The role of trees in flood water management
Trees play a hugely important part in preventing flood water from damaging our homes and crops. Tree planting in the landscape can be managed in order to decrease the incidence of surface flood water. Trees also have a role in protecting the quality and quantity of water in our landscapes both rural and urban.
Justin Milward, Policy Officer of the Woodland Trust says;
'Trees alone can't stop flooding, but there is growing evidence that natural solutions can have a big long term impact, at Pont Bren in rural Wales targeted tree planting led to a 40% drop in water run off at peak times, while work at Manchester University showed tree canopies in urban areas can stem the surface flow by as much as 80% compared with asphalt.'
The Woodland Trust has three very good leaflets available;
'The RBC Blue Water Project. Planting trees to protect water.'
'Trees in our Towns.'
'Stemming the Flow'
All are found on www.woodlandtrust.org.uk/water or www.woodlandtrust.org.uk/plant-trees or https://www.woodlandtrust.org.uk/plant-trees/why-plant-trees/ or https://www.woodlandtrust.org.uk/plant-trees/help-and-support/
Shropshire Wildlife Trust is also actively engaged with projects to alleviate flooding, these include work done in Corvedale to Slow the Flow into Ludlow and a new project to introduce beavers north of Shrewsbury to encourage wet lands in the right place which will also help alleviate flooding in the wrong places. The aim of these projects is to alleviate flooding through natural means, one of which is planting trees along the hillside contours withing the water catchment area. If you have land are interested in helping in this project then please read below and get in touch either with ourselves or Shropshire Wildlife Trust for more information.
https://www.shropshirewildlifetrust.org.uk/
Read more about.........
Advantages of tree planting for farmers
We have recently planted a shelter belt of 1000 hedging trees at Cuan House Wildlife Centre, the trees have been planted along the contour lines at the base of the slopes which form the hills at one side of the Stretton Rd. We are looking for more landowners to help with this scheme.
This is a very interesting study recently seen in the Journal of Hydrology
ARCHER, N. A. L., BONELL, M., COLES, N., MACDONALD, A. M., AUTON, C. A. & STEVENSON, R. 2013. Soil characteristics and landcover relationships on soil hydraulic conductivity at a hillslope scale: A view towards local flood management. Journal of Hydrology, 497, 208-222.
There are surprisingly few studies in humid temperate forests which provide reliable evidence that soil permeability is enhanced under forests. This work addresses this research gap through a detailed investigation of permeability on a hillslope in the Eddleston Catchment, Scottish Borders UK, to evaluate the impact of land cover, superficial geology and soil types on permeability using measurements of field saturated hydraulic conductivity (K-fs) supported by detailed topsoil profile descriptions and counting of roots with diameters >2 mm.
K-fs was measured at depth 0.04-0.15 m using a constant head well permeameter across four paired landcover sites of adjacent tree and intensely grazed grassland. The measured tree types were: 500-year-old mixed woodland; 180-year-old mixed woodland; 45-year-old Pinus sylvestris plantation; and 180-year-old Salix caprea woodland. The respective paired grids of trees and grassland were compared on similar soil texture and topography.
This study highlights the significant impact of broadleaf woodland at a hillslope scale on K-fs in comparison to grassland areas: median K-fs values under 180-year-old S. caprea woodland (8 mm h(-1)), 180-year-old mixed woodland (119 mm h(-1)) and 500-year-old broadleaf woodland (174 mm h(-1)) were found to be respectively 8, 6 and 5 times higher than neighbouring grazed grassland areas on the same superficial geology. Further statistical analysis indicates that such K-fs enhancement is associated with the presence of coarse roots (>2 mm diameter) creating conduits for preferential flow and a deeper organic layer in the topsoil profile under woodlands. By contrast the P. sylvestris forest had only slightly higher (42 mm h(-1)), but not statistically different K-fs values, when compared to adjacent pasture (35 mm h(-1)). In the grassland areas, in the absence of course roots, the superficial geology was dominant in accounting for differences in 4, with the alluvium floodplain having a significantly lower median K-fs (1 mm h(-1)) than surrounding hillslope sites, which had a range of median K-fs from 21 to 39 mm h(-1).
The data were used to infer areas of runoff generation by comparing K-fs values with modelled 15 min maximum intensity duration rainfall with a 1 in 10 year return period. Infiltration prevailed in the 180- and 500-year-old mixed and broadleaf woodland, whereas some grassland areas and the floodplain were inferred to generate overland flow. The significantly higher K-fs under broadleaf mature forests suggests that planting broadleaf woodlands on hillslopes in clusters or as shelterbelts within grasslands would provide areas of increased capacity for rainfall infiltration and arrest runoff generation during flood-producing storm events. (C) 2013 Elsevier B.V. All rights reserved.
Below is a description of a successful flood alleviation scheme at Pontbren in Wales.
The Report of the Much Wenlock Tree Forum visit to the Pont Bren scheme to see how the planting of trees can assist with the prevention of flooding.
At the heart of the Pont Bren Scheme is Tyn-y-Bryn Farm in Mid-Wales, a 2,500 acre hill sheep farm owned by Mr. and Mrs. Roger Jukes. Ten neighbouring farming families are involved in the scheme, Mr. Jukes being the catalyst for the project.
A visit was made to look at the scheme by members of the Much Wenlock Tree Forum on Tuesday, 19th August 2008. Accompanying them and Mr. Jukes was Mr. David Jenkins, Director of Coed Cymru.
The following is the Tree Forum's Report written immediately following the visit.
What is the Pont Bren Scheme ?
The published literature about Pontbren has been produced by the Centre for Ecology and Hydrology and the University of Wales Bangor. It states: "There is growing concern that modern farming practices are increasing the risk of flooding. Changes to these practices, such as planting trees and lowering stocking rates increase the thickness of surface vegetation and improve soil structure; thus reducing the flood risk by allowing more rainwater to soak through the soil. Ten farming families in mid-Wales are changing their farming practices to ensure a sustainable future for their children. The Pontbren Group, as they are known, live in a small community near Llanfair Caereinion and farm 1000 hectares in the upper reaches of the River Severn……..The Pontbren Group noticed that planting trees reduced the amount of rainwater flowing off the land to the local streams, reducing the risk of floods it seemed. To find out if this was true, they asked the Centre for Ecology and Hydrology (CEH) and the University of Wales Bangor (UWB) to study how their new farming methods are affecting the environment." CEH and UWB "took soil measurements in grazed grassland (pasture) and in areas where trees had been planted. In the planted areas the soil structure had changed, and water moved through the soil sixty times more rapidly than in the grassland. (They) looked at a range of tree ages and were surprised to find that these changes started with trees as young as 2 years old. A longer project will investigate further how tree planting affects flood risk. There will also be research to find out if the wider variety of habitats provided by the new trees is encouraging wildlife at Pontbren."
The visit found that…….
The initial reasons for planting trees were not for conservation but for practical farming advantages. First was the desire to reduce farm running costs by planting the trees for shelter and so allowing sheep to be kept on the hill all year round. Secondly, the peat bog on the high ground was drained. By doing this, it was found that the regulatory control exerted by the bog was removed which led to increased flow, erosion of the river banks and flooding.
What has been achieved ?
Sheets of run-off rainwater over sheep-compacted pasture and from where the peat bog had been drained were causing erosion along the banks of the main river flowing through the land. When trees were planted to stabilise the banks, water absorption was found to be high in comparison to that on the pasture. This simple observation was what led to more scientific research on the land. Research has found a 60-fold increase in absorption of rainwater where trees have been planted and are now 2 to 7 years old. Where trees are 10 years old, a 100-fold increase in absorption was found.
This absorption of rainwater changed the timing of the flow of rainwater off the land. "Instead of one big rush, some water takes 3 to 4 days to run off." The biggest impact is close to the surface but this may be because the trees are still young and many have not yet penetrated with their roots so deeply into the soil.
A simple experiment conducted by Mr. Jukes with Mr. Jenkins, was to insert glass tubes into the ground in areas of newly planted trees and again into areas of compacted pasture. Within a very short time, the water in the tubes near the trees had been absorbed into the soil whilst the water in the tubes on the pasture was still to the same depth in the tubes two days later.
Compaction of soil is noticeable on pasture land where animals constantly graze but can also be found on arable land because of the use of heavy equipment: tractors, combines, etc. although not to the same degree.
New tree planting on Tyn-Bryn Farm
As mentioned above, new tree planting has taken place along the banks of the river which flows through the farm land. In addition, new tree planting has taken place in the form of linear copses along the contours of the land.. These can be used in a number of ways. They intercept the overland flow and, with the better soil structure they develop, cause the flow to pass within the soil. The same effect is not produced if the linear copses are planted at right angles to the contours.
Another benefit is that these belts provide shelter for livestock and the livestock can move around them for shelter when the wind changes direction. The most effective planting for these copses is when there is also a shrubby lower layer. The advice is to plant the shrub layer at the same time as the trees.
Further tree planting has been done to improve the existing hedge field boundaries and to create new field boundaries. Ditches have also been dug alongside the hedges and have been allowed to overgrow with vegetation. This, again, intercepts run-off of rainwater.
Hence, tree planting at Tyn-y-Bryn has taken place in the following ways:
Regeneration of native woodlands 9.73ha.
New planting of shelter woodlands 16.51ha.
Protection of streamside corridors 3.73ha.
Restoration of hedgerows 16.35km
Planting new hedgerows 4.67km
What tree species have been planted ?
Originally, Sitka Spruce were planted on the higher land as part of the tax avoidance scheme in the 1970s. To plant these, deep trenches were dug to drain the land first and the trees were planted in plantation form on the ridges between the trenches. Any surface rainwater simply ran through the trenches without percolating through the ground.
The more recent tree planting, started in the late 1990s, was mainly of local provenance native broadleaved species. Ash and Alder have proved themselves to be very effective "clay busters". They drive their roots through the clay producing a very rapidly improved soil structure. These species, together with Rowan, Silver Birch and various species of Willow (mainly Goat Willow) have proved to be pioneer species, easily establishing themselves on this land and allowing other species, such as Oak and Field Maple to take to this soil more readily.
The pioneer species, particularly Ash, can be used as "nurse" trees which are planted alongside species that are more difficult to establish. The Ash grow rapidly and shade the other species, forcing them to grow upwards towards the light more rapidly than they would normally do.
Mr. Jukes explained that, firstly, the farmers looked around the local environment to discover which native trees grew better than others. The above were chosen for planting because they were actually seen to establish themselves well under local conditions.
As mentioned above, shrub species, such a Hazel, require to be planted at the same time as the trees in the shelter woodlands and the hedges.
Have any species proved themselves to be more successful than others in ameliorating the soil structure ?
To break the soil quickly, Alder has been found to be the "champion", closely followed by Ash. Alder will also grow to 2 metres in 10 years on this land, a good rate considering the altitude and exposure of this hill farm. The roots of these two species go deeper into the soil than those of the other species mentioned.
Silver Birch produces a dense root mass throughout the whole length of its root system, also a good structure to break up the soil.
However, all the species mentioned above are good at fulfilling their role.
Trees with a bushy habit lower down their stems are also good for the shelter woodlands, giving shelter at a lower level for the livestock. Hence, many conifers do not fit this requirement. As they grow, they become bare at lower trunk levels, especially if planted close together, and produced a wind-tunnel effect exactly at the height required for sheep shelter.
Which academic institutions are involved with Pontbren ?
The Centre for Ecology and Hydrology (based at the Orton Building, Deiniol Road, Bangor, LL57 2UP) has been involved in researching the effects of the tree roots on the soils. They have examined the changes in soil structure over many sites on the Pontbren lands.
Imperial College, London University, has been involved with research into the movement of water off the land. They have investigated a number of mathematical modelling systems to compare the Pontbren rainwater run-off with expected rates.
Nottingham University Geomorphology Department has been looking at the effects of water movements on the erosion of the main stream.
Apart from the prevention of flooding, what are the other benefits for local farmers that derive from planting trees ?
Firstly, it must be said, that only a small part of the farm needs to be given over to tree planting for the above effects to be seen. At Mr. Jukes' farm, Tyn-y-Bryn, only 5% of the land has been planted.
Shelterbelts for livestock are a secondary benefit from planting the copses along the contours.
The pasture below the shelter woodlands is drier, as the rainwater drains more quickly from the land, thus allowing livestock to stay outside longer. They can feed outside on the drier pastures rather than the farmer incurring the expenditure of providing feed and lighting costs in the farm buildings.
Such sheep diseases as foot-rot can be avoided or reduced.
The benefit of adding hedgerows and creating smaller fields is an advantage for a livestock farmer. The farmer can rotate his livestock around the fields more easily and leave fields to "grow on". The hedgerows also provide shelter.
The contour planting of "game crops" (e.g. kale, sorghum, sunflowers, maize, artichokes, etc.) has the benefit of producing longer roots that can break up the soil more than some other crops but this also provides the farmer with opportunities to diversify to provide game shooting on his land.
The planting of trees on farmland can easily be seen as an opportunity for the farmer in improving his yields with, sometimes, less outlay.
Roger Jukes mentioned that he would start coppicing in the wooded areas soon. Trees coppiced on a regular cycle can provide material for bedding for animals and material to produce woodchip for domestic heating avoiding oil costs and reducing carbon emissions.
The Pont Bren Cooperative has also set up a nursery to provide trees of local provenance for further planting schemes in their district.
Roger Jukes explained that there had been an increase in wildlife as a result of the tree planting. Numbers of nuthatches and flycatchers, for example, have increased although he did add, "We didn't do it for conservation; we did it for survival."
A Way Forward
David Jenkins was particularly helpful in this area. He had brought maps of the Much Wenlock district with him, together with an aerial photograph and he felt that action could be taken in the same way as at Pont Bren to alleviate any flooding issues. He said that "for Much Wenlock, what is happening is not an urban issue."
He emphasised that any project in the Much Wenlock area should not be seen as any farmer's responsibility but as an opportunity for farmers.
David Jenkins was also very helpful on aspects of funding for tree planting. He said that tree protection was the most expensive part of any scheme and cited the following suggestions as possibilities for approaches to be made about funds:
Government agencies: (a) DEFRA through their farm stewardship scheme.
(b) Natural England. The Chief Executive, Helen Phillips, knows all about the Pontbren Project having visited it and seen its advantages for herself.
(c) Forestry Commission grants
(d) Environment Agency
(b) Shropshire Council
Charitable Funding: e.g. The Woodland Trust
Other: Confederation of British Insurers - who stand to gain considerably from the reduction in claims if homes are less likely to flood.
Finally, this was an immensely profitable and relevant visit and the Much Wenlock Tree Forum owe a huge debt of thanks to Roger and Mrs. Jukes and David Jenkins for the time they spent discussing the Pontbren Scheme, taking us over the site and answering our questions.
[Members of the Much Wenlock Tree forum who attended this visit: Catherine Benbow, Simon Ross, Colin Ryder, John Tuer]
The above report was written by John Tuer on 21st August 2008.