flood modelling and compaction studies for the … · flood modelling and compaction studies for...

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Flood modelling and compaction studies for theUpper Ure

Project UUCP1/2015

Jihui Gao, Mike Kirkby, Richard Grayson and Joseph Holden

water@leeds, School of Geography, University of Leeds

March 2015

Report to be cited as: Gao, J., Kirkby, M.J., Grayson, R. and Holden, J. (2015) Flood modellingand compaction studies for the Upper Ure. Final report to Yorkshire Dales National Park Authority,Project UUCP1/2015, University of Leeds, Leeds.

Contact: Professor Joseph Holden, water@leeds, School of Geography, University of Leeds,Leeds, LS2 9JT; [email protected]

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1. Executive summary

This report presents work from two subcatchments of the Upper Ure with a view to

determining whether land management change in those catchments may have beneficial

impacts on flood risk and hydrological processes. The project also acts to demonstrate the

effectiveness of the research methods with a view to wider roll out across the Upper Ure

system to comprehensively assess where targeted actions might yield greatest benefits.

Two activities were undertaken: 1) Flood modelling in Coverdale using a new spatially

distributed form of TOPMODEL, involving a number of spatial experiments to test different

scenarios of land-cover change and their associated flood risk benefits under different

magnitudes of rainfall; 2) Soil compaction surveys for a large land-holding in Bishopdale

showing how compaction varies in different parts of the system and also testing the impact

of buffer strips on soil compaction.

The key findings were:

Coverdale flood modelling

Changes in the land cover in riparian zones have the potential to make very large

differences to flood peaks emerging from Coverdale. Stripping vegetation from

10 % of the catchment area along riparian zones could increase peak flows by

19 %. Placing denser vegetation in these areas could potentially reduce flood peaks

by 12 % during a 15 mm per hour rainfall event.

It is almost as effective to have a roughened buffer zone along riparian areas that

cover 10 % of the catchment as it is to have wider buffer zones that cumulatively

cover 20 % of the catchment area. Therefore, efficiency savings can be made by

investing in narrower buffer zones that cover a longer length adjacent to stream

channels.

Heavy grazing in ELS and HLS areas could theoretically enhance flood risk in the

Coverdale catchment. The impact of the combined ELS and HLS areas could be as

great as 32 % during a 15 mm per hour storm event. The impact on the flood peak

of such changes in HLS areas alone would be 25 %.

Bare peat covers around 5.8 % of Coverdale. Revegetation of this bare peat in the

headwaters of Coverdale could reduce peak flow by over 4% and 3% respectively

for either 15 mm or 30 mm per hour rainfall events.

In a very extreme storm event (150 mm in 5 hours), the proportional impact of land-

cover interventions on flood peak magnitude in Coverdale is likely to be small.

However, there will still be impacts on the timing of the rising and falling limbs of the

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hydrographs which may have impacts on the overall flood peak in the Ure as this

will depend on the synchronicity of flood peaks from all the different tributaries down

the main Ure channel. For bare soil, enhanced grazing and more frequent moorland

burning scenarios, there would be earlier rising and falling limbs on the flood

hydrograph than is currently the case, while delayed rising and falling limbs would

occur if there was revegetation of bare areas and enhanced roughness (e.g.

reduced grazing, less frequent burning, creation of buffer zones).

The results show that the new distributed version of TOPMODEL can be applied to

test relevant scenarios of land cover change and to show how different spatial

configuration of change in the landscape can influence the flood peak.

It would be desirable to develop and run the model across the whole of the Upper

Ure to determine the full scale of possible management intervention effects in

different locations upon the overall flood peak in the main river channel. It may be

that some tributaries are more or less sensitive than Coverdale to changes in

surface cover and management and it would be possible to evaluate locations in the

Upper Ure system where efforts ought to be focussed.

Bishopdale compaction survey

This report provides one of the few studies of soil compaction that exists for the UK

uplands.

There are areas of high soil surface compaction (e.g. >100 N m-2 of soil resistance)

across some of the slopes on the study site in Bishopdale and these have been

mapped. These occur on both steep and gentle gradient slope areas.

After 5 years, the exclosure zones near the main stream channel had significantly

lower soil compaction than areas without exclosure, or areas that had been in

exclosure status for 2 or 3 years. Thus, the use of such measures may reduce

compaction in some areas of the study site after several years.

Data collected in this survey provide an essential baseline against which future

change through management interventions can be evaluated.

A compaction survey has been shown to provide a cost-effective way of determining

locations in the upper Ure system where there may be compaction problems. This is

because it does not require costly lab analysis which may be required for other soil

measures such as bulk density. It is therefore recommended that such surveys are

rolled out across other areas of interest in the Upper Ure.

Future work could consider measuring compaction in the subsoil for a subset of

sampling points as well as the topsoil.

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Combining the two types of study

Combining results from the compaction surveys with the use of TOPMODEL would

be very useful in showing what impact different spatial hotspots of compaction may

have on the flood peak. As TOPMODEL results demonstrated for Coverdale,

changes in grazing intensity that increase or reduce soil compaction can greatly

affect the flood peak at the catchment outlet. Therefore, by combining compaction

data with hydrological modelling it would be possible to show which compaction

zones may be having the largest impact on flood peaks in the Ure system and

therefore which locations should be prioritised with interventions to reduce

compaction.

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2. Flood modelling in Coverdale

2.1 Objectives

This research seeks to show, for a tributary in the Upper Ure catchment, how different

spatial configurations of land management may influence local flood peaks and to

demonstrate how the new distributed version of TOPMODEL, developed by water@leeds,

can be used to predict the effect of land-cover change on flood flows. Such work could

support decision-makers in understanding the scale of flood benefits possible from land-

cover change and also how to target interventions spatially for maximum effect.

Specifically the work was commissioned to:

Adjust the modified spatially distributed version of TOPMODEL developed by Gao

et al (2015) for use in Coverdale.

Run the model for three types of rainfall magnitude

Test possible impacts of different levels of compaction in ELS areas on the flood

peak at the Coverdale catchment outlet

Test possible impacts of bare peat restoration in Coverdale on the flood peak

downstream

Test possible impacts of moorland burn rotation scenarios on the flood peak

downstream

2.2 Study site and methodology

The Coverdale catchment (54◦16’ N, 2◦43’ W) located in the Yorkshire Dales National Park

has an area of 84.0 km2 at elevations between 97 m and 675 m AOD. The River Cover is

a tributary of the Ure. The climate is cool and wet with a mean annual precipitation of 1757

mm based on the Environment Agency rainfall record (station number: 047281) between

1986 and 2014. Figure 2.1 shows a rainfall frequency analysis for the EA gauge data.

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Figure 2.1. Frequency of maximum rainfall for each year (a) 15-min rainfall, (b) hourlyrainfall, (c) daily rainfall. A frequency of 0.8, for example, would indicate that themaximum rainfall in 80 % of all years is above that value.

The hydrological model, TOPMODEL, was developed at the University of Leeds in the

1970s. It is now used worldwide. Recently, a new fully distributed version of the model has

been produced (Gao et al., 2015). TOPMODEL was already suited to upland catchments

but this new development has major advantages because: 1) it can predict where overland

flow will occur, showing greater detail for the rates and pathways of overland flow

production; and 2) it makes use of measured overland flow rates to forecast how the

speed of overland flow depends on the density of vegetation cover. These properties

mean that land management or surface-cover change in different parts of the catchment

can be evaluated with regard to impacts on the river flow at the catchment outlet.

To model the impact of land management on downstream flow in the catchment,

illustrative scenarios are needed to represent different land management options. Groups

of scenarios were designed to determine basic factors that might have the largest effect on

flood peaks for Coverdale. These scenarios are described in turn in section 2.3. It should

be noted that many other scenarios could be chosen and tested if stakeholders require.

However, due to the project being a pilot exercise with limited funding then we have

restricted the work to a handful of scenarios to be modelled. The distributed version of

(a) (b)

(c)

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TOPMODEL was used to simulate river flow for each scenario under a series of rainfall

events.

The size of the digital elevation model grid-cell used in the study was 20 m by 20 m. The

time step was set as 0.1 hr (i.e. a discharge result for every 6 minutes) in scenario

modelling runs to identify possible minor differences between outcomes on river flow. For

rainfall input, a number of typical rainfall events with different rainfall intensities were

employed to demonstrate the impacts of land-cover change scenarios on river flow in

different rainfall conditions. These rainfall intensities were selected based on the results

from Figure 2.1 using the ~30 year rainfall record and were 15 mm/hr for one 1 hr (roughly

equivalent to a one in 10-year event), 30 mm/hr for 1 hr (approximating the maximum

recorded in the whole rainfall timeseries) and 30mm/hr for 5 hrs (approximating the highest

total daily rainfall in the record). The modelling was done by running a 10-step (1 hour)

warming-up stage for the model at the very beginning of each scenario run prior to the

rainfall event. Another 80 steps follow each rainfall period in the entire modelling period.

Most scenario hydrographs shown within the figures in this report just present the first 80

or 100 time steps to focus on the rising and falling limbs around peak flow conditions and

ignore the last steps which just contain the low flow recessional part of the hydrographs.

Due to the lack of river flow data in the Coverdale catchment (monitoring by water@leeds

has only recently begun in one part of the catchment as part of a different project, and

there are only a few months of stage data to date), it is difficult to optimize the parameters

of the distributed TOPMODEL in this catchment. A nearby catchment in the Upper Ure

system, Snaizeholme Beck (54◦17’ N, 2◦15’ W), has a long-term flow record (2003-2014,

15-min interval), and was employed to optimize the parameter set of the catchment

hydrological model. Two 3-day periods were selected in summer as calibration and

validation periods in order to avoid confusion due snowmelt events, and the two periods

(i.e. [0:00 17th Aug 2012 - 23:59 19th Aug 2012] and [0:00 8th June 2011 - 23:59 10th June

2011]) contain the largest hourly rainfall intensities in the rainfall record. Each period had

288 time steps with 15-min intervals which matches the interval of rainfall and river flow

records.

A total of 20 testing runs of the model were operated through the calibration period to

identify a well-performing set of parameters (m = 14 mm, K = 100 m hr-1, kv = 30). There

was a good correspondence between simulated and observed flow in the calibration

period (the Nash-Sutcliffe efficiency is 0.88, see Figure 2.2). This parameter set was then

used to run the model in the validation period and the simulation corresponds well to the

observed flow with an efficiency of 0.82 (Figure 2.3). Thus the model performed well

against observed data.

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Figure 2.2. Time series of observed and simulated runoff in the calibration period for thecatchment of Snaizeholme Beck.

Figure 2.3. Time series of observed and simulated runoff in the validation period for thecatchment of Snaizeholme Beck.

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2.3 Results

2.3.1 Riparian buffer areas

Cells with a cumulative upslope area of over 1.2 km2 were defined as river channels.

Riparian strips of theoretical bare soil and rough vegetation cover were placed parallel to

water courses as scenarios, covering either 10 % (thinner strips) or 20 % (thicker strips) of

the catchment area as shown in Figure 2.4.

Figure 2.4. Riparian buffer strips covering 10% and 20% of Coverdale.

The results of the modelling suggests that the bare soil strips give a higher flow peak and

reduce the delay to flow peak, while there is a reduction and delay in flood peak

associated with rough vegetation strips (Figures 2.5-2.10). Table 2.1 shows the change of

flow peak in each scenario compared to the current baseline (‘normal’) scenario.

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Figure 2.5. Hydrographs of the riparian buffer strip scenarios (10 % area) under a 15 mmrainfall event.


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Figure 2.10. Hydrographs of the riparian buffer strip scenarios (20 % area) under a 150mm rainfall event.

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Table 2.1. Modelling comparison of riparian buffer strip scenarios.

Land cover scenario

Peak flow change compared normalPeak delaycompared tonormal(time step)Absolute change Relative

change(mm/6min) (m3/s)

15-mmrainfallevent

10% area

Bare soil 0.070 16.33 18.9 % 3

Roughvegetation

-0.046 -10.73 -12.4 % -4

20% area

Bare soil 0.076 17.73 20.5 % 5

Roughvegetation

-0.044 -10.27 -11.9 % -3

30-mmrainfallevent

10% area

Bare soil 0.19 44.32 14.6 % 1

Roughvegetation

-0.16 -37.33 -12.3 % -3

20% area

Bare soil 0.20 46.65 15.4 % 2

Roughvegetation

-0.17 -39.66 -13.1 % -3

These results indicate that loss of vegetation in riparian areas of the Coverdale catchment

may increase the flood risk substantially. Conversely, vegetation regeneration with dense

cover to slow flows of water in those areas would have a considerable effect on flood

attenuation. Importantly, from a management perspective, it is almost as good to have a

buffer zone that covers 10 % of the catchment area as it is to have one that covers 20% of

the catchment area. Therefore, efficiency savings can be made by investing in narrower

buffer zones that cover a longer length of stream channels. There was a slight decline in

the relative effects of the vegetation change on flood peak for each scenario when the

storm event increased from 15 mm to 30 mm. For the massive 150 mm event (Figures 2.9

and 2.10), there is a convergence of flood peaks at about the same level suggesting that,

for the very extreme events, land cover would not affect the magnitude of the flow peak.

However, even here the bare soil scenarios have early rising limbs in hydrographs, while

delayed rising limbs can be seen in rough vegetation scenarios.

2.3.2 ELS and HLS areas

There is 24.4 km2 of land under Entry Level Stewardship (ELS) and 42.6 km2 under Higher

Level Stewardship (HLS) in the Coverdale catchment. Grazing in these areas may

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compact soil and reduce the storage space for water. This may lead to greater and earlier

occurrence of overland flow on hillslopes during storms and thus increase flood risk in the

catchment. Scenarios for the ELS area, the HLS area, and both together were designed

and are shown in Figure 2.11. A half value of m, which is a parameter representing the

active water storage in soil in the model, was set in the target areas for each scenario to

represent the impact of heavy grazing and compaction.

Figure 2.11. ELS and HLS areas used in the modelling scenarios.

The modelling results show that increased grazing could lead to larger flow peaks

compared to the baseline. However, these changes do not impact the timing of the flow

peaks and this could have important meaning at a larger scale when considering how

flows from Coverdale interact with flood flows in the main Ure channel. Hydrographs of the

scenario results are shown in Figure 2.12 to 2.14, while Table 2.2.2 compares the scenario

results on the flood peak at the Coverdale outlet. The HLS grazing scenario results in

more than twice the relative change in flood peak compared to ELS areas alone, even

though the area under HLS is less than twice that of the ELS areas. The scenario of heavy

grazing in both ELS and HLS regions has a still larger impact on river flow with a greater

than 30 % increase in flow peak for a 15 mm storm event. While the experiment conducted

here is theoretical in its interpretation of how grazing impacts soil water properties, the

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message is very clear: Any changes in land management in the ELS or HLS areas that

could reduce compaction and increase soil water storage and flow through the soil (rather

than as overland flow) have the potential to significantly reduce flood peaks in Coverdale.

In Coverdale, the areas under HLS appear to be more sensitive to such changes than

those under ELS. In other words, the flood benefits could be greater from encouraging

change in HLS areas than the same change in the ELS areas of this catchment, but that is

not to say that the potential effects of ELS change would not also be large.

Figure 2.12. Hydrographs of the ELS and HLS scenarios under a 15 mm rainfall event.

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Table 2.2. Modelling comparison of ELS scenarios.

ELS scenario

Peak flow change compare to the normalscenario

Peak delaycompared to thenormal scenario(time step)Absolute reduction Relative


15 mm rainfall

ELS 0.043 10.03 11.6 % 0

HLS 0.091 21.23 24.5 % 1

ELS andHLS

0.118 27.53 31.8 % 2

30 mm rainfall

ELS 0.09 21.00 6.9 % 0

HLS 0.20 46.67 15.4 % 0

ELS andHLS

0.29 67.66 22.3 % 0

2.3.3 Burning

Rotational burning regions were determined from aerial photos (Google Map) for the

Coverdale catchment. For the purpose of the scenario testing, here we assumed a more

frequent burning cycle of 1 in 10-years with a burn effect on dense surface vegetation

lasting 4 years. Hence 40% of the moorland burning area would have ‘recent’ burn

patches (7.3 km2). For these areas we have assumed a reduction of surface roughness in

recently burnt area by half compared to more normal conditions outside the recently burnt

patches. Figure 2.15 illustrates the burning patch scenario and the size of each patch was

set as 100 m×100 m.

Figure 2.15. Burning patch scenario.

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The modelling results show that having burning patches gives higher flow peaks under

every rainfall event we tested. The results of the scenario runs are shown in Figure 2.16 to

2.18 with a summary comparison presented in Table 2.3.

Figure 2.16. Hydrographs of the burning scenarios under a 15 mm rainfall event.

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Table 2.3. Modelling comparison of rotational burning scenarios.

Burning scenario




15 mm rainfall 0.011 2.57 3.0 % 0

30 mm rainfall 0.03 7.00 2.3 % 0

The total area of the Coverdale catchment under recent burning in our scenario was 8.7 %

with these areas being concentrated in the headwaters where the moorland is more

extensive. In Coverdale, the relative effect of such moorland burn patches on flood peaks

appears to be a lot smaller than for the role of riparian vegetation for a similar proportion of

the catchment under consideration (i.e. 10 % of the catchment was tested under the

riparian buffer strip scenarios). We conclude that the impact on flood peaks in Coverdale

of reducing burn frequency or ‘stopping’ burning would be relatively small, although

potentially still important (e.g. reduced peak by 7 m3 s-1 in a 30 mm rainfall event).

However, we suggest that the result may be different in other catchments with a different

spatial configuration of moorland burning and that if the burning area extended to

downslope areas closer to main channels it would be expected to have a greater impact

on river flow.

2.3.4 Bare peat restoration scenarios

We consider the case of what might happen if we revegetate the areas of bare peat that

exist in the Coverdale catchment. The bare areas were digitized (approximately) using

aerial photos of the catchment from Google Map (a more accurate bare soil distribution

map would need a UAV or field survey). Most bare areas are concentrated in the peaty

headwaters and they cover 5.8 % of the catchment (Figure 2.19). To explicitly evaluate the

impact of bare soil restoration on river flow, a scenario with all bare soil areas and a

scenario representing revegetating all these areas were simulated to see the differences in

hydrographs before and after revegetation.

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Figure 2.19. General locations of bare soil areas in the Coverdale catchment.

Revegetation of bare areas would decrease river flow peak and postpone the start of the

rising limb of the flow peak but the overall time to peak would not be delayed (see Figure

2.20 to 2.22 for the different hydrographs). Table 2.4 provides a comparison of the bare

soil revegetation scenario results. As rainfall intensity increases, the relative change in the

flood peak caused by revegetation decreases. However, the absolute change in flood peak

becomes greater, meaning that revegetating bare soil has a greater absolute effect in

reducing the flow peaks under heavier and rarer rainfall events.

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Figure 2.20. Hydrographs of the bare soil revegetation scenarios under a 15 mm rainfallevent.


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Table 2.4. Modelling comparison of bare soil revegetation scenarios.

Revegetation scenario




15 mm rainfall 0.015 3.50 4.04 % 0

30 mm rainfall 0.04 9.33 3.08 % 0

2.4 Conclusions and recommendations from the flood modelling

To evaluate the impact of potential land-management activities on flood peaks in the

Coverdale catchment, scenario modelling was conducted under a series of rainfall events

using the distributed TOPMODEL. The main conclusions are as follows:

Changes in the land cover in riparian zones have the potential to make very large

differences to flood peaks emerging from Coverdale. Stripping vegetation from

10 % of the catchment area along riparian areas could increase peak flows by

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19 %. Placing denser vegetation in these areas could potentially reduce flood peaks

by 12 % during a 15 mm per hour rainfall event.

It is almost as good to have stream corridor buffer zones that cover 10 % of the

catchment area as it is to have wider buffer zones that cumulatively cover 20 % of

the catchment area. Therefore, efficiency savings can be made by investing in

narrower buffer zones that cover a longer length adjacent to stream channels.

Heavy grazing in ELS and HLS areas might greatly increase flood risk in the

Coverdale catchment. The combined impact of ELS and HLS change could be as

great as 32 % during a 15 mm per hour storm event. The impact on the flood peak

of such changes in HLS areas alone would be 25 %.

Any changes in land management in the ELS or HLS areas that could reduce

compaction and increase soil water storage and flow through the soil (rather than as

overland flow) have the potential to significantly reduce flood peaks in Coverdale.

The areas under HLS appear to be more sensitive to such changes than those

under ELS but that is not to say that the potential effects of ELS change would not

also be large.

Areas associated with bare peat cover around 5.8 % of Coverdale. Revegetation of

this bare peat in the headwaters of Coverdale could reduce peak flow by over 4 %

and 3 % respectively for either 15 mm or 30 mm per hour rainfall events.

In a very extreme storm event (150 mm in 5 hours), the proportional impact of land-

cover interventions on flood magnitude in Coverdale is likely to be small. However,

there will still be impacts on the timing of the rising and falling limbs of the

hydrographs which may be important for the overall flood peak in the Ure but this

will depend on the synchronicity of flood peaks from all the different tributaries down

the main Ure channel. For an extreme 150 mm event for the bare soil, enhanced

grazing and more frequent moorland burning scenarios tested, there would be

earlier rising and falling limbs of the flood hydrograph than is currently the case,

while delayed rising and falling limbs would occur if there was revegetation of bare

areas and enhanced roughness (e.g. reduced grazing, less frequent burning,

creation of buffer zones).

The new distributed version of TOPMODEL was successfully applied to test

relevant scenarios of land-cover change to show how different spatial configurations

in the landscape can influence the flood peak. The work shows clear promise in

determining how spatially specific land management might affect flood peaks under

different types of rainfall events in the wider Ure catchment. Clearly, it would be

desirable to run the model across the whole of the Upper Ure to determine the full

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scale of possible management intervention effects in different locations upon the

overall flood peak in the main river channel. It may be that some tributaries are

more or less sensitive than Coverdale to changes in surface cover and

management and it would be possible to evaluate where in the Upper Ure system

efforts ought to be focussed.

This work could involve:

Additional field campaigns to measure flow rates across major surface cover types

not already captured by existing data used in the model.

Inclusion of a wider range of surface cover change scenarios to be tested.

Inclusion of a wider range of soil types and drainage data.

Coupling the model to the streamflow models in the catchment.

Comparing land-cover change scenario impacts with potential impacts of other flood

reduction measures such as woody debris dams, water retention features etc.

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3. Compaction study in Bishopdale

3.1 Objectives

The objective was to conduct a survey of parts of Bishopdale to measure soil compaction

to understand different levels of compaction in the system. Specifically the work was

commissioned to undertake:

A diffuse survey covering valley bottom, valley sides and upland parts of the

catchment.

A concentrated survey around areas of buffer zone implementation to test

differences between buffer zones of different ages and non-buffer zone areas.

Compaction results in greater bulk density and reduced soil pore space. Compaction is

known to affect soil flora and fauna, including plant community composition and diversity in

grasslands (Godefroid and Koedam, 2004; Defra, 2007), may negatively impact water

quality in relation to loss of applied nutrients and agrochemicals, and may also negatively

impact some species of birds (Defra, 2007). There is some evidence of upland soil

compaction impacts on flood risk (Meyles et al., 2006; Zhao, 2007), although there has

been little work at the catchment scale to evaluate such effects (O’Connell et al., 2007; but

see section 2 above for new evidence from Coverdale). Despite the potential effects of

compaction outlined above, data on soil compaction are very scarce, particularly in the

uplands.

3.2 Study site and methodology

Howsyke Farm (Figure 3.1) was selected for the study as a large land-holding where

permissions had been agreed for us to undertake the work. The site covers 4.6 km2 and

ranges in altitude from 550 to 174 m. Nine zones are shown in Figure 3.1 which relate to

Natural England (2010) Environmental Stewardship Scheme index map codes for the

farm. This numeric coding is used throughout this report to refer to different parts of the

system. Howskye is dominated by organic and organo-mineral soils (Table 3.1) with

blanket peat being dominant in zone 9 with some coverage over areas 6 and 7. Peaty soils

(such as humic rankers) dominate in zones 1, 4, 5 and 6 while gleys (with a peaty surface

layer) dominate in zones 3 and 8. At the lower end of the site, adjacent to the main water

course, some narrow buffer strips have been installed, excluding sheep and encouraging

grass and shrub growth. Four of these areas were installed on the site being 1, 2, 3 and 5

years old respectively. Of these buffer strips, the 1, 2 and 3-yr strips are quite narrow (with

1 and 3-yr strips being rather small in total area), while the 5-yr strip is wider (see Figure

3.12).

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Three types of measurement were made across the study site: soil resistance (i.e.

compaction), soil moisture and the geo-referenced sampling location. Sampling was

generally conducted in a grid format across the site with 1201 compaction measurements

making this one of the richest studies of surface soil compaction in the UK to date.

Sampling was densest in valley side and bottom areas where grazing was concentrated

while the peat was sampled less intensively. The buffer strips were each sampled,

although as the yr-1 buffer strip was very small indeed, the results are shown but they

were not included in statistical analysis which aimed to test whether the buffer strips

resulted in reduced compaction. As the buffer strips are of different sizes, a different

number of samples were obtained from each so that the density of sampling was about the

same in each case. To aid statistical analysis a control area without fencing, but adjacent

to the buffer strips with similar soil and slope setting was sampled with the same density

as the buffer strips.

Soil surface compaction was measured using a Van Walt penetrometer. Two readings

were taken at each point. If the values were within 10 % of each other (i.e. lowest value

was at least 0.9 times the highest value) then the mean of those values was taken. If the

values were more than 10 % different then a third reading was taken and the median value

was used for that point. Readings were not taken when the sampling point hit a large

stone. The final values of compaction for each sampling point are provided in the

accompanying excel spreadsheet with GPS location points. These data provide an

excellent baseline by which change over time can be determined in future years.

Soil moisture was measured to aid interpretation of the compaction sampling and in case

there were large differences between sampling days. However, as the sampling was

carried out in February and March 2015 then the soil moisture was consistent from day to

day and so a correction factor was not applied to the compaction data. A time domain

reflectrometry (TDR) probe was used to measure soil moisture in the field. Data from the

probe were checked against laboratory soil moisture tests (oven mass loss) resulting in a

strong and robust calibration equation. A hand-held GPS was used to record the location

of each sampling point. These are accurate to +/- 4 m in the x and y co-ordinates.

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Figure 3.1 Howsyke Farm within Bishopdale. Internal boundaries and area numbers referto Natural England mapped areas and these are used for mapping purposes and analysesin the report below.

1

2

3

45

6

7

89

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Table 3.1 Proportion of each zone on Howsyke covered by different soil types

% of zone covered by soil typeHowsyke Farmzone

Total area,km2

Humicranker

Brownalluvial soil

Brownearth

Stagnogley

Stagnohumicgley

Blanketpeat

1 1.076 55.9 0.0 0.0 0.0 42.7 1.4

2 0.536 0.0 13.4 53.6 32.9 0.0 0.0

3 0.106 0.0 0.0 0.0 100.0 0.0 0.0

4 0.291 99.2 0.0 0.0 0.0 0.0 0.8

5 0.509 69.3 0.0 0.0 0.0 10.1 20.5

6 0.273 56.2 0.0 0.0 1.7 0.8 41.3

7 0.320 0.0 79.3 20.7 0.0 0.0 0.0

8 0.813 0.0 0.0 22.4 61.2 7.1 9.3

9 0.666 0.0 0.0 0.0 18.5 16.4 65.1

3.3 Results

Surface compaction varied from 5 to 158 N m-2. The spatial distribution of compaction is

shown in Figure 3.2 for the whole area. As would be expected the least compacted areas

correspond with locations of blanket peat, and these are also the areas with highest

moisture content. The zones of high compaction were found on the central and

southwestern parts of zone 8, parts of zone 7 and zones 1 and 5. These maps should

allow managers to focus soil remediation efforts in those locations with high compaction.

Maps for each zone are shown in Figures 3.3 to 3.11. Summary data for each zone is

shown in Table 3.2. However, as some zones have areas of both high and low compaction

averages can mask the fact that there are concentrated areas with high compaction within

some of the farm zones.

Table 3.2 Summary compaction statistics for each zone on Howsyke Farm, N m-2 (notincluding buffer zones)

Zone n Median Max Min Interquartile range

1 15 80 102 44 20

2 178 62 158 32 23

3 31 74 100 22 47

4 49 68 113 38 26

5 13 70 108 50 20

6 5 64 100 50 19

7 679 54 132 28 20

8 11 72 124 36 26

9 8 52 80 36 15

30

Figure 3.2 Compaction and soil moisture across Howsyke Farm, February-March 2015

31

Figure 3.3 Compaction and soil moisture across zone 1 on Howsyke Farm.

32


33


34


35


36


37


38


39


40

The soil compaction maps for the buffer zones which are located near the mean stream in

zone 7 of Howsyke Farm are shown in Figure 3.12. A Kruskal-Wallis H-test showed that

buffer zone age was a factor controlling soil compaction (p<0.001). Post-hoc pairwise

comparisons showed that the soil compaction in the 5-yr buffer zone was significantly less

than that of the 3-yr, 2-yr or control (no buffer zone) sites. There were no significant

differences in soil compaction between the control site and the 2-yr or 3-yr buffer zones

(p>0.05). These results suggest that it may take > 3 years for buffer zones to have an

effect on soil compaction at the study site, but that this effect can be significant after buffer

zones have been in existence for 5 years. Thus, if sheep are fenced off from other areas of

the site it may take several years for soil compaction to recover. However, we have

evidence that exclosures could lead to recovery of soils to compaction and could be a

worthwhile intervention if there was a desire to reduce compaction in some areas.

Figure 3.12 Soil compaction on buffer strips on Howsyke Farm

Table 3.3 Summary compaction statistics for buffer zones on Howsyke Farm, N m-2

Buffer n Median Min Max Interquartile range

Year 2 60 44 32 62 12

Year 3 17 46 5 58 11

Year 5 116 40 16 76 10

Control 19 44 38 74 18

41

3.4 Conclusions and recommendations from compaction surveys

This report provides one of the few studies of soil compaction that exists for the UK

uplands.

There are areas of high compaction (e.g. >100 N m-2 of soil resistance) across

some of the slopes on the study site in Bishopdale and these have been mapped.

After 5 years, the exclosure zones near the main stream channel had significantly

lower soil compaction than areas without exclosure, or areas that had been in

exclosure status for 2 or 3 years. Thus, the use of such measures may reduce

compaction in some areas of the study site after several years.

Data collected in this survey provide an essential baseline against which future

change through management interventions can be evaluated.

A compaction survey provides a cost-effective way of showing locations in the

upper Ure system where there may be compaction problems. This is because it

does not require costly lab analysis which may be required for other soil measures

such as bulk density. It is therefore recommended that such surveys are rolled out

across other areas of interest in the Upper Ure.

Combining results from the compaction surveys with the use of TOPMODEL would

also be highly useful in showing what impact different spatial hotspots of

compaction may have on the flood peak. As TOPMODEL results demonstrated for

Coverdale, changes in grazing intensity that increase or reduce soil compaction can

greatly affect the flood peak at the catchment outlet. Therefore, by combining

compaction data with hydrological modelling it would be possible to show which

compaction zones may be having the largest impact on flood peaks in the Ure

system and therefore which ones should be prioritised for interventions to reduce

compaction.

Acknowledgements

We thank Daniel Turner of the Yorkshire Dales Rivers Trust for provision of information

and land-access arrangements in Bishopdale. We thank Robert Brown of Howsyke Farm

for kindly allowing land access for the compaction survey and Stuart Dent for supporting

the access arrangements. We thank Karen Scott, David Ashley, Sarah Hunt, Robert

Peachey, Michael Dorrington, Shengmiao Jiang, Rudi Barker and Olivia Desmond for

undertaking the field surveys.

42

References

Defra (2007) Scoping study to assess soil compaction affecting upland and lowlandgrassland in England and Wales. Final report, BD2304, Cranfield University.

Gao J, Holden J, Kirkby MJ. (2015). A distributed TOPMODEL for modelling impacts ofland-cover change on river flow in upland peatland catchments. HydrologicalProcesses. DOI: 10.1002/hyp.10408.

Godefroid S, Koedam N. (2004) Interspecific variation in soil compaction sensitivity amongforest floor species. Biological Conservation 119, 207-217.

Meyles EW, Williams AG, Ternan JL, Anderson JM, Dowd JF. (2006). The influence ofgrazing on vegetation, soil properties and stream discharge in a small Dartmoorcatchment, southwest England, UK. Earth Surface Processes and Landforms 31,622-631.

Natural England (2010) Environmental Stewardship Scheme index map, Howsyke Farm,application AG00302894.

O'Connell E, Ewen J, O'Donnell G, Quinn P. (2007) Is there a link between agriculturalland-use management and flooding? Hydrology and Earth Systems Sciences 11,96-107.

Zhao Y. (2007) Livestock impacts on hydrological connectivity. PhD thesis, University ofLeeds, Leeds.

Appendix

An Excel spreadsheet has been provided showing sample locations and compaction

values for the Bishopdale survey

flood modelling and compaction studies for the … · flood modelling and compaction studies for...

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