Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

Figure

17-22.

Clyde Pumping

Station,

(a)

Plan;

(b)

Section

A.

Courtesy

of

C.S. Dodson

&

Associates,

Consulting

Engineers.

Unlike

the

Vallby

pumping station, which ejects

sludge whenever

a

pump

is

turned

on and

which

can

be

programmed

to

eject scum automatically,

an

opera-

tor

must visit

the

Clyde pumping station

to

clean

it

—

no

real disadvantage

if the

policy

is

always

to

have

an

operator present

at

pump-down.

At

Clyde,

the

operator must climb down into

the

pump

vault

to

operate

the

valves.

An

improvement

would

be to

install

a

removable grating about

0.6

m

(2

ft)

below grade

and

extend

the

valve stems through

the

grating

to

just below

the

hatch.

The

operator could

then

manipulate

the

valves without entering

the

vault.

This improvement

is now

(1996) being incorporated

into

the

California Maritime Academy Pumping Sta-

tion

at

Vallejo, California.

Example

17-9

Black

Diamond

Pumping

Station

This station

at

Black Diamond, Washington,

was

designed

to

meet most

of the

concepts devel-

oped during this research project

for C/S

duplex pumps

in a

round, self-cleaning pump sump.

The

plans

are

shown

in

Figure 17-23.

The

400-mm

(16-in.)

approach

(influent)

pipe slopes

at a

2%

gradient

for 61 m

(200

ft) but is

horizontal

for 10

pipe diameters before entering

the wet

well. There

are two

self-priming

pumps

of 63 L/s

(230

m

3

/h

or

1000

gal/min)

each housed

in a

nearby

building with only

the

250-mm

(10-in.)

pump suction pipes

in the wet

well.

The

diame-

ter

of

each bell

is 400 mm (16

in.),

so the

entrance velocity

is

only 0.49

m/s

(1.6

ft/s).

The

layout

of the

pump room (not shown) allows

for the

utmost

flexibility of

operation—a

requirement

of the

owner.

A

diesel

gen-set

furnishes

enough power

to

operate

the

station

in the

event

of a

power outage.

Critique

The

behavior

of the

steeply sloping

(2%

gradient)

approach pipeline

is

entirely satisfactory,

but the LWL

shown

on the

plans

is too low to

prevent

the

hydraulic

jump

in the

approach pipe

from

reaching

the

sump.

Of

course,

the LWL can be

readily changed.

In

the

sump,

the

smooth sides sloping

at 60°

keep

solids

from

sticking during pump-down.

The

pumps

broke suction when

the

submergence

of the

bells

was

about

1 D. At

this water depth,

the

area

of the

water surface

was too

large

to

confine

the

scum suf-

ficiently

so

that

all

scum could

be

ejected

on a

sin-

gle

pump-down,

and a

second pump-down

was

needed. During

an

inspection,

it

appeared that oper-

ating

the two

pumps simultaneously would possibly

remove

the

scum

in a

single pump-down,

but

that

operation

was not

tested.

The

designer

has

stated

that,

in a

redesign,

the

walls would

be

made vertical

from

the floor to 1 D

above

the

pump intakes

so as

to

restrict

the

area

to

which scum would

be

confined

upon pump-down.

Figure

17-23.

Black

Diamond

Pumping

Station

sump,

(a)

Plan;

(b)

Section

A-A.

The

pump entrance velocity

is

very

low and

might

not

suck

out

sand

and

gravel quickly. Nevertheless,

cleaning

is

accomplished

with

reasonable dispatch.

However,

the

designer stated

he

would follow

the

more

restrictive

dimensions

of

Figure 12-59 next time.

The

success

of the

Black Diamond Pumping Sta-

tion demonstrates

the

practicality

and

usefulness

of

the

concepts explained

in

Chapter

12.

17-8.

References

1.

ASCE Manual

37,

Design

and

Construction

of

Sanitary

and

Storm Sewers, American Society

of

Civil Engineers,

New

York

(1970).

2.

Metcalf

&

Eddy,

Inc.,

Wastewater

Engineering:

Collection

and

Pumping

of

Wastewater,

McGraw-Hill,

New

York

(1981).

3.

AISI,

Modern

Sewer Design, American Iron

and

Steel

Institute,

Washington,

DC

(1980).

4.

Parmakian,

J.,

Waterhammer

Analysis, Dover

Publ.,

New

York

(1981).

5.

Dicmas,

J.

L.,

Vertical

Turbine,

Mixed

Flow,

&

Propeller

Pumps,

Figures

6.6, 6.8, 6.9,

and

6.10,

McGraw-Hill,

New

York

(1987).

6.

Sanks,

R.

L.,

G. M.

Jones,

and C. E.

Sweeney.

"Improvements

in

pump intake basin design."

EPA

600/

R-95/041,

RREL-CR. Order

No. PB

95-188090.

National

Technical Information Service, 5285 Port Royal Road,

Springfield,

VA

22161

(1995).

The

purposes

of

this chapter

are

(1)

to

describe meth-

ods for

determining

the

required capacity

and

pressure

for

a

water pumping station,

(2) to

present recommen-

dations

on

selecting

and

sizing

the

pumps,

(3) to

dis-

cuss

control methods

for

operating pumps,

and (4) to

provide representative examples

of

pumping station

design. Examples

of

pumping station design

may be

worked

only

in the

units originally used, whereas some

may

be

worked

in

both

SI and

U.S. customary units.

1

8-1

.

Types

of

Water

Pumping

Stations

For the

purposes

of

discussion, water pumping sta-

tions

are

considered

to

fall

into

five

general categories

or

systems:

•

Source (such

as a

well) pump discharging into

an

elevated tank

Chapter

1

8

System

Design

for

Water

Pumping

BAYARD

E.

BOSSERMAN

Il

RICHARD

J.

RINGWOOD

MARVIN

DAN

SCHMIDT

MICHAEL

G.

THALHAMER

CONTRIBUTORS

Pat H.

Bouthillier

Roland

S.

Burlingame

A. L.

Charbonneau

Allen

W.

Peterson

David

M.

Reeser

Joseph

W.

Steiner

Jerald

D.

Underwood

Eric

L.

Winchester

James

R.

Wright

Eugene

K.

Yaremko

• Raw

water pumping

from

a

river

or

lake

•

In-line booster pumping into

an

elevated tank

•

High service pumping

of finished

water

at

high

pressure

•

Distribution system booster without

a

storage tank

in

the

piping system.

Functionally, there

may be

little

difference

between

high

service

and

distribution

or

in-line booster pump-

ing

or

between

low

service

(raw water pumping

to a

treatment plant)

and

booster pumping.

18-2.

Pumping

Station

Flow

and

Pressure

Requirements

Water

pumping stations

are

fundamentally

different

from

wastewater pumping stations because they

do

not

have

to be

sized

to

pump

at

high peak

flowrates.

A

wastewater pumping station must

be

able

to

pump

whatever

sewage

flow

enters

it, but a

water pumping

station

can be

designed

to

take advantage

of

water

storage

reservoirs

in the

system.

Flow

Requirements

Water

pumping stations

are

usually designed

to

supply

water

to an

area

in

which

the

required demand

is

reason-

ably

well defined

or can be

projected

to a

reasonable

degree.

In a

water distribution system,

the

demand

is a

combination

of

customer needs

and fire flow

require-

ments. Average annual

per

capita water consumption,

peak hour,

and

maximum daily demands

vary

widely

depending

on

factors

such

as

climate, income levels,

population,

and the

proportions

of

residential, commer-

cial,

and

industrial users. Typical water consumption

values

are 560 to 760

L/cap

• d or 150 to 200

gal/cap

• d.

More

detailed data

on per

capita water demand

is

readily

available

[1-5].

Most water supply utilities

and

cities

also have extensive records

on

water consumption.

Fire

flow

requirements

are

usually dictated

by the

fire

code

adopted

by the

local jurisdiction. Formulas

based

on

population

or on the

individual buildings

[where area, height, type

of

construction, occupancy,

installation

of

automatic sprinklers,

and

proximity

(and

thus exposure)

to

other structures]

may be

useful.

For

example,

the fire flow

requirement

can be

esti-

mated

using

an

early National Board

of

Fire

Under-

writers formula.

In SI

units,

the

formula

is

Q

=

2327^(1-0.01^)

(18-la)

in

which

Q is the fire

demand

flow in

cubic meters

per

hour

and P is the

population

in

thousands.

In

U.S. cus-

tomary

units,

the

formula

is

Q

=

1020

JP(\-

0.01

JP)

(18-lb)

where

Q is fire

demand

flow in

gallons

per

minute

and

P,

again,

is the

population

in

thousands.

Be

wary

of

computing

fire flows

based

on

formulas

that include

only

the

population

factor

because they

can

give

unreal-

istically

low flow

values

in

comparison

to a fire flow

demand

that might actually occur.

For

example,

in a

town

of

1000 people,

P

would

be 1 and Q

would

be 230

m

3

/h

(1000

gal/min),

which

may be

inadequate.

The

presence

of a

lumber yard,

for

example, would require

a

high water

flow

allocated

for fire.

Furthermore, design

flows

for

pumping stations

and

water mains depend

on

the

reservoir storage capacity available. Generally,

the

design

flow

should

be the

larger

of (1)

peak hourly

demand

or (2)

maximum daily demand plus

fire flow.

For

commercial areas,

the flow

demand

is

usually

estimated

as flowrate per

area. Demands

of

18.7

to 47

m

3

/ha

• d

(2000

to

5000

gal/acre

• d) are not

unusual

in

mixed

commercial

and

residential areas

[2,3,4,5].

Zoning classifications greatly

affect

these unit

flows.

In

one

large water agency

on the

East Coast

of the

United States, water consumption varies

from

18.7

to

700

m

3

/ha

• d

(2000

to

75,000

gal/acre

• d)

depending

on

the

zoning classification.

Pressure

Requirements

Service connection pressures during normal system

operations should

lie

between

the

following limits:

•

Minimum pressure

=

210-280

kPa

(30-40

lb/in.

2

)

•

Maximum pressure

=

410-550

kPa

(60-80

lb/in.

2

)

Some

cities

and

agencies

further

define

acceptable

minimum

pressures

as, for

example,

• 276 kPa (40

lb/in.

2

)

for

maximum daily

flow

• 207 kPa (30

lb/in.

2

)

for

peak hourly

flow

• 138 kPa (20

lb/in.

2

)

for

maximum daily

flow

plus

fire flow.

A

frequent

practice

in

water districts

is to

have zones

with

a 45- to

60-m

(150-

to

200-ft)

difference

in

eleva-

tion

from

top to

bottom. This corresponds roughly

to

the

maximum

410 to 550 kPa (60 to 80

lb/in.

2

)

pres-

sure

range above.

A

major

reason

for the

maximum

pressure

of 550 kPa (80

lb/in.

2

)

is

that household

plumbing

fixtures

—

especially

water

heaters

—

cannot

withstand

greater pressures.

In

sparsely populated areas,

it is

possible

to

estab-

lish zones with

a

90-m

(300-ft)

difference

in

elevation

and

to

serve

the

lower areas through pressure-reducing

valves.

It is

also

possible

to use

booster pumping

and

storage

in

alternating zones

(e.g.,

pump

from

zone

1 to

3 to 5) and to

service intermediate zones

(e.g.,

zones

2

and

4)

through pressure-reducing valves until addi-

tional

pumping

and

storage facilities

can be

justified.

Example

18-1

Flow

Requirements

in a

Small

Town

Problem:

A

town

has a

population

of

10,000.

Find

the

requirements

of the

water supply system

if

(1) an

adequate reservoir

is to be

built,

(2) an

existing

but

somewhat inadequate reservoir

exists,

and (3)

there

is no

reservoir.

Solution:

Note that data

and

calculations

are

rounded

off

to the

customary

2 (or

sometimes

3)

significant

figures.

The

assumptions

are as

follows:

Item

Sl

Units

U.S. Customary

Units

Population

10,000

Water

consumption

758 L/d • cap 200

gal/d

• cap

Fire

flowrate

(from

Equation 18-1)

197 L/s

3100

gal/min

Fire

flow

time

8 h 8 h

So the

demand

flow

rates

are as

follows:

Average

daily

10,000

X

758/(1440

X 60)

10,000

X

200/(1440

min/d)

=

88 L/s =

1400 gal/min

Maximum

daily

2.5 X Avg

daily

= 220 L/s 2.5 X Avg

daily

=

3500

gal/min

Peak hour

4 X Avg

daily

= 352 L/s 4 X Avg

daily

=

5600

gal/min

Average

daily

+ fire flow 280 L/s

4500

gal/min

Maximum

daily

+ fire flow 420 L/s

6600

gal/min

(1)

Storage

reservoir.

If a

properly sized water storage reservoir

is

provided

in the

system,

it

would

be

reasonable

to

design

the

pumping station with

a firm

capacity equal

to the

maximum

daily

demand—in

this example,

220 L/s

(3500

gal/min).

The

reservoir would supply additional

water

to the

system during peak hour

and fire flow

demands.

Although

the

detailed sizing

and

design

of

water storage reservoirs

is

beyond

the

scope

of

this book, some general

criteria

are as

follows:

•

Peaking storage equals

25 to 50% of the

average daily demand

•

Fire

flow

storage equals

fire flow for 3 to 8 h

duration

•

Emergency storage required

for

loss

of

power supply with

no

standby power supply

or

alter-

native water supply available equals

2 to 3

days

of

maximum daily demand.

The

total required storage, then,

can

range

from

the

volume required

by one of

these criteria

to the

volume

required

by the sum of all of the

criteria.

The

actual size

of the

storage reservoir depends

on

the

pumping station capacity

and

design considerations, such

as

whether standby power

is

pro-

vided

for the

pumping station.

In

this example,

if

standby power

is

omitted,

the

required storage

is

Storage

Sl

Units U.S. Customary

Units

For

peaking

0.25(88

L/s)(86,400

s/d)

X

0.25 (1400

gal/min)(1440

min/day)

=

IQ-

3

m

3

/L

=

1900

m

3

504,000

gal.

For fire flow

(197 L/s)

(3600

s/h)

(8 h) X

(3,100

gal/min) (609

min/hr)

(8 h) =

10-

3

m

3

/L

=

5700

m

3

1,500,000

gal.

For

emergency

(2

days

of

storage

at

maximum daily demand)

(2 d)

(220 L/s)

(86,400

s/d)

X (2 d)

(3500

gal/min) (1440 min/d)

=

10-

3

m

3

/L

=

38,000

m

3

10,100,000

gal

Total

45,600

m

3

12,100,000

gal

Notice that

by far the

largest storage volume

is due to the

emergency storage requirement,

which

results

from

the

assumption that

the

pumping station lacks standby power

or

that

the

water

supply

is

temporarily lost.

Selection

of

pumps. When considering this reservoir,

a

reasonable design would

be to

provide

four

320

m

3

/h

or 89 L/s

(1400 gal/min) pumps identical

for

ease

in

stocking

the

same spare parts.

• One

pump would normally operate

for the

average daily

flow.

• The

second

and

third pumps would come

on as

demands increase

and the

pressure conse-

quently

decreases

up to the

maximum daily

flow. The

nominal peak

flow

would

be 3 x 320 =

960

m

3

/h

or 267 L/s (3 x

1400

=

4200

gal/min).

• The

fourth

pump would serve

as

standby when

one of the

pumps must

be

taken

out of

service

for

repairs.

In

this example,

the

installed maximum operating capacity

of 960

m3/h

(4200

gal/min)

is

slightly greater than

the

required maximum daily

flow of 220 L/s

(3500

gal/min)

because

of the

decision

to use

pumps

of the

same

size.

The

actual total capacity would

be

less than

the

installed

maximum

because

of the

increased headloss

at

higher

flows.

(2)

Inadequate

reservoir.

Reservoir capacities

are

often

marginal

or

even substandard.

In a

congested urban area,

it may not be

feasible

to

construct additional reservoir capacity. Even

in

relatively open areas, topography

may be so flat

that additional reservoirs

are

unwarranted.

In

such

situations, constructing additional pumping facilities

may be the

only feasible alternative.

Consider

the

previous example,

but

assume

it has

been determined that

sufficient

reservoir

capacity

is

available

to

satisfy

peak hour

and

short-term maximum daily demands

but not

aver-

age

daily plus

fire flows. In

such

a

situation,

the

pumping station must

satisfy

both

(1)

maximum

daily

flow and (2)

average daily

flow

plus

fire flow. A

possible

design might provide

Demand

Sl

Units U.S. Customary Units

Average

daily

One

pump

at 88 L/s One

pump

at

1400 gal/min

Fire

flow One

pump

at

197

L/s One

pump

at

3100

gal/min

Maximum

daily Both

of the

pumps operate Both

of the

pumps operate

Standby

Another pump

at

197

L/s

Another pump

at

3100

gal/min

(3)

No

reservoir

in the

system.

The

difference

between

the first

part

of

this example,

18-1(1),

and

this part,

18-1(3),

is

that

in

this part,

the

pumping station must supply

all of the

water

demands—

there

is no

reservoir

to

make

up

peak hour

and fire flow

demands. Thus,

the

pumping station

should

be

designed

to

supply

the

larger

of (1) a

peak hour demand

of 352 L/s

(5600

gal/min)

or

(2)

a

maximum daily demand plus

fire flow of 220 + 197 = 417 L/s

(3500

+

3100

=

6600

gal/min).

A

reasonable design might consist

of the

following number

and

sizes

of

pumps.

One

pump

at 88 L/s One

pump

at

1600

gal/min

Two

pumps

at 159 L/s Two

pumps

at

2500

gal/min

One

standby pump

at 159 L/s One

standby pump

at

2500

gal/min

In

all of the

examples, consideration must also

be

given

to

system operation

at

very

low

flows,

such

as

might occur

at

night.

In the first and

second parts

of

this example,

the

pumps

could

be

shut

off and the

reservoir could supply

the

small amount

of

water required.

In

this

part, there

is no

reservoir,

so a

pump must continue

to

operate.

It is

usually

not

feasible

to

allow

a

pump

to

operate

at flows

less than,

say,

33% of its

optimum capacity,

so

there

are

three alternatives.

•

Install

a

small pump that

is

sized

to

provide

the low flow

needed.

•

Provide

a

variable-speed drive

on the

smaller pump

to

reduce

its

output.

The

variable-speed

drive would

be

controlled

to

maintain

a set

minimum system pressure.

• Add a

hydropneumatic tank

(see

Example

18-5).

Controls

for the

pumping station

in all

three parts

of

this example

can be

based either

on the

pressure sensed

in the

pumping station discharge

or on the

change

in the

level

of a

reservoir.

Pressure

at a

pumping station could, depending

on

system geometry,

be

difficult

for

control.

A

control scheme might

be as

follows:

•

Demand

on the

system causes

a

decline

in

water level

in the

reservoir

or a

decrease

in the

pressure

in the

system.

• As

soon

as the

pressure

at the

pumping station

falls

to Pl, the first

pump comes

on.

• If the

pressure continues

to

fall

to

P2,

the

next pump

comes

on.

•

This procedure continues until

(if

necessary)

all

pumps

are

running.

•

Eventually,

the

water level

in the

storage tank

rises

and fills the

tank

and the

system pressure

starts

to rise.

• As the

pressure

rises, the

pumps

are

stopped.

The

order

in

which

the

pumps

are

shut

off

can

be as

simple

as

shutting them

off in the

order

in

which they started.

A

more complicated

scheme

is to run

combinations

of

pumps

of

various capacities

to

provide small changes

in

1

8-3.

Raw

Water

Pumping

from

Rivers

and

Lakes

Raw

water

may be

pumped

from

a river, a

natural lake,

or an

artificial

reservoir.

A

potable (treated) domestic

water

supply

may be

pumped directly

or

indirectly

through

a

distribution reservoir

or

into

a

water

distri-

bution

system.

A raw

water supply

may be

pumped

to

a

water treatment plant either

from

the

source

or

before

or

after

passing through desilting basins.

Depending

on the

source

and

ultimate

use of the

water,

raw

water pumping facilities

are

generally

a

combination

of

only three basic components:

(1)

the

raw

water intake structure,

(2) the

pumping facilities,

and

(3) the

screening facilities, which

may or may not

be

required. Design variations

and

arrangement

of

these

basic components depend

on the

imagination, ingenu-

ity,

experience,

and

judgment

of the

design

engineer.

Most

raw

water pumping facilities

are

shore instal-

lations.

The

intake

may be

placed below

the

lowest

water

level

on

record

or the

lowest level

of

reservoir

drawdown

determined

by

storage requirements.

The

intake

may be

placed

(1)

at a

point

in

deep water such

as

a

lake

or

impoundment,

(2)

near

the

shoreline

in

shallow

water,

or (3)

directly

on

shore with

an

exca-

vated

channel

to

deeper water. Compared with waste-

water

pumping, there

is

greater

flexibility in

placing

the

pumping facilities

for a

water supply.

The

pumping

station

may be

built

in the

intake structure

at any of the

points cited above,

or it may be

located remotely

from

the

intake.

The

connection between

the

intake

and the

pumping

station

may be any of the

following:

• A

suction pipe

in a

shallow trench

to a

higher eleva-

tion

and

directly connected

to the

pumps.

The

pump

would

not be

more than

20 ft

(including pipeline

losses) above

the

lowest water level

at the

intake.

The

pumps must

be

equipped with adequate prim-

ing

devices.

• An

excavated open channel leading

to the

pump

suction

well.

• A

horizontal pipeline

from

the

intake directly

to the

pumps.

• A

tunnel

(in

larger installations) connecting

the

intake

to a

remote pumping station.

•

Combinations

of any of

these.

It

may be

desirable

to

incorporate

the

intake works

and/or

the

pumping

and

screening facilities into

a

water supply dam.

In an

earth dam,

the

intake

may be

• A

simple gate structure built into

and

parallel

to the

upstream

slope, near

the toe of the

dam, with

a

con-

duit

leading

to a

pumping station

at or

beyond

the

downstream

toe.

• A

tower

inlet,

located near

the

upstream toe, with

an

access bridge

from

the top of the

dam.

The

tower

can

include pumps

and

have trash racks,

screens,

or

multiple inlet

ports.

With this type

of

Note that

the

lack

of a

reservoir results

in

operating

inefficiencies.

Large pumps must

be

pro-

vided

for

peak hour

and fire flows and

must consequently operate

at

lower

flows and

less

effi-

ciency

at

average

and

intermediate demands.

Utilizing

either

(1)

another smaller pump (one-fourth

to

one-half

the

size

of the

smallest

pump

above)

or (2) a

variable-speed

drive

on the

smaller

unit

can

result

in an

even

finer

degree

of

control

in

response

to

system demand

and

pressure.

All

these

flows are

nominal.

Increases

in

friction

head reduce

the

higher

flows.

Accurate

flows

can

be

calculated only

by due

consideration

of the

system

H-Q

curve.

Average

daily:

Maximum

daily:

Peak hour

or

maximum

daily

plus

fire flow:

One

pump

at

100

L/s

or

one

pump

at 159 L/s

One

pump

at

100

L/s

or

one

pump

at 159 L/s

Total

of 259 L/s

or

two 159 L/s

pumps

for

a

total

of

3

18

L/s

Two

159 L/s

pumps

=

318

L/s

plus

one 100 L/s

pump

= 100 L

Total

of 418 L/s

One

pump

at

1600

gal/min

or

one

pump

at

2500

gal/min

One

pump

at

1600

gal/min

or

one

pump

at

2500

gal/min

Total

of

4100 gal/min

or

two

2500

gal/min pumps

for

a

total

of

5000

gal/min

Two

2500

gal/min pumps

=

5000

gal/min

plus

one

1600

gal/min pump

=

1600

gal/min

Total

of

6600

gal/min

flow.

In

this part

of the

example,

the

following

combinations

of

pumps

can be run to

meet dif-

fering

demands.

inlet,

a

conduit

may

also

be

carried

through

the

dam

to a

pumping station downstream.

• A

concrete

dam

intake section

may be in the

dam's

upstream face, with

or

without racks, screens,

pumps,

and

multiple ports.

Because many options

and

alternatives

are

avail-

able,

the

choice

is

based

on the

siting considerations

of

the

land

and

water environment.

The

most impor-

tant

factor

influencing

design

is the

depth

of the raw

water source. Depending

on the

design

flows

(pump-

ing

rates)

and

pumping heads,

it may be

difficult

to

deal with

a

large variation

in

water surface elevation

due

to

reservoir drawdown

or

natural hydrological

variations. Water quality variations

at

different

levels

in

a

deep lake

or

reservoir

may

require multiple port

inlets

to the

intake structure

to

allow taking

the

best

quality

of

water available

at all

times.

Deep reservoirs

in

temperate climates

turn

over

(sometimes rapidly)

in the

fall

or

spring

or

both. Turnover

is

caused

by

cold

and

dense

surface

water overlaying

warmer

(less dense) layers.

The

unstable condition causes

bottom layers

to be

suddenly brought

to the

surface,

often

with

bottom mud. Normally,

the

water quality

at or

near

the

bottom

of a

deep reservoir

is

inferior

to

mat

of

shal-

lower

depths.

It may be

devoid

of or low in

oxygen

and

high

in

manganese

and/or

iron

due to

decayed vegetation.

Because

the

water

quality

at any

given depth undergoes

seasonal variations, multiple ports

in the

inlet works

of

deep reservoirs

are

desirable

to

draw

the

best

water.

Top

and

bottom waters

can be

continuously mixed

to a

uni-

form

quality

by

extending

a

perforated

air

hose

or

pipe

across

the

bottom

of a

reservoir

and

supplying

air

from

a

compressor

on

shore,

an

arrangement

that

may

allow

a

single inlet

in the

intake structure.

A

sampling program

should

be

carried

out

throughout

the

year

to

determine

water

temperatures

and

quality

at

various depths.

Surface

conditions

in

lakes

and

impoundments

influence

the

intake design.

In

warm, humid climates,

aquatic plant growth, such

as

water hyacinth

and

duckweed,

is a

problem.

A floating

boom arrangement

may

prevent plant growths

and

surface trash

from

entering

the

intakes

and

pumps.

In

cold climates

the

intake works

and

pumping arrangements must also

be

designed

to

prevent damage

from

ice and

freezing.

River Intake

Design

Considerations

This subsection

is

essentially

an

abridgement

of

pages

1173

to

1214

and

1247

to

1256

of

Proceedings

[6].

Low

Water

The

determination

of

design

low

water

is

important

for

an

intake that must operate year-round. Design

low

water

is an

important factor

in the

design

of a

direct

intake,

in the

layout

of an

infiltration gallery,

and in

the

pump setting

and wet

well

floor

elevation. Design

low

flow can be

obtained through

a

routine

low-flow

frequency

analysis

of

water survey data.

If

there

are no

data

for the

stream, either

a

regional analysis

or the

analysis

of

transposed data

may be

required.

Cross-section

data

can

then

be

used

to

calculate

a

design

low

stage.

The

calculations must begin

at a

downstream control

section

or at a

control reach

in the

river.

Consider

the

likelihood

of

channel changes that

could significantly change

low-flow

levels

over

the

expected

life

of the

intake structure.

The

degradation

of

a

rapids

or a

downstream control section

or the

migration

of

rapids

from

downstream

to

upstream

of

the

site would result

in

lower water levels,

as

would

the

development

of a

downstream

cutoff.

The

devel-

opment

of a

large

bar

downstream,

or a

cutoff

just

upstream, could cause higher water levels.

The

likeli-

hood

of

such

an

occurrence

and the

magnitude

of the

effect

cannot

be

determined analytically.

Any

allow-

ance made must

be a

matter

of

judgment based

on a

study

of

comparative aerial photographs

and

observa-

tions made

in the field.

High

Water

The

elevation

of

design

flood

high water

is

also

an

important

factor

in

determining

the

elevation

of the

working

floor of the

pumphouse

and in

determining

the

design

of any river

training works that might

be

associated with

the

intake.

In

many instances, design

scour depths

can

also

be

determined

from

mean

flood

depths.

The

discharge

of the

design

flood can be

estimated

by

analyzing

the

water survey data.

The

correspond-

ing

stage

can be

computed

by

using

the

cross-section

data

and the

channel

and flood

plain roughness esti-

mates. Direct stage measurements

at one or two

high

flows

of

known magnitude

are

most

useful

for

increas-

ing the

accuracy

of the

calculated design high water.

High-water marks, information

from

local residents,

and

backwater computer programs

are

also very use-

ful

in

simulating water

levels

once

the

channel charac-

teristics have been determined.

The

choice

of

design high-

and

low-water levels

should

be

based

on the

aggradations

of the river

reach

under consideration because estimated water levels

and

streambed

levels

can be

dramatically reduced

at

an

intake

if the river

reach

is

subjected

to

degradation.

Alternatively,

on an

aggrading reach such

as the

back-

water above

a

storage dam, water

and

streambed lev-

els

will

rise,

which results

in

more frequent

flooding

during periods

of

high

flow and

possible

inundation

of

the

intake with sediment.

Once

the

frequencies

of the

low-

and

high-water

levels have been ascertained,

the

probability

of

levels

above

or

below these values during

the

life

of the

structure

can be

determined.

For a

50-yr

design

life

of

the

structure

and

design high-

or

low-water levels with

a

return period

of 100 yr,

there

is a 60%

assurance that

the

high-

water

levels will

be

exceeded

at

least once

in

the

life

of the

structure (see Figure

18-1).

For the

low-

water-level condition (for

a

similar design

life

and

return period),

the

assurance

of

lower water

levels

would

also

be

60%.

Trash

and

Debris

Large amounts

of

trash

and

debris,

floating or

sus-

pended,

are

likely

to be

carried

by the

stream

at

high

discharges. River structures that project high above

the

bed can

trap

the

debris

and can

sometimes cause

the

formation

of an

adjacent

floating

island that

impedes

the

operation

of the

intake.

Log

booms

can be

con-

structed

to

deflect

the floating

debris away

from the

intake,

but

booms

are

effective

only

if the

angle

between

the

direction

of the

surface

velocity

and the

boom

is

small. Suspended debris, such

as

waterlogged

wood

and

coal,

can be

drawn

into

the

system

and can

clog screens

or

trash racks. These materials

can be

removed

by

adequate

backflushing,

and

frequent back-

flushing

is

usually required during periods

of

high dis-

charge. Trash racks should

be

designed with horizontal

bars exposed

to the flow. The

racks should

be

recessed

into

the

face

of the

intake structure

to

minimize

the

hang-up

of

trash

on the

upstream side

of the

rack.

Ice

Jams

Design

high water

in

regions

of

below-zero weather

may

often

result

from

ice jam flooding. For

example,

the

water levels during spring break-up

on the

Atha-

basca River

at

Fort

McMurray,

Alberta,

can be 20 ft

above

the

summer high

flood

levels because

of ice

jams. Evidence

for

such

high-

water

levels

may be

dis-

covered during

the field

survey,

from

either

ice

scars

on

tree trunks

or

information

from

local

residents.

Ice-

jam

data

can be

found

in the

records

of

various gov-

ernment

departments

and

public

and

private agencies,

in

the

Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

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