Pentland Firth

 


Series Title:- The Orkney Riddle

5/29

Blog Title:- 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). 

......... 

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. 

......

 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)

.....also.....

"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)


A long feature stretching into the North Sea,  from the Pentland Firth, is the Sandy Riddle. This may be a remnant of a coastal beach that may have joined Caithness to Orkney in the years after the end of the last ice age.

The SandyRiddle is discussed in:- "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)

 


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.



Next blog:- "Walkable Land in the North Sea"

Back to the beginning of the Orkney Riddle

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 

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