LSPIV Measurements of Two‐Dimensional Flow Structure in Streams Using Small Unmanned Aerial Systems: 2. Hydrodynamic Mapping at River Confluences

Abstract

Although past field work at stream confluences has relied on velocity information at specific cross sections to examine flow structure, detailed characterizations of spatial and temporal variations in the hydrodynamics of confluences are lacking. This study uses large‐scale particle image velocimetry (LSPIV) obtained from small unmanned aerial systems (sUAS), a method evaluated in a companion paper, to map surficial patterns of mean flow and turbulent structures at two small stream confluences in unprecedented levels of detail. LSPIV reveals two‐dimensional flow patterns within different hydrodynamic zones in each confluence as well as similarities and differences in hydrodynamic conditions between the confluences. As expected based on extant conceptual models of confluence hydrodynamics, the spatial arrangement of characteristic hydrodynamic zones varies with confluence planform geometry and with changes in incoming flow conditions. However, local morphological features, such as bars and irregularities in channel banks, also exert a strong influence on the spatial structure of flow and in some cases influence confluence hydrodynamics to an extent comparable to changes in incoming flow conditions. The usefulness of sUAS‐based LSPIV is demonstrated by the correspondence between patterns of flow curvature revealed by this method and patterns of helical motion documented at cross sections within the confluences using acoustic Doppler velocimetry. The method can also be used to characterize the structure of turbulent vortices within the mixing interface between confluent streams under appropriate conditions. Hydrodynamic mapping using sUAS‐based LSPIV enriches the interpretation of traditional in‐stream velocity data acquired in the field and provides information on surface velocity patterns in rivers at a resolution similar to that of numerical models.

Advective Lateral Transport of Streamwise Momentum Governs Mixing at Small River Confluences

“…As flow moves downstream, the separate high‐velocity cores of the incoming flows quickly merge into a single core. A region of flow separation develops along the left bank (right side of the cross section) downstream of a large bank protrusion on this side of the confluence (Figure 2; see also Lewis & Rhoads, 2018b). The pattern of secondary‐flow vectors reveals that pronounced counterclockwise helical motion develops over the middle part of the downstream channel in conjunction with curvature of flow from the Saline into the confluence.…”

“…More measurements of mixing rates and patterns at particular confluences over a suite of flow conditions are needed, and studies that assess mixing at numerous confluences are also recommended. Because of the difficulty in measuring detailed cross‐sectional velocities, studies that assess mixing at many confluences may benefit from remote sensing and noncontact methods to determines patterns of flow and mixing, at least at the surface (Biron et al, 2019; Lewis & Rhoads, 2018a, 2018b). However, as this study has shown mixing can be highly three‐dimensional, especially when influenced by helical motion of the flow.…”

“…As the name suggests, the mixing interface is an important hydrodynamic feature related to lateral mixing at confluences. Turbulence within this interface is generated by lateral shear associated with differences in velocity between the two flows (Babarutsi & Chu, 1998; Guillén Ludeña et al, 2017; Sukhodolov & Rhoads, 2001; Sukhodolov & Sukhodolova, 2019; van Prooijen & Uijttewaal, 2002) and by shedding of vortices from interacting shear layers that bound the stagnation zone (Lewis & Rhoads, 2018b). Large velocity differences between the incoming flows result in Kelvin‐Helmholtz (KH) instability and the generation of co‐ (co‐ rotating)rotating vertically oriented vortices within the mixing interface (KH mode) (Constantinescu, 2014; Rogers & Moser, 1992; Sukhodolov & Rhoads, 2001).…”

LSPIV Measurements of Two‐Dimensional Flow Structure in Streams Using Small Unmanned Aerial Systems: 1. Accuracy Assessment Based on Comparison With Stationary Camera Platforms and In‐Stream Velocity Measurements

“…A quantitative assessment of the LSPIV results provides insight into limiting factors of both sUAS and fixed systems, while examples and analyses of flow structure at river confluences demonstrate the utility of the method in highly complex flows. A companion study explores high‐resolution hydrodynamic mapping at river confluences, using results from this paper, to fundamentally improve understanding of these regions of complex 2‐D flow (Lewis & Rhoads, ). The results of this study help to better inform the value of LSPIV as a tool for measuring complex patterns of river flow in the field.…”

Experiments on the Morphodynamics of Open Channel Confluences: Implications for the Accumulation of Contaminated Sediments

“…The height of the flume walls was 40 cm. Different widths were employed for the upstream and downstream channels to mimic the channel expansion in recent confluence field studies (Biron et al, 2002; Boyer et al, 2006; Imhoff & Wilcox, 2016; Rhoads et al, 2009; Rhoads & Sukhodolov, 2001; Rhoads & Sukhodolov, 2008; Riley et al, 2015; Riley & Rhoads, 2012; Lewis & Rhoads, 2018; Sukhodolov et al, 2017; Sukhodolov & Sukhodolova, 2019). Two stacked tube grids were set behind the outlets of the upstream tanks in order to minimize the turbulence at the outlets.…”