Fig 1.
(A) Photograph of the field site showing the location of the floodplain patch scanned and replicated in this study. (B) 10-cm resolution DEM of the Highland Water study reach derived from Terrestrial Laser Scanning (TLS) data. The approximate location of the patch replicated in this study is indicated by the red box, with the orientation of the photograph indicated by the red arrow, stream flow is indicated by the white arrows and differ from the direction of floodplain flow (see also Fig 3). (C) Distribution of the ADV measurement nodes (black dots) over the 2×1.37 m area of interest. The greyscale is a hillshade such that darker areas reflect shaded (steeper) terrain. (D) Photograph of the replicated floodplain (Physical Terrain Model; PTM) and ADV instruments as deployed within the flume.
Fig 2.
Conceptual diagram of flow structures over commonly investigated roughness elements.
Fig 3.
Results of Hydro2de simulations for a total flow discharge Q = 1.12 m3/s, showing the distribution of (left) simulated flow depth and (right) simulated flow velocity in the Highland Water study reach around the investigated floodplain patch.
The flow discharge of 1.12 m3/s corresponds to an estimated Recurrence Interval of 6 years, based on a 5 year flow record. Model simulations were undertaken based on a 10cm resolution DEM of the reach (see Fig 1) that was acquired via TLS survey in May 2007.
Fig 4.
Three-dimensional structure of the flow over the floodplain patch (see Fig 1) visualised with streamlines and profiles of the downstream velocity (a-e) and cross-stream velocity (f-j).
The colour-scale is the same for downstream and cross-stream velocities. Note the partial exchange of the fluid from the top and bottom of the time-averaged flow as visualised by the streamlines (coloured by elevation).
Fig 5.
Three-dimensional structure of the time-averaged velocity field over the floodplain patch (see Fig 1) illustrated by vertical profiles and isosurfaces of selected high and low velocities.
The histograms represent the velocity distributions, and the lines indicate the values of the isosurfaces. The colour-scale in the profiles is the same for downstream, cross-stream, and vertical velocities. (A) downstream velocity with isosurfaces of the recirculation zones (0 m/s, blue), and isosurfaces of the largest downstream velocities (99th percentile, transparent red); (B) cross-stream velocity, indicating flow to the left (95th percentile, yellow) and flow to the right (5th percentile, transparent red); (C) vertical velocity with isosurfaces that indicate the largest upward flow (95th percentile, green) and downward directed flow (5th percentile, blue).
Fig 6.
(A) Relationship between the reattachment lengths and the height of the roots. Typical relations between obstacle height and reattachment lengths (1:4 to 1:6) are indicated by the red dashed lines. (B) Relationship between the reattachment lengths and the maximum slope of negative step (left). Root 1: closed circles. Root 2: open circles. (C) Details of the flow structure over and around an oblique depression in root 1, showing strong cross-stream currents that affect the presence of flow separation (see Fig 5A, blue isosurface). Note that the black dashed lines in (A) and (B) delineate the minimum values for which flow separation was observed for the different roots.
Fig 7.
(A) Deviations from a logarithmic downstream velocity profile across the floodplain patch (see Fig 1) as indicated by the Pearson’s correlation coefficient between the downstream velocity and the log of the height above the bed: r2 > 0.5: good fit with logarithmic velocity profile; r2<0.5: significant deviation from logarithmic velocity profile. Individual log-linear and linear velocity profiles (left and right in inset plots) from key zones are plotted alongside the map. Key zones include the upstream area, which has well-established logarithmic velocity profiles (highlighted in the red box), and areas with r2 values below 0.5: over the roots; and in the zone where the velocity maximum is lowered towards the bed by secondary circulation. (B) Map of the maximum downstream velocity. (C) Elevation of the downstream velocity maximum as a fraction of the average flow depth (height/flow depth). Note that the elevation of the maximum velocity does not necessarily coincide with lower velocities: high maximum velocities may occur near the bed and low maximum velocities may occur in the free flow.
Fig 8.
(A) Distributions of roughness height (H0) as calculated for individual profile. Note the strong skewness and orders of magnitude variation in the roughness height values. (B) Distributions of roughness height calculated for different sampling volumes. (C) Convergence of the roughness height values as a function of increasing the sampling volume. Note that the skewness in local roughness height values causes and initial underestimation of the average roughness height.
Fig 9.
Three-dimensional structure of the Reynolds stress over the floodplain patch (see Fig 1), illustrated by vertical profiles and isosurfaces of selected values of elevated and zero Reynolds Stress.
(A) Three-dimensional shape of Reynolds stress exceeding 1.25 Pa, which indicates the zones within the flow where Reynold stresses are high, is indicated by the red isosurface. At the truncation at the top of the flow, colours are interpolated following the Reynolds stress colorbar. (B) Zero Reynolds stress in the UW plane is indicated by the blue isosurface. The contents of the blue volume represent negative Reynolds stresses: a reversal in the direction of the momentum flux within the turbulent velocity fluctuations. (C) Downstream and cross-stream slices of Reynolds stress in the UW plane over the roots. (D) Three-dimensional shape of elevated and negative Reynolds stress in the VW and (E) UV planes.
Fig 10.
Quadrant analysis quantifies the relative contribution of large instantaneous velocities with different orientations to the overall Reynolds stress.
Values are given here in percentage time that instantaneous velocities exceed of a velocity threshold value of 2σ. At the truncation at the top of the flow, colours are interpolated following the quadrant % colorbar. (A) vertical profiles and isosurfaces visualise the three-dimensional structure of the largest contributions of all four quadrants in the UW plane. The isosurface of the quadrant values is set to 5% (as in the colorbar) for all four quadrants; (B) Downstream and cross-stream slices of the four quadrants in the UW plane. Note that bursts and sweeps (quadrants 2 and 4) are dominant in their contribution to the Reynolds stress, and that the elongated shapes of the isosurfaces indicate significant advection of turbulence.
Fig 11.
Conceptual diagram of the principal structure of A) time-averaged velocity (Figs 4 and 5) and B) turbulence (Figs 9 and 10).
Q2 indicates the dominance of quadrant 2 events (sweeps) and Q4 indicates the dominance of quadrant 2 events (bursts) (see Fig 10).