Asymmetric Effects of Subaerial and Subaqueous Basement Slopes on Self‐Similar Morphology of Prograding Deltas

Abstract

Deltas form over basements of various slope configurations. While the morphodynamics of prograding deltas over single‐slope basements have been studied previously, our understanding of delta progradation over segmented basements is still limited. Here we use experimental and analytical approaches to investigate the deltaic morphologies developing over two‐slope basements with unequal subaerial and subaqueous slopes. For each case considered, the scaled profiles of the evolving delta collapse to a single profile for constant water and sediment influxes, allowing us to use the analytical self‐similar profiles to investigate the individual effects of subaerial/subaqueous slopes. Individually varying the subaerial/subaqueous slopes exerts asymmetric effects on the morphologies. Increasing the subaerial slope advances the entire delta; increasing the subaqueous slope advances the upstream boundary of the topset yet causes the downstream boundary to retreat. The delta front exhibits a first‐retreat‐then‐advance migrating trend with increasing subaqueous slope. A decrease in subaerial topset length is always accompanied by an increase in subaqueous volume fraction, no matter which segment is steepened. Applications are presented for estimating shoreline retreat caused by steepening of basement slopes, and estimating subaqueous volume and delta front using the observed topset length. The results may have implications for real‐world delta systems subjected to upstream tectonic uplift and/or downstream subsidence. Both scenarios would exhibit reduced topset lengths, which are indicative of the accompanied increases in subaqueous volume and signal tectonic uplift and/or subsidence that are at play. We highlight herein the importance of geometric controls on partitioning of sediment between subaerial and subaqueous delta components.

Morphodynamics of River Deltas in Response to Different Basin Water Depths: An Experimental Examination of the Grade Index Model

“…Rivers reaching terminal basins produce delta dynamics that vary with different basin bathymetries (Edmonds et al, 2011;Ethridge & Wescott, 1984;Lai et al, 2017). For example, shelf-top deltas (Ethridge & Wescott, 1984) are more likely to experience avulsion than shelf-edge deltas in which channels tend to be less distributive but converge to effectively funnel sediment into the deep basin (Porębski & Steel, 2003).…”

Self‐Similar Morphodynamics of Gilbert and Hyperpycnal Deltas Over Segmented Two‐Slope Bedrock Channels

“…The effects of Q / I ratio on different types of delta are shown in Figure , where each subplot includes three delta profiles associated with different Q / I ratios. For Gilbert deltas, increasing Q / I advances the entire delta, leading to a smaller topset length L t and a greater subaqueous volume V f (Lai, Hsiao, & Wu, ). These effects are clearly demonstrated in Figures a–c, where the increase of Q / I invariably accelerates the migration of shoreline λ (2) , reduces the topset length L t , and increases the subaqueous volume V f .…”

“…For Part 1, the geomorphic actions of fluvial flow over the topset are modeled as a single‐diffusion process while the bedrock‐alluvial transition and shoreline are treated as two moving boundaries (Figure a). This has been presented previously (Lai, Hsiao, & Wu, ), thus is only briefly summarized (Text S1). For Part 2, the geomorphic actions of turbid streamflow over the topset and dense underflow over the foreset are modeled as a two‐diffusion process with two moving boundaries (Figure b).…”

“…We extend the analytical framework of Lai and Capart () by incorporating the two‐slope setting (Text S1). A similarity solution for the evolving delta profile z ( x , t ) is obtained following a scaling approach used in the prior works (e.g., Capart et al, ; Lai & Capart, ; Lai, Hsiao, & Wu, ; Lorenzo‐Trueba et al, ; Voller et al, ), which may be expressed in a general form: where I is the volumetric sediment supply rate per unit width (Figures a and b); and are, respectively, the scaled (normalized) horizontal coordinate and bed surface elevation (Figures c and d). Equation indicates that the length and height of the delta evolve according to the self‐similar profile governed by the dimensionless shape function and time‐varying length scale .…”

“…We extend the analytical framework of Lai and Capart (2009) by incorporating the two-slope setting (Text S1). A similarity solution for the evolving delta profile z(x,t) is obtained following a scaling approach used in the prior works (e.g., Capart et al, 2007;Lai & Capart, 2009;Lai, Hsiao, & Wu, 2017;Lorenzo-Trueba et al, 2009;Voller et al, 2004), which may be expressed in a general form:…”