Delta size and plant patchiness as controls on channel network organization in experimental deltas

Abstract

Understanding the feedbacks between water, sediment, and vegetation in deltas is an important part of understanding deltas as ecomorphodynamic systems. We conducted a set of laboratory experiments using alfalfa (Medicago sativa) as a proxy for delta vegetation to investigate: (1) the effects of plants on delta growth and channel network formation; and (2) the timescales controlling delta evolution in the presence of plants. Experiments were conducted with fluctuating discharge (i.e. flood and base flow periods) and variable seeding densities. We found that when deltas were small, channels had no memory across flood cycles, as floods could completely fill the incised channel network. When deltas were large, the larger channel volume could remain underfilled to keep channel memory. Plant patches also helped to increase the number of channels and make a more distributive network. Patchiness increased over time to continually aid in bifurcation, but as vegetation cover and patch sizes increased, patches began to merge. Larger patches blocked the flow to enhance topset deposition and channel filling, even for the case of large deltas with a high channel volume. We conclude that both plant patchiness and delta size affect the development of the channel network, and we hypothesize that their influences are manifested through two competing timescales. The first timescale, Tv, defines the time when the delta is large enough for channels to have memory (i.e. remain underfilled), and the second, Tp, defines the time when vegetation patches merge, amplifying deposition and blocking channels. When run time is between these two timescales, the delta can develop a persistent distributary network of channels aided by bifurcation around plant patches, but once Tp is reached, the channel network can again be destroyed by vegetation. © 2018 John Wiley & Sons, Ltd.

Influence of Floods, Tides, and Vegetation on Sediment Retention in Wax Lake Delta, Louisiana, USA

“…This patch-scale feedback may strongly influence sediment retention on deltas, like WLD. Consider that the side channels that allow for water and sediment to enter island interiors (Figure 1a, inset) are important for sediment and water exchange between channels and islands and may be the result of patch-scale feedback (Piliouras & Kim, 2019;Wright et al, 2018). Hence, the patch-scale feedback that creates channels and connectivity counterbalances the buffering effect that limits flow onto the islands.…”

“…Previous work has shown that dense vegetation enhances sediment deposition and retention in both salt marshes and deltaic freshwater marshes by decreasing water flow velocities, enhancing bed roughness and shear strength, and directly capturing sediment in the vegetation canopy (Chen et al, 2012;Christiansen et al, 2000;Fagherazzi et al, 2012;Gedan et al, 2011;Larsen, 2019;Leonard & Luther, 1995;Lightbody & Nepf, 2006;Ma et al, 2018;Nepf, 1999;Nepf, 2012aNepf, , 2012bNeumeier & Ciavola, 2004;Tanino & Nepf, 2008;Zong & Nepf, 2010). However, in some instances, vegetation also has been shown to divert flow away from patches of densely vegetated area, resulting in reduced sediment deposition within these patches and increased erosion along their edges due to locally enhanced flow velocities (Chen et al, 2012;Nardin et al, 2016;Nardin & Edmonds, 2014;Nepf, 1999;Nepf, 2012aNepf, , 2012bPiliouras & Kim, 2019;Temmerman et al, 2005Temmerman et al, , 2007Van Dijk et al, 2013;Zong & Nepf, 2010).…”

Biogeochemical and Hydrological Variables Synergistically Influence Nitrate Variability in Coastal Deltaic Wetlands

“…The timing and magnitude of this transition are strongly controlled by external drivers and the extent and density of vegetation coverage. Previous work shows that vegetation patterns can significantly influence flow and sediment in wetlands (Ma et al., 2018; Piliouras & Kim, 2019; Piliouras et al., 2017; Wright et al., 2018) and are linked to insights into nitrate removal (Knights et al., 2020), making vegetation a driving mechanism on transitions in hydrological and biogeochemical process connections.…”

Hydrological Connectivity Controls Magnitude and Distribution of Sediment Deposition Within the Deltaic Islands of Wax Lake Delta, LA, USA

“…Recent work has pointed to the importance of patchy vegetation in creating flow variability that can lead to channelization. Numerical experiments show patches of sufficient size and density concentrate flow between them leading to erosion, and channelization (Yamasaki et al, 2019), a process which may create and maintain the secondary channels on the islands of WLD (Piliouras & Kim, 2019;Wright et al, 2018). Future work would benefit from looking at how patchy vegetation influences the patterns of sediment deposition, and whether this is more important than the contrasting effects of rivers and tides described in Figure 8.…”