Flood control strategies in a time of climate change.
The Leeds Flood Alleviation Scheme
[This a case study that may be useful to students of the Water cycle. It refers to flood management in both rural and urban settings.]
Introduction
In 2020 the Environment Agency (EA) published a report entitled the National Flood and Coastal Erosion Risk Management Strategy for England. Its main purpose was to outline a vision of a nation resilient to the future climate impacts, or at least for the remainder of this century.
The last major EA report on this issue was published in 2011, when it tentatively wrote ‘The climate is changing, and this is likely to have an impact on flooding and coastal erosion.’ Fast forward to 2020, and the tone of the new report was different: ‘It is essential that we get ready for the unavoidable impacts of climate change.’ The EA has stated that England (and presumably the rest of the UK) needs to be climate ready to be resilient to climate hazards in the future, and to the potential consequent economic shocks that will follow.
Ambitions
The EA’s new strategy has three core ambitions concerning future risk and investments:
· The creation of climate resilient places – i.e. bolstering resilience to flooding
· The need to plan for economic growth and infrastructure development that will be resilient and sustainable in tomorrow’s climate
· A nation ready to respond and adapt to flooding and coastal change, wherever it may occur – involving understanding risk and knowing responsibilities.
The EA recognised that it must change its approach to flooding. For many years, investment was placed in trying to stop flooding, with varying degrees of success [Figure 1]. It now states that all flooding cannot be stopped, not least because of climate change and rising sea levels. So, the approach is more of ‘adaptive pathways’ – in other words making areas more resilient to the events that befall them.
Natural flood management
Building resilience – using an adaptive pathways approach – should no longer just mean large, complex (hard) engineering projects. Strategies should now include storing water in the drainage basin of a river, making properties more resilient to floods when they occur, and protecting key infrastructure, such as water treatment plants and electricity substations. The Report states that for every house that is flooded, another 16 people lose access to key utilities. Roads and telecoms for instance should also become flood resilient.
The key to this is natural flood management (NFM), largely a soft engineering strategy. NFM aims to mimic natural processes to ‘slow the flow’ of water into the main river. NFM also involves lots of small-scale interventions, which all interconnect and contribute across a whole catchment. Furthermore, water should be managed on a catchment basis - water does note respect local authority boundaries.
Figure 1. Significant national flooding events since 2007: properties flooded, properties protected, and economic losses
*Best estimate of economic damages (£ million, 2018 prices)
Source: EA (2020)
The Leeds Flood Alleviation Scheme.
The first phase of the Leeds Flood Alleviation Scheme, centred in the city centre alongside the River Aire, was completed in October 2017. The works significantly reduced the risk of floods that had the potential to cause devastating physical and economic damage to the heart of the city. Previously, 3,500 residences and 500 commercial properties were at high risk, as were key access routes to the railway station area, telecommunications infrastructure, electricity sub-stations and more than 300 acres of open, but potentially development, land.
The scheme comprised of three main elements: the removal of the river’s two main navigation weirs, replacing them with moving mechanical weirs that allow the water level to be better controlled [Figure 2]; installing 10km of flood defence walls along 5km of the city centre waterfront, embankments and landscape improvements; and the removal of a 600m-long man-made island that separated the river and canal, thus eliminating a bottleneck for water flow.
Phase 2 of the Scheme, at a cost of £112 million, was completed in 2022. Alongside engineering works such as flood walls and a flood storage area, an NFM approach will reduce flood risk and provide better protection for 1,048 homes and 474 local businesses. There are two elements to Phase 2:
· The first runs along an 8km stretch of the River Aire, upstream of Leeds railway station and focussing on the Leeds industrial Museum at Armley Mills, Kirkstall Abbey and Kirkstall Meadows. Here NFM strategies will include the creation of new areas of woodland, land management to reduce the flow of water into the river during heavy rain periods, and river and flood plain restoration such as re-channelling and the use of small storage ponds. All of these are designed to ‘slow-the-flow’ of rainwater into the river.
· The second involves the creation of a large flood storage area at Calverley, to the northwest, making use of a large area of existing flood plain. [Figure 3]
Throughout the length of Phase 2 there were several other challenges. Much of the work took place parallel to the Leeds to Skipton railway line which needed to continue to operate; there are several listed buildings in the valley (notably Kirkstall Abbey); and it was important not to disrupt fish and other wildlife corridors.
Figure 2. The new Crown Point weir in the centre of Leeds
Source: ARUP Journal Issue 1 (2019)
Figure 3. Zones of Phase 2 of the Leeds Flood Alleviation Scheme
Conclusion
Perhaps surprisingly, the EA states that Phase 2 of this scheme was the first to use natural processes (NFM) over such a large area. Similar schemes elsewhere are smaller in scope. Engineers also used a GIS database that suggested and automated woodland design, and it is hoped that additional data sets from similar interventions around the world can be inputted.
As well as slowing the flow of water into the river, the scheme will introduce opportunities to sequester and store carbon in the valley, as well as increase biodiversity. The City of Leeds Authority declared a climate emergency in 2019, and it sees this scheme playing a key role in its response to the challenges of climate change.