Extended Uhf Radar Observations of River Flow Velocity and Comparisons with In-situ Measurements

A RiverSonde radar system, operating at a frequency of approximately 350 MHz in the ultrahigh frequency (UHF) band, was installed on the Cowlitz River at Castle Rock, Washington, USA during October 2003 and has been operating for several months. Using techniques and equipment developed over several decades for measuring ocean surface currents at much lower frequencies, this radar system continuously measures the river streamflow from a location on one bank of the river by utilizing Bragg scattering from naturally-occurring water waves of 0.5-m wavelength. Data are processed in real time on a portable laptop computer and are available through a dial-up modem. The radar data provide hourly estimates of mean flow and cross-channel variations in the flow. Mean values of the radar flow profile track very closely continuous in-situ stage height measurements. River flow velocities of 0.8 m·s-3.5 m·s were observed in the first five months of the experiment, with a nearly linear relationship between radar-inferred flow velocity and stage height of 9 m–14 m. The radar velocity also appears to have a weak correlation with the local wind and several tidal frequencies. The strong correlation between surface velocity and stage height suggests that – with refinement – surface velocity could replace stage height in river gaging, as well as offering additional flow information.