The accuracy of discharge measurements derived from surface velocities are highly dependent on the accuracy of conversions from surface velocity ${\mathrm{u}}_{\mathrm{s}}$ to depth-averaged velocity $\mathrm{U}$. This conversion factor is typically known as the ‘velocity coefficient’, ‘velocity index’, ‘calibration factor’, ‘alpha coefficient’, or simply ‘alpha’, where $\alpha ={\mathrm{U}/\mathrm{u}}_{\mathrm{s}}$. At some field sites detailed in situ measurements can be made to calculate alpha, while in other situations (such as rapid response flood measurements) alpha must be estimated. This paper provides a review of existing methods for estimating alpha and presents a workflow for selecting the appropriate method, based on available data. Approaches to estimating alpha include: reference discharge and surface velocimetry measurements; extrapolated ADCP velocity profiles; log law profiles; power law profiles; site characteristics; and default assumed values. Additional methods for estimating alpha that require further development or validation are also described. This paper then summarises methods for accounting for spatial and temporal heterogeneity in alpha, such as ‘stage to alpha rating curves’, ‘site alpha vs. local alpha’, and ‘the divided channel method’. Remaining challenges for the accurate estimation of alpha are discussed, as well as future directions that will help to address these challenges. Although significant work remains to improve the estimation of alpha (notably to address surface wind effects and velocity dip), the methods covered in this paper could provide a substantial accuracy improvement over selecting the ‘default value’ of 0.857 for alpha for every discharge measurement.

A novel aerial tracer particle distribution system has been developed. This system is mounted on an unmanned aerial vehicle (UAV) and flown upstream from where surface velocimetry measurements are conducted. Tracer particles can be rapidly deployed across the full channel width and distributed in a crescent shape (for larger channels), so that they reach the measurement cross section at a similar time. This enables surface velocimetry techniques to be applied in rivers and channels lacking sufficient natural tracer particles or surface features. Lack of tracers is a common problem during low flows, in lowland rivers, or in artificial channels. Techniques for colouring tracer particles with biodegradable dye have also been developed, along with methods for extracting them from Red Green Blue (RGB) imagery in the Hue Saturation Value (HSV) colour space. The use of coloured tracer particles enables flow measurements in situations where sunglint, surface waves, moving shadows or dappled lighting on riverbeds can interfere with and corrupt results using surface velocimetry techniques. These developments further expand the situations where surface velocimetry can be applied, as well as improving the accuracy of the results. The tracer particle distribution system has been successfully deployed in multiple rivers and canals in New Zealand and has enabled surface velocimetry measurements where they were not previously possible due to a lack of natural tracers.

This paper reports the morphology of a natural patch of Ranunculus penicillatus and presents high-resolution measurements of flow velocities in its wake using a stereoscopic PIV field measurement system. The patch was 3.80 m long, 1.24 m wide and caused substantial changes to downstream mean velocities and turbulence. Vertical profiles of streamwise mean velocity were not logarithmic and flow was redirected under the positively buoyant canopy, enhancing vertical turbulent mixing in the wake and generating a large region where the velocity covariance $\overline{u^{\mathrm{\prime }}{w}^{\mathrm{\prime }}}$ was positive. Turbulent kinetic energy was enhanced downstream from the patch lateral shear layer, but not at the centre of the wake. Spectra downstream from the patch showed that turbulence was neither dominated by fine-scale nor large-scale structures, likely due to the low energy of the flow conditions and lack of a developed vortex street within the measurement domain. Sedimentation was observed at the upstream end of the patch, but not underneath the floating canopy. The methods and results of this work will be useful for planning other in situ studies. Also, the reported data on macrophyte geometry and biometrics will assist with the design of more realistic replicas for use in laboratory studies.

Aquatic macrophytes are a key component of river systems around the world. Surveys of macrophyte surface cover, cross-sectional blockage and plant/patch sizes provide data for river managers to assess in-stream habitat, hydraulic resistance, and the success of stream restorations. Manual surveying techniques are labour intensive, provide low spatial detail and are predominantly applied at the cross-section scale, resulting in a lack of published data on macrophyte size distributions. In this study, 1099 Ranunculus penicillatus patches were surveyed using a UAV-mounted digital camera. Geometric properties such as patch area, length, aspect ratio and orientation were determined from the aerial orthophotos. These data were then coupled with hydraulic measurements. Macrophyte abundance corresponded to specific ranges of velocity, Froude number and stream power, indicating clear patterns of hydraulic habitat use (and preferential modification) by Ranunculus. At the reach scale, flow redirection around dense vegetation clusters was observed, with implications for localised sedimentation and bank erosion. The reported data can improve the design of laboratory experiments to represent Ranunculus characteristics in the field. The aerial surveying techniques can be used to efficiently estimate vegetation abundance, surface area blockage factor and also to visualise flow through patch mosaics, enabling targeted management of aquatic vegetation.

The paper reports the results of laboratory experiments to investigate the effect of vegetation patch mosaics on hydraulic resistance. Experiments were run for seven levels of vegetation coverage with square patches of flexible plastic grass in aligned and staggered configurations and a wide range of hydraulic conditions. Hydraulic resistance was substantially higher for staggered than aligned configurations, particularly for intermediate ranges of vegetation coverage. The results indicate that hydraulic resistance differs between regimes of isolated roughness flow, wake interference flow, and skimming flow. Two types of models are proposed to predict hydraulic resistance (i.e. Manning’s coefficient n) for aligned and staggered configurations, one as a function of the nondimensional spatially-averaged hydraulic radius and another as a function of relative submergence and surface area blockage factor. To account for the effects of vegetation patch alignment, an additional factor α is introduced. This work provides comprehensive datasets and models that can be used to improve the prediction of hydraulic resistance in open-channel flows with vegetation patches.