Seeing the sea-bed

‘The ground’s too rough – the shrimp-boats avoid it like the plague, it does too much damage to their nets’, says David Dobson, now retired from the NW-Inshore Fisheries and Conservation Authority at Whitehaven, talking about the sea-bed in Allonby Bay. We’ve been discussing the proposal to make Allonby Bay a Marine Conservation Zone and what effects, if any, it would have on the fisheries. Around Allonby and Beckfoot, on the English side of the water, there are many rocky scaurs stretching out into the Firth – but when we think of the Solway, it’s usually the turbid waters and changing channels and sand-banks that come to mind .

Photo with thanks to E.On (see the Solway Shore Stories' piece about Robin Rigg)

Looking down the column. Photo with thanks to E.On (see the Solway Shore Stories’ piece about Robin Rigg)

Before and during the construction of the offshore wind-farm at Robin Rigg, several important surveys looked at the geology of the sea-bed and shore, and the behaviour of the tides. Most of the interesting information that I quote here, about the soft sediments of the sea-bed, is summarised in a 2010 report by COWRIE (ABPmer Ltd et al.(2010) A Further Review of Sediment Monitoring Data  (project reference ScourSed-09).

For example, “The sandbanks in the Solway Firth have mobile superficial deposits with much denser more compact cores …The seabed material is varied consisting of fine to medium sand and sandy muds”, which overlie sediments formerly scoured out by glaciers and deposited by rivers and tides.
In the region of the wind-farm, the seabed is in fact a series of banks (Dumroof Bank, Robin Rigg, Two Feet Bank and Three Fathoms Bank) orientated in a northeast – southwest direction. Here 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”.
There are mega-ripples, too, on Robin Rigg bank – up to to 1m high, with variable orientation and wavelengths between 6m and 20m.”
And so we begin to see a picture of the sandy bed of part of the Firth: the bottom isn’t just plain ‘sand’, it’s a complex mix of sizes and shapes.

Next, when we look at the descriptions of the hydrology, how the water moves around the Firth, we start to see just how easily these sediments could get moved around.

The Solway is an area of high tidal energy and strong currents.

At neap tides, the tidal range (the difference between high and low water) may be as little as 3 metres, but for the largest spring tides, the range can approach 10 metres; the range for mean spring tides is about 8 metres. That’s a lot of water that has to shift in and out of the Firth every 12 hours or so – and the speed for mean spring tides has been “shown to be around 1.9m/s between Dubmill Point and Southerness Point and around 2.4m/s at Annan to Bowness” (COWRIE report).
And “information on near-bed currents at Silloth and Annan indicate … the maximum recorded flood and ebb velocities [are] 2.0m/s and 1.5m/s, respectively (Ove Arup & Partners, 1993), creating greater opportunity for sediment transport during the flood tide and hence sediment transport into the estuary.” (COWRIE report)

A 3D image of the scour-pits around the bases of the Robin Rigg piles (see COWRIE report for details of the bathymetry)

A 3D image of the scour-pits around the bases of the Robin Rigg piles (see COWRIE report for details of the bathymetry)

Scour-pits (see footnote 1)
You can feel this effect of ‘sediment transport’ on a small scale if you wade out into the sea from a sandy shore – the tide picks up the sand from around your feet (I felt this to alarmingly greater effect when I was haaf-netting, standing up to my chest in the water of the Firth!). If the foot of a wind-turbine is sunk into the sand, the same will happen – ‘scour-pits’ form around the pile.
Engineers and hydrologists carry out a great deal of research and effort into trying to ameliorate this potentially awkward situation, and now we know not only about the composition of the sea-bed around Robin Rigg, but also about the effect of tidal currents on the sand, through a couple of bathymetric surveys that have used sonar to construct profiles, and fascinating 3D images that help us easily visualise the sea-floor.
The tides in and out of the Firth travel mainly east-west, and there seems to be a ‘residual current’ at the sea-bed that travels east. Data show, for example that “At [turbine] position G3, there has been back-filling of the scour hole immediately adjacent to the pile location and general scouring downstream of the pile (relative to the dominant tidal flow direction) … there has been a general lowering of the seabed along the principal flow axis passing through the pile location.”(COWRIE report)

But of especial delight to anyone interested in sea-bed fauna, underwater surveys have shown that many of the scour-pits contain concentrations of starfish.

Reefs on the Solway sea-bed
E.On have commissioned a series of surveys of benthic fauna, sea mammals and sea-birds during and after the construction of Robin Rigg; methods and results are summarised in their Marine Environmental Monitoring Plan for the Scottish Government. Dr Jane Lancaster and colleagues at Natural Power have monitored ‘biofouling’ of cables and piles, and the diversity and distribution of benthic fauna (see page 5 of the MEMP report), and have found no adverse effects on marine invertebrates. Through their use of Drop-Down Video cameras (DDV), they found that the abundance of marine life that had colonised the bases of the turbines and substations after only 4 years was impressive.

Starfish and mussels on a Robin Rigg turbine base (from Natural Power's DDV survey)

Starfish and mussels on a Robin Rigg turbine base (from Natural Power’s DDV survey)

The intertidal sections of the foundations were “colonised with typical rocky shore organisms, with an abundance of barnacles as well as occasional limpets and green seaweed, and a dense carpet of edible mussels around the low water mark.”
Below low-water mark, “the mussels continued, providing a rich feeding ground for starfish, as well as green sea urchins which feed on the anemone-like colonial animals [hydroids]that live on the mussel shells. On foundations in shallower waters this mussel-dominated habitat extended down to the sea bed, but on those foundations located in deeper waters the lower sections were crowded by a forest of brightly coloured plumose anemones as well as pink oaten ear [oaten pipe? Tubularia?] hydroids. Mussels and plumose anemones were even seen thriving on the export cable by the substation before it is buried beneath the sand. Throughout the surveys shore crabs and hermit crabs were seen crawling on the foundations and on the sandy seabed beneath them, and fish such as whiting swimming around the foundations.” (quoted from Natural Power’s press release.)

Mussels densely colonising the base of a turbine (from Natural Power's DDV survey)

Mussels densely colonising the base of a turbine (from Natural Power’s DDV survey)

The foundations are acting as artificial reefs, providing marine animals and algae with solid substrata to which to attach and form new micro-habitats in which other animals can live and feed and shelter.

In summary, the researchers note that the fauna are of species found naturally in the Firth, ‘typical of a rocky reef in the Irish Sea with a moderate or strong current’.

Hidden worlds in four dimensions

I’ve written elsewhere in this blog (with reference to Allonby Bay’s designation as a Marine Conservation Zone) about research on people’s perceptions of the sea-bed: that most people have no idea at all what it might look like — with the follow-on that,  if there’s no great feeling as to whether it might be interesting or important, then why bother to protect it?

Now though, these various surveys of the Solway, plus anecdotal evidence from ships’ pilots and shrimpers, show us a fascinating undersea world of ripples and sandbanks, changing channels and hollows, as well as rocky scaurs and reefs. Sub-surface four-dimensional worlds, of animals in place and time; neither static nor lifeless, constantly changing with the seasons and the weather, as the sea washes in and out, around and overhead.

***

(1) Scour-pits: an update October 2016. This month a side-loading hopper barge, the HAM-602, with assistance from the cargo boat Visnes, has been involved in ‘rock-dumping activities for scour protection’ at the bases of turbines B1, B2, A2, C1 and D1.

 

 

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