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

Abstract

This paper contributes a field study of suspended sediment transport through aquatic vegetation. The study was run over a 3 month period which was selected to coincide with scheduled weed cutting activities. This provided the opportunity to obtain data points with no vegetation cover, as well as to investigate the effects of weed cutting on Suspended Sediment Concentrations (SSC), particle size distributions and river hydraulics. Aquatic vegetation cover was quantified through remote sensing with Unmanned Aerial Vehicles and biomass estimated from ground truth sampling. SSC was highly dependent on aquatic vegetation abundance, and the distance upstream that had been cleared of aquatic vegetation. The data indicates that fine sediment was being trapped and stored by aquatic vegetation, then likely remobilised after vegetation removal. Investigation of suspended sediment spatial dynamics illustrated changes in particle size distribution due to preferential settling of coarse particles within aquatic vegetation, for example D50 decreased from 36.08 μm to 15.64 μm after suspended sediment travelled 304.2 m downstream and passed ~3700 kg of aquatic vegetation biomass. Hydraulic resistance in the study reach (parameterized by Manning's n) dropped by over 70% following vegetation cutting. Prior to cutting hydraulic resistance was discharge dependent (likely due to vegetation pronating at higher flows), while post cutting hydraulic resistance was approximately invariant of discharge. Aerial surveying captured interesting changes in aquatic vegetation cover prior to vegetation cutting, where some very dense regions of aquatic vegetation were naturally removed (without any high flow events) leaving behind unvegetated riverbed and fine sediment. The weed cutting boat had a lower impact on SSC than was originally expected, which indicates that it may offer a less damaging solution to aquatic vegetation removal in rivers than some other approaches such as mechanical excavation. This paper contributes valuable field data (which are generally scarce) on the research topic of flow‐vegetation‐sediment interactions, to supplement laboratory and numerical studies.

Longitudinal dispersion affected by willow patches of low areal coverage

“…In fact, patches located near to the banks created notably longer residence times than channel centre patches and non‐vegetated conditions, while their flow resistance was substantially lower than for the centre patches and only slightly higher than for the non‐vegetated conditions. Thus, leaving vegetation on channel margins while cutting it from the channel centre, as also proposed by Errico et al (2019), provides a potential scenario for environmentally preferable vegetation management as an alternative to the common complete vegetation cut to ensure flow conveyance (Bączyk et al, 2018; Biggs et al, 2021). These implications cannot be directly extended to mean velocities other than the examined range of ~ 0.3–0.4 m/s as the influence of u m on the modification of flow resistance and hydrodynamics at the patch mosaic scale is not fully clear (see also Barcelona et al, 2021; Licci et al, 2019).…”

“…Careful patch definition is particularly important in measuring and modelling, as different delineations are associated with different functions of the patches (Schoelynck et al, 2018). Vegetation patches can have areal coverages of 0–1 (Biggs et al, 2021; Verschoren et al, 2017; Yamasaki et al, 2019). A coverage of 0 represents an area devoid of vegetation, for example after hydraulic engineering works, vegetation maintenance or geomorphological events uprooting plants or burying vegetation (Wilcox & Shafroth, 2013).…”

Scale-dependent effects of vegetation on flow velocity and biogeochemical conditions in aquatic systems

“…For longer patches, the within-patch velocity was always reduced, except for a patch of intermediate length at the BC site, where u 20 was positive, indicating flow acceleration above the canopy at the depth where velocity was measured. In these patches, the sediment grain size (d0.5) was smaller, indicating accumulation of fine sediment, probably due to the reduction in velocity within the patches that favours the sedimentation of smaller particles (Liu and Nepf, 2016;Licci et al, 2019;Biggs et al, 2021). Some modifications of withinpatch sediment grain size may also be due to the retention of suspended and bed-transported particles inside plant patches by collision with stems and leaves (Hendriks et al 2008;Pluntke and Kozerski 2003).…”

“…Patches are porous structures through which flow can partially pass, but with a reduced velocity relative to the upstream conditions (Sand-Jensen and Pedersen, 2008;Folkard, 2011;Vandenbruwaene et al, 2011;Nepf, 2012;Marjoribanks et al, 2017;Licci et al, 2019), which causes the flow to deflect and accelerate above and next to the canopy, locally increasing water velocity and turbulence at the edges of the patch (Sand-Jensen and Mebus, 1996;Sand-Jensen and Pedersen, 2008;Sukhodolov and Sukhodolova, 2010;Folkard, 2019). As a result, inside plant patches, the potential for resuspension and erosion is reduced Schulz et al, 2003;Hendriks et al, 2009), and fine sediment tends to accumulate compared to bare areas Biggs et al, 2021), whereas flow acceleration next to the patch contributes to particle entrainment and transport Schoelynck et al, 2013). As a consequence, plant growth and thus patch expansion could be locally enhanced inside or immediately downstream of a patch due to reduced hydrodynamic stress, a lower risk of mechanical damage for plants (breakage, uprooting) and accumulation of fine particles (Wharton et al, 2006;Jones et al, 2012;Biggs et al, 2021;Reitsema et al, 2021), which in turn increases nitrogen and phosphorus concentrations Clarke and Wharton, 2001;Sanders and Trimmer, 2006;Schoelynck et al, 2017) and enhances nutrient availability for plants.…”

“…As a result, inside plant patches, the potential for resuspension and erosion is reduced Schulz et al, 2003;Hendriks et al, 2009), and fine sediment tends to accumulate compared to bare areas Biggs et al, 2021), whereas flow acceleration next to the patch contributes to particle entrainment and transport Schoelynck et al, 2013). As a consequence, plant growth and thus patch expansion could be locally enhanced inside or immediately downstream of a patch due to reduced hydrodynamic stress, a lower risk of mechanical damage for plants (breakage, uprooting) and accumulation of fine particles (Wharton et al, 2006;Jones et al, 2012;Biggs et al, 2021;Reitsema et al, 2021), which in turn increases nitrogen and phosphorus concentrations Clarke and Wharton, 2001;Sanders and Trimmer, 2006;Schoelynck et al, 2017) and enhances nutrient availability for plants. Reciprocally, plant growth and thus patch expansion could be inhibited next to the patch due to higher hydrodynamic stress and coarser sediment, leading to the formation of regular patterns Schoelynck et al, 2012;Cornacchia et al, 2019).…”