Backwater Length Estimates in Modern and Ancient Fluvio-Deltaic Settings: Review and Proposal of Standardized Workflows

The backwater effect (i.e. adjustments in open-channel flow as a response to proximity of standing water) is used to predict down-dip changes in fluvial morphodynamics and consequent sediment distribution on delta plains. However, there is currently no standardized method to obtain input parameters to estimate backwater length, nor consensus on where to measure these variables, for both modern and ancient settings. This study reviews existing methods for estimating backwater lengths in ancient and modern settings and proposes workflows to minimize ambiguity in resultant estimates.

The proposed workflows are tailored to both modern and ancient settings and are prioritized based on practicality, accuracy, smallest uncertainty ranges and allow different data types as input parameters. For the first time, application of multiple methods, i.e. the proposed workflows, is tested on a single modern and ancient river system. In the modern case study, riverbed intersection with sea level matches previously documented major changes in sedimentary trends. However, backwater lengths based on h/S (h = bankfull thalweg channel dept, S = slope) plot downstream of this zone characterized by major changes, when input parameters are derived from discharge and grain size. In the ancient case study, backwater lengths based on bankfull thalweg channel depth and slope based on Shields’ empirical relation with grain size, match changes in fluvial architectural style interpreted as a result of backwater effects. The proposed workflows facilitate comparability and applicability of future backwater length estimates and their corresponding influence on the hydrodynamic environment and ultimately quantification of source-2-sink segments.