urbanization and sedimentation

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

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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.

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Page 1: Urbanization And Sedimentation

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

Page 2: Urbanization And Sedimentation

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

Page 3: Urbanization And Sedimentation

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

Page 4: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 4

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.

Page 5: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 5

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

Page 6: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 6

Individual discrete storms: big events can cause catastrophic changes to cross-section, gradient etc. etc.

Short-term

Page 7: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 7

‘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

Page 8: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 8

Rills on a bare slope

Accelerated erosion

Sedimentologicalchanges

Page 9: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 9

Construction sites like this one generate huge sediment loads into thedrainage channels

Page 10: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 10

Notice the concentration of suspended sediment in the streamflow – leading to heavy sediment discharge

Page 11: Urbanization And Sedimentation

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

The sediment generated at the disturbed sites are transferred to the drainage channels

Page 12: Urbanization And Sedimentation

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

Page 13: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 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)

Page 14: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 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)

Page 15: Urbanization And Sedimentation

3/11/2008 Dr. K. Chatterjea, NTU. 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)