Print Email Facebook Twitter Hydrodynamics of partially vegetated channels: Stem drag forces and application to an in-stream wetland concept for tropical, urban drainage systems Title Hydrodynamics of partially vegetated channels: Stem drag forces and application to an in-stream wetland concept for tropical, urban drainage systems Author Buckman, L.J. Contributor Uijttewaal, W.S.J. (mentor) Faculty Civil Engineering and Geosciences Department Water Management Programme NUS-TUD Double Degree Programme Date 2013-08-27 Abstract Introduction: The addition of vegetation to the banks urban drainage channels is an increasingly common measure for improving water quality, enhancing ecological health, and improving aesthetic appeal. An in-stream wetland concept is proposed for channels in Singapore to just that, but faces a major challenge in that high flow rates during storm events will increase the risk of damage to the vegetation. Problem definition: In high-flow situations, flow around the vegetation causes a region of turbulent shear to develop along the interface between the vegetation and main, open-channel flow. This is expected to impact the hydraulic resistance in the channel and the drag forces experience by individual plants, especially those nearest to the interface. For design it is important to understand the effects of this turbulent shear on drag to optimize design to limit upstream flooding risk and assess the risks of damage to plants. Research: The aim of the research was to increase understanding of the effects of lateral, turbulent shear on the channel resistance and forces experienced by individual stems in a uniform patch of vegetation. A patch of uniform vegetation was modelled by an array of rigid cylinders in an experimental flume. Measurements of the flow field and the drag forces on individual cylinders were recorded. Using this data, answers to the following questions were found: 1) What is the effect of lateral turbulent motion on channel resistance? 2) How do drag forces on individual stems vary spatially over the patch and in time, with special attention to local maximums? 3) What is the effect of lateral turbulent motion on fluctuations in the stem drag force? 4) What are the implications for estimation of the mean and maximum stem drag forces? Results: Analysis of the experimental results revealed the following: 1) The presence of a lateral shear layer significantly increased channel resistance, by 175% when compared to similar conditions when the shear layer was not allowed to develop. 2) Drag forces on stems mirrored the velocity distributions in both time and space, showing both higher mean and maximum values near the interface between the stem array and open channel. 3) Similar periodicity in the velocity and force signals gave evidence of coherent, turbulent structures as the primary means of momentum transport across the vegetation interface. This motion causes a sweep-ejection pattern in the flow at the interface with a net flux of momentum towards the vegetation, resulting in a skewed distribution of stem forces towards higher extreme values. Conclusions and recommendations: Lateral turbulent shear is an important factor in both the channel resistance and stem drag forces in a partially vegetated channel. Coherent structures at the vegetation interface were determined to be the main factor in stem force distribution within the region of shear. The mean stem force can be derived directly from the mean velocity given adequate assumptions of the vegetative drag coefficients. A conceptual model was developed to describe the maximum force in the vegetation patch as a function of the mean velocity at the vegetative interface and a fraction of the difference in velocities between the vegetated and open channel sections. Subject vegetationhydrodynamicsdrag force To reference this document use: http://resolver.tudelft.nl/uuid:71f7e38d-a05b-4e6e-bb84-88fa4d6f31b9 Part of collection Student theses Document type master thesis Rights (c) 2013 Buckman, L.J. Files PDF MScThesis_Buckman.pdf 27.8 MB Close viewer /islandora/object/uuid:71f7e38d-a05b-4e6e-bb84-88fa4d6f31b9/datastream/OBJ/view