Fluvial sediment supply and pioneer woody seedlings as a control on bar‐surface topography

Abstract

Plants influence river channel topography, but our understanding of the interaction among plants, flow, and sediment is limited, especially when sediment supply is variable. Using laboratory experiments in a recirculating flume with live seedlings in a mobile sand bed, we demonstrate how varying the balance between sediment supply and transport capacity shifts the relationship between plants and bar‐surface topography. Each experimental trial contrasted two sediment conditions, in which initially supply was maintained in equilibrium with transport via sediment recirculation, followed by sediment deficit, in which transport capacity exceeded supply, which was set to zero. For both sediment balances, the topographic response was sensitive to plant size, with larger plants inducing greater aggradation relative to a baseline condition. During sediment equilibrium, the positive relationship between plant size and topographic change also depended on species morphology (multi‐stemmed shrubs versus single‐stemmed plants). Plant morphology effects disappeared when the sediment balance shifted to a deficit, but the presence of plants had a greater impact on the magnitude of change compared to the topographic response under sediment equilibrium. Our results suggest that the interactions among sediment supply, plants, and topography may be strongest on rivers with a balance in sediment supply and transport capacity. Because of the large variability in fluvial sediment supply resulting from natural and anthropogenic influences, these interactions will differ spatially (e.g. longitudinally through a watershed) and at different temporal scales, from single flood events to longer time periods. Copyright © 2016 John Wiley & Sons, Ltd.

The influence of a vegetated bar on channel-bend flow dynamics

“…Elevated sediment deposition within patches of woody seedlings, with variations depending on plant characteristics, has been documented in meandering (Kui et al, 2014) and straight (Diehl et al, 2017b) flumes. Gorrick and Rodríguez (2012), working in a flume in which vegetation patches were simulated with dowels, documented elevated fine-sediment deposition within the patches (Gorrick and Rodríguez, 2012).…”

Can environmental flows moderate riparian invasions? The influence of seedling morphology and density on scour losses in experimental floods

“…All of these collinear traits increase plants’ ability to stabilise the substrate and reduce water velocities and scour forces. Our experiments showcased this ability of plants to engineer their local environment (Diehl, Merritt, et al., ; Diehl, Wilcox, et al., ), thereby controlling the rate of scour and eventual plant dislodgement. Given that only 1% of tamarisk that occurred in tall patches were lost, the lowest rate of any of the configurations, we would expect that bankfull and other moderate floods would not be likely to dislodge tamarisk seedlings larger than 40 cm in height ( tall tamarisk in our experiment), particularly those occurring in high densities along river margins.…”

“…As sediment was evacuated from the flume, the bed degraded, reducing the bed slope and changing the hydraulic and sediment transport conditions (Manners et al., ). The rate at which the bed slope changed during a run depended on the plant configuration (Diehl, Wilcox, et al., ).…”

“…To evaluate the relationship between plant dislodgement and local patterns of bed scour (Bywater‐Reyes et al., ), we calculated each plant's scour ratio , which was the maximum scour depth at the plant's location derived from pre‐ and post‐flood laser scans of the bed (Diehl, Wilcox, et al., ) divided by its root length. We used ANOVA tests to compare the scour ratio between species and between groups of plants that dislodged versus those that remained rooted in place.…”

“…() and Diehl, Wilcox, et al. (), who analysed within the same experiment the interactions of plants and sediment transport conditions on flow hydraulics, sediment transport, and bed morphology. This present study focuses on the fate of the plants themselves during floods, specifically how differences in root and crown architecture influence seedlings’ vulnerability to flood scour.…”