Hydrological thresholds and basin control over paleoflood records in lakes

The scarcity of long-term hydrological data is a barrier to reliably determining the likelihood of floods becoming more frequent and/or intense in a warmer world. Lake sediments preserve characteristic event layers, offering the potential to develop widely distributed and unique chronologies of historical floods. Inferring flood magnitude remains a greater challenge, previously overcome in part by analyzing sharply laminated polar or alpine sequences. Here we demonstrate an approach to obtain flood frequency and magnitude data from an unexploited resource, the largely visually homogeneous, organic sediments that typify most temperate lakes. The geochemical composition and end-member modeling of sediment trap and adjacent short core particle size data for Brotherswater (northwest England) discriminates the signature of infrequent, coarse-grained flood deposits from seasonal and longer-term allogenic (enhanced discharge and sediment supply during winter) and autogenic (summer productivity, thermal mixing) depositional processes. Comparing the paleoflood reconstruction to local river discharges shows that hydrological thresholds censor event signature preservation, with 4 yr recurrence intervals detectable in delta-proximal sediments declining to 9 yr in the lake center. Event threshold (discharge) and process characterization are essential precursors to discerning flood magnitude from sediment archives. Implementation of our approach in globally prevalent temperate lakes offers a vast, unique repository of long-term hydrological data for hydrologists, climate modelers, engineers, and policy makers addressing future flood risks

PyShoreVolume 1.0.0: A Python based Shoreline Change and beach Volumetric Change Analysis tool

Shoreline Change Analysis (SCA) and Volumetric Change Analysis (VCA) are of growing importance to coastal managers throughout the world. The volume, resolution and accuracy of shoreline configurations are gradually improving, which demands tools for efficient processing and analysis. The limited number of systems that combine the two analysis types have lengthy workflows or require commercial software licences, with no current tool that automates VCA from a time series of Digital Elevation Models (DEM’s). We present a new, dedicated and open-source package for automating SCA and VCA in the Python environment. It is designed with a user-friendly interface and workflow, delivers efficient processing speeds, automatically generates map based graphical outputs and computes a full range of positional statistics. We verify the package delivers equivalent outputs to existing tools (AMBUR, DSAS) and demonstrate strong performance by reconstructing erosion and accretion over the last twenty years along the beaches of the Taw and Torridge Estuary, North Devon, UK, a region known to have a complex sediment and morphometric dynamics

Lake sediment records of flood frequency and magnitude

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 >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 (>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

Pluridisciplinary analysis and multi-archive reconstruction of paleofloods: societal demand, challenges and progress

Floods are one of the gravest natural hazards for societies, worsened by population growth, unchecked development, and climate change. From a Global Change perspective, past extreme events merit particular interest because they can be linked to wider climate and environmental changes, introduce perturbations. During the last decade, knowledge of long-term flood frequency and magnitude has been improved by extracting data from different types of archive. But, despite advances in dating methods, proxies and statistical techniques and efforts to identify atmospheric drivers, some fundamental questions remain unresolved. The Special Issue entitled "Pluridisciplinary analysis and multi-archive reconstruction of paleofloods" in the journal Global and Planetary Change addresses these uncertainties and complexities by assembling a selection of studies, which were first presented at the Past Climate Changes (PAGES) Open Scientific Meeting held at Zaragoza in 2017. In this introductory paper, the guest editors outline the 17 research contributions and meta-data from the 17 paleoflood studies were systematically analyzed in terms of i) geographical distribution; ii) methodologies applied; iii) types of archives; iii) numbers of flood series compiled and iv) spatial and temporal resolution of paleoflood data. The data indicate that paleoflood studies focused on fluvial depositional environments show a higher rate of integration with other types of paleoflood archive (mean of 4.5 types of archive) than studies focused on documentary sources (mean of 3.5) and lake sediments (mean of 2.4). We suggest that this strategy of archive integration has been adapted to effectively compensate for the higher uncertainties of fluvial deposition in floodplains

Macronutrient processing by temperate lakes: a dynamic model for long-term, large-scale application

We developed a model of the biogeochemical and sedimentation behaviour of carbon (C), nitrogen (N) and phosphorus (P) in lakes, designed to be used in long-term (decades to centuries) and large-scale (104 – 105 km2) macronutrient modelling, with a focus on human-induced changes. The model represents settling of inflow suspended particulate matter, production and settling of phytoplankton, decomposition of organic matter in surface sediment, denitrification, and DOM flocculation and decomposition. The model uses 19 parameters, 13 of which are fixed a priori. The remaining 6 were obtained by fitting data from 109 temperate lakes, together with other information from the literature, which between them characterised the stoichiometric incorporation of N and P into phytoplankton via photosynthesis, whole-lake retention of N and P, N removal by denitrification, and the sediment burial of C, N and P. To run the model over the long periods of time necessary to simulate sediment accumulation and properties, simple assumptions were made about increases in inflow concentrations and loads of dissolved N and P and of catchment-derived particulate matter (CPM) during the 20th century. Agreement between observations and calculations is only approximate, but the model is able to capture wide trends in the lakewater and sediment variables, while also making reasonable predictions of net primary production. Modelled results suggest that allochthonous sources of carbon (CPM and dissolved organic matter) contribute more to sediment carbon than the production and settling of algal biomass, but the relative contribution due to algal biomass has increased over time. Simulations for 8 UK lakes with sediment records suggest that during the 20th century average carbon fixation increased 6-fold and carbon burial in sediments by 70%, while the delivery of suspended sediment from the catchments increased by 40% and sediment burial rates of N and P by 131% and 185% respectively