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SPATE ENGINEERING GASH RIVER - SUDAN

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SPATE ENGINEERING GASH RIVER - SUDAN Lesson 4 IanMcAnderson_at_aol.com Gash River in north west Sudan The Gash River Known in the past to be fearsome with sharp ... – PowerPoint PPT presentation

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Title: 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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