Sediment generation during the process of urbanization is accelerated many folds, affecting the stream channel morphology. Rate of this sediment generation depends on the landuse and land cover. Regions in the humid tropics are particularly vulnerable to such accelerated sediment generation due to the characteristic high-intensity rainfall. The presentation examines the salient points related to the geomorphological processes under such conditions with special references to humid tropical environments. References to relevant lietrature and field data are included.

1. Dr. K. Chatterjea, NIE/ NTU 3/11/2008 Dr. K. Chatterjea, NTU. 1

2. 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

3. 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

4. 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

5. 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.
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6. Short-term
Individual discrete
storms: big events can
cause catastrophic
changes to cross-section,
gradient etc. etc.
3/ 2008 Dr. K. Chatterjea, NTU.
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7. 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.
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8. Sedimentological
changes
Rills on a
bare slope
Accelerated erosion
3/ 2008 Dr. K. Chatterjea, NTU.
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9. Construction
sites like this
one generate
huge
sediment
loads into the
drainage
channels
3/ 2008 Dr. K. Chatterjea, NTU.
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10. Notice the concentration of suspended sediment
in the streamflow – leading to heavy sediment
discharge 3/ 2008 Dr. K. Chatterjea, NTU.
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11. The sediment
generated
at the disturbed sites
are
transferred to the
drainage channels
3/ 2008 Dr. K. Chatterjea, NTU.
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12. 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.
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13. 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)
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14. 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.
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15. 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)
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