programmes in a river catchment upstream from the intake will not rapidly reduce the river sediment loads. The reason is the vast store of sediments available for

remobilization in catchments that have suffered from high rates of soil erosion in the

past, this store will continue to contribute to downstream sediment loads for long

periods, decades or even centuries. In addition, measures such as re-forestation

programmes take time to implement and establish their impacts.

2. Location of River Intakes

Moving the location of an intake is an expensive option but worth considering,

especially for coarser sediments. Ideally a river intake should be sited at the outside

of a river bend, just downstream from the section of maximum curvature. At the

outside of a bend the deep water channel is established close to the bank, which

usually ensures trouble free abstraction of low flows. Meanwhile the secondary

currents minimise the abstraction of coarse sediments, which are swept along the

channel bed towards the inside of the bend, see Fig 1. It is the relatively sediment

free surface flow which is abstracted.

Fig 1. Secondary Currents at a river bend sweep sediment from the intake.

Sediment shoals often form at the entrance to a badly sited intake, for example

intakes located at the inside of a river bend. These hinder diversion of low flows, and

in extreme cases can cut off the intake from the river. Correct siting of an intake is

the first, and usually the most important step to minimise the diversion of coarse

sediments.

At intakes where there is no barrage or weir across the river to control the flow, then

there is no head difference available to drive sediment control measures, hence the

intake location becomes even more significant.

3. River training to improve the alignment of the flows approaching the intake.

It might be feasible to induce beneficial flow curvature using embankments and

spurs in a river. An example is at Sukkur Barrage, Pakistan. Fig 2 shows the training

works implemented in the 1940s in response to sedimentation in the right bank

canals, note the curvature of the “Approach Channel”. The original structures are

shown in black and the 1940s works in green. The new training works involved

closing several barrage gates which in theory would reduce the ability of the barrage to safely pass historic floods. The engineers assessed that the risk was acceptable, subsequent experience and analysis using numerical models have so far proved them correct.

Fig 2 Training works at Sukkur Barrage to introduce flow curvature just upstream of the diversion.

At Sukkur the training works did not re-align the whole Indus river, just the flows

approaching the right bank intakes. Re-training a whole river to induce the beneficial

curvature seems impracticable.

The advantage of intakes which divert water at both banks of rivers is how one

diversion weir or barrage then supplies two canal systems, as at Sukkur. But it can

be difficult to provide the correct approach flow conditions at both sides of the river. Artificial islands have been constructed to create curved approach flows for both sides of a river.

In steep rivers the upstream bed levels rapidly rise to the crest level of the diversion

weir, which can lead to problems in stabilising the position of the low flow channel.

This can be overcome by regular operation of the sluice gates, when sufficient

sluicing capacity, and excess water, is available. Weirs with a small cross-fall

towards the sluice and canal intake have been used to ensure that the main river

channel develops close to the canal intake.

Training works more immediately local to the intake may also be considered, such as

Kings Vanes as shown in Fig 3. The vanes protrude up from the river bed, but the

variations in river bed level in a morphologically active river will make their impact

variable and perhaps not reliable.

Fig 3 Kings vanes used to divert the sediment near the river bed away from an intake

4. Closing the intake gates during river floods

Often a large proportion of the annual sediment load entering an intake occurs

during river floods. The cost-benefit balance may be in favour of closing intake gates during periods of high river discharge. In the case of irrigation, the floods may be associated with high rainfall in the irrigation service area and so the penalty of closing canal head gates is diminished.Often a large proportion of the annual sediment load entering an intake occurs during river floods. The cost-benefit balance may be in favour of closing intake gates during periods of high river discharge. In the case of irrigation, the floods may be associated with high rainfall in the irrigation service area and so the penalty of closing canal head gates is diminished.

5. Sediment exclusion at intakes

Sediment excluders are structures which actively divert the high sediment flow near

the river bed away from the intake and ensure only relatively sediment-free water in

the upper layer is withdrawn into the intake. Typical river intakes incorporating

sediment excluders are shown in Figs 4 and 5.

Fig 4 Narora Sediment Excluder, India, where tunnels take the high sediment bed flow past the intake

Fig 5 Curved channel excluder at Kapunga Intake, Tanzania, where flow curvature sweeps bed material past the intake

Sediment excluders are designed for continuous operation and they are used in

rivers where the flow is larger than the abstraction requirement during periods when

high sediment loads are transported.

Separation of the upper and lower layers of the flow is achieved by:

Incorporating a tunnel under-sluice, (Tunnel excluder, Fig 4), so that it is impossible

for water and sediment from the bed layers to be diverted to the canal.

Incorporating a curved channel, (Curved channel excluder, Fig 5), so that flow and

coarse sediments in the bed zone are swept away from the intake gates. Structures

in the river can also be used to induce flow curvature, Fig 6, but these may change

behaviour as conditions change, such as in high river flow.

Setting the sill level of the canal intake well above the level of the sluice-way,

(conventional intake), and operating with the sluice gates partially open so that the

lower layer of river flow passes below the canal intake and through the sluice gates.

If the intake is a weir, this arrangement is called a skimming weir. This is

considerably less effective than using tunnels or curved channels, as bed flow can

still rise up and flow into the intake.

Fig 6 Schematic of a structure intended to introduce flow curvature at an intake

Sediment excluders do little to exclude finer sediments that are suspended

throughout the depth in the river flow. They are thus only likely to offer a solution to

sedimentation problems when the sediments settling in the channels or damaging

turbines are fairly coarse.

Sediment exclusion options where there is no weir or barrage are restricted to

inducing flow curvature, skimming weirs or overshot intake gates so that the top

water carrying the smallest sediment concentrations and the finest sediments are

diverted. Pumped intakes can be designed to take near-surface flow, such as with

floating inlets.

6. Flushing and “still pond operation”

Another method of managing sediment at the intake is where sediment flushing is

carried out intermittently, it is often called “still pond operation”. It is used at intakes

with conventional sluice gates. An essential feature is a Sluicing Channel that forms

a settling area upstream from the intake and sluice gates, see Fig 7. At intakes

designed for this operation the water velocity in the Sluicing Channel is lower than

the average velocity in the river upstream. Coarser sediment fractions in the flow

approaching the canal intake gates thus settle in the channel.

Fig 7 An intake with a sluicing channel to trap sediment and flush it away, Mae Tang, Thailand

After a period of sediment deposition in the sluice channel the bed levels will rise

such that sediment will start to be drawn into the intake, the sluice gates are then

opened. The sediment deposits are thereby flushed through the sluice gates. During flushing the intake is temporarily closed. This style of operation works best in rivers where the sediment loads are not too large, so that sediment flushing is not required more often than about once a day. The temporary intake closures must be

acceptable.

Fig 8 An intake with still pond operation but the divide wall is too short to be effective

The term “still pond operation” is common in south Asia and is usually associated

with a layout such as sketched in Fig 8. The embankment dividing the sluicing

channel from the river in Fig 7 is replaced by a wall in Fig 8, called the Divide Wall.

The sluicing channel itself is called the Pocket. However, a common feature is the

inadequate length of the Divide Wall which produces insufficient storage for the still

pond operation to provide any significant sediment control.

7. Physical screens and grilles

In mountain streams the sediment can be dominated by very large material which

can be kept from being abstracted with a physical barrier. A good example is the

Tyrolian weir type intake, Fig 9, the weir itself is shown in long-section in Fig 10. The

bars sit on the back face of the weir and align with the flow, so large stones, rocks

and boulders roll above the bars and only finer material passes between the bars.

In this arrangement the flow passes laterally into a chamber beside the weir and then into the intake.

Fig 9 Tyrolian type intake in plan showing bars to physically prevent large material entering the intake

Fig 10 Longitudinal weir section of a Tyrolian intake

Finer screens or filters can also be used, often with movement or back washing, but

they are unlikely to be appropriate for large scale abstractions.

8. Settling basins or chambers

Settling chambers can be used immediately after the point of diversion within the

intake structure, as shown above in Fig 9. Or settling basins can be placed at the

head of the offtake channel and are formed as an enlarged canal cross section. A

typical settling basin layout at the head of an irrigation canal is shown in Fig 11.

Fig 11 A typical settling basin layout with trapped material flushed back to the river

Within the basin or chamber, the flow velocities are reduced to ensure that sediment

settles and is trapped in the basin. Where the conditions are suitable sediment

deposits are flushed back to the river via a low level sluicing outlet located at the

downstream end of the basin. Where flushing is not feasible sediment is removed

using mechanical plant. Twin basins are sometimes used, so that sediment removal

can be carried in one basin while the second continues the supply of water.

For a certain abstracted discharge, increasing the size of the basin or chamber

causes finer material to be trapped, thus these structures are suitable for controlling the fine sands which other methods cannot address.

9. Sediment extractors

Sediments in the sand size range are normally transported in suspension in rivers at

medium and high discharges, and as they are quite well mixed in the river flow, are

typically not controlled effectively by sediment excluders. If sediments in the sand

size range are settling in a canal network, then sediment extractors can be a suitable

option.

A tunnel type sediment extractor is shown in Fig 12, it consists of a row of tunnels

placed at the bed of a canal that divert water and sediment flowing near the bed.

High velocities are necessary in the tunnels to prevent sediment deposition. The

basic principle is the same as for tunnel type sediment excluders, which is that

significantly higher sediment concentrations flow near the channel bed. But the flow conditions in the canal headreach are more quiescent than in the river, which causes extractors to have higher performances than excluders for sand sizes.

Fig 12 A tunnel type sediment extractor diverting the channel bed flow away

Fig 13 shows another means of extraction, the vortex tube sediment extractor, which

consists of one or more slotted tubes laid flush with the canal bed. One end of the

tube is closed, the other is open and connected to an escape channel. Water and

sediment flowing near the bed of the channel upstream is diverted through the vortex tube. A strong vortex flow is developed in the tube, which, provided the tube

dimensions have been chosen correctly, prevents sediment from settling and

blocking the tube.

Fig 13 A vortex tube sediment extractor which helps keep the diverted sediment in suspension

In most cases the extracted flow is returned to the river in an escape channel, see

Fig 14. The layout shows how a long near-straight canal reach is needed upstream

from the extractor to enable suspended sediments to settle near the channel bed.

Fig 14 A typical vortex tube arrangement whereby the extracted material is returned to the river

10. Canal de-silting

In some cases de-silting will be the only feasible option or the most cost-effective

means of sediment management in irrigation systems, and in other cases it may be

one component of a sediment control strategy.

If the desilted material can be sold or removed free of charge then it will become a

more attractive option.

Preliminary Screening Matrix

Click to download the Preliminary Screening Matrix.

Note: This matrix provides a high-level comparison for preliminary screening. The

suitability of each option depends heavily on site-specific factors and constraints.

Future articles will cover these.

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