HYDROGRAPHIC

Now you see it, now you don’t

A new initiative by the PLA to encourage cleaner, greener ships has been welcomed in the industry.

Latest technology: new 3DChirp scanner deployed from survey boat, Maplin.Latest technology: new 3D Chirp scanner deployed from survey boat, Maplin.

Imagine a group of visitors taking time to watch port operations on the Thames. What would they see? Ships, boats, quays, cranes, bridges – in short, all the things that can be seen above the water.

They probably wouldn’t consider that there’s a whole other dimension but that’s the job of the PLA’s hydrographic team. Along the entire length of the tidal Thames, the hydrographic team are responsible for ‘seeing’ what is underneath the water, and their skills are a vital part of ensuring safe, cost-effective, environmentally sound shipping and port operations.

If a sand bank has been shifted by a storm, or sediment has been swept into a berth by tidal flows, or a container has gone overboard and might be obstructing the shipping channel, or there’s a suspicion of unexploded ordnance on the river bed, the hydrographic team have the surveying expertise and equipment to identify exactly what is happening.

They can demonstrate with absolute precision where any dredging is required. Equally, if an old anchor or other obstruction needs to be recovered, they can direct divers to the spot with pinpoint accuracy, saving valuable time and costs in a river where the natural sediment build-up means a diver really cannot see his hand in front of his face.

When the world’s biggest, deepest ships come into the Port of London, they need to have complete confidence in the UK Admiralty charts and the tide tables on which they depend. All of this information comes direct from the PLA’s hydrographic department.

As conservancy authority, the PLA is responsible for a 95-mile stretch of the River Thames. This encompasses 400 square miles of river and seabed, where water depths, tidal heights and tidal flows must be accurately measured and predicted, with all the information published in clear and timely fashion both electronically and on traditional charts.

The whole river is surveyed, bank to bank, on a rolling programme over five to 15 years, depending on the particular area. About 90 survey areas are subject to greater change and consequently have higher navigational significance and these are surveyed much more frequently, as often as once a month in some cases.

As well as serving the needs of the PLA as the statutory harbour and conservancy authority, the hydrographic department works closely with port and terminal managers and a range of other customers on a commercial basis, carrying out surveys, sampling and monitoring water quality, and providing data to support them in their operations and developments.

This support work was very much at the forefront during the major dredge carried out in the run-up to the opening of the DP World London Gateway container port in 2013 and now the hydrographic team is providing vital surveying services to the Tideway project.

The whole river is surveyed, bank to bank, on a rolling programme over five to 15 years, depending on the particular area.

Regular baseline studies have been undertaken since 2013, using both multibeam and laser technologies, and these continue. “We are undertaking bank to bank surveys annually as well as detailed work site surveys quarterly; this provides historical records that are important for measuring any change,” says port hydrographer John Pinder.

“Because we have been doing this monitoring work for three years since well before any construction work was done, we have built up a large bank of data. This baseline of natural seasonal changes gives a picture of the natural regime, so that we can make sure the Tideway work isn’t causing any scouring or silting, undermining river walls or causing any other impacts.”

On top of that, the hydrographic team have provided a wide range of technical information in support of Tideway’s works, such as core penetration tests to check ground strength at key sites, analysing samples to check for contamination, and carrying out 3D riverbed and shoreline surveys.

“We have taken on extra admin support in the office – we are gathering a great deal of data and all of this needs to be checked and processed,” says John Pinder.

First year for the Maplin – and another new boat on the way

The next tranche of proposed Marine Conservation Zones is due to be announced in early 2017. The PLA has worked closely with Defra to provide economic data about the port and emphasise the need to minimise the impact of any designation on port operators.

The Maplin, a purpose-built new hydrographic survey vessel, was delivered to the PLA at the beginning of 2016. Built at the Colchester yard of CTruk, this 17-metre catamaran has had a successful first year of operation.

“Maplin is economic to run and has been performing well, both in our safety of navigation surveys and also new capabilities including seismic survey work such as 3DChirp technology,” says John Pinder. “3DChirp allied with our existing systems is an excellent solution for sub-riverbed Geophysical investigations and clearance. Using 3D imaging and analysis, we can detect items down to 12 cms in size, identify whether it’s a piece of iron, wood or a tyre, and then be specific in targeting safe inspection and removal by divers”.

This year a new 11-metre river survey boat will be ordered and this will focus primarily on Tideway-related work. The delivery of this craft will allow the Yantlet to be retired, after more than 20 years of excellent service.

Lights, camera… image

The hydrographic team draw on a range of technologies in their work, including multibeam, side-scan sonar and LiDAR (laser) surveying as well as GeoChirp 3D.

In the past year, another specialist service has been increasingly in demand – the use of very high-resolution photography for civil engineering surveys.

“Using 3D imaging and analysis, we can detect items down to 12 cms in size, identify whether it’s a piece of iron, wood or a tyre, and then be specific in targeting safe inspection and removal by divers”. John Pinder Port Hydrographer

The department is using state-of-theart high-res camera equipment, together with sophisticated software to enhance photographs so that structural engineers can see precisely what they need. Safety and timing are crucial here. The photographer needs to get close to the subject in order to achieve forensic quality pictures and the work must be carefully planned around tidal conditions as well as light and weather.

The PLA hydrographic team provides marine asset surveys - above and below waterThe PLA hydrographic team provides marine asset surveys - above and below water

The results can be astonishing in terms of capturing minute details and enabling measurements of any possible cracks or damage to a structure.

“As a department, we can offer photography, vessel-mounted, laser or bathymetric surveys,” says John Pinder. “We have also done trials using a drone to take photographs and this could be useful in asset management. We also continue to gather a great range of data on the bridges over the Thames for TfL.”