River Cross-Sections With a Moving Water Surface
You are mapping 3.2 miles of the Platte River for a bridge replacement. The project datum is NAVD88, but the river stage changes 2.4 ft during the day. You have RTK GNSS on a boat, a single-beam echosounder with a 200 kHz transducer, and a UAS with topo-bathy LiDAR. The bridge plans require bed elevations at 50-ft stations and a cross-section every 500 ft. A check section on a calm side channel gives a GNSS antenna height of 5.12 ft above the water surface, transducer draft 0.85 ft, and sound velocity 4,820 ft/s. The echosounder default velocity is 4,800 ft/s. The water-surface elevation from a nearby gauge is 1,102.36 ft NAVD88 at 10:15 and 1,104.72 ft at 14:40. The UAS LiDAR point cloud shows the water surface as a flat plane at 1,103.10 ft.
1. For a measured depth of 12.00 ft at 10:15, with the GNSS antenna at 1,107.48 ft NAVD88, compute the corrected bed elevation. State all assumptions.
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At 10:15 the gauge water-surface elevation is 1,102.36 ft. The transducer is 0.85 ft below the water surface, so its elevation is 1,101.51 ft. The antenna elevation of 1,107.48 ft minus the 5.12-ft antenna height and 0.85-ft draft gives the same transducer elevation. The echosounder assumes 4,800 ft/s but actual sound velocity is 4,820 ft/s, so true depth = 12.00 times (4,820/4,800) = 12.05 ft. Bed elevation = 1,101.51 - 12.05 = 1,089.46 ft NAVD88. Assumptions: vertical beam, no significant transducer motion, constant draft, sound velocity representative of the water column, and the gauge represents local water surface. If the antenna elevation is referenced to the ellipsoid, convert with the geoid model before using NAVD88.
2. How should the 2.4-ft stage change and the LiDAR water plane be handled in the final surface?
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All soundings must be reduced to a common vertical datum using the water surface at the time of each sounding. Use the gauge record or RTK water-surface elevation, interpolate stage between readings, and account for travel time and local slope. Do not use the UAS LiDAR flat water plane at 1,103.10 ft as the bed datum; it is a single surface from one flight and may not match the river stage at the sounding time. The 2.4-ft stage change is larger than the bed-feature accuracy needed for bridge design, so uncorrected soundings would create a false slope. Use LiDAR for the water-surface mask, banks, and shallow-water check, but derive bed elevations from the echosounder with draft and sound-velocity corrections.
3. What QC checks and deliverable contents are needed to support bridge design?
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Run bar checks and sound-velocity casts, verify draft, and compare RTK positions with a total station or known control. Repeat a cross-section and compare with the LiDAR in shallow water. Check that each 500-ft cross-section closes to the bank and that 50-ft stations are correctly spaced. Deliver XYZ soundings, a TIN, contours, cross-sections, a water-surface table, vertical accuracy, datum, and method. State that the bathymetric surface is not a legal boundary and that the bridge designer must account for scour and stage. Include a QA log documenting calibration, corrections, and check-section results.