The tidal Thames is flanked by a complex of industrial terminals and dock entrances which are used by a wide variety of vessels, ranging from deep-sea bulk carriers to tugs handling strings of barges. One of the biggest growth trades is oil, the inland distribution of which is by tank barge. There is a variation of the pattern of movement governed both by the tide and commercial considerations. Navigation on the river will be affected by the size of opening since the level of movement needs the widest possible channel and for preference any constriction should be as far up river as practicable. Allied with this requirement is the need for maximum size of opening and the 61 m proposed, while adequate, introduces limitations, particularly for the larger vessel. During construction, special arrangements for navigation will be required to ensure the minimum of disruption to traffic both transitting the area and using terminals in the vicinity of the barrier site. The closure of the barrier will halt all vessels bound through it and is likely to delay them for up to 12 h. It will also have some effect on vessels bound for docks below the barrier. Special arrangements will be required to enable all such vessels to wait. Running costs of vessels likely to be delayed range from pound 1500 to pound 300 per day and so the overall sum could be high. Membership of the Navigational Working Party includes representation from all concerned with the movement of vessels on the river. Their initial deliberations were to conclude which site would be acceptable and whilst factors other than navigational had also to be considered, acceptance of the Woolwich site was governed by the forecast of the likely rate of closures to be initially no more than 2 per annum. With opening widths of 61 m, a long straight approach is needed to ensure ease of navigation and the piers of the barrier must be alined to the direction of the tidal stream. To minimize disruption of navigation, the level of the sill must be low enough to ensure that vessels are not hindered. Sill level also needs to be considered in relation to the regime of the river. Use of the barrier other than for flood control could create problems of siltation and pollution and it is essential before using the barrier for other than its prime purpose to establish that there is no such possibility. Arrangements for control of navigation remain to be finalized, but special signals, radio, visual and aural will be required. Also requiring further consideration is the precise timing of closures. Because of the vital need to prevent flooding, the number of closures may exceed the forecast. This is apparent from experience to date and the tidal pattern of initially indicating a surge which subsequently does not reach flood level. While economic pressures have resulted in a design not necessarily ideal from navigational considerations, it is hoped that the Working Party's deliberations can minimize the impact on the trade of the Port while ensuring flood protection to London.