Arches and changing formations on the seabed – the underwater world of the quays is revealed
“Of the City center harbours in Helsinki, approximately 5.6 kilometres of quay structures belong to the Port of Helsinki. The oldest quays date from pre-war times and the most recent quay LHD in Hernesaari was taken into use in the spring of 2019,” says Planning Engineer Tero Sievänen from Port Technical Services.
The quays are constantly being worn down by the sea and the weather conditions. Maintaining quays in the Port of Helsinki requires skill and a good concept of the condition of the underwater structures.
Underwater steel sheet walls and reinforced concrete poles as well as the erosion protection structures at the front of the quays bear the brunt of the rough sea and northern weather conditions. The erosion protection structures are designed to shield the port basin and quays from the erosion caused by slipstreams generated by vessels.
Digital Scanning allows for an area to be studied quickly
The underwater condition of quays can be examined using several methods, from traditional condition surveys performed by divers to digital multi-beam probes and modelling.
Condition surveys performed by divers are slow and relatively expensive, but result in detailed and comprehensive reports. Diving will not work very well, however, if the water is murky and the diver cannot see farther than their own hand.
Digital surveys, on the other hand, can be used to gain a more comprehensive perspective more quickly.
Single-beam echo sounding provides information on the depth of water, but by comparing surveys from different years, we can estimate the erosion of the seabed.
Multi-beam echo sounder surveys provide a considerably improved image of the seabed and the changes caused to the front of the quay by erosion. In addition to measuring depth, depressions caused by erosion, sedimentation and loose objects embedded in the seabed, the multi-beam echo sounder can also be used to determine quantity (dredging planning).
When multi-beam echo sounding is carried out with the sounder directed towards the quay, the survey will also yield a detailed image of the quay structure. If you have access to a laser sounder or camera, you will also be able to observe the structure above the water level.
Multi-beam echo sounding can cover an area in an hour that would take days to cover by diving.
“Echo sounding will not provide as much information as diving would, but it allows us to target detailed surveys by diving to critical locations,” Sievänen says.
Modelling images with technically fascinating methods
All of the quays in the Port of Helsinki have been surveyed by diving at least once. Check-ups using multi-beam echo sounding are performed every few years by port area.
Last spring, all old port structures in central port areas were sounded. The sounding was carried out by VRT Finland Oy. In addition to the sounding material, an examination report was drafted for each quay.
The sounding was performed on an inspection vessel equipped with a multi-beam echo sounder. The vertical structures of the quays were sounded using a tilted multi-beam echo sounder, which uses a frequency of 700 kHz. The high frequency limits the opening angle of the sound beam and allows more accurate observations to be made. A lower frequency of 400–700 kHz was used in the seabed areas.
The measurement material is tied to Helsinki’s height system N2000 and coordinate system ETRS-GK25.
The point clouds of the 3D point cloud material were combined and the data was cleaned manually.
The sections above the water level were photographed and scanned using a mobile laser scanner.
Different materials have been combined when possible, and they are used for quay condition reports, the planning of repairs, seabed depth maps and the determination of quantity for dredging planning.
“Underwater modelling is fascinating. It provides us with an accurate view of the condition of erosion plates, for example. And I like to think that modelling images are also very beautiful,” says the engineer turned aesthetician with a laugh.