed turbulent kinetic energy, k[subscript t], the Q[subscript s], measured in both bare and vegetated channels agreed within uncertainty, suggesting that k[subscript t] may be a more universal predictor of Q[subscript s] than [tau]. The Einstein-Brown [tau]-based bed load transport model was reinterpreted as a k[subscript t]-based model. The new kt-based model predicted the Q[subscript s] measurements for both bare and vegetated channels. The dependence of Q[subscript s] on k[subscript t] was explained by the statistics of individual grain motion, which showed that Q[subscript s] was predominantly controlled by the number of grains in motion, which correlated with k[subscript t]. The proposed k[subscript t]-based sediment transport model can be used to simulate large-scale landscape evolution and to help ecologists design better coastal restoration strategies. Third, the impacts of vegetation on bed-form characteristics and migration rate were studied. After the measured bed load transport rate converged to an equilibrium value, the bed topography was scanned by a laser topography system. Bed-forms with height less than 2cm were observed and characterized as ripples. For low vegetation solid volume fraction ([phi]
Description
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Civil and Environmental Engineering, 2018.
Cataloged from PDF version of thesis.
Includes bibliographical references (pages 179-188).
Subjects
Civil and Environmental Engineering.
MIT Department
Massachusetts Institute of Technology. Department of Civil and Environmental Engineering
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