Taking time's sands

One of this winter's mad floods in the hillside streams left a rich drift of sand across the road for the morning school bus to plough through. I was out soon after with the wheelbarrow and a shovel: when you want a bit of fresh-washed sand, you never have any.

I wasn't long thinking of a use for it - a couple of steps at the plank-bridge in The Hollow. First, an armature of rocks and cobbles from the bed of the stream, then a timber frame for the mix of sand and cement. With a nice cello concerto on Lyric FM, what labour could be more blissful above the babbling water?

Steps in a stream-bank, new office-block towers for Dublin, new estates for the workers - it's all making shapes with gravel, sand and cement. Just to build one standalone bungalow, from the foundations to the concrete tiles on the roof, takes up to 400 tonnes of aggregate. This is not just some mysterious product builders use, but the very marrow of this island, suitably crushed and graded. Even five years ago, we were using nearly 50 million tonnes of Ireland's rock each year. About 35 million tonnes of it came from hard rock quarries, extracted by drilling and blasting, and the rest was sand and gravel dug out from glacial deposits, such as eskers and drumlins.

Aggregates have to be cheap: they sell at a few pounds per tonne. If the journey to the customer is more than about 25km, extraction becomes uneconomic. Production in the Republic is shared out between more than 200 quarries and pits - limestone from the great raft of rock beneath the Central Plain, hard rock from the quarries in the island's mountain rim. Inland sources of aggregate were already seriously depleted, especially around the towns and cities, long before the current massive boom in construction.

Natural gravel has been giving way to crushed limestone - a much more costly product. Now, the Irish aggregate industry is looking offshore, to the great banks of gravel and sand off the east coast, from Dublin Bay to Carnsore Point. They may do this at great cost to the capital and the coastline.

In the late 1980s, the Geological Survey of Ireland made the first surveys of the Irish Sea resources. The Marine Institute has since funded research into sand and gravel resources to a sea depth of 30 metres around the whole Irish coast. The idea of such offshore "mining" can be attractive. Here is a badly-needed resource, deposited in the retreat of the glaciers, well-sorted by currents, and left in huge banks on the seabed. Excavating it would keep down building costs and help to limit environmental damage ashore.

In Britain, up to a quarter of sand and gravel used in construction is dredged from the sea floor (in London, 35 per cent, almost all of it for concrete). Apart from its commercial value, the royalties from dredging make £10 million a year for the Crown. As British and Dutch dredging companies look covetously at the resources mapped off Dublin, Wicklow and Wexford, the benefits to our own State exchequer are tempting .

So far, Irish extraction has been small, but the construction of Cork's new tunnel and harbour works gives some idea of demand: in 1997, an estimated 1 million cubic metres of sand and gravel were dredged from the harbour to use as back-fill for the project. New tunnels, bridges and motorways in the National Development Plan, to say nothing of housing, will need colossal quantities of aggregrate.

The main banks off the east coast - about a dozen of them - lie parallel with the shore, from the Bennet Bank off Howth to the Long Bank off Carnsore Point. They consist of mobile sand, added to by coastal erosion, and are part of what are called coastal "cells". These are virtually closed systems, in which a local budget of sand keeps circulating between the shore and its sand dunes and the hidden offshore reserves. The coastline is continually trying to shape itself so as to absorb wave energy in the most efficient manner. Take sand from anywhere in the cell, and the sea bites into the dunes or cliffs to make up the loss. As GSI geologists William Warren and Raymond Keary reported in 1989, the offshore sand bars "must be regarded as an integral part of the coastal system." Removal or interference with them "would likely alter the coastal dynamic system and lead to severe coastal erosion in areas that are now relatively stable". Their comments took no account of sea-level rise, since accepted as a consequence of global warming, and the threat this offers to low-lying parts of Dublin and coastal towns to the south.

Discussing it in 1991, the late Professor Bill Carter said: "The role of these linear shoals or banks in controlling shoreline processes cannot be underestimated".

Biggest prize among the offshore resources would be the Codling Bank off north Wicklow, where sands and gravels cover more than 200 square kilometres and reach a thickness of more than 50 metres. The bank did not get its name for nothing, and conflict between fishermen and dredgers already seen in Cork Harbour and off Waterford, could be the first of many serious confrontations.

In the North Sea, where 25 million tonnes of gravel are sucked up every year, herring and sand-eel spawning grounds have been ravaged and herring larvae are suffocated and blinded by sediment plumes. With this experience in mind, Warren and Keary called for environmental impact studies, and protection of biologically precious seabed zones.

A careful weighing of damage against benefit before any major extraction is licensed has been urged in two recent documents: the Marine Institute's own environmental assessment of Ireland's marine and coastal areas, and the draft government policy for Coastal Zone Management. The latter, prepared by consultants Brady Shipman Martin, calls for a "national strategy" on seabed extraction, and a voice for Duchas on licensing applications. There could be stormy arguments ahead in deciding which stones should be left unturned.