Pentland Firth

 

The Pentland Firth 

The Pentland Firth is the channel of water that separates the Orkney archipelago from Caithness,  the northern most county of Scotland. It is deep, wide, and famously dangerous to cross,  even with modern boats and navigation systems. From the ferry crossing from Caithness to South Ronaldsay interesting surface phenomena indicate what is happening beneath the hull of the boat. Flat shiny areas of water commonly suggest an upwelling mix of currents , while whirlpools imply water being drawn down into the murky deeps.

 

"The Pentland Firth’s eastern approach hosts intricate sediment transport pathways. Time-averaging the modelled flow field revealed the recirculatory nature of the residual flow also reflected by the inferred sediment transport field identified via timeseries bathymetric analysis. 

The Pentland Firth (PF), located between mainland Scotland and Orkney, is an area of the UK continental shelf (UKCS) that experiences extreme tidal flows. These are demonstrated partly by the occurrence of mobile sedimentary bedforms (e.g dunes). 

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Tidal flows of the PF result from a tidal phase difference between the east and west approaches to the channel. A resulting barotropic pressure gradient leads to flow accelerations that locally exceed 5m/s. The extreme tidal setting of the PF’s eastern approach was found to promote distinct bedforms that are spatially varied in geomorphology with distance from the PF. 

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 The energetic tidal flows of the PF’s eastern approach interact with the highly irregular coastline, generating residual tidal counter currents leeward of flow obstructions. These counter currents (i.e. RTC vortices) are reflected in bedform migrations and thence the sediment transport pathways. As the PF is considered as a bedload parting site (BLP), the RTC vortices are expected to influence the rate of erosion at the BLP, by recirculating sediment back upstream as a counter current inshore of the main flow." (Armstrong)

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"Penthland Firth is the name of the treacherous strait between Caithness and the Orkney Islands. It was- and is - a very dangerous area for shipping because it has one of the most powerful tidal currents in the world. This can create tidal swell waves several metres in height. In trhe eastern part of the strait, currents up to 16 knots have been measured.

Areas where the current and the tidal swell are particularly pronounced have been given names such as The Merry Men of May and The Swilkie. The latter also sometimes creates a maelstrom or whirlpool. The name Swilkie has Nordic origins meaning something like “the swallower”. According to a legend, said to date back to Viking times and also known from the Younger Edda, the maelstrom is created by a sea witch rotating the quern that grinds salt for the sea – hence the word maelstrom (mael = grind; strom = current). The Swilkie originates from a point on the small island of Stroma, whose name also has Nordic origins: “Strøm Ø” (Current Island). The place names clearly speak of the dangers of the strait!

Right up to the demise of sailing ships, sailing through the Pentland Firth was avoided if possible. The great ocean-going voyages, for example from Denmark and the Netherlands to the Colonies in the West and East Indies, went far out beyond the Orkney Islands. Since 1822 it has been possible to sail through Scotland via the Caledonian Canal, at least with ships of up to 45 m in length." (Thomsen)

 

Sandy Riddle

Offshore Energy SEA January 2009 Page 261 Appendix 3 – Environmental baseline

APPENDIX 3b - GEOLOGY, SUBSTRATES & COASTAL GEOMORPHOLOGY 

A3b.3.2 Sandbanks and sandwaves

The Sandy Riddle is a large carbonate gravel and sandbank set at the east end of the Pentland Firth (Figure A3b.3). The area is characterised by high current velocities generated from tidal streams which have profoundly affected the regional distribution and composition of seabed sediments. Sediment transport and the resulting geomorphology of the Sandy Riddle are determined by the complex pattern of eddies generated over the area under the influence of tidal and wave-induced currents. Maximum east-travelling surface tidal streams of 5.3m/s are recorded on the west margin of the Pentland Skerries, at which time a strong tidal eddy extends some 3.2km to the south east. Near-bed spring tide currents are more than 2.75m/s near the head of the Sandy Riddle and decrease rapidly to around 0.875m/s further to the south-east (Holmes et al. 2004). Though mentioned in minutes of the third meeting of the Marine Natura 2000 project group in 2003 as part of a survey under the heading, ‘Selection of offshore SACs for Annex 1 habitat interest features’ (JNCC 2003), the Sandy Riddle has not been selected for either of the two possible Annex 1 habitats, Sandbanks which are slightly covered by sea water all the time, or Reefs for which it may qualify.

To the north and west of the Sandy Riddle, areas with the strongest tides are swept clean of sediments, exposing bedrock. Cobbles and boulders are also largely swept clean of sandy sediments except in the spaces between the rocks. In this sediment-starved environment the surfaces of the pebbles and cobbles are characterised by abundant attached biota. In areas of weaker currents the seabed is still characterised by cobbles and pebbles but also by mobile bedforms with coarse-grained sands. The mobile sands are thick enough to migrate as sediment waves over the seabed and periodically bury the underlying pavement of cobbles and pebbles. This process appears to prevent the establishment of abundant permanently attached biota found on the pebbles and cobbles in areas of weaker current.

Sand and gravel carbonates accumulate in areas of weak or convergent currents. Other sandbanks in Regional Sea 1 include those located within the Moray Firth SAC as a qualifying Annex 1 habitat.

Highlights 

The Sandy Riddle Bank is an exceptional example of a large banner bank system whose morphology is influenced by the interaction of very strong tidal streams. The outstanding nature of this bedform means the key geodiversity area may be considered internationally important. Associated with the bank are a complex series of bedforms including very large mobile sediment waves that comprise shelly carbonate gravel. The bank is one of the thickest deposits of shell-derived carbonate known from any shelf sea, and this and nearby banks have been described as ‘carbonate factories’ (Farrow et al., 1984; Light and Wilson 1998). The area in the neighbourhood of the bank is also scientifically important for furthering understanding of shelf bedform systems. 

Introduction 

The Sandy Riddle Bank lies to the east of the Pentland Firth and south-east of the Pentland Skerries. It is a linear, carbonate, sandy gravel bank covering an area of 34 km2, rising from a water depth >80 to <20 m (Figure 29a,b). The bank is tied to the southern side of the Little Skerry islet and two smaller banks are tied to rocky islets that lie just to the north; Louther Skerry and Clettack Skerry (Holmes et al., 2004). Detailed surveys were made of the Sandy Riddle Bank as part of the SEA Area 5, commissioned by the DTI (Holmes et al., 2004; Leslie and Stewart, 2004), whilst aspects of the morphology of the Bank have also been discussed in Andrews et al. (1990). 

Description 

The Sandy Riddle Bank and the adjacent seabed have been investigated using a range of surveying techniques including multi-beam survey, sidescan sonar and seabed grab samples. The Bank is 10 km in length, 1-2 km wide and about 60 m high for much of its length and is situated in an area of very high current speeds generated from tidal streams that are constricted between the headlands of the Scottish mainland and the Orkney archipelago. The Bank is generally asymmetric in cross-section, with the steepest side facing south-west, but the profile becomes less distinctive south-eastwards. The northern end is separated from the rocks of the Pentland Skerries by a gap of about 1 km

The currents are among the strongest of any sea, and near-bed spring-tide currents are more than 2.75 m/s near the head of the Bank. These currents are at times augmented by wave-induced currents and storm surge currents, making the top of the Bank one of the most active places for bedload transport in Scottish or any other waters8. Individual sand grains are believed to cross from one side to the other of the Bank crest on each tidal cycle, following subcircular counterclockwise directed paths. The net, long term, sand transport paths converge towards the Bank crest from either side. Seabed slope breaks of between 2° and 3° and higher angle slopes sharply define the Bank margins. The sediments on the Bank consist mainly of carbonate-rich, very coarse to coarse-grained, shelly sand and gravel. The gravels are 100% carbonate and made up of broken and whole shell fragments. Carbonate content in the sand varies between 23-29%, with an average value around 15% (Black, 2004). Samples collected during the DTI survey are almost completely free of mud and are consistent with earlier samples collected by BGS which indicate that the coarsest sediments are found towards the northern end of the Bank. A number of very large sandy gravel waves have also been identified on the Bank itself (Figure 29a,b). In places, these features are up to 10 m high with wavelengths of 80 to 200 m and are superimposed on the north-eastern flank of the sand bank. Megaripples of up to 0.5 m have been identified on these sandy gravel waves and these run parallel or slightly oblique to their strike (Andrews et al., 1990). 

Interpretation 

The analysis of seabed sediments coupled with the orientation of seabed bedforms suggested to Holmes et al. (2004) that the head of the Sandy Riddle Bank originated from the deposition of shell carbonate in a cell of an anticlockwise gyre occurring to the west and south of the Pentland Skerries. However, it has been demonstrated that such banks extend well beyond any gyre (e.g. Bastos et al., 2004). Like all such headland-tied banks, it is stationary despite the great mobility of the near surface sand. The Bank is formed because there is a line of convergence of sand transport that coincides with the crest of the Bank. This line is predetermined by the physics of tidal flow as it emerges from a restriction. By analogy with better studied headland-tied banks such as the Shambles, Portland Bill (Bastos et al., 2002), the line of convergence should extend for a considerable distance beyond the Bank. It is expected that some sand escapes to the east and is deposited, in areas where the peak tidal current has dropped to about 40 cm/s, as a relatively thin sand sheet, but this area has not yet been surveyed in detail. 

The clastic carbonate from which the Bank is comprised originates from the exceptionally productive ‘carbonate factory’ on the seafloor that lies along the sand transport paths converging towards the Bank. The longest transport path is that coming eastwards out of the Pentland Firth (Kenyon and Stride, 1970). 

Because of the strong currents, the areas to either side of the Sandy Riddle Bank are swept clean of sediments to expose fault- and joint-related lineations in bedrock and also gullies. These gullies cut across bedrock boundaries and are considered by Holmes et al. (2004) to relate to glacial scour. Nearby, in the areas of weaker currents, the seabed is characterised by cobbles and pebbles, but also by mobile bedforms with coarse-grained sands. These mobile sands migrate as sediment waves over the seabed. Seismic profile data suggest that the bulk of the Bank comprises sediments of Holocene age. It has been suggested that the Bank may have a core of glacial material (Allen, 1983). This is unlikely as there is no glacial material covering the nearby strongly scoured rocks, and the location of the Bank is in the same relationship to its anchoring point as the 100 or so other banner banks known around the British Isles. 

Conclusions 

The Sandy Riddle Bank is an exceptional example of a large, complex banner bank system and may be considered to be of international importance. This bedform is outstanding mainly because the extremely high current speed, operating in relatively deep water (60m or so), has caused this particular bank to build up to near the sea surface and hence become one of the thickest bodies of mobile clastic sediment known from any shelf sea. It is also a major deposit of clastic carbonate, that comes from the exceptionally productive ‘carbonate factory’ on the seafloor that lies along the sand transport paths converging towards the Bank. The bulk volume of all such banner banks is stable, being tied in this case to the southern-most of the Pentland Skerries, although the extremely high current velocities also make the top of the Sandy Riddle Bank one of the most active places for bedload transport in Scottish waters. The Bank is associated with a number of smaller bedform assemblages such as megaripples and large sandy-gravel waves and the bedform facing directions indicate that bank stability is partly tied to the re-circulation of almost pure shelly carbonate gravel and well-sorted very coarse-grained sand. The area is scientifically important since it represents a key area for furthering understanding of shelf bedform systems.

The geology of the Moray Firth. United Kingdom Offshore Regional Report By I J Andrews

The largest sand wave in the area, called Sandy Riddle. occurs at the eastern entrance to the Pentland Firth; it may be considered as either a sandbank or a shoal. it is 10 km in length, 1 to 2 km wide, up to 60 m high, and is composed predominantly of sandy gravel (Figure 63). Very strong tidal currents of up to 5.25 m/s occur within the Pentland Firth, but decrease rapidly south-eastwards in the open water to the east of the Pentland Skerries. Over Sandy Riddle, there is a complex pattern of eddies during periods of south-going tidal stream. The sandbank is generally asymmetric in cross-section (Figure 63), with its steep side faring south-west. Sand waves with heights of 10 m and wavelengths of 80 to 200 m are superimposed on the north-eastern flank of the sandbank; these are orientated north-south with their steeper slopes facing west. Megaripples on their backs run parallel or slightly oblique (up to 20°) to their strike, and the steep south-western face of the bank has large megaripples about 0.5 m high. The bank's profile becomes less distinct south-eastwards, and the northern end is piled against the rocky ridge of the Pentland Skerries. Seismic profiles suggest that the bulk of the shoal comprises Holocene sediments. although it is possible that there is a core of glacial material (Allen. 1983

Sandy Riddle

 

This long, narrow bank extends south-eastwards from the Pentland Skerries between Orkney and the Scottish mainland (Figure 63). Sandy Riddle has a high carbonate content (94%) rich in barnacle fragments (37%) and bivalves (32%), with lesser amounts of serpulids (12%) and bryozoa (10%). The proportion of bryozoan debris increases southwards, inversely to the serpulid distribution. The coarsest material in the north includes a significant proportion of well-rounded pebbles of the local Devonian flagstones. The accumulation rate is 581 g/m2/year.

Sources

"Sediment pathways and bedform mobility: considerations for offshore construction from the Pentland Firth, northeast UK" by Christian Armstrong et al

Mikkel H. Thomsen (www.vikingeskibsmuseet.dk)

Jeffery Nicholls 

South Ronaldsay 

Orkney 

Jiffynorm@yahoo.co.uk