Measurement of lateral and wake flows associated with stream-scale willow patches

Flow hydrodynamics and transport processes are expected to be highly complex on floodplains, riverbanks and channel bars growing distinct patches of shrubs and trees. Current understanding on patch-scale hydrodynamics is largely based on flume experiments with simplified vegetation of a limited physical scale. This paper reports preliminary results on the lateral and wake flow characteristics of stream-scale willow patches having dimensions of meters. We investigated emergent patches of Salix subfragilis in an experimental channel with a water surface width of ~6 m and water depth of ~1 m. The experiments covered both leafless and foliated conditions at the two ambient mean velocities of 0.3 and 0.5 m/s. The flow field was measured with an array of nine Acoustic Doppler Velocimeters. The flow deflection around the patch was stronger under the foliated compared to the leafless conditions. The greatest velocity deficit downstream of the patch was recorded under foliated condition at the lower mean flow velocity. At the higher mean velocity, the depth-averaged turbulence intensity within the patch was higher for the foliated condition while the opposite was observed for the lower ambient velocity. Generally, the depth-averaged turbulent kinetic energy and turbulence intensity showed large variability within the patches. The wake flows were found to extend up to several tens of meters downstream from the patches, indicating that real-scale riparian patch mosaics are characterized by highly complex flow fields. In the future, the data will be used to develop the numerical modeling of shrub-vegetated flows at real scale.