K. Hayes

Use of radars to monitor stream discharge by noncontact methods

[1]xa0Conventional measurements of river flows are costly, time-consuming, and frequently dangerous. This report evaluates the use of a continuous wave microwave radar, a monostatic UHF Doppler radar, a pulsed Doppler microwave radar, and a ground-penetrating radar to measure river flows continuously over long periods and without touching the water with any instruments. The experiments duplicate the flow records from conventional stream gauging stations on the San Joaquin River in California and the Cowlitz River in Washington. The purpose of the experiments was to directly measure the parameters necessary to compute flow: surface velocity (converted to mean velocity) and cross-sectional area, thereby avoiding the uncertainty, complexity, and cost of maintaining rating curves. River channel cross sections were measured by ground-penetrating radar suspended above the river. River surface water velocity was obtained by Bragg scattering of microwave and UHF Doppler radars, and the surface velocity data were converted to mean velocity on the basis of detailed velocity profiles measured by current meters and hydroacoustic instruments. Experiments using these radars to acquire a continuous record of flow were conducted for 4 weeks on the San Joaquin River and for 16 weeks on the Cowlitz River. At the San Joaquin River the radar noncontact measurements produced discharges more than 20% higher than the other independent measurements in the early part of the experiment. After the first 3 days, the noncontact radar discharge measurements were within 5% of the rating values. On the Cowlitz River at Castle Rock, correlation coefficients between the USGS stream gauging station rating curve discharge and discharge computed from three different Doppler radar systems and GPR data over the 16 week experiment were 0.883, 0.969, and 0.992. Noncontact radar results were within a few percent of discharge values obtained by gauging station, current meter, and hydroacoustic methods. Time series of surface velocity obtained by different radars in the Cowlitz River experiment also show small-amplitude pulsations not found in stage records that reflect tidal energy at the gauging station. Noncontact discharge measurements made during a flood on 30 January 2004 agreed with the rated discharge to within 5%. Measurement at both field sites confirm that lognormal velocity profiles exist for a wide range of flows in these rivers, and mean velocity is approximately 0.85 times measured surface velocity. Noncontact methods of flow measurement appear to (1) be as accurate as conventional methods, (2) obtain data when standard contact methods are dangerous or cannot be obtained, and (3) provide insight into flow dynamics not available from detailed stage records alone.

River discharge measurements by using helicopter‐mounted radar

[1]xa0The United States Geological Survey and the University of Washington collaborated on a series of initial experiments on the Lewis, Toutle, and Cowlitz Rivers during September 2000 and a detailed experiment on the Cowlitz River during May 2001 to determine the feasibility of using helicopter-mounted radar to measure river discharge. Surface velocities were measured using a pulsed Doppler radar, and river depth was measured using ground-penetrating radar. Surface velocities were converted to mean velocities, and horizontal registration of both velocity and depth measurements enabled the calculation of river discharge. The magnitude of the uncertainty in velocity and depth indicate that the method error is in the range of 5 percent. The results of this experiment indicate that helicopter-mounted radar can make the rapid, accurate discharge measurements that are needed in remote locations and during regional floods.

Remotely sensed river surface features compared with modeling and in situ measurements

[1]xa0Images of river surface features that reflect the bathymetry and flow in the river have been obtained using remote sensing at microwave, visible, and infrared frequencies. The experiments were conducted at Jetty Island near the mouth of the Snohomish River at Everett, Washington, where complex tidal flow occurs over a varied bathymetry, which was measured as part of these experiments. An X band (9.36 GHz) Doppler radar was operated from the river bank and produced images of normalized radar cross sections and radial surface velocities every 20 min over many tidal cycles. The visible and infrared instruments were flown in an airplane. All of these techniques showed surface evidence of frontal features, flow over a sill, and flow conditioned by a deep hole. These features were modeled numerically, and the model results correspond well to the remote observations. In situ measurements made near the hole showed that changes in measured velocities correlate well with the occurrence of the features in the images. In addition to tidal phase, the occurrence of these features in the imagery depends on tidal range. The surface roughness observed in the imagery appears to be generated by the bathymetry and flow themselves rather than by the modulation of wind waves.

Non-contact flood discharge measurements using an X-band pulse radar (I) theory

Abstract This article describes a non-contact method for measuring surface velocity and discharge in a natural channel. The X-band pulse (9.36 GHz) radar, developed by the Applied Physics Laboratory of the University of Washington, was used to scan instantaneously the lateral distribution of surface velocity across a river section, according to Bragg scattering from short waves produced by turbulent boils on the surface of the river. Based on the assumption that the vertical velocity distribution follows a universal power or logarithmic law, the discharges were estimated.

Non-contact flood discharge measurements using an X-band pulse radar (II) Improvements and applications

Abstract This paper presents and applies an improved method of determining cross-sectional depth and discharge of a river. The method used with the universal law and Darcy-Weisbach friction factors to obtain the lateral variation of the roughness height. This method of measurement was successfully used at the Kaoping River during the Xangsane typhoon in Taiwan, and the results show that the surface velocity obtained using an X-band pulse radar system were close to that obtained by the float method. The estimated discharges at four stages were within 3% of the recorded values of the stage-discharge rating curve in the gauging station.

Rainfall and river currents retrieved from microwave backscatter

Since October of 2002, eight CW microwave sensors have been operating on a bridge over the Cowlitz River in western Washington State in the USA. These sensors measure the river surface velocity via Doppler shifts at eight locations across the river; a ninth unit pointing upwards measures scattering from rain drops.

Observed space-time structure of radar backscatter from the ocean surface

Space-time radar images of the ocean surface were collected from an airship off the Oregon coast in the fall of 1995, using a coherent, short pulse, HH-polarized X-band system. The radar successfully mapped wind waves, long swell waves and internal wave features. The radar images are converted to two-dimensional spectra of microwave backscatter and the surface velocity field. These spectra are used to examine space-time properties of surface waves. Examples of the radar maps and their corresponding spectra are presented. It is demonstrated that the linear dispersion relation gives the relationship between the time and spatial dependence of the long gravity waves. Also, evident in the spectra are features that dont obey the linear wave theory.

Measurement of river surface currents using rough surface scattering

Simple Bragg scattering is often the type of rough surface scattering applicable to rivers. Thus the center frequency of the Doppler spectrum provides information on the velocity of the surface of the river. We have developed a simple continuous wave microwave system called Riverscat to take advantage of this effect to measure river surface currents. Measurements with this velocity sensor show that surface velocity is well related to the depth of the river. Therefore on unstable streams where the bottom changes either at the measurement site or downstream of it, Riverscat offers the potential for improved monitoring of stream discharge.