EWS Unit 5 Surface Water Intakes

8/13/2019 EWS Unit 5 Surface Water Intakes

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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES

Edition 1.1 GJM WEDC Loughborough University U K 5.1

UNIT 5

SURFACE WATER INTAKES

What this unit is about

This unit is about the development and rehabilitation of surface water intakes in

emergency situations.

What you will learn

On completion of this unit you should:

be familiar with the criteria for siting and protection of surface water intakes

understand how different types of temporary bulk surface water intakes can be

designed and constructed to provide optimal raw water qualities and quantities,

and how they can be protected from physical damage

know how pumps can be used in surface water abstraction

understand how submerged weirs can be used to raise water levels at intake

points

be familiar with the potential impacts of emergencies on existing surface water

intake structures, and the tasks involved in the repair and rehabilitation of such

facilities and

be familiar with the arrangements that can be made for users to safely collect

water directly for surface water sources in the early stages of an emergency

response.


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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY

5.2 Edition 1.1 GJM WEDC Loughborough University UK

Contents

5.1 Introduction....................................................................................... 5.3

5.2 Siting and protecting intakes........................................................... 5.3

5.3 Bulk intakes.......................................................................................5.3

5.3.1 Single point intakes ............................................................................................... 5.4

5.3.2 Sub-surface infiltration wells and galleries.......................................................... 5.10

5.3.3 Weirs ................................................................................................................... 5.17

5.3.4 Rehabilitation of existing intake structures.......................................................... 5.18

5.4 Individual collection........................................................................ 5.19

Figures in this unit

Figure 5.1. The problems caused if the pipe inlet is too near the surface or

too close to the source bed ................................................................................... 5.4

Figure 5.2. Simple arrangements of pump suction inlets........................................................ 5.6

Figure 5.3. Intake using a side channel and screen................................................................ 5.7

Figure 5.4. A prefabricated inlet strainer for a suction pipe .................................................... 5.7

Figure 5.5. A square of mesh heated on a metal plate can be pressed

against the end of the pipe .................................................................................... 5.8Figure 5.6. Submersible pump unit.......................................................................................... 5.9

Figure 5.7. A pumping unit mounted on a floating platform .................................................. 5.10

Figure 5.8. An infiltration well ................................................................................................ 5.11

Figure 5.9. Hand dug well close to river bank drawing water from the aquifer

connected to the river.......................................................................................... 5.12

Figure 5.10. Infiltration trench filled with sand and gravel.......................................................5.13

Figure 5.11. Infiltration well made of concrete rings sunk in the river bed.............................. 5.14

Figure 5.12. Infiltration well using a pipe with screened intake............................................... 5.15

Figure 5.13. Bank side infiltration gallery ................................................................................ 5.15

Figure 5.14. Temporary weir made of sand bags ................................................................... 5.17

Figure 5.15. Water supply intake on a small stream destroyed by floods .............................. 5.18

Figure 5.16. Examples of water collection platforms............................................................... 5.19

Figure 5.17. Support rail around water collection platform .....................................................5.20


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We will now to look at how these techniques are incorporated into two kinds of

surface water intake structure and at how pumps can be used in surface water

intakes.

5.3.1 Single point intakes

Design

Depth of inlet

Inlet pipes should be positioned:

at least 0.3 m below the surface of the water, in order to protect the inlet from

floating debris and, when pumping, to avoid the creation of vortices that result in

air being entrained into the intake pipe and

at least 0.3m above the sources bed to reduce the intake of sediments (Figure

5.1).

Figure 5.1. The problems caused if the pipe inlet is too near the surface ortoo close to the source bed

Where water levels are not sufficient to meet these criteria, they can be raised by

constructing a temp subsurface weir or dam discussed later.

Inlets can be correctly positioned using a number of techniques including: suspending

the inlet from a float securing the inlet in a rigid structure such as a bucket or crib


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using rigid pipe work or in freezing conditions, suspending the inlet through the ice

(Figure 5.2).

Suspending the inlet from a float has the advantage of allowing the inlet height to be

automatically adjusted with changes in water level. In deep water, such as lakes it

also allows the upper level of water (which will often have lower levels of sediments)to be accessed. Floating inlets should be securely anchored to the bed and bank of

the water source. A variation of this technique is used when a floating pontoon is

used to support a pump (see later in this section).

Securing the inlet in a perforated bucket, drum, crib, or concrete ring filled with coarse

stones allows the inlet to be protected against physical damage and can provide

coarse screening around the inlet reducing the intake of debris and detritus.

Inlets can also be positioned using rigid pipe work secured to the bank or bed a

technique which is likely to be more applicable for longer term installations.

Use of side channels

Inlets can be positioned in a side channel offset from the main body of water, or in a

pond or tank fed by the channel. Side channels can be used to reduce the velocity of

the water around the inlet, and in doing so allow water with lower sediment loads to

be abstracted: sediment loads can be further reduced when a side channel is

combined with coarse screening. A side channel also provides protection to the inlet

by removing it from the main watercourse (Figure 5.3).

Channels can be excavated by hand and, do not have to be lined unless the ground

is extremely sandy or porous. Where coarse screening using stones is combined withthe side channel the stones will provide structural support to the channel.

Screening

Screening should be used to protect the intake from floating and suspended debris.

They normally consist of:

a coarse/primary screen positioned before/around the inlet

a fine screen/strainer over the end of the inlet pipe.

A coarse or primary screen is used to remove larger floating and suspended debris.

In conventional intake works bar screens (grates, often set at an angle) are often

used for primary screening. In and emergency a coarse screen can be improvised

using stones positioned around the inlet (Figure 5.2).

Alternatively the open end of the inlet pipe can be fitted with a prefabricated strainer

to act as a coarse screen (Figure 5.4).


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Figure 5.2. Simple arrangements of pump suction inlets


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Figure 5.3. Intake using a side channel and screen

Figure 5.4. A prefabricated inlet strainer for a suction pipe


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A fine screen can be made by securing fine-wire mesh over the end of the inlet pipe.

When plastic pipe is used, the wire-mesh can be heat welded across the end of pipe

by placing a square of mesh onto a hot heating plate, and then pressing the end of

the pipe against the screen and heating plate (Figure 5.5). Alternatively the inside of a

prefabricated inlet strainer can be lined with fine mesh.

Figure 5.5. A square of mesh heated on a metal plate canbe pressed against the end of the pipe

If the water has a high silt load the fine screen may quickly block and reduce the

pumping efficiency. Make sure the inlet point is easy to access for cleaning.

Pumping from single point intakes

If topographical conditions allow, water can be fed by gravity from inlets to storage

and treatment facilities. More commonly, especially, in the initial stages of an

emergency, pumps will be used. This allows relatively large volumes of water to be

quickly delivered to storage and treatment facilities, which will often be situated on

higher ground.

Different kinds of pumps can be used to draw water from inlets but most commonly

centrifugal pumps will be used (Units 11). These are able to draw water from up to 7m

below the pump (although pump performance will become inefficient at height

differences greater than 3m). Where the pump can be securely positioned within this

height range the pump can be mounted on the surface, with the suction pipe

connected to the inlet: (Figure 5.1).

Where the height difference between the water surface level and the pump inlet is

greater than 0.7m, submersible centrifugal (or other) pumps, positioned in the water,

can be used (Figure 5.6).


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Positioning surface mounted pumps

Surface mounted pumping units can be positioned by the side of the water source, or

on a platform/pontoon floating on top of the water.

Figure 5.6. Submersible pump unit

When positioned by the side of the water source, on a bank for example, a firm

section of land should be chosen, and one that is not liable to be flooded. The

pumping unit can be mounted on a wooden pallet, or a more permanent concretefoundation can be installed (Unit 11).

Pumps and pumping units can also be mounted on a floating platform/pontoon

(Figure 5.7), an arrangement that will reduce the height between the pump and the

inlet, and in doing so increase pump efficiency. Where such an arrangement is used

the floating platform should be securely anchored, and the mooring ropes, and

pumps delivery pipe must be long and well secured to allow for changes in water

level and, when used on a stream or river, increased flow rates.


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In freezing conditions the pumping unit can be positioned on the bank, and the inlet

placed through the ice. Alternatively if the ice is strong enough, the pump can be

mounted on the ice, and the power unit on the bank - Figure 5.2

Figure 5.7. A pumping unit mounted on a floating platform

Additional measures should be taken in freezing conditions to protect equipment f rom

the cold including:

constructing protective housing around the pumping unit, and raising the unit

above the ground

removing the unit when its not in use

raising pipe work above the ground/ice (on wooden blocks for example) at a

gradient that will allow pipe work to be drained when not in use.

5.3.2 Sub-surface infiltration wells and galleriesInfiltration well and gallery intake structures (Figure 5.8) typically have two

components:

a collector well in the bank of the water source and

an inlet a well or gallery of perforated pipe work buried in the sources bank or

bed.

Although often involving longer construction times than the rapid installation intake

designs described above, such structures have the advantage of being able to access


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sub-surface flows, which can allow water to be abstracted from apparently dry

ephemeral watercourses, and, in freezing conditions, from water sources where the

source is frozen above ground.

We will now look in more detail at the collector well and inlet components of infiltration

wells and galleries.

Figure 5.8. An infiltration well

Collector well

Collector wells should be positioned in the bank of the water source, in locations not

liable to flooding. They should be sunk to a depth below that of the lowest point of the

inlet: additional depth should be given to allow the bottom of the well to act as a sump

for siltation. Collector wells can be constructed using the techniques for handdug well

construction: temporary well lining materials, such as corrugated steel rings, are well

suited to the rapid lining of collector wells (Unit 6).

In some cases water can be pumped directly from the inlet without the need for acollector well - for example screened infiltration inlets (see later in this unit)

Although similar to a shallow well positioned in the bank of a surface water source

(sometime called a riverside well), a collector well differs in that it is connected to the

source by a manmade inlet/conduit. However where bank soils have high levels of

hydraulic conductivity (i.e. are made up of coarse sand or gravel), simple wells

without manmade inlet/conduit structures, can be sunk (Figure 5.9).


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Figure 5.9. Hand dug well close to river bank drawing water from the aquiferconnected to the river

Infiltration inlets

Sand and gravel filled trench inlet

When a sand and gravel filled trench (Figure 5.10) is used:

a trench isdug between the source and the collector well

the trench is then backfilledwith coarse sand to allow basic filtration to take place

- the sand can be graded into coarser gravel closer to the collection well, in order

to minimise siltation inside the well.


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Figure 5.10. Infiltration trench filled with sand and gravel

Care should be taken to ensure that excavations are conducted in a safe way, and

that, where necessary, temporary trench supports are used Appendix 21 of Davis

and Lambert (2002) gives further information on the temporary support of

excavations.

Concrete ring inlet

Pre-cast concrete rings can be sunk into the bed of a water source (Figure 5.11) -

techniques for the fabrication and sinking of rings are covered in Unit 6. Rings

positioned below the bed should ideally be porous, and graded layers of sand and

gravel placed in the base of the bottom ring to reduce the intake of sediments (to

prevent damage and encourage the intake of water that has been filtered by the river

bed). The top of the rings should be sealed with a concrete cover. Alternatively thecolumn can be extended above the top of the flood water level.

A connector pipe or sand/gravel f illed trench can be used to transmit flows to a

collector well, alternatively water can be pumped directly from inside the concrete

rings.


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Figure 5.11. Infiltration well made of concrete rings sunk in the river bed

Borehole screen inlet

Pre-fabricated or improvised borehole screens (Unit 6) can be sunk into the beds ofsurface water sources, and have been used particularly successfully to access sub-

surface flows in ephemeral watercourses during the dry season. Screens can be

inserted by manual excavation or drilling techniques such as hand auguring. The

procedures described for borehole design for: selecting screen slot size identifying

the need for, and, if necessary designing an artificial gravel pack and developing the

gravel pack around the screen by pumping, can all be applied to the design and

development of well screens sunk in the bed of surface water sources (Unit 6).

Water can be abstracted from the inlet by connecting the suction side of a surface

mounted centrifugal pump.


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Figure 5.12. Infiltration well using a pipe with screened intake

Figure 5.13. Bank side infiltration gallery


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Infiltration galleries

The inlet surface area, and therefore yield, can be increased by developing infiltration

galleries sections of perforated or loosely jointed pipe laid in the sources bank

(Figure 5.13) or bed (Figure 5.8).

Galleries in the bank can be constructed throughout the year, and should be used

where the hydraulic conductivity of river bank soil is high - i.e. in clay and silt free

sand and gravel. Test excavations and drilling can be used to assess bank soil type,

and select optimum sites.

Where the hydraulic conductivity of the bank soil is low, infiltration galleries can be

constructed in the beds of ephemeral sources when water levels are low.

Construction is unlikely to be feasible at other times of year, or in perennial

watercourses.

Borehole screen, slotted, drilled or open jointed pipe can be used for the infiltration

gallerys pipe work. The principles described in Unit 6 for borehole intake design can

be used for designing an infiltration gallery, these include:

analysis of the soil to see if an artificial gravel pack is required (it may be possible

to use the sediments of the river bank/bed - if these sediments are too small and

too uniform in size it will be advantageous to develop and artificial gravel pack to

improve hydraulic conductivity around the pipe, and reduce the intake of

sediments)

selection of screen slot size and

establishing a diameter and length of intake pipe where, in order to reduce headlosses in the pipe, entrance velocities do not exceed 0.3 l/s and flow velocities 1.5

m/s for the known slot size and design abstraction rate.

The reference cited below provides further information and an example of how

desired abstraction rates can be used to size infiltration galleries. The data quoted for

a 100mm diameter infiltration pipe gives a yield of 1 litre/minute/meter of infiltration

pipe. This is equivalent to 6m/10hours/10meters.

It is recommended that pipes are:

positioned between 0.5m and 1m below the sources bed and

buriedon 0.3m of gravel media

Trench width is recommended to be about twice the depth of the pipe (giving widths

of around 1.6m). When more than one pipe is laid in parallel in the sources bed, the

pipes should be laid at least 3m apart to reduce interference (Davis and Lambert,

2002).

An example of an infiltration gallery design is given in the Self-Asessment Questions

for this Unit.


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Further reading:Davis and Lambert (2002), Infiltration galleries, p.216-218.

5.3.3 Weirs

Submerged weirs positioned below a stream or small river intake, can be used to

increase the depth of water at the intake site, and reduce the flow velocity above the

weir, allowing heavier solids to settle out of the water (Figure 5.14).

Temporary weirs can be constructed from dense impermeable media such as rocks,

boulders, sandbags, or gabion baskets. Woven fabric sacks, made from materials

such as hemp or jute, filled with a dry concrete mix (1:6:8, cement:sand:gravel) and

sewn shut can also be used: the sacks can be placed directly in the water the

sacking will hold the concrete mix in place until it sets the weight of the bags will

allow them to mould themselves together so that they interlock solidly (Jordon, 2000).

Care must be taken when raising the water to level that land upstream of the weir will

not be flooded even during periods of high flow. Consideration should also be given to

the downstream impacts of structural failure.

Figure 5.14. Temporary weir made of sand bags


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5.3.4 Rehabilitation of existing intake structuresExisting surface water intake structures can be damaged or destroyed in

emergencies, reducing the capacity, or rendering inoperable, the supply systems that

are dependent on them (Figure 5.15). Structurally damaging disasters such as floods,

landslides, earthquakes and volcanic activity are particularly likely to cause damage

to intake works. Intake screens and inlets may be broken, blocked or buried by

debris foundations may be eroded or destroyed, causing structural damage such as

cracking, subsidence or collapse to intake structures.

Temporary rapid installation intakes, such as the ones that we have studied above,

can be set-up in parallel with damaged ones, in order to supplement, or substitute

supplies, while cleaning, repair and reconstruction takes place.

Blockages should be cleared, and where possible structural damage made good.

Foundations and protective structures may have to be strengthened. Longer term

rehabilitation should consider the causes of damage, and evaluate the possibility ofrelocating and/or redesigning intake structures to take account of the damage that

has been caused, and the vulnerability that has been exposed.

Figure 5.15. Water supply intake on a small stream destroyed by floods


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5.4 Individual collection

Direct contact between users and surface water sources should be minimised in order

to reduce the risk of source contamination. Ideally this should be achieved by

developing water supply arrangements such as those previously described. However

in the initial stages of an emergency response, and in situations with a low density of

affected users, people may collect water directly from surface water sources. When

this is the case, collection areas should be designated as soon as possible and

structures such as platforms, steps and ramps constructed so that users can collect

water without coming in direct contact with the water source - Figure 5.16

As we have seen, water collection points should be carefully located with

consideration given to water quantities, quality, the risk of physical damage, and the

impact of collection from these points on downstream users.

Collection platforms can be constructed using a number of materials such as sand orconcrete filled bags, wood, or stone. Consideration should be given to the possibility

of fluctuating water levels - floating platforms can be used in these situations. The

needs of physically vulnerable users (children, elderly, disabled, pregnant women

etc.) should also be considered through such measures as providing a handrail for

safety and support (Figure 5.17).

Figure 5.16. Examples of water collection platforms

In crowded situations water point supervisors may be needed to manage access.

Supervisors can also be used to conduct bucket chlorination in order to disinfect

collected water (Unit 8).


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Figure 5.17. Support rail around water collection platform

Documents

EWS Unit 5 Surface Water Intakes