Population Growth and Urbanization Chapter 12 Population Growth and Urbanization.
Urbanization and Sedimentation
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Transcript of Urbanization and Sedimentation
3/11/2008 Dr. K. Chatterjea, NTU. 1Dr. K. Chatterjea, NIE/ NTU
3/11/2008 Dr. K. Chatterjea, NTU. 2Dr. K. Chatterjea, NIE/ NTU
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.
References
3/11/2008 Dr. K. Chatterjea, NTU. 3Dr. K. Chatterjea, NIE/ NTU
• Urbanization
• Soil erosion
• Sediment generation
• Sediment movement
• Surface wash
• Rills and gullies
• Sheetflow
• Runoff
• Tropical rainforest
Key words for this topic:
• Suspended sediment
• Accelerated erosion
• Human activities and
temporary equilibrium
• Sediment yield
•Urban development and streamflow alteration
•Sedimentological impacts of development
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Channel morphology adjusts to changing water and sediment discharges
1. Long-term
2.Medium-term
3. Short-term
ClimaticHydrologicTectonic
Human activities
Temporary disequilibrium
The channel passes through “transient phases”till it reaches a new “equilibrium”.
Individual discrete storms: big events can cause catastrophic changes to cross-section, gradient etc. etc.
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Medium-termHuman activities Temporary disequilibrium
The river tends to readjust to the changed conditions.
This leads to changes in the channel morphology
and flow conditions
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Individual discrete storms: big events can cause catastrophic changes to cross-section, gradient etc. etc.
Short-term
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‘Direct’ Impacts (planned):•Management of water Control of stream flow
Regulate water flowIrrigationNavigation
‘Indirect’ Impacts (unplanned) Caused by alteration of landuse
Both result in changes in Sediment yieldRunoff production
The stream needs to adjust to such changes
Medium term effects
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Rills on a bare slope
Accelerated erosion
Sedimentologicalchanges
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Construction sites like this one generate huge sediment loads into thedrainage channels
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Notice the concentration of suspended sediment in the streamflow – leading to heavy sediment discharge
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The sediment generated at the disturbed sites are transferred to the drainage channels
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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
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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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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)
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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)