South East Rivers Trust (& the Wandle Trust)

Introduction to Leaky Woody Structures – NFM in action.

In Natural Flood Management we seek to use the landscape to store water during flood events, 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 more.

Leaky Woody Structure in action – Storm Dennis Feb 2020

One of the main techniques SERT is testing for this purpose is the creation of ‘Leaky Woody Structures’ in watercourses. The aim of these is to temporarily resist the flow of water during flood events so that it’s held back and stored, rather than contributing to a sharp flood peak.

In normal conditions structures like these don’t interfere with the flow of water, which is allowed proceed naturally. For this reason the term ‘Leaky Woody Structures’ is preferred over ‘Leaky Wooden Dams’ since we don’t wish to create permanent dams of any kind – read our previous blog for the reasons why. However in an extreme storm event, as the water rises they kick in and start to hold back and store a large volume of water. They also take some of the kinetic energy out of the water flow, and encourage infiltration into the soil, which should also have beneficial effects.

In our Medway Natural Flood Management pilot project we’ve created over a hundred Leaky Woody Structures (LWS) in three target sub-catchments. Before we started, we researched a number of other pilot projects across the U.K. which raised a number of questions:

  • How can we best apply learning from elsewhere to the different landscapes of our region?
  • Where is the best place to locate such structures?
  • What works best – a few large ones, or many small ones?
  • What are the most effective designs?
  • How can these be created safely, without themselves contributing to flood risk, or causing other problems?
  • How can we create them cost-effectively?
  • How can we persuade landowners to allow us to build LWS on their properties?
  • What’s their impact on the ecology of watercourses and the surrounding land?
  • What’s their real impact on flooding?

Through our work we think we’ve found answers to some of those questions. Others are quite tricky and will take longer to answer – especially the last two, which is a challenge for everyone involved in Natural Flood Management. This is why we’ll be monitoring our work over a number of years (see our previous blog on ‘Monitoring Natural Flood Management’).

In future blogs I’ll go into more detail about some of the solutions we’ve found to those questions, for now I’ll show a few examples of the LWS we’ve built and discuss our thoughts on the relative merits of larger and smaller ones.

Large Woody Structures

These are certainly the most impressive kind!

Woodland contractors David and Ben at a LWS on the Alder Stream
Harry at a LWS on the School Stream near Headcorn
Harry at a LWS on the School Stream near Headcorn
LWS at Church Wood

The advantages of such large structures are clear:

  • They are capable of holding back large volumes of water
  • The ones we have built are very robust, and hopefully long-lasting
  • They are great at explaining the principle to communities, and gaining support for the Natural Flood Management approach

On the other hand:

  • They have to be very robust indeed to resist the ferocity of water flow we’ve witnessed in recent storm events
  • They are potentially higher risk should they collapse during a flood event, or if timbers wash away downstream
  • There are limited spatial opportunities for them – all of ours are within woodlands and they tend to be on the fewer, larger watercourses downstream.
  • They intercept water relatively late in a flood event compared to smaller structures created in the headwaters
  • They require the help of skilled woodland contractors, and work is often in challenging conditions
  • They are relatively expensive to create – sometimes an entire day or two’s work for a team
LWS at Church Wood

Small Leaky Woody Structures

These can range in size from more modest versions of the above examples to simple ‘gully stuffing’ – where side channels and drains are blocked with brash:

Small LWS at Bedgebury of ‘classic’ design
’’Gully stuffing” –in this instance we used traditional hedgelaying techniques so the cut material survives providing a lasting solution
One of our project officers, Ed, beside a small LWS at Bedgebury
A very small LWS. Over time these collect material behind them creating a ‘sponge effect’
A small LWS holding back a surprisingly large volume of water

The advantages of these smaller structures are:

  • They are quick and simple to create
  • They can be easily created by unskilled labour and volunteers
  • You can create large numbers in a day, and they are therefore inexpensive
  • There are many opportunities for them, and they can cover a large area of the landscape
  • They are usually created in headwaters, meaning water is immediately intercepted and slowed at source, early in the cycle
  • They are generally low risk, in the sense that if one were to collapse they wouldn’t release much water, and the debris would be picked up by the next one downstream
  • Even small diameter wood can be used, the idea being that over time this picks up leaf litter and debris creating a marshy ‘sponge’ on the woodland floor over time, which should be self-sustaining
  • They contribute to creating ‘wet woodland’ which is under threat in the region as woods dry out under the pressure of climate change.
  • There are good opportunities for them in the woodlands of the High Weald, and in farm ditches downstream
  • They are a great way to directly engage the public in Natural Flood Management

On the other hand:

  • Each individual structure generally stores smaller volumes of water
  • It’s harder to quantify their effects without a very large monitoring effort

Conclusion – large or small – which is best?

The simple answer for now is: why not do both? It will take a number of years to measure and assess the different approaches to Natural Flood Management, and we’re part of a national effort to add to the evidence base for this. Our current feeling is that we should exploit every opportunity which presents itself, and trial a wide variety of NFM measures. These include not only the construction of Leaky Woody Structures, but also the creation of storage ponds and water meadows, soil management and tree planting. The hope is that these will work together in combination to deliver the desired results of flood relief and ecological restoration. It’s only by working at a landscape scale we’re likely to realise the full benefits, and the signs nationally are that this may be a very cost-effective approach to tackling our flooding problems.

LWS in action during Storm Dennis – Feb 2020

We’ve had the opportunity to see some of our Leaky Woody Structures put to the test in the severe spring storms of February 2020. The signs so far are very encouraging: not only were the LWS capable of withstanding the ferocity of water flow, it was clear that they were holding back and slowing the flow of water just as we’d hoped. For a video of this, and the impacts on local communities, see this previous blog.

Have any Question or Comment?

One comment on “Introduction to Leaky Woody Structures – NFM in action.

[…] Through our ongoing Natural Flood Management project (FRAMES), I’ve recently been working as part of a team in Kent focusing on construction of leaky wooden structures (LWS) – you can learn more about them, and how they work, in Dean’s blog. […]

Comments are closed for this post !!