Mechanics of upland erosion

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MECHANICS OF UPLAND EROSION

By

Sewa Singh

A Thesis submitted to the

Indian Institute of Technology, Delhi for the award of the degree of DOCTOR OF PHILOSOPHY

Department of Civil Engineering Indian Institute of Technology, Delhi

1979

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ACK NO WLEDGEMENT S

I gratefully acknowledge my indebtedness to

DR. P.NATML4JAIN, Assistant Professor, Department of Civil Engineering, Indian Institute of Technology, Delhi for

supervising the research work reported in this thesis. I recall with the keenest pleasure the inspiration derived from the indefatigable energy and the painstaking guidance of Dr.P.

Natarajan. The suggestions, encouragement and the freedom of work given by him throughout the doctoral programme are highly appreciated.

I wish to express my gratitude to Prof. Saranjit Singh and Prof. Subhash Chander in particular and to all faculty members in general of the Civil Il1gineering Department,

I.,I. T. , Delhi for their all round help, all these years.

I wish to record my deep appreciation for the

suggestions and help I have received in various forms from Prof. R.J. Garde, Department of Civil Engineering, University of Boorkee, Roorkee, Prof. A.J. Raudkivi., Civil Engineering Department, University of Aickland, New Zealand and Dr.D.J.

Paintsr,Lincoln College, Canterbury.

Sincere thanks are extended to the staff of Fluid Mechanics Laboratory and Civil Engineering Workshop for their cooperation and assistance at all levels during the research work.

I take this opportunity to thank my beloved wife, whose love, care and encouragement have counted immensely

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towards my success and my father whose sacrifice and love have in many ways contributed to my present being.

Finally, I am grateful to the Government of Punjab for sponsoring me for research work under the Quality Improvement Programme under the Government schemes and to

the Principal, Guru Nanak Engineering College Ludhiana, Punjab, who relieved me to enable me to pursue the research programme.

Dated ,1979 (Sewa Singh)

Civil Engineering Department, Indian Institute of Technology, Delhi

New Delhi-29 (INDIA)

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CERTIFI CATE

This is to certify that the thesis entitled

"MECHANICS OF UPLAND EROSION' being submitted by Shri Sewa Singh to the Indian Institute of Technology, Delhi for the award of the degree of Doctor of Philosophy in Civil

Engineering is a record of bonafide research work carried out by him under my guidance and supervision. To the best of my- knowledge it has reached the requisite standard fulfilling the . requirements of the regulations relating to the said degree.

The material contained in this thesis has not been submitted, in part or fUll, to any other University or Institute for the award of any degree or diploma.

(P . NATARAJ AN)

Assistant Professor, Civil Engineering Department,

Indian Institute of Technology, De1bi New Delhi-110029

(INDIA)

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SYNOP

The present study on "Mechanics of Upland Erosion's has been primarily intended to evaluate erosion rates under

controlled conditions of simulated rainfall and controlled conditions of erodible target sediment bed surfaces. The

study has been carried out on unigranular non-cohesive sediments forming the target sediment bed for a laboratory set-up of rain-simulator, sand trough to carry the sediment bed, a trap to collect the eroded material, etc., which has specifically been designed and fabricated for the purpose.

Six bed slopes, viz., 0.005, 0.010, 0.015, 0.020, 0.025 and 0.030, provided to the target sediment bed have been tested.

A specific feature of this study is that the erosion rates have been studied by keeping the slope of the top surface of the target sediment bed at the same slope as set originally by an inbuilt device to adjust the bed surf-ice slope as the erosion progressed under the rainfall. The target sediment bed has been exposed to varying intensities of rainfall for each of the two rainfall forming tubing tips of different sizes (producing two different raindrop sizes of 1+.2 mm and 3.3 mm respectively). The height of fall of the raindrops has been adjustable so as to incorporate the effects of varying fall velocities (in the vicinity of sediment target bed) of rain. Five sieved sand fractions of uniform grair

sizes with mean particle size of 1.x-75, 1.001, 0.71k, J. j02 and 0.35 mm have been used as the sediment bed m ,terjal for the experimentation.

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(v) Reported literature on amounts

of soil erosion

(dealing

with sheet erosion) under artificial or natural rainfall conditions is relatively

scarce and even that needs refinements in the explanation of the basic phenomenon, The

review of literature also depicts

a lack of distinction

that

exists in respect of the effects of

'impact' and 'kinetic energy'

of rainfall. In most of the literature

the terms

'the impact forcer

^and ⁽

the kinetic energy'

of the rainfall are used as though no distinction exists _between

the two.

Fundamentally, the

present state of

knowledge would have demanded a clearer demarcation

of the above two effects.

The

effects

of these two basic phenomena

have

been segregated

in the present study and a better identification of the two has been brought out.

At least

three different stages or 'Regimes', viz., 'Regime-1', 'Regime-2t and 'regime-3f have been identified

from the results of the present work. ?Regime-1

^f ^pertains

to impact-induced erosion where impact of the raindrops felt

at the target sediment bed controls

the erosion phenomenon.

In 'Eegime-3' the erosion

rates are controlled by diffusing kinetic energy of rain through the overflowing water depth.

In 'Regime-2' which falls in between the

above two Regimes,

the erosion rates are controlled solely neither by 'Regime-1' nor by 'Regime

-3';

^rather

they

are controlled by the joint

effects

^{of these}Regimes moderated by increasing depths of the (spatially varied) overland flow.

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The experimental data have been analysed by making use of non-dimensional paraL eters which have been derived

from theoretical considerations, though only qualitatively.

These pertain individually to each of the situations

mentioned above. The erosion rates in each of the threee Regimes have been expressed in terms of weight rate of

sediment transport function, 0 (of the• same form as ,instein ► s 0-function, Kalinske function, Bagnold function, etc.) in the form 0 ₌W

SgV_ S_~j~gd

where W = dry weight of eroded material in Iigf/m width/s, )s and are mass densities of sediment and water, respectively, in msl/m3 , d = mean particle size in in and g = acceleration due to gravity in m/ s2. The impact functions L'%, much in the same manner as ' nsteinf s

^T-

function (or Bagnold► s 9-function) in Fluvial Hydraulics, has been evolved to escribe the

impact-controlled erosion in 'Regime-1'. Likewise the kinetic energy fuznction, e 9 again in the same way as

Einstein t s i -function has been evolved and this is found to describe the erosion when it is controlled by diffusion of kinetic energy through the overflow depth of water in

TR.egime_3t. The forms of these

~lli-,

and ) - functions

are 1, i - 1 ~~ jgd2/((-fir)

an

d [. ` )/ 3/ 2g 3/ 2 d 5/ 2/ (Qvr )

Te: s J

where Q(=LI) is the discharge intensity through the rain- simulator in m3/m width/s, and L = length of test bed in m

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and I = intensity of rain in m/sec over a unit area (one metre square) of the test bed, and v. fall velocity of raindrops in the vicinity of target sediment bed in m/ s.

In 'Regime-I t where the depth of flow i s small (not quantified in this study), the impact of rainfall controls the erosion rates. The analysis of the experimental data in this Regime has been presented through graphical relations

between 0 and Z and the results have been found to depend on target sediment bed slope, S and the relative ratio of the several raindrop diameters, (dr1/dr2) which can be more

effectively considered as (d/ dr2)/ (d/dr1)' where dr1 and d r2 are the mean diameters of the raindrops for different

raindrop forming tubing tips and d, as before, i s the mean particle size. It has been indicated that the representation of data through such non-dimensional parameters can be a much better form of studying the erosion rates under the effect of the impact of the rainfall.

The analysis of the experimental data in 'Regime-31 ^, where the diffusion of kinetic energy of the rainfall through the water depth (when depths are relatively larger) seems to

control the rates of erosion, has been presented through graphical relations between 0 and ' and found to depend also on target sediment bed slope, S. It has been inferred that the individual raindrop sizes do not seem to have any influence on the erosion rates in this Regime.

The results of the analysis of data, in 'Regime-2' where the impact force as well as the kinetic energy of the

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raindrops jointly control the erosion, through

0

and ₁

as well as ~ have also ben represented graphically and the transitional phenomenon in this case has been scrutinized.

Equations reported in literature relating erosion rates particularly to bed slope, particle size, etc. and also to the terosion indext (defined by El where E is the total energy of a rainstorm and I is the maximum 30-minutes intensity of rain) have been reviewed, Attention has been

given to the doubts that had been r wised about adopting

rain-simulators for studying rainfall-induced erosion (when

flowing as sheet flow) and only tentative suggestions have

been possible.

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CONTENTS

Page

ACKNOWLEDGEI NTS (I)

CERTIFTCATE (iii)

SYNOPSIS (iv)

CONTENTS (ix)

LIST OF FIG RES (xiii)

LIST OF TABLES (xx)

NOINCLATURE (xxiii)

CHAPTER I INTRODUCTION AND P1 BLEM STATEMENT 1

1.1 Background of the Study 1

1.2 The Geological Process of Sedimentation 3

1.3 Soil Erosion Process 5

1.4 Classification of Soil Erosion 7 1.5 The Ill-effects of Soil Erosion 9

1.6 Control of Soil Erosion 11

1.7 Mechanics of Soil Erosion 11 1.8 The proach in the Present Study 13

CHAPTER II REVIEW OF LITERATURE 17

2.1 General 17

2.2 Soil Erosion under Condition of Rainfall 17 2.3 Effect of Rainfall on Overland Flow 4g 2.4 Rainfall Characteristics and Rainfall

Simulation 59

2.5 Estimation of Drop-size and Drop-velocity

of Artificial Rainfall 64

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CHAPTER III THEORETICAL CONCEPTS

73

3.1

Defining the Variables influencing Soil

Erosion

73 3.2 Summary of the

Variables of the Problem

76 3.3 The deight

Rate of Transport

77 3.4

The Parameters causing the Sediment Transport

78

3.4.1

Non-dimensional Parameter for the

Rainfall Impact

79

3.4.2

Non-dimensional Parameter for the Diffusion of Kinetic Energy of

Rainfall 80

3.4.3

Other Non-dimensional Parameters

82 3.5

Summary of the pertinent Non-dimensional

Parameters 87

3.6 Other Possible Non-dimensional Parameters 88 CHAPTER IV EXPERIMENTS AND PRELIMINARY CONSIDERATIONS 90

4.1 Component Facilities 90

4.2 Variables to be considered 91 4.3 Features taken into account 91

4.4 Experimental at-up

9 2

4.4.1 The

Flume and General Lay-out 92 4.4.2 Details of the Components 104 4.4.2.1 The Rain-simulator(Fig.4.3) 104 1+.1+,2.2 The Supporting Structure 107

4.4.2.3

The Sand Trough (Figs. 1+.1+.a

and

4.4.b) 108

x

.1-. 2.4 The Sand Trap (Figs. 4.5.1

and

4.5.b)

₁₁₀

4.5 Preliminary ,Details of the Experiments 113

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4.5.1 Calibration of the Manometer and

the Performance of the Rain-i.mulator 113

4.5.2 The Bed Slopes 11)+

4

.5.3 Sediment Grains used for the Tests 115

4.5.4 Raindrop Sizes 117

4.5.5 Ranges of Raindrop Velocities 121

4.5.6 Bed Slopes adopted 125

1+,5,7 Placement of Rain-sinmulator and

Nominal Fall Height

¹

~5

4.5.8 Adjustment of Eroding Surface Slope

during Test Runs 126

4.5.9 Preparation of Sand Bed for any Test

Ran 127

4.5.10 Discharge Rates used for the Tests 128 1+.6 Test Procedure and collection of Eroded Sand 130

4.7 Bxperimental Results and Observations of the

Response of the Bed during Test R ns 173 4.7.1 Presentation of Basic Data 173 1+.7.2 1~sponse of the Bed: Bed Patterns,

Rates of Erosion, Mechanics of Erosion 174 4.7.2.1 'Regime-1': Erosion

controlled by the Impact of

the Rainfall 174

4.7.2.2 'Regime-2t : Transition in

Control of Erosion from Impact of Rainfall to Diffused Kinetic Energy of the Rainfall 177 4.7.2.3 'Regime-3': Erosion Controlled

by Diffusion of Kinetic Energy

of the Rainfall 178

4.8 Presentation of the Basic Data of the

Experiments 182

CHAPTER V ,ANALYSIS OF EXPERIMENTAL DATA AND U[SCUSSION 181+

5.1 General 184

5.2 Analysis of Data in 'Regirne-1' 185

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5.3 Analysis of Data in ?Regime-3R 2)+8 5.4 Consideration of QS versus tie in tRegime-1

1

269 5.5 Consideration of 56 versus ~~ in (Regime-3+ 281 5.6 Analysis of Data in 'Regime-2t 283

5.7 A Review of the 0 versus Y`-i Curves 303

5.8

A Review of the 0 versus

Ire

^Curves ³⁰⁹

5.9 An .Advantage in using two different Parameters,

and G'e: Other related Comments 309

CH PTER VI CONCLUSIONS 312

6.1 Summary and Conclusions 312

6.2 Points for + rther Study 317

BIBLIOGRAPHY 318

BIO-DATA 323