Muirburn and Flooding: What's the deal?

BURNING ISSUES
Moorland Management
and Flooding
Retired hydrologist John Anderson assesses
impacts muirburn may have on flood risk.
Grouse moors in the catchment of the River Braan in Perthshire.
I
was asked recently to assess the potential impact of rotational
heather burning on run-off to water courses and its likelihood
to enhance downstream flood risk. This has been a subject
which has attracted a fair degree of debate in recent times.
Rainfall, geology, land form, soils and
vegetation are all factors influencing the
quantity and time distribution of runoff
from any given area. There is a level of
interdependence among them which
means it is difficult to develop universal
relationships between runoff and any
one.
Time distribution of runoff is determined
principally by the distribution and
intensity of effective rainfall. The volume
of flow plotted against time is modified
by topography -including stream
density and bed gradient or slope -, by
soil infiltration rates and catchment
storage capacities and the base flow
characteristics of the catchment.
As a general rule it is difficult to isolate
simple relationships between vegetation
type or land use and runoff. The reason
for this is that the other physical factors
will tend to be dominant, particularly
when it comes to high flows and extreme
flood discharges.
Afforestation and runoff
Limited research has been carried out
into the relationships of vegetation
and land use and flood flows in the UK.
Most has been related to commercial
afforestation.
There have been several projects
dedicated to this, notably Plynlimon
in Central Wales and Balquhidder in
Scotland. The primary concern of these
has been to assess the impact of the
forest cycle on water yield from preplanting,
through growth and canopy
closure to felling.
Commercial afforestation probably
represents the most significant potential
land use intervention (other than
urbanisation) that occurs in this country
in terms of its physical impact and areal
extent on a catchment scale.
Generally the impact of afforestation is
to reduce mean annual stream flow from
that experienced in grassland dominated
catchments, largely through enhanced
evaporation losses. This effect becomes
more pronounced as the conifer canopy
closes from about 15 years after
planting.
The effect on flood flows also varies
through the life of the plantation,
with initial phases following drainage
enhancement leading to reduced time to
peak and increased peak flows with later
impacts tending to reduce runoff during
small to moderate flood events (less than
20% of the mean annual flood).
These effects will tend to disappear
during prolonged or extreme rainfall
events as saturation occurs throughout
the catchment. No significant difference
in peak flows was found at Plynlimon
between grassland and afforested
catchments during high flood flows.
The EMBER Project
This is probably the only comprehensive
project to study impacts of moorland
burning on streamflow runoff. The
project was funded by the Natural
Environment Research Council (NERC)
with support from Yorkshire Water.
The field work was conducted between
March 2010 and November 2011 and
assessed the impacts of prescribed
vegetation burning by comparing 5
burned and 5 unburned sub-catchments
across the Pennines. The sub-catchments
were all first or second order headwater
streams with catchment sizes ranging
from 0.7 to 3.1 square kilometres in
size. Interestingly the catchments were
all dominated by deep blanket peat,
typically over 1 metre in depth. This
would not necessarily be the dominant
substrate for all heather moors. The most
extensive and vigorous heath communities
tend to occur on more freely-drained
soils. Generally larger upland catchments
would have a more varied moorland
substrate than simply deep peat.
The unburnt catchments had no history
of burning for at least 30 years. In the
burnt catchments, plots were selected
from age ranges, 2 years since burning
up through 4 years, 7 years and greater
than 10 years.
The principle finding was a reduction
in water table depth following burning.
This reduction was greatest in the most
recently burned sites and generally
recovered to close to that of unburned
sites after a 10 year period.
The reduction in water table coincided
with significantly less overland flow
recorded for recently burned sites; a
reflection of the increase in time to
saturation at these sites during rainfall
events. The project appeared to find that
river flow in catchments where burning
had taken place were “slightly” more
prone to higher peaks during “heavy”
rain but suggested that these variations
could have been due to other catchment
characteristics and that it was not
conclusive.
Overall the study does not appear to
offer conclusive evidence that moorland
burning has a significant impact on
increasing flood peaks or duration. The
study also concentrated on very small
scale watershed catchments and did not
assess impact on larger scale downstream
catchments where a flood risk to property
might exist. The study also concentrated
on catchments with deep peat substrate
which is not necessarily characteristic of
all heather moor catchments.
The Problem of Scale
At small spatial scales (less than 20
square kilometres) some studies have
shown evidence that land cover and land
management can have some effect on
small to moderate flood flows but not for
extreme floods.
However, data for land-use impacts in
larger catchments are lacking. When
considering the impact and cost of flood
damage it is important to note that this is
rarely coincident with small scale upland
catchments but is generally associated
with areas further down the catchment
where people tend to live in villages and
towns.
In small catchments the peak of the flood
hydrograph (flow plotted against time) is
dominated by run-off from hillslopes in
response to storm rainfall. The response
time of the catchment might be quite
limited. In larger catchments, the river
channel network determines which
areas contribute to the peak of the flood
hydrograph to cause flooding.
The pattern and passage of the storm
over the catchment and the catchment
morphology in relation to stream density
and slope can be significant factors
controlling the scale and shape of the
flood hydrograph at the point where
damaging flood effects are experienced.
Different storm patterns can affect
the coincidence of flood peaks from
the various sub catchments leading
to variations in the shape of the flood
hydrograph and the scale of the flood
26 | Scottish Gamekeeper

BURNING ISSUES
Flooding at Hebden Bridge was
blamed on muirburn by critics such as
George Monbiot but was kicked into
touch by MPs at Westminster.
peak at the downstream point of flood
risk.
Small scale and highly distributed land
management interventions are unlikely
to have a significant impact at the
catchment scale during extreme events
and there is currently no evidence for this.
Interestingly the two nationally
recognised hydrological tools for flood
estimation produced over the years by
first, The Institute of Hydrology and then
its successor The Centre for Ecology
and Hydrology (CEH) namely, The Flood
Studies Report and then the Flood
Estimation Handbook concluded (from
studies of 553 and 943 catchments,
respectively) that urban extent was
the only land cover factor that was
significantly related to the magnitude of
the mean annual flood in UK rivers.
A large scale study within the Hodder
catchment, a tributary of the River
Ribble, looked at interventions over
25 square kilometres of a 58 square
kilometre sub-catchment. These
interventions included moorland ditch
blocking in areas of blanket peat, tree
planting and reduction in livestock
stocking density.
The possible consequences of these
changes for downstream flooding were
evaluated at multiple scales down
the catchment, to test how smallscale
impacts propagate through the
river network. Data indicated that the
interventions had minimal short-term
effects on the pattern of flood flows at
the sub-catchment level and no effects
were found at the larger scale of the
entire Hodder catchment (260 square
kilometres).
To try and obtain some perspective of
the relevance of the scale of heather
burning within a larger catchment we
can look at the example of the River
Braan catchment in Highland Perthshire.
The only point of flood risk is effectively
the village of Inver at the foot of the 210
square kilometre catchment.
The land use within the catchment
is 32.4% woodland (dominantly the
Griffin coniferous forest), 0.3% arable,
25.8% grassland and 40.6% mountain/
heath and bog. That is some 85 square
kilometres falling in the category of
mountain/heath and bog. Of this only a
proportion is of uniform heather stands
sufficient to justify control by prescribed
heather burning, probably no more than
a third.
That gives an area of about 28 square
kilometres that might be subject to
heather burning in the catchment. In
this catchment a burning rotation of 15
to 20 years is usual. That means around
2 square kilometres of new heather
burning per annum. That is equivalent to
400 patches of burnt heather about 100
metres by 50 metres.
Again, if we assume that any effect of
burning on elevating the flood flow
discharge lasts for say 5 years before the
vegetation cover has reasserted itself,
then at any time we have a potential area
of about 10 square kilometres making
some level of increased contribution
to the flood volume or about 5% of the
catchment.
In practical hydrological terms it is
difficult to conceive of how a widely
dispersed small plot based activity could
enhance flood run-off sufficiently from
only a few per cent of the total catchment
to become the critical factor in elevating
an existing flood risk or increasing its
frequency.
During the sort of exceptional
meteorological conditions required to
generate a severe flood risk to Inver it is
unlikely that the run-off from the areas
of burnt or recently burnt heather is
sufficiently enhanced from that of other
ground of similar area and gradient
which is subject to the same level of
rainfall.
Conclusion
Prescribed heather burning that follows
the muirburn guidelines is a small,
plot based activity which is dispersed
across wide areas. There is currently no
evidence from the only in depth study
(the EMBER project) that run-off from
these plots can be elevated sufficiently
to increase the likelihood of downstream
flooding at the larger catchment scale
where significant flood risk exists. Whilst
there is no detailed study available to
determine the impact of heather burning
on flood risk at the large catchment scale
the frequently small proportion of this
activity at this scale and its dispersed, plot
based nature suggests that it is unlikely
to be a significant factor in elevating
downstream flood risk.
References
Ecology of Heathlands. C.H. Gimingham,
1972.
The effects of forestry on the quantity
and quality of runoff in upland Britain,
J.R. Blackie and M.D. Newson in Effects of
Land Use on Fresh Waters, agriculture,
forestry, mineral exploitation,
urbanisation. Editor J.F. de L. G. Solbe.
Effects of Moorland Burning on the
Ecohydrology of River Basins. Key
findings from the EMBER project. 2014
NERC, Yorkshire Water, Water at Leeds –
University of Leeds.
Impact of prescribed burning on blanket
peat hydrology, 2014. J. Holden, S.M.
Palmer, K. Johnston, C. Wearing, B. Irvine
and L. Brown. AGU Publications, Water
Resources Research.
A restatement of the natural science
evidence concerning catchment-based
‘natural’ flood management in the UK.
2017 S. J. Dadson et al. Proceedings of
the Royal Society.
Moors in the Braan
catchment will see
controlled burning but its
rotational nature, extent
and frequency is unlikely
to lead to flooding.
SGA Backs Dogs on Leads Campaign
The SGA has thrown
its weight behind
the ‘Take a Lead’
campaign, initiated by
Scottish Farmer and
NFU Scotland.
Emma Harper MSP
has brought forward
a consultation on a Members’ Bill
to tackle livestock worrying and to
protect wildlife.
Sheep worrying incidents have
rocketed since The Land Reform Act
2003 and MSP Harper believes there
is a case to review the wording around
responsible dog ownership in the
Outdoor Access Code.
SGA Chairman Alex Hogg campaigned
at the time of the 2003 Act for dogs
to be on leads around livestock and
nesting wildlife and is happy to offer
the SGA’s backing.
The organisation will meet with
Emma Harper and other rural
stakeholders shortly.
Writing to Emma Harper, Alex said:
“With the benefit of experience, we
have seen the rise in dog attacks on
sheep, which is extremely distressing
for the shepherds. In our sector, we
see the same with hill sheep and deer.
With declining wildlife always in the
news, sensible options around dogs
on leads need to be looked at. These
places are there to be enjoyed but they
are also working environments which
are producing food and sustaining
families.”
Scottish Gamekeeper | 27