Hydrogeomorphic characteristics of streams in Bukit Timah Nature Reserve, Singapore

Abstract

Field data and information from the literature on hydrology, hydraulic and geomorphic characteristics of the ten streams in Bukit Timah Nature Reserve were collected to form a baseline dataset with the aim of evaluating various functions that these streams perform, as well as the stresses and disturbances that they experience. Preliminary results show that majority of the channel reaches studied are functioning well, except that moderate to intense erosion was observed at the middle reaches of Lasia and Dairy Farm, and the lower reaches of Fern Valley and Wallace streams, where instability, undercutting and failure of banks have caused significant channelisation (bank widening and channel down-cutting). In general, the streams in Bukit Timah have little sediment, with low and small-sized bars visible only at the lower reaches of Fern Valley, Lasia and Wallace streams, which should not significantly affect the stream function if culverts and lined drains downstream are well maintained. However, several forced step-pools (artificial ponds) near the outlets of Taban, Seraya and Catchment streams and the middle of Jungle Fall stream continuously receive upstream sediments loads and accumulation of sediment with distinct siltation have obviously degraded the instream habitats. Follow-up actions on stream rehabilitation, adaptive management, future research and future monitoring are discussed.

Physicochemical characteristics of streams in Bukit Timah Nature Reserve, Singapore

“…Water samples from 20 of the 87 sites (Table 2) were collected for ex-situ analysis. The points were chosen corresponding to the at-a-reach stream channel morphology sampling (described by Cai, 2019). The sampling sites were taken to represent water quality in the chosen reach of the stream channel, without further distinguishing the influence of inflows from side channels entering at intermediate positions between sampling points.…”

“…Results of the spatial survey are presented in Table 1. The ten streams originating in BTNR range from only 250 to 1100 m in length (Cai, 2019), downstream of which they flow into the concretised surface drainage system of urban Singapore and they lie within an elevation of between 158 and 37 m above sea level (Cai, 2019). There are few reports on the physicochemical characteristics of small lowland streams in the Southeast Asian region, most studies having been either on large rivers, or on small headwaters at considerable elevations.…”

“…In spite of their overall similarity and their close proximity on different faces of a single hill, each of the streams shows individual characteristics. These are related to catchment size for each stream (Cai, 2019), the intensity of human impacts in different parts of BTNR (Chatterjea, 2019) that differentially affect the stream catchments, the current vegetation structure (primary, old secondary and maturing secondary forest) (Chan & Davison, 2019) affecting tree size, rainfall interception versus stemflow, leaf litter depth, root density and root penetration, as well as details of slope, bedrock, and soil chemistry. These factors play out through the expected interactions between soil, groundwater and rainfall.…”

Synthesis of results from the Comprehensive Biodiversity Survey of Bukit Timah Nature Reserve, Singapore, with recommendations for management

“…In general, the streams in Bukit Timah have little sediment, with low and small-sized sediment bars visible only at the lower reaches of Fern Valley, Lasia and Wallace streams, which should not significantly affect the stream function if culverts and lined drains downstream are well maintained. However, several artificial pools near the outlets of Taban, Seraya and Catchment streams and the middle of Jungle Fall stream continuously receive upstream sediments loads and accumulation of sediment with distinct siltation have obviously degraded the instream habitats (Cai, 2019).…”

“…Stream enhancement initiatives should mirror those for the terrestrial habitat, for the survival of freshwater fish, decapods, dragonflies and other aquatic plant and animal species (Cai, 2019;Cai et al, 2019;Khoo et al, 2019;Li et al, 2019;Nguyen & Cai, 2019).…”