laboratory study on baffled apron as an integral …

LABORATORY STUDY ON BAFFLED APRON AS AN INTEGRAL PART OF AN ENERGY DISSPATOR SYSTEM

NURFADILA BTE RAAIBI

A thesis submitted in partial fulfillment of the requirement for the award of the degree of Bachelor of Civil Engineering

FACULTY OF CIVIL AND EARTH RESOURCES

UNIVERSITI MALAYSIA PAHANG

NOVEMBER 2010

ABSTRACT

High water velocity can cause bad Impacts such as erosion, scour,

sedimentation and cavitations around spillway toe, weir bed and downstream of

river. In order to overcome these problems, the flow velocity has to be reduced. This

can be done through constructing an energy dissipating structure at the affected area.

Blocks were used in the experimental energy dissipating structure in this study.

Various arrangements of blocks at spillway of the model were tested to obtain the

most effective result in reducing the water flow velocity. The relation between the

percentage of velocity decrease and various block arrangements were determined

through the plotted graphs. The decrease in flow velocity varied and depended on the

size of blocks, spacing between blocks and the number of rows of blocks. Generally,

this study proved that blocks could be used as an energy dissipating structure.

V

ABSTRAK

Halaju air yang tinggi berupaya menyebabkan hakisan, kerosakan dan

mendapan di kaki limpah, dasar saluran dan hilir sungai. Masalah mi perlu diatasi

dengan mengurangkan halaju air. Bagi mencapai matlamat mi, pembinaan struktur

pelesap tenaga di kawasan yang mengalami impak halaju air perlu dilakukan. Di

dalam kajian ini,blok digunakan sebagai struktur pelesap tenaga. Pelbagai susunan

blok di dasar laluan model dikaji untuk memperoleh keputusan yang paling efektif

dalam pengurangan halaju air. Hubungan antara peratus pengurangan halaju dengan

pelbagai susunan blok dapat diperolehi melalui graf. Pengurangan halaju aliran

adalah berbeza-beza bergantung kepada saiz block, jarak antara barisan blok dan

bilangan barisan blok. Secara umum, kajian mi telah membuktikan bahawa blok

boleh digunakan sebagai struktur pelesap tenaga

VI

TABLE OF CONTENTS

VII

PAGE CHAPTER TITTLE

THESIS STATUS DECLARATION

SUPERVISOR DECLARATION

TITTLE PAGE

DECLARATION

DEDICATION

ACKNOWLEDGEMENT

ABSTRACT

ABSTRAK

TABLE OF CONTENT

LIST OF TABLES

LIST OF FIGURES

LIST OF SYMBOLS

11

111

iv

V

Vi

vi'

vi"

vix

Xi"

CHAPTER 1

INTRODUCTION

1

2

3

3

4

1.1 Introduction

1.2 Problem Statement

1.3 Objectives

1.4 Study Scope

1 .5 Important of Study

CHAPTER 2 LITERATURE REVIEW

2.1 Introduction

2.2 Energy Dissipator

2.3 Hydraulic Structure

2.4 Hydraulic Jump

2.5 Types of Energy Dissipating Structure

5

6

6

7

10

VIII

2.5.1 Stilling basin 10

2.5.1.1. USBR Type I Stilling Basin 11

2.5.1.2 USBR Type II Stilling Basin 11

2.5.1.3 USBR Type III Stilling Basin 12

2.5.1.4 USBR Type VI Stilling Basin 13

2.5.1.5 USBR Structure Type V 14

2.5.1.6 SAF Stilling Basin 14

2.5.1.7 Expansion and Depression for

Stilling Basins 15

2.5.2 Stepped spillway 16

2.5.3 Ski jump 19

2.5.4 Baffle Block 19

2.4.4.1 Baffle chutes 20

2.5.5 Riprap 21

2.5.5.1. Plunge Pool 23

2.5.5.2 Outlet Channel 24

2.5.5.3 Waterway Drop Structure 25

2.5.5.4 Gabion Baskets 26

CHAPTER 3 RESEARCH METHODOLOGY

3.1 introduction 27

3.2 Model Setup 29

3.3 Blocks Construction 31

3.4 Laboratory Experiment 33

3.4.1 Category A 33

3.4.l.lArrangement in Spacing 33

3.4.1.2 Size of Blocks 34

3.4.2 Category B 35

3.4.2.1 Number of Rows 35

3.5 Discharge Measurement 38

3.6 Laboratory Experiment Procedure 40

CHAPTER 4 DATA COLLECTION AND ANALYSIS

IA

4.1 Discharge and Flow Velocity Measurement 41

4.2 Data Collection 42

4.3 Data Analysis for Category A 44

4.3.1 Data Analysis of 50mm Block 44




4.3.5 Comment 52

4.4 Data Analysis for Category B 55

4.4.1 Data Analysis for Number of Rows Using

Block 125mm 55

CHAPTER 5 CONCLUSION AN RECOMMENDATIONS

5.1 Conclusion 58

5.2 Recommendations 60

REFERENCES

61

APPENDIX 64

LIST OF TABLES

A

TABLE NO. TITTLE PAGE

2.1 Jump characteristics according to Froude number 9

4.1 Data Analysis of Flow Depth for Blocks 50mm 45

4.2 Data Analysis of Flow Depth for Blocks 75mm 47

4.3 Data Analysis of Flow Depth for Blocks 1 00m 49

4.4 Data Analysis of Flow Depth Blocks 125mm 51

4.5 Summaries of all Size of Blocks 53

4.6 Data Analysis for Number of Rows Using Block 56

125mm

LIST OF FIGURE

FIGURE NO. TITTLE PAGE

2.1 Hydraulic Jumps on a Horizontal Rectangular Channel can

be Classified According to the Froude Number in the

Incoming Flow 8

2.2 Flow Profile According to Froude Number (Philip H.

Burgi, 2002) 8

2.3 USBR Type II Stilling Basin (DROP Design Manual,

1997) 11

2.4 USBR Type III Stilling Basin(OHIO Department of

Resources, 1999) 12

2.5 USBR Type VI Stilling Basin(DROP Design Manual,

1997) 13

2.6 Sketch of an Expansion and Depression for Stilling

Basin(Aquaveo Water Model Solution 16

2.7 Nappe Types (Lakshamana Rao 1975) (a) The Impinging

Jet, (b) Breaking Wave (c) Surface Wave and (d) Surface

Jet (Wu & Rajaratnam 1996). 17

2.8 Skimming Flow Regime of a Stepped Spillway(Hubert

Chanson, 1995) 18

2.9 Baffled Chutes(OHIO Department of Resources, 1999) 21

2.10 Riprap Revetment 23

2.11 Plunge Pool(OHIO Department of Resources, 1999) 24

2.12 Waterway Drop Structure( C.Mary Jones and Christopher

D.Lidstone, 1996) 26

Al

2.13 Gabion Baskets(FritZ, Doyl M, 1996) 27

3.1 Experiment Design Flow 29

3.2 Model of Channel 31

3.3 Block Construction 32

3.4 Making Formwork of Different Size to Form Blocks 33

3.5 Filling of Cement into the Formwork 33

3.6 Arrangement of Spacing 34

3.7 Sizes of Blocks Used 35

3.8 Flows in Conducting Category A 36

3.9 Number of Rows 37

3.10 Flows in Conducting Category B 38

3.11 Dimension of the Model Use in Laboratory 39

4.1 Percentage of Velocity Reduction (%) Versus Spacing

between Rows of 50mm Bloc 46


between Rows of 75mm Bloc 48


Between Rows of 100mm Blocks 50


Between Rows of 125mm Blocks 52

4.5 Summaries of All Block Size According to their Velocity

Reduction 53

4.6 Summaries of All Different Spacing According to their

Number of Rows 57

A Spacing Between Block to Block is 75mm 66

B Arrangement of Block at a Certain Spacing Between Rows

For A Given Number Of Row, n=4 66

C Analyze the Type of Jump at 50mm Size of Block 67

D Method Used to Measure the Depth, y2 67

E Installation of Pump 68

F Different Size of Blocks Used 68

XI

LIST OF SYMBOL

V - Volume of the container

A - Hydraulic cross section area

B -Width of channel

Q - Discharge

h -Height of block

S - Spacing between rocks

n -Number of rows

t - Time duration

vo - Initial velocity at

V, - Velocity at downstream of energy dissipating structure

yo - Original flow depth in channel

yn - Flow depth at downstream of energy dissipating structure

XIII

CHAPTER I

INTRODUCTION

1.1 Introduction

High velocity flows of water caused environmental problems such as flood,

damaged of roads and building and even destroys crops. High water velocity is

where water moves through a distance in a short period of time. As water moves

downwards, the kinetic energy in water increase causing the spillway toe and weir

bed get a bad impact such as erosion, scour and cavitations. Many culverts and

detention basin outlet structures undergoes damages due to erosion. Erosion forces

in natural drainage network often make it worst by construction of highway and by

urban development. Unstable overland flow results in increased of erosion potential.

In order to protect the culvert and other outlet devices, sometimes it needs to

rnploy an energy dissipator. Energy dissipators are any device designed to protect

iownstrearn areas from erosion by reducing the velocity of flow to acceptable limits

and by reducing the kinetic force (Albert, 1990).

2

However, energy dissipator structure can operate more efficiently using

hydraulic jump which occurs when water flows from supercritical to subcritical. The

design of the energy dissipator is largely dependent base on the characteristic of the

site and the magnitude of the energy. Thus, design dimensions for energy dissipation

structures are important because an inappropriate design can worsen erosion and

scouring problem (Gary P.Merkley, 1995).

Due to this, a channel model is made so that experiment can runs in a

laboratory. A static object must be placed at the water passage to change the flow

rate and reduces its velocity. Blocks are common object used in this experiment as it

is cheap and economical. However, hydraulic jump can also be created to reduce the

velocity of water at the downstream of the model channel where turbulent change in

flow from a low stage below critical depth to the conjugate depth above critical.

1.2 Problem Statement

Spillway, dams and weir are manmade structures built to deal with scour

and erosion cause by the high velocity flow. Increasing of kinetic energy occurs

when water flows from a high altitude slope area. As water approaches downstream,

velocity tend to increase causing the highway structure and adjacent area to damage

due to erosion. Due to this, studies need to be conducted to preserve and protect the

environment and these structures.

Application of hydraulic jump as a mean of energy dissipator can

effectively solve this problem. Hydraulic jump phenomena occurs when a sudden

rise in the water surface with a significant loss of energy. By creating an energy

dissipator in a passage for water helps prevent undue acceleration of flow and

limiting the velocity so that the structure can handle the flow force.

3

1.3 Objectives

The objective of this study is to

1. To determine the most effective size and block spacing with regards to

effective velocity reduction

2 To analyze the relationship of effective size and block spacing to be used

with the number of rows to be provided.

1.4 Scope of Study

The scopes of study included the following:

i. Rectangular channel with fixed gradient of 30 degree.

ii. Blocks made of concrete with different sizes

iii. Distance of concrete blocks from spillway toe.

L}

1.5 Importance of Study

The study seeked to observe the effectiveness of using blocks as an energy

dissipating structure to create hydraulic jump. It is not only exposes the civil

engineers on various types of energy dissipator that have been designed and

constructed but also makes them aware of erosions, sedimentation and other further

problems despite finding various solutions to these problems. The study also

provides the useful information about design and construction of energy dissipator

that fulfills specifications, standards or technical needs.

9

CHAPTER II

LITERATURE REVIEW

2.1 Introduction

In this chapter, types of hydraulic structure and energy dissipators for

culverts and channels are introduced. The background studies of the past energy

dissipator were discussed in this chapter. Under many circumstances, discharges

from culvert and channels may cause erosion problems. To mitigate this erosion,

discharge energy can be dissipated prior to release downstream.

6

2.2 Energy Dissipator

An energy dissipator is a concrete structure designed to reduce the velocity

of the flow existing culvert to prevent erosion of the streambed and banks. It is a

device for reducing the energy of storm water flows by mean of friction, impact and

turbulence. "Energy dissipate" means a structure shaped channel section with

mechanical armoring placed at the outlet of pipes or conduits to receive and break

down the energy from high velocity flow (Albert, 1990)

2.3 Hydraulic Structure

Generally, hydraulic structures are built to control and transport water and

maintain the water level of the river or waterway (Siti Aishah, 2003). Another

important function of the hydraulic structure is to dissipate the high water energy.

The energy dissipating structure is normally built at hill slope or dams. Energy of

water flowing over weirs, dams and other hydraulic structure can be dissipated

through practical application of the hydraulic jump. Another example that can be

related to the energy dissipator is stepped construction paved drain and spillway

(Ven Te Chow, 1959).

7

2.4 Hydraulic jump

A hydraulic jump is a sudden and turbulent transition in flow from a low

stage below critical depth to the conjugate depth above critical whilst the velocity

changes from supercritical to subcritical with a significant loss of energy (DROP

Design Manual, 1997). The abrupt rise of water depth through hydraulic jump takes

place as the fast flow velocity decreases to critical flow speed, which is the speed of

gravity waves (Clare Steedman, 2000). The formation of surface roller from the

beginning of the jump produces turbulent eddies which extract energy from the mean

flow to break it down into small eddies. The mean flow energy together with kinetic

energy is dissipated rapidly into heat towards the downstream (K.G.Ranga Raju,

1984)

Hydraulic jump is very common and effective way of dissipating energy at

downstream of the spillway toe. Energy dissipation and erosion control of the

incoming flow is depended on the available tail water depth (yi) and the conjugate

depth (Y2) (C.Nailuri, 2001). Figure 2.1 shows hydraulic jumps on a rectangular

channel that can be classified according to the Froude number and Figure 2.2 shows

flow profile according to Froude number as stated in Table 2.1.

VI

A

Vi V,

3T : - '2 /7

B

. V2 TWi

8

yI/

V1T2 (1) (2)

Tf± 1-TW •. TW

V2 V2

C D

Figure 2.1 Hydraulic Jumps on a Horizontal Rectangular Channel can be Classified

According to the Froude Number in the Incoming Flow. (C. Nailuri, 2001)

1: 1 Between 1.7 and 25 A - Prejiimp Stage

OsciIatinj Jet

TT F, Between 2.5 and 4.5

B - Transition Stage

IFr Between 4.5 and 9.0

C - Range of Well-Balanced Jumps

I- _J - ----- -

Fr Higher than 9.0 D - Effective Jump but Rough Surface Oowstream

9

Figure 2.2 Flow Profile According to Froude Number (Philip H. Burgi, 2002)

Table 2.1 Jump Characteristics According to Froude Number ( C.Nalluri, 2001)

- F roude Jump Characteristics

-iTöi.T Surface waves (undular)

Weak jump. Low energy loss

Oscillating jump. Large irregular waves

Steady jump

>9.0 Strong jump

According to DROP Design Manual (1997), there are three important

parameters of the jump in the design of energy dissipating structures:

i. The ratio of the incoming depth to the sequent depth

ii. The magnitude of the energy loss for any given set of incoming flow

conditions.

iii. The length of the jump

Referring to Belanger momentum equation, the ratio can predict the

conjugate depth base on Froude number derived from discharge per unit width (K.G.

Ranga Raju, 1984). Energy toss is generally more significant for higher Froude

number. Length of the jump and the end of the jump which can be defined as the

point where the reverse surface roller ends or the point of maximum water depth

(Subash C. Jam, 2001)

10

2.5 Types of Energy Dissipating Structure

Various inventions of energy dissipating structures have been developed to

prevent scouring downstream from the structure. They consist of-

i. Stilling basin

ii. Steeped spillway

iii. Ski jump

iv. Baffle block

v. Riprap

2.5.1 Stilling Basin

The stilling basin as an energy dissipator is based on the concept of

developing a hydraulic jump on a concrete pad immediately downstream of a

spillway to dissipate the energy of the turbulent flow before it reaches the

downstream channel (Philip }-1.Burgi, 2002). Several considerations need to be made

in the design of stilling basin which is location, gradient and stabilization of the

inflow channel.

The hydraulic laboratory developed seven unique stilling basin designs

where a hydraulic jump is used to dissipate energy. A widely used energy dissipator

in this 9"OU1) is the impact basin often used at the downstream end of pipe. It is very

effective energy dissipator that is commonly seen today in urban flood control project.

11

2.5.1.1 USBR Type I Stilling Basin

Structure type I consists of a horizontal apron without blocks or end sill.

The basin is suited to incoming flows characterized by Froude number in the range

1.7 to 2.5 (pre-jump condition). The structure is not commonly used as it tends to be

uneconomically long and the hydraulic jump can also be sweep out at tailwaters only

marginally below design level (DROP Design Manual, 1997)

2.5.1.2 USBR Type II Stilling Basin

Structure type II is suited to high velocity flows with Froude number above

4.5 conditions occurring typically below high spillways and it is therefore not

considered here in detail. The jump length is reduced over the unrestrained case by

around 33% through the introduction of chute blocks and a dentate end sill. Sweep

out of the jump can be a problem so the USBR recommends that the apron level is

fixed assuming that the tailwater depth is 5% less than expected (DROP Design

Manual, 1997)

Pui. blocks0.02Dt-,i

0*

410

'. •)

CO,

-i

Apr ort> 0.50,

6. -.

Figure 2.3 USBR Type 11 Stilling Basin (DROP Design Manual, 1997)

2.5.1.3 USBR Type III Stilling Basin

The structure is suited to locations where the Froude number exceeds 4.5.

The incoming velocity does not exceed 1 Sm/s and the discharge intensity is less than

18M 3/S per meter width. The restriction is imposed to avoid cavitations damage to

baffle blocks, chute blocks and the solid end sill which are fundamental to the design

of the structure. Introduction of these appurtenances shortens the jump and the basin

about 60% over the comparable Type 1 structure. The structure is suited to lower

head conditions small spillway, outlet works and canal structure (DROP Design

Manual, 1997)

Figure 2.4 USBR Type III Stilling Basin (OHIO Department of Resources, 1999)

13

2.5.1.4 USBR Type VI Stilling Basin

Structure Type VI is intended for Froude number in the range 2.5-4.5

conditions which may typically occur at small drops in open channel systems. The

design is intended to reduce the generation of surface waves which can travel large

distances downstream and erode canal banks. For this reason the chute blocks are

enlarged in order to direct the incoming jet into the base of the jump roller to help

stabilize it. For situations where it is essential to minimize surface turbulence, the

USBR has also developed designs for surface wave suppressors which can be placed

downstream of the main structure (DROP Design Manual, 1997)

U

Proc lionol spoce

w -Max. tooth 4',O'M Di

'Top surface on 5* slope Si H option

:ZD I -i h4 See figure

Figure 2.5 USBR Type VI Stilling Basin (DROP Design Manual, 1997)

14

2.5.1.5 USBR Structure Type V

The Type V structure is designed to provide effective performance when the

jump sequent depth and the tail water cannot be matched across the full range of flow

conditions. A sloping apron allows the jump to form at different points, depending

on the discharge. This solution is not often adopted for small structures, probably

because it is less simple to construct than a horizontal apron (DROP Design Manual,

1997).

2.5.1.6 SAF Stilling Basin

This small structure design was developed at the Saint Anthony Falls

laboratory of the University of Minnesota, Blaisdell, 1948. The aim was to design an

energy dissipating structure with the minimal practical basin length, suitable to a

wide range of tail water conditions and Froude number.

The structure can produce reductions of up to 40% on the length of a simple

apron structure and 40% on the calculated sequent tailwater depth. The penalty is

that construction becomes more complex. An endsill, chute blocks and baffle blocks

are include in the design. Transported debris is liable to become lodged in the basin.

The required tailwater depth for satisfactory performance across the range of Froude

number was determined from test (Varshney et al, 1977).

By the 1970s the trend for spillway terminal structure had returned to the

flip bucket where the principle used was to direct the flow away from the structure

and downstream a sufficient distance where the water could erode its own plunge

pool. The energy dissipated within the basin at the end of the tunnel spillway up to a

laboratory study on baffled apron as an integral …

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