Snippets 9: ‘Seeing’ the Solway’s bottom

chart with buoys outlined

“Between Solway Buoy and Corner Buoy, it’s a critical region, the region that gives us the most trouble. At Corner Buoy there’s a narrow corridor – that channel is our window [to Silloth], to the East of it are big boulders, to the West are shifting sands.’ Captain Ed Deeley, harbourmaster at the Port of Silloth, and a ship’s pilot, explains one of the major problems of navigating the Solway Firth – the sea constantly re-sculpts the sea-bed.

The Admiralty charts for the Firth show areas of sandbanks and ‘changeable depths’. Sediment is constantly on the move, being lifted and deposited, eroded and accreted. This affects fishermen – trawlers, scallop-boats and shrimp-boats, lobster-pot men and haaf-netters – and vessels that service the windfarm, as well as the larger ships that transport goods around the coasts. Ports and harbour entrances need to be dredged, the depths of channels charted and surveyed.

In recent years, the route up to Silloth has been periodically surveyed by the Association of British Ports (ABP), who own the ports of Silloth and Barrow on the Cumbrian coast. Ed Deeley had previously shown me some of ABP’s bathymetric charts for the ‘critical region’ and we were both intrigued by a flat-bottomed, apparently unchanging area South of Corner Buoy. Could it be a layer of peat or boulder clay such as occasionally gets exposed on the shore?

 

Chris Heppenstall, ABP’s Hydrographic Surveyor, was very willing to show me the data and talk about the surveying, so I went down to Barrow to meet him. Chris grew up in nearby Ulverston; he told me that he originally trained as a land surveyor, studying at Newcastle University. ‘Then I read that offshore survey companies were recruiting and I thought “that sounds quite interesting”.’ He took a job surveying for an offshore construction company, but then in 2011 saw that ABP in Barrow were advertising for hydrographical surveyors, so he applied and was accepted.

The Port Office is a red sandstone Victorian building, now almost hidden from its raison d’etre by a large office block built on an area where once there would have been warehouses and cranes. Barrow is still a large and important port and home to a ship- and submarine-building industry and Chris explained that the deep, straight channel from the docks, out past Walney Island, needs surveying ‘every few months, and dredging once a year, sometimes more’.

But as well as surveying the approaches to his home port, Chris goes up the Solway to Silloth. His first survey, using a multi-beam echo-sounder, was in early 2013. ‘We were using the [ABP’s] harbour tug, it’s 20 metres long with a 2.5 metre draught but we found it was affected by the strong currents and progress was very slow. Also we always work from Whitehaven, it’s just not viable to work from Silloth due to the limited access. We sometimes finish off on the last day in Silloth to survey the dock, but most of the work is done from Whitehaven, then the incoming tide pushes us towards the survey site, and the falling tide helps us on the way back.’

Despite the difficulties, they had good weather, and a good Spring tide; on that first survey, Chris said, they looked at a wider area than the pilotage channel to get a general idea about the surrounding sea-bottom.

A multibeam scanner sends out a broad fan-shaped pulse of sound, which bounces back, the length of the time delay creating a ‘picture’ of the topography of the sea-bed. Accurate postitioning of the device is necessary (eg by GPS), and the received signals have to be corrected for factors such as wave height, and pitch and yaw of the boat.

‘There are swathes of coverage, we try to overlap the corridors. The width of the beam is about four times the water depth – so the area scanned depends on water depth, it’s greater when there’s more water. Obviously, if you’re vertically above a spot, the beam has a smaller footprint, so you get greater detail. At Barrow, we do overlapping scans, so each bit gets done twice and this gives much greater resolution. But at Silloth this is too difficult, because of the currents and the limited access.’

The ways in which the scanner results can be displayed is impressive. I had already seen the large print-outs of the charts showing each recorded depth point. The chart below shows the position of the survey relative to the three buoys; the paler the colour, the deeper the water. For obvious reasons, the depth in metres is corrected to the depth of the Sill at the entrance to Silloth port.

sept15

September 2015 survey, with position of the 3 buoys marked

In Chris’s office we were able to look at multi-coloured 3D images on the computer, the colours relating to the measured depth. Images from different surveys could be stacked and tilted, to show changes in height of the seabed.

On these images, the colour range from red through yellow through green to blue indicates increasing depth.

2-18 jan14

January 2014 survey: rocky scaurs and sandwaves

Though not at the greatest possible resolution, the detail is excellent, showing the large regular sandwaves in the NW corner of the survey (for much more about sandwaves and sand-ripples see elsewhere on this blog). Parallel bands of unchanging material trending NE-SW near the southern-most part of the survey are probably part of the same rocky bands that form Dubmill and nearby scaurs; later surveys show them dotted with a scattering of boulders.

And of course, what was of greatest interest was to compare the position of the sandwaves and sandbanks relative to the pilotage channel throughout the three years of periodic surveys. Looking at the shifts in position of the red-orange (shallow, ie sandbanks) and blue-green (deeper) colours between January and September 2014, it’s easy to see how the sandbanks have been moved towards the East, filling in the channel.

panorama 3 surveys

from left to right: January 2014; September 2014; September 2015 (thanks to Chris Heppenstall and ABP)

Between September 2014 and September 2015, however, the sandbanks in the NE have been moved in a westerly direction again and the (green) channel is more extensive. In September 2015, there are obvious boulders (seen more clearly in the previous image) scattered over the surface of the rocky scars towards the South – probably revealed as sand has been swept away.

sept14-15

However, there doesn’t seem to be much evidence for the flat-bottomed channel which Ed had mentioned, and which I had surmised (and hoped) might be peat or clay. On reflection, peat would have been unlikely anyway, as the peat ‘horizon’ on the shore is much higher; but glacially-derived clay remains a possibility. The only relatively unchanging smooth area is to the very South of the surveys, seen in January and September 2014. Now what we need is a diver!

 

My grateful thanks to Chris Heppenstall of ABP for so generously sharing these data and images.

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