Morphodynamic equilibrium of lowland river systems during autoretreat

Abstract

Lowland river systems (with channel slopes of 10–5 to 10–4) inevitably shift away (retreat upstream) from the receiving basin under a sustained rate of base-level rise, even if the system can maintain a period of advance at the onset of rise. This autogenic pattern of transition from progradation to retrogradation through steady base-level rise and sediment supply is termed “autoretreat.” Using a morphodynamic model of autoretreat, this study explored the varying channel hydrodynamics of lowland fluvial systems and associated stratigraphic record under sustained base-level rise and constant sediment supply. Results from the numerical simulations show that a fluvial system will reach a state of dynamic equilibrium during autoretreat where both the backwater length and the morphodynamic adjustment of the downdip channel profile become steady. Moreover, when this dynamic equilibrium state is realized, simulated systems display a persistent twofold downstream deepening of flow depth across the backwater zone, a pattern that is also present in many natural systems. In general, backwater effects play a key role in the morphodynamics of a lowland fluvial-deltaic system during autoretreat, and this hydrodynamic condition is therefore critical for predicting river responses to sea-level change.

Climate‐Change Controls on River Delta Avulsion Location and Frequency

“…This finding is consistent with historical observations of the Yellow River delta, where the avulsion location has intermittently migrated downstream with shoreline progradation over the past century (Ganti et al, 2014). Upstream migration of the backwater zone and avulsion node during sea-level rise has been observed in recent models (Moran et al, 2017;Wu et al, 2020). However, these models only considered the special case where the backwater zone originates from sea-level rise under constant discharge, which is not applicable to most lowland deltas (Chadwick et al, 2019).…”

“…Our results suggest that the onset of autoretreat will be associated with more frequent avulsions at an upstream-migrating node (Figure 13). So long as there is sufficient sediment to prograde the active lobe, the delta will continue to build back-stepping lobes and avulse during shoreline transgression-a response that is not captured in 1-D models of autoretreat (Tomer et al, 2011;Wu et al, 2020 foreset necessitates that the active lobe will eventually drown, at which point avulsions will cease to occur and the system will experience non-deltaic transgression (Tomer et al, 2011). We expect deltas with different initial basin depths and river lengths to feature altered autoretreat trajectories (Muto & Steel, 2002) and avulsion frequencies (Chadwick et al, 2020), but that large-scale response will remain consistent across deltas.…”

“…Scaled physical experiments and subsequent modeling supported the backwater hypothesis, showing deltas produced a backwater‐scaled avulsion node when subjected to variable flood regimes with subcritical Froude numbers (Chadwick et al., 2019; Ganti, Chadwick, Hassenruck‐Gudipati, Fuller, et al., 2016; Ganti, Chadwick, Hassenruck‐Gudipati, & Lamb, 2016). Numerical simulations have also isolated special cases where rapid sea‐level rise (Chadwick et al., 2019; Moran et al., 2017; Wu et al., 2020) or the assumption of a horizontal deltaic floodplain (Chadwick et al., 2019; Ratliff et al., 2021) can give rise to backwater‐scaled aggradation and avulsion without flood variability.…”

The Longitudinal Profile of a Prograding River and Its Response to Sea Level Rise

“…Note that the river mouth progradation rate could be proportional to the sediment discharge (Aadland & Helland‐Hansen, 2019); however, the dependency can be mediated by nearshore water depth, baselevel changes, waves, etc. (Bijkerk et al, 2016; Gao et al, 2020; Swenson et al, 2005; Wang et al, 2019; Wu et al, 2020; Wu & Nittrouer, 2020). In deflected river mouths under waves, the progradation rate can be insensitive to fluvial sediment load but more dependent on alongshore sediment load (Gao et al, 2020; Nienhuis et al, 2016).…”

“…As one of the key morphological characteristics of a river channel, the curvature of its longitudinal profile influences important processes, such as channel avulsion (Chadwick et al, 2019) and sediment delivery in deltaic systems (Bijkerk et al, 2016), and therefore has been extensively studied (Blom et al, 2016; Bolla Pittaluga et al, 2014; Ferrer‐Boix et al, 2016). It is well known that the evolution of a river longitudinal profile is subject to both upstream boundary conditions, including river discharge and sediment load (Blom et al, 2017; Bolla Pittaluga et al, 2014; Chatanantavet et al, 2012; Fasolato et al, 2009; Zaprowski et al, 2005), and downstream controls, including river mouth progradation and sea level rise (Blum & Törnqvist, 2000; Fagherazzi et al, 2015; Muto & Swenson, 2005; Swenson, 2005; Wu et al, 2020). Given the increasing intervention of human activities and climate change in the progradation of river mouths (Besset et al, 2019; Gao et al, 2019), understanding its effects on the evolution of rivers, particularly the change in the longitudinal profile curvature, becomes imperative.…”