ABSTRACT
Vegetation in channels is beneficial to river systems in that it acts as an ecological buffer in natural and restored systems, but can suffer severe damage when subjected to high discharges. This paper presents the results of an experimental study of the lateral distribution in drag force on individual stems in a patch of uniform, simulated vegetation subjected to lateral, turbulent shear and presents a conceptual model for estimation of the mean and maximum forces experienced by rigid vegetation at the open channel interface. An array of cylinders in a laboratory flume was used to model a partially vegetated channel common of riparian floodplains and urban renewal applications. Direct measurements of the stem drag force on individual cylinders within the array were collected using submersible force gauges attached to the rods and embedded in the channel bed for both emergent and shallowly submerged arrays. The experiments revealed a highly dynamic flow with strong gradients at the interface between the experimental array and the open channel. The mean stem drag force exhibited a similar pattern to the mean velocity, with the highest mean forces recorded near the interface, reducing in magnitude moving into the stem array for both emergent and submerged conditions. The fluctuation in forces was also greatest at the interface, up to twice the deviation from the mean as recorded within the array. Maximum forces at the interface were recorded up to 30 times the patch-averaged drag. The periodicity of the stem force is related to the timescale of the lateral sweep-ejection pattern across the array interface. A conceptual model was developed to derive the entire lateral mean velocity profile for any partially vegetated channel given information of the flow rate, water depth, and vegetative and bed resistance factors. With this profile known, the maximum stem force is approximated, providing a basis for the evaluation of the risk of damage to plants within patches of vegetation subjected to lateral, turbulent shear.
