Hydraulic Structures

Weirs – Spillways – Stilling Basins

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 Weirs
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Weirs have been used for measuring discharge in open channel flow for over
two hundred of years both in laboratory and in the field.

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They can be classified as sharp crested or broad crested

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Sharp crested weir : A thin vertical plate is fixed to the channel bottom and
sides

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Broad crested weir: comprises of a sudden rise of the channel bottom over
some distance

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 Sharp-Crested Weirs
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A sharp-crested weir usually comprises of a thin plate
mounted perpendicular to the flow direction. The top
of the plate has a beveled, sharp edge which makes
the nappe spring clear from the plate.
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These weirs may be rectangular or triangular in
shape. The latter are used mainly for measuring
small rates of discharge.
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The basic theoretical development on weirs are
based on the assumption that the pressure above
and below the nappe is atmospheric. Therefore, it is
necessary to vent the underside of the nappe so that
the lower side of the nappe is at atmospheric
pressure.

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 Broad-Crested Weirs
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The channel bottom is raised of
sufficient length in the direction
of the flow.
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Critical Depth occurs over the
jump
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If W is the height of the weir
above the channel bottom, then
V = Q∕[B(H + W)].

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 Hydraulic Jumps
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a hydraulic jump is formed whenever flow changes from
supercritical to subcritical flow.
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In this transition from the supercritical to subcritical flow, water
surface rises abruptly, surface rollers are formed, intense mixing
occurs, air is entrained, and usually a large amount of energy is
dissipated.
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By utilizing these characteristics, a hydraulic jump may be used to
dissipate energy, to mix chemicals, or to act as an aeration
device.

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 Jump types
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Hydraulic jump occurs in four distinct forms [Peterka, 1958] depending upon
the approach Froude number, Fr1. Each of these forms has a distinct flow
pattern, formation of rollers and eddies, etc. The energy dissipation in the jump
depends upon the flow pattern and the strength of the rollers. The range of
Froude number listed in the following paragraphs for various types of jumps is
not precise, and there is some overlap from one type to the other.

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 Jump Types
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Weak jump – sequent depths
are approximately equal to
each other and only a slight
ruffle is formed on the surface.
This undulation results in very
little energy dissipation.
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However, as Fr1 approaches
1.7, a number of small rollers
are formed on the water
surface, although the
downstream water surface
remains smooth. The energy
loss is low in this jump.
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Water Depth Hydraulic Jump-
Rectangular Channel
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Rectangular
channel
 Hydraulic Jump
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a hydraulic jump is formed whenever flow
changes from supercritical to subcritical flow.
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Length of Jump: The length of a jump is
needed to select the apron length and the
height of the side walls of a stilling basin.

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 Energy Dissipation in Jump

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 Spillway Design
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Energy dissipation at dams and weirs is closely associated with spillway
design
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The passage of water from a reservoir into the downstream reach
involves a whole number of hydraulic phenomena such as the transition
into supercritical flow, supercritical non-aerated and aerated flow on the
spillway, possibly flow through a free-falling jet, entry into the stilling
basin with a transition from supercritical to subcritical flow, and echoes
of macroturbulence after the transition into the stream beyond the basin
orplunge pool. It is, therefore, best to consider the energy dissipation
process in five separate stages,

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 Spillway Design
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A spillway is used to release surplus or flood water or for other controlled
releases, such as for irrigation, navigation, or environmental considerations
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may be classified into different categories using different criterion for such
classification. For example, based on function, a spillway may be classified as
service, auxiliary, or emergency;
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Based on the structural components, it may be called an overflow, chute, or
tunnel spillway.
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Utilizing the type of inlet, a spillway may be classified as orifice, siphon, side
channel, or morning glory.
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The overflow spillway is one of the common types of spillway

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 Overflow spillway
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An overflow spillway is used on concrete-gravity, arch and
buttress dams where part of the dam length may be used for
spillway. Because of the shape, it is also called an ogee spillway.
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An overflow spillway has three main parts: the crest, the sloping
face, and the energy dissipator at the toe.

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 Crest of overflow spillway
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The pressures on the crest
are atmospheric if the crest
shape is the same as the
underside of the nappe of a jet
over a sharp-crested weir.
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These pressures may be
above atmospheric (positive)
or sub-atmospheric (negative)
depending upon the shape of
the crest relative to the
underside of the nappe over a
sharp-crested weir
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 Shape of the spillway crest
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The shape of the crest is based on the design head, H
d, which is selected for a given site such that the minimum
pressure at the crest is higher than − 6 m in order to prevent
cavitation.
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Usually, the design head is selected such that 1.3 < H∕Hd < 1.5. In
this range, acceptable levels of sub-atmospheric pressures are
produced on the spillway face. This results in an increase in the
discharge capacity of the spillway and at the same time does not
result in cavitation on the spillway
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H maximum head on the crest – maximum upstream of spillway
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Hd Design level – design head
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 Shape of spillway crest

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 Shape of spillway crest
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Rating Curve
A curve between the
upstream reservoir level
and the spillway discharge
is called the rating curve.

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Discharge through an
overflow spillway under a
given total head on the
spillway crest may be
written
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 Discharge coefficient

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 Pier coefficient

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 Stilling basin with chute blocks and baffles

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 Energy Dissipator
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The flow velocity at the toe of a high-head spillway is usually high and may cause
serious scour and erosion of the downstream channel if proper precautions are not
taken.
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For this purpose, energy dissipators are provided to dissipate sufficient amount of
energy before water enters the downstream channel.
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In order to have an idea about the amount of energy dissipation, let us compute its
value at the toe of the Grand Coulee dam, located on the Columbia River in the
United States of America. The design discharge for the spillway is 28,320 m3/s, and
the upstream and the tailwater levels for this flow are 393.8 m and 308.23 m,
respectively. Assuming no losses on the spillway face, the amount of energy at the toe
= ρgQH, where H is the difference between the headwater and tailwater levels.
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Substitution of the values of different variables into this expression yields an energy
of 23 GW. This should give the reader an idea about the amount of energy involved
and clearly shows that even excellent rock may be eroded if proper measures are not
taken
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 Energy Dissipators
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Three types of energy dissipators [Hager 1992; Hager
and Vischer, 1995] have been commonly used:

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stilling basins,

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flip buckets,

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and roller buckets

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 Stilling Basins
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The hydraulic jump is used for energy dissipation in
a stilling basin.
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Typically, this basin may be used for heads less
than 50 m. At higher heads, cavitation becomes a
problem.
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The chute blocks serrate the flow entering the
basin and lift up part of the jet
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The baffle blocks stabilize the jump and dissipate
energy due to impact
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The sill mainly stabilizes the jump and inhibits the
tendency of the jump to sweep out

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 Design of Stilling Basins
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To design this basin, first the design and standard project flood
are selected. The design flood may be less than the probable
maximum flood, and the standard project flood may be less than
the design flood.
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By assuming the head losses on the spillway face to be a certain
percentage of the total head (say 5-10 per cent), the values of V
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1 and y 1 are computed for the design flood. Then, the values of
Froude number, and the sequent depth y2 are determined.
Similarly, the sequent depth, is computed for the standard project
flood.
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The dimensions of the basin and of the appurtenances are then
determined from the following relationships:
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 Design of stilling basins

The baffle block width is less than or equal to h. The spacing between the blocks is to be at
least equal to the baffle block width.
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 Flip Bucket
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The flip bucket energy dissipator is suitable for sites
where the tailwater depth is low (which would
require a large amount of excavation if a hydraulic
jump dissipator were used)
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and the rock in the downstream area is good and
resistant to erosion.
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The flip bucket, also called ski-jump dissipator,
throws the jet at a sufficient distance away from the
spillway where a large scour hole may be produced.

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 Roller Buckets
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A roller bucket may be used for
energy dissipation if the
downstream depth is
significantly greater than that
required for the formation of a
hydraulic jump.

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