Delivered by ICEVirtualLibrary.com to: IP: 158.125.70.141 On: Thu, 22 Jul 2010 13:24:55 Proceedings of the Institution of Civil Engineers Water Management 163 May 2010 Issue WM5 Pages 247–254 doi: 10.1680/wama.2010.163.5.247 Paper 900016 Received: 27/02/2009 Accepted: 03/06/2009 Keywords: field testing & monitoring/floods & floodworks/ hydraulics & hydrodynamics Xin Sun Associate Professor, School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Shaan Xi, PR China Koji Shiono Professor of Environmental Hydrodynamics, Department of Civil and Building Engineering, Loughborough University, Loughborough, UK Jim H. Chandler Professor of Geomatics, Department of Civil and Building Engineering, Loughborough University, Loughborough, UK Ponnambalam Rameshwaran Research Scientist, Centre for Ecology and Hydrology, Wallingford, UK Robert H. J. Sellin Formerly Professor of Hydraulic Engineering, Department of Civil Engineering, the University of Bristol, Bristol, UK Ichiro Fujita Professor, Department of Civil Engineering, Kobe University, Kobe, Japan Discharge estimation in small irregular river using LSPIV X. Sun PhD, K. Shiono PhD, J. H. Chandler PhD, P. Rameshwaran PhD, R. H. J. Sellin PhD and I. Fujita PhD This paper reports on an estimation method used to quantify discharge in the small irregular channel of the River Blackwater, using large-scale particle image velocimetry (LSPIV). The test reach (250 m 3 60 m) consists of relatively straight and multiple meander channels, with a bankfull depth of 0 . 90 m and a base width of 4 . 25 m. Water surface velocities were measured by LSPIV at three sections along the river. Acoustic Doppler velocimeter (ADV) was also used on two occasions to measure velocity profiles for the validation of water surface velocities of LSPIV. In addition to this field work, velocity data derived from a 1:5 physical model of the river were available to compare with the field data. A comparison between the LSPIV data and ADV data was made and agreement was confirmed. The discharge correction factor method was suggested to estimate the discharge in the river. Discharge correction factors of 0 . 90–1 . 05 and 1 . 10–1 . 15 were proposed for inbank and overbank flow conditions respectively. 1. INTRODUCTION River discharge is traditionally measured using electromagnetic current meters and acoustic sensor methods in the area where there are no flow measurement devices. These methods require special devices to put the measurement instruments into the river and have some inherent drawbacks of flow disturbance. Moreover, it is very dangerous to use these methods during river floods. Image-based velocity measurement techniques, such as large-scale particle image velocimetry (LSPIV), is a potential alternative and has gained increasing popularity in the river engineering community. Following Fujita’s pioneering work (Fujita and Komura, 1994; Fujita et al., 1998), the LSPIV technique has been improved (Fujita and Tsubaki, 2002; Fujita and Aya, 2000) and applied to measure free-surface velocities in large-scale rivers spanning up to 45 000 m 2 under different velocity ranges (Creutin et al., 2003; Fujita et al., 1998; Meselhe et al., 2004). For example, LSPIV was successfully applied to measure the discharges in the Yodo river (Fujita et al., 1998) with a width (W) of 230 m and a depth (H) of 8 m, Kino River (Fujita and Aya, 2000) and Uono River (Fujita and Tsubaki, 2002) in Japan, Iowa River (Creutin et al., 2003) (W70 m 3 H6 m) in the USA and River Arc (Jodeaua et al., 2008) (W50 m) in France. However, little work has been done in small irregular rivers. Previous studies of LSPIV focused on the relationship between the surface velocity from LSPIV and depth-averaged velocity. Most researchers introduce a velocity index k ¼ (U d /Us), which is the ratio of the depth-averaged velocity (U d ) to the surface velocity (U s ). In wide large rivers the flow is thought to be two-dimensional, and the vertical velocity profile follows either the logarithmic law or Prandtl’s seventh power law over the water depth (Chanson, 2004). In such a flow case, the theoretical index for the seventh power law is 0 . 875, which more or less agrees with k ¼ 0 . 85 by Creutin et al. (2003) who carried out field measurements using an acoustic Doppler current profiler (ADCP) in the River Iowa. In small irregular rivers, vertical velocity profiles across a cross-section neither follow the seventh power law nor the logarithmic law, since the flow is three-dimensional. The complex secondary currents also make the velocity index k vary from location to location across cross-sections. Thus the use of a k value of 0 . 85 might yield inaccurate discharge estimation in such rivers. In order accurately to estimate discharge for small irregular rivers using LSPIV, an easier and more convenient approach to estimate the river discharge using the surface velocity data from LSPIV needs to be explored. The three-year research project entitled ‘New approaches to estimating flood flows via surface videography and 2D & 3D modelling’, funded by the UK Engineering and Physical Sciences Research Council (EPSRC), makes it possible to explore the new approach in a small irregular river, namely the River Blackwater, Hampshire, UK. In the River Blackwater, the water surface velocities and the vertical profiles of velocity were measured using LSPIV and acoustic Doppler velocimeter (ADV) respectively. Free water surfaces were measured using a digital photogrammetry (DP) technique. River discharges were measured using an ADCP technique. In addition to the River Blackwater data, data representing a 1:5 scale physical model were available. This replicate was built in the Flood Channel Facility (FCF) at HR Wallingford, from which the velocities and boundary shear stresses were measured by Lambert and Sellin (1996). The measured data will be used to understand flow behaviour in relevant sections of the River Blackwater since the flow structure was not able to be sufficiently explained using the limited ADV and LSPIV measurement data. This paper also examines the water surface velocity vectors obtained from LSPIV under various conditions of vegetation and furthermore Water Management 163 Issue WM5 Discharge estimation in small irregular river using LSPIV Sun et al. 247