Dynamics of fluvial and slope processes in the changing geomorphic environment of singapore

Abstract

The nature and rates of fluvial and slope processes change over time and space as urbanized areas replace forested land in Singapore. Storm-based and time-based data, from undisturbed rainforests, heavily disturbed construction sites, urban grass-covered slopes and an experimental plot, are collected to observe the impact of rainwater on the soil moisture conditions, surface microtopography, runoff generation, sediment movement, and ground lowering in the three different categories of land use.The undisturbed forested environment is characterized by high throughfall (58% of total rainfall) and frequent negative soil moisture suctions. The slow and unconcentrated overland flow during heavy storms is restricted by the forest floor microtopography. No rills develop. Ground lowering is recorded as 3-2-3.4mm aC1. But sediment movement is episodic and suspended sediment concentrations in overland flow are 172-222 mg 1-' . During urban construction, gully development is rapid on the bare slopes, runoff generation, voluminous, and sediment-laden discharges (5200-75 498 mg 1-') lead to sediment plumes at channel mouths. Ground lowering rates are measured at 132.4 mm a-l. Once grass-covered, runoff carries less suspended sediment (800 mg 1-') and ground lowering rates are reduced, but depend on the condition of the cover, ranging from 0.2 to 8.2 mm a-l.As urban development continues, environments are altered both in time as well as spatially.

Assessment and demarcation of trail degradation in a nature reserve, using GIS: case of Bukit Timah Nature Reserve

“…With slopes ranging from 15 to 20 degrees such surface wash leads to substantial sediment movement, albeit for a short distance (Chatterjea, 1994), scouring topsoil, creating potholes, exposing roots and washing out entire layers of topsoil and thus exposing an even more compacted subsurface layer higher in clay content (Figure 11), as also observed by Sutherland et al (2001). The trails soon get degraded and trail users create detours through forested areas at the verges of the trails.…”

“…Of the 36 sites measured, an average of 64Á3 per cent surface was exposed and bare, indicating movement of loose dry litter due to either surface disturbance such as visitor movement or due to excessive surface runoff during rainstorms. Chatterjea (1994) mentioned that there are instances of generated saturation overland flow on the Bukit Timah forest slopes. If that is assisted by a compacted surface, the trails will be subjected to much surface wash.…”

Severe wet spells and vulnerability of urban slopes: case of Singapore

“…1 a Rezaur et al (2002Rezaur et al ( , 2003; b Rahman (1992); c Chatterjea (1989aChatterjea ( , 1994; d Author-current field data, obtained from 50 samples at each site; e Chatterjea (1989bChatterjea ( , 1991 safety. Data from various sources are looked at in detail with a view to establishing and emphasizing the relationship between rainfall events and slope failures, as observed by many researchers on this topic.…”

“…Characteristically, in Singapore, rainstorms rarely last beyond 1 h and an intensity-duration-frequency curve based on a 30-year annual maximum series shows that a typical 30-min 2-year rainstorm has an intensity of about 90 mm/h (Chatterjea 1994). Chatterjea (1998) reports that a consecutive 2-year record of all rainstorms showed 38% of all rainfall events in Singapore lasted for 30 min or less, while 13.4% and 7.9% of storms lasted for 20 and 10 min, respectively.…”

“…Singapore, situated between latitude 1°09 0 and 1°29 0 N and longitude 103°38 0 and 104°06 0 E, is a hot and humid tropical country, with an average temperature of 26.6°C, and a high and year-round rainfall of 2400 mm, with December-January being the wettest period (Chatterjea 1994). The constantly warm and humid conditions are conducive to the development of deeply weathered regolith of varying depths (10-45 m: Agus et al 2001, p. 285) on bedrocks of Bukit Timah formation granite and Jurong formation sedimentaries.…”