How do we use wood in river management?

Bl8AGtyIMAArXc2[1]

A large, natural type logjam installed in woodland (© Alistair Driver Twitter:@AliDriverEA)

The presence of in-stream wood in rivers is a fairly complex and chaotic management issue. It’s not neat and tidy and to a degree wood in a channel is unpredictable, both in its effects and its behaviour. Wood moves around in the channel, slows the flow of water and can drive local bank erosion. It can also restrict human activities in the river, blocking passage for canoes and boats and making it physically more difficult to for fishing. However, we know from the scientific literature and numerous management examples from around the world that wood can have enormous and varied benefits to stream ecosystem functioning and diversity. In addition to benefiting the ecosystem there are many linked benefits in varying patterns of erosion and deposition which alter channel geomorphology, as well as wood potentially being part of an integrated flood risk management programme.

Goyt-tree-kickers-5-Andrew-Parker[1]

“Tree Kickers” on the River Goyt, nr Stockport. Trees are cut partly through, ‘hinged’ into river and secured with steel cables. (© Andrew Parker, http://www.urbantrout.net)

It’s a natural human impulse to want to control and stamp our mark on the landscape, so potentially unpredictable elements of the landscape, such as wood in rivers, can create problems and frictions. In the past we wanted neat, tidy river systems, not wild, untamed riverscapes. And there is plenty of evidence that society still resists and is afraid of natural rivers. More recently river managers have tried to reintroduce in-stream wood as part of river restoration projects and the concept of “rewilding” landscapes is beginning to get more attention. In the long-term, such projects need riparian woodland to be restored in order to provide a sustainable input of new wood to the channel as the old pieces rot away. However, in the UK the scope for planting woodland or “rewilding” is restricted by develop and issues of site suitability. In that context we can’t plant woodlands everywhere and so a lot of in-stream wood projects can only ever be short-term.

concept

Conceptual model showing how engineering and natural processes are on the same spectrum of river management and different parameters changes along this spectrum

So how do river managers square this circle of competing and contradictory demands? How can we ensure a project using wood has desired benefits but isn’t ‘messy’ or unpredictable? And how can a project achieve long term goals without riparian woodland to supply new wood? Part of the answer is that in-stream wood management is not a simple ‘one-size fits all’ solution, there are a myriad of ways of using wood to achieve specific goals which will have different effects (see sketch opposite). If we are talking semantics, many (most?) of these projects are not really ‘restoration’, but provided limitations are known in advance they can be viable management options.

BqWWGaNIUAAJi-4[1]

Logjam constructed to slow flood waves in Pickering N.Yorks. This structure has been designed to stay fixed in place (© Sim Reaney, Durham University, Twitter: @simreaney)

B_L5qd9WsAAGjjt[1]

Another example using stakes to lock a structure in place. Again this structure uses natural processes but doesn’t replicate a natural structure. (© Love your River Telford, twitter: @LYR_Telford)

A key concern with natural in-stream wood is its mobility, something I covered in a recent post and journal article. Managers want wood to stay in the same place both to harness its effects in a desired location, but also to stop it moving off and causing problems elsewhere. The example to the right is of a logjam in Pickering, part of the “slowing the flow at Pickering” project. This structure is likely to prove effective and relatively long-lasting, but it is not really representative of a natural structure. Fundamentally this is engineering with natural materials, harnessing what we know about in-stream wood to successfully deliver a very specific effect in a particular location. However, it is only going to deliver this effect for the length of time the wood takes to rot, so would need ongoing monitoring and management.

Anchoring a structure in place is one way of overcoming the unpredictable nature of wood. Such an approach requires a fairly detailed geomorphological and hydrological assessment, as the local hydraulics will change to accommodate the structure. By being fixed in place the effects on erosion may be intensified, for example if a fixed logjam persists in a location for a very long time it may generate a substantial scour pool. Likewise a staked or cabled piece of bank reinforcement may focus flow towards the centre of the channel and cause bed scour and channel deepening.

BZl6z5GIYAEpr1V[1]

Staked brushwork being installed to deflect flow. (© Chilterns Area of Outstanding Natural Beauty, Twitter: @ChilternsAONB)

Such unwanted effects can be particularly problematic where wood is being used as an erosion control measure (see post on erosion control). The issue here being that the wood is doing nothing to address the imbalance in stream power which is driving the erosion. In a crude sense the river has ‘too much’ energy and is expending it on eroding its bed/banks. Reinforcing in one place (whether with wood or concrete) even if successful, often pushes the problem up or downstream.

A popular method of creating areas of slower flow and/or arresting erosion is to use brush or willow spilling. This can be a very effective technique in the right place, particularly where the willow can subsequently  become established as riparian trees, sprouting out of the cut wood.

Awk12TwCMAEglgQ[1]

Brushwork installed to prevent erosion on outside of meander bend (© Tom Dawson, Twitter: @TomDawson83)

So do these distinctions between engineering and “restoration” really matter? I would argue they do insofar as it is important for people managing (and often volunteering) on projects to understand the goals, the mechanisms, the outcomes and the likely time-scales the project is working over. Where a project is a true “restoration” to a new quasi-stable state (in reality extremely rare), once the project is completed (and if successful) the river should not require any further substantial management. However, where a project is fundamentally engineering based it follows that the project will either need continuous monitoring and upkeep, or the project is intended as a one-off site specific intervention designed to last for a given time before the structure fails. Either way there needs to be an acceptance the project is a long-term commitment to continuing low-level management of the channel using natural resources (potentially indefinitely). This then raises the issue that goals and likely success of a given piece of work need to be understood and communicated, in order that the stakeholders can absorb any potential individual project failures without losing commitment to the overall management scheme. It is in light of this that I think “restoration” can be an unhelpful and misleading term when applied to all management using wood. Potentially raising false expectations in stakeholders and volunteers of the outcomes and workload/commitment in improving a river using in-stream wood management.

There is no doubt that using wood as a method of enhancing the river environment is a useful and effective management tool. Debates around what terms we use for various measures can seem a bit academic. However, when so many projects to improve stream health are dependent on the goodwill and efforts of stakeholders and volunteers it is vital project goals are understood and clearly communicated and part of this is using clear language.

I’m always excited to hear about new projects using wood for river management so please let me know in the comments or on twitter if you are involved in one.

Advertisements
This entry was posted in Geomorphology, Hydrology, River Management, River restoration and tagged , , , , . Bookmark the permalink.

5 Responses to How do we use wood in river management?

  1. Hi Simon,
    We are using wood in a variety of ways (debris, deflectors, attenuation creation, impoundment of seasonal channels) as part of our Natural Flood Management project in the Stroud Valleys. We have installed over 50 wood based structures over the winter and it would be good to discuss some of the implications and issues in light of your experience.

    We have based some of our installation on your “rough” gudiance on ratio of log length/channel width. We are also fixing some in place. We are mostly working in headwaters streams.
    You will have seen some of our work on Twitter, but if you are able to, it would be good to have a longer chat. We are also only 90mins from where you are based, so if you fancy a day our in the Stroud Valleys looking at wood in streams, then let me know.

    Cheers

    • Simon Dixon says:

      Chris
      That sounds really interesting, I like to come down and see some engineered logjams in action. It’s probably going to have to be a bit later in the year; end of April or May I think as I have loads to do over the next month or so. I’ve jotted down your contact details (and deleted them from the comment in case you’d prefer not to have them out in public!) so I’ll be in touch.

      Cheers
      Simon

      • Hi Simon,
        Thats great, we can show you around anytime in May, just let me know a few weeks in advance. We are hopefully doing some more work before the end of the woodland management season on another site, so should be plenty to see by then (circa 70 structures). I’m quite happy for people to see my email contact details to be honest, as its a public funded project and I work for the council.
        Cheers
        Chris

  2. Mike Potter says:

    Glad to see you are making some progress with NFM Chris. Was it much of an uphill battle to convince councillors to stump up the funding? Was it any help to show them what has been achieved in Pickering & district?

  3. Hello Mike,
    We managed to get some funding from the local capital FRM budget in the end, but it took abit of cajoling. I think letters from people who had been flooded played more of a role than project examples, but I’ve certainly been using the evidence generated by Pickering and other projects. We had the Forestry Commission out the other day actually (Vince Carter who I believe was involved in Pickering). I’m hoping we can direct some of their project funding and grants for producing management plans to encourage woodland owners to consider water management when drafting their woodland management plans. This will make it easier for them to attract funding to actually pay for some of the measures we are developing.

    If you have a twitter account, you will be able to see photos of what we are doing.
    Cheers
    Chris

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s