Water and Wastewater Engineering
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3-4 WATER AND WASTEWATER ENGINEERING
Traveling crane
Traveling
screen
Sluice gates
River
Pump column
To water
treatment
plant
Stairway
Bar rack
Pump
motor
FIGURE 3-2
River intake.
Criteria Remarks
Water quality See Section 2-3 and Table 2-14
Lake and stream currents
Wind and wave impacts
Variation with water depth due to stratification
Infiltration galleries “under the influence” of surface
water must comply with surface water regulations
Water depth Maximum available
Adequate submergence over inlet ports
Avoid
ice problems
Silt, sand Locate to minimize impact
Treatment facility Minimize conduit length to treatment plant
Cost Minimize consistent with long-term performance
and operation & maintenance requirements
TABLE 3-1
Considerations in locating water intakes
A dapted from Foellmi, 2005.
Type of Intake
In a broad sense, intake structures may be classified into two categories as noted in Table 3-2 .
Many varieties of these types have been used. The selection of the type of intake is highly depen-
dent on local circumstances. Because the circumstances are fundamentally
different, the systems
are often classified as either river intakes or lake/reservoir intakes.
INTAKE STRUCTURES 3-5
Category Design type Remarks
Exposed Tower integral with dam Applicable to large systems, expensive
Tower in lake Navigational impact
Shore inlet Design for floating debris and/or ice
Floating or movable
Good access for O&M
a
Siphon well Applicable to small systems, flexible, easy
to expand
Submerged Plain-end pipe or elbow Applicable to small systems
Screened inlet crib No navigational impact
No impact from floating debris or ice
b
Not flexible
Difficult O&M
a
Gravel-packed well(s) No navigational impact
No impact from floating debris or ice
b
Must have favorable geology
Horizontal collection systems (also
called infiltration gallery or
infiltration bed)
No navigational impact
No impact from floating debris or ice
b
Must have favorable geology
TABLE 3-2
Types of intake structure
a
O&M operation and maintenance.
b
With sufficient depth, ice impacts are minimized.
Adapted from Foellmi, 2005.
Lakes and Reservoirs. Because of their navigational impacts as well as severe winter weather
and consequent difficulties in their operation and maintenance (O&M), exposed structures are
not often used in the Great Lakes and other large cold-climate lakes. On the other hand, exposed
intake structures have been widely use
d in more warm-climate lakes and in reservoirs. A classic
tower design ( Figure 3-3 ) includes multiple intake ports at different elevations, screens for each
port, and access for maintenance. The tower may be divided into two sections or cells to provide
redundancy. It is a
ccessed by a bridge, causeway, or boat.
S ubmerged intake structures avoid many of the problems of the exposed systems but are
significantly more difficult to maintain because of lack of access. On the other hand, the lack of
exposed mechanical parts lowers
the amount of maintenance time required. A typical submerged
inlet structure is shown in Figure 3-4 . With a favorable geologic strata of sand and gravel on the
shore or the bottom of the lake or reservoir, either an infiltration gallery as shown in Figure 3-5
or a horizontal collection system (called a “Ranney well” after its inventor) under the lake bottom
( Fig
ure 3-6 ) may be appropriate.
Rivers. Both exposed and submerged inlet structures have been used in rivers. In large rivers
that are controlled by locks and dams, the variation in flow and consequent variation in water
surface elevation are of less concern than in unregulated waterways. For most water supplies,
3-6 WATER AND WASTEWATER ENGINEERING
Bar screen
winch
Railing
Bar screen
Winch
Traveling
screen
Overhead crane
Pump motor
Bridge
Discharge via
bridge to water
treatment plant
Pump
column
Sluice gate
Section
Low water
surface
elevation
Intake ports
at three
elevations
Bar screens
Exterior view: outside
of intake port
FIGURE 3-3
C l a ssic tower design with multiple ports.
Protective blocks
Horizontal baffle
Intake screens
Granular fill
15.2 m to lake
surface
Intake crib
2.74 m
3.3 m
Intake conduit
2.3 km to low-lift pump station
FIGURE 3-4
Lake intake crib. Crib is steel octagonal.
INTAKE STRUCTURES 3-7
River
Perforated pipe
Collector wells
River
Perforated pipe
Collector wells
FIGURE 3-5
Two arrangements of infiltration galleries with wells.
Pump house,
pump motors,
and controls
River
Reinforced concrete caisson
Horizontal screens
Bedrock or confining impervious layer
Distribution
Water level
Sand and gravel
aquifer
FIGURE 3-6
Collector well with horizontal groundwater collection screens.
shore-based systems provide the best combination of access for operation and maintenance and
reliable supply (Bosserman et al., 2006).
Unlike lakes and reservoirs, special consideration must be given to the impact of floods and
droughts on river intakes. In the first instance, structural integrity, availability of power, and
acc
ess must be considered in the design. In the second instance, provision must be made for
3-8 WATER AND WASTEWATER ENGINEERING
alternative access to water when drought conditions lower the water level below the lowest intake
port.
While a reservoir or lake will have suspended matter during high wind events, it will seldom
have the quantity or quality of the grit produced during flood events on rivers. The river intake
structure must be designed to protect the pumps and valves in the transmission system from
undue wear from grit.
Conduits
The intake conduit connects the inlet works with the low-lift pump station. Either a tunnel or a
pipeline may be used. Although tunnels have a high degree of reliability, they are expensive to
construct. For large water systems, they may be the more economical choice when both capital
and long-term maintenance costs are considered
.
3-3 DESIGN CRITERIA
Design Capacity
The design process to select a design flow rate ( Q ) is based on a forecast demand. With Q, the
hydraulics of the intake structure design are based on the worst case estimate of friction loss,
an estim ate of potential sand intrusion into the conduit, the all-time historic low water level,
and a life ex
pectancy of 60 years. Some hydraulic design capacities are listed in Table 3 -3 .
Becau se the life expectancy is very long, prudent engineers use the ultimate flow to des ign the
hydraulic structures (intake tower or crib, conduit, gates, etc.). The de
sign flow is used to select
pumps and motors. Space is provided for additional pumps that will be required to meet the
ultimate flow.
Layout
Division of the intake system into two or more independent cellular or parallel components is
recommended for all but the smallest systems. This enhances reliability, provides flexibility in
operation, and simplifies maintenance. The operating deck (also called the operating floor and
pump station floor ) that houses the
motors, control systems, and so on should be located 1.5 m or
more, depending on the maximum wave height, above the high water level of a lake or reservoir
or the 500-year flood level of a river supply. The area of the operating deck should be sufficient
to allow for the installation and servicing of the pumps, intake gates , and screens. Overhead
cranes are an essential featu
re (Foellmi, 2005; Kawamura, 2000).
Flow criteria Capacity Remarks
Design flow
Q
Capacity at design life under worst case conditions
Minimum flow
0.10 Q to 0.20 Q
System specific
Ultimate flow
2.0 Q or higher
At life expectancy
TABLE 3-3
Hydraulic criteria
Adapted from Foellmi, 2005.
INTAKE STRUCTURES 3-9
Intake Tower
Location. Intake towers should be located as close to the shore as possible, consistent with the
variation in water depth. With the exception of very small intakes, the minimum depth should be
3 m (Kawamura, 2000).
Intake Ports. Gated ports are provided at various depths to allow for changes in water eleva-
tion and changes in water quality due to wind
/wave action, stratification, and lake turnover. Typi-
cal design criteria are listed in Table 3-4 .
The port area may be estimated using the relationship
QvA
(3-1)
where Q flow rate, m
3
/ s
v velocity of flow, m/s
A cross-sectional area of flow, m
2
Example 3-1. A two-cell intake tower located in a cold climate reservoir is being designed for
a winter design flow rate of 6,000 m
3
/ d. The tower will have three ports at three different eleva-
tions in each cell. Each port must be able to deliver the design flow rate operating alone. Deter-
mine the area of each port opening.
Solution. For a cold climate reservoir, the intake velocity should be limited to less than 0.10
m/s ( Table 3-4 ). Using an intake velocity of 0.08 m/s and Equation 3-1,
A
Q
v
6 000
0 08 86 400
0 868
3
,
.,
.
m /d
m/ss/d()( )
or about m09
2
.
This area is a preliminary estimate that will have to be enlarged because it does not take into
account the area of the screen that has to be installed to prevent debris from entering the tower.
Comment. Note that the design flow rate specified was for winter conditions. Summer flow
rates generally are higher than winter flow rates, and the velocities will be higher.
Criterion Typical recommendation
Number Multiple; three minimum
Vertical spacing 3 to 5 m maximum
Depth of lowest port 0.6 to 2 m above bottom depending on “muck” quality
Depth of top port Variable; 5 to 9 m below surface to avoid wave action
Ice avoidance At least one port 6 to 9 m below the surface
Port velocity Gross area of ports at same elevation sized to li
mit velocity to less
than 0.3 m/s. To avoid ice buildup, limit velocity to less than 0.1 m/s.
TABLE 3-4
Intake port design criteria
Sources: Foellmi, 2005, and Kawamura, 2000.
3-10 WATER AND WASTEWATER ENGINEERING
Gates. Sluice gates may be used on either the interior or exterior of the tower. Historically, gate
valves have been preferred because the other valves become fouled with debris.
Coarse Screens. Also known as bar racks, these screens are provided to prevent leaves, sticks,
and other large pieces of debris from entering the tower. They are located as shown in Figure
3-3 .
Rack and screen characteristics are summarized in Table 3-5 . Bar rack screenings are taken to a
licensed landfill.
T o maintain the desired inlet design velocity, the design of the port must take the area
occupied by the bars into account.
Example 3-2. Revise the area estimate made in Exam
ple 3-1 to take into account the area occu-
pied by a bar rack with a clear opening of 4 cm and a bar 1 cm in width. Assume that the opening
is a square.
Solution. The arrangement of a bar and opening is shown in the sketch below.
Individual
bars
4 cm
1 cm
TABLE 3-5
Typical rack and screen characteristics
Type Remarks
Coarse screens
Bar racks Clear opening 2 to 8 cm between bars
Steel bars 1 to 2 cm in diameter
Inclined or vertical; if inclined, angle is 20 to 30 from vertical
Trash racks Clear opening 8 to 10 cm between bars
Steel bars 2 to 4 cm in diameter
Inclined or vertical; if inclined, angle is 20 to 30 from vertical
Fine screens Clear opening 0.5 to 1 cm between elements
Me
chanically cleaned on either a continuous or intermittent schedule
Inclined or vertical; equipment supplier specifies angle
Provide head space or other means to raise the screen entirely
out of the water for inspection and maintenance
A dapted from Foellmi, 2005, and Kawamura, 2000.
INTAKE STRUCTURES 3-11
The unit space occupied by a bar and the adjacent opening is 4 cm 1 cm 5 cm. The fraction
of the area occupied by the bar is 1 cm/5 cm 0.20 or 20 percent. To account for the bars, the
area of the port opening must be increased by 20 percent or
A ()( )120 09 108 11
22
.. . .m or m
Fine Screens. A fine screen is placed downstream of the coarse screen. Its purpose is to collect
smaller material that has passed through the coarse screen but is still large enough to d amage
downstream equipment. Generally, it is placed in the low-lift pump station ahead of the pump
intake. Fine screen
characteristics are given in Table 3-5 .
Intake Crib
Location. The desired location of the intake crib is in deep water where it will not be buried
by sediment, be washed away, be a navigational hazard, or be hampered by problems associ-
ated with ice. The minimum suggested depth is 3 m from the surface. In rivers, where the depth
exceeds 3 m, the top of the intake should
be 1 m above the river bottom. In cases where the water
depth is less than 3 m, the crib is buried 0.3 to 1 m (Kawamura, 2000).
Structure. The Milwaukee, Wisconsin, intake crib shown in Figure 3-4 is a classic design. It is
octagonal in shape. A similar circular design was used for the Cincinnati, Ohio, intake ( Figu
re 3-7 ).
Other structures include hydraulically balanced inlet cones and inlet drums. The intake is protected
by riprap or a concrete slab.
Intake Ports. In warm climates, the intake crib ports are sized to provide a maximum velocity
of less than 0.3 m/s (Kawamura, 2000). In col
d climates, where frazil ice is anticipated, the intake
velocity is limited to less than 0.1 m/s.
Screens. Submerged intakes are screened with coarse screens. The T-screen ( Figure 3-8 ) has
found application in both river and reservoir applications. It is especially employed to prevent
small fi
sh from entering the intake structure. It is cleaned by a burst of air at a pressure of
1,000 kPa.
Conduit
The conduit may be either a tunnel or a pipeline. Generally, it is designed to flow by gravity.
Size. The conduit is sized to carry the maximum design flow rate ( Table 3-3 ). To minimize the
accumulation of sediment the flow velocity should be greater than 1 m/s (Kawamura, 2000). The
3-12 WATER AND WASTEWATER ENGINEERING
Bar screen cover
Bar screen
River bottom
River
Peripheral bar
screens
Tunnel conduit to
wet well
Normal submergence
approximately 7.6 m
FIGURE 3-7
River intake.
Hazen-Williams equation is the one most commonly used to describe the flow of water in pipes.
In terms of flow rate, it is given as
QCDS 0 278
263 0 54
.
..
(3-2)
where Q flow rate, m
3
/ s
C Hazen-Williams coefficient of roughness
D diameter of pipe, m
S slope of energy grade line, m/m
Williams and Hazen (1905) investigated a number of different types of pipe to determine C. A
table of updated values is given in Appendix C.
INTAKE STRUCTURES 3-13
Conical
debris
deflection
75 m diameter
piping
Air
backwash
piping
A
A
Section A-A
Section A–A
1 m dia. screen
cylinder w/ 3 mm
slot openings
Access
manhole
Intake
screen
1.5 m 2.3 m
8.35 m
2.3 m 1.5 m
0.75 m
3 m
6 m
20 cm air
backwash piping
Channel
bottom
1.8 m steel raw
water pipeline
1.7 m
1.7 m
1.5 m
1.2 m
1.4 m
1.4 m
2.8 m
1.0 m dia.screen cylinder
w/ 3 mm slot openings
Channel
flow
Intake screen
Water
surface
FIGURE 3-8
S ubmerged intake T-screens with typical dimensions.