Hydrographic Survey – Yellowstone River, Montana

CRA was part of a team headed by a major aerial photogrammetric company who was selected by the Yellowstone River Authority (with assistance from the U.S. Army Corps of Engineers in Omaha) to conduct a survey of over 130 miles of the Yellowstone River in Stillwater, Yellowstone and Dawson Counties in Montana. While an aerial photographic and lidar survey was carried out, CRA was tasked with providing an automated hydrographic survey on the river. This was to determine the location of the main channel withing the river and then to mesh the river elevation data with the lidar data to provide a complete terrain model over the whole floodplain. As the yellowstone is a typical uncontrolled western river, it is really only navigable with a survey boat during periods of snow melt from the nearby Yellowstone Park.

This made it very difficult to survey with fast running rapids and barely submerged pebble banks. CRA proposed a relatively new methodology of using a GPS system in real time kinematic mode to give real time x,y,z of the fathometer transducer on the boat, which had the advantage of taking out any variations in water surface elevation due to either the hydraulic gradient, or to variations in flow during the survey. It required many base station set ups in order to maintain radio link to the boat. Each base location was surveyed in by recording several hours of dual frequency GPS data and submitting this to the NGS’s OPUS service, which processes the data against the three nearest CORS sites to give a result accurate to a few centimeters.

A 20 foor jet-drive aluminmum boat was used and fitted out with a special “well” in the hull in which the transducer for the digital fathometer system was placed so that it would shoot through the hull and be safe from damage. A zig-zag was then steered up-river for maximum control of the boat, and better coverage of the bottom contours. As the hydrographic data was acquired at higher water levels than the lidar, the overlap between the two surveys was to be part of the overall QC process. The hydro crew also overlapped each day’s data, again for QC purposes. Final data was delivered in ascii x,y,z data in state plane coordinates in meters.

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