Dual purpose reservoirs

How dual-purpose reservoirs are providing new benefits and reduced costs to multiple stakeholders including a water company.

Can we make better use of our stock of raw water reservoirs?

[edit] Introduction

The cost of protecting properties from river flooding in the UK is set to increase in the face of climate change, ageing infrastructure and development in flood-prone areas. Can we make smarter use of our national stock of reservoirs to store water and protect people, properties and livelihoods? Was the water privatisation in 1989 a lost opportunity to apply some joined-up thinking in managing our water resources to address flood risk as well as drought risk?

The recent flooding in November 2019 highlights the continuing need for increased action to prevent or reduce the impact of floods. The Environment Agency recently published a guide on flood storage reservoirs and adapting existing reservoirs for flood risk management (EA,2016). This research highlights the challenges in making better use of existing reservoir storage to mitigate the highly damaging effects of flooding on properties and people’s lives.

Fundamentally, water companies operate reservoirs to provide drought resilience. There is no driver to make reservoir storage available for floodwater ahead of storms, or to maintain a proportion of the total storage available for flood events within key river basins facing flood management issues.

[edit] Reducing bespoke flood storage costs

Where flood storage is considered necessary to protect lives and property in England, typically it is the Environment Agency who will construct and operate a bespoke flood storage reservoir upstream of the area at risk to reduce the peak flow rates through vulnerable areas. At the national scale the expenditure on constructing, operating and maintaining such reservoirs in the future would be reduced if we could use water supply reservoir storage more flexibly to generate storage for floodwater in advance of predicted flood risk.

With long-range flood forecasting better than ever before, we could consider how water companies might be rewarded (or at least not penalised) for making choices that better align with our national needs. Thinking longer term, it might even be a place where new markets could be created for new forms of value between stakeholders.

[edit] Pilot project at Gorpley reservoir

Action is already being taken. One major water company has an agreement with a local flood action group to operate one of their reservoirs to better manage flood risk. It was recognised that the risk of flooding at a town downstream of the reservoir was largely dependent on the prevailing reservoir water level.

An informal agreement was made to make reservoir releases during the autumn months to maintain the reservoir more than 3m below the full supply level. Water companies care about the environment and want to look after their customers in the wider sense. If they choose not to lower the water level and there was flooding they could suffer bad publicity even though there is no legally-binding agreement in place.

Equally if they fail to secure supply to cope with drought conditions there could be consequences through the regulatory system as well as poor publicity. In such cases, it seems to be “heads we win, tails you lose”. Is this the sort of predicament we want our water companies to face when they have no responsibility for flood risk management?

We must recognise that there are some parts of the country, such as the south-east, where water scarcity would clearly prevent use of supply reservoirs for flood storage. However, elsewhere we have water companies with literally hundreds of redundant reservoirs (mostly at the small end of the scale) which might be adapted to serve a flood protection function.

It is therefore really encouraging to see Yorkshire Water and their agreement with the Environment Agency to operate Gorpley Reservoir to reduce flood risk in the Calder Valley. Their decision to enter this agreement and the other, recently announced natural flood risk management measures planned within the catchment area is an excellent example of the sort of thinking we need at a national scale to make better use of reservoir storage. It will be really good to see others follow suit.

[edit] References

• Environment Agency, 2016, “Design, operation and adaptation of reservoirs for flood storage”, Report SC120001/R.) Yorkshire Water

This article originally appeared as 'The benefits of dual purpose reservoirs' in the ICE civil engineering blog, published on 6 February, 2020. It was written by Alan Warren, Technical Director at Mott MacDonald.

--Institution of Civil Engineers

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