Effect of flexible woody vegetation patches on river flow. Flume investigation of patch-scale drag and velocity distributions
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School of Engineering |
Master's thesis
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Date
2024-09-20
Department
Major/Subject
Water Resource Management
Mcode
Degree programme
Nordic Master Programme in Environmental Engineering
Language
en
Pages
54
Series
Abstract
Riparian vegetation plays a crucial role in shaping water flow dynamics and the ecosystem health of rivers. Naturally varied vegetation patterns, organized into distinct patches, introduce uncertainties in hydraulic modeling and management practices. This study investigated the drag and velocity distribution of flexible woody vegetation patches with understory grass arranged in different configurations of shape (elongated and compact) and density (Leaf Area Index (LAI) of 1.0-2.4). A new force instrumentation setup was developed to measure patch-scale drag forces, while an Acoustic Doppler Velocimeter (ADV) captured point-based velocities. Drag measurements were conducted for mean velocities of 0.1-0.6 m/s and at two relative submergences (H/hd = 1-2, where H is the water level and hd is the patch deflected height), representative of conditions in low-land rivers. Velocity measurements focused on H/hd = 1 and a mean velocity of 0.25 m/s. The patch shape significantly impacted the patch-scale drag (Fp) for H/hd = 1, with high-density elongated patches exhibiting higher drag than the compact counterparts. The variability in Fp was lowered by scaling with the horizontal ground area of the patch, signifying the importance of the areal extent. The elongated patches showed steeper streamwise velocity gradients between vegetated and non-vegetated areas, which coincides with the higher Fp of this patch shape. Lateral velocity distribution showed high sensitivity to the patch position, affecting near-wall velocity and wake flow position. Low-density patches exhibited high wake flow variance, indicating induced flow through the patch. Variance in Fp between patch densities corresponded to LAI differences and was reduced by scaling with leaf area, aligning with prior studies. High-density patches revealed a reconfiguration threshold (> 0.5 m/s) for H/hd = 1, suggesting that the patchy structure influence plant characteristics. Understory grass contribution to bulk drag depended on relative submergence and velocity, with the highest contribution at H/hd = 2 and low velocity, indicating it should not be neglected. The patches were arranged in a staggered pattern along the reach, where the approaching velocities between patches of the same stream side decreased by 14-21 % from upstream to downstream. In contrast, the approaching velocity increased between patches of opposite stream side, indicating that the staggered pattern confines flow paths. Future studies should explore a broader range of patch shapes and densities, as the patchy effect induces high variability in flow and drag, which is crucial for developing effective nature-based solutions and improving river management strategies.Description
Supervisor
Lotsari, EliisaThesis advisor
Järvelä, JuhaFuhrman, David
Keywords
hydrodynamics, riverine vegetation, patch-scale drag, flow field, experimental flume investigations, river flow