Obtaining timely, accurate information on streamflow in Alaska’s rivers is difficult because gaging stations are sparse, with many located in remote inaccessible areas. Even for established gages, the maintenance and periodic measurements required to operate a gage are logistically challenging and can place personnel at risk, particularly during high flows. The core objective of this study is to develop and test airborne remote sensing methods for non-contact measurement of river discharge. The work focuses on estimating surface flow velocities from optical and thermal image time series acquired from helicopters. Direct field measurements of velocity from boats and bridges are also collected to assess the accuracy of image-derived estimates. This research is important because most Alaskan rivers are ungaged, and efficient remote sensing methods of measuring discharge could provide valuable streamflow information for water resource management and flood hazard mitigation.
In 2019, the USGS Geomorphology and Sediment Transport Laboratory collaborated with the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory to acquire remotely sensed data from six rivers in Alaska during a field campaign from July 21–28: the Knik, Matanuska, Susitna, Tanana, Chena, and Salcha Rivers. A pod mounted on the landing skid of the helicopter was equipped with 1) a lidar sensor for measuring topography and water-surface elevations and slopes; 2) a thermal camera for recording image time series from which surface flow velocities can be inferred via Particle Image Velocimetry (PIV) of features expressed at the water surface as subtle differences in temperature; and 3) a high-resolution (100 megapixel) digital mapping camera for producing georeferenced, orthorectified image mosaics and for PIV in sediment-laden rivers. In addition, a second mounting system on the front of the helicopter housed a video camera that acquired data at a higher frame rate of 30 frames per second that is more conducive to PIV. The visible cameras also might be useful for estimating water depth in the relatively clear-flowing Salcha River. These remotely sensed datasets will provide information on surface flow velocities that, in combination with a priori knowledge of channel geometry (i.e., cross-sectional area), will enable calculation of river discharge. An additional objective of the 2019 effort is to produce continuous maps of water temperature from thermal images.
Left: Helicopter with nose mount and pod for deploying remote sensing instruments. Right: Acoustic Doppler current profiler used to make direct measurements of flow velocity on the Salcha River and five other rivers in Alaska.