Bathymetric surveys: when you need one and how it is done
What lies under the water can be surveyed too. A bathymetric survey gives the designer the bed in the same way a survey plan gives the ground.
When it is needed
- Ports: port works studies, dredging and checks of depth in berthing areas
- Lakes: jetties, shore works and studies of the basin
- Reservoirs and farm ponds: volume calculations and monitoring of sedimentation
- Rivers: cross sections of the bed for hydraulic studies and demarcations
How it is done
The echo sounder sends a sound pulse towards the bed and measures the time it takes to return. A single-beam echo sounder measures the depth directly below the boat, so the bed is recorded along dense lines or cross sections. The position of each measurement comes from GNSS RTK.
We work mainly with the CHCNAV Apache 3 Pro uncrewed surface vessel, which carries a single-beam echo sounder and GNSS RTK. It follows programmed lines on autopilot and reaches shallow water where a conventional boat cannot go. For longer distances we use our own inflatable boat with the portable OHMEX SonarMite BTX echo sounder.
Why an uncrewed boat
A conventional crewed boat works slowly and at some risk on rivers with currents, islets and logs, and does not easily enter shallow water. The autonomous boat holds the lines planned in the office, so the density of measurements is known in advance, and the team stays on the shore or in an escort boat.
An example: the Axios river
On the Axios, with currents of up to 2 m/s, islets and logs, the Apache 3 Pro followed programmed cross sections perpendicular to the flow, with a 200 kHz echo sounder connected in real time to HEPOS (the Greek GNSS reference network). An inflatable boat with the team escorted it, for safety and to deal with obstacles. Bed heights were computed directly from RTK, so they were not affected by changes in water level during the day. The result was 26 cross sections in GGRS87 (ΕΓΣΑ87), the national coordinate system of Greece, as DWG and KMZ files, with a technical report on the method.
Linking with the land survey
A bathymetric survey rarely stands alone. The study also needs the shore, the banks and the works around the water, in the same coordinate and height system. So we link the bathymetry with a ground survey and, where needed, with drone photogrammetry or LiDAR. For the jetty at Psarades on Lake Prespa, for example, land, air and lakebed data gave the study a single base.
What affects the survey
Currents, waves, vegetation in the water and very shallow areas all affect how the work is planned. For each site we propose the line spacing and the vessel that fit, and points that a boat cannot reach are completed with measurements from land where possible.
Deliverables
- Bathymetric chart with depth contours
- A single model of land and bed
- Cross sections and volume calculations
- XYZ, DWG and GeoTIFF files, in GGRS87
- Technical report on the method
What you need / steps
- The area and the purpose of the study: volume, cross sections, depth chart or a combination
- Whether a land survey is also needed, and over what area
- Access to the water to launch the boat
- Planning of the lines, survey, processing and checks
- Delivery of the files to the designer
Tell us which body of water you need surveyed and for what study, and we will propose the method and the deliverables.