Island Formation Resulting From Radially Symmetric Flow Expansion

Abstract

Island formation and distributary channel branching are important processes in prograding river deltas. We develop and test a new theory predicting the distance to islands and channel bifurcations based on fluid mass conservation and radially symmetric transport conditions. We analyze channelization and island formation using nine new and five existing delta experiments as well as four field deltas. The new experiments were designed to produce islands from initial deposition of a mouth bar. Before island formation, each bar evolved into a radially symmetric deposit with unchannelized flow over its top previously described as a topographic flow expansion. This morphology was stable to topographic perturbations, and its distal limit prograded basinward while maintaining a characteristic flow depth. Island formation and channel branching occurred on top of this deposit. We hypothesize that this distance (Ψ) is set by the location where boundary shear stress applied by expanding, radially averaged flow falls below the threshold of sediment motion. The model predicts that the distance to the first island scales with water discharge, scales inversely with flow depth, and scales with the inverse square root of median grain diameter. From experiment to field scales, distances to island locations are predicted within a factor of two.

On the incipient formation of bars and channels on alluvial fans

“…The green line represents the radial distance Ψ m of the islands (bar apices) from the inflow cross section according to the formula proposed by Shaw et al. () [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…We note several features common to the experimental observations of Shaw et al. (). Firstly, the maximum bar elevation occurs at a given distance from the inflow cross section (in our case r ≃ 11).…”

“…Shaw et al. () proposed that the transition from unchannelized expanding flow to island formation occurs when the expansion causes the shear stress to drop below critical, and used this idea to predict the radial distance of the islands from the inflow cross section.…”

“…Similar experiments have been more recently performed by Shaw et al. (). In this case, the formation of a series of islands was observed to arise from the initial deposition of a mouth bar.…”

Measuring Subaqueous Progradation of the Wax Lake Delta with a Model of Flow Direction Divergence

“…The difference arises because the Wax Lake Delta has an extensive delta front deposit that lies below low tide. While the subaerial and shallow land area are where marshes are established (Johnson et al, 1985), the subaqueous delta forms the platform upon which subaerial islands grow (Cahoon et al, 2011;Shaw et al, 2018). Hence, the subaqueous platform extent is important as a leading indicator of future marsh growth, necessary data for navigation, and the key area metric for estimating delta volume and volume 30…”