our report on `natural flood management`

COLDEN UPPER CATCHMENT : FLOOD RESILIENCE PATHFINDER
ISSUES REPORT
robin.gray@pennineprospects
March 2015
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CONTENTS
1.0 Background
2.0 Acknowledgements
3.0 Findings from Evaluating upland land-cover change impacts on flood risk : the Colden Water
case study, Upper Calder.
4.0 The Upper Colden Catchment : Vegetation
5.0 Erosion issues in the Upper Colden Catchment
6.0 Erosion features and their impact on peak flow
7.0 Work to date
8.0 Practical issues & solutions
9.0 Generic costings :
10.0 References
Appendix 1 : Changes in land cover and impacts on flood risk:
Colden Water case study, Upper Calder
1.0 Background
Pennine Prospects has been working directly with the Environment Agency & Calderdale Council
through the DEFRA Flood Resilience Community Pathfinder. The upper Colden Catchment 5.7 km²
has been chosen as a sub set of the wider Hebden Water Catchment for detailed modelling by
Water@Leeds, part of the University of Leeds 1.
Pennine Prospects has been asked to ‘ground truth’ the model with regards to on the ground
delivery/ legacy . The intention of the report is to consider the conclusions from the report and
suggest practical applications based on these conclusions to take forward in any future programmes
of work .
This report has been written by Pennine Prospects to be read in conjunction with the report
produced by water@Leeds : Gao, J., Kirkby, MJ and Holden, J (2014) Evaluating upland land-cover
change impacts on flood risk: the Colden Water case study, Upper Calder.
2.0 Acknowledgements
The report is based on a number of site visits and consultation undertaken with various
organisations.
Rob Twigg, Dr Jonathon Walker &Chris Fry,
Viki Hurst, Jihui Gao
Jen Richardson
Glyn Haworth
Moy Cash
Andrew Coen
1
Moors for the Future
Water@Leeds, University of Leeds
Natural England
United Utilities
Calderdale Council
Environment Agency
Water@leeds are working with the DEFRA Pathfinder Project, Calderdale MBC and the Environment Agency
to assess the impacts of land use management on floods in the Upper Calder catchment.
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3.0 Findings from Evaluating upland land-cover change impacts on flood risk : the Colden Water
case study, Upper Calder.
Water@Leeds applied TopModel [ Topography Model ] – a distributed hydrological model to identify
the impact of land management upon flow to the 5.7 km² headwater tributary for the River Calder.
Re-vegetation modelling was conducted under a series of rainfall events. The model has been
adapted to predict overland flow and how it can be accelerated or decelerated depending upon
vegetation cover. Different land cover regimes in different parts of the catchment have been
evaluated with regards to impacts on flow.
The model takes explicit account of the different velocities of overland flow over different types of
moorland vegetation, ranging from the most rapid flow over bare soil surfaces to increasingly slow
rates of flow over densely vegetated land. Even when the land is almost saturated by previous
rainfall events , these differences modify the shape of the hydrograph and the rate of flow over time
at a specific point in a river, for water flowing through a catchment. They could provide some
potential for a modest reduction of flood peaks through appropriate catchment management.
Main conclusions [ see appendix 1]
4.0 The Upper Colden Catchment : Vegetation
Figure #1 : Study area
The area of study covers 5.7 km² of mostly blanket bog with elevation ranging from 299 metres to
476 metres. The area is almost entirely within the South Pennines Site of Special Scientific Interest
[SSSI] and the Special Protection Area.
There are a number of causes for concern with regard to the condition of the SSSI within the study
area. Indeed units 58, 59, 62 & 91 /H7310 are considered to be ‘Unfavourable Recovering’.
Reasons for failure include :
 Cover of dwarf shrubs
 Frequency of individual species
 Presence/evidence of erosion
 Frequency of positive indicator species
 Cover of graminoids
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Blanket bog is based upon deep peat dominated by sedges and grasses with significant areas of
calluna and molinia grass and areas of cotton grass [ Eriophorum sp] and Nardus stricta on the
shallower peat soils to the south of the study area.
Sphagnum perceived as the building block for blanket bog is present but not in the extent that would
be considered that the blanket bog was in ‘favourable condition’. Sphagnum papillosum and
capillifolium, both good bog building species are localised in their extent. ( S. subnitens and S.
cuspidatum were also noted on the site with S.fallax also present ). ( Reference: Natural England).
There are significant areas of unstable bare peat and gullying based on previous drainage ‘grips’ and
evidence that these have caused significant erosion [ see below] .
5.0 Erosion issues in the Upper Colden Catchment
Exposed bare peat in upland regions is caused by a number of factors including; livestock
poaching and overgrazing, wild or badly managed fires or illegal off roading . In the past
drainage ditches or ‘grips’ in the uplands were funded through the Common Agricultural Policy
in a misguided attempt to improve agricultural productivity by lowering the surface water table
and therefore creating drier conditions.
Illegal or careless wildfire is a significant cause of damage . Not only does this reduce the surface
vegetation but high temperatures can lead to a reduction in the viability of seed and there can
also be long term damage to the structure of peat and its ability to act as a growing medium.
The site was the scene of a large moorland fire in 2010.
As well as the conservation interest in terms of the plant material and the species that blanket
bog supports peat also supplies some other important ‘ecosystem services.’ . Peat is the largest
store of carbon on the planet. Once in solution the carbon will eventually be released into the
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atmosphere and indirectly lead to the build up of greenhouse gases. At the very least degraded
blanket bog will lose the capacity to store significant quantities of carbon.
Peat in upland watercourses also impacts water quality. Silt getting into streams as result of
erosion is settling out in reservoirs and presenting serious problems for water companies as the
siltation depths build up. Water supply companies have to address dissolved oxygen carbon that
results from peat degradation.
Areas of bare peat are unstable – subject to wind and water erosion as well as poaching by
livestock often down to the subsoil/mineral/bedrock. Eroded peat is usually washed into
watercourses along with silt from any mineral base material that has become exposed as the
peat is removed. This has implication for water quality on catchments as well as adding to the
debris that can increase flood events.
Land use and management can make a significant contribution to reducing runoff by:
1.Changing the amount of water which flows off the hills.
2. Changing the timing and reducing peak runoff - slowing the rate of flow and / or by storing water:
The Making Space for Water Project managed by Moors for the Future and funded by DEFRA
identified the following headlines: 1.Peat restoration in upland blanket peat systems reduces storm flow in headwaters and can
contribute to the reduction of downstream flood risk
2. Restoration of bare and gullied peat significantly altered storm flow release from headwater
micro-catchments, reducing peak flows by 30% and increasing lag times by c.20 minutes
3. Model upscaling suggests that re-vegetation across 12% of a 9 km2 catchment would reduce
flood peaks of severe storms by up to 5% and, with additional gully blocking, by up to 8%
6.0 Erosion features and their impact on peak flow
Figure#2 : Main types of peat erosion adapted from ( Bower,1961). Slope is the major factor with a
series of gully systems cut into moorland plateaux [1]and gullies generated on steeper slopes [ 2].
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Figure#3: View towards Moss Crop Hill with drainage ‘grips’ these ditches have created significant
hags and resulted in increased overland flow and peat erosion. They could in time develop into
larger gullies. Traditional ‘peat dams’ would not work in blocking up these gullies. Drains at
approx. 100 metre centres [check]
Figure #4 : Areas of bare peat on moorland plateaux
Figure#5: Hags near Wolf stones
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Figure#6: Evidence of erosion down to the mineral layer.
Figure#7: Peat hags on the side of gullies can be the source of further erosion by wind or water
continuing to undermine the edge and further eroding the gully. The best way to reduce this
erosion is to ‘reprofile’ the hag to create a slope of between 30 – 40 % .pereferably by restoring the
vegetation. Moors for the Future have experimented with geojute and reseeding which on shallow
slopes can be effective.
•Gullies result in more rapid runoff and typically have a positive feedback leading to increased
erosion and export of particulate and dissolved organic carbon (Evans and Warburton 2007).
•Gully edge peats provide a key linkage between the hill slope hydrological system and channel flow
so that their influence on the hydrological functioning of the peatlands is disproportionate to their
aerial extent within the catchment (Daniels et al. 2008).
•Future climate change may lead to further degradation of the bogs and a reinforcement of the
importance of erosion gullies to runoff generation and water quality (Daniels et al. 2008).
Peat Pipes
•Peat pipes are large macropores, often many centimetres in diameter, via which water may move
through the soil (Holden et al. 2012).
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•These pipes can often be several hundred metres in length and typically form branching networks.
Holden (2005a) found land management (moorland gripping) to exert the most important control on
hillslope pipe frequency in blanket peats, and that management practice in peatlands may therefore
induce more rapid subsurface erosion and carbon loss. Further, Holden (2005b) demonstrated that
heather (Calluna) species are one causative factor of piping in blanket peat catchments; pipe
occurrence was significantly higher where bare peat (149 pipes/km) and heather (87 pipes/km) were
present compared to other plant species (67 pipes/km).
•In the blanket bog of the Moor House NNR, North Pennines, Holden and Burt (2002) found pipes to
have a prolonged recession limb such that they maintain low flow for longer periods than most other
runoff production processes; pipeflow contributed ~10% of the streamflow but did, at times,
contribute up to 30%.
Water tables
•Catchment water tables are significantly higher in the intact site than at the bare peat or late-stage
restored reference sub-catchments.
•There is also evidence that restoration by re-vegetation results in higher water table conditions;
mean water tables at the late-stage restored reference site are 75 mm higher than mean water
tables at the bare/eroded control catchment.
7.0 Work to date
Landownership of the SSSI/SPA land is divided between United Utilities and Yorkshire Water
catchments with various shooting rights and tenancies .
There has been previous investment from both Yorkshire Water [working with Moors for the Future]
and United Utilities [ SCaMP – Sustainable Catchment Management Programme ] .
Lime, seed and fertiliser restoration works on Heptonstall Moor has led to colonisation of large areas
of previously bare peat with graminods
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Figure#8: Extent of lime, seed and fertiliser on Heptonstall Moor [ Reference : Moors for the
Future].
The land use changes suggested by water@leeds would require both access to site and the ability to
tackle some of these larger erosion edges through techniques such as reprofiling. This would require
significant operations with an impact to the peat through the use of plant using a very low ground
pressure 360o excavator with wide tracks. Even with these measures, however, the total
machine weight should be less than 10 tonnes and portable “bog mats” will be needed to
traverse areas of wet deep peat. All machine operators must be able to demonstrate a high level
of expertise in working in a bog environment. The best available access to the site would be via
United Utilities land through the Long Causeway.
Figure#9: The report by Water @ Leeds identifies creating 10 or 20 % riparian strips of sphagnum
as a measure to increase flood resilience of the Colden Catchment reducing peak flow by 2.7 % or
4.5%. .
Figure#10: North Grain on White Hill : In reality a number of these riparian strips are highly incised
and might require significant re-modelling.
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8.0 Practical issues & solutions
Moorland plateaux :
Peat pans often develop after damage by wildfire often marking the limit of wildfire but also
increasing erosion. They are generally flat with only small amounts of flow in small rills and
sheet wash. Water flow needs to be reduced and the bare peat re-vegetated before they
deteriorate further into gullies by dividing the area into smaller ‘patches’ by staking heather
bales with chesnut pailing stakes to dam sediment and water wand use this basis for cotton
grass planting using plugs. Where the land is gently sloping then geotextile is used to stabile the
slope. The sides of the hags should be reprofiled where the sides are steep or undercut with
overhanging vegetation (Yorkshire Peat Project, 2013).
Grip blocking :
Grips are classified into six categories each with a different technique if they are flowing they
need blocking to reinstate a natural hydrology and reduce erosion starting from the top of the
grip and working downhill with either peat dams at no greater than 12 metre centres but
reducing to no more than 5 metre centres on steeper areas. (Yorkshire Peat Project, 2013)
Re-vegetation:
Commonly using heather brash delivered by helicopter in ‘dumpy’ sacks and spread by hand in
autumn or early winter using hay forks.
Figure#11: Stone dams installed by helicopter ( Photo : Moors for the Future).
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9.0 Generic costings :
Heather Bales
£ 250 per unit incl. installation
Moors for the Future
Reseeding bare peat :
Lime, seed and fertiliser
Stone dams
Notional £ 25,000
Moors for the Future
Cost per unit incl. installation £
200
1 hectare of bare peat requiring
165 dump sacks
£ 1.10 per linear metre
£ 3.82 per linear metre
Moors for the Future
Revegetation costs
Grip blocking : peat dams
Wooden dams
Revegetation costs : nurse crop, £ 675 per hectare
lime and fertiliser ( assuming 2
year repeat treatment, 100%
bare peat).
Yorkshire Peat Project
Yorkshire Peat Project
Exmoor/ Crichton Centre /
DEFRA
Moors for the Future
10.0 References
Daniels. S.M, , C.T. Agnew, T.E.H. Allott, M.G. Evans :Water table variability and runoff generation in
an eroded peatland, South Pennines, UK ( 2008)
DEFRA/ Crichton Centre, 2013 Valuing nature's services: moving towards payments for ecosystem
services and conservation credits in the English Uplands - NE0136
http://ecosystemsknowledge.net/resources/programmes/pes-pilots/english-uplands
Evans. M , Jeff Warburton, Juan YangEroding blanket peat catchments: Global and local implications
of upland organic sediment budgets
Holden and Tim P. Burt Runoff production in blanket peat covered catchments;
Holden, J., Kirkby, M., Gao, J., Hirst, V., Wright, N. (2014) Flood modelling in Upper Calderdale.
Calderdale Council, unpublished report.
Yorkshire Peat Project , 2013 , Technical Guidance Notes,
http://www.yppartnership.org.uk/restoration/technical-guidance-notes/
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Appendix 1 :
Changes in land cover and impacts on flood risk:
Colden Water case study, Upper Calder
Water@ leeds have developed a model to predict the effect of vegetation land cover change on flood
attenuation. This is important because the amounts and velocities of overland flow in the uplands, influence
flood peaks lower down the valleys.
Working with the DEFRA Pathfinder Project, Calderdale MBC and the Environment Agency we trialed this
approach on the upper Colden catchment, an area of 5.7km2. It is hoped that over time we can apply this
approach to the whole of the upper Calder and more widely, to influence land use to help manage flood risk.
Key findings
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There is a considerable bare peat in the headwaters of the Calder, close to watercourses. Well
vegetated buffer strips would reduce flooding in the upper Calder. Many of these bare areas are
within protected conservation sites hence regeneration with Sphagnum would also improve
conservation attributes.
Obtaining 10% Sphagnum cover in the catchment could reduce flood peaks by between 5.1 and 7.4%
during a 20mm/hr rainfall event.
Sphagnum regeneration in riparian buffer areas, rather than on gentle slopes, is more efficient for
flood attenuation in frequent rainfall events (eg. 20mm rainfall, 5% frequency).
It is almost as good for flood attenuation, to have a Sphagnum buffer zone that covers 10% of the
catchment as 20%, provided this is adjacent to watercourses. This is good for catchment planning as
more benefits can be gained from managing a smaller area.
In extreme events of 60 to 80mm/hr rainfall Sphagnum establishment in riparian buffer areas delays
flow peaks more than Sphagnum on gentle slopes. Sphagnum establishment on gentle slopes has a
greater effect of reducing river flow peaks.
Coupling these findings with existing Environment Agency flow models for the main river could illustrate
significant benefits for flood reduction for towns further downstream. Also the model could be used to
improve forecasting of drought extremes, which are strongly influenced by upland management.
This diagram shows an example from the model
of how reduction and delay in flood peaks
following a 20mm rainfall event could be
achieved.
1. if 10% (green line) Sphagnum cover was
present in riparian buffer strips the
flood peak is reduced by 7.4%
2. or 20% (blue line), flood peak reduced
by 8%.
Also the flood peak is reduced by one time step
ie 6 minutes, compared with normal conditions.
This type of management across the uplands
could potentially yield significant benefits for
flood risk downstream.
For a full copy of the report or further information please contact [email protected].
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