Flood Frequency and Magnitude Estimates Based on Valley Floor
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FLOOD FREQUENCY AND MAGNITUDE ESTIMATES BASED ON VALLEY FLOOR MORPHOLOGY AND FLOODPLAIN SEDIMENTARY SEQUENCES: THE TYNE BASIN, N.E. ENGLAND :i: 'LE:I; I: ......V L.. ]. EF1' Z9J. 514:Lé 3 Lqo Submitted in candidature for the degree of Doctor of Philosophy to the University of Newcastle upon Tyne, September, 1991 By BARBARA RUMSBY, B.Sc. (Hons) ABSTRACT This study examines the sedimentary and morphological flood record of the River Tyne, northern England, over the last 400 years. Temporal trends in flooding and spatial variations in the timing and nature of river response to floods (flood "effectiveness") are investigated at 3 reaches representative of upland, piedmont and lowland locations in the Tyne catchment. Deposition of coarse flood sediments in headwater tributaries of the River Tyne appears to be strongly related to flood water:sediment ratios that is primarily controlled by sediment supply. Cobble - boulder bars, sheets and splays are deposited under Newtonian flow conditions from flows with relatively low sediment concentrations. Boulder berms and lobes result from floods with high sediment loads, associated with slope-channel coupling. Flood sedimentation in lower reaches of the Tyne comprises vertical accretion of fine-grained, well-bedded sediments at overbank and within-channel locations. Larger floods can be identified on the basis of textural discontinuities that reverse the overall fining-upward sequence. Estimation and interpretation of flow magnitude in each setting is complicated by progressive river bed incision over the period of investigation and significant changes in sediment supply which have affected the nature of flood sedimentation. Trace metal dating of fine-grained, and lichenometric dating of coarse- grained, flood units has enabled detailed and temporally well-defined records of channel and floodplain response to flooding to be established within the Tyne catchment. Since 1600 A.D. fluvial activity at all 3 study reaches has been characterised by alternating periods of river bed incision and stability. Episodic changes in vertical channel stability appear to coincide with non-random decadal- scale fluctuatiions in flood frequency: 3 periods with distinct clustering of large floods (1740-1790, 1860-1899, 1940-1965) were associated with vertical instability (channel trenching) throughout the Tyne basin, while intervening periods with relatively few exceptional floods were associated with channel accretion and floodplain rebuilding in general, but with significant upstream/downstream variation. Distinct clustering in the timing of large floods correspond with short- term hydroclimate changes associated with fluctuations in the configuration of the upper atmosphere. These findings have significant implications for longer-term fluvial development and interpretaion of Holocene alluvial sequences. ii ACKNOWLEDGEMENTS Many people have provided help and encouragement over the last four years. Firstly, I would like to thank Dr Mark Macklin and Professor Malcolm Newson for their supervision and guidance during my Ph.D studies. The ideas expressed in this thesis have benefitted considerably from helpful and insightful discussion with Mark Macklin, and with Dave Passmore. Thanks to Tim Heap for lichen hunting, and for access to unpublished pollen data from Thinhope Burn. I am also indebted to Watts Stelling for invaluable help in the lab, Dave Archer at Northumbrian Water for access to unpublished documentary flood information and the Reverend G.G. Graham for lichen identification. Thanks must also go to Tony Stevenson for letting me monopolise his laptop for so long, and Watts, Brian, Tony, Dave, Phil, Neil and all the other "Bridge Club" members for numerous lunchtime sessions, without which I would have finshed sooner! Finally, thanks to my parents for their constant support and encouragement throughout all my academic studies. iii CONTENTS Abstract 1 Acknowledgments ii List of figures viii List of tables xiii Chapter 1 Introduction Research objectives 1 Choice of study area 2 Research strategy 4 Chapter 2 Review of literature Introduction 7 Flood frequency analysis 7 Conventional methods of extending flood series 10 Documentary flood information and historic flood chronologies 11 Limitations 13 Geomorphological flood histories 14 Flood geomorphology in Britain 16 Floods in fluvial geomorphology 18 Floods and climate 21 Climate change 23 Summary 23 Chapter 3 Background to the study area: The Tyne Basin, north east England Introduction 26 Geology and physiography of the Tyne Basin 26 iv Archaeology and land-use history 29 Climate and hydrology 33 Previous fluvial geomorphological studies in the Tyne catchment 39 Chanter 4 Historic flood sedimentation at Low Prudhoe in the lower Tyne Valley Introduction 44 River channel change and vertical accretion processes 47 Field sampling and laboratory analysis of flood sediments 48 Sedimentary sequence and grain-size characteristics of flood sediments 48 Trace metal dating of flood sediments 53 Flood frequencies at Low Prudhoe 1890-1990: a comparison of the documentary and sedimentary flood record in the Lower Tyne valley 56 Vertical accretion rates at Low Prudhoe 1890-1990 62 Conclusions 66 Chapter 5 Upland flood histories and recent valley floor development: Thinhope, Whitewalls and Knar Burns Introduction 68 Study sites 69 Methods 70 Lichenometric dating of coarse flood sediments 70 Geomorphic, sedimentologic and hydraulic properties of Thinhope Burn flood deposits 76 Cobble-boulder splays and sheets 81 Cobble-boulder bars 81 Boulder berms 81 Boulder lobes 83 V Hydrodynamic interpretation and environment of deposition of flood sediments 83 Flood frequency and magnitude reconstruction, 1766 to the present 85 Age of flood deposits and large flow frequencies 85 Discharge estimates of rare large floods 88 Floods and longer-term valley floor development 94 Conclusions 102 Chapter 6 Response of a piedmont reach of the River Tyne to variations in flood magnitude and frequency, c.1600-1990 Introduction 104 Background to the study site 105 Sampling strategy and methodology 105 Sedimentary characteristics of flood deposits 110 Age of alluvial units 118 Map and aerial photographic evidence 118 Chemostratigraphy 119 Discussion: flood frequency estimates and sedimentation rates 125 Flood frequency 125 Relative elevation of depositional surfaces 129 Sedimentation rates 130 Loss-on-ignition 132 Timing, magnitude and nature of river response to floods at Broomhaugh Island, c.1600-1990 132 River response 134 Impact of land-use and channel modification 137 Conclusions 138 vi Cha pter 7 Overview: Channel and floodplain response to flood frequency and magnitude variations in the Tyne Basin since c.1600 A.D. Introduction 139 Climate, land-use changes and channel and floodplain adjustment at upland, piedmont and lowland locations, 1600-1990 140 Flood regime, channel entrenchment and sediment transfer 147 1. High magnitude floods and channel entrenchment 147 2. Periods of low flood frequency: episodic erosion and transfer of sediment 148 Short-term climate change and secular trends in flood frequency and magnitude 150 Implications for longer-term alluvial development and interpretation of Holocene alluvial sequences 158 Response of the River Tyne to future climate changes 161 Cha pter 8 Conclusions 162 References 167 Appendices 191 vii List of_Figures 1.1 Map showing location of the Tyne catchment 3 3.1 Map of the Tyne Basin showing location of the study sites, gauging stations and long-term precipitation records 27 3.2 Relief map of the Tyne Basin 28 3.3 Geology map of the Tyne Basin 30 3.4 Map of the Tyne Basin showing average annual precipitation and potential evaporation 34 3.5 Variations in mean annual temperature at Durham Observatory since 1847 (from Harris, 1985) 36 3.6 Long-term precipitation records from the South Tyne (Allenheads), North Tyne (Catcleugh, Camphill) and Lower Tyne (Whittle Dean) catchments 37-38 3.7 Plots of peak-over-threshold flows for selected gauges in the Tyne catchment 40 3.8 Documented floods in the Tyne Basin since 1600 A.D. 41 4.1 Map of the Tyne Basin showing location of the study reach at Low Prudhoe, drainage network and metal mines 45 4.2 (a) Maps showing channel change at Low Prudhoe between 1860 and 1978, and location of section, (b) Channel cross-sections upstream and downstream of Ovingham Bridge. Relative heights of 1771 and 1815 floods are indicated 46 4.3 Sedimentary log and summary moment statistics of vertically accreted alluvium at Low Prudhoe 49 viii 4.4 Grain size curves for type I (medium-fine sand), type 2 (fine-very fine sand) and type 3 (silty fine-very fine sand) flood units 50 4.5 Metal concentrations and organic matter content in vertically accreted alluvium at Low Prudhoe showing major flood units with their probable dates 54 4.6 Lead and zinc production in the Tyne Basin 1845-1938 (after Dunham, 1944) 55 4.7 Comparison of floods documented in the lower Tyne Valley (between 1890 and 1989) and the sedimentary flood record at Low Prudhoe 58 4.8 Sediment accumulation curve for Low Prudhoe 1890-1989 63 5.1 Location map showing stream networks and catchments of Knar Burn, Thinhope Burn and Whitewalls Burn. The 4.1 km study reach in Thinhope Burn is indicated 69 5.2 Morphological map of Thinhope Burn showing valley floor terraces, palaeochannels, alluvial fans, boulder flood deposits and valley side rotational landslips. The position of surveyed cross-sections, boulder and lichen measurement sites, logged river bank sections,