Underwater Bridge Pier Inspection with the Apache 4 USV and HN-400M Multibeam Echosounder
2026-08-12
At a busy railway crossing over the Huaihe River, a survey team needed to inspect the underwater foundations of the bridge piers, measure the scour and siltation around them, and map the riverbed upstream and downstream, all without putting a crewed boat into a heavily trafficked channel. They ran the survey with the CHC Navigation Apache 4 unmanned surface vessel carrying an HN-400M multibeam echosounder, and turned the soundings into a full three-dimensional picture of the riverbed. This is how the survey came together.
The site: railway bridge piers on the Huaihe River
The survey area sits at the Huaihe River railway bridge in Bengbu, Anhui Province. The work had a clear brief: inspect the bridge piers below the waterline, measure the scour and siltation around each pier, and carry out a topographic survey of the riverbed upstream and downstream of the bridge. An accurate record of the riverbed shape across the whole span was needed to judge the condition of the foundations.
Huaihe river railway bridge in Bengbu, Anhui Province, China, where the underwater bridge pier inspection project was carried out.
The challenge: inspecting piers from a crewed boat
In this setting, running the survey from a manned vessel raised three problems:
Frequent vessel traffic on the channel makes crewed operation near the piers a navigation safety risk.
A large boat cannot get close enough to the piers to capture detailed data at the foundations.
There is no suitable dock at the site, which limits where a crewed vessel can launch and operate.
What the team needed was a compact, highly maneuverable platform that could work safely in traffic, approach the piers closely, and launch from the bank without a dock.
The CHCNAV Apache 4 USV equipped with the HN-400M multibeam echosounder performs underwater bridge pier inspection and riverbed mapping at the Huaihe river railway bridge.
The solution: the Apache 4 USV with the HN-400M multibeam
The team paired the Apache 4, a compact and purely electric unmanned surface vessel, with the HN-400M, a lightweight dual-frequency multibeam echosounder. The USV is small and highly maneuverable, so it can approach the bridge foundations closely to capture detailed data, and its compact design lets the team launch and survey from the bank with no dedicated dock. Millimeter-wave obstacle-avoidance radar lets the vessel steer clear of passing boats automatically, and on-board cameras let the operator watch the equipment in real time, so the survey runs safely in an active channel.
The HN-400M combines 400 kHz and 550 kHz operation with FM CHIRP, so the 550 kHz mode captures high-resolution detail in the shallows around the piers while 400 kHz improves penetration in deeper and more turbid water. A 0.03° integrated IMU provides real-time motion compensation, a 145° swath covers up to six times the water depth in a single pass, and at 3.5 kg the sensor is light enough for rapid USV deployment. Together the platform and sensor produce dense coverage of the riverbed, on the order of 5,000 points per square metre with point spacing under 0.03 m, rather than a set of single-point measurements.
In the field: autonomous survey to a 3D point cloud
The workflow was straightforward. The team planned the survey lines based on the water depth of the area and let the Apache 4 run them autonomously, while the HN-400M recorded a continuous swath of soundings across the riverbed. The soundings were then output as a three-dimensional point cloud of the survey area, giving a full-coverage model of the bed and the structures on it.
Bathymetric results of the bridge survey area with depth values visualized by color. The multibeam data clearly reveals the riverbed topography, scour pits around the bridge piers, and siltation patterns across the surveyed area.
The results: a full underwater picture of the piers
From the three-dimensional point cloud, the survey delivered four types of result for the bridge and the channel:
The underwater condition of the bridge piers, with the scour and siltation around each foundation.
The state of siltation across the bed, useful for planning and checking river dredging.
The distribution of dredged and deposited material and river sand across the survey area.
The location of underwater obstacles that could affect navigation safety in the channel.
Left: Three-dimensional bathymetric model of a bridge pier foundation showing the surrounding scour depression reconstructed from multibeam echosounder data. Right: Detailed underwater point cloud visualization around two bridge pier foundations, showing the scour pits and riverbed changes captured through multibeam surveying.
Where else this applies
The same approach carries across to other structural and channel work. It suits the underwater foundation inspection of highway and railway bridges, waterway maintenance and dredging surveys, and the inspection of port and dock structures, wherever detailed underwater data is needed in water that is busy, shallow near structures, or awkward for a crewed boat to reach.
Small, maneuverable platform works close in for detailed data
Launching on site
Needs a suitable dock
Compact and lightweight, launched from the bank
Coverage of the bed
Sparser single-point measurement
Dense multibeam swath, full three‑dimensional coverage
Equipment safety
Harder to monitor near structures
On‑board cameras give real‑time monitoring
For teams planning inland and structural bathymetric surveys, the wider range of CHC Navigation marine surveying solutions covers USV platforms and multibeam payloads for hydrographic and bathymetric work.
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About CHC Navigation
CHC Navigation (CHCNAV) develops advanced mapping, navigation, and positioning solutions designed to increase productivity and efficiency. Serving industries such as geospatial, agriculture, machine control and autonomy, CHCNAV delivers innovative technologies that empower professionals and drive industry advancement. With a global presence spanning over 140 countries and a team of more than 2,200 professionals, CHC Navigation is recognized as a leader in the geospatial industry and beyond. For more information about CHC Navigation [Huace:300627.SZ], please visit: https://geospatial.chcnav.com/about/overview
