I have a bit of cutting edge science for you today in the shape of a summary of a recently accepted paper of mine from the journal Water Resources Research. This paper covers a small side project from my PhD in which I used numbered metal tags and repeat surveys in order to track the movement of individual pieces of dead wood in a small river. The motivation for this research was to try and provide some guidance for river restoration and river management on the sizes of wood that should be used for logjam/deflector structures to maximise stability.
The main aim of this research was to try and establish a) how big a piece of wood needs to be before it doesn’t move and b) for pieces that are moving how far they go. I managed to answer both of these questions during the study and also came across some other interesting observations concerning logjams which I’ll go into in a future post.
This work was carried out from Summer 2010 through to May 2013 and fortunately this meant we had two fairly dry winters followed by an extremely wet one. This allows us to look at the results in the context of flood magnitude on pieces of wood moving.
To give you some vital context the work was conducted in the New Forest National Park in the UK, the study river is fairly small, with an average width of just under 5m. The forests alongside the stream are mature, but not especially complex, there are between 4-8 trees on the immediate river bank every 100m and the mature tree density on the floodplain is 20-50 trees per hectare. Essentially the setting is a typical mature beech forest with fairly open, almost parkland like, woodland. I monitored wood in five sections of river that gave a good balance between meandering and channelized streams and open and denser woodland.
In total I attached tags to 162 pieces of wood ranging from 1m to >15m in length. Of these only 39 (24%) remained in place for the whole 32 months of the survey. Using statistical modelling I was able to show that wood length, wood species class (e.g. “conifer”, “broadleaf”) and the complexity of branching explained 65-74% of the variability in movement. This means that these three factors control the majority of the variance, or likelihood of a given piece of wood moving. I was fortunate to get data from two other published studies allowing me to look across different rivers up to 8m wide, and this confirmed that the length of a piece of wood relative to the channel width is the key control on mobility. There are two reasons longer wood is less mobile; it has a greater weight so can better resist the forces acting to float and drag it, and it is more like to become caught or wedged at bends or against obstructions.
I found that a piece of wood needs to 2.5 times the channel width in length in order to be considered functionally immobile, so for a small, 4m wide channel that’s a piece of wood 10m long. The results show that mobility begins to drop off at around 1-1.5 channel widths in length, but that is still 4-6m of wood in a 4m wide stream. These lengths are substantially longer than anything typically used in river restoration projects.
The key implications here for river management is any wood shorter than 2.5 channel widths in length should be considered potentially mobile and anything less than the channel width should be considered potentially highly mobile. The data supports this for channels up to 8m in width (the largest we have data for), but given that covers pieces of wood up to 25-30m in length it is reasonable to assume it would apply to larger rivers (as trees/wood have a finite size in nature).
Of the logs that moved I was able to relocate 86 and thus by establishing the start and end points work out a transport distance for them. The range of movement was from 0.36m up to 5.6km, with 6 pieces moving further than 0.5km. This is substantially further than we were expecting. Perhaps unsurprisingly the data shows shorter pieces of wood move further; all the wood moving over 0.5km was less than 3m long. Crucially the results show wood transport is possible across logjams (there were multiple logjams between the start and end points of furthest moving wood) and over the top of the floodplain in big floods.
We have synthesised all the results into a conceptual model, reproduced from the paper below, showing that the primary geomorphological controls on wood mobility are channel planform and density of forest cover.
So what is the take home message from this research?
Wood is much more mobile in small forest streams than previously thought.
The implications for river management are that longer pieces of wood may need to be used if features such as deflectors and logjams need to be fixed without some form of anchoring. Potentially this could take the form of pieces of wood extended some distance onto the floodplain, particularly in the case of flow deflector structures. This could create issues with riverside footpaths, but the message is “go big, or go home”!
Another implication is the storage/disposal of “waste” wood on the floodplain. I have seen many examples of wood removed from the channel being cut up into small pieces and placed on floodplains to provide terrestrial habitat, these results suggest such wood could not only be swept away but could move kilometres downstream in a flood. This need not be a problem, but managers need to be aware of the possibility and perhaps leave removed wood intact given greater size reduces likelihood of movement.
The full paper can be assessed on the Wiley Blackwell website in review form (full typeset version will follow from the publishers) and I encourage people to read it if they want to find out more about the methods and the context (relative to other research). Please note it is NOT open access, however the AGU support the “green” open access route and thus I can put the paper on a website after a 6 month embargo. Thanks to the UK Environment Agency for funding the research.
I will summarise some interesting finding about logjams from the same paper in a later post. If you want to cite the article please do so as:
Dixon, S.J., Sear, D.A. (2014) The influence of geomorphology on large wood dynamics in a low-gradient headwater stream. Water Resources Research, DOI: 10.1002/2014WR015947