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The most common occurrence of this terrain is as the broad flat base to large valleys such as at Sha Tin.However, some areas of alluvium occur as small isolated plains drained by streams with restricted outlets. Anexample of this type of terrain occurs on the high plateau below the Hunch Backs.2.5 Surface HydrologyThe natural drainage regime has been greatly modified by development and the need to provide watersupplies to the urban areas. From the mean annual rainfall data incorporated in the GEOTECS data base it isestimated that an average of 329 million m$ of water falls within the Central New Territories study area perannum. Major modifications to the drainage system have included the creation of six large reservoirs, theconstruction of extensive catchwaters, development on footslope terrain and the formation of large areas ofreclamation.The main modification to the drainage system that is associated with development involves the covering oflarge areas with effectively impermeable materials. This has resulted in larger volumes of surface runoff asinfiltration is reduced, and a shorter time to peak discharge following the onset of rainfall. A high density ofdrains to remove the large volumes of storm runoff are provided within the area. In addition, some naturaldrainage paths are channelised to improve their efficiency of discharge beyond the developed areas.There are six reservoirs in the study area, the Shing Mun (Jubilee), the Lower Shing Mun and four smallerreservoirs in the Kam Shan Country Park. These are supplied by an extensive network of catchwaters and withthe aid of several water tunnels form part of a complex water management system.The boundaries of the major natural catchments are indicated on the Engineering Geology Map and areclassified in accordance with the method devised by Strahler (1952) which is described in Appendix A. 8.The largest catchment is formed by the Shing Mun and Sha Tin valleys. The southeastern slopes of Tai MoShan, the southwestern slopes of Grassy Hill and the western slopes of Needle Hill form the naturalcatchment for the Shing Mun (Jubilee) Reservoir. Apart from the upper valley, the Lower Shing MunReservoir has only a small natural catchment. The majority of streams that flow into the lower reaches of theShing Mun River are subparallel, with long elongate catchments which drain moderately steep sideslopes.Two fifth order catchments exist in the Sha Tin valley, one draining the eastern slopes of Grassy Hill and theother draining the slopes from Tate's Cairn to Turret Hill. Another fifth order catchment drains the slopes fromTurret Hill to Ma On Shan and currently discharges into Tide Cove, however further reclamation will result inthis stream discharging into an extension of the Shing Mun River Channel. These three catchments areapproximately equal in size and shape with a well developed pattern of dendritic drainage. Runoff isconcentrated along the lower reaches of the streams with resultant high peak discharges.The Kam Shan Country Park has a very high drainage density, producing a fifth order catchment within acomparatively small area. However, the resultant discharge within the natural drainage system throughButterfly Valley has been modified by the presence of the four reservoirs within the drainage basin.Directly south of the summit of Tai Mo Shan an elongate fourth order basin drains through Lo Wai. Steepslopes and the high annual rainfall on Tai Mo Shan (Appendix C.4) result in high discharges from thiscatchment The catchments which drain the ridge that trends southwest from Tai Mo. Shan have more gentleslope angles and longer drainage paths. Times to peak discharge are expected to be longer. The drainagepattern is slightly elongate in nature resulting in a spreading of the times to peak discharge in the mainchannel and nullah that carries the flow through Tsuen Wan.For the most part, Tsing Yi Island has a radial drainage pattern. However, one small fifth order catchmentexists which drains the northeast of the island and discharges into Tsing Yi Bay.Two subcatchments exist on the northern slopes of Tai Mo Shan and Grassy Hill. These catchmentsultimately drain through Tai Po. Slopes are steep, with a moderately dense drainage pattern. The remainder ofthese catchment areas are contained with in the North New Territories (GASP V) Study Area.2.6 VegetationThe vegetation of the study area is considerably affected by human activities and the present patternrepresents significant variation from the natural regime. In this Report, a nine class classification system isused to distinguish broad categories of vegetation type. The spatial distribution of these groups is illustratedin the GEOTECS Plot in Figure? whilst Figure 4 shows their relative proportions. About 25.9% (3458 ha) ofthe study area is devoid of vegetation due to man's disturbance. Denudation within the study area is generallythe result of site formation and, to a lesser extent, man induced soil erosion. The data is presented in Table B7at Appendix B.The vegetation classes are as follows:(i) Grass/andThis class generally consists of indigenous or introduced grass species which occur naturally or afterthe clearing of shrubland or woodland. Grassland occupies 3 696 ha (27.6%) of the study area andoccurs on the slopes of Tai Mo Shan, Ma On Shan and Pyramid Hill. However, scattered areas ofgrassland occur throughout the undeveloped areas of the district.20