River response to recent environmental change in the Yorkshire Ouse basin, northern England. Sean Anthony Longfield, B.Sc. (H ons) 'N/ Submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds School of Geography July 1998 The candidate confirms that the work submitted is his own and that appropriate credit has been given where reference has been made to the w'ork of others II A b s t r a c t Longfield, S.A. 1998. River response to recent environmental change in the Yorkshire Ouse basin, northern England. University of Leeds, Ph.D. This study examines historical variations in flood frequency and magnitude in the Yorkshire Ouse basin, northern England, over the last 900 years. The causes of temporal and spatial variations in flooding are evaluated through investigation of climatic and land-use controls. Documentary evidence of flooding and climate suggests that a series of large floods between 1263 and 1360 were associated with climatic deterioration from the Medieval Optimum. A shift to generally milder conditions between 1361 and 1549 resulted in no floods being documented in the Ouse basin The frequency of large magnitude floods increased dramatically between 1550 and 1680, as a result of low temperatures, increased surface wetness, more frequent snowfall and a southward shift of prevailing storm tracks over middle latitudes, associated with the onset of the "Little Ice Age’. In contrast, during a wanner phase of the Little Ice Age, between 1681 and 1763, the frequency of localised summer flooding increased in the Ouse basin due to more frequent high intensity, short duration convective storms. Extensive lowland flooding became more common between 1764 and 1799 due to an increase in heavy rainfall, followed by a 50-year period characterised by relatively moderate flood frequencies and magnitudes. The later half of the nineteenth century experienced high flood frequencies and magnitudes, particularly in the 1870s and early-1880s, coinciding with high rainfall totals and a high incidence of cyclonic flood generation. Gauged flood and climate data, and land-use records indicate that the period between 1900 and 1916 was characterised by very low flood frequencies and magnitudes, associated with low rainfall, warm temperatures, and an increase in westerly flood generation. Between 1916 and 1943 there were marked variations in flood magnitude between the rural northern rivers and southern industrialised rivers. Magnitudes generally increased on northern rivers, whilst on some southern tributaries of the Ouse, flood magnitudes declined as a result of widespread channel improvement and flood defence schemes. Around 1944 a marked and sustained increase in flood frequency on northern rivers was associated with an increase in the incidence of heavy daily rainfall, greater westerly flood generation and large-scale upland and lowland drainage. Very low flood frequencies and magnitudes between 1969'and 1977 resulted from extremely low rainfall totals. Whereas the most recent period, between 1978 and 1996 has experienced some of the highest flood frequencies and magnitudes on record, associated with an increase in the frequency of floods generated under cyclonic and south-westerly synoptic situations, and a number of land-use changes promoting more rapid runoff including, large increases in upland livestock numbers, an increase in the area under winter-cereals and the cumulative effects of moorland gripping. A cknowledgements First I would like to thank my supervisors. Dr Mark Macklin and Dr Barbara Rumsby for their support and guidance over the last three years. Thanks are also extended to all those who helped with many different aspects of the project, especially : Professor Mike Kirkby for assistance with flood frequency analysis, data analysis techniques and suggestions for time saving programs. Professor Adrian Mcdonald for constructive advice during Research Support Group meetings Dr Mark Taylor for his boundless enthusiasm and encouragement Special thanks also to Professor John Sheail for help with historical data sources and collection. Staff at the Environment Agency for supplying flood and climate data, with special thanks to lan Ward, Martin Wilson, Robin Bailey and Mark Fuller at Coverdale House in York, and Ian Hampson from the Leeds office Numerous staff at the Institute of Hydrology including, Terry Marsh and Adrian Bayliss for supplying flood data, Dr Mark Robinson for advice on land drainage data in the Ouse basin, Isabella Tindell for supplying climate data and Frank Law for valuable suggestions relating to historic flood data Staff at the Meteorological Office in Bracknell for climate data, particularly the Climate Consultancy Section, the Meteorological Archive and Library staff. Dr Andrew Brooks for data on channelization Mark Rutter of Yorkshire Water Authority for information on Yorkshire reservoirs Hilary Arnold of the Yorkshire Philosophical Society for suggestions during the historic flood data collection phase. All the staff of the multitude of Libraries and Archive Offices which I visited, for assistance in gathering flood information. Alistair French and Alison Manson of the graphics unit for producing numerous figures and maps. I am also grateful to NERC and the LOIS project for my studentship and funding during the project. I would also like to thank a number of my friends who have helped me to unwind' on many occasions, thanks Matt, Ash, Mark, Marc, Gus, Steve, Tom and not least Glenn. Finally, thanks to Sadie for many years of support and patience, and to my parents and family for their encouragement and assistance throughout my academic studies. IV CONTENTS Abstract ii Acknowledgements iii List of figures ix List of tables xiii List of abbreviations xiv Chapter 1 Introduction 1.1 Tire Land-Ocean Interaction Study (LOIS) 1 1.2. Research objectives 2 1.3. Research scale 2 1.4. Research Strategy and thesis structure 4 Chapter 2 Review of literature 2.1. Introduction 6 2.2. Recent trends in UK flooding: 1750-1995 6 2.3. Climatic controls of flooding 10 2.3.1. Introduction 10 2.3.2. Controls of regional climate in the UK 10 2.3.2.1. The Lamb (1972) catalogue of atmospheric circulation types 1 1 2 3 .2 .2 . Circulation types : relationships with precipitation and recent trends 13 2.3.3. Atmospheric circulations and flooding 15 2.4. Land-use change and flooding 18 2.4.1. Introduction 18 2.4.2. Agricultural practice and land-use change 18 2.4.3. Agricultural land drainage 22 2 4.3.1. Types of drainage system used in the UK 22 2.4.3.2. The effects of land drainage on floods 23 2.4.4. River channelization : methods and hydrological consequences 25 2.4.5. The effects of reservoirs on flood flows 28 2.5. Summary of literature review 3 0 Chapter 3 Background to study area : the Yorkshire Ouse basin 3 1 Introduction 32 3 2 Late Quaternary glacial history 32 3 .3 Basin geology 34 3.4 Basin relief 34 3 5 Basin climate 34 3 .5 1. Causes of precipitation in the British Isles and Ouse basin 37 3.6. Basin hydrology 38 3.6.1 Introduction 38 3.6.2. Recent flooding in the Ouse basin 40 3.7 Basin land-use 43 Chapter 4 Flood history of the Yorkshire Ouse basin since the eleventh century 4.1. Introduction 45 4.2. Gauged flood histories in the Ouse basin 45 4.2.1. Data sources 45 4 .2 .2. Preparation of gauged flood data 53 4.2.3. Techniques of data analysis 54 4.2.4. Variability in flood frequency, magnitude and seasonality in the Ouse basin 57 4.2.41. Gauging station histories 57 4.2.4.2. Annual maximum series (AM) 58 4.2.4.3. Annual mean exceedance (AME) 67 4.2.4.4. Annual flood frequency (AFF) 72 4.2.4.5. Annual flood frequency over various thresholds 78 4.2.4.6. Flood seasonality 82 4.3 Documentary flood histories in the Ouse basin 94 4.3.1. Introduction 94 4.3.2. Data sources 94 4.3.3. Variations in flood frequency and magnitude in the Ouse basin inferred 96 from documentary sources 4.3.2.1. Documentary flood history at York : 1263-1878 97 4.3.2.2. Documentary flood history of the Ouse basin 103 4.3.2 .3. Seasonality of documentary floods 107 vi 4.4 Summary of flood variability in the Yorkshire Ouse basin since the eleventh century 110 4.4 1 Variations in flood frequency and magnitude 1 10 4 4 2. Variations in flood seasonality I 1 I Chapter 5 Climatic variability in the Ouse basin since the eleventh century 5 1 Introduction 113 5.2 Daily rainfall records in the Ouse basin 1873-1996 1 14 5.2 1 Introduction 114 5.2.2. Datasources 115 5.2.3. Preparation of rainfall data 115 5.2.4 Variation in annual and seasonal rainfall series 118 5.2.5 Variation in POT rainfall series 118 5.2.5.1 Annual POT rainfall frequency 118 5.2.5 2. Rainfalls over various thresholds 125 5 .2 5.3. Seasonality of POT rainfall 128 5.2.6. Links between rainfall and flood series 128 5.3. Variability of snowfall 1875-1995 and links with flood series 132 5.4. Flood generating atmospheric circulation types 134 5.4.1. Introduction 13 4 5.4.2. Method for determining dominant flood generating circulation types 135 5.4.3. Flood generating circulation types in the Ouse basin 138 5.4.3.1. Weather associated with dominant flood generating circulation types 140 5.4.3.2. Relationships between flood magnitude and generating circulation 141 types 5 .4.3.3.Temporal variation in the incidence of floods generated by 144 individual circulation types 5.4.4.