Transcript Sediment Overview - Spate Irrigation

6.2
Sedimentation Issues: Overview
Phil Lawrence ([email protected])
STATEMENT:
IN SPATE IRRIGATION
MANAGING SEDIMENTATION IS AS IMPORTANT AS
MANAGING WATER
Exercise – differences between spate and
conventional irrigation
 Make a list of between spate and conventional
irrigation, focussing on:






Sediment loads
Water availability
Type of canals and water control structure structures
Sediment management
Command issues
“Ownership” and O & M
Overview of sedimentation issues
in spate irrigation systems
Sediment concentrations carried by some
Sediment concentration ppm
perennial rivers
25000
22041
20000
15000
10000
6750
5000
1849
1720
602
325
204
0
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R
Sediment Concentration ppm
Compare with sediment concentrations
transported by wadis
180000
160000
140000
120000
100000
80000
78000
60000
40000
38000
20000
22041
204
325
602
6750
1720 1849
0
Typical sediment size range transported
in wadis
bed load,
suspended
sand
silt and clay
Features of sediment transport in wadis

Very high sediment loads carried by flash floods, but in some wadis an
appreciable proportion of the annual run off occurs at relatively low
discharges in flood recessions and seasonal base flows.

Total load sediment concentrations exceeding 10 percent by weight are
transported by floods in some wadis.

Wadi bed materials can range from boulders and cobbles to silts, but
sediment transport is dominated by the finer sediment fractions, silts
clays and fine sand transported in suspension. Large sediments,
coarse sand, gravel, cobbles, and small boulders, typically represent
only 5 percent or less of the annual sediment load.

Very large quantities of fine silts delivered the fields
Sediment management in spate systems
 Silt wanted by farmers on the fields (fertility)
 But larger sediments transported at high wadi
discharges flows will, if diverted, settle and block
canals
 Ideally all fine sediments (silts) will be transported to
the fields, while large sediments, coarse sand and
larger, should be excluded from canals.
Now is this possible?
Traditional spate systems
 Intakes are made in such a way that they
avoid taking in the very large floods

Intakes (both guidebunds and diversion bunds)
are washed away by large floods, preventing the
ingress of the coarse sediments carried by high
wadi flows
 Intakes are located in outer bends
Traditional Intake – Yemen
Traditional intake – Pakistan
Sediment management in traditional systems
 Traditional spate canals are very steep
compared with conventionally designed
perennial canals, flow at high velocities, and
have a very high sediment transporting
capacity for fine sand and silts.
Sediment management in traditional systems
 Usually all the flow in a canal is diverted to at
a single point and then flows from field to
field. A high sediment transporting capacity is
maintained right through to the fields and
flows are not headed up at water control
structures.
Sediment management in traditional systems
 When command starts to be lost intakes can
easily be moved further upstream, by
extending a diversion spur and or moving the
canal intake.
Summary of sediment management features in
traditional systems

Diversion spurs are washed away by large floods, preventing diversion of high
concentrations of coarse sediments. (As multiple intakes are used diversion from
floods usually still possible at downstream intakes)

Diversion bunds spanning a wadi are only used in lower reaches of wadis where the
coarse sediments have settled on the wadi bed and only fine sediments are
transported.

Canals are very steep compared with canals in conventional perennial irrigation
schemes – providing a high sediment transporting capacity.

Usually all the flow in a canal is diverted fields at a single point, a high sediment
transporting capacity is maintained from the wadi through to the fields.

When command starts to be lost by rising field levels intakes can easily be moved
further upstream.
Sediment problems in modernised spate
systems
 Permanent diversion structures enable larger
discharges to be diverted from spate flows: that was
often the main purpose of building them. Diversion
from wadi flows carrying very high sediment loads is
possible. Sediment sluices, canal closures in flood
peaks and sediment exclusion/extraction facilities
are used to reduce the loads of coarser sediment
fractions entering canals.
Wadi Rima Intake (Yemen)
Timely operation of manual sluice and intake
gates in spate flows difficult or impossible
 Water levels change very rapidly at flood
peak
 Operators do not how big a flood peak is
going to be
 Multi peak spates common
 Farmers resist ”wasting” water by operating
scour slices etc.
Manually operated gate
Rising command levels
 Adequate provision for rising command levels
must be made when permanent diversion
structures with reasonably long anticipated
design lifetime are adopted.
 In Wadi Zabid (Yemen) the upstream weirs
have been raised twice since the early 1980’s
Summary sediment management strategy for
modernised schemes

Limit the diversion of coarser sediments by appropriate intake
sitting and design, use of scour sluices and, where feasible
provide sediment control structures

Transport fine sediments through canals to the fields, (steep
canals, maintain high flow rates to fields, don’t head up flows at
water control structures.)

Make provision for the inevitable rise in command levels

Anticipate and quantify the need for canal de-silting and plan for
it
Exercise – differences between spate and
conventional irrigation – the answers??
 Make a list of between spate and conventional
irrigation, focussing on:






Sediment loads
Water availability
Type of canals and water control structure structures
Sediment management
Command issues
“Ownership” and O & M
Sediment loads
Spate irrigation
Sediment concentrations up too
and exceeding 10% by weight
Perennial / seasonal irrigation
Sediment concentrations rarely
exceed 0.2 % by weight
Water availability
Spate irrigation
Perennial / seasonal
irrigation
Spate floods with highly variable Reasonably predictable seasonal
and unpredictable timing number flows allowing continuous
and magnitude. In some cases
diversion at controlled flow rates.
supplemented with low seasonal
base flows
Canals and water control structure
structures
Spate irrigation
Steep canals operating a high
flow velocities
Flows supplied to very limited
number of field off-takes (only
1 at a time in most traditional
systems)
Perennial / seasonal
irrigation
Low sediment transporting
capacity provided by
conventional canal design
methods
Water supplied to numerous
outlets simultaneously
Sediment management
Spate irrigation
Many sediment management
options, particularly those
involving sediment flushing, not
appropriate for spate schemes
Perennial / seasonal
irrigation
Range of tried and tested
sediment management options
available.
Command issues
Spate irrigation
Command levels in upstream
areas that receive most water
and the largest sediment loads
may be rising by 5 cms./year or
more
Perennial / seasonal
irrigation
Command levels usually fixed
over the design life of diversion
and water control structures .
“Ownership” and O & M
Spate irrigation
Perennial / seasonal
irrigation
Traditional systems are farmer
constructed operated and
managed. Some spate irrigation
systems are the largest farmer
managed systems in the world.
Customary water rights and
arrangements and obligations for
O and M well developed.
Larger systems often agency
designed operated and
maintained, with turnover to
artificially created farmer groups at
some point within the water
distribution system.
Based on work of Philip Lawrence