SPATE ENGINEERING
GASH RIVER - SUDAN
Lesson 4
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Gash River in north west Sudan
The Gash River
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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.
The Gash River
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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
The irrigation system in the Gash
Agricultural Scheme (GAS)
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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
river[5% 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
Design considerations
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Maintain velocity
Minimise annual O&M
Do not provide blockages
Use limited available water as efficiently as
possible
Adapt application of water to new cropping
patterns