Flow resistance of woody vegetation and velocity estimation for flow over wooded floodplains
Horn, R. P.
PublisherUniversity of Cambridge
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This thesis examines the velocity, patterns and resistance of flow through rigid vegetation by analysing experimental studies, and presents a method for measuring the mean velocity of flow within a vegetated floodplain where access during flood conditions may be difficult. Flume studies investigated the influence of vegetation parameters and flow properties on the velocity profile, flow resistance and drag coefficient of emergent and submerged artificial rigid vegetation in the form of cylinders. The mean drag coefficients of the vegetation stands were found to vary with array density, pattern and spacing in the flow and transverse directions, which relate to the shielding effects within the array and flow cross-section contraction due to the dowels. An expression is developed which represents the variation in the mean drag coefficient due to shielding and cross-section contraction. A method using conductivity and dye tracing techniques was developed for measuring mean velocity on vegetated floodplains. Video analysis of the dye plume advection and dispersion enabled estimation of an approximate surface velocity and showed that the general surface flow tended to migrate towards the edge of the floodplain furthest from the main stream. Analysis of the conductivity traces of the passage of a plume of saline solution past an array of conductivity sensors on the floodplain indicated that the sub-surface flow tended to migrate down the floodplain towards the main channel. The velocity increased from the bed to the surface, and a circulatory motion within the floodplain flow from the main channel towards the floodplain at the surface and returning to the main channel closer to the bed was suggested.