Quantifying bankfull flow width using preserved bar clinoforms from fluvial strata

Evan Greenberg,Vamsi Ganti,Elizabeth Hajek

doi:10.1130/g48729.1

Evan Greenberg, Vamsi Ganti + Show 1 more

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https://doi.org/10.1130/g48729.1

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Abstract

AbstractReconstruction of active channel geometry from fluvial strata is critical to constrain the water and sediment fluxes in ancient terrestrial landscapes. Robust methods—grounded in extensive field observations, numerical simulations, and physical experiments—exist for estimating the bankfull flow depth and channel-bed slope from preserved deposits; however, we lack similar tools to quantify bankfull channel widths. We combined high-resolution lidar data from 134 meander bends across 11 rivers that span over two orders of magnitude in size to develop a robust, empirical relation between the bankfull channel width and channel-bar clinoform width (relict stratigraphic surfaces of bank-attached channel bars). We parameterized the bar cross-sectional shape using a two-parameter sigmoid, defining bar width as the cross-stream distance between 95% of the asymptotes of the fit sigmoid. We combined this objective definition of the bar width with Bayesian linear regression analysis to show that the measured bankfull flow width is 2.34 ± 0.13 times the channel-bar width. We validated our model using field measurements of channel-bar and bankfull flow widths of meandering rivers that span all climate zones (R2 = 0.79) and concurrent measurements of channel-bar clinoform width and mud-plug width in fluvial strata (R2 = 0.80). We also show that the transverse bed slopes of bars are inversely correlated with bend curvature, consistent with theory. Results provide a simple, usable metric to derive paleochannel width from preserved bar clinoforms.

Highlights

Reconstruction of formative channel geometry from fluvial strata is critical to constrain the ancient hydrology and terrestrial mass fluxes on Earth and other planets (e.g., Bhattacharya et al, 2016), unravel fluvial responses to past climate change (Foreman et al, 2012), and aid hydrocarbon exploration (e.g., Miall and Tyler, 1991)
To test model sensitivity to these effects, we assumed that the bar clinoform preservation in fluvial strata is randomly distributed with respect to cross-section obliquity and bend position, and we generated channel cross sections at varying angles, θ, to the centerline direction across Trinity River (Texas, USA) bars, yielding a data set of channel cross sections at θ ∈[0°, 90°]
Using high-resolution lidar data and field observations of Wbar and Bbf from 19 rivers, and concurrent field measurements of preserved bar clinoform width and mud-plug widths from fluvial strata, we showed that the formative channel width is 2.34 ± 0.13× the measured channel-bar width

Summary

Introduction

Reconstruction of formative channel geometry from fluvial strata is critical to constrain the ancient hydrology and terrestrial mass fluxes on Earth and other planets (e.g., Bhattacharya et al, 2016), unravel fluvial responses to past climate change (Foreman et al, 2012), and aid hydrocarbon exploration (e.g., Miall and Tyler, 1991). Bankfull channel width predicted with Equation 3 showed good agreement with the measured Bbf at the cross-sectional (R2 = 0.64), bend-averaged (R2 = 0.73), and reach-averaged (R2 = 0.93) scales (Fig. 3B). Our model (Equation 3) predicted Bbf for the compiled natural and numerical rivers (R2 = 0.79; Fig. 3D) and performed significantly better than the existing power-law (R2 = 0.55) and linear (R2 = 0.23) width-to-depth scaling relations.

Results

Conclusion