When you walk along the Thames Pathway between the Thames Barrier and Greenwich, you walk through what appears to be a very large building supply yard, with a web of conveyor belts over one’s head leading to enormous tapering hoppers and mounds of gravel and sand. An information board tells you that you are walking through Angerstein and Murphy Wharves, Europe’s largest sea-dredged aggregates terminal. Angerstein Wharf was named after an early 19th Century entrepreneur and art collector who was chairman of Lloyd’s from 1790-1796.
Both Angerstein and Murphy Wharves have been given safeguarded status by the Mayor of London and the London Port Authority, ensuring that they are retained as working wharves and are protected from redevelopment into non-port use.
They annually receive 2,5 million tonnes of marine dredged aggregates from licensed areas for dredging around the UK coastline. The adjacent 33 acres of land, called the “Aggregate Zone” is used by CEMEX, Day Aggregates, Tarmac and Aggregate Industries UK Ltd to process these deposits into materials for the construction and road building industries.
I decided to do a little more research into this industry, an anthropocenic network that mobilises and re-distributes marine sands and gravels into the UK’s cities and towns and beyond. It now satisfies almost one quarter of the UK’s total aggregate need, where it has been used in many major recent construction projects, such as the Olympic Park, Heathrow Terminal 5, Cross Rail Stations, the Channel Tunnel, the Thames Barrier and Court One at Wimbeldon, and is exported to Holland, Belgium and France.
I learned that since 1955, a total of around 500 million tonnes of aggregates have been dredged from the sea around the UK, with an estimated 50 years remaining at present levels of extraction. The sands and gravels were deposited by fast flowing rivers about 20,000 years ago during the planet’s last glacial period towards the end of the Pleistocene, when sea levels were 100 metres lower then than they are today and much of today’s seabed was dry land.
Gravelly deposits are locked in these ancient river terraces and channels, which often form extensions of rivers on land today, or on beaches submerged when sea levels rose about 8,000 years ago. Sediments were reworked by the action of the sea, leaving them clean and well sorted, or they were mobilised into sand banks, as is found for example off Great Yarmouth or in the Bristol Channel. Most marine gravels are smooth and rounded, given the distance they travelled in their geological past and due to the constant pounding by the sea. Their chemical composition is no different from material quarried on land, though their shell and chloride content is carefully monitored.
The British Crown owns the mineral rights to the seabed and issues licenses to explore and extract sand and gravel.
Licenses also have to be approved by the Department for Communities and Local Government in England, the Welsh Assembly or the Scottish Executive. A royalty is paid to the Crown Estate for every tonne of dredged material. Identifying a license area involves surveying and sampling of a large area of the seabed by the British Geological Survey. This involves both seismic and sampling techniques. Seismic profiling data produces a cross section and plan of the seabed that enables marine geologists to understand how it was formed. Sampling cores establish aggregate quality and provide environmental information about the sea bed. Licensed dredging areas typically lie six miles offshore and in water more than 20 metres deep, to avoid any possibility of coastal erosion. The most important of these are off the east and south coasts of the UK and in the outer Thames Estuary, with smaller deposits in the Bristol Channel and Liverpool Bay.
Of some 1344 square kilometres of seabed licensed for marine aggregate extraction in 2007, only 134.7 square kilometres was actually dredged, equivalent to just 0.016% of the UK continental shelf area.
The British marine aggregate industry is represented by the British Marine Aggregate Producers Association (BMAPA). It has a fleet of about 30 vessels operating around the clock, 365 days a year, employing 2,500 people. A large dredger loads 5,000 tonnes of sand and gravel in around 3 hrs and discharges it in the same time. Vessels use satellite navigation systems accurate to less than five metres to locate a dredging area, and then one of two types of dredging techniques – anchor dredging over deep deposits or trailer dredging, requiring the dredger to trail its pipe along the seabed at speeds of up to 3 knots, for more distributed deposits.
After the dredge pipe is lowered, powerful pumps draw the sand and gravel into the ship’s cargo hold, displacing the seawater previously loaded as ballast.
Clearly there are issues here. The practice is opposed by MARINET an volunteer organisation that campaigns against, amongst other things, offshore strip mining for sand and gravel because of the adverse impact that the operation has upon the coastal environment and the marine ecosystem. MARINET is particularly concerned about the threat to sea defences and coastal communities, to the fish spawning areas and marine life on the seabed which are both intimately connected to the health of commercial fish stocks and the livelihood of fishermen. Not least, it is worried by the threat that aggregate dredging places upon sensitive coastal habitats such as salt marshes, sand dune systems, sand cliffs and inland habitats which are vulnerable to coastal erosion. Needles to say, BMAPA has aggressively countered all such accusations and claims that they actively engage fishing communities and archeological interest groups on all aspects of dredging and that any negative impacts of operations are reversed in two to five years.
A variety of techniques are employed for discharging dredged sands and gravels, including bucket wheels, scrapers, wire-hoisted grabs and pumps, which place the aggregate onto a conveyor system for delivery to the wharf or processing plant, as was evident in my walk alongside the Angerstein and Murphy Wharves.
The sand and gravel is then subject to screening, washing and grading into different sizes.
One of the great benefits of the industry is its ability to deliver large volumes of aggregates close to urban areas. Over 7 million tonnes of marine sand and gravel are delivered to wharves along the Thames each year, requiring on average 4 cargoes of 5,000 tonnes to be landed each day. The contribution of marine aggregate supplies in the region is equivalent to 1000 lorry loads of sand and gravel being delivered by sea, close to the point of demand, 365 days a year.Go here for a British Geological Survey map that shows all the sand and gravel resources for London Boroughs, including all the wharves along the Thames that receive marine sands and gravels: london_borough sand and gravel resources.
Information drawn from:
D .E. Highley, L. E. Hetherington, T. J. Brown, D. J. Harrison and G. O. Jenkins. The strategic importance of the marine aggregate industry to the UK. Nottingham: British Geological Survey, 2007.
British Marine Aggregate Producers Association. Aggregates from the Sea. London: BMAPA, no date.
The Crown Estate and BMAPA. The Area Involved: 15th Annual Report, 2012.
Safeguarded Wharves on the River Thames. London Plan Implementation Report. Greater London Authority, 2005.
The Crown Estate and BMAPA. The Area Involved: 15th Annual Report, 2012.