The re-shaping of the Firth

Stories from geological and historical time have described the Making of the Firth - but since those embryonic days the Firth has continued to change its character and behaviour. Here, then, are the short-term stories of its development, stories whose beginnings and endings change with the months and days. 

In the intertidal region on the shore the changes are on a scale where you can reach out and touch them, or cover them with a footstep -  patterns of sand-ripples varying from metre to metre, locked into shape until the tide returns to re-mould them. Chevrons, fish-scales, dimples: their shapes can tell us something about the wind and waves that shaped them.

Ripple patterns in the sand (photos: Ann Lingard)

 

 

 

 

 

But further out, hidden and rarely uncovered by the tide, large sand-waves and megaripples are mutated by the currents. So how can we see the Solway’s ‘internal configurations’? The Admiralty Chart doesn’t give many clues, there is so much ‘sand and mud’ and ‘changeable depths’ amongst the larger sandbanks.

Section of Admiralty Chart, with approximate position of navigation buoys added

Those who attempt to cross its 'waths' on foot are very aware of the changeability of its channels. The channels can shift in a matter of days, as I discovered when I crossed from Bowness to Annan Waterfoot, with Mark Messenger as my guide.

For those on the sea, knowledge of the route up to the Port of Silloth is particularly important, and local expertise is crucial; cargo vessels going up the channel must wait off Workington to take on a pilot. Both Captain Chris Puxley and Captain Ed Deeley, former Harbour Masters at Silloth, told me how quickly the safe channel can change. Chris also acts as a pilot, and he told me, “We take soundings with the vessel’s echo-sounder as we come up the channel, and we note the changes for the next time.”

Computer images of hydrographical data, Jan. & Sept.2014, Sept. 2015 (my thanks to Chris Heppenstall and ABP)

More recently, the channel has been surveyed by the hydrographer from Associated British Ports (ABP), based at Barrow. “They used a multibeam echosounder to give a 3D picture and they went up and down between the Solway Buoy and Corner Buoy, at the top end of Allonby Bay. That’s a critical region, the region that gives us the most trouble,” Ed Deeley told me. After the winter storms of 2013/14, “a lot of sand had moved around. There was an area 180-200 metres West of the buoys, of shifting sand. The channel used to be 3.8 metres deep and had gone to 1.3 metres deep – the sandbanks had spilled over.”

When I visited ABP’s hydrographer, Chris Heppenstall, he told me about the difficulties of surveying in the face of the Solway's powerful currents. Despite this, he has carried out several surveys of the most difficult part of the channel to Silloth, and he showed me the 3D images of the seabed on his computer, coloured for different depths. The heights of the regular sand-waves and the bands of rock jutting SW towards Dubmill Scaur were dramatically clear. The images could be overlaid, showing graphically how one of the major sandbanks to the North-West had been whittled away and then rebuilt further to the East.

Aerial view from Dubmill looking North at very low Spring tide (thanks to Andrew Lysser for this photo); many sandbanks are visible

Seeing from above

In August 2015 I flew over the English coast of the Solway during an Extreme Low Water Spring tide, because I wanted to better understand the Firth’s edges. I was passenger in a gyroplane, which has the advantage that it can fly ‘low and slow’, making it easier to take photos (and the disadvantage that there is no roof or sides – I was held in only by the seatbelt).

To see so much of the ‘bare bones’ of the Firth was a revelation. I could appreciate much more clearly, from above, that this skeleton was not fixed, it could shape-change like a contortionist (and see also note [2]).

I’d planned the flight because I'd wanted to get a different perspective of the rocky scaurs and boulders whose names Ronnie Porter had told me, but now I also saw the great arrays of sandwaves, the braided currents in the stream – and the massive domes of sandbanks along the centre of the Firth.

Tug and barque off Maryport in a storm. Painting by local artist William Mitchell (at Maryport Maritime Museum)

The presence of the sandbanks is often betrayed on a flood tide by the shoaling of white-crested waves, but now, seen from above and at such a low tide, their true size was remarkable.

In the days of sail, ships were often driven into the Solway by storms and occasionally came to grief on these shoals. It was on Barnhousie Sand off Southerness Point that the barque William Leavitt was stranded in 1889, and even now, the shrimp-boats from Silloth can make a mistake.

Jack-up barge MPI Adventure working at Robin Rigg, Autumn 2015 (thanks to Eddie Studholme for this photo)

E.On’s Robin Rigg windfarm is built close to the Scottish coast on a group of four undersea sandbanks – Dumroof Bank, Robin Rigg, Two Feet Bank and Three Fathoms Bank – where there are “finer grained granular sediments … ranging from laminated sands, silts and clays to organic silts and clays”; across the whole of the turbine array area sediments “generally comprise mobile, shelly fine to medium grained sands” (see link for the survey report).

Recent ‘natural movements of the sand’ at the North end of the array, have had the unfortunate result that two turbines have had to be ‘decommissioned’ and taken down. Anecdotes about the Solway’s sandbanks are common: I have been told that the Robin Rigg sandbanks were visible at low Spring tides before the windfarm was built...

The tides and currents

Tidal chart for the week beginning Sept. 2015 (from UKHO Admiralty Easytide website)

Height in metres, scale 0-10m

The bore came rushing up the Firth, we heard its rasping chatter before we saw it. It was barely half-a-metre high, but the water quickly and silently flooded the shore in its wake. Twenty minutes later, I stood up to my waist in the Firth, gripping one bar of the great haaf-net; the water kept rising, up to my chest, it sucked the sand from beneath my feet: Mark Messenger and I left our place at the tip of the boak and waded back towards the inner end. We were haaf-netting for salmon, fishing in the Inner Solway, way up by Bowness, but even on a quiet day the power of the water was obvious.
At the biggest of the Spring tides, the range in the Outer Solway can be from a few centimetres above Chart Datum (3) to nearly 10 metres – twice a day. That’s a lot of water shifting in and out during each six hour period; it’s what has attracted schemes for using the power of the Solway's tides to generate electricity.

The flood (incoming) tide moves faster than the ebb, and in the inner reaches of the Firth, the slack period between the ebb and flow is longer, the change in height around the marshes is smaller.

At the Port of Workington, harbourmaster Russell Oldfield had been telling me about the vast amounts of débris and mud that the River Derwent carries down during the big storms. We talked about the currents in the Firth and how they too dumped sediment outside the port. He brought out the Admiralty Tidal Stream Atlas (NP256, Irish Sea and Bristol Channel), which shows the pattern of tidal flow into and out of the Firth. Arrows thick and thin, long or short, bend around the coasts; numbers next to them show the mean speed in knots (1), changing from hour to hour. From page to page the tidal cycle is captured on paper; you can see how water pours into, swirls around and drains out of that big bath tub, the Irish Sea, in several directions at once, and how its patterns are influenced by the immovable bath-toy of the Isle of Man.

Extract from the Admiralty Tidal Stream Atlas, 3 hours before High Water at Liverpool, to show direction and strength of flow of currents

And the long finger of the Firth imposes a lag.

When the tide is on the turn and starting to rise along the Irish Sea coasts, it is still draining out of the Solway for another hour or so. The sea starts pushing in round the toe of the Mull of Galloway, but although it has also started flooding in – more slowly – along the centre of the Firth, the main tidal stream to the South is being shunted South-East, down towards Fleet.

As the speed of the flood picks up around the Mull of Galloway (there’s an excellent view of the fierce current from the lighthouse), to means of 2.5kts and 4.5kts (equivalent to 1.3 metres/sec and 2.3 m/s, Neaps and Springs respectively), in the Firth it is not yet so fast.

And then a moment arrives when the south-easterly stream swings to the North, up into the Firth - and the flood tide along the Firth speeds up to means of 2.1 knots (1.1 m/s) and 3.8 knots (2 m/s).

The sea continues to pour into the Firth for another hour beyond high water in the western Irish Sea. High water at Silloth is 35 minutes later, at Annan Waterfoot 60 minutes later, than high water at Liverpool.

Meanwhile, what is happening at the sea-bed?

When I stood still in the waters of the Firth  - to fish for salmon or while crossing from England to Scotland - the sand was sucked from beneath my feet. The near-bed velocities of flood and ebb tides have been recorded as high as 2.0m/s and 1.5m/s, respectively.
So we should not be surprised that sediment is picked up and carried elsewhere, that sand-waves form, that sandbanks are eroded and re-built, and channels twist and turn.

Seacroft Farm at Dubmill Point, Allonby, at high water in a storm

As Eddie Studholme, Operations Manager at Silloth RNLI, told me, “The Solway’s an unusual area. People think they know it, but they don’t. You can be as scientific [about it] as you like, but it can always kick you in the pants.”

On a high Spring tide when a gale is driving the breakers against the Cumbrian shore and Seacroft Farm at Dubmill Point has shuttered its windows, the sea hurls sand and rocks across the road and batters at the dunes. The Solway shape-changes yet again.

'Arriving home'. Copyright James Smith Photography, to whom thanks for this beautiful image.

But on a quiet evening at Allonby, the first indication that the tide has turned is a pale scum of bubbles at its leading edge. The sea comes in softly, filling the ripple-marks, with only the faintest sussuration.

Sunset in midsummer: the sun slowly dropping in the North-West, over Criffel’s flank, lighting the sky and the wet shore with the flaming colours for which Solway sunsets are famous. “An aggressive estuary”? At such a time, that’s hard to believe.
 

(C) June 2016


(1)  One knot (kt) is 1 nautical mile per hour=1.151 miles per hour (approximately), 1.852 kilometres per hour (exactly), 0.514 metres per second (approximately)

(2) Since I wrote this article, photographer James Smith has made a beautiful video, using a UAV ('drone'), of aerial views of both sides of the Firth; called The Light of the Solway it is well worth watching, not only for the atmospheric views but also as a help to understanding the Solway's importance to us and to the wildlife that shares it with us.


 

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