PATH ASSESSMENT GROUP

MINISTRY OF TRANSPORT

KJARTAN THORS AND PÁLL EINARSSON:

EVALUATION OF TWO ROUTES FOR SUBMARINE CABLES WEST OF THE WESTMAN ISLANDS

FEBRUARY 2007 KJARTAN THORS JARÐFRÆÐISTOFU KJARTANS THORS EHF

1. Introduction

This report is a contribution to planning for the laying of a submarine cable from to the British Isles. At present (February 2007), the cable is projected to land on the Icelandic mainland in proximity to the Westman Islands area.

The Path Assessment Group, commissioned by the Icelandic Ministry of Transport, is responsible for recommending a route for the cable. Further research will result from the recommendation of a route. This overview, created at the request of the Path Assessment Group, is an appraisal of the strengths and weaknesses of two possible routes leading southward from the mainland south coast, west of the Westman Islands. The initial plan called for a cable landing at Landeyjar or possibly Þykkvabær. A later additional assessment was requested projecting a landing near Þorlákshöfn.

Threats from both natural and human-induced factors were considered in the recommendation of a route for a submarine cable. These include:

a. Glacial lake outburst flooding from Katla (or Eyjafjallajökull) and current turbidity b. Underwater volcanic eruptions c. Commercial fishing using otter-trawl/seine nets d. Contour and composition of seafloor (showing risk of a suspended cable and necessitated ploughing) e. Marine currents

The authors of this report are:

Dr. Páll Einarsson, Professor Dr. Kjartan Thors Institute of Geological Sciences Jarðfræðistofu Kjartans Thors ehf University of Iceland Borgartúni 18 Sturlugötu 7 105 Reykjavík 101 Reykjavík ,

2. Outlining of route

The reviewed routes are shown in Chart 1. The commonality shared by the two routes is a path over the continental shelf west of .

The eastern route lies to the north between and Súlnasker towards Þrídrangar. However, this route turns towards the coast after passing east of Þrídrangar. If the landing in Þorlákshöfn is viable, the sea route would turn north of Þrídrangar and follow the coast towards Þorlákshöfn.

The western route lies west of (and Skötuhryggur) running to the north and turning towards the landing point at Rángársandur. The route potentially could follow the coast towards Þorlákshöfn.

The threats posed to a submarine cable along these routes were assessed in determination of a preferable recommendation. The benefits of these two routes over others, such as one transecting Selvogsbanki to Þorlákshöfn, are discussed as well.

3. Threats

3.1. Glacial Lake Outburst Flood (GLOF) 3.1.1. Frequency of GLOF’s and affected regions Consequences of GLOF’s are clearly evident in their impact on seafloor and submarine cables to the south of Iceland. These effects extend further south than the immediate coastal area. Recent multi-beam sonar surveys show fluvial deposition on the seafloor extending from Iceland (see Anon., 2004). Neritic-zone depositions have origins traceable to floods from Katla and from Skeiðarársandur. Depositional paths from Katla are trifurcated: one flowing from Markarfljót, another from Sólheimasandur, and the last and largest from Mýrdalssandur.

Chart 1. Navigational chart for the western Westman Islands area.

They combine into one channel offshore. Volcanic eruptions in Katla are clearly the single largest risk factor present in laying submarine cable through this region.

A statistical chronology of eruptions in Katla was made by Jónas Elíasson et. al. (2006) based on historical data from the past 1 100 to 1 200 ybp and fluvial deposition from Markarfljot over the last 8000 years. Under a simple model with random distribution of eruptions, a 93% chance results for an eruption occurring in the next 10 years. However, the likelihood of an eruptive event in the next 10 years is reduced to 20% taking into account the proportionality of the interval between eruptions to the size of the previous eruption. This reduced risk stems from the last large eruption in 1918 being unusually significant and, consequently, an extended intermittent period should result.

The study by Jonas, et al., may not have considered recent data on the stability of the volcano. Such records elevate the eruptive risks as greater than that indicated by statistical models. Seismic activity has increased in frequency around Katla, measurements indicate a slow expansion of the magma chamber under the caldera, and increased ground temperatures (Sturkell, et.al., 2007) have been recorded. Such Indicators favour an eruptive event with subsequent GLOF within in the submarine cable’s lifetime.

3.1.2. GLOF and turbid currents In the report, Kötluhlaup og sæstrengir, prepared by Jarðfræðistofu Kjartans Thors ehf for Síminn in 2004, turbid currents are described as resultant from unstable masses of moving sediment mixing with overlying water and forming a liquid layer of suspended particles with a high specific-gravity. Turbid currents move down inclines potentially reaching significant velocity (tens of kilometers per hour). Such particle streams present significant erosive capacity and pose a potential risk to submarine cables. The authors submit that turbid currents, flowing from Reynisdjúp or Álinn and Háfadjúp, present an elevated certainty for damage to the CANTAT-3 cable at locations along the slope of the south-coast continental shelf. This would apply to other submarine cables in this area as well.

3.1.3. The effect of GLOF on cable placement A cable between Ireland and Iceland, landing on the southern Icelandic coast between the Westman Islands and Þorlákshöfn, would directly intersect the flow of turbid currents originating in Iceland. Such intersections would likely occur if the cable were located significantly further south from the coast than the present CANTAT-3 route. However, the velocity of turbidity currents will diminish with an increase of distance from the coast. The most southerly route possible should be given to such a cable, ideally buried where turbidity currents flow.

3.2. Volcanic eruptions 3.2.1. Frequency of volcanic eruptions in the Westman Islands A direct volcanic effect on submarine cables is implausible unless located in an active volcanic region. For the routes under consideration, The Westman Islands area is the only region of volcanism. The following threat-factors are important in evaluating the probability of volcanic activity in that region adversely affecting a submarine cable:

1. The Westman Islands constitute a young and underdeveloped volcanic region, practically in its birth. Volcanic activity is relatively dispersed, but does roughly align along an axis of passing through Heimaey in the direction of Surtsey.

2. The volcanic activity in historic times, that is to say in the last 10 000 ybp, has been rare. This low frequency of eruptive activity presents insufficient data for meaningful statistical analysis. All statistical data should be taken with a degree of scepticism. At best, the data only offer the probable magnitude of an eruption.

Volcanic eruptions in the Westman Islands and surroundings have been taking place since the last glacial maximum. The area is active, as the recent eruptions in Surtsey and Heimaey attest. In a 1979 publication, Sveinn Jakobsson describes the activity of the Westman Island region in recent times (i.e. in the last ten thousand years). In the essay, he lists volcanic regions that he believes to have been active in this period:

Stórhöfði, Sæfell-Helgafell and Eldfell on Heimaey, inclusive of Bjarnarey, Elliðaey, Suðurey, Álsey, Brandur, Hellisey and Súlnasker.

He is includes Faxasker, Smáeyjar, Geirfuglasker, Þrídrangar and Einadrangi as formed by recent eruptions along with Surtsey, Syrtlingur, Jólnir and Surtla. A number of the later eruptions, if grouped together, were at least partially terrestrial.

An analysis (Sveinn Jakobsson, 1982) of seafloor irregularities proximal to the Westman Islands added other volcanic hot spots to this list. These are Skötuhryggur west of Surtsey, an anonymous peak south-east of Surtsey, Rófuboði to the north-east of Elliðaey, and another anonymous peak in Háfadjúp.

Based on the above, eruptions have occurred recently in at least 20 locations in the high- activity region by the Westman Islands. Sveinn Jakobsson was, however, able to show that the insensitivity of volcanic activity in flow-volume was relatively small contrasted to other volcanic regions in Iceland. His interpretation was that volcanism occurred episodically between long intervals; the eruptions of Surtsey and Heimaey occurred after 5 000 to 6 000 years of dormancy. Chart 2 depicts locations where eruptions are believed to have taken place in 10 000 ybp. The chart also shows the two cable paths in question.

3.2.2. Probability of eruptions along cable paths We calculated the probability for a submarine cable damage by multiplying the total probability of volcanic activity in the whole of the Westman Island region (Pv) by the ratio of surface area of the region affecting the cable (Fs) over the surface area of the region of volcanic activity (Fv).

PS = PV (FS /FV)

The surface area of the cable-affected region was obtained by multiplying the length of the segments crossing active regions by the width of that region assumed to be affected by an eruption. The Surtsey eruptions, as an example, have a width of approximately 4 km (see Norrman and Erlingsson, 1992). This is of course a rough estimate.

The surface area of the region of volcanic activity is estimated by drawing a polygon encompassing known eruptions in recent times according to Sveinn Jakobsson (1982). This yields an Fv of 800 km2. The length of the easternmost path through the region is around 25 km. Fs is therefore 100 km2. The probability of an eruption within the affected region of the cable is then one-eighth of the probability of an eruption taking place in the whole region. This is again a rough estimate. For example, the model assumes that the eruptions are evenly distributed throughout the region, which is a considerable statistical compromise.

Chart 2. Navigational chart for the western Westman Islands area. From the assessment of the number of eruptions in recent times, There is an estimated average From the assessment of the number of eruptions in recent times, There is an estimated average interval between eruptions of approximately 500 years for the whole region, giving a probability of 2 × 10-3 events per year. This assumes that the eruptions are evenly distributed across time, which is hardly the case. The probability of an eruption taking place within the area affecting the cable are, then, on the order of magnitude 10-4 per year. If the cable is laid well west of Surtsey, the chance reduces by at least one order of magnitude, given a minimal probability of an eruption taking place west of Skötuhryggur.

Chart 3. Fishing methods off the south coast of Iceland (2006)

3.3 Demersal fishing Fishing in Iceland is intensely monitored with detailed information available on fishing activities at any given time. Mandatory catch reports, filed on-board by captains, provide time, place, and method data. Such data is compiled graphically as shown in Chart 3 above. The graphic, prepared by Hoskuldur Bjornsson at the Marine Research Institute, shows the fishing regions south and south-west of Iceland in 2006. The fishing method used is represented by different colours.

As shown by the illustration, diverse types of fishing methods are used in Westman Islands’ waters. Otter trawls are deployed close to the edge of the continental shelf, lobster trawls closer to land, and seine nets, lines and other nets in shallow waters.

Chart 3 shows that it is impossible to select a path across the continental shelf based on the avoidance of fishing activities. Reduced interactions with commercial fishing gear requires either counteractive protections, ploughing, or both.

3.4. Seafloor composition Detailed information on the properties of the seafloor for the two proposed cable routes is not available. At a certain period (1975 to 1981), the Marine Research Institute collected bottom samples off the southern coast, including around the Westman Islands. Markings on sea maps also provide limited information on seafloor composition. Additionally, the Sjómælingar Íslands agency conducted multi-bean sonar surveys around the Westman Islands. These measurements only cover a small portion of the routes under study. A comprehensive understanding of the seafloor is, therefore, not available. The two routes, as depicted on Chart 1, are situated on an apparently flat bottom according to charts. It is probable that the bottom is “soft”, i.e. made up of lose sediments easily ploughed. This is, however, an unscientific method of selecting a cable route and further information prior to cable deposition is recommended.

Based on existing charts, it is likely that an optimal route exists in proximity to the paths suggested here. The seafloor to the west of the westernmost route at Selvogsbanki has an unfavourably variable topography and presents an suboptimal choice. Along the coast between Þorlákshöfn and the Westman Islands it is likely that the seafloor at 60 m depth is “soft” and easily ploughed for cable deployment.

3.5. Currents The southern continental shelf presents a wide range of currents. The common forms are tidal currents and permanent currents flowing west. Winds also significantly affect sea motion. Currents can become quite intense by the Westman Islands. Tidal currents, flowing both east and west, intensify with compression between the islands, becoming sufficiently intense to adversely affect an exposed submarine cable. Theoretically, cables lying between the Westman Islands and the coast receive a degree of protection through current-induced “sandblasting” with sediment dislodged from the seafloor.

Wind in this region intensifies the tidal currents significantly. Current velocity data are not available for this area, but overall intensity likely diminishes westward from the Westman Islands.

4. Conclusions

The conclusions drawn from our deliberations are: A route for a submarine cable west of the Westman Islands is well worth considering. The two routes presented here both have merits. In evaluation, the westernmost route has several advantages over its easternmost counterpart. The risk posed by volcanic eruptions is minimal (this risk is also small on the easternmost path), and the western route is safer with regard to GLOF’s. A cable passing by the easternmost route risks subjugation to the effects of GLOF’s, especially if landed at Landeyjarsandur. Finally, currents are projected to have less of an effect on the westernmost route resulting from the impact of the Westman Islands.

5. Sources

Anon, 2004. Kötluhlaup og sæstrengir. Jarðfræðistofa Kjartans Thors.

Elíasson, J., Larsen, G., Gudmundsson, M.T., Sigmundsson, F. 2006. Probabilistic model for eruptions and associated flood events in the Katla caldera, Iceland. Computational Geosciences, 10, 179-200.

Kjartan Thors og Jóhann Helgason, 1988. Jarðlög við . Fjölrit Hafrannsóknastofnunarinnar,1.

Norrman, J. O., and U. Erlingsson. The submarine morphology of Surtsey volcanic group. Surtsey Research Progress Report X: 45-56, 1992.

Sturkell, E.,Einarsson, P., Roberts, M.J., Geirsson, H., Gudmundsson, M.T., Sigmundsson, F., Pinel, V., Guðmundsson, G.B., Ólafsson, H., & Stefánsson, R., Seismic and geodetic insights into magma accumulation at Katla subglacial volcano, Iceland: 1999 to 2005, Submitted to Journal of Geophysical Research.

Sveinn P. Jakobsson,1979. Petrology of Recent basalts of the eastern volcanic zone, Iceland. Acta naturalia Islandica, 26.

Sveinn Jakobsson, 1882. Dredge hauls from Vestmannaeyjagrunn, Iceland. Surtsey Research Progress Report, IX, 142 – 148.

Translation by: Sol Squire, MPhil, MSc