Post on 01-Mar-2016
ZM UTP ENERGY DISSIPATORS 1
HYDRAULIC STRUCTURES
ENERGY DISSIPATORSby:
Dr. Zahiraniza Mustaffa
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General Contents:
Introduction
Energy Dissipators
Stilling Basins
Design considerations
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Recall the layout of a dam!
Damn you dam!
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Dam
Energy Dissipator
Structures
Hydraulic jump
Reservoir
Q
Typical Layout of a Dam
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Introduction
Problem Statement:
When water is released through the
spillway, it carries a significant amount of
energy.
If no structures were built at the downstream
section of the spillway, this huge amount
of energy will destroy the river bed (or
anything located near the area).
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Solutions:
Use energy dissipators, which are able to:
Dissipate the amount of energy released from the spillway.
Slow down (reduce the velocity) the tailwater flow
Protect any structures located at the downstream section of the dam
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Energy Dissipator
Structures
High flow
(supercritical)
Q
Hydraulic
jump
Low flow
(subcritical)
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Energy Dissipators
Examples of energy dissipators:
Stilling basins
Drops/Steps e.g. Stepped spillway
Roller/Flip Buckets
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Baffle Blocks
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Pergau Dam, Kelantan
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Stilling Basins
Stilling basin is a basin comprises small structures like baffles, sills and chute blocks.
When heavy flow (from u/s of dam) hits these structures, the energy of the water will be
dissipated. Thus, water flowing d/s will be
weaker than the u/s. Is it good? Why?
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Why do we need to have the basin?
The basin is a platform used to convert supercritical flows into subcritical flows. This condition will form a hydraulic jump. Is it good to have a hydraulic jump?
Hydraulic jump equation,
1812
1 21
1
2 Fy
y
1
11
gy
vF
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Q y1
y2
F1
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There are FIVE types of stilling basins Basin I, Basin II, Basin III, Basin IV and Basin V.
They are designed with respect to Froude number (F) of the flow at the downstream
section of the spillway.
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BASIN TYPE DESCRIPTION
Basin I 1.7 < F < 2.5
No special stilling basin required
Basin II F > 4.5 and V > 15 m/s
Chute blocks (u/s end) and dentated sill (d/s end).
Used at high dam, earth dam spillways & canal structures.
Basin III F > 4.5 and V < 15-18 m/s
Chute blocks (u/s end), baffle blocks (middle) and end sill
(d/s
end).
The basin length is 60% shorter than Basin II.
Used at small spillways, outlet works & small canal
structures.Basin IV 2.5 < F < 4.5
Chute blocks (u/s end) and end sill (d/s end).
Used at low dams (small spillways), small outlet works &
diversion dams.
Hydraulic jump not fully developed but lots of waves formed
from the jump.
Basin V Built on sloping aprons
Used at high dams spillways
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Design Considerations
Some concerns when designing stilling basin:
To determine basin width and elevation so that a stable hydraulic jump is formed within the
basin.
To avoid the jump neither swept out of the basin nor drowned.
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What are the stilling basin parameters that you need to design?
Chute block dimensions
Baffle block dimensions
Dentated sill dimensions
End sill dimensions
Length of the basin
Distance between the blocks
Number of blocks
etc.
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How do you design these parameters?
i. Determine the Froude number, F of the approaching flow before the placement of the stilling basins.
ii. Determine the corresponding hydraulic jump conjugate depths (y1 and y2) by using the hydraulic jump equation.
iii. Determine the tailwater depth located at the d/s of the basin (TW).
iv. Use the graphs given to find the parameters.
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What do you need to know?
Q
1
11
gy
vF
y2TW
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Q1. Select and design the type of stilling basin for a spillway with crest length of 100 ft, discharge of 15,000 cfs with elevations as shown in the figure.
Now, fun time!
Elev. 100 ft
Elev. 50 ftElev. 20 ft
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Q2. Select and design the type of stilling basin for a spillway that discharges flow with Froude number of 5.0 and velocity of 55 ft/s.
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