Transcript Part 3 - Sediment management structures and canal design

6.3
Sediment management options
Phil Lawrence ([email protected])
Summary sediment management strategy
 Limit the diversion of coarser sediments
 Transport fine sediments through canals
to the fields
 Make provision for the inevitable rise in
command levels
Limiting the diversion of coarser sediments: four
options
1. Locate intakes at outside of bends
2. Sediment excluding intakes
3. Limit diversion when wadi flows high –
throttling structures or close gates
4. Secondary sediment control
(1) Location of intakes at bends
 The low flow channel carrying flood recession
flows forms at the outside of a wadi bend.
(Traditional intakes are placed at the outside
of wadi bends for this reason.)
 In floods bed load sweep will move the
largest sediments towards the inside of a
bend and away from an intake.
Bed load sweep at a channel bend
Traditional intake showing location at the
outside of a wadi bend
Limiting the diversion of coarser sediments
 Locate intakes at outside of bends
(2) Sediment excluding intakes
 Limit diversion when wadi flows high –
throttling structures or close gates
 Secondary sediment control
Example of a spate sediment excluding intake
Features of the “spate” intake
 No divide wall, flows can approach from any
direction including parallel to the weir
 Intake aligned to minimise the diversion angle
 Curved channel with floor set lower than the
intake gate sill encourages coarser sediments
to move through the sluiceway
 In this case a fuse plug was used
Limitations of sediment excluding intakes in
spate schemes
 Spate intakes divert all the wadi flows except for the
short periods, sometimes only minutes, during flood
peaks when wadi flows exceed the intake capacity.
Sediment exclusion only effective during these
periods.
 Sluice gates have to be operated in response to
rapidly varying spate flows – mechanised gates
desirable but not often affordable.
 It is not unusual for farmers to block the sluice gate
as they do not want to loose water – in some systems
however the downstream farmers take their water
from the sluice gate and this makes the sluice gate
operated
Downstream area irrigated from scour sluice
Limiting the diversion of coarser sediments
 Locate intakes at outside of bends
 Sediment excluding intakes
(3) Limit diversion when wadi flows high –
throttling structures or close gates
 Secondary sediment control
Basic intake
Limit diversion from flood peaks
 For simple un-gated intakes use flow
throttling structures with a rejection spillway to
limit the flows entering a canal.
 For gated intakes consider closing canal
gates during short periods of high flow. (There
are problems of responding to rapidly varying
flows, and farmers reluctance to “waste”
water) Flow throttling structures with a
rejection spillway are also used with gated
intakes to ensure that canals are not
damaged by excessive flows if the gates are
left open during very large floods .
Limitation to throttling structures
 May not be like by farmers – too much of the flow is
lost
 May attract trash and become clogged
Limiting the diversion of coarser sediments
 Locate intakes at outside of bends
 Sediment excluding intakes
 Limit diversion when wadi flows high –throttling
structures or close gates
(4) Secondary sediment control
Secondary sediment control
 Settling basins
 Canal sediment extractors
Wadi Mawr settling basins
Models are used design settling basins/gravel
traps
Model predictions include:

Variation in sediment concentrations and grain sizes passing
through a basin it fills with sediment.
 Estimates of the frequency of sediment sluicing or de-silting
operations.
 The time period required to flush the basin and the volume of
water needed for flushing.
 The dimensions of an escape channel to convey sediment
flushed from a basin to the river or disposal point.
Minimising trapping fine sediments
A disadvantage of settling basins in spate schemes is their high trap
efficiency for fine sediments at low flows or when basins are empty.
To minimise the trap efficiency for fine sediments:
•
Basins should be relatively narrow, with sediment storage obtained
by increasing the length, rather than the width or depth of the basin.
•
If it is considered necessary substantial reductions in the trap
efficiency for fine sediments can be made if the tail water level in
the basin is lowered for very low basin discharges. One possibility
is to provide a notched weir at the basin exit, so that tail water
levels are substantially lowered when the basin discharge is very
low.
Secondary sediment control for spate schemes

Settling basins – Mechanically excavated or flushed basins can provide
high sediment trap efficiencies with a low, or in the case of
mechanically excavated basin, zero, “wastage” of water for sediment
flushing. But sediment trap efficiency varies as a basin fills, and also
with the basin discharge which varies from zero to full supply discharge
in spate schemes.

Canal sediment extractors – Trap coarse sediment with a relatively
constant trap efficiency but require continuous flushing flows of
between 10% and 15 % of the canal discharge. Conventional extractors
not suitable for use in spate schemes.
Limitations to secondary sedimentation control




Not easy to get it right – i.e. catching the coarse and not the fine
sediment
Requires space, esp. if sediment loads are high – this space may not
be there
Cleaning requires adequate organization
In case of flushing systems, farmers may object on water being used
for this
Desilting a small basin
Based on work of
Philip Lawrence