Guidelines for Siting and Operating Surface-water Velocity Radars
The goal … To collect channel velocity and cross-section data on Monday and operationally transmit real-time stage, velocity, area, and discharge on Tuesday regardless of (1) how surface-water velocities were measured (LSPIV or radar) or (2) the hydraulic conditions that existed on Monday.
The United States Geological Survey (USGS) and Environment and Climate Change Canada (ECCC) are exploring the use of velocity radars to measure surface-water velocities and compute real time mean-channel velocity and discharge in streams and rivers. Velocity radars can (1) deliver real-time discharge at new stations where stage-discharge, index-velocity, or slope-discharge ratings are not available; (2) extend ratings; (3) corroborate indirect measurements; and (4) provide an alternative for measuring discharge at sites with complex ratings . The ultimate goal of this effort is to transition the use of surface-water velocity radars from a proof-of-concept to an operational mode and to more clearly determine operational limitations.
These guidelines are an extension of the non-contact methods initiated by HYDRO 21 (Costa et al., 2006). The USGS Hydrologic Instrumentation Facility (HIF), Project Chiefs from 6 USGS Water Science Centers (WSCs), and two vendors (Stalker and Hydrological Services of America; HSA) participated in proof-of-concept testing. Fourteen (14) sites, which exhibited different hydraulic flow regimes, were identified and collocated with existing USGS streamgages. Laboratory testing was conducted in parallel with field deployments by the HIF (Fulford, 2015) and the Switzerland Federal Institute of Metrology (METAS). Testing at the HIF and METAS was conducted in the spring 2015 using standard methods including flumes, carriage-tow tanks, and tuning forks with known frequencies. Stalker and HSA offered their units gratis in exchange for an assessment on their performance. Similarly, OTT Hydromet (OTT) offered their radar for testing; however, the unit is still under development.
Quick Start Guide
You’ll need to acquire a small amount of velocity and channel data beyond what a normal field trip demands. Data should be collected at the cross section-of-interest and in the vicinity of the radar footprint. Keep in mind that the radar’s ability to return a surface-water velocity is influenced by (1) the quality of scatterers or waveforms on the water surface, (2) the air gap or the distance between the bridge deck and the water surface, and (3) the potential noise imposed by wind drift, eddies, secondary flows, and macro turbulence.
Follow the same principles used to site a conventional streamgage (Site Selection, p. 9; Turnipseed and Sauer, 2010):
Straight channels with parallel streamlines
Streambed free of large rocks, weeds, obstructions that would create turbulence/slack water
Sections that are parabolic, trapezoidal, or rectangular
Avoid variable flow conditions downstream of piers or channel obstructions (Please note it is important they we target surface scatterers to achieve sufficient radar returns, but highly turbulent conditions should be avoided)
Velocities greater than .5 to 1 feet per second (fps) and depths greater than 0.5 feet (ft)
Avoid sections influenced by tributaries or contributing drainage
Collect the following streamflow and channel data at the cross section-of-interest:
Station number and measurement number
Date of measurement
Lat/long of the starting and ending edge of water
Lat/long of the vertical (termed the “y-axis”) where the maximum in-stream or maximum surface-water velocity is measured
At the y-axis, record the surface-water velocity and point velocities near the water surface, close to the channel bottom, 0.2D, 0.3D, 0.4D, 0.5D, 0.6D, 0.7D, 0.8D, 0.9D using a current meter, FlowTracker, or Stationary Moving Bed Analysis with an ADCP
Confirm the location of the y-axis by repeating this procedure to the left and right of the y-axis
Water depth at the y-axis
Wind speed and direction
To estimate the stationing of the y-axis, rely on the location of the maximum-surface water velocity; it generally coincides at the same vertical as the maximum-instream velocity
Develop a stage-area rating using AreaComp (https://hydroacoustics.usgs.gov/indexvelocity/AreaComp.shtml)
Generally, data collection and radar deployments point should be upstream of bridges or structures to avoid wind-dominated reaches, eddies, secondary flows, and macro turbulence
Velocity radars can be deployed by hand or fixed on bridges, light cableways, or cable stays