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), 452-466] 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 sediment 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. © 2007 International Association for Hydraulic Engineering and Research, Asia Pacific Division.