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West Lamma Channel These may be attributed to increased pore (gas) pressure in theunderlying sediments, or, more likely, to sea bed dumping. A study to assess the effect of shallowgas on the engineering properties of sediments (Premchitt et al. 9 1990) indicated that someoccluded gas bubbles exist in the areas of acoustic turbidity. The free gas mainly consisted ofnitrogen and methane, while carbon dioxide was the dominant dissolved gas. Free gas was notdetected where acoustic turbidity was not seen on seismic records. It was concluded that the gasdid not adversely effect the geotechnical properties of the sediments.Seismically, the formation consists of very extensive, laterally continuous, horizontal tosub-horizontal, moderate to low amplitude reflectors. The base of the formation is generallyplaced at a well-developed high amplitude reflector. The horizontal to sub-horizontal reflectorstend to infill any small hollows and channels, with little or no evidence of truncation and overlap.In some areas, such as south of Lantau Island, the amplitude of reflectors increases within thebasal section of the formation with a more transparent unit above. An intra-formational reflectorhas been identified in many of the channels. This high amplitude reflector runs across the tops ofchannels, joining each channel crest (Figure 22). It marks the change in current activity as thechannels filled up with sediment and the sea bed became relatively planar. The reflector couldmark a hiatus in sedimentation due to erosion or non-deposition, or may form part of aravinement surface truncating the surrounding formations.The banks at the margins of the main tidal current channels display a characteristic internalreflector style. Long, low-angled reflectors predominate, with low-angled truncations formed byreactivation. An extensive, prominent reflector with a very long, sinuous wavelength has beenidentified south of the Urmston Road - Brothers proto-channel. This reflector may be the resultof a major hiatus or a change in current activity. South of Sha Chau, the formation exhibitsstrong clinoform reflectors dipping to the south. These are inclined at a high angle directly abovethe basal reflector of the formation. The whole formation in this area is virtually one foreset andtopset unit, with no bottomset development, indicating very rapid sedimentation.DetailsNorth and east of Lantau Island. Over most of the area, the sea bed comprises soil, clayey silts of the formation.Coarser sandy sediment is associated with the tidal current pathway that runs along the Urmston Road - BrothersChannel through Kap Shui Mun (240 225) and southwards past Green Island (290 160). Tidal current velocitiesexceed 1 m s" 1 in Kap Shui Mun where the sea bed is floored by rock. As the current velocity gradient decreases tothe west and south of Kap Shui Mun (Figure 23), so sedimentation has occurred, firstly as sand banks, and withcontinuing loss of velocity the sand has given way to the ubiquitous mud blanket. A sand bank occurs off thenortheast coast of Lantau, to the north of which the sea bed is rock floored, This sand bank widens and thinswestwards to be concealed by a thickening cover of marine mud. A second sand bank occurs immediately south ofKap Shui Mun, that is subsumed southwards by a thickening mud cover. A third sand bank occurred southwest ofTsing Yi, extending under a mud cover to Green Island, but was removed for reclamation fill Considerablemodification of the sea bed has taken place in this area. Much of the sand from the Kap Shui Mun bank has beendredged for reclamation fill, the Urmston Road Channel is regularly dredged for navigation, and the site of the newChek Lap Kok airport has had most of the marine mud removed, and replaced by reclamation fill. A number oflarge seabed pits have been dredged east of Sha Chau as a repository for dredged, contaminated marine mud andmost of the northeast Lantau coast and littoral zone has been extensively modified by reclamation works for theNorth Lantau Expressway.South Lantau Island. To the south of Lantau Island, the sea bed is a layer of marine mud that is generally up toc.l 5 m thick. A sand bank, which has now been dredged, occurred west of the Soko Islands, and there is some sandat the seabed immediately to the east of the Soko Islands. Both of these occurrences are related to accelerating tidalcurrents passing the islands, keeping finer material in suspension while sandier material is deposited on the leeside. A large gazetted dumping ground to the south of Cheung Chau (Sheet 1A) has received a variety of dumpedwaste, and most recently the dredged mud from the Chek Lap Kok airport and other reclamations throughout theTerritory. A former sea bed dumping ground (240 170) is located north of Siu Kan Yi Chau,138