Research paper Surface discharge of negatively buoyant effluent in unstratified stagnant water O. Abessi a , M. Saeedi b, * , T. Bleninger c , M. Davidson d a Department of Water and Environmental Engineering, School of Civil Engineering, Iran University of Science and Technology, Tehran, Iran b Environmental Research Laboratory, Department of Water and Environmental Engineering, School of Civil Engineering, Iran University of Science and Technology, PO Box 16765-163, Tehran, Iran c Department of Hydraulics and Sanitary Engineering (DHS), Federal University of Parana´(UFPR), Curitiba, Parana, Brazil d Department of Civil Engineering, University of Canterbury, Christchurch, New Zealand Received 1 April 2011; revised 4 May 2012; accepted 8 May 2012 Abstract A series of laboratory experiments have been conducted for negatively buoyant effluents discharged through a protruding surface channel into unstratified stagnant water. The mixing behavior of the flow has been analyzed using digital video recordings of the dye colored discharge together with micro conductivity probe measurements. Plume trajectories and geometries have been determined by image processing and dilutions from the conductivity probe data. Comparison of non-dimensional results for plume characteristics showed similar features to those of submerged positively buoyant jets. However, notable differences are observed in the flow behavior prior to it plunging away from the free surface. As expected the overall dilution is lower when compared to submerged discharges. These differences are quantified and the new results provide information that can be used for the preliminary design of dense surface discharges. Ó 2012 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved. Keywords: Dense jet; Brine disposal; Desalination; Dilution; Surface discharge 1. Introduction Disposal of effluents with higher density than the receiving water, such as the brine (or concentrate) produced as a byproduct of the reverse osmosis process in seawater desa- lination plants, have recently increased; particularly in the Middle East (Safrai and Zask, 2008; Purnama and Al-Barwani, 2004). If disposed to the sea, the dense brine sinks towards the sea floor and spreads over the bed, where it may cause envi- ronmental impacts on local benthic communities. These impacts are mainly related to high salt concentrations, but are also related to chemical additives introduced to improve the desalination process (Lattemann and Hopner, 2008; Bleninger et al., 2010). The potential impacts associated with the construction and operation of seawater desalination plants, particularly large-scale facilities, are of increasing public concern (Einav and Lokiec, 2003; Lattemann and Hopner, 2003; Hashim and Hajjaj, 2005). When assessing the poten- tial environmental impacts of brine discharges from such facilities, it is essential to have a sound understanding of the transport and mixing behavior of the effluent in the receiving water body, so that the substance concentrations and their distribution (geometrical plume properties) can be predicted with some confidence. Surface discharges, in the form of open channel effluent releases with free surface (Fig. 1), are still a common and a very cost effective method for the disposal of large volumes of effluent into the sea (Jones et al., 2007, 1996; Doneker and Jirka, 1997). This is despite the fact that such discharges are * Corresponding author. Tel.: þ98 9121900228; fax: þ98 (021)77240398. E-mail addresses: Abessi@iust.ac.ir (O. Abessi), Msaeedi@iust.ac.ir (M. Saeedi), Tobias.dhs@ufpr.br (T. Bleninger), Mark.davidson@canterbury.ac. nz (M. Davidson). Please cite this article in press as: Abessi, O., et al., Surface discharge of negatively buoyant effluent in unstratified stagnant water, Journal of Hydro-environment Research (2012), doi:10.1016/j.jher.2012.05.004 Available online at www.sciencedirect.com Journal of Hydro-environment Research xx (2012) 1e13 www.elsevier.com/locate/jher + MODEL 1570-6443/$ - see front matter Ó 2012 International Association for Hydro-environment Engineering and Research, Asia Pacific Division. Published by Elsevier B.V. All rights reserved.