title: Lake sediment records of flood frequency and magnitude
Creators: name: Schillereff, Daniel
Publisher: name: University of Liverpool Repository
Year: 2015

description: The recent spate of floods in many parts of Britain has stimulated substantial interest among scientists, policy makers and the public concerning contemporary trends in flood frequency and magnitude, in particular questioning whether these events exceed historical extremes. However, detecting a clear signal of recent intensification in the flood regime is hampered by the relatively short timescales covered by meteorological and river flow data. Lake sediment sequences have proven a valuable archive of historical flooding over centennial and millennial timescales elsewhere in Europe and globally, but this thesis presents one of the first attempts to extract flood histories from the sediments of lakes in Britain. It adapts a detailed field and lab-based approach to test the hypothesis that discrete layers can be distinguished from long sediment cores (1 – 3.5 m) that were probably deposited by high-magnitude floods based on their particle size and geochemical signatures. The programme of research was developed and applied at Brotherswater (River Eden catchment), northwest England, and then further tested at the Loch of the Lowes (River Tweed catchment), southern Scotland. A detailed literature review led to the creation of a conceptual model to guide field site selection, based on the catchment-lake configuration and hydrological regime of individual sites. Characteristics deemed critical include a high catchment:lake area ratio to maximise sediment availability, limited pre-lake sediment storage and an effective sediment conveyor, as well as simple lake bathymetry. The viability of µXRF scanning to characterise flood laminations within wet sediment cores was assessed in detail. A new method of calibrating geochemical concentrations where the water content varies substantially (50%) down-core based on x-ray scattering is described, and the first inter-comparison between different core scanners is performed. The analytical resolution of the ITRAX can reveal laminations too thin to manually sub-sample but the dataset can be noiser, especially after correction to the dry-mass basis. Caution should be used when interpreting ITRAX data at the sub -mm scale to ensure peaks and troughs represent real changes in sediment composition. Inspection of multiple sediment cores extracted along a delta-proximal to distal transect in Brotherswater, coupled with high-resolution (0.5 cm) particle size measurements revealed silt-dominated (90th percentile grain size (P90) ~ 16 µm) sediment matrices frequently punctuated by coarser-grained sand layers (P90 &gt;100 µm) that are lighter in colour. These layers cannot be easily characterised geochemically, most likely due to the volcanic bedrock in the catchment, but constitute the stratigraphical signature diagnostic of repeated high-magnitude floods for the River Eden catchment. Sediment supply varied substantially through the late-Holocene at Brotherswater, complicating the particle size record. A normalisation approach was tested to remove the background trend and identify notable particle size peaks (&gt;1 standard deviation from the longer-term moving window) and the similar profiles produced for three cores from Brotherswater suggests the technique has been able to produce a record of major floods in the Patterdale Valley spanning the last 1500 years. Geochemical profiles at Brotherswater are dominated by enhanced metal deposition during the last 300 years, especially Pb, which mimics and could be used as a surrogate for production data for the nearby Hartsop Hall Lead Mine. The down-core pattern of Pb deposition is strongly reproducible in twelve cores extracted from different parts of the lake and reveals reveals the pattern, rate and controls over sediment deposition, with post-1860 accumulation rates four times greater near the inflow. A coherent chronology that integrates short-lived radionuclide dating (137Cs, 241Am, 210Pb), 14C ages and the geochemical contaminant markers for Brotherswater reveals that the last millennium was characterised by flood-rich and flood-poor phases generally 50 – 100 years in duration. The timing of these phases is significantly correlated with annual precipitation reconstructions, indicating a link between rainfall and flood generation. Some temporal correspondence between flood occurrence and phases of the North Atlantic Oscillation is observed but the relationship frequently breaks down. At Brotherswater, flood deposits have been most frequent during the 20th Century but isolating the influence of human activity on the sediment record is challenging. Sixteen months of process monitoring at Brotherswater using sediment traps confirms the mechanics of sediment delivery and shows a strong continuity of current process to the late-Holocene sediment record. Particle size signatures were identified for a major winter flood, low-flow conditions and the regular flushing of the sediment system by successive low magnitude flood flows. Scaling the sediment trap data to an equivalent annual accumulation show how event delivery of coarse material may be masked by more regular fine-grained sedimentation through the year. The revealed anatomy of the annual sediment accumulation cycle has significant implications for palaeoflood research with process understanding at each site clearly a critical precursor to any sediment-based flood investigation. Laminations interpreted as palaeoflood deposits in the Loch of the Lowes sequence also exhibit prominent peaks in particle size but differ in colour and geochemical composition from Brotherswater, which highlights the importance of developing site-specific protocols for interpreting the sediment record. The coarse bands are much darker in colour and the Zr/Rb ratio appears to be an effective proxy of particle size in this case, with higher values reflecting coarser material. Preliminary chronological information suggest a finely-resolved palaeoflood record has been recovered extending approximately 350 years at the delta-proximal zone and perhaps twice as long in the central basin. The thesis demonstrates that particle size data, supported by appropriately calibrated geochemical analyses, can generate high-resolution flood stratigraphies from upland UK lakes and these span millennia, extending our understanding of regional flood frequency.