SPATE ENGINEERING GASH RIVER SUDAN

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SPATE ENGINEERING GASH RIVER - SUDAN

• Lesson 4

IanMcAnderson_at_aol.com
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Gash River in north west Sudan
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The Gash River

• Known in the past to be fearsome with sharp
  uncontrollable floods.
• Historically the river flowed further to the west
  with the original Delta area close to what is now
  the New Halfa irrigation scheme.
• It rises in Eritrea, some 15 miles south of
  Asmara, and as the river Mareb, it flows through
  a relatively narrow valley until it reaches
  Haykota, where it gradually widens into a
  sandy-clayey plain, the Tesseney-Omhajer plain.
  From there it flows in a westerly direction to
  Tesseney where the river turns northwards into
  Sudan.
• The Gash/Mareb is dry for much of the year, but
  is subject to sudden floods during the rainy
  season (July to September).
• The river transports considerable amounts of
  sediment (fine sand and silt) and has been
  estimated to transport about 5.5 millions tons of
  sediment annually at Kassala Bridge.

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The Gash River

• The river continual deposits and picks up
  material from the river bed and the neighbouring
  banks to stay in regime.
• When the river crosses the border, its slope
  reduces remarkably and the river gradually
  deposits the sediment load in its course and the
  lower Delta.
• This gives rise to a very unstable river that
  that has changed direction many times and that
  undergoes continuous morphological changes.
• The sediment deposits have led to a gradual build
  up in river channel level (sometimes 1-2 m),
  especially where irrigation offtakes are located.
  
• In Kassala City, the existing river bed is now
  some two meters above much of the ground level in
  the City and thus when breaches occur,
  considerable flood damage results

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The irrigation system in the Gash Agricultural
Scheme (GAS)

• Command area comprises main, link and some branch
  canals fed from the Gash River by means of
  gravity offtakes fitted with stop logs.
• The flows in these gravity canals are divided
  into a number of Misga or distributary canals
  that directly supply the agricultural land
  through breaches in the canal banks.
• Unit flows are high and these flow across the
  Misga blocks (of about 3000 feddans) as a wetting
  front following the line of least resistance and
  guided by Misga water workers who are instructed
  by the Sheikh al Misga or water master
• The main factors affecting its progress are
  slope, nature/texture of soil, surface
  characteristics its covering.

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• The design of these systems was first developed
  in the 1930s and improved upon in the late 1950s.
  
• They were originally developed for the
  cultivation of cotton using flush irrigation
  techniques (this is a variation on spate
  irrigation that utilise fluctuating run-off in
  the seasonal Gash River) and the design details
  are reported by C.H. Swan (1959) in the Recorded
  Behaviour of the River Gash in Sudan.
• The considerable amount of sediment that is
  transported by this river5 by Volume is
  deposited within the canal systems and gives rise
  to increased annual maintenance costs necessary
  to provide the design slopes and flows within
  each of the six blocks that utilise the river
  water

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Design considerations

• Maintain velocity
• Minimise annual OM
• Do not provide blockages
• Use limited available water as efficiently as
  possible
• Adapt application of water to new cropping
  patterns

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