PLENARY LECTURES
RIVERINE SEDIMENT CONNECTIVITY
Ellen Wohl (Department of Geosciences, Colorado State University, USA)
Sediment, along with water, is a fundamental driver of river process and form. Sediment is more difficult to manage than water, however, because of the diverse processes, volumes, grain-size distributions, and other characteristics (e.g., contaminants) of sediment inputs, fluxes, and storage within watersheds and river corridors. The absence of direct measurements of sediment fluxes in most rivers also makes it more challenging to quantify natural or altered sediment regimes than flow regimes on gauged rivers. However, sediment budgets and the characterization of natural and balanced sediment regimes provide a conceptual framework for understanding sediment dynamics in contemporary environments, including direct and indirect human alterations of sediment dynamics. Connectivity within watersheds and within river networks strongly influence sediment dynamics. Focusing on river corridors (channels and floodplains), I review current understanding of how sediment and water inputs interact with geomorphic context to govern sediment connectivity from scales of the entire watershed to the river network to a river reach. Current understanding includes recognition of connectivity as occurring along a continuum, such that disconnectivity can be critically important in some contexts. I discuss the contexts in which understanding of riverine sediment connectivity is important for management, including hazards, water supply, and environmental sustainability.
CAPTURING VARIABILITY: BEDFORM MORPHODYNAMICS AND THE DEPOSITIONAL RECORD
Daniel Parsons (Loughborough University, UK)
Riverine flows are inherently non-stationary, varying on a range of timescales. These variations in flows field strength alter a suite of morphodynamic processes. For example, during flow field unsteadiness bedforms change in size and shape over time and in space, altering bed roughness and imparting system hysteresis between flows and form. However, our knowledge of how these bedforms adapt to changing flows remains inadequately understood. Moreover, how these variations, and hysteresis between flow and form, manifest at broader scales and control the morphodynamic and sedimentary evolution of the wider system is also poorly understood. Herein I will present a mix of laboratory flume experiments, field case studies and numerical experiments to explore some of these relationships and their controls. The results indicate how changes in primary sediment transport mechanisms can dominate how bedforms change with unsteady flows and how sediment redistribution over greater spatial scales can also play a significant role. Where substrate sediments are comprised of mixed sand and mud, the results indicate how this can play a first-order control on bedform size and aspect ratios, also altering adaption styles and rates as well as impacting the levels of hysteresis between flow and form. How this work extends our knowledge on the impact of variable flows on riverine and estuarine processes will be discussed and the broader impact and significance of the findings for a wide variety of purposes, such as improving morphodynamic modelling, environmental and engineering management, more reliable flood predictions and our understanding of the sedimentary deposits produced will also be highlighted.
UNDERSTANDING DISCHARGE CHARACTERISTICS IS THE FOUNDATION FOR CLASSIFYING ALLUVIAL DEPOSITS
Christopher R. Fielding (Department of Earth Sciences, University of Connecticut, USA) and
Jan Alexander (School of Environmental Sciences, University of East Anglia, UK)
As an alternative to using fluvial planform as the primary basis for classifying alluvial channel deposits, Fielding and others (2018, Sedimentary Geology 365, 1-20) proposed a classification based on a measure of interannual discharge variability. A suite of modern rivers for which both long-term discharge records and subsurface stratigraphic data are available was used to demonstrate that discharge variability exerts control on alluvial deposit characteristics. Deposits of rivers with low coefficients of annual peak discharge variation (CVQp) were found to be dominated by dune-scale cross-bedding and macroform stratigraphic patterns (channel forms, bar forms). With increasing CVQp, rivers tended to deposit and preserve greater proportions of transcritical flow structures, fewer macroforms, increased lithological variability and also increased evidence of in situ trees and large woody debris. Recognition of ancient examples of these variants confirms that discharge variability at various timescales (including “flashiness”) is a primary control on alluvial deposit character. Using this approach provides a means of assessing aspects of palaeoclimate in ancient alluvial successions, and in changes in palaeoclimatic conditions over time. This talk will review the method, present some recent developments in its use, and highlight the need to consider antecedent conditions.
TRACING SEQUENCE STRATIGRAPHIC SURFACES ACROSS ALLUVIAL DEPOSITS: CONSTRAINTS FROM THE QUATERNARY RECORD
Alessandro Amorosi (Department of Biological, Geological and Environmental Sciences, University of Bologna, Italy)
Understanding controls of fluvial architecture from chronologically constrained Quaternary subsurface successions provides tremendous opportunities to use sequence-stratigraphic conceptual models in a variety of applications beyond the petroleum industry, including aquifer characterization, offshore groundwater research, carbon capture and storage, subsurface engineering, groundwater contamination, and seismic hazard assessment.
Milankovitch-scale glacioeustatic oscillations shape stratal geometries and fluvial architecture of late Quaternary depositional sequences in a particular and characteristic manner. Repetitive changes in lithofacies and channel stacking patterns define rhythmically stacked, high-accommodation (floodplain-dominated) and low-accommodation (amalgamated channel-belt) systems tracts that are used as a diagnostic signature to characterise high-amplitude sea-level fluctuations. Packages of peat-bearing strata delineate the updip (freshwater to brackish) equivalents of transgressive depositional systems. Sediment core analysis supplemented by palynologic, chemostratigraphic and geotechnical data can add refinement to sequence-stratigraphic interpretations of non-marine strata.
Across the most proximal segments of Quaternary sediment routing systems, weakly developed paleosols represent commonly unrecognized key stratigraphic markers that can be traced laterally into genetically related fluvial channel-belt deposits. Downstream, at the interface with coastal depositional systems, such immature paleosols form regionally mappable interfluve sequence boundaries that correlate to coeval basal surfaces of coastal-plain paleovalley systems.
Undertaking alluvial stratigraphic systems using surfaces and facies shifts, with an emphasis on Quaternary datasets, may serve to enhance recognition of sedimentary heterogeneity in subsurface successions, which is critical in developing a useful and predictable tool to support decision-making. All these notions will be illustrated using outcrops, sediment cores and well logs from both ancient and Quaternary records.