Henshaw, Alexander J. (2009) Impacts of land use changes and land management practices on ...

286 Pages·2017·13.96 MB·English

by  Christine Middleton

IMPACTS OF LAND USE CHANGES AND LAND MANAGEMENT PRACTICES
ON UPLAND CATCHMENT SEDIMENT DYNAMICS:
PONTBREN, MID-WALES

Alexander J. Henshaw, BSc.

Thesis submitted to the University of Nottingham
for the degree of Doctor of Philosophy

December 2009
ABSTRACT

There is growing concern that the adoption of intensive agricultural land
management practices in upland areas of the UK over the past 50-60 years may have
affected hydrological responses and sediment transfer regimes in river catchments
and could, therefore, be contributing to increased levels of flood risk and ecological
disturbance. However, recent evidence from a research catchment at Pontbren in
mid-Wales indicates that the implementation of a more sustainable livestock
farming strategy could help to mitigate some of these impacts, raising the possibility
that strategic land use planning could be used as a cost-effective, multi-functional
river management option. The impacts of historical land use changes and land
management practices on contemporary sediment dynamics in the study area are
explored in this thesis through a system approach which acknowledges the
importance of interrelationships between hydrological and geomorphological
processes. Results from hydrological experiments and modelling exercises are used
to inform analyses of spatial and temporal variation in sediment production and
transfer from a variety of potential sources. Grazed, agriculturally-improved
pastures were found to supply fine material to stream channels via both surface
runoff and field drains. In particular, drain-derived sediment is likely to represent an
important component of the total fine sediment yield in subcatchments where
agricultural intensification has been widespread. Agricultural drainage ditches were
also found to act as sources of sediment in such areas, along with eroding channel
banks. Sediment production from bank sources may relate to historical changes in
peak flows caused by agricultural intensification. Stream sediment yields are
strongly related to differences in sediment supply from the aforementioned sources
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and could therefore be reduced by limiting mobilisation at the point of origin within
the landscape. In terms of channel-derived material, this could be achieved through
peak flow reductions associated with woodland and hedgerow restoration.

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ACKNOWLEDGEMENTS

This research forms part of a multi-disciplinary programme undertaken by the Flood
Risk Management Research Consortium (http://www.floodrisk.org.uk).
The Consortium is funded by the UK Engineering and Physical Sciences Research
Council (EPSRC) under grant GR/S76304/01, jointly with NERC, the Joint Defra/EA
Flood and Coastal Erosion Risk Management R&D programme, the Scottish
Executive, the Rivers Agency (Northern Ireland) and UK Water Industry Research.
The work was supported by a three year EPSRC studentship (EP/P5011431/1) and
completed during employment as a research associate at the School of Geography,
University of Nottingham.

I would like to express my gratitude to Colin Thorne for the opportunity to conduct
this investigation in collaboration with a fantastic group of researchers from a
variety of academic disciplines; Nick Clifford for thought-provoking discussions,
advice, and generous donations of field equipment; and Ian Conway, Graham Morris
and Teresa Needham for assistance in the workshop and laboratory.

Access to field sites at Pontbren and accommodation was kindly provided by local
residents and landowners. I am particularly grateful to Roger Jukes, Keith and Terri
Blanchard, Alun Davies, David Rowlands, Margaret Hughes, Joyce Cornes and UPM
Tilhill in this respect. I would also like to thank the following members of the
Pontbren research team: Miles Marshall (Imperial College) and Matt Clifford (CEH
Bangor) for collecting hydrological data; Beth Jackson (Imperial College/Victoria
University of Wellington) for patient explanations of hydrological models and
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MATLAB; Tim Pagella (University of Wales Bangor) for collating local knowledge of
erosion and changes in channel morphology; and David Jenkins (Coed Cymru) for
invaluable background information and historical photographs.

This research would not have been possible without the fantastic support I have
received from my family and friends. Particular thanks go to my parents, Chris and
Margaret, my grandparents, Jonny, Ellie, Chris, Nick and Gem for all their
encouragement, help, ideas and guidance, especially when things got tough.

This thesis is dedicated to “Jack” Morton.
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CONTENTS

List of Figures ix
List of Tables xvi

1 INTRODUCTION 1
1.1 Introduction 1
1.2 Aims and objectives 3
1.3 Thesis structure 4

2 POTENTIAL EFFECTS OF AGRICULTURE ON SEDIMENT 7
DYNAMICS IN BRITISH UPLAND CATCHMENTS: BACKGROUND
TO RESEARCH AT PONTBREN
2.1 Introduction 7
2.2 Agricultural intensification in the British uplands 7
2.3 Geomorphological impacts of agricultural intensification in the 13
British uplands
2.4 Background to experimental research at Pontbren 19
2.4.1 Overview of the Pontbren study area 19
2.4.2 Historical changes in land use and land management in the 25
Pontbren study area
2.4.3 Implications of changes in land use and land management for 32
erosion and sediment transfer in the Pontbren study area:
preliminary assessment
2.4.4 Research approach 44

3 DYNAMICS AND CONTROLS OF SOIL EROSION IN A GRAZED, 49
AGRICULTURALLY IMPROVED UPLAND PASTURE AND THE
TRANSFER OF MOBILISED SEDIMENT TO STREAM CHANNELS
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3.1 Introduction 49
3.2 Field site 52
3.3 Data collection 55
3.1.1 Experimental infrastructure and hydrometeorological data 55
3.3.2 Event-scale sediment yield measurement 57
3.3.3 Long-term sediment yield measurement 62
3.4 Results and discussion 64
3.4.1 Meteorological characteristics of the field site during the 64
experimental monitoring period
3.4.2 Runoff characteristics of the field site 69
3.4.3 Event-scale sediment yield dynamics 79
3.4.4 Long-term sediment yields 92
3.5 Summary 99

4 DYNAMICS AND CONTROLS OF SEDIMENT SUPPLY FROM 101
ERODING CHANNEL BANKS TO STREAM CHANNELS IN THE
UPPER PONTBREN SUBCATCHMENT
4.1 Introduction 101
4.2 Methodology 103
4.2.1 Measurement of bank erosion and estimation of sediment 103
supply from bank sources in the Upper Pontbren subcatchment
4.2.2 Exploration of temporal and spatial variation in bank erosion 109
rates in relation to hydrometeorological variables and site-
specific parameters
4.3 Results 115
4.4 Discussion 136

5 DYNAMICS AND CONTROLS OF BEDLOAD YIELDS FROM 155
HEADWATER SUBCATCHMENTS AT PONTBREN
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5.1 Introduction 155
5.2 Methodology 156
5.2.1 Bedload yield and particle size measurement 156
5.2.2 Bedload transport dynamics 165
5.3 Results and analyses 170
5.3.1 Spatial and temporal variation in bedload yields from 170
headwater streams in the Pontbren study area
5.3.2 Relationships between specific stream power-based indices of 181
bedload transport potential and bedload yields from headwater
streams in the Pontbren study area
5.3.3 Influence of sediment supply on bedload yields from headwater 193
streams in the Pontbren study area
5.4 Discussion: the influence of historical land use changes and 196
land management on bedload yields from headwater streams
in the Pontbren study area

6 DYNAMICS AND CONTROLS OF SUSPENDED SEDIMENT YIELDS 207
FROM HEADWATER SUBCATCHMENTS AT PONTBREN
6.1 Introduction 207
6.2 Methodology 207
6.2.1 Suspended load measurement and sediment yield calculation 207
procedure
6.2.2 Assessment of controls on suspended sediment transport in 210
headwater streams at Pontbren
6.3 Results and discussion 213
6.3.1 Suspended sediment transport in the Nant Pen-y-cwm and 213
Nant Melin-y-grûg
6.3.2 Exploring spatial and temporal variation in suspended sediment 219
transport at Pontbren
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7 CONCLUSIONS 233
7.1 Conclusions 233
7.2 Recommendations for further research 238

REFERENCES 240

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LIST OF FIGURES

Figure 1.1 Known and potential process-response linkages relating to 6
agricultural intensification in upland Britain and thesis
structure.
Figure 2.1 Distribution of land drained prior to 1939 in England and 9
Wales.
Figure 2.2 Historical changes in the sheep population of Wales (1867- 12
2006).
Figure 2.3 Location of the Pontbren study area and drainage network 20
of the Afon Einion.
Figure 2.4 Long-profiles of major streams in the Pontbren study area. 22
Figure 2.5 Map of soil series in the Pontbren study area. 23

Figure 2.6 Pontbren Rural Care Project features: (a) mature 27
shelterbelt; (b) artificial lake.
Figure 2.7 Spatial distribution of historical changes in land use in the 31
Pontbren study area.
Figure 2.8 Spatial distribution of woodland and wetland features 33
created through the Pontbren Rural Care Project.
Figure 2.9 Nant Melin-y-grûg at (a) Hirrhos Bridge and (b) Hirrhos 37
Uchaf.
Figure 2.10 (a) Exposed bridge buttresses and (b and c) steep banks 39
due to channel incision in the Upper Pontbren
subcatchment.
Figure 2.11 Land surrounding the upper reaches of the Nant Melin-y- 41
grûg.
Figure 2.12 (a) Livestock feeding area alongside Nant Pen-y-cwm and 42
(b) trampled ground in improved pasture in Upper
Pontbren subcatchment (Winter 2005).
Figure 2.13 Contrasting (a) agricultural and (b) forestry ditches in the 43
Upper Pontbren subcatchment.
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