EWS Unit 5 Surface Water Intakes
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Transcript of EWS Unit 5 Surface Water Intakes
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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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
5.4 Edition 1.1 GJM WEDC Loughborough University UK
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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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES
Edition 1.1 GJM WEDC Loughborough University U K 5.5
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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
5.6 Edition 1.1 GJM WEDC Loughborough University UK
Figure 5.2. Simple arrangements of pump suction inlets
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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES
Edition 1.1 GJM WEDC Loughborough University U K 5.7
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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
5.8 Edition 1.1 GJM WEDC Loughborough University UK
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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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES
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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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
5.10 Edition 1.1 GJM WEDC Loughborough University UK
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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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES
Edition 1.1 GJM WEDC Loughborough University U K 5.11
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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
5.12 Edition 1.1 GJM WEDC Loughborough University UK
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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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES
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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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
5.14 Edition 1.1 GJM WEDC Loughborough University UK
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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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES
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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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
5.16 Edition 1.1 GJM WEDC Loughborough University UK
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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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES
Edition 1.1 GJM WEDC Loughborough University U K 5.17
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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
5.18 Edition 1.1 GJM WEDC Loughborough University UK
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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EMERGENCY WATER SUPPLY UNIT 5: SURFACE WATER INTAKES
Edition 1.1 GJM WEDC Loughborough University U K 5.19
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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UNIT 5: SURFACE WATER INTAKES EMERGENCY WATER SUPPLY
Figure 5.17. Support rail around water collection platform