An experimental approach to characterising vegetation roughness in forested floodplains using dilution tracing and terrestrial laser scanning

Abstract:

The vegetation on forested floodplains plays an essential role in floodplain flow routing, flow dynamics and therefore the transfer of sediments and associated nutrients and contaminants. The flow resistance introduced by the presence of vegetation assemblages leads to changes in flow velocity and depth that, in turn, modulate sediment transport. Attempts to quantify these processes remain elusive due to difficulties in accurately estimating the altered flow parameters around and through vegetation.

Here we employ complementary techniques of characterising the bulk flow properties and vegetation structure. Physical modelling of a prototype scale (2 x 45 m) shallow earthen channel containing small trees, herbaceous plants and leaf-litter was undertaken to explore the bulk flow characteristics. Vegetation structure and fluid retention was measured in controlled discharge experiments for: (1) natural cover of herbs and trees; (2) trees only and; (3) earthen channel only. Dilution-curve data were analysed within the Aggregated Dead Zone (ADZ) framework to yield bulk flow parameters including dispersion, fluid retention and flow resistance parameters. Vegetation structure is recorded using TLS, with data processed to represent vegetation structure as a 3D volume porous media.

The main response of flow to vegetation removal was an increase in bulk velocity, with depth and wetted width decreasing slightly. Retentiveness was more prominent during low flow and all three experimental conditions tended to converge on a constant low value for high discharges. Reach mean travel times and the advective time delays decreased very slightly from experiments (1) to (2). The ADZ analysis and TLS data shows that in these two initial experiments, the trees provided the majority of the resistance in contrast to the aggregate effect of herbaceous plants and litter. Removing the trees further decreased travel times such that the ADZ residence time was more than halved moving from condition (1) to (3). The overall bulk flow effect of tree cover on retention is here expressed by the dispersive fraction parameter, which reduced from 0.4 to 0.2 when vegetation was removed.