Deforestation and Floods in the Central Nepal Himalaya

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Deforestation and Floods in the Central Nepal
Himalaya

• Lecture outline
• Problem statement and background
• Methods
• Results and conclusions
• Reference
• Marston, R.A., Kleinman, J. and M. Miller. 1996.
  Geomorphic and forest cover controls on monsoon
  flooding, central Nepal Himalaya. Mountain
  Research and Development 16 257-264.

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Problem Statement and Background

• April 1991 Tropical storm Gorky roars out of Bay
  of Bengal and hits landfall on Ganges-Brahmaputra
  delta at high time

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Problem Statement and Background

• Created storm surge 15 feet high
• Winds up to 145 mph
• Most areas of delta lt 10 feet above MSL

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Problem Statement and Background

• Population density in delta gt 2000 people per
  square mile
• Flimsy construction
• Death toll gt 125,000
• Similar storms in
• 1970 500,000 deaths
• 1876 1737 300,000 deaths in each

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Problem Statement and Background

• Lowland floods have been blamed on subsistence
  farmers in Nepal Himalaya
• Population Growth
• ?Wood Demand ?Deforestation ?Floods

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Problem Statement and Background

• Annual monsoon floods (May-Sept.) pose severe
  hazard to mountain communities
• Bridge wash-outs

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Problem Statement and Background

• Floods trigger streamside mass movement by
  undercutting hillslopes

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Problem Statement and Background

• Water and sediment from floods damages
  agricultural land along rivers

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Problem Statement and Background

• Aggradation buries streamside terraces

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Research Objectives

• Estimate bankfull discharge from field
  measurements
• Examine what watershed characteristics (including
  forest cover) affect floods

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Central Nepal Himalaya

• Land of extremes
• Highest mountains
• Deepest gorges
• Most erosive rivers

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Central Nepal Himalaya

• Land of extremes
• Strong winds
• Great changes in humidity
• Great changes in temperature
• Leeches!!

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Central Nepal Himalaya

• Rapid runoff from long-duration monsoon
  rainstorms
• Steep slopes
• Shallow soils
• Impermeable metamorphic rocks

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Methods

• Two scientific expeditions to central Nepal
  Himalaya
• 120 miles over 3 weeks in 1984
• 150 miles over 6 weeks in 1987
• Worked between 1000 and 5000 meters elevation

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Methods

• Bankfull discharge estimated at 22 stream
  crossings
• Q WDV

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Methods

• Estimated bankfull stage

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Methods

• Measure bankfull width depth
• Estimate velocity using equation based on
  particle roughness

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Methods

• Range of values for Bankfull discharge 22.5 to
  1864 cms

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Methods

• Geomorphic characteristics of watersheds measured
  from topographic maps

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Methods

• Percent coniferous forest cover estimated in each
  watershed from SPOT imagery
• Values ranged from 25-40

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Results
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Results
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Conclusions

• Bankfull discharge in central Nepal Himalaya can
  be accurately estimated using watershed area
  alone as a predictor
• Forest cover was not a useful predictor

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Conclusions

• Dont blame the subsistence farmers for floods in
  Himalaya and in the lowlands of Bangladesh
  India
• Deforestation, wood cutting, floods are
  problemsbut links between them cannot be
  supported by data