The Interplay between Flow Field, Suspended Sediment Concentration, and Net Deposition in a Channel with Flexible Bank Vegetation

Abstract

This paper investigates the interplay between the flow, suspended sediment concentration (SSC), and net deposition at the lateral interface between a main channel and riverbank/floodplain vegetation consisting of emergent flexible woody plants with understory grasses. In a new set of flume experiments, data were collected concurrently on the flow field, SSC, and net deposition using acoustic Doppler velocimeters, optical turbidity sensors, and weight-based sampling. Vegetation largely affected the vertical SSC distributions, both within and near the vegetated areas. The seasonal variation of vegetation properties was important, as the foliage strongly increased lateral mixing of suspended sediments between the unvegetated and vegetated parts of the channel. Foliage increased the reach-scale net deposition and enhanced deposition in the understory grasses at the main channel–vegetation interface. To estimate the seasonal differences caused by foliation, we introduced a new drag ratio approach for describing the SSC difference between the vegetated and unvegetated channel parts. Findings in this study suggest that future research and engineering applications will benefit from a more realistic description of natural plant features, including the reconfiguration of plants and drag by the foliage, to complement and replace existing rigid cylinder approaches.

Interactions between aquatic vegetation, hydraulics and fine sediment: A case study in the Halswell River, New Zealand

“…In the Halswell river it is likely that fine sediment was preferentially transported down flow paths between the dense vegetation, then laterally transported and/or diffused into the dense stands of vegetation where accumulation was observed. This is a similar configuration of vegetation, flow and sedimentation to Cotton et al (2006) and Box et al (2019), rather than the isolated vegetation patches of Sand-Jensen (1998), Biggs et al (2019) and Przyborowski et al (2019). At the study sites of Cotton et al (2006) an accumulation of 6-16 cm of fine sediment was reported within dense macrophyte stands.…”

Flow dynamics and sediment transport in vegetated rivers: A review

“…In these studies, some controversy appeared. For example, sediment deposition in the vegetation region is twice or triple as large as that in the region without vegetation [142] : however, in another experiment [140] , the sediment deposition in the vegetated region is nearly the same as the non-vegetated region. One of the probable factors is the different types of vegetation used.…”

“…When vegetation covers the half width of the river, not only the profile of deposition along the streamwise but also along the cross-section must be considered [139][140][141][142] . In these studies, some controversy appeared.…”

“…One of the probable factors is the different types of vegetation used. Rigid cylinders were used in the study of Zong and Nepf [142] while flexible foliated vegetation were used by Box et al [140] . The explicit reason for these differing experimental results is not clear and further elaboration still awaits.…”

Floodplain flow resistance in case of a sparse mixture of plants at low relative submergences

“…Artificial plants consisting of grasses and flexible foliated woody plants were selected to represent similar flexibility and reconfiguration behaviour as natural floodplain plants, as confirmed in prior investigations by (Box et al 2019, Caroppi et al 2019. Five different types of flexible woody plants were fabricated (S3, S4, T3, T4, T7, see Table 2 for details).…”