Influence of vegetation on sediment accumulation in tidal saltmarshes

400 Pages·2016·31.72 MB·English

by  Darryl Price

Influence of vegetation on sediment accumulation
in tidal saltmarshes: An integrated field and
modelling study

Darryl Christopher Price

Coastal & Estuarine Research Unit
UCL Department of Geography

A thesis submitted for the degree of
Doctor of Philosophy

October 2016
Declaration

I, Darryl Christopher Price, confirm that the work presented in this thesis is my own.
Where information has been derived from other sources, I confirm that this has been
indicated in the thesis.

Name: …………………………………
Signature: …………………………………
Date: …………………………………

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Abstract
Tidal saltmarshes in the UK, and especially in the estuaries of southeast England, have
been subject to degradation and erosion over the last few decades, primarily caused by
sea-level rise and coastal squeeze. This is of great concern as saltmarshes play a key
coastal defence role and function as important habitats for a variety of plant and animal
species. Sediment accumulation is critical for the maintenance of marsh elevation within
the tidal frame and for delivering the aforementioned functions and services. Key
questions still remain, however, regarding the processes that govern deposition and the
role of vegetation in enhancing sedimentation. The research presented in this thesis
focuses on a case study at the managed realignment site and adjacent natural marshes at
Tollesbury, in the Blackwater Estuary, Essex, UK. An innovative combination of
secondary data analysis, intensive field campaigns and numerical modelling is used to
advance our understanding of the processes controlling sedimentation.
Results from a series of hierarchical deployments of sediment traps indicate the
role of vegetation in marsh development is less clear-cut than previously thought. Gross
sedimentation rates were statistically higher in non-vegetated areas, and vegetation has
no overall influence over trapping efficiency. However, sediment retention was higher
at the vegetated sampling points. This implies that vegetation acts primarily to inhibit
resuspension by waves rather than by facilitating deposition from tidal flows.
The performance of the realignment site in terms of vegetation development has
been poor compared to other schemes, with only half of the site having been colonised
after 18 years of tidal inundation. No evidence was found for erosion of the natural
marshes surrounding the site, and it is shown that previous estimates of marsh erosion
are erroneously high. It is thus speculated whether the realignment of large stretches of
coastal defences are actually necessary or worthwhile.

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Acknowledgements
First and foremost, I would like to thank my parents, Lianne and Bob Price for their
unwavering help, support and encouragement throughout the entirety of my PhD.
Without their assistance, I would have been unable to undertake this research, as they
helped to plan and accompanied me on the majority of my fieldwork and carried out
half of the seasonal data sampling. A particular thanks to my brother, Justin, who was
also made to endure getting up in the early hours of many Sunday mornings in order to
trudge around a very muddy saltmarsh in Essex with us.
Thanks must go to my primary supervisor, Prof Jon French, who inspired me to
get this research off the ground, and provided expert guidance throughout the lifetime of
this project. As well as assisting with the planning of fieldwork, he also helped
considerably with the hydrodynamic modelling that was undertaken.
A big thank you to Dr Helene Burningham, my second supervisor, for the many
hours spent poring over my fieldwork results trying to tease out relationships between
the multitude of variables in my data. Her suggestions for which analyses to carry out
were particularly gratefully received. Helene also assisted with data collection during
the two intensive fieldwork monitoring periods.
A particular mention must be given to Ian Patmore, the fieldwork technician in
the Geography Department for his considerable support and ideas to ensure the smooth
running of my entire fieldwork programme. From coming up with a simple tool to make
around 1,000 pins for my sediment traps to meticulously planning the installation and
retrieval of the ADCP, Ian was always enthusiastic and willing to help. The data
collection aspect my research would certainly have suffered without him. Janet Hope
and Tula Maxted also aided in sourcing fieldwork equipment and provided advice on
laboratory methods.
Many thanks to Miriam Fernandez Nunez, Mandy Green and Lizzie Gardner for
helping out during the intensive fieldwork periods. My fieldwork programme was partly
funded by grants from the British Society of Geomorphology, and I was awarded a one-
year studentship by the Geography Department.
I’d also like to thank David Welsh, from the Environment Agency for supplying
the aerial photography of the study area from 1997, 2008 and 2013; and Timothy Poate,
from Plymouth University for providing detailed tidal predictions for fieldwork
planning.
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Table of Contents

Abstract 3
Acknowledgements 4
List of Tables 8
List of Figures 12
List of Acronyms 21

1. Introduction 23
1.1 Importance of Saltmarshes 23
1.1.1 Human Uses 23
1.1.2 Habitat 25
1.1.3 Ecosystem Functions and Services 29
1.1.4 Carbon Sequestration 32
1.1.5 Coastal Defence Value 34
1.2 Saltmarsh Biogeomorphology 38
1.2.1 Controls on Saltmarsh Occurrence and Type 38
1.2.2 Initial Formation and Sediment Dynamics 41
1.3 Resilience of Saltmarshes to Change 45
1.3.1 Historic Degradation and Loss 46
1.3.2 Future Marsh Loss and Methods of Restoration 50
1.4 Some Unresolved Questions in Saltmarsh Science 54
1.4.1 Role of Vegetation 55
1.4.2 Sediment Availability and Trapping Efficiency 56
1.5 Aims and Objectives 58

2. Methodology 60
2.1 Research Design 60
2.2 Case Study Location 62
2.2.1 Tollesbury Marshes 64
2.3 Historic Data Analysis 69
2.3.1 Aerial Photography 69
2.3.2 Elevation Data 71
2.4 Field Data Acquisition 78
2.4.1 Sampling Strategy 78
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2.4.2 Sediment Deposition 82
2.4.3 Suspended Sediment Concentration 85
2.4.4 Sediment and Vegetation Characteristics 87
2.4.5 Tidal and Meteorological Monitoring 88
2.5 Modelling Study 89

3. Longer-Term Evolution of Tollesbury Marshes 91
3.1 Vegetation Changes 91
3.1.1 Realignment Site 91
3.1.2 Natural Marshes 109
3.2 Marsh Fragmentation 122
3.3 Topographical Development 127
3.3.1 LiDAR 127
3.3.2 Ground-based Elevation and Bathymetry Surveys 147
3.4 Summary 154

4. Sediment Accumulation Processes in the Tollesbury Marshes 155
4.1 Summary of Tidal and Weather Conditions 155
4.2 Temporal Variation in Sediment Deposition and Retention 163
4.2.1 Suspended Sediment Concentration 174
4.2.2 Tidal Factors 179
4.3 Spatial Variability in Sediment Deposition and Retention 185
4.3.1 Realignment Site versus Natural Marshes 188
4.3.2 Distance from Sediment Source 190
4.3.3 Elevation 195
4.4 Influence of Vegetation 198
4.4.1 Presence of Vegetation 203
4.4.2 Vegetation Abundance 207
4.4.3 Species Composition 211
4.5 Summary 215

5. The Role of Vegetation in Sedimentation within the Tollesbury Realignment Site 217
5.1 Summary of Tidal and Weather Conditions 217
5.2 Environmental Controls on Sediment Deposition and Retention 221
5.2.1 Temporal and Spatial Variation in Sedimentation 221
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5.2.2 Tidal Controls on Sedimentation 230
5.2.3 Spatial Controls on Sedimentation 239
5.3 Influence of Vegetation 247
5.3.1 Presence of Vegetation 247
5.3.2 Vegetation Gradient 252
5.3.3 Vegetation Abundance 256
5.3.4 Species Composition 259
5.3.5 Trapping Efficiency 263
5.4 Summary 268

6. Hydrodynamic Modelling in the Tollesbury Realignment Site 270
6.1 Modelling Approach 270
6.1.1 The Telemac Modelling System 271
6.1.2 Model Formulation 273
6.1.3 Model Implementation 274
6.1.4 Validation and Sensitivity Analysis 279
6.2 Hydrodynamic Model Results 285
6.2.1 Effect of Vegetation on Flow Velocity 285
6.2.2 Comparison of Flood and Ebb Velocities 303
6.2.3 Influence of Species Composition 311
6.3 Summary 321

7. Discussion 323
7.1 Longer-Term Changes in the Tollesbury Marshes 323
7.1.1 Natural Marshes 324
7.1.2 Realignment Site 334
7.2 Sedimentation Processes at Tollesbury 339
7.2.1 Controls on Sediment Deposition 339
7.2.2 Role of Vegetation 341
7.3 Summary 347

8. Conclusions 348
References 353

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List of Tables

Table 1.1: Sea wall construction costs associated with levels of saltmarsh protection 36
Table 1.2: Wave height and wave energy reduction in different saltmarshes 37
Table 1.3: Saltmarsh erosion in Essex estuaries between 1973 and 1998 48
Table 2.1: Aerial photography analysed 69
Table 2.2: LiDAR datasets covering the Tollesbury marshes 73
Table 2.3: Horizontal difference between map control points and LiDAR surveys 74
Table 2.4: Difference between 2013 LiDAR control point elevations and prior surveys75
Table 2.5: Seasonal fieldwork campaign dates 78
Table 2.6: Intensive monitoring field campaigns 80
Table 2.7: Dafor scale for vegetation cover 88
Table 3.1: Area of pixels classified as vegetated and unvegetated in the realignment
sitea 95
Table 3.2: Change detection statistics for classified maps of the realignment area 103
Table 3.3: Annualised change detection statistics for the realignment area 103
Table 3.4: Net change statistics for the realignment site, 1997 – 2013 105
Table 3.5: Class extents for 1997 and 2011 outer marsh test area 112
Table 3.6: Change detection statistics for the outer marsh test area 114
Table 3.7: Class extents for 1997 and 2011 inner marsh test area 117
Table 3.8: Change detection statistics for the inner marsh test area 118
Table 3.9: Comparison of quoted vertical accuracies and those derived from residual
analysis 137
Table 4.1: Seasonal deployment campaign periods 156
Table 4.2: Mean high water levels (m OD) during the seasonal campaigns 156
Table 4.3: Tidal surge statistics during the seasonal campaigns 158
Table 4.4: Wind speed statistics during the seasonal campaigns 160
Table 4.5: Rainfall totals (mm) during the seasonal fieldwork campaigns 162
Table 4.6: Probability results from Kruskal-Wallis tests performed on weekly sediment
flux data groups 165
Table 4.7: Description of variables included in PCA biplots 167
Table 4.8: Results of multiple linear regression analysis with sediment flux as the
dependent variable 171
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Table 4.9: Results of multiple linear regression analysis with sediment retention as the
dependent variable 173
Table 4.10: List of species recorded during vegetation sampling 201
Table 4.11: Dafor scale for vegetation cover 201
Table 4.12: Descriptive statistics for sediment flux recorded across the full campaign
period at vegetated and bare sampling points 204
Table 4.13: Probability results from Kruskal-Wallis tests performed on sediment flux
between each vegetation abundance group 209
Table 4.14: Summary of theorised and measured relationships between sediment flux
and retention and the variables investigated in Chapter 4 216
Table 4.15: Explanation of Indicators used in Table 4.14, with associated qualitative
descriptors and r2 values 216
Table 5.1: Deployment periods for intensive monitoring field campaigns 217
Table 5.2: Tidal and weather statistics for May 2012 219
Table 5.3: Tidal and weather statistics for March 2014 219
Table 5.4: Description of variables included in PCA biplots 228
Table 5.5: Results of multiple linear regression analysis with sediment flux as the
dependent variable 229
Table 5.6: Results of multiple linear regression analysis with sediment retention as the
dependent variable 229
Table 5.7: Strength of relationships between sediment flux and hydroperiod for grouped
sampling points during intensive fieldwork periods 237
Table 5.8: Strength of relationships between sediment retention and hydroperiod for
grouped sampling points during intensive fieldwork periods 237
Table 5.9: Strength of relationships between sediment flux and distance from sediment
source for grouped sampling points during intensive fieldwork periods 239
Table 5.10: Strength of relationships between sediment retention and distance from
sediment source for grouped sampling points during intensive fieldwork periods 243
Table 5.11: Strength of relationships between sediment flux and elevation for grouped
sampling points during intensive fieldwork periods 245
Table 5.12: Strength of relationships between sediment retention and elevation for
grouped sampling points during intensive fieldwork periods 247
Table 5.13: Descriptive statistics for sediment flux (g m-2 tide-1) recorded at vegetated
and bare sampling points during intensive monitoring deployments 248
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Table 5.14: Descriptive statistics for sediment retention (%) recorded at vegetated and
bare sampling points during intensive monitoring deployments 250
Table 5.15: Descriptive statistics for sediment flux g m-2 tide-1) recorded across the
vegetation gradient 254
Table 5.16: Probability results from Kruskal-Wallis tests performed on vegetation
gradient sediment flux data 254
Table 5.17: List of species recorded during March 2014 deployment 259
Table 5.18: Summary of theorised and measured relationships between sediment flux
and retention and the variables investigated in Chapter 5 269
Table 5.19: Explanation of Indicators used in Table 5.18, with associated qualitative
descriptors and r2 values 269
Table 6.1: Model scenarios used to test effect of vegetation on flow velocity 286
Table 6.2: Reduction in flow velocity across transect 1 (% m-1) 292
Table 6.3: Reduction in flow velocity across the vegetation margin and the rest of the
vegetated area in transect 2 (%) 295
Table 6.4: Velocity attenuation (% m-1) for selected scenarios along transects 3 and 4299
Table 6.5: Strength of relationship (r2) between simulated water levels and flow
velocities for selected friction scenarios across transects 3 and 4 301
Table 6.6: Velocity attenuation (% m-1) across various sections of transect 5 302
Table 6.7: Average maximum velocities (ms-1) on the flood and ebb tides during the
high flow period within the mudflat and vegetated area of transect 1 307
Table 6.8: Average maximum velocities (ms-1) on the flood and ebb tides during the
high flow period within the mudflat and vegetated areas of transect 2 308
Table 6.9: Average maximum velocities (ms-1) on the flood and ebb tides during the
high flow period within the mudflat and vegetated areas of transects 3 and 4 309
Table 6.10: Average maximum velocities (ms-1) on the flood and ebb tides during the
high flow period across various sections of transect 5 310
Table 6.11: Vegetation structure and Manning’s n friction coefficients used for each
material zone 312
Table 6.12: Maximum flood velocity attenuation across transect 6 (% m-1) 317
Table 6.13: Average maximum velocities (ms-1) on the flood and ebb tides during the
low and high flow periods for transect 6 317
Table 6.14: Average maximum velocities (ms-1) on the flood and ebb tides during the
low and high flow periods for transect 7 318
Table 6.15: Maximum flood velocity attenuation across transect 7 (% m-1) 319
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