Morphodynamic modeling of fluvial channel fill and avulsion time scales during early Holocene transgression, as substantiated by the incised valley stratigraphy of the Trinity River, Texas

Abstract

The Trinity River system provides a natural laboratory for linking fluvial morphodynamics to stratigraphy produced by sea-level rise, because the sediments occupying the Trinity incised valley are well constrained in terms of timing of deposition and facies distribution. Herein, the Trinity River is modeled for a range of base-level rise rates, avulsion thresholds, and water discharges to explore the effects of backwater-induced in-channel sedimentation on channel avulsion. The findings are compared to observed sediment facies to evaluate the capability of a morphodynamic model to reproduce sediment deposition patterns. Base-level rise produces mobile locations of in-channel sedimentation and deltaic channel avulsions. For scenarios characteristic of early Holocene sea-level rise (4.3 mm yr À1 ), the Trinity fluvial-deltaic system progrades 13 m yr À1 , followed by backstepping of 27 m yr À1 . Avulsion is reached at the position of maximum sediment deposition (located 108 km upstream of the outlet) after 3,548 model years, based on sedimentation filling 30% of the channel. Under scenarios of greater base-level rise, avulsion is impeded because the channel fill threshold is never achieved. Accounting for partitioning of bed-material sediment between the channel and floodplain influences the timing and location of avulsion over millennial time scales: the time to avulsion is greatly increased. Sedimentation patterns within the valley, modeled and measured, indicate preference toward sandy bed material, and the rates of deposition are substantiated by previous measurements. Although the results here are specific to the Trinity River, the analysis provides a framework that is adaptable to other lowland fluvial-deltaic systems. appropriate over the late Holocene and time scales of a few centuries Nittrouer et al., 2012;Ganti et al., 2014]. However, during the early and middle Holocene, fluvial-deltaic systems would have been MORAN ET AL.MODELING OF CHANNEL FILL AND AVULSIONS 215Key Points:• Fluvial channel sedimentation and avulsion location backstep during sea-level rise • Time to avulsion scales with increasing rate of sea-level rise • Floodplain capture of sediment is necessary to model influence of base-level rise on stratigraphy (2017), Morphodynamic modeling of fluvial channel fill and avulsion time scales during early Holocene transgression, as substantiated by the incised valley stratigraphy of the Trinity River, Texas,

Origin of a Preferential Avulsion Node on Lowland River Deltas

“…In contrast, constant discharge numerical experiments tend toward a graded state without strong backwater effects or a preferential node location. Our results suggest that previous numerical models that lacked variable discharges and produced backwater‐scaled avulsions (Moran et al, ; Ratliff, ) were likely affected by initial conditions of uniformly sloped initial surfaces. A backwater‐scaled avulsion node under these conditions is a geometric consequence of assessing superelevation of a prograding channel or lobe relative to a planar seaward sloping landscape.…”

“…In particular, the model of Chatanantavet et al (2012) requires flow variability to produce a persistent avulsion node, consistent with available experimental data, but the elimination of river mouth progradation in their model might have biased their results. In contrast, more recent models (Moran et al, 2017;Ratliff, 2017) can produce persistent avulsion nodes with constant discharge, but they impose an unrealistic initial condition. To address these potentially problematic assumptions, we constructed a quasi-2-D numerical model that allows for repeated lobe construction and avulsion such that lobes build on top of one another, thereby minimizing the role of initial topography as the delta evolves.…”

Impacts of backwater hydrodynamics on fluvial–deltaic stratigraphy

“…This work explored how backwater and base‐level adjustments combine to influence sediment deposition patterns. Moran, Nittrouer, Perillo, Lorenzo‐Trueba, and Anderson () developed a morphodynamic model for the Trinity River (Texas) by considering Holocene sea‐level rise. This work documents a consistent back‐stepping of the backwater zone, accompanied by bed material sediment filling of the Trinity River incised valley.…”

“…Since the 1D model is width‐averaged and width changes in the along‐stream direction for the channel and floodplain is not considered, the inclusion of , , and in the model helps to account for deposition in floodplain and flood intermittency of natural rivers so that the model simulation has application to real rivers (Parker, ). Short of measuring these parameters from field data, standard values from modern lowland systems that scale with the ancient basins are utilized (Table ) (Moran et al, ; Parker et al, ). The channel bed profile evolves iteratively with each time step by solving Equations (1)–(3), over the horizontal distance ( x ).…”

Modeling Deltaic Lobe‐Building Cycles and Channel Avulsions for the Yellow River Delta, China

“…where H bf is a characteristic channel bankfull flow depth and is a coefficient that varies between 0.3 and 1 on modern delta systems but may be >1 for fan-delta systems (Ashworth et al, 2004;Bryant et al, 1995;Ganti et al, 2014Ganti et al, , 2016aGanti et al, , 2016bJain & Sinha, 2004;Jerolmack & Mohrig, 2007;Heller & Paola, 1996;Martin et al, 2009;Moran et al, 2017;Mohrig et al, 2000;Stouthamer & Berendsen, 2001;Törnqvist, 1994). Ganti et al, 2014, after Pang & Si, 1979van Gelder et al, 1994).…”