Drag force of submerged shrubs and trees in open-channel flows

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Koji Shiono, Edgar Ferreira, Sho Yonekura, Jim Chandler, Rene Wackrow, Taisuke Ishigaki, R Giles

Friday 3 july 2015

9:00 - 9:15h at North America (level 0)

Themes: (T) Hydro-environment, (ST) Ecohydraulics and ecohydrology

Parallel session: 14F. Environment - Ecohydraulic

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Large scale protrusions (e.g. emergent aquatic vegetation) are often found in rivers, enhancing flow resistance and hence, the risk of flooding. Over the years, established strategies to ensure the safety of flood prone communities mainly relied on civil engineering measures combined with streams desnagging, sometimes leading to adverse hydrological, morphological and ecological effects (Erskine and Webb, 2003). Consequently, in recent times, river management plans highlight the necessity to preserve the ecosystem services provided by aquatic vegetation, therefore reinforcing the need for a deep understanding of vegetation and flow interactions (Nepf, 2012). This paper examines the relation between drag coefficients and specimens and flow structure and compares the performance of the model proposed by Whitakker et al. (2013) and the traditional drag formula. Flow velocities and drag forces were recorded using a 2D/3D side looking Nortek Vectrino and a load cell, respectively. Plants projected area was estimated using the procedure described in Shiono et al. (2014) and their volume and flexural rigidity was determined using the water displacement technique and the beam bending theory. Two approaches (considering or not the vertical distribution of the product of the projected area and velocity) were employed to determine the drag coefficient of two species (Buxus and Euonymus). Main differences arose whenever the cumulative area profile of the species was not linear over a significant portion of the flow depth. Moreover, we have tested Whitakker et al. (2013) formulation using submerged braches of Salix and Conifer trees. Our results seem to point to a significant improvement of the drag prediction using this formula. References: Erskine D.W. and Webb A.A. (2003). Desnagging to resnagging: new directions in river rehabilitation in southeastern Australia. River Res. Appl., 19, 233-249. Nepf H. (2012). Hydrodynamics of vegetated channels. J. Hydraul. Res., 50(3), 262-279. Shiono K., Ferreira E., Chandler J.H., Wackrow R. and Ishigaki T. (2014). Measurements of drag force of cylindrical rods and trees in a river. River Flow 2014, International Conference on Fluvial Hydraulics, Lausanne, Switzerland. Whittaker P., Wilson C., Aberle J., Rauch H.P. and Xavier P. (2013). A drag force model to incorporate the reconfiguration of full-scale riparian trees under hydrodynamic loading. J. Hydraul. Res., 51(5), 569-580.