Published on March 12, 2008
Dr. K. Chatterjea, NIE/ NTU 3/11/2008 Dr. K. Chatterjea, NTU. 1
References Chatterjea, K. (1989). ‘Surface wash: the dominant geomorphic process in the surviving rain forest of Singapore’, SINGAPORE JOURNAL OF TROPICAL GEOGRAPHY, Vol.10, No. 2, Singapore, 95-109. Chatterjea, K. (1991). ‘Effectiveness of grass cover in reducing risk of sediment generation on urban slopes: an example from Singapore,’ SINGAPORE JOURNAL OF TROPICAL GEOGRAPHY, Vol. 12, No. 2, Singapore, 69-81. Chatterjea, K. (1994). ‘Dynamics of Fluvial and Slope Processes in the changing geomorphic environment of Singapore’, EARTH SURFACE PROCESSES AND LANDFORMS, Vol. 19, Great Britain, 585-607. Dr. K. Chatterjea, NIE/ NTU 3/11/2008 Dr. K. Chatterjea, NTU. 2
Key words for this topic: • Urbanization • Suspended sediment • Soil erosion • Accelerated erosion • Sediment generation • Human activities and • Sediment movement temporary equilibrium • Surface wash • Sediment yield • Rills and gullies •Urban development and • Sheetflow streamflow alteration • Runoff •Sedimentological impacts of development • Tropical rainforest Dr. K. Chatterjea, NIE/ NTU 3/11/2008 Dr. K. Chatterjea, NTU. 3
Channel morphology adjusts to changing water and sediment discharges 1. Long-term Climatic Hydrologic Tectonic 2.Medium-term Human activities 3. Short-term Temporary disequilibrium Individual discrete storms: big events can cause catastrophic changes to cross-section, gradient etc. etc. The channel passes through “transient phases” till it reaches a new “equilibrium”. 2008 Dr. K. Chatterjea, NTU. 3/11/ 4
Medium-term Human activities Temporary disequilibrium The river tends to readjust to the changed conditions. This leads to changes in the channel morphology and flow conditions 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 5
Short-term Individual discrete storms: big events can cause catastrophic changes to cross-section, gradient etc. etc. 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 6
Medium term effects ‘Direct’ Impacts (planned): •Management of water Control of stream flow Regulate water flow Irrigation Navigation ‘Indirect’ Impacts (unplanned) Caused by alteration of landuse Both result in changes in Sediment yield Runoff production The stream needs to adjust to such changes 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 7
Sedimentological changes Rills on a bare slope Accelerated erosion 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 8
Construction sites like this one generate huge sediment loads into the drainage channels 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 9
Notice the concentration of suspended sediment in the streamflow – leading to heavy sediment discharge 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 10
The sediment generated at the disturbed sites are transferred to the drainage channels 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 11
Changes in the channels caused by urbanization: Sedimentological changes: •Deposition of relatively coarse textured sediment in the channel •Very high concentration of suspended sediment - sharply increased sediment load - this will be followed by a reduction in the amount of load. Hydrological Changes: •Changes in Flow duration - modification of hydrographs - lower base flows, larger floods •Changes in flood frequencies - more frequent - also bigger floods •modification of hydrograph characteristics - decreased lag time, increased peak Q, increased peak velocity Morphological Changes: •Changes in channel size - width, depth and bedforms 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 12
Some data on sediment yield from drainage basins in the Tropics: Place Landuse Sediment yield Source (m3/m2/y) Australia Rainforest 5.65 Douglas (1967) Malaysia Rainforest 21.1 Shallow (1971) Malaysia Rainforest & 144 Douglas (1970) Agriculture Indonesia Shifting cultivation1362 Leigh & Low (1973) India Mostly agriculture 1310 Lal et al. (1977) 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 13
Some Singapore data on sediment yield: (under forest cover) Place Landuse Sediment concentration Source (mg/l) Bukit Timah Rainforest 222 Chatterjea(1994) Bukit Timah Rainforest 192 Chatterjea(1989) Bukit Timah Rainforest 172 Chatterjea(1994) (all undisturbed) Bukit Timah Rainforest 8679 Chatterjea(1994) (disturbed) Bukit Timah Rainforest 5200 Chatterjea(1989) (disturbed) Bukit Timah Rainforest 1714 Chatterjea(1989, 1994) (disturbed) 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 14
Some local data on sediment yield: (in areas undergoing construction work - bare surface) Place Type of flow Sediment concentration Source (mg/l) NUS Campus Runoff 5,600 Chatterjea(1991. 1994) NUS Campus Sheet flow 75,398 Chatterjea(1991, 1994) 34,113 Chatterjea(1994) 48,913 Chatterjea(1994) Clementi Rd Rill 11,000 Chatterjea(1991, 1994) Sheet flow 11,200 Chatterjea(1991, 1994) Yishun Runoff 7,800 Chatterjea(1991, 1994) Malaysia Channel flow 81,230 Douglas (1978) Malaysia Channel Flow 15,343 Leigh (1982) 3/ 2008 Dr. K. Chatterjea, NTU. 11/ 15
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