Reduction of deltaic channel mobility by tidal action under rising relative sea level

Abstract

As Holocene river deltas continue to experience sea-level rise, sediment carried by distributary channels counteracts delta-plain drowning. Many deltas worldwide are subject to tidal action, which strongly affects the morphology of distributary channels and could also influence their mobility. Here we show, through physical laboratory experiments, that distributary-channel mobility can be dramatically reduced in systems affected by tides in comparison to an identical system with no tides, and that the mobility of distributary channels decreases as the ratio of tidal to fluvial energy increases. This effect occurs even if new accommodation space is created by rising relative sea level. By analyzing synthetic stratigraphy derived from both digital elevation data and time-lapse photography, we show also that the reduction of channel mobility in tidal deltas increases channel stacking and connectivity in the stratigraphic record.

Experimental delta evolution in tidal environments: Morphologic response to relative sea‐level rise and net deposition

“…Here a value of 0.9 was used for f, based on test experiments conducted at SAFL (Baumgardner, 2015;Lentsch et al, 2018). Here a value of 0.9 was used for f, based on test experiments conducted at SAFL (Baumgardner, 2015;Lentsch et al, 2018).…”

“…We have shown in this study that the bypassing effect of tides on bedload sediment Figure 9. Pethick, 1969;Knighton et al, 1991;D'Alpaos et al, 2005;Hughes et al, 2009;Coco et al, 2013;van Maanen et al, 2015) in deltas that aid in evenly distributing water and sediment load across the delta plain, thus reducing fluvial avulsion frequency and enhancing channel stability thanks to the reduced topographic gradient that is generated (Lentsch et al, 2018). Note that exaggeration in panel (g) is ten times larger than in all the other panels.…”

“…Such features have been documented for major tidal delta systems such as the Ganges-Brahmaputra and Yangtze (Kuehl et al, 1997;Chen et al, 2000), and are typically associated with tidal acceleration, which causes strong shear stresses on the inner shelf to form a region of limited deposition separating the subaerial and subaqueous clinoforms (Swenson et al, 2005;Goodbred and Saito, 2012). In addition, tidal forcing exerted a strong control on the mobility of deltaic distributary channels, with tide-influenced and, above all, tide-dominated experimental deltas exhibiting a reduced number of distributary channels, much more planimetrically stable when compared to purely fluvial systems (Baumgardner, 2015;Rossi et al, 2016;Lentsch et al, 2018) (Figure 6; see also Figure S1). In addition, tidal forcing exerted a strong control on the mobility of deltaic distributary channels, with tide-influenced and, above all, tide-dominated experimental deltas exhibiting a reduced number of distributary channels, much more planimetrically stable when compared to purely fluvial systems (Baumgardner, 2015;Rossi et al, 2016;Lentsch et al, 2018) (Figure 6; see also Figure S1).…”

“…The rise in relative sea level (RSL) currently being experienced by most deltas worldwidedue to eustatic sea-level changes, as well as natural and anthropogenically induced subsidencethreatens the stability of these landscapes and subsequently endangers the populations and human activities they support (Syvitski and Saito, 2007;Syvitski et al, 2009;Vörösmarty et al, 2009;Giosan et al, 2014). While many recent studies investigate the role played by tides in the morphodynamic evolution of fluvial deltas under steady RSL (Goodbred and Saito, 2012;Rossi et al, 2016;Hoitink et al, 2017), little research exists on the response of deltas subject to the combined actions of tidal processes and RSL rise (Jerolmack, 2009;Lentsch et al, 2018). This may be partially due to the limited number of tidal deltas identified in the rock record (Plink-Björklund, 2012), as several characteristic tidal features that might help one to recognize the signature of tidal processes in fluvio-deltaic deposits (e.g.…”

“…funnel-shaped tidal channels, tidal mouth bars, etc.) Physical experiments can help bridge this knowledge gap, allowing one to capture delta evolution at a spatial and temporal resolution otherwise impractical in the field (Malverti et al, 2008;Paola et al, 2009;Kleinhans et al, 2012Kleinhans et al, , 2014aStefanon et al, 2012;Braat et al, 2018;Lentsch et al, 2018;Leuven et al, 2018). Physical experiments can help bridge this knowledge gap, allowing one to capture delta evolution at a spatial and temporal resolution otherwise impractical in the field (Malverti et al, 2008;Paola et al, 2009;Kleinhans et al, 2012Kleinhans et al, , 2014aStefanon et al, 2012;Braat et al, 2018;Lentsch et al, 2018;Leuven et al, 2018).…”