Science Spin May 2008

Harvesting sand and gravel from the sea

By Sean Duke

Aggregates are essential for the development and maintenance of Ireland's infrastructure, such as roads, bridges, and rail lines. As such it is in our national strategic interest that aggregates are readily available and at a reasonable price.

Ireland's national demand for aggregates is about 30 million tonnes per annum, a figure which is significantly higher than practically everywhere else in the world. The land sources that supply aggregates are getting more expensive to operate, and at the same time, tighter enforcement of planning stipulations means that new sources are harder to come by.

Against this background, the significant marine aggregate deposits in the sediment layers at the bottom of the southern Irish Sea come into play. Many experts believe that this 'national resource' can be exploited in a cost-effective and environmentally stable way. There are significant deposits of aggregates that lie close to the Irish east cost.

If these were mined from the sea bed, the experts state, it could help to reduce traffic congestion - given that land sourced aggregates are supplied by boat directly into city centres by fleets of trucks - and thereby reducing noise, dust, and carbon dioxide fumes, and positively impacting the drive to reduce greenhouse gases. In addition, extracting aggregates from the sea is cheaper than extracting them from quarries on land.

Policy

Ireland has no national policy for the development of our marine aggregates. The exploitation of these will not be without environmental impacts, and these need to be looked at. The technology used is to pull a dredge across the seabed, thus, lifting up the sands and gravels to the surface, but this process also lifts up everything else that's there on the seabed, for good or ill.

Another issue is that the initial cost of getting into the marine aggregates business is high, with boats capable of dredging up aggregates from the sea floor costing in the region of €30 million. Then there is also the shore-side facilities that must be in place. For all of these reasons, industry that might be interested in extracting the marine aggregates are waiting until government provides more clarity on whether they intend to integrate this activity as part of the national development plan.

IMAGIN

The Irish Sea Marine Aggregates Initiative, or IMAGIN, was set up three years ago, to look at the issue surrounding the possibility of extracting aggregates from the Irish Sea. The project has a wide remit. Gerry Sutton, IMAGIN co-ordinator, a researcher based at the Coastal and Marine Resources Centre, or CMRC, at UCC, and explained its mission statement at the INFOMAR conference held at the Marine Institute, Galway, in February.

"To facilitate the evolution of the strategic framework within which the development and exploitation of marine aggregate resources from the Irish Sea may be sustainably managed with the minimum of impact on marine or coastal environments, ecosystems and other marine users." The goal is to make recommendations to government about how policy should be formulated so that aggregates are extracted from the sea, with the minimum of impact on the marine environment. There are many experts involved with input from the Marine Institute, the GSI, the CMRC as well as experts based in Wales.

IMAGIN is supported by Interreg, and it is focussed on five study areas, four in Irish waters in the Irish Sea and one on the Welsh, or UK, side of the Sea. The initiative brings together an Irish/Wales scientific consortium, made up of organisations with expertise in coastal processes, geology and marine resource management. The idea is to provide a scientific underpinning for future policy for the development of Irish Sea aggregates.

Mapping

One of the key questions for IMAGIN, which finishes its work this year, was how much aggregates were out in the Irish Sea to be exploited. This meant geological surveys needed to be conducted using various technologies. In addition to this, biological surveys were also done to determine the potential impact of aggregate extraction on marine life.

The survey strategy was that data was generated in a grid formation, which covered 500 km², but left some gaps in between that had to be interpreted. The team took 'grab samples' from the seabed for analysis, as well as using multi-beam technology (similar to the way a bat uses echolocation, where sound waves are sent and received to learn more about the environment - multi-beam does this to learn more about seafloor geology). In addition, video footage was collected in key places, where a change in the sub-floor sediments was noted. This footage was integrated with the multi-beam data and put together in a software system. This allowed the data to be looked at in a number of ways.

It was already known that sedimentary structures that resembled ancient sand dunes were located on the Irish Sea bed, and these areas were targeted for particularly close study. The scientists wanted to determine how thick the sand and gravel layers were beneath the dune structures, which in turn would help to quantify the amount of aggregates present.

The researchers identified a set of locations that deserved further investigation, and that would be useful to ground truth (where information is collected at a particular point in great detail to check that theory matches the actual features that are seen). The Granuaile vessel was used to take about 36 cores through the seabed, which were chopped up into one metre lengths and processed back at the lab. "The results highlights," said the Gerry Sutton, "included a finding of six metres of really nice sand - you would be happy to have that on your wall or in concrete blocks."

A map was produced which showed in simple terms where the sand deposits, gravel deposits and mixed sand and gravel deposits were located. All the data that has been collected by Imagin was put together with all the data collected by the GSI in the same areas since the 1970s, and a data model was developed. There was a nice correlation found between satellite imagery of sediment concentrations, and the IMAGIN model.

Cost-benefits

A number of cost-benefit analysis were investigated as part of the Project. The scenarios that were looked at were replacing five, 10 and 20 per cent of current land-sourced aggregates with marine-sourced aggregates. "Very briefly the positive findings from this were, in general, that they supported the marine aggregate option" said Gerry Sutton.

"The principal basis for that was CO², as you can imagine replacing trucks with ships is the key to that. There is also cost saving, and recent work we have undertaken with relation to the Port of Cork's intention to use marine aggregates as a fill source has shown that there are significant cost savings available, particularly in a fill situation where you can take stuff directly from the seabed, and pump it into a big hole which is on the shore."

The Imagin project is due to finish up shortly, and after three years, Gerry Sutton said, it has produced many broad scale achievements and 'deliverables'. There is a team of people in Ireland now that understand a lot about marine aggregates, where they are, and how they can be extracted. There are a number of online data tools which can act as a decision support system for future regulation of this area. People in industry, interested in extracting marine aggregates, now have somewhere to go, to get specific information. A cost-benefit analysis has shown that savings are to be made by moving offshore. The benefits in terms of reducing CO₂ emissions have been demonstrated, and the full extent of Ireland's marine aggregate resource defined: between 8 and 10 million m³.

Future

The responsibility for developing offshore resources lies within the Department of the Environment, so engagement with this Dept. is crucial for future progress. The Project people wish to now work with them to shape the marine aggregate policy for Ireland. Meanwhile, the gaps in the data need to be filled, and more needs to be understood about the sedimentary environment in the Irish Sea. The groundwork has been done, the benefits proven, and now it is up to the Irish government to move things forward.