River Irwell flood control scheme


River Irwell flood control scheme


Delivering the River Irwell flood control scheme

The River Irwell flood controls combine flood defences and a new basin into which water can run when levels get high. The river’s embankments have been raised: some are earthen, some are hard structures, and some a combination of the two. This provides flood protection to a one-in-40 year standard. However, the addition of the new basin at Littleton Road, into which water can run when levels get high, increases the protection to a one-in-75-year standard. The basin, which has a capacity of 650,000m3, fills automatically via a simple overtopping side weir and has been designed to overtop at a one-in-15 year flood event. There is a downstream outlet back into the river, with valves that can be operated to prevent backflow from the river or the retention of water in the basin. The basin is designed to increase the protection of 3,300 properties.

The basin is only used as a flood defence when water levels get high and most of the time it is public open space and a park. The space includes playing fields and the new Salford Sports Village, a state-of-the-art sports and training facility. Alongside this is the new Riverbank Park, developed in close consultation with local residents, who now take part in a friends’ group to help ensure that the park remains well looked after.

Conducting the strategic flood risk assessment

PPS25 recognises that climate change is likely to increase the risk of flooding in some areas and UKCIP scenarios indicate that winters in the North West of England may become as much as 20 per cent wetter by the 2050s. For Salford City Council, one of the key reasons for doing the SFRA was to identify more precisely which parts of the city were at risk of flooding from the Irwell and other watercourses. The council added a climate change allowance of 20 per cent to predicted peak river flows. Undertaking the SFRA gave the council the opportunity to model the impact of floods, taking account of topography, the depth of the water, the speed of flow, and the direction and destination of flow.

Data obtained from it would not only be useful regarding existing properties, but also help to inform decisions about the location and design of new buildings. This is particularly relevant as a key priority of the council is to regenerate existing deprived communities that are also subject to a risk of flooding and had attracted two major government funding streams, the new deal for communities and housing market renewal, to do so. The SFRA set the broad strategy to mitigate flood risk and new development in the Lower Irwell Valley. It was essential that the council demonstrated to the Environment Agency that new development needed to regenerate existing communities could proceed and safely manage the risk of flooding at the same time.

Implementing the findings of the SFRA

Salford City Council incorporated the findings of the SFRA into a practical document to guide planners and developers, the ‘flood risk and development planning guide’, which supplements its UDP policy EN19 (flood risk and surface water). Drawn up in close consultation with the Environment Agency, it contains planning requirements including ensuring that: the floor level of new development is above predicted flood level; new development does not cause risk of flooding elsewhere; open spaces are used as temporary flood water storage areas; identified flood water routes are kept free of obstructions; surface water run-off is captured or slowed before it reaches sewers or drains; new developments are designed to be flood-resilient; and new development includes emergency escape routes that remain dry during flooding.