One of the key objectives of my PhD research at University of Southampton was to try and assess how changing volumes of wood in rivers affects the likelihood and magnitude of flooding. The amount of wood in rivers can increase for various reasons, such as artificial insertion in the river for river restoration, river managers choosing to cease clearing natural deadwood from channels, or the growth/restoration of streamside (riparian) forests. We know that if wood is put into a river then flood water moves slower through/around it, and thus for a short distance downstream flood water will have a longer “travel time”. What no-one has done before is look at the effects upon flooding at a distant downstream location (such as a town) of changing the speed water moves through a small sections of the river catchment upstream of it.
My research in this area was sponsored by the Environment Agency in the UK. Currently my thesis is awaiting examination, and for various reasons the publication of a key set of results in the academic literature is not likely to appear until the later part of this year at the earliest. Given the current interest the subject with articles in The Guardian and BBC in the past few days I thought it timely to write a short blog post giving a précis of the main findings of a computer modelling study I conducted on the effects of adding wood to rivers as a flood control method.
I used a computer model (OVERFLOW written by Dr Nick Odoni) also used for the experimental “soft engineering” flood defence project at Pickering, N.Yorkshire mentioned in Roger Harrabin’s article for the BBC. This model allowed me to run thousands of model variations to look at the effects of changing wood in rivers at a wide range of scales, but also different spatial arrangements within a catchment. What I am interested in is the effect upon flooding at a downstream urban location when wood is inserted into rivers through the catchment in various arrangements.
I am concious I need to restrain myself as I could (literally and demonstrably) write over 70 thousand words on this subject!
The catchment I modelled is within the New Forest National Park and has fairly low slopes, it is around 100km² and contains around 100km of stream length. The outflow from the catchment is the Lymington River at the town of Brockenhurst.
The key points that emerged from the modelling study are:
- Adding artificial logjams to stretches of river as short as 500m can change the height of a flood peak in a downstream town, but these changes are very small and have highly variable magnitude and directionality.
To unpack this point a little more – putting logjams into a river in some locations can increase the depth of flooding downstream.
- Broadly speaking steep headwater reaches with wood in them had little to no effect on downstream flooding – in very simple terms this can be put down to the water flowing in steep streams moving faster and having more energy, and thus being less susceptible to being significantly slowed by additional wood. I found streams with a slope of >0.005m/m or greater showed little change in flood peak upon adding wood.
- In middle to lower parts of the catchment changes in flood height at the town are observed, but there is little predictability in response. I.E. with my data I am unable to predict whether putting wood into a river channel around 3-15km upstream of a town will decrease or increase flood risk.
- Generally as the length of river with wood in it increases, the magnitude of change in the flood peak height also increases – i.e. wood in more river channels results in bigger changes to flooding. However the direction of this change remains highly variable.
The overall conclusion is that just inserting wood and/or logjams into rivers within a catchment as part of flood control for a downstream location is highly unpredictable. The reason for this is that locally logjams force water out onto the floodplain, if this floodplain is grass or scrub then the flood water is capable of still moving fairly rapidly across it, indeed in some cases it can flow directly down-valley and bypass bends in the river.
The real benefits in “rewilding” for flood control comes from restoring floodplain forests. Complex forested floodplains dramatically slows water moving over them as they have an irregular surface covered by tree roots, upright tree trunks and dead wood. My model runs for scenarios where wood is inserted into the river and a forest is allowed to grow on the floodplain show substantial and predictable responses in downstream flood height.
- Restoring floodplain forests to short sections of river of 1-2km can reduce downstream flood height by 1% after 25 years growth.
- There is predictable pattern of response with floodplain forest in lower parts of a catchment (e.g. near a town), increasing flood height. However when used in the middle and upper reaches of a catchment a reduction in flood height is modelled.
- As the extent of restoration increases the magnitude of change in downstream flood peak increases and displays a similar spatial pattern (near the town increases flood height, further upstream reduces flood height)
The most promising scenarios, and the real take home message is the restoration of floodplain forests to entire “sub-catchments” of the main catchment (a tributary of the main river and all streams draining to it) always decreases flood peak height after 25 years growth, and can have dramatic effects. If this is done for areas of 20-35% of the catchment reduction in flood peak height of 10-15% are modelled after 25 years of forest growth.
As a simple analogy during a flood many “packets” of water are delivered to the main trunk river from all its tributaries. If the delivery of a single large “packet” of water can be significantly delayed it will then arrive at the main river after the peak of the flood, and thus the main flood peak height has less “packets”of water in it and is lower. Imagine it as a tapas meal where you’ve ordered too many dishes, your table is full already, but thankfully the waiter brings your albondigas after the gambas pil-pil are finished and thus your table doesn’t overflow with tapas.
The two most important implications for flood control using river restoration or re-wilding are:
- Applying wood and/or logjams on their own to short stretches of river of 1-5km has a highly variable effect on flood peak height at a downstream urban location of ±4%. The (well known) local flood wave attenuation effects of logjams do not always translated to reduced downstream flood risk. Given the difficulty in predicting the response before installation this is a highly risky approach and should probably be avoided unless there is extensive site experience/local knowledge/investigation beforehand.
- The most promising (and practical) scenarios are to restore small headwater sub-catchments representing 10-15% of the total catchment area, where reductions of 5-6% in flood peak height can be seen after 25 years, with this reduction increasing to 7-8% after 50 years growth. If the area is increased to a large sub-catchment representing 25-35% of the catchment area reductions of 10-15% in flood peak magnitude can be seen after 25 years (with again bigger reductions as the forest ages and matures); although restoration of such large areas may prove impractical.
It is important to note that this modelling only looks at the speed of water moving through the river network and off hillslopes, it does not take any account of the reductions predicted in the amount of water reaching rivers through trees increase infiltration rates of rain/runoff into soils, as mentioned by George Monbiot last week. So we could expect to see even greater reductions in flood peak discharge downstream than predicted just from flood wave travel time modelling.
In conclusion although governmental and public interest in the concept of “rewilding” rivers for flood control is promising, it is important to recognise that the local effects of wood in rivers slowing flow can have surprising and counter-intuitive effects when looked at in the context of a whole river catchment. We need to do a lot more work in this area and in the meantime the insertion of logjams and dead wood into rivers for flood control should be used with caution and extensive site analysis.