IMPACTS OF NON-UNIFORM FLOW HYDRAULICS ON FLUVIAL-DELTAIC STRATIGRAPHY: LINKING FIELD INVESTIGATIONS AND HYDRODYNAMIC MODELING

Non-uniform “backwater” flow develops where a system approaches its terminal basin. Boundary shear stress decreases at the onset of backwater flow in response to increasing cross-sectional flow area of the channel. As a result, sediment transport capacity decreases and the coarser fraction of load is deposited on the channel bed. Although backwater conditions are common and a recognized feature for the morphodynamics of modern fluvial-deltaic channels, it’s impact on the stratigraphic record remains unclear. Here we explore stratigraphic response to non-uniform flow under various allogenic forcing using a one-dimensional morphodynamic model for river flow. Data from the Tullig Sandstone in the Western Irish Namurian Basin were used to constrain the sediment transport model and test the relevance between backwater hydrodynamics and stratigraphic signatures.

The model results show that: (1) modeled fluvial-deltaic systems never reach an equilibrium state, whereby flow hydraulics are steady and the onset of non-uniform flow propagates upstream beyond the backwater transition that is typically estimated with backwater length scales. (2) Base level fluctuations enhance the development of non-uniform flow. (3) Non-uniform flow hydrodynamics produce a spatial variation in flow depth, channel bed slope, and grain size over the delta, so that paleohydraulic conditions are potentially preserved in the rock record.

Field data show that the average cross bed set thickness increases and then decreases, progressing downstream over the backwater reach; taken as a proxy for channel size, these observations indicate a similar trend in paleoflow depth. The variation in the distribution of cross bed set thickness informs backwater hydrodynamics for the formative river flow, which are consistent with the model results. Vertical changes in grain size and bed thickness suggest that backwater hydrodynamics of the ancient river co-evolved with base level change. Therefore, with a proper tuned morphodynamic model, these signals (i.e., vertical and lateral changes in grain size, sandstone bed and cross bed set thicknesses) could be a useful tool for differentiating between short-term autogenic and long-term allogenic processes for ancient fluvial-deltaic systems.