Clear-water scour depth below underwater pipelines Subhasish Dey * , Navneet P. Singh Department of Civil Engineering, Indian Institute of Technology, Kharagpur 721302, West Bengal, India Received 29 October 2006; revised 14 June 2007; accepted 12 July 2007 Abstract The paper presents (1) a semi-theoretical model for the computation of maximum clear-water scour depth below underwater pipelines in uniform sediments under steady flow and (2) an experimental investigation on the influence of upward seepage on clear-water scour depth below underwater pipelines. Based on the ‘‘image method’’ of the potential flow theory for the flow through the gap between a circular pipeline and the scoured bed, a theoretical model is presented for the computation of maximum scour depth below underwater pipelines in clear-water condition considering the wall correction. Also, a simple theoretical calculation, that involves Chiew’s (1991) [Chiew, Y.M., 1991. ‘‘Prediction of maximum scour depth at submarine pipelines.’’ J Hydraul Eng 117(4), 452e466] gap flow curve, is proposed to estimate the maximum scour depth considering the wall correction. The computed results agree satisfactorily with the experimental data. In addition, the potential predictors of the maximum scour depth given by different researchers are compared with the experimental data. The result of an experimental investigation on the scour depth below pipelines under the influence of upward seepage through the bed sed- iment is presented. In general, the scour depth below pipelines with upward seepage is smaller than that without seepage. There exists a critical value of the ratio of seepage velocity to main flow velocity that corresponds to a minimum scour depth. Ó 2007 International Association for Hydraulic Engineering and Research, Asia Pacific Division. Published by Elsevier Ltd. All rights reserved. Keywords: Erosion; Scour; Sediment transport; Open channel flow; Seepage 1. Introduction Removal of riverbed sediment below underwater pipelines that are commonly laid on and across the riverbeds in India (also in other countries of Asia-Pacific region) to convey water, oil or gas takes place by the scouring action of stream flow. Fatigue failure due to flow-induced oscillation caused by the wake-vortex shedding may occur, if scour exposes the pipeline unsupported over a considerable distance. Therefore, an accu- rate estimation of scour depth below pipelines is important in designing an underwater pipeline. Chiew (1991) and Sumer and Fredsøe (1992) gave a survey of the researches on scour be- low pipelines. A large number of equations or methodologies for the prediction of maximum scour depth below pipelines were proposed by various investigators. However, a few are found to be suitable or easily applicable. Following the potential flow theory, Chao and Hennessy (1972) put forward a theoretical model for the computation of maximum scour depth below pipelines under steady flow assuming that the scour takes place until the bed shear stress below the pipeline reaches the threshold bed shear stress. Kjeldsen et al. (1973) investigated scour below pipelines for live-bed condition and expressed scour depth as a function of approach flow velocity and pipe diameter. A Dutch re- search group (Bijker and Leeuwestein 1984) stated that the scour depth below pipelines depends on the pipe diameter, sediment size, approach flow velocity and flow depth, and gave an empirical equation for the estimation of the scour depth. Mao (1986) studied the scour profiles below pipelines for different approach flows and found that the equilibrium scour depth, if less than one pipe diameter, is a weak function of the Shields parameter. He also differentiated two stages of * Corresponding author. E-mail address: sdey@iitkgp.ac.in (S. Dey). 1570-6443/$ - see front matter Ó 2007 International Association for Hydraulic Engineering and Research, Asia Pacific Division. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jher.2007.07.001 Available online at www.sciencedirect.com Journal of Hydro-environment Research 1 (2007) 157e162 www.elsevier.com/locate/jher