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Lagan Weir - Why it exists

Looking out on the River Lagan today, it is hard to believe its waters and riverbanks were ever smelly, dirty and polluted.

The water quality of the river was poor due to several factors:

  • There was a large amount of "domestic" pollution entering the river system from an outdated sewage system.
  • Poor intermixing of the fresh water from upstream with the denser seawater from Belfast Lough.
  • Nitrates contained in the fresh water.
  • The Lagan’s water level varied by three metres between high and low tide so that at low tide unsightly smelly mud flats were exposed as far up the river as the Ormeau embankment.

The bad smell was due to decomposition in the mud releasing unpleasant smelling gases into the air. This increased during the summer months as the higher temperatures increased the rate of decomposition.

The odour was particularly noticeable upstream of the old McConnell Weir, at the outlet of the Blackstaff River near the Gasworks site. At this point, because seawater is denser than fresh water, a pool of stagnant seawater was trapped under the river water. This reduced the oxygen levels in the river, so increasing the rate of decomposition in the mud.

The banks of the river were overgrown and deserted, wildlife was practically non-existent and there were few or no fish to be seen in the murky waters.

Something needed to happen to bring new life to the river and re-establish the River Lagan as a focal point for the city of Belfast, re-emerging from over three decades of the Troubles.

Laganside undertook a successful infrastructure led approach, which started with the building of the Lagan Weir. The aim of the weir is to create an attractive reach of water ensuring a minimum level, which will cover the mud flats at every stage of the tide. Additional measures were also taken to improve the water quality, with a programme of river dredging and the installation of an aeration system.

In the event of a dangerously high tide, the weir also acts as a tidal barrage and can be used to protect the city from flooding.

At a cost of £14 million, which was jointly funded by Laganside Corporation and the European Commission, the Lagan Weir was one of the largest civil engineering projects ever to be undertaken in Northern Ireland.