Mapping River and Lake Bottoms With Multibeam Sonar

U.S. Geological Survey Fact Sheet 2018–3021, June 2018. By Richard J. Huizinga and David C. Heimann.

Seeing with sound

A multibeam echosounder (MBES) is a sonar that uses sound to "see" even through murky water. A single-beam echosounder — a depth sounder or fathometer — sends one narrow pulse and listens for its echo; a multibeam system sends a fan of beams that covers a whole swath at once. The timing and direction of the returning echoes reveal the water's depth and the shape of the river channel, lake bottom or anything on it. The USGS uses it to make high-resolution maps of river and lake beds efficiently.

The system

A multibeam echosounder mapping system (MBMS) combines:

• the echosounder — for many USGS surveys in Missouri, a Teledyne RESON SeaBat 7125-SV2, run at 200 or 400 kilohertz;
• an inertial navigation system, with two satellite-navigation antennas and an inertial motion unit, that fixes the boat's position in three dimensions and measures its heave, pitch, roll and heading;
• a computer to collect and process the data.

A real-time kinematic correction often improves the live display, but the navigation data are usually post-processed anyway to correct for moments of poor positioning. Afterward, the soundings are cleaned of spikes and stray points, georeferenced, and shown as a 3D surface or point cloud.

What it is used for

Bridge scour. Scour — flowing water carrying away the bed and banks — is the leading cause of bridge failures in the United States. It comes from long- and short-term changes in the river and from the local effects of piers and footings, and worsens in high flows, when water runs faster and deeper. Because it can be dangerous, bridges over water are inspected routinely, and multibeam surveys show scour around their foundations.

Scour around a pier of the Interstate 635 bridge over the Missouri River at Riverside, Missouri, where the flow has undermined the seal course and exposed a drilled shaft. USGS.

Channel-bed features and habitat. River beds reflect the constant interplay of flowing water and loose sediment, creating varied habitat for fish and invertebrates — information fisheries managers value. Multibeam maps, combined with water-velocity data from an acoustic Doppler current profiler, were used to assess pallid sturgeon habitat in the Mississippi River near a proposed bridge. Surveys at many bridges on the Missouri and Mississippi found everything from a flat bed — little sediment moving — to large dunes showing heavy movement, and bare rock where the sand had washed away. Repeat surveys measure how the bed changes — where it scours and fills — and how that relates to changing flow.

Change in the bed of the Missouri River at Kansas City between surveys in 2011 and 2015, around the Interstate 635 bridge. Reds show scour, blues deposition. USGS.

Measuring moving sand. Bedload — sediment that slides, rolls or skips along the bed — is not measured consistently in most rivers (as of 2018); direct measurement in big sand-bed rivers is highly inaccurate, so untested theoretical estimates are common. Repeating multibeam surveys about an hour apart lets scientists estimate bedload from how far the dunes move and where the bed scours and fills.

Lakes and reservoirs. Sediment slowly fills lakes and reservoirs, cutting flood storage and water supply and shrinking habitat. Detailed multibeam surveys give accurate volumes for lakes in Missouri and neighbouring states — information for judging water supply, sedimentation rates and a reservoir's expected lifespan, and for mapping aquatic habitat.

The bottom of Reservoir 3 near Cameron, Missouri, surveyed in July 2013. USGS.

Sources

• Huizinga, R.J., and Heimann, D.C., 2018, Hydrographic surveys of rivers and lakes using a multibeam echosounder mapping system: U.S. Geological Survey Fact Sheet 2018–3021. https://doi.org/10.3133/fs20183021 · https://pubs.usgs.gov/publication/fs20183021
• The figures come from the fact sheet's PDF.
• Rewritten in hubnx's own words.