SANDWAVES
	Formation
	Sandwaves are found where water is moved rapidly by strong tidal streams or heavy seas over a seabed covered by a sufficient depth of unconsolidated sediment. No sandwaves of any significance are found where the seabed is predominantly mud, but they are found where it is sand or gravel. Extensive sandwave fields are known to exist in the S North Sea, including the Dover Strait and parts of the Thames Estuary, in the Persian Gulf, in the Malacca and Singapore Straits, in Japanese waters, and in the Torres Strait. (Sand Dunes [52 KB]) Sandwaves are analogous to sand dunes formed by wind action on land. The action of the water movement forms the seabed into a series of ridges and troughs, most of which are thought to be virtually stationary, but others are known to move and alter significantly in height. Recent investigations have shown that sandwaves build to their maximum vertical extent, and therefore to their most critical navigational condition, following periods of relatively calm weather or neap tides. The Mariner should be prepared for changes from charted depths in any area where sandwaves are known to exist. Even in recently surveyed areas, it is possible that the surveys were not carried out when the sandwaves reached their greatest height. Sandwaves form fields which may be several miles in extent, with the waves in primary and secondary patterns. The waves vary in size from ripples seen on a sandy beach at low water to waves up to 20 m in amplitude and several hundred metres in wavelength. The waves forming the primary pattern may be several miles long. They usually lie nearly at right angles to the main direction of water movement, but small waves are sometimes found lying parallel to it. Secondary patterns are usually superimposed on the primary pattern, often at an angle; it is where the crests of the patterns coincide that the shoalest depths can be expected. Detection A line of soundings run at right angles to a navigational channel to fix its sides will usually run parallel to any primary pattern of sandwaves, and thus may well fail to obtain the least depth over the waves, or even to locate them at all. Further lines of soundings at right angles to the others will increase the chances of obtaining the least depth, but even these may be inadequate if the secondary pattern is complicated. Navigation Areas where the bottom is liable to change because of the movement of sandwaves are indicated on Admiralty charts by the appropriate symbol, or a suitable legend. Known details of such areas are given in Admiralty Sailing Directions. Since the position of sandwaves and the depth of water over them are liable to change, ships with little under-keel clearance should treat the areas in which they are known to exist with due caution.