Transcript SPATE ENGINEERING AN OVERVIEW

SPATE ENGINEERING
AN OVERVIEW
Lecture 1
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What is Spate Irrigation?
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Diversion of flashy spate floods running off steep often
denuded mountainous catchments
Flood flows usually last for only a few hours with
appreciable discharges and very short times to peak
Recession flows lasting for only one to a few days
Flows channelled through short steep wide canals to
irrigated areas, usually bunded basins,
Cultivated areas pre-irrigated through flooding to certain
depths determined by farmers through long experience.
Characterised by wide variations in areas irrigated
annually due to changes in annual Wadi flows.
Supplementary Irrigation
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Now included under Spate Irrigation definition
in some countries
Used in “rain shadow” areas to supplement
water requirements for already planted crops
Much greater use than in past
Traditionally used in Southern Ethiopia
Design Approaches different
Principles
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Utilise irregular peak flood flows for crop
cultivation
Divert through means bunds or splitters into
over-sized channels
Fill soil moisture reservoir either through
ponding or through successive wetting fronts
flowing over land
Types of Spate Irrigation
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Pre irrigation (Eritrea – Red Sea; Gash
Barka; Yemen (Tihama Plains); Sudan
(Tokar and Gas )
Supplementary irrigation
Trends
Understanding difference as this impacts on
capacity of canals
Main Categories
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Traditional
– Respect what is there
– Do not block flow but work with it
– Traditionally to split spate flow into
manageable quantities
– Maintaining velocity important
Main Categories
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Improved
– Provide durable engineering
structures to support traditional
approaches
– Manageable by communities
– Simulate their traditional techniques
Water Management
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risk-prone
requires high levels of cooperation between
farmers to divert and manage the distribution
of flood flows
Cohesive community or organisation
Sharing resources available according to
agreed plans
Water Management
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Locations and designs developed from long
local experience
Siting and constructing intakes flexible to
adjust to changing river alignments etc
Management of high flood waters and heavy
sediment loads
Organizing water distribution and annual
repair and maintenance
Traditional intakes and canals
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Diversion bunds across part or all of the channel;
Deflecting spurs – towards the top of a system – most
vulnerable to the peak floods
Canals are usually short and rarely include a secondary
distribution system.
Water flows as a wide wetting front over relatively uniform
land with manual guidance
or water collected on bunded fields and then passed
down to next field by breaking dividing field bunds when
the ponded water reaches a predetermined depth.
Some spate systems have their own supply channels.
Improved traditional systems
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Flow throttling/limiting structures
Rejection spillways/breach sections to
reduce impact of excessive large floods
Drop structures and flow division structures
in main canals.
Proportional dividers
Modernised Systems
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Numerous traditional intakes replaced with large
concrete diversion weirs.
Gated canal intakes and sediment sluices.
High costs of permanent structures results in single
permanent weir replacing many traditional intakes.
Design capacity equivalent to the combined capacity
of each replaced local diversion
Steep canals and sediment management structures
provided to minimise sedimentation.
Design tailored to management structure and
experience of farmers
Purpose of Engineered structures
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To reduce the labour involved in re-building
diversion spurs and bunds
Provide more secure structures that have
longer life than those using traditional
measures
Abstract for use a greater proportion of flow
in Wadi and avoid losses in Wadi beds
Reduce annual costs and time to facilitate
better water efficiency
Options
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Schemes on large rivers/wadis treated as a
series of independent small systems
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Traditional arrangements remain unchanged
Combine into one larger system that
respects traditional water delivery systems
and flow rates
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Requires high level of engineering expertise and
good hydrological data
Options
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Large modern weir across whole river
More traditional gabion weir across whole
river
Lower cost permanent weir plus breach
section – fuse plugs
Capital Cost and maintenance
considerations
Disappointing performance attributed
to:
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An increased inequity of water distribution
Command and diversion problems due to
high rates of sediment deposition
reduced the WUAs/farmers’ role in diverting
and distributing water
unrealistic assumptions concerning levels
and costs of operation and maintenance
failures to achieve an expected increase in
irrigated area