IAHR, founded in 1935, is a worldwide independent member-based organisation of engineers and water specialists working in fields related to the hydro-environmental sciences and their practical application. Activities range from river and maritime hydraulics to water resources development and eco-hydraulics, through to ice engineering, hydroinformatics, and hydraulic machinery.
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You are here : eLibrary : IAHR World Congress Proceedings : 35th IAHR Congress - Chengdu (2013) : THEME 4 - HYDRO-ENVIRONMENT : The Impact of Macroalgae on Mean and Turbulent Flow Fields
The Impact of Macroalgae on Mean and Turbulent Flow Fields
Author : Robert E. Thomas, Stuart J. McLelland and Lynne E. Frostick
Over the past century, laboratory flume experiments have been used extensively to explore and elucidate micro- and meso-scale hydraulic and sediment transport processes. In recent decades, flume experiments have been increasingly applied to the study of ecohydraulic interactions: the interactions and feedbacks among organisms, fluid flow and the sediment substrate. However, questions remain regarding the comparability of observations gained in the field and the laboratory, and the limitations and applicability of laboratory experiments are not yet fully understood.In this paper, we describe research carried out by the participants in the PISCES work package of the HYDRALAB IV project that addresses s ome of these questions. To do this we quantified: 1.) the mean and turbulent flow field within a vegetated field prototype; 2.) the locations and biomechanical properties of the plants (see Paul and Henry, 2013); and 3.) the mean and turbulent flow field after plants were completely removed. These measurements will be repeated within a 1:1 replica of the prototype in the 11 m long 6 m wide Total Environment Simulator of the University of Hull.We selected an 11 m long 6 m wide region of a tidal inlet, the Hopavgen Bay field station of NTNU, Sr-Trndelag, Norway to replicate the flume dimensions. Two Laminaria digitata specimens ~0.30 m apart were selected for detailed study and a 2 m long 8 m wide frame was oriented around them by enforcing zero cross-stream discharge at the upstream edge, which was ~0.40 m upstream of the algae. Within this frame, a profiling ADV (Nortek Vectrino II) was used to collect velocity profiles, each composed of up to seven 35 mm-high profiles collected for 240 s at 100 Hz, at a streamwise spacing of 0.25 m and cross-stream spacing of 0.20 m. These were collected at 45 planform locations. Within the 11 6 m region surrounding the frame an ADCP survey was undertaken at streamwise intervals of 1.5 m. Velocity profile measurements were repeated over a sparser grid after the region had been completely cleared of vegetation and major roughness elements (i.e., boulders).Upstream of the algae, the streamwise (?????) velocity profile was approximately logarithmic with maxima of ~0.10 m -1 s , while cross-stream (????) and vertical (?????) flows were minimal. Between the algae was a region of -1 downwelling (?????-velocity -0.035 m s ), while at the outer edges of the algae, counter-rotating velocity -1 cells were generated (|????| ~0.04 m s ). Near-bed ?????-velocities were reduced to almost zero near the algae, but recovered quickly downstream of them, as the retarded region was advected downstream and upwards. -4 2 -2 ????? This advection was manifest in elevated magnitudes of ????'????' (~4 10 m s ) downstream of the algae. After removal of the vegetation, the dominant difference was a reduction in roughness height from ~0.085 m to ~0.005 m. This resulted in the formation of ripples within the area. Cross-stream (????) and vertical (?????) -4 2 -2 ????? velocities were small within the footprint of the frame and values of ????'????' were uniformly <1 10 m s .
File Size : 677,595 bytes
File Type : Adobe Acrobat Document
Chapter : IAHR World Congress Proceedings
Category : 35th IAHR Congress - Chengdu (2013)
Date Published : 18/07/2016
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