Morphodynamics and sedimentology of the tidally-influenced fluvial zone (TIFZ)

Lead Research Organisation: University of Exeter
Department Name: Geography

Abstract

All rivers across the globe that exit to the ocean contain a zone, which can be 100s of kilometres long, which is transitional between river and tidal environments (termed here the Tidally-Influenced Fluvial Zone, or TIFZ). This zone is one of the most complex environments on the surface of the Earth because it is an area where both river flow and tidal currents are significant, and these competing forces vary daily, seasonally and annually. These regions are important to mankind and form some of the areas of highest population density: they are strategically important in the present day because these zones are at the interface of competing demands for shipping, aquaculture, land reclamation and nature conservation. Thus in order to better maintain, manage and protect these fragile zones, we must understand how and why these regions change and what factors control such change. Additionally, the sediments of ancient TIFZ's may contain significant volumes of hydrocarbons which are increasingly the target for many energy companies. For example, the Athabasca oil sands form the largest petroleum deposit on Earth and these bitumen tars are locked up with ancient TIFZ sediments. Understanding the internal nature of such TIFZ sediments is thus of paramount importance when attempting to extract the maximum quantity of oil (or gas) from such ancient hydrocarbon reservoirs - we need to know what controls the geometry and internal characteristics of these reservoirs, and thus better plan efficient and maximal hydrocarbon extraction strategies. Thus all of these interests in both modern and ancient TIFZ environments depend on a detailed knowledge of the fluid flows in these areas, how such flows transport their sediment and critically how the form (or morphology) of these environments changes through time. However, due to the extraordinary challenges of working in such a complex and dynamic environment, few high-resolution, spatially-representative, field datasets exist and remarkably little work has been undertaken on the diagnostic internal sedimentary structure of such TIFZ deposits. Additionally, whilst there has been progress on the mathematical modelling of estuarine flow and sediment transport, these models remain largely untested. There is therefore a pressing need to link the processes and deposits of the TIFZ through an integrated study of their flow, morphology and sediment movement to quantify the key processes and how these are represented within the subsurface sedimentary record. This proposal outlines an integrated field and mathematical modelling study that seeks to achieve a step-change in our understanding of the TIFZ, using the very latest techniques in field survey and mathematical modelling. These techniques will yield unrivalled high-resolution datasets of bathymetry, flow, sediment transport and sedimentary structure that will then be used to construct and validate new numerical models of the TIFZ. This will ultimately allow evaluation of key unknowns with respect to the TIFZ, such as how such environments evolve under changing scenarios of tidal and fluvial contributions associated with sea-level change, and whether it is possible to differentiate between 'fluvial' and 'tidally' influenced deposits. Such results will transform our understanding of how such TIFZ zones behave in modern environments and critically how these changes may be recognized within ancient sedimentary successions.

Publications

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Prokocki E (2015) Fluvial-Tidal Sedimentology

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Sandbach S (2018) Hydrodynamic modelling of tidal-fluvial flows in a large river estuary in Estuarine, Coastal and Shelf Science

 
Description The key findings are: (i) due to the strong lateral contrasts within the Columbia River grain size is not a reliable indicator of downstream location within the TIFZ (ii) few unique tidal signatures are preserved in areas even with a significant tidal range (iii) The upstream part of the minimum EFD (energy flux divergence) region records a type of IHS (Inclined Heterogenous Strata) not commonly reported in the literature. Bars containing these pervasive low-angle sandy-silt surfaces may be indistinguishable from purely fluvial fining-up successions are hence display only a subtle tidal influence (iv) More typical TIFZ deposits are largely restricted to the embayments and very downstream part of the minimum EFD region.
Exploitation Route See summary of key findings provided by lead institution on this split award (PI Ashworth, University of Brighton)
Sectors Environment