Slopes processes and stability

River action often begins on the uplands as soil erosion. Slope erosion, which includes rainsplash (splash erosion) and sheet erosion, occurs naturally and it can be accelerated by human activity. The density of vegetation cover is an important factor in determining the nature and rate of erosion on slopes. The removal of vegetation from slopes accelerates erosion and favors the development of rills and gullies.

Although the morphology of Singapore is generally low, with more than 60% of the land surface below a height of 30 m and only 10% above 30 m (Wong, 1968), there is an indication that slope stability problems are increasing in Singapore, which include failures in rock slopes. The influence of human activity on the landscape of Singapore is apparent from the large number of slope failures and landslides which contribute material to the rivers.

Urban development also influences sediment supply in the river channels. The sediment load is increased by the removal of protective vegetation cover during the phase of rapid urban development. During the initial phase of urban development, an urban stream receives a significant pulse of sediment eroded from upland construction sites despite the implementation of sediment control measures. Once upland development is stabilized, sediment source from construction sites gradually declining (WEFASCE, 1998).

Singapore is subjected to constant pockets of residential and industrial construction development, and hence has the potential of higher sediment load in its river systems. The slopes have often been re-graded as part of a programme of landscaping, road construction, residential development or quarrying (Figure 9). The excavation of these slopes has re-generated weathering processes and accelerated soil erosion. Many slopes have been steepened during engineering works and they cannot be sustained in certain cases.