Variability in Stream Erosion and Sediment Transport

SOME TITLES RECENTLY PUBLISHED BY The International Association of Hydrological Sciences (IAHS)

In the IAHS Series of Proceedings and Reports

Erosion and Sediment T r a n s p o r t Measurement. Proceedings of a symposium held at Florence, June 1981 Publ.no. 133 (1981), price $20

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Drainage Basin Sediment Delivery. Proceedings of a symposium held at Albuquerque, August 1986 edited by R. F. Hadley Publ. no. 159 (1986). price $45, ISBN0-947571-80-9

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Regional Characterizat ion of Water Quality. Proceedings of a symposium held during the Third IAHS Scientific Assembly at Baltimore, May 1989 edited by S. Ragone Publ. no. 182 (1989), price S45, ISBN 0-947571-02-7

Sediment and the Envi ronment . Proceedings of a symposium held during the Third IAHS Scientific Assembly at Baltimore, May 1989 edited by R. F. Hadley & E. D. Ongley Publ. no. 184 (1989), price $40, ISBN0-947571-12-4

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Hydrology of Mountainous Areas . Proceedings of a workshop held at Strbské Pleso, Czech Republic, June 1988 edited by L. Molndr Publ.no.190 (1990). price S45, ISBN 0-S47571-42-6

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Research Needs and Applications to Reduce Erosion and Sedimentation in Tropical Steeplands. Proceedings of a symposium held at Suva, Fiji, June 1990 edited by R. R. Ziemer, C. L. O 'Loughlin & L. S. Hamilton Publ.no.192 (1990). price $50, ISBN 0-847571-52-3

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Hydrogeological Processes in Karst T e r r a n e s . Proceedings of a symposium and field seminar held at Antalya, October 1990 edited by G. Giinay, A. 1. Johnson & W. Back Publ.no.207 (1993), price $60, ISBN 0-847571-28-0

Erosion, Debris Flows and Environment in Mounta in Regions. Proceedings of a symposium held at Chengdu, July 1992 edited by D. E. Walling, T. R. Davies & B. Haslwlt Publ.no.209 (1992), price $75, ISBN 0-847571-38-8

Erosion and Sediment Transpor t Monitoring Programmes in River Basins. Proceedings of a symposium held at Oslo, August 1992 edited by J. Bogen, D. E. Walling & T. J. Day Publ.no.210 (1992), price $75, ISBN0-847571-43-4

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Variability in

Stream Erosion and

Sediment Transport

Edited by

L. J. OLIVE Department of Geography and Oceanography, University College, University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia

R. J. LOUGHRAN Department of Geography, University of Newcastle, Callaghan, New South Wales 2308, Australia

J. A. KESBY Department of Geography and Oceanography, University College, University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia

Proceedings of an international symposium held at Canberra, Australia, 12-16 December 1994. The symposium was organized by the Department of Geography and Oceanography, University College, University of New South Wales, Australian Defence Force Academy, and the International Commission on Continental Erosion of the International Association of Hydrological Sciences.

IAHS Publication No. 224 in the IAHS Series of Proceedings and Reports

Published by the International Association of Hydrological Sciences 1994. IAHS Press, Institute of Hydrology, Wallingford, Oxfordshire OX10 8BB, UK. IAHS Publication No. 224. ISBN 0-947571-19-1

British Library Cataloguing-in-PublicationData. A catalogue record for this book is available from the British Library.

IAHS is indebted to the Department of Geography and Oceanography, University College, University of New South Wales, Australian Defence Force Academy, and the Department of Geography, University of Newcastle, Callaghan, New South Wales, for the support and services provided that enabled the Editors to function effectively and efficiently.

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Special thanks are due to everyone who helped with the production of this Proceedings volume, in particular, Paul Ballard for drawing and modifying many of the figures and producing them camera ready, Sabina Clark and Eileen McEwan for retyping some of the manuscripts and Jai-shu Shen for his assistance in editing papers from Chinese authors,

The final camera-ready copy for the papers was finished, printed out, and assembled at IAHS Press by Penny Kisby, using edited files in WordPerfect 5.1 produced at Department of Geography and Oceanography, University College, University of New South Wales, Australian Defence Force Academy.

Printed in Great Britain by Redwood Books, Trowbridge, Wiltshire.

Preface

In recent years the International Commission on Continental Erosion (ICCE) of the International Association of Hydrological Sciences (IAHS) has organized a number of international symposia focusing on themes concerning erosion and sediment yield. Recent symposia include the Symposium on Sediment Budgets (Porte Alegre, 1988), the Symposium on Erosion and Sediment Transport Monitoring Programmes in River Basins (Oslo, 1992) and the Symposium on Sediment Problems: Strategies for Monitoring, Prediction and Control (Yokohama, 1993). In addition, there have been joint symposia held in conjunction with the IAHS International Commission on Water Quality — the Symposium on Sediment and Stream Water Quality in a Changing Environment: Trends and Explanation (Vienna, 1991) and the Symposium on Tracers in Hydrology (Yokohama, 1993). In most of these symposia, papers have raised the issue of variability in erosion and stream sediment transport. The Symposium on Variability in Stream Erosion and Sediment Transport arose from this background. It is fitting that the symposium was held in Canberra, Australia, as this is possibly the most hydrologically variable continent on Earth.

On a global scale soil erosion is one of the major contemporary environmental issues, while stream sediments provide the most common host for contaminants and nutrients. As a result, there is a growing need to understand better the processes involved in sediment erosion and transport in order to isolate sediment sources, characterize transport behaviour and determine the fate of eroded material. In many cases, an understanding of the processes involved is clouded by the natural variability within the sediment system at a range of spatial and temporal scales. Research designs and methodologies must be capable of accurately characterizing the sediment system especially with respect to its variability. This becomes more important as human impacts on stream systems increase and also if we are to make meaningful predictions of changes related to global environmental changes. In both cases we must be able to disentangle natural variability and that associated with basin change.

The 54 papers in this proceedings volume cover a wide range of topics. They have been grouped into six main themes. The papers on Soil erosion, sediment transport and sediment tracers examine variability in erosion and transport, especially the implications of variability on the techniques used. This theme continues in the next section on Flood plains and lake sedimentation which focuses on determining the fate of eroded material and the use of such deposits in the interpretation of past processes and environmental history. The next two sections, Large basins and regional variation and Small basins, concentrate on variability at a range of spatial scales. The penultimate section

vi Preface

examines Human impacts on erosion and sediment transport systems. The final section titled Techniques outlines the application of new research techniques. It is hoped that these papers will lead to greater consideration of the role of variability in erosion and sediment transport especially in the establishment of adequate research and monitoring designs.

Laurie Olive & Julie Kesby Department of Geography and Oceanography,

University College, University of New South Wales, Australian Defence Force Academy, Australia

Bob Loughran Department of Geography, University of Newcastle, Australia

Contents

Vil

Preface by Laurie Olive, Julie Kesby & Bob Loughran

1 Soil Erosion, Sediment Transport and Sediment Tracers

Variability of gravel movement on the Virginio gravel-bed stream (central Italy) during some floods Corrado Cencetti, Paolo Tacconi, Mario del Prête & Massimo Rinaldi 3

The significance of extreme events in the development of mountain river beds Carmen de Jong 13

Interpreting the temporal and spatial dynamics of bed load transport phases according to FAST (Fluid and Sediment Transfer Model) Carmen de Jong & Peter 25 Ergenzinger

Relationships between rill sediment and flow time varying with freezing, groundcover, compaction and slope on a Prince Edward Island (Canada) fine sandy loam Linnell Edwards, J. R. Burney & P. A. Frame 33

The susceptibility of valley slopes and river beds to erosion and accretion under the impact of climatic change — Alpine examples Peter Ergenzinger 43

Quantifying soil erosion and sediment transport in drainage basins; some observations on the use of 137Cs /. D. L. Foster, H. Dalgleish, J. A. Bearing & 55 E. D. Jones

Origins of variability in the 230Th/232Th ratio in sediments Jon M. Olley & Andrew S. Murray 65

Some observations on bed load movement in a small stream in Hong Kong M. R. Peart & A. W. Jayawardena 71

A comparison of the roles of tillage and water erosion in landform development and sediment export on agricultural land near Leuven, Belgium Timothy A. Quine, Philippe J. J. Desmet, Gerard Govers, Karel Vandaele & Desmond E. Walling 77

Sediment movement on hillslopes measured by caesium-137 and erosion pins Michael J. Saynor, Robert J. Loughran, Wayne D. Erskine & P. F. Scott 87

Measuring soil movement using 137Cs: implications of reference site variability P. J. Wallbrink, J. M. Olley & A. S. Murray 95

Sediment production and storage in an urbanizing basin, Lake Macquarie, New South Wales, Australia Barbara Whitelock & Robert J. Loughran 103

Flood Plains and Lake Sedimentation

Cyclical construction and destruction of flood dominated flood plains in semiarid Australia Mary C. Bourke 113

Vll l Contents

Lake sediments as indicators of recent erosional events in an agricultural basin on the Canadian prairies Dirk H. de Boer 125

The record of extreme hydrological and geomorphological events inferred from glaciolacustrine sediments Joseph R. Desloges & Robert Gilbert 133

Sand slugs generated by catastrophic floods on the Goulburn River, New South Wales Wayne D. Erskine 143

Variability in the physical, chemical and magnetic properties of reservoir sediments; implications for sediment source tracing I. D. L. Foster & S. M. Charlesworth 153

Use of Chernobyl-derived radiocaesium to investigate contemporary overbank sedimentation on the flood plains of Carpathian rivers W. Froehlich & D. E. Walling 161

Temporal variability in streambank response to individual flow events: the River Arrow, Warwickshire, UK D. M. Lawler 171

Rates of formation of forms in a river channel hierarchy: the case of the River Yana in northeast Russia Boris Matveev, Andrey Panin & Alexey Sidorchuk 181

Post-glacial temporal variability of sediment accumulation in a small alpine lake Phil Owens & Olav Slay maker 187

Channel processes and erosion rates in the rivers of the Yamal Peninsula in western Siberia Alexey Sidorchuk & Boris Matveev 197

Rates of overbank sedimentation on the flood plains of several British rivers during the past 100 years D. E. Walling & Q. He 203

Temporal and spatial variations in erosion and sedimentation in alternating hydrological regimes in southeastern Australian rivers Robin F. Warner 211

3 Large Basins and Regional Variation

Sediment transport in the Rio Grande, an Andean river of the Bolivian Amazon drainage basin Jean Loup Guyot, Jacques Bourges & Jose Cortez 223

Sediment transport in the Fly River basin, Papua New Guinea Andrew Markham & Geoffrey Day 233

Spatial variation in suspended sediment transport in the Murrumbidgee River, New South Wales, Australia L. J. Olive, J. M. Olley, A. S. Murray & P. J. Wallbrink 241

Spatiotemporal variability of sediment transport in arid regions K. D. Sharma, N. S. Vangani, M. Menenti, J. Huygen & A. Vich 251

Sediment transport observations in Switzerland Manfred Spreafico & Christoph Lehmann 259

Annual and decadal variation of sediment yield in Australia, and some global comparisons R. J. Wasson 269

Temporal and spatial variation of sediment yield in the Snowy Mountains region, Australia Bofu Yu & David Neil 281

Contents j x

Sediment transport in the Yangtze basin Zhou Gangyan & Xiang Zhian 291

4 Small Basins

Temporal variability of suspended sediment transport in a Mediterranean sandy gravel-bed river Ramon J. Batalla & Maria Sala 299

The contribution of gully erosion to the sediment budget of the River Leira J. Bogen, H. Berg & F. Sandersen 307

Long-term variability of sediment transport in the Ombrone River basin (Italy) Anna Frangipane & Enio Paris 317

Water and sediment discharge from glacier basins: an arctic and alpine comparison A. M. Gurnell, A. Hodson, M. J. Clark, J. Bogen, J. O. Hagen & M. Tranter 325

Variability in discharge, stream power, and particle-size distributions in ephemeral-stream channel systems L. J. Lane, M. H. Nichols, M. Hernandez, C. Manetsch & W. R. Osterkamp 335

Recent changes in rates of suspended sediment transport in the Jôkulsâ a Sôlheimasandi glacial river, southern Iceland D. M. Lawler 343

Comparative modelling of large watershed responses between Walnut Gulch, Arizona, USA, and Matape, Sonora, Mexico M. H. Nichols, L. J. Lane, H. M. Arias & C. Watts 351

Debris flows in northeastern Victoria, Australia: occurrence and effects on the fluvial system Ian D. Rutherfurd, Paul Bishop & Tim Loffler 359

5 Human Impacts

Erosion and sediment yields in the Kakadu region of northern Australia Kate Duggan 373

Analysis of sediment transport developments in relation to human impacts Helmut M. Habersack & Hans P. Nachtnebel 385

Land-use effects on magnitude-frequency characteristics of storm sediment yields: some New Zealand examples D. Murray Hicks 395

Suspended sediment from two small upland drainage basins: using variability as an indicator of change R. C. Johnson 403

The impact of run-of-river hydropower plants on temporal suspended sediment transport behaviour W. Summer, W. Stritzinger & W. Zhang 411

A study of the accumulation rate of the lower Yellow River in the past 10 000 years Xu Jiongxin 421

Variability of sediment load and its impacts on the Yellow River Long Yuqian, Qian Yiying, Xiong Guishu & Xu Mingquan 431

X Contents

6 Techniques

LISEM: a new physically-based hydrological and soil erosion model in a GIS-environment, theory and implementation A. P. J. de Roo, C. G. Wesseling, N H. D. T. Cremers, R. J. E. Offermans, C. J. Ritsema & K. Van Oostindie 439

A methodology for quantifying river channel planform change using GIS S. R. Downward, A. M. Gurnell, A. Brookes 449

Spatial distribution of sediment discharge to the coastal waters of South and Southeast Asia Avijit Gupta & Parthiphan Krishnan 457

Long-term records of sediment yield from coral skeletons: results from ion beam analyses David Neil, Stan Newman & Peter hdale 465

Estimating the impacts of global change on erosion with stochastically generated climate data and erosion models A. D. Nicks, R. D. Williams & G. A. Gander 473

Flocculation of fine-grained suspended solids in the river continuum M. Stone & 1. G. Droppo 479

Temporal variations of environmental pollutants in channel sediments W. Symader, M. Schorer & R. Bierl 491

1 Soil Erosion, Sediment Transport and Sediment Tracers

Variability in Stream Erosion and Sediment Transport (Proceedings of the Canberra Symposium December 1994). IAHS Publ. no. 224, 1994.

Variability of gravel movement on the Virginio gravel-bed stream (central Italy) during some floods

CORRADO CENCETTI, PAOLO TACCONI Environmental Engineering Institute, University of Perugia, S. Lucia Canètola, 1-06125 Perugia, Italy

MARIO DEL PRETE Technical-Economical Department, University of Basilicata, Via N. Sauro 85, 1-85100 Potenza, Italy

MASSIMO RINALDI Department of Earth Sciences, University of Firenze, Via G. La Pira 4, 1-50121 Firenze, Italy

Abstract A continuous-operation bed load measuring station was installed on the Virginio gravel-bed stream (central Italy), which has a median grain size of about 16 mm (-5 phi) and a straight channel with alternate bars. The slope of the stream at this point is about 0.008 and the basin has an area of 40 km2. The introduction of a marked sample of about 4000 pebbles upstream from the measuring station made it possible to study the relationships between bed load transport and the passage of pebbles through the reach above the station. It was possible after each flood to measure the position of the pebbles remaining on the surface of the river bed. The results of this study confirm the pulsating nature of bed load transport and its poor correlation with grain-size distribution and water discharge. For each grain-size class, the distance travelled by the pebbles, and their embedding were measured.

INTRODUCTION

The processes of pebble transport in gravel-bed rivers are little known, as is the transport rate. Measurement is very difficult and, apart from small amounts of data gathered in experimental drainage basins and in laboratory flumes, it has been almost impossible to assess either processes on a large scale with great enough accuracy using present-day methods (Emmett, 1981; Tacconi, 1982).

A vortex-tube bed load trap (phi > —1) was installed on Virginio Creek, in the Arno basin, near Baccaiano in Tuscany (Fig. 1). The drainage basin has an area of 40 km2 and is underlain by Pliocene marine sediments composed of silts, sands and conglomerates. The area is used for cultivating grain, vines and olive trees. It has a Mediterranean climate with an average annual precipitation of 900 mm (Tacconi & Billi, 1987).

The river bed upstream from the measuring station is straight with alternate bars. The banks are thickly vegetated. The average channel width is 12 m and the channel gradient is 0.008.

4 Corrado Cencetti et al.

Fig. 1 Location of study area.

OBSERVATIONS AND MEASUREMENTS

The 1988 experiments consisted of the introduction of a marked sample into a section 250 m upstream from the measuring station and the subsequent analysis of the distances travelled by all visible clasts (not those buried in the river bed) in each grain-size class after the four flood events. For operational reasons, not all the measurements of sediment transport could be continued throughout the entire duration of the observation period.

The marked sample was prepared and called "YELLOW". It ranged in size between - 4 and - 7 phi and was composed of 3935 pebbles weighting a total of 337 kg. Besides this "YELLOW", a "RED" sample was introduced composed of 300 pebbles with sizes between -4.5 and —5.5 phi and representative of the three forms in Folk's shape classification (spherical, discoidal, elongate). The pebbles were taken from Virginio Creek in order to ensure identical lithological features (limestones) to those normally found in the river bed. The clasts of the "RED" sample were numbered, enabling close study of their movements.

Table 1 shows the grain-size data of the samples of the river bed and of the marked sample.

OBSERVED FLOOD EVENTS

Flood no. 1 (14 April 1988)

This was a small event with a maximum discharge of about 3 m3 s"1 and a duration of 24 h. The sediment discharge was not measured.

Variability of gravel movement on the Virginio, central Italy

Table 1 Grain-size features of sediments in the river bed and the marked sample.

Mean diameter (phi)

Standard deviation (phi)

Skewness

Kurtosis

D10 (phi)

D50 (phi)

D90 (phi)

Grain-size classes

YELLOW sample

-4 .0

-4 .5

-5 .0

-5 .5

-6 .0

-6 .5

-4 .0

RED sample

-4 .5

-4 .5

-5 .0

-5 .5

-6 .0

-6 .5

-7 .0

-7.0

-5 .5

Bars:

Armour

-5.96

0.687

0.558

3.413

-5.05

-6.03

-6.83

Number of clasts

2000

1000

500

250

125

60

3935

300

Subarmour

-4.93

1.287

1.394

5.187

-3.28

-5.18

-6.28

Weight (kg)

20

28

32

50

80

127

337

8.5

Channel armour

-6.38

0.634

0.679

3.150

-5.51

-6.48

-7.18

Mean weight of clasts (g)

10

28

64

200

640

2117

85.6

28.3

YELLOW sample

-6.03

0.77

0.87

2.64

-4.74

-6.24

-6.87

Morphometry

n. 100 elongate n. 100 discoidal n. 100 spherical

Flood no. 2 (20 May 1988)

This was the largest flood of the four observed and was associated with four precipitation events. A first peak of about 4 m3 s"1 was registered followed by the mean peak with a discharge of 17 m3 s"1. The record of sediment discharge is not complete; in fact, by the time that the measuring station was made operational, some marked pebbles were found to have entered the vortex-tube and these must have travelled along a section of the order of 250 m. Unfortunately, 30 minutes after the start of the recordings, and due to operational problems connected with the high rate of bed load transport, measuring was stopped for 50 minutes. A maximum bed load transport of 580 kg min"1 was recorded, and 2110 kg in 5 minutes was recorded. This is the highest level of sediment discharge ever recorded at the measuring station since its installation in 1983. The total sediment discharge of this flood was about 43 260 kg. Although

6 Corrado Cencetti et al.

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20

18

16

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0

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900

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300

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20 May 1988 FLOOD

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100 200 300 400

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500 600

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STATISTICAL PARAMETERS OFBEDLOAD GRAIN-SIZE DISTRIBUTION

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Sd (phi)

Sk

Ku

010(phi)

D50 (phi)

D90 (phi)

=̂T̂

_ i _

300 400

Time (min)

(b)

Fig. 2 20 May 1988 flood: (a) rainfall, discharge and bed load; (b) statistical parameters range for the bed load samples. Dm: mean diameter; Sd: standard deviation; Sk: skewness; Ku: kurtosis; D10, D50, D90: diameters for 10, 50, 90 percentiles.

interrupted, measuring continued for more than 48 h, during which further data was not gathered. The recorded data are shown in greater detail in Fig. 2.

Flood no. 3 (28 May 1988)

Despite being smaller, the third flood carried all clast sizes. The length of the flood wave was 7 h, with a peak discharge of 6 m3 s"1. The maximum sediment discharge measured was 125 kg min"1 while the total sediment discharge was 2780 kg. A few isolated arrivals of bed load preceded a real state of continuous motion. The initial

Variability of gravel movement on the Virginio, central Italy

o -

2 -

4 -

6 -

8 -

1 o -6 "

. 3" I 2" o

5 ! -

0 -

3 0 0

— 2 0 0 -

V 200

Time (min)

STATISTICAL PARAMETERS OF BED LOAD GRAIN-SIZE DISTRIBUTION

Dm (phi)

Sd (phi)

Sk

Ku

D10(phi)

D50(phi)

D90(phi)

(b)

Fig. 3 28 May 1988 flood: (a) rainfall, discharge and bed load; (b) statistical parameters range for the bed load samples. Dm: mean diameter; Sd: standard deviation; Sk: skewness; Ku: kurtosis; D10, D50, D90: diameters for 10, 50, 90 percentiles.

condition of sediment movement was easily recognizable, and at a water discharge of little more than 2 m3 s"1 (Fig. 3). The sediment supply was exhausted quickly in this event soon after the peak discharge.

Flood no. 4 (5 June 1988)

This event lasted a considerable amount of time and had a peak discharge of 4.9 m3 s"1. Although instantaneous data for sediment discharge are not available, it was observed that, at the end of the event, the vortex-tube contained 450 kg of material instead of its

8 Corrado Cencetti et al.

maximum capacity of 700 kg. This result may be taken as a measure of total bed load transport. This material was composed of fine sediments and of mud balls with a few pebbles of medium size. It may be concluded that the event was characterized by high concentration of suspended sediment linked to pronounced bank and slope erosion, and to the supply from the tributaries situated upstream to the measuring station. The small amount of bed load may be explained by a higher degree of armouring of the river bed. However, the hydrograph does not display abrupt variations in discharge even during the rising stage and this may also account for low bed load discharge. Moreover it could be a bed load from lateral sedimentary supply, which was then deposited and remained stable on the armour. This is supported by means of the marked pebbles analysis. In fact the number of marked pebbles after flood no. 4 is much less than before; this may be due to the pebbles burying previously transported ones.

MOVEMENT OF THE MARKED PEBBLES

The clasts that were visible after each flood were examined in order to determine their dimensions and the distance they had travelled.

During periods in which bed load transport rates were being measured, any marked clasts arriving in the vortex-tube were recorded and thrown immediately back into the flow downstream from the measuring station; during periods in which the measuring station was not in operation all clasts, except those filling the vortex-tube passed freely downstream from the measuring station. This enabled us to follow the "cloud" of clasts even in the section downstream from the measuring station. The position of the clasts related to each flood event is shown in Fig. 4 while Fig. 5 shows the distance range of the marked pebbles for each grain-size class.

Flood no. 1 brought about only a slight redistribution of the marked sample, with a maximum travel distance recorded of 18 m. Bed load discharge was not determined

V -7,0

20 May 1988 flood © 28 May 1988 f lood-B—

5 June 1988 flood Q

100 200 300 400 500 600 700 800

Distance of the marked pebbles (m) from the initial point

9 0 0 1000

Fig. 4 Distribution of marked pebbles after the floods as a function of their relative size.

Variability of gravel movement on the Virginio, central Italy

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<̂ Number of clasts K gathered

CD

ii

• Illl

'r

D Flood n°2

0 Flood n°3

H Flood n°4

in co m OÎ Ol O

$§fe

-4,5 -5 -6.5 -5.5 -6.0

Size of marked pebbles (phi)

Fig. 5 Distance range of marked pebbles (m) as a function of their relative size.

-7

and this flood was used to move the marked sample into positions that would be closer to a natural condition.

Flood no. 2 caused the greatest movement of marked pebbles as a corollary of the greater sediment discharge; the marked clasts were discovered on the surface up to 666 m away from the introduction point (i.e. they had moved through the bed load trap).

Following floods no. 3 and no. 4, the maximum distances travelled by marked pebbles were 744 m and 776 m respectively. The centroid of the pebble cloud was 235 m from the introduction point after the second flood; 246 m after the third flood and 192 m after the fourth flood.

The river bed survey continued for another 1000 m downstream from the last pebble found in the river bed; consequently, that part of the sample that was not found must have been buried in the bed of the channel.

OBSERVATIONS AND DISCUSSION

Observations of bed load

The two events for which bed load discharge measurements are available, confirm its extreme temporal variability and its pulse-like nature (Tacconi & Billi, 1987).

During flood no. 3, a few short bursts of bed load may be noted which greatly precede the general wave of mobile sediment. These early arrivals could be a result of either partial lateral erosion of bars and/or banks, or contributions from tributaries, or the movement of clasts deposited on top of the armour during the terminal stage of the preceding flood (20 May 1988).

10 Corrado Cencetti et al.

Observations of the YELLOW sample

There are many observations to be made here, yet it should remembered that during flood no. 4 there was an anomalous deposition of sediments and very low bed load transport rates.

Number of rediscovered pebbles

The first observation concerns the number of pebbles rediscovered in the river bed after floods (Fig. 6). This amounts to 8.6% of the total after flood no. 2; 8.4% of the total after flood no. 3 and 4.7% of the total after flood no. 4. This last percentage may be explained by deposition of sediments as described earlier. This means that 91.2 % of the

diameter of pebbles (phi) diameter ol pebbles (phi)

60 - \ m

<N Number ot *" rediscovered pebbles

o Weight (g) of ^ rediscovered pebbles

0 j , , , . .

- 7 , 0 - 6 , 5 -6 ,0 -5 ,5 -5 ,0 - 4 ,5 - 4 , 0

diameter of pebbles (phi)

Fig. 6 Relationships between size of marked pebbles (phi) and percentage of pebbles rediscovered in relation to the marked sample (%) for each grain-size class.

Variability of gravel movement on the Virginio, central Italy 11

pebbles remained buried after flood no. 2; 91.6% after flood no. 3 and 95.3% after flood no. 4. This rate of burial is very high.

If we look at this phenomenon in relation to each grain-size class, we see how this rate is very high for the small grain-sizes and diminishes exponentially with an increase in clast size (Fig. 6). The percentage of the clasts rediscovered after flood no. 2 in terms of weight is 34.3 % (115 752 g); after flood no. 3 it is 31.3 % (105 449 g) and after flood no. 4 it is 29.2% (98 572 g).

Shifting of marked pebbles

A synthetic view of the clasts' movements of the YELLOW sample is shown in Figs 4 and 5, where the statistical parameters of the movements of marked pebbles, subdivided into grain-size classes, are displayed.

Regarding the three observed floods, in all the grain-size classes there are pebbles that hardly moved during the floods. The average distances away from the introduction point are almost alike for the grain-size classes, below to D50, with a slight rise with the increase in phi and a sharp decrease when phi is greater than D50. There is similar behaviour, though less accentuated, in the maximum distances.

The correlation between bed load transport obtained experimentally in natural rivers and hydraulic parameters such as discharge are much lower than expected (Billi & Tacconi, 1987). The variability of coarse sediment transport is very high and is characterized by pulses that have been both to morphological-sedimentary features of the river bed and to variations in discharge (Klingeman & Milhous, 1970; Hayward & Sutherland, 1974; Meade et al., 1981; Reid et al., 1985; Tacconi & Billi, 1987).

Acknowledgements This research is part of a study on "Problems of Solid Transport in Natural Water Channels" supported by the Ministry for University Research in Science and Technology (40% funds) and by the National Research Council (National Group for Defence against Hydrogeological Catastrophes). The authors wish to thank both organizations.

REFERENCES

Billi, P. & Tacconi, P. (1987) Bed load transport processes monitored at Virginio Creek measuring station, Italy. In: Int. Geomorphology, Part. 1, 549-559. John Wiley, Chichester, UK.

Emmett, W. W. (1981) Measurement of bed load in rivers. In: Erosion and Sediment Transport Measurement (Proc. Florence Symp., June 1981), 3-15. IAHS Publ. no 133.

Hayward, J. A. & Sutherland, A. J. (1974) The Torlesse Stream vortex-tube sediment trap. J. Hydrol. 13(1), 41-53. Klingeman, P. C. & Milhous, R. T. (1970) Oak Creek vortex bed load sampler. Proc. 17th Annual Pacific North-West

Meeting of the Am. Geophys. Union (Univ. of Puget Sound, Tacoma, Washington, October 1970). Meade, R. H., Emmett, W. W. &Myric, R. M. (1981) Wavelike movement of bed load, East Fork River, Wyoming, USA

(Abstract). Conf. on Modern and Ancient Fluvial System, Sedimentology and Processes (University of Keele, UK). Reid, I., Frostick.L. E. & Layman, J.T. (1985) The incidenceand nature of bed load transport during flood flows in coarse

grained alluvial channels. Earth Surf. Proc. andhandforms 10, 33-44. Tacconi, P. (1982) La misura del trasporto solido nei corsi d'acqua (Bed load transport measurements in river beds). In:

Proc. Final Congress of "Prog. Final.: Conservazione del Suolo" (CNR, Rome), 103-128. Tacconi, P. & Billi, P. (1987) Bed load transport measurements by the vortex-tube trap on Virginio Creek — Italy. In:

Sediment Transport in Gravel-bed Rivers, 583-615. John Wiley, London.