hydraulic structures unit 7 - energy dissipators.pdf

ZM UTP ENERGY DISSIPATORS 1

HYDRAULIC STRUCTURES

ENERGY DISSIPATORSby:

Dr. Zahiraniza Mustaffa


General Contents:

Introduction

Energy Dissipators

Stilling Basins

Design considerations


Recall the layout of a dam!

Damn you dam!


Dam

Energy Dissipator

Structures

Hydraulic jump

Reservoir

Q

Typical Layout of a Dam


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).


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


Energy Dissipator

Structures

High flow

(supercritical)

Q

Hydraulic

jump

Low flow

(subcritical)


Energy Dissipators

Examples of energy dissipators:

Stilling basins

Drops/Steps e.g. Stepped spillway

Roller/Flip Buckets


Baffle Blocks


Pergau Dam, Kelantan


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?


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


Q y1

y2

F1


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.


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


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.


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.


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.


What do you need to know?

Q

1

11

gy

vF

y2TW


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


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.

hydraulic structures unit 7 - energy dissipators.pdf

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