Results are presented from a series of laboratory experiments in which the intrusion and arrest of a salt wedge in a smooth circular pipe have been investigated over a range of densimetric Reynolds (Re ?) and Froude (F ?) numbers. Horizontal and inclined pipes have been considered, in order to model intrusion in the outlet ports and tunnels of long sea outfalls. An empirical estimator expression has been derived to represent the functional dependence of wedge length upon densimetric Reynolds and Froude numbers respectively, and pipe slope. The agreement between the predicted wedge lengths and the measured values is generally good for a wide parameter range (Re ?: 7,300-13,200; F ?: 0.25-0.68) typical of prototype operation. Comparisons between the pipe data and measurements of salt wedges in open channels reveal the pitfalls of applying two dimensional results to a three dimensional configuration.