Bridging The Gap Between Morphodynamic and Stratigraphic Models: A Study of The Formative Conditions of Anabranching Rivers

Abstract

Morphodynamical modeling is a useful quantitative approach to explore the effect of boundary conditions in fluvial systems as well as to examine river evolution and deposition. While morphodynamic models are traditionally used for short time-scale engineering approaches, they can also be applied to explore longer time-scale stratigraphic problems. Here we explore the effect of contrasting boundary conditions on anabranching rivers to determine which conditions are necessary for their development. The aim of this study is to show how morphodynamic models can lead to useful, testable stratigraphic hypotheses. Our modeling approach is to use boundary conditions from four different field scale anabranching rivers, spanning different climatic and geologic settings, to drive modeling experiments. In this way, our model simulations have the same general set of dynamics as the field cases but are not reproductions of them. The boundary conditions we used in the simulations are: down-valley floodplain slope, characteristic minimum and maximum discharge, median grain size, and average valley-width. These values were obtained from field measurements on the Neales River, Australia and from published literature for the Columbia, Negro, and Congo Rivers. We envision our model setup as representing the river valley prior to the development of anabranching. For each run the initial conditions consist of a flat, sloping floodplain with 1 cm white noise bumps randomly placed on the surface. This condition would correspond to a scenario where water and sediment have escaped through a crevasse and entered the floodplain. Results from all anabranching simulations show that bed instability develops first, followed by a transient stage from which anabranches develop and become fully stable. We propose that anabranching develops in our simulations chiefly because the valley-width is uncoupled from discharge. This arises because during flood stage the width of the inundated plain is uncorrelated with discharge. Once the anabranching configuration is achieved, the individual channels are large enough to carry all the discharge at the same flood discharge (preventing wall to wall flooding). This study provides a valuable quantitative approach for understanding the formation of anabranching rivers. These formative conditions for anabranching systems should be recorded in the stratigraphy of natural systems; our future research will focus on evaluating this.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015