South East Rivers Trust (& the Wandle Trust)

Natural Flood Management: How does it work?

Part 2: How does Natural Flood Management Work?

In part 1 of this series, I described some of the reasons why groups across the UK have come up with the idea of working with nature to reduce flooding. The challenge for SERT is to work out how we can apply these techniques in the Medway catchment and the wider South-East, and tailor them to local conditions. For us to do this, it’s helpful to examine the thinking behind Natural Flood Management (NFM), and how it might work.

In a way the concept is very simple – if you hold back water before it hits people’s homes, then you can prevent flooding – but where to start?

Traditional flood defences, such as flood walls and storage reservoirs, tend to be built downstream near the point of impact and aim to hold back huge volumes of water through costly engineering solutions. Natural Flood Management techniques tend to start at the source of the problem in the headwaters.

Chris Uttley, the inspirational leader of the Stroud NFM project describes it like this:

“Think of it like a hose pipe,” said Chris. “Flooding occurs when the pipe is turned on for too long and the water levels get too high.

“Rather than just trying to put our finger in the end of the hose to stop the flow of water coming out, we want to tackle the flow at its source. We try and turn down the tap.”

By “turning down the tap” Chris means making many small (and inexpensive) interventions upstream to hold the water at the source, rather than trying to tackle the problem when in some ways it’s already too late. NFM seeks to work in the upper and middle parts of the catchment – as illustrated at the top right of this diagram:

Environment Agency 2017

There’s another analogy I find useful – and which most people in Kent can relate to – traffic! If everyone who has to drive to London for their morning commute set off at exactly the same time, the first part of their journey from small villages and farms might be relatively smooth and fast – but it’s pretty obvious what the consequences are when they join the major roads and bottlenecks along the way – traffic jams.  If on the other hand people set out at different times,  and perhaps some obstacles along the way such as speed bumps and traffic lights – then the overall flow of traffic would be smoother, as the traffic ‘downstream’ has a chance to get to its destination before the next wave comes along.

This essentially is what NFM seeks to do with water. By holding back and delaying flow in the minor tributaries we can relieve the build up of congestion (flooding) downstream. This analogy is actually more than a superficial one – traffic engineers actually use models based on water flow to try to solve traffic problems!

In more scientific terms what we aim to do is to ‘flatten the peaks’ on the hydrograph.

Source: Calderdale Slow The Flow project

The graph shows what happens to water flow and flood peaks at a particular point after a heavy rainfall event. In simple terms what we seek to do with NFM is to ‘attenuate the flow’, or try to move the nature of the water flow from the blue line towards the red line. The same volume of water is released more gradually avoiding the flood peaks which cause problems. That’s the theory anyway!

How do we plan to go about this in the Medway Catchment?

Here are just some of the types of NFM interventions that have been tried elsewhere.

  • Leaky woody dams
  • Ditch blocking
  • Flood storage areas
  • Recreating water meadows
  • Field corner bunds
  • Cross–slope hedge planting
  • Riparian tree planting (along riverbanks)
  • River restoration and ‘re-meandering’
  • Woodland planting and management
  • Soil management to improve capacity to absorb water (infiltration)

The evidence for the benefits of these approaches has been gathered together by the Environment Agency in their document “Working with Natural Processes – the evidence base”.  There are also very useful one page summaries of each technique here. We’re aiming to try as many of these approaches as possible, starting roughly with the ones at the top of the list above.

The EA also produced a map of where such approaches are most likely to be beneficial. This pointed to Kent and the Medway Catchment as being particularly promising for NFM – mainly because of our heavy clay soils and many woodlands – here’s a sample from around Yalding:

We’re aiming to try as many of these approaches as possible so we can learn which work in this area, and how best to apply them. We can then add to the growing national evidence base for NFM in the UK, and add our experience from a rather different part of the country to the ones it’s been tried in before.

In the time we have available, we’ve decided that applying NFM to the entire Medway catchment is way too big a task. We’ve set ourselves a target to help protect at least 51 properties in our pilot project areas.  After much discussion with our partners and help from the Environment Agency we’ve used a mixture of local knowledge and data to focus our efforts on two target sub-catchments:

  • The Alder Stream – protecting properties in Five Oak Green
  • The Hogg Stream – protecting properties in Headcorn

As well as this, we plan two demonstration projects to test our methods and to develop a core of professional expertise we can apply elsewhere. These are:

In the rest of this series, I’ll focus on each of the pilot areas, and look at the techniques we’re applying there, starting with our work installing leaky woody dams at Bedgebury with Forestry England.

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2 comments on “Natural Flood Management: How does it work?

[…] of our previous blogs explained what Natural Flood Management is and how it works. But how do we know that Natural Flood Management (NFM) is having the desired effects? These […]

[…] to reduce peak flows downstream, and make damage to homes and businesses less likely. See our ‘Natural Flood Management – how does it work?’ blog to find out […]

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