Models to estimate hydraulic resistance due to vegetation in emergent wetlands are crucial to wetland design and management. Hydraulic models that consider vegetation rely on an accurate determination of a resistance parameter such as a friction factor or a bulk drag coefficient. At low Reynolds numbers typical of flows in wetlands, hydraulic resistance is orders of magnitude higher than fully turbulent flows and resistance parameters are functions of the flow regime as well as the vegetation density and structure. The exact relationship between hydraulic resistance, flow regime and vegetation properties at low-Reynolds number flows is unclear. The project goal was to improve modeling of emergent wetlands by linking vegetation and flow properties to hydraulic resistance. A 12.2-m x 1.2 m vegetated flume was constructed to evaluate seven models of vegetated hydraulic resistance through woolgrass (Scirpus cyperinus (L.) Kunth), a common native emergent wetland plant. Measurements of vegetation geometry and structure were collected after each set of flume runs. Study results showed at low stem-Reynolds numbers (<100), the drag coefficient is inversely proportional to the Reynolds number and can vary greatly with flow conditions. Empirical models that were developed from data collected in natural wetlands predicted flow velocity most accurately. Using data from this flume study, regression models were developed to predict hydraulic resistance. Results indicated stem Reynolds number, stem diameter, and vegetation area per unit volume were the best predictors of friction factor. Vegetation flexibility and water depth were also important parameters but to a lesser extent. The spatial distribution of hydraulic resistance was estimated in a small floodplain wetland near Stephens City, VA using the regression models developed from the flume data. MODFLOW was used to simulate a 4-hour flood event through the wetland. The vegetated open water surface was modeled as a highly conductive aquifer layer. On average, MODFLOW slightly underpredicted the water surface elevation. However, the model error was within the range of survey error. MODFLOW was not highly sensitive to small changes in the estimated surface hydraulic conductivity caused by small changes in vegetation properties, but large decreases in surface hydraulic conductivity dramatically raised the elevation of the water surface. / Ph. D.

Yes / The failure of swirling ship propellers in marine environments can lead to huge repair costs. One of the main causes of such failure is when propellers tangle with vegetation, especially in shallow flow environments like ports, harbours, or shipyards. In order to understand the above-mentioned issue, this study proposes an analytical approach to explore efficient predictions and provide a flow guideline with respect to the co-existence of vegetation and propeller swirling effects. More specifically, we intend to investigate the full-depth theoretical velocity profile to represent propeller-induced flow under submerged vegetation conditions. This paper first investigates the modified logarithmic law approach to determine its suitability to represent the regional vegetated flow zone before implementing it into a three-layer analytical model. It was found, using the benchmark of literature measurements, that the modified log law improved the near-bed velocity calculation by nearly 70% when compared to an existing model. A propeller jet impact computation coupled into the vegetation analytical model was then investigated in different locations within the vegetated flow, i.e., at free-flow, water–vegetation interface, and vegetation-hindered zones, to study their complex velocity distribution patterns. The results demonstrate adequate validation with the vegetated flow and measured propeller jet data from the literature. This proves the potential of the proposed analytical approach in representing a real-world propeller jet event submerged in water flow with the existence of vegetation. The proposed novel method allows costless computation, i.e., as compared to conventional numerical models, in representing the complex interaction of the propeller jet and vegetated flow.

This research is intended to determine the role of riparian vegetation in stream morphology. This experiment examined the effects of riparian vegetation on boundary shear stress (BSS) by completing the following objectives: (1) evaluating the effects of streambank vegetation on near-bank velocity and turbulence; (2) determining a method for measuring BSS; and, (3) examining the effects of streambank vegetation on BSS using an existing model. A second order prototype stream, with individual reaches dominated by the three vegetation types (trees, shrubs, and grass) was modeled using a fixed-bed Froude-scale modeling technique. One model streambank of the prototype stream was constructed for each vegetation type in addition to one bank with only grain roughness. Velocity profiles were measured using an acoustic Doppler velocimeter (ADV) and a miniature propeller (MP). A flush-mounted Dantec MiniCTA system was used to measure shear stress at the streambank wall. The addition of vegetation on a sloping streambank increased the streamwise free stream velocity and decreased the near-bank streamwise velocity. The turbulence caused by the upright shrub treatment increased turbulent kinetic energy and Reynolds stresses near the streambank toe, an area susceptible to fluvial erosion. The presence of dense, semi-rigid vegetation may encourage the formation of a wider channel with a vertical streambank. The small range of CTA shear stress measurements (0.02—2.14 Pa) suggested that one estimate can describe a streambank. The law of the wall technique is not appropriate because the velocity profiles did not follow the necessary logarithmic shape. Vegetative roughness present in channels created secondary flow; turbulence characteristics more appropriately estimated BSS. The BSS model predicted velocity fields in similar distribution to that measured by the ADV and MP. BSS calculated using the ray-isovel method for both velocity measurement devices were different than the measured BSS values, likely due to distortions in the measured velocity field. In general, the predicted BSS distribution increased with water depth and decreased with increasing vegetation density. The predicted BSS at the shrub toe indicated a spike in shear stress consistent with TKE estimates. / Ph. D.

Yes / This study presents results from a vegetation-induced flow experimental study which investigates 3-D turbulence structure profiles, including Reynolds stress, turbulence intensity and bursting analysis of open channel flow. Different vegetation densities have been built between the adjacent vegetations, and the flow measurements are taken using acoustic Doppler velocimeter (ADV) at the locations within and downstream of the vegetation panel. Three different tests are conducted, where the first test has compact vegetations, while the second and the third tests have open spaces created by one and two empty vegetation slots within the vegetated field. Observation reveals that over 10% of eddies size is generated within the vegetated zone of compact vegetations as compared with the fewer vegetations. Significant turbulence structures variation is also observed at the points in the non-vegetated row. The findings from burst-cycle analysis show that the sweep and outward interaction events are dominant, where they further increase away from the bed. The effect of vegetation on the turbulent burst cycle is mostly obvious up to approximately two-third of vegetation height where this phenomenon is also observed for most other turbulent structure. / The full text will be available at the end of the publisher's embargo period: 1st Feb 2025