6. What happens when we slow the flow? 7. What does this mean for

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References and further reading. Lane, S. N., Tayefi, V., Reid, S. C., Yu, D. and Hardy, R. J. (2007) "Interactions between sediment delivery, channel change, ...
How can slowing river flow alleviate flood risk downstream? I-Hsien Porter, Research Postgraduate (MSc), Department of Geography [email protected] www.durham.ac.uk/i.r.a.porter

6. What happens when we slow the flow?

4. How does it work?

• River flooding is a natural process, but one that has hugely damaging impacts on lives and livelihoods. • Conventionally, flooding is managed locally. Barriers and artificially straightened channels (right) increase the speed at which floodwater is transferred away.

• In this example, the entire river network upstream of a point has been made rougher...

Water is temporarily stored in one location (e.g. low value land)... ... in preference to others (e.g. where people live).

• But by speeding up the movement of floodwater downstream, the risk of flooding for people living downstream increases.

The volume of water is spread over a longer period of time.

Depth, metres

1. What’s the problem?

This poster is about: Flooding River management Computer modelling of floods

So the maximum depth is lower.

Direction of flow Time from start of event, hours

• If we slow the flow, then floodwater is released downstream over a longer period of time.

We need ways to manage flooding that don’t create problems elsewhere.

2. What can be done? This research looks at whether slowing the flow in some places (land where flooding is acceptable) can be used to reduce flooding downstream.

• If floodwater doesn’t arrive downstream all at once, then the maximum depth of water will be reduced. • Flow speed depends on the roughness of the river channel. Reinstating vegetation, natural river bends and putting woody debris into rivers are all techniques we can use to slow the flow.

Maximum flood depth downstream (metres)

Before slowing the flow

After slowing the flow

4.24

3.70

7. What does this mean for flooding? A small reduction in flood depth (in our example, half a metre) ...

5. How will we know that it works? 3. Field site: River Seven, North Yorkshire North York Moors

Sinnington

• The River Seven causes problems when it emerges from the foot of the North York Moors, at the village of Sinnington (left).

• Overflow allows us to specify a number to represent the roughness of the river, anywhere in the river system, and then calculates what will happen in a flood.

• The area has a long history of flooding, most recently in December 2009.

• Later in the project, these results will be compared with standard modelling techniques used by flood consultants and the Environment Agency.

• In rural areas, like this, settlements are often very dispersed. It is difficult to construct flood defences without causing problems for other people. • Also, the value of land activities is rarely enough to justify building defences everywhere. Great Britain Outline Map: Reproduced from Ordnance Survey map data by permission of the Ordnance Survey © Crown copyright 2010.

• This research uses a computer model called Overflow, developed at Durham University.

... can lead to a large reduction in the area of land that is inundated.

8. Unanswered questions... 1.

We can’t slow the flow everywhere, so which locations are best?

2.

Does it work even in the biggest floods?

3.

How do we make decisions about which places to sacrifice in floods, to benefit downstream areas?

References and further reading Lane, S. N., Tayefi, V., Reid, S. C., Yu, D. and Hardy, R. J. (2007) "Interactions between sediment delivery, channel change, climate change and flood risk in a temperate upland environment." Earth Surface Processes and Landforms 32 (3): 429-446. Ryedale Flood Research Group (2008) "Making Space for People: Involving Local Knowledge in Flood Risk Research and Management in Ryedale, Yorkshire.“ Acknowledgements This project is supervised by Prof. Stuart Lane and Dr. Richard Hardy, Department of Geography, Durham University. I would also like to thank Dr. Nick Odoni, Department of Geography, for his work in developing the Overflow model and his continued guidance.