The Lagan Weir, completed in 1994 at a cost of £14m, is located in Belfast, Northern Ireland and crosses the River Lagan between the Queen Elizabeth Bridge and the M3 cross-harbour bridge. The Lagan Weir was jointly funded by the Laganside Corporation and the European Commission. It was designed by Ferguson and McIlveen and constructed by Charles Brand Ltd. The "Lagan Lookout" housing the weir controls, CCTV surveillance system and visitors centre also formed part of the project and is located on the Donegall Quay side of the river. The visitor's centre is located upstairs in the Lagan Lookout building and houses a public exhibition to provide information on the function of the weir and the history of the River Lagan. The weir gate control centre, security, CCTV system and welfare facilities for River Management personnel (employed under the Department for Communities) are located on the ground floor. Equipment storage is located in the basement. The Lagan Weir consists of five weir gates and four intermediate gatehouses. An underground tunnel traverses under the River Lagan, which connects to both quaysides and to each of the gatehouses. Primarily, this provides access to the gatehouses for maintaining the weir gate motors.
The five weir gates can each be operated independently. They are usually raised as the tide retreats in order to keep the river at a specific impoundment level. The gate operations are controlled by the River Manager, with assistance from the River Warden. Without the operation of the weir gates, the river would be subject to the tidal fluctuations from the harbour. The tidal range is typically up to three metres between high and low tide, but the maximum tidal range can be as much as four metres. Prior to the building of the weir, low tide would expose mudflats, which were unsightly and emitted a strong odour, particularly in the summer months. On occasion, at high tide, the weir can operate as a barrage. This has the opposite effect of ensuring the river levels remain lower than the harbour levels. If the river level is too high this can have such negative effects as causing erosion of the river banks, reduce air draft for vessels needing to pass under any of the river's road bridges and in extreme cases, increase the risk of flooding to low areas.
Construction of the weir, installation of an underwater aeration system and maintenance dredging has led to environmental improvements in terms of water quality and biodiversity. The last maintenance dredge was carried out by GRAHAM Construction in 2010 to 2011. Social developmentment along the riverside has also led to the construction of new residential towers, commercial and retail blocks, restaurants, and improved public towpaths.
A pedestrian footbridge is constructed over the weir and connects Donegall Quay with Queen's Quay. The original bridge, which was narrow and required spiral access ramps at either end, was located above the weir gates and supported by the weir gatehouse structures. This was demolished in 2014 in order to be replaced in 2015 by a new structure, which would provide access for both cyclists and pedestrians. The new bridge was erected by GRAHAM Construction at a cost of £5,000,000. It is 120m long and 10m wide at its widest point.