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Many factors combine to determine the way in which sediments are distributed throughout an estuary. Most fundamental are those which produce the natural rhythm of diurnal (or semi-diurnal), bi-monthly and seasonal fluctuations due to predictable variations in tide and weather. This group includes tidal discharge, fresh river flow and the resultant distribution of saline water. When considered together with such factors as the availability and properties of sediments within and beyond the landward and seaward limits of an estuary, they determine how the available material shall be eroded, transported and deposited during the course of the natural cycle. Superimposed on these regular fluctuations are the effects of other factors which may or may not be predictable, are not necessarily regular in occurrence and may be either natural or man-made. These include secular trends, such as long-term adjustments in land/sea levels or climatic conditions, which have a small but continuing effect on some of the factors in the first group. They also include sudden, short-term events like earthquakes or hurricanes which impose a shock to the system that may involve the movement of large quantities of material during the subsequent period of readjustment.

A brief review has been made of five hydraulic investigations of port siltation carried out by various international laboratories. The ports studied are distributed throughout the tropics from South America to Indonesia and the factors affecting sediment supply and distribution either directly or indirectly have been summarized. In spite of local variations, the similarity of many features suggests the existence of a common mechanism of sediment transport.Twenty months of field measurements at one of the ports demonstrated that turbidity in the water column is low from surface to 0.15 m above the bed. These measurements, together with those from flume tests on the behavior of local mud in currents, indicated that siltation is not solely due to deposition of silt suspended in the main body of flow. Further wave flume tests showed that under certain conditions, a shallow, highly turbid layer forms on the bed, capable of being transported by weak tidal currents without significant vertical mixing. Supplementation of field measurements of water movement with more detailed hydraulic model measurements permitted the formulation of a siltation mechanism.Appraisal of results suggests the most important immediate source of the silt to be the shallow coastal mud banks. Waves produce the turbid, near-bed layer which is transported by littoral currents into port approach channels for redistribution by the prevailing residual current, itself a function of fluvial/tidal interaction.Key words: siltation, dredging, silt movement, flocculation, waves, tides, river flow, residual currents

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