Chapter 7 spillway and energy dissipators

CHAPTER 7: SPILLWAY AND ENERGY

DISSIPATORS

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0401544 - HYDRAULIC STRUCTURES

University of Sharjah

Dept. of Civil and Env. Engg.

DR. MOHSIN SIDDIQUE

ASSISTANT PROFESSOR

SPILLWAY

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LEARNING OUTCOME

After taking this lecture, students should be able to:

(1). Obtain in-depth knowledge on various types of spillways

used in dams and their design guide lines

(2). Apply the design guide lines for the design of selected

Spillway

3

References:

Khatsuria, R. M., Hydraulics of Spillways and Energy Dissipators,

Novak, A.I.B. Moffat, C. Nalluri, R. Narayanan, Hydraulic Structures, 4th Ed. CRC PressSantosh, K. G., Irrigation Engineering and Hydraulic Structures, Khanna Publishers

BULU, A., Lecture noted of water resources, Istanbul Technical University

SPILLWAY

A spillway is a structuredesigned to 'spill' flood watersunder controlled (i.e. safe)conditions.

� The Spillways can be

� Uncontrolled (Normally)� Controlled

� Note: Concrete damsnormally incorporate an over-fallor crest spillway, butembankment dams generallyrequire a separate side-channelor shaft spillway structurelocated adjacent to the dam.

Sketch of conventional weir/spillway

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CLASSIFICATION OF SPILLWAYS

I. According to the most prominent feature

• A. Ogee spillway

• B. Chute spillway

• C. Side channel spillway

• D. Shaft spillway

• E. Siphon spillway

• F. Straight drop or overfallspillway

• G. Tunnel spillway/Culvert spillway

• H. Labyrinth spillway

• I. Stepped spillway

II. According to Function

• A. Service spillway

• B. Auxiliary spillway

• C. Fuse plug or emergency spillway

III. According to Control Structure

• A. Gated spillway

• B. Ungated spillway

• C. Orifice of sluice spillway

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CLASSIFICATION

OF SPILLWAY

Classification of Spillway (Vischer et al, San

Francisco,1988). 6

ANALYSIS OF EXISTING STRUCTURES

Semenkov (1979) analyzed more than 400 projects in terms of parameters L/H and N for the three main types of spillways: gravity spillways, chute spillways, and tunnel spillways for concrete and earth-fill dams.

Where, L and H are the length and height of the dam crest respectively, and

N is the power of the flow

Types of spillways for concrete and earth-fill dams. T: Tunnel spillways, C:

Chute spillways, G: Gravity spillways (Semenkov, 1979).

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VARIOUS ASPECTS INVOLVED IN A

SPILLWAY DESIGN

The following aspects are involved in the design of spillways:

1. Hydrology

• Estimation of inflow design flood

• Selection of spillway design flood

• Determination of spillway outflow discharge

• Determination of frequency of spillway use

2. Topography and geology

• Type and location of spillway

3. Utility and operational aspects

• Serviceability

4. Constructional and structural aspects

• Cost-effectiveness

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ECONOMIC ANALYSIS

Comparative costs: spillway-dam combinations. A:Minimum cost: gated

spillway, B: Minimum cost: ungated spillway (USBR,1960).

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SPILLWAY DESIGN FLOOD

Probable Maximum Flood (PMF)

This is the flood that may be expected from the most severecombination of critical meteorological and hydrological conditions thatare reasonably possible in the region. This is computed by using theProbable Maximum Storm.

Standard Project Flood (SPF)

This is the flood that may be expected from the most severecombination of hydrological and meteorological factors that areconsidered reasonably characteristic of the region and is computed byusing the Standard Project Storm (SPS).

In US, generally, large dams are designed for PMF, intermediate for SPF/PMF, and small dams for floods of return period of 100 years to SPF.

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ESTIMATION OF SPILLWAY DESIGN FLOOD

The estimation of spillway design flood or the inflow design flood is an exercise involving diverse disciplines of hydrology, meteorology, statistics and probability.

There is a great variety of methods used around the world to determine exceptional floods and their characteristics. ICOLD (1992) groups all these methods under the two main categories:

1. Methods based mainly on flow data.

2. Methods based mainly on rainfall data.

(discussion on the methods is not scope of this course)

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SPILLWAY DESIGN

Ogee or Overflow Spillways

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OGEE OR OVERFLOW SPILLWAYS

The ogee or overflow spillway is the most common type of spillway. It has a control weir that is Ogee or S-shaped. It is a gravity structure requiring sound foundation and is preferably located in the main river channel.

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The basic shape of the overfall (ogee) spillway is derived from the lower envelope of the overall nappe flowing over a high vertical rectangular notch with an approach velocity, Vo,=0 and a fully aerated space beneath the nappe (p=po)

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DISCHARGE CHARACTERISTICS

Similar to the crest profile, the discharge characteristics of the standard spillway can also be derived from the characteristics of the sharp crested weir. The weir equation in the form:

If the discharge, Q, is used as the design discharge in above Eq, then the term

He will be the corresponding design head (Hd) plus the velocity head (Ha). i.e.,

He= Hd +Ha

For high ogee spillways, the velocity head is very small, and He≅ Hd.

2/32 eLHgCQ =

He

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Overflow spillways are named as high-overflow, and low-overflow depending upon to the relative upstream depth P/HD.

In high-overflow spillways, this ratio is (P/HD>1.33) and the approach velocity is generally negligible.

Low spillways have appreciable approach velocity, which affects both the shape of the crest and the discharge coefficients.


Definition sketch of overflow spillway cross-section


Figure gives variation of CD, the value of C when H equals the designhead HD, with the relative upstream depth P/HD. Here P is the height ofthe spillway crest with respect to the channel bed.


Overflow spillways frequently use undershot radial gates for releases over the dam. The governing equation for gated flows:

Where C is a coefficient of discharge, and H1 and H2

are total heads to the bottom and top of the gate opening. The coefficient Cis a function of geometry and the ratio d/H1, where dis the gate aperture.


THE SPILLWAY CREST PROFILE

On the crest shape based on a design head, HD, when the actual headis less than HD, the trajectory of the nappe falls below the crest profile,creating positive pressures on the crest, thereby reducing thedischarge. On the other hand, with a higher than design head, thenappe-trajectory is higher than crest, which creates negative pressurepockets and results in increased discharge.

H=HD

H>HD

H<HD

OGEE OR OVERFLOW SPILLWAYSTHE SPILLWAY CREST PROFILE



Accordingly, it is considered desirable to under design the crest shape of a high overflow spillway for a design head, HD, less than the head on the crest corresponding to the maximum reservoir level, He (~Hmax).

However, with too much negative pressure, cavitation may occur. The U.S. Bureau of Reclamation (1988) recommendation has been that He/HD should not exceed 1.33.

The Corps of Engineers (COE) has accordingly recommended that a spillway crest be designed so that the maximum expected head will result in an average pressure on the crest no lower than (-4.50m) of water head (U.S. Department of Army, 1986). Pressures of (-4.50m) can be approximated by the following equations (Reese and Maynord, 1987).



He/HD <=1.33



Crest shapes have been studied extensively in the USBR hydraulic laboratories with various approach depths. On the basis of the USBR data, the US Army Corps of Engineers, WES (1952)** has developed several standard shapes, designated as WES standard spillway shapes, represented on the downstream of the crest axis by the equation:

**WES Spillway for Genegantslet dam,. New York, Tech Memo 2–351, 1952.


THE SPILLWAY CREST PROFILE (typical values)


In the revised procedure developed by Murphy (1973), using the same basic data of USBR, the upstream quadrant was shaped as an ellipse with the equation

and the downstream profile conformed to the equation

Where K is a parameter depending on the ratio approach depth and design head

For vertical u/s face

origin at the base of apex


Figure. Coordinate coefficients for spillway crest (USACE, 1986)


Typical WES crest profiles.


In a high-overflow section, the crest profile merges with the straight downstream section of slope α, as shown in Fig. (i.e., dy/dx = α). Differentiation and expressing that in terms of x yield the distance to the position of downstream tangent as follows:

where

xDT = Horizontal distance from

the apex to the downstream

tangent point

α = Slope of the downstream

face.


With respect to origin at the apex, the equation of the elliptical shape for upstream quadrant is expressed as,

where

x = Horizontal coordinate, positive to the right

y = Vertical coordinate, positive downward

A, B = One-half of the ellipse axes, as given in Fig. above for various values of approach depth and design head.


For a inclined upstream face of slope FS, the point of tangency with elliptical shape can be determined by the following equation.


The coefficient of discharge (or say discharge) is influenced by a number of factors such as

(1) the relation of the actual crest shape to the ideal nappe shape,

(2) the depth of approach,

(3) the inclination of the upstream face,

(4) the contraction caused by the crest piers and abutment,

(5) the interference due to downstream apron, and

(6) the submergence of the crest due to downstream water level.


(1). The relation of the actual crest shape to the ideal nappe shape,

R. M. Khatsuria, Hydraulics of Spillways and Energy Dissipators,


(2) the depth of approach



(3) the inclination of the upstream face



(4) The effective length (L’) of Ogee spillway

Crest piers and abutments cause contraction of the flow, reduction in the effective length of the crest, and cause reduction in the discharge as compared to that of an otherwise uncontrolled crest. The following relationship applies:

The values of KP and Ka depend mainly upon the shape of the piers and that of the abutments.



(5 & 6): Submerged Discharge on Overflow Spillways

The coefficient of discharge decreases under the condition of submergence. Submergence can result from either excessive tailwaterdepth or changed crest profile.

The effect of tailwater submergence on the coefficient of discharge depends upon the degree of submergence defined by hd/He and the downstream apron position, (hd+d)/He shown in Fig. (7.5).

For a value of (hd+d)/He up to approximately 2, the reduction in the coefficient depends on the factor (hd+d)/He and is independent of hd/Heas shown in Fig. (7.5.a), i.e., it is subject to apron effects only.


(5 & 6): Submerged Discharge on Overflow Spillways

Atıl BULU, Lecture noted of water resources, Istanbul Technical University


When (hd+d)/He is above 5, the reduction depends only on hd/He as shown in Fig. (7.4.b), i.e., tailwater effects control.

For (hd+d)/He between 2 and 5, the reduction of the coefficient depends on both factors, given in Fig. (7.5.c).


SPILLWAY TOE

The spillway toe is the junction between the discharge channel and the energy dissipator. Its function is to guide the flow passing down the spillway and smoothly in the energy dissipator

A minimum radius of 3 times the depth of flow entering the toe is recommended.


EXAMPLE 7.1: Design an overflow spillway section for a design discharge of 1500 m3/sec. The upstream water surface level is at elevation 240m and the upstream channel floor is at 200 m. The spillway, having a vertical face, is 50 m long.


Solution:

1. Assuming a high overflow spillway section, for P/HD ≥ 3, discharge coefficient CD =0.49 from Fig.

2. From the discharge equation


5. Calculate height of the crest,

P = 40.00 − 5.73 = 34.27m

6. Calculate design head

Since He=5.76 m<10m

Design head=HD=0.7He=0.7*5.76=4.03m

7. Calculate P/HD

P/HD=34.27/4.03=8.5 >1.33 high overflow


8. Shape of downstream quadrant

for P/HD=8.5 ��K= 2 (from Fig)

Therefore,


Coordinates of the downstream shape computed by the equation are as follows:

9. Calculate point of tangency: Assume a downstream slope of (2/1). From Eq.


10. Shape of upstream quadrant:

Eq.

Therefore ,


Coordinates of the downstream shape computed by

the equation are as follows:


sketch of overflow spillway cross-section


EXAMPLE 7.2: A spillway has been designed for a head of 2.80 m with a length 200 m. The discharge coefficient is C = 0.49. Calculate the discharge for this head.

What will the discharge be for heads of 0.20 m and 1.50 m?

What is the maximum discharge that can be passed over this spillway without cavitation?


Solution:

At the design head,


Similarly,


Maximum head:


EXAMPLE 7.3: Determine the length of an overflow spillway to pass 60 m3/s with a depth of flow upstream not to exceed 1.50 m above the crest. The spillway is 2.50 m high. The upstream face is sloped 1/1. For 60 m3/s, the tailwater rises 1.00 m above the crest. The spillway is designed for the maximum head.


1. Since the spillway is designed for maximum head,

HD= He = 1.50 (without the approach velocity head)

2. From the given figure,

>2 but <5


Problem 1:

Design a suitable section for the overflow portion of a concrete gravity dam having the downstream face sloping at a slope of 0.7H: 1V. The design discharge for the spillway is 8,000 m3/s. The height of the spillway crest is kept at RL 204.0 m. The average river bed level at the site is 100.0 m. Thickness of each pier may be taken to be 2.5 m.

(Take He=HD)


Problem 2:

Design a suitable section for the overflow portion of a concrete gravity dam having the downstream face sloping at a slope of 0.7H: 1V. The design discharge for the spillway is 8,000 m3/s. The height of the spillway crest is kept at RL 204.0 m. The average river bed level at the site is 100.0 m. The spillway length consists of 6 spans having a clear width of 10 m each. Thickness of each pier may be taken to be 2.5 m.

(Take He=HD)

THANK YOU

Slides are prepared from various sources(References). It may have discrepancies/ inconsistency. If you find any, kindly rechecked with sources list in “references” .

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ENERGY DISSIPATERS

(STILLING BASIN)

LEARNING OUTCOME

After taking this lecture, students should be able to:

(1). Obtain knowledge on energy dissipators (stilling basin)

used in hydraulic structures and their design guide lines

(2). Apply the design guide lines for the design of selected

energy dissipators

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References:

Khatsuria , R. M., Hydraulics of Spillways and Energy Dissipators, Novak, A.I.B. Moffat, C. Nalluri, R. Narayanan, Hydraulic Structures, 4th Ed. CRC Press

Santosh, K. G., Irrigation Engineering and Hydraulic Structures, Khanna PublishersMays, L. W., Hydraulic design handbook (CHAPTER 18), Mcgraw hills

ENERGY DISSIPATION

Dissipation of the kinetic energy generated at the base of a spillway is essential for bringing the flow into the downstream river to the normal—almost pre-dam— condition in as short of a distance as possible.

This is necessary, not only to protect the riverbed and banks from erosion, but also to ensure that the dam itself and adjoining structures like powerhouse, canal, etc. are not undermined by the high velocity turbulent flow.

Low velocity

Very high velocity

V1=(2gH1)0.5

y1=q/V1

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ENERGY DISSIPATION

CLASSIFICATION1. Based on hydraulic action: Turbulence and internal friction as in hydraulic jump stilling basins, roller buckets, and impact and pool diffusion as with ski jump buckets and plunge pools.

2. Based on the mode of dissipation: Horizontal as in the hydraulic jump, vertical as with ski jump buckets/free jets, and oblique as with spatial and cross flows. The vertical dissipation may be in the downward direction as with free jets and plunge pools and in upward direction as with roller buckets.

3. Based on geometry or form of the main flow: Situations involvingsudden expansion, contraction, counter acting flows, impact, etc.

4. Based on the geometry or form of the structure: Stilling basin employs hydraulic jump with or without appurtenances like chute blocks, baffle piers, etc. Buckets (ski jump or flip buckets) include special shapes like serrated, dentated buckets, and roller buckets that are either solid roller bucket or slotted buckets.

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ENERGY DISSIPATION

PRINICIPAL TYPES OF ENERGY DISSIPATORS

The energy dissipators for spillways can be grouped under the following five categories:

1. Hydraulic jump stilling basins

2. Free jets and trajectory buckets

3. Roller buckets

4. Dissipation by spatial hydraulic jump

5. Impact type energy dissipators

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ENERGY DISSIPATION

ANALYSIS OF PARAMETERS

Eg

Vy

g

Vy

g

Vy o

o ∆++=+=+222

2

22

2

21

2

∆E= Energy dissipation between

u/s and d/s

Energy equation:

Mass conservation:Q1=Q2=Q3

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ENERGY DISSIPATION

In case of hydraulic jump at the d/s

V1=(2gH1)0.5

y1=q/V1

Thus, q/y1=(2gH1])0.5

+−

+==∆

g

Vy

g

VyE

22

2

21

2

22

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Energy dissipation

Assumption of Horizontal bed !!!

ENERGY DISSIPATION

Hence, for a given discharge intensity and given height of spillway, y1 is fixed and thus y2 (required for the formation of hydraulic jump) is also fixed.

But the availability of a depth equal to y2 in the channel on the d/s cannot be guaranteed as it depends upon the tail water level, which depends upon the hydraulic dimensions and slope of the river channel at d/s.

The problem should, therefore, be analyzed before any solution can be found by plotting the following curves:

Tail Water Curve (TW Curve): A graph plotted between q and tail water depth,

Jump Height Curve (JH Curve) also called y2 curve: A curve plotted on the same graph, between q and y2,

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ENERGY DISSIPATION

(1)

IdeaI condition

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ENERGY DISSIPATION

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ENERGY DISSIPATION

(1). When TW curve coincides with y2 curve

This is the most ideaI condition for jump formation. The hydraulic jump will form at the toe of the spillway at all discharges. In such a case, a simple concrete apron of length equivalent to length of jump (e.g.,5 [y2

- y1]) is generally sufficient to provide protection

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ENERGY DISSIPATION

(A). When TW curve is above the y2 curve

When y2 is always below the tail water, the jump forming at toe will be drowned out by the· tail water, and little energy will be dissipated.

The problem can be solved by:

(i). constructing a sloping apron above the river bed level

(ii). providing a roller bucket type of energy dissipator

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ENERGY DISSIPATION

iii. Providing a higher apron level followed by a drop

ENERGY DISSIPATION

(B). When TW curve is below the y2 curve

When the tail water depth is insufficient or low at all discharges, the following solution can be applied:

(i). Ski jump bucket type: This type of energy dissipator requires sound and rocky river bed, because a part of the energy dissipation takes place by impact, although some of the energy is dissipated in air by diffusion and aeration

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ENERGY DISSIPATION

(ii). Providing of a sloping apron as below the river bed

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ENERGY DISSIPATION

(iii). Constructing a subsidiary dam below the main dam

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ENERGY DISSIPATION

(iv) Providing upward slope

ENERGY DISSIPATION

(D). When TW curve is above the y2 curve at low discharges and below the y2 curve at high discharges: In this case, at low discharges, the jump will be drowned and at high discharges, tail water depth is insufficient. The following solutions can be applied by: (i). Providing a sloping apron partly above and partly below the river bed (ii). A combination of energy dissipator performing as a hydraulic jump apron for low discharges and flip bucket for high discharges

At low discharges, the jump will form on the apron above the river bed.Similarly, at high discharges, the jump will form on the apron below the river bed

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ENERGY DISSIPATION

(C). When TW curve is below the y2 curve at low discharges and above the y2 curve at high discharges (inverse of case D)

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The following solutions can be applied: (i). Sloping-cum-horizontal apron such that the jump forms on the horizontal portion for low discharges and on the sloping portion for high discharges

ENERGY DISSIPATION IN HYDRAULIC JUMP

Hydraulic jump can be used as Energy Dissipator

+−

+=∆

g

Vy

g

VyE

22

2

21

2

22

yqV /=

However, the real problem in the design of stilling basins, is not the absolute

dissipation of energy, but is the dissipation of this energy in as short a length

as possible.

−=∆

21

12

4 yy

yyE

=

gy

VF

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V1=(2gH1)0.5

y1=q/V1

Thus, q/y1=(2gH1])0.5

STILLING BASIN

• In general, a stilling basin may be defined, as a structure in which the energy dissipating action is confined.

• If the phenomenon of hydraulic jump is basically used for dissipating this energy; it may be called a hydraulic jump type of stilling basin.

• The auxiliary devices may be used as additional measures for controlling the jump, etc.

• Stilling basins are placed at the ends of dam spillways and at the ends of steep-sloped canal sections where elevation change has generated high kinetic energy.

• Stilling basin come in a variety of types and can either contain a straight drop to a lower elevation or an inclined chute

• Inclined chutes are the most common design for stilling basins and the most used inclined chutes are: USBR Stilling Basins Type II-IV, SAF Stilling Basins

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STILLING BASIN

In practice, the following types are highly recommended:

• USBR Type II basin for large structures and Fr > 4.5;

• USBR Type III basin and the SAF basin for small structures;

• USBR Type IV basin for oscillating jump flow conditions

The designs are selected based on the Froude Number of the flow and the flow velocity:

1

1

1

11

y

qV

gy

VFr

=

=

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STANDARD STILLING BASINS

Elements of Stilling Basin

Chute blocks

Baffle blocks

End sill or Dentated Sill

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• Chute blocks -concrete blocks built into the inclined sections of the spillway. These features are commonly placed at the head of the stilling basin to create turbulence prior to the hydraulic jump

• Baffle blocks -freestanding concrete blocks built in the main basin. These blocks are only used for flows <20m/s due to the high force they are subjected to and the potential for cavitation

• End sills -a built-up lip at the tail of the basin, with or without blocks. The sill height has the most significant impact on energy dissipation and taller sills are used to reduce the overall length of the stilling basin

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USBR Stilling Basin Type II

Fr1 > 4.5

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USBR Stilling Basin Type III

Fr1 > 4.5 & V<18m/s

D1=y1

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USBR Stilling Basin Type IV

Fr1=2.5-4.5

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Saint Anthony Falls

Effective for Fr1= 1.7 and 17

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d=y1

dconj=y2

Summary

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ENERGY DISSIPATION

DEFLECTOR BUCKETS

Sometimes it is convenient to direct spillway into the river without passing through a stilling basin. This is accomplished with a deflector bucket designed so that the jet strikes the riverbed a safe distance from the spillway and dam. This type of spillway is often called a flip bucket or ski jump spillway.

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ENERGY DISSIPATION

The trajectory of the jump

Where,

hv = Velocity head

d = Thickness of the jump

When the free jet discharging from the deflection bucket falls into an erodible riverbed, a plunge pool is eroded to a depth, D, given by:

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THANK YOU

Slides are prepared from various sources(References). It may have discrepancies/ inconsistency. If you find any, kindly rechecked with sources list in “references” .

96

Chapter 7 spillway and energy dissipators

Engineering

Transcript of Chapter 7 spillway and energy dissipators