Numerical and experimental study of unsteady salt water purging in Hong Kong sea outfall model

Abstract

Based on experiments on the 1:20 Hong Kong model diffuser, a numerical model for simulating seawater intrusion and purging process in an invertconnected outfall is developed. An extension of the theory of Guo and Sharp (1996), this is essentially a one dimensional unsteady flow model with locally 2D techniques to account for stratification and density changes at riser-tunnel junctions. Numerical predictions of purging flow and times are in good agreement with measurements; key features of the purging sequence are also well-supported by the synoptic observed riser flows. Both experiments and calculations demonstrate that, unlike a soffit-connected outfall, the seawater at the bottom of the tunnel is initially purged, and the mixing between effluent and seawater is much stronger. The purging sequence in invert-connected situation is usually from seaward end to landward end for unsteady and quasi-steady purging; however for steady purging the sequence is from landward end to seaward end as in soffit-connected outfalls. The presence of an effluent layer at the top of the tunnel (due to e.g. pump shutdown) results in a persistent circulation in the system. The numerical model also shows that shortening the outfall tunnel length by an artificial bend has an effect on the purging sequence and time, but not the purging flow. Two-layer purging occurs in steady purging process and can help reduce purging discharge rate, especially with a large number of risers. Salt-assisted purging is effective only if the duration of the process is sufficiently long; however, the effluent volume required can be larger than that in the pure effluent purging process.