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

Abstract

Delta progradation over segmented two‐slope bedrocks is prevalent in nature, where the bedrock slope upstream or downstream of the slope‐break knickpoint is steepened by continued tectonic uplift or subsidence. Understanding the morphodynamics of the most common types of delta in different slope settings is important for interpreting the observed delta progradation into reservoir and predicting delta evolutions under future tectonic/climate scenarios or anthropogenic interventions. Here, we present an experimental and analytical study demonstrating the morphological responses of Gilbert‐type and hyperpycnal deltas to variations of bedrock slopes. Steepening the upstream slope accelerates shoreline migration; steepening the downstream slope decelerates shoreline migration. In either case, the subaqueous volume is enhanced yet the subaerial volume is reduced. Both types of delta exhibit self‐similar morphologies when evolving over segmented bedrocks. Hyperpycnal deltas, through enhanced sediment fluxes driven by dense underflows, develop larger subaqueous volumes. We further demonstrate the underlying self‐similarities in sediment flux and bed growth rate that come into play for attaining the self‐similar morphologies. The combined effect of flowrate (Q) and sediment supply rate (I) may be characterized by a dimensionless Q/I ratio. Increasing Q/I advances the entire delta. For Gilbert deltas, shoreline migration accelerates with Q/I. For hyperpycnal deltas, shoreline migration exhibits a “first‐accelerate‐then‐decelerate” trend with Q/I in a limited range of slope combinations close to the single‐slope setting, indicating that the effect of Q/I emerges only when the two‐slope effect is weak or absent. Away from this near‐single‐slope range, the two‐slope effect becomes dominant, thus suppressing the effect of Q/I.

Two‐Stage Transition From Gilbert to Hyperpycnal Delta in Reservoir

“…Experiments were conducted in a small flume (270 cm long, 1 cm wide) installed in an external water tank, modified from Lai et al. (2019), Lai, Hsiao and Wu (2017) (see Figure for details). Constant inflows per unit width (mm 2 /s) of clear water and saturated brine (density 1,200 kg/m 3 ) were used, respectively, as homopycnal and hyperpycnal flows (Lai et al., 2016, 2019, Lai, Hung, et al., 2017), fed with constant sand influxes per unit width (mm 2 /s), with that established supply limited conditions (Lai et al., 2019).…”

“…(2019), Lai, Hsiao and Wu (2017) (see Figure for details). Constant inflows per unit width (mm 2 /s) of clear water and saturated brine (density 1,200 kg/m 3 ) were used, respectively, as homopycnal and hyperpycnal flows (Lai et al., 2016, 2019, Lai, Hung, et al., 2017), fed with constant sand influxes per unit width (mm 2 /s), with that established supply limited conditions (Lai et al., 2019). Ottawa standard sand (with median grain size mm, density kg/m 3 , porosity , angle of repose ) was used as bedload material for deltaic deposits.…”

“…The above-stated transitional processes apply to all slope angles used and all experiments either starting with Gilbert or hyperpycnal delta (see Figures S2 and S3 for full demonstration of six transitional runs). Because transition from hyperpycnal to Gilbert delta is a simple depositional process over the topset and LAI AND WU foreset of hyperpycnal delta, such transition is not substantially different from Gilbert delta progradation over two-slope bedrock presented in earlier studies (Lai et al, 2017a(Lai et al, , 2019. Thus, when answering the second research question posed in Section 1, we focus on quantitative descriptions of the two-stage transitional processes from Gilbert to hyperpycnal delta.…”

“…Examples of Gilbert‐type delta in reservoir have been documented in the side tributary of Wushe Reservoir (central Taiwan), Englebright Lake of northern California, and Lake Mills of western Washington State (Ke & Capart, 2015; Snyder et al., 2006; Stratton & Grant, 2019). Well‐known hyperpycnal deltas in reservoir have been exemplified by the Colorado River delta in Lake Mead, Tarbela delta in the upper Indus River (northern Pakistan), and rapid infill of Wushe Reservoir by the trunk river delta (Ke et al., 2019; Kostic et al., 2002; Lai & Capart, 2009; Lai, et al., 2019).…”

“…Given the highly probable shifts in flow/sediment regimes and in relative density that could be triggered by natural or anthropic disturbances (Lai et al., 2019; Stratton & Grant, 2019), we need a better understanding of the corresponding morphological responses to forecast the spatial distribution of deltaic deposits in reservoirs. So far, however, no dedicated study has been conducted to investigate how such transitions occur and whether these processes can be quantitatively described.…”