AN ASSESSMENT OF FLOOD MITIGATION OPTIONS
FOR THE CITY OF BRECHIN
Darren Lumbroso
HR Wallingford Ltd, Howbery Park, Wallingford, Oxfordshire OX10 8BA, UK
Abstract
The city of Brechin is located on the River South Esk in Angus. There is a long history of
flooding in Brechin. In the past 100 years Brechin has been subject to flooding on at least 18
occasions. Following severe floods in November 2002 Angus Council initiated a series of
studies to evaluate the most sustainable and cost effective flood mitigation option for the city.
This paper details the hydrological, hydraulic and geomorphological modelling carried out for
the River South Esk, together with an assessment of the proposed flood mitigation options.
Key words:
Brechin, flood mitigation options, River South Esk
INTRODUCTION
Overview of the River South Esk catchment
The River South Esk drains an area of some 488 km2 to Brechin. The catchment is rural in
nature and has an average annual rainfall of 1,080 mm. The River South Esk catchment is
long and narrow, draining the south-eastern flank of the Grampians. The upper two-thirds of
the catchment are steeply sloping rising to a maximum altitude of 958 m AOD. Land use in
the catchment comprises a mixture of rough grazing on open moorland, forestry and, at lower
levels, arable farmland. The lower half of the catchment lies on Old Red Sandstone with the
rest being underlain by metamorphic rock. The combination of the geology and topography
results in a “flashy” response. During periods of high precipitation, flows in the River South
Esk can increase by several hundred m3/s in a few hours.
Background to flooding in Brechin
There is a long history of flooding in Brechin, the location of which is shown in Figure 1.
River Street, which runs along the northern bank of the River South Esk is particular
susceptible to being inundated. In May 1913 it was reported that River Street was flooded to
a depth of approximately 3 m. Attempts to alleviate flooding began in December 1967 with
the removal of a weir constructed to provide water for a mill that was located downstream of
Brechin Bridge. This weir was undoubtedly a principal factor in the flooding prior to 1967.
However, since 1967 River Street has been inundated on at least five occasions. The most
serious of these events occurred on 26 November 2002, shown in Figure 2, when local
residents had to be evacuated from their properties. Following this flood Angus Council
decided to investigate a variety of flood mitigation options. This paper describes the
hydrological, hydraulic and geomorphological modelling carried out to assess which option
was most appropriate and sustainable for Brechin.
The legislative background and sustainable flood management
Under the Flood Prevention Act 1967 Local Authorities may promote a flood prevention
scheme to defend areas subject to flooding. Such schemes must be confirmed by an order
made by the relevant Scottish Minister. In addition under the Water Environment and Water
Services (Scotland) Act 2003 responsible authorities, such as Angus Council, must promote
sustainable flood management.
Sustainable flood management means different things to different people. However, the
Scottish Executive National Technical Advisory Group (NTAG) on flooding has made
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considerable efforts to define sustainable flood risk management and proposed the following
concise high-level definition or ‘vision’ of it in 2004:
“Sustainable flood management provides the maximum possible social and economic
resilience against flooding, by protecting and working with the environment, in a way which
is fair and affordable both now and in the future.” (Reference 8)
Study area
N
Inch
Gauging station
River Street
Brechin Bridge
Reproduced from Ordnance Survey with the
permission of the Controller of Her Majesty’s
Stationery Office. Crown Copyright Reserved.
South Esk River
Figure 1
Location of Brechin
Figure 2
River Street, Brechin during the November 2002 flood
In Scotland, it has been recognised by the Government that there is a need to develop
solutions that integrate risk management as part of integrated catchment management
2
including land use change. This is largely due to drivers such as the Water Framework
Directive (WFD) and commitments related to water management made at the World Summit
on Sustainable Development in Johannesburg in 2002. The WFD states that water bodies
must achieve good ecological status by 2015. Presently, the River South Esk has been
identified as at risk of not meeting this requirement, so any development taking place along
its course must therefore take this into account and avoid deterioration to the ecological
habitat.
A CATCHMENT APPROACH TO FLOOD MITIGATION
Background to flood mitigation in Scotland
Recently there have been significant developments in policy on flood alleviation in Scotland.
The process of dealing with flooding in Scotland is changing towards looking at the processes
that generate floods rather than just simply mitigating the consequences of flooding. The
objective of the Scottish Executive is to ensure that flood mitigation measures are not only
cost effective but also sustainable. This paper details the flood mitigation options that were
considered for Brechin detailing the catchment scale approach that was undertaken.
Part of the emerging philosophy is to engage with stakeholders and statutory agencies such as
the Scottish Environment Protection Agency (SEPA) and Scottish Natural Heritage (SNH)
during the development of a scheme. Angus Council convened a series of workshops with
stakeholders and undertook a number of public consultations that informed the process of
examining options for the scheme.
Estimation of flood flows at Brechin
A hydrological assessment was undertaken to derive the 1 in 100 year and 1 in 200 year
design hydrographs. To reduce the uncertainty in the derivation of the flood flow
hydrographs a number of techniques were employed. These included:



A single site analysis of the 22 year annual maximum flow series at the flow gauge at
Brechin, which generated a 1 in 200 year flow of 320 m3/s;
A FEH rainfall-runoff analysis to establish the design hydrograph for Brechin.
Adjusting this method using observed data produced a 1 in 200 year flow of 446 m3/s;
A Flood Estimation Handbook (FEH) statistical analysis based on a pooling group
approach which produced a 1 in 200 year flow of 355 m3/s.
Observed rainfall intensity and flow data were used to define the design hydrograph shape for
Brechin. A recent piece of research carried out by Birmingham University into the use of the
FEH rainfall-runoff method indicated that even when the time to peak and standard
percentage runoff of the FEH unit hydrograph are adjusted using observed data, the resulting
peak flows are still generally 37% higher than those estimated using the FEH statistical
method (Reference 2). In the case of Brechin it was found to 26% higher than the FEH
statistical analysis.
The 1 in 200 year flow generated by the FEH statistical pooling group analysis has the lowest
degree of uncertainty. As a result the ordinates of the FEH rainfall-runoff generated
hydrograph were adjusted using the results of the FEH pooling group analysis (i.e. scaled so
that the resulting hydrograph had a peak of 355 m3/s). The 1 in 100 and 1 in 200 year design
hydrographs used in the analysis for Brechin are shown in Figure 3.
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400
350
300
Flow (m3/s)
250
200
150
100
50
0
0
5
10
15
20
25
30
35
40
45
Time (hours)
1 in 100 year hydrograph
Figure 3
1 in 200 year hydrograph
1 in 100 and 1 in 200 year design hydrographs for Brechin
Hydraulic modelling of the River South Esk
Background
A hydraulic model of 3.7 km of the River South Esk in the vicinity of Brechin was
constructed using the river modelling software ISIS. The uncertainty in the estimates of
design flood levels were reduced by calibrating and verifying the model against four historical
flood events, which occurred in November 2002, April 1999, November 1997 and January
1993. The November 2002 event has a return period of between 1 in 25 and 1 in 30 years and
proved useful for calibrating the levels in the floodplain. The mean difference between the
modelled and observed levels at eight locations in Brechin for the November 2002 flood was
29 mm. The hydraulic model was thus considered to form a good basis from which to
estimate the 1 in 200 year water levels.
The ISIS hydraulic model was also used to not only predict flood extents and depths in
Brechin, and to assess different flood mitigation options, but also as a basis for carrying out
geomorphological modelling of the River South Esk.
Flood mitigation options considered for Brechin
A number of flood mitigation options, generated from the workshop and consultation process,
were investigated for Brechin, these included:



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


Removal of a redundant weir structure;
Removal of a gravel shoal upstream of Brechin Bridge;
Upstream off line and on line storage;
Flood relief channel;
Relocation of commercial and residential properties;
Catchment land use change;
Flood defences along the north bank of the river.
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These options were presented to interested stakeholders at a number of meetings over the
course of the studies. This paper outlines the advantages and disadvantages of the above
options.
Removal of a redundant weir structure
Kinnairds Mill Weir, shown in Figure 4, is a redundant structure located some 2 km
downstream of Brechin. There was a perception amongst many people in Brechin that
Kinnairds Mill Weir exacerbates flooding in the city. The hydraulic model of the River South
Esk was extended downstream to this weir to assess its effect. The model was run for a 1 in
200 year flood event with and without the structure in place. The results showed that during
the 1 in 200 year flood the backwater effect of Kinnairds Mill Weir extends some 1.25 km
upstream. Hence the removal of Kinnairds Mill Weir could not be justified on a flood
mitigation basis.
Removal of a gravel shoal upstream of Brechin Bridge
Immediately upstream of the Brechin Bridge there is a gravel shoal attached to the right bank.
There is anecdotal evidence to suggest that there has been a gravel shoal at this location for a
considerable period of time. However, there is a perception amongst the citizens of Brechin
that this gravel shoal has a significant effect on the 1 in 200 year flood levels. The hydraulic
modelling indicated that the 1 in 200 year water level would only be reduced by 100 mm if
the shoal and its associated vegetation were removed. This option was not taken forward as
the loss of habitat on the gravel shoal would have been difficult to support for a relatively
small decrease in the design flood level. Geomorphological modelling also indicated that
should the shoal be removed it was likely to reform within a period of 10 to 20 years.
Figure 4
Aerial photograph of Kinnards Mill Weir
Off line storage
Many stakeholders believed that additional storage could be created in the River South Esk
floodplain upstream of Brechin to significantly attenuate the 1 in 200 year flood hydrograph.
For off line flood storage to be practicable it should fulfil the following criteria:
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The value of the land used should be low;
The storage area should be close to the area being protected;
There should be wide flat floodplains with riverside embankments that prevent the
floodplain from being inundated during rare flood events.
The catchment upstream of Brechin is essentially rural in nature. In some areas farmers have
built low level, informal embankments to protect their land from flooding. However, simple
modelling indicated that the removal of these embankments would have negligible effect on
the 1 in 200 year flood levels in Brechin, meaning off line storage was not a viable option.
On line storage
The use of on-line flood storage was considered comprising an earth embankment and sluice
gates located upstream of Brechin. The ISIS hydraulic model of the River South Esk was
used to:


Estimate the volume of storage and height of the embankment required to prevent
inundation during a 1 in 200 year flood in Brechin;
Define the operating rules of the outlet gates.
It was estimated that a reservoir volume of some 10 million m3 would be required with an
embankment height of some 15 m above the ground level to attenuate the 1 in 200 year flow
to a bankfull level in Brechin. The outlet to the reservoir would comprise three 9 m wide and
15 m high radial sluice gates. Figure 5 shows details of this option.
This option was not taken forward for a number of reasons including:


The capital, operation and maintenance costs;
The long term effects of such a scheme on the aquatic environment, since it would
introduce a new barrier on the River South Esk which goes against the principles of the
WFD.
Flood relief channel
The possibility of constructing a flood relief channel running to the south of Brechin was also
considered. Using the ISIS hydraulic model of the River South Esk it was established that
such a channel would require a capacity of 200 m3/s to alleviate a 1 in 200 year flood event in
Brechin i.e. 57% of the flood peak. This option was ruled out on the basis of cost and
sustainability issues such as sedimentation of the channel, and the impact on the aquatic
environment.
Relocation of commercial and residential properties
The relocation of commercial and residential properties was also considered as an option.
This option would have required 129 local authority houses, 35 private sector houses, an
industrial estate and a high technology engineering factory to be moved out of the 1 in 200
year floodplain. The associated social and economic costs of this were estimated to be
£57.7 million. This option was ruled out as the social disruption together with the costs was
deemed excessive in comparison with the benefits.
Catchment land use change
Many stakeholders were under the misconception that changes to rural land use in a large
catchment, such as the River South Esk, could significantly reduce the 1 in 200 year flood
level in Brechin. It was found that 88% of the recorded flood events at River Street had
occurred in the winter months (October to March).
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View 1
Cross-section through sluice gates
View 2
Plan view of the on-line storage area upstream of Brechin during a 1 in
200 year flood event
Figure 5
Details of the on-line storage option
The majority of the River South Esk catchment is an upland catchment subject to high
precipitation. This means that the soils will be saturated for much of the year and during
some flood events during the winter will be frozen. As a consequence flood generation will
be mainly controlled by the amount of precipitation that falls not by the antecedent moisture
conditions. With respect to afforestation of the catchment, a recent research report states that
for the UK “overall no clear evidence has emerged to show that forests either mitigate or
increase flooding to a significant extent” (Reference 3). As a consequence changes in
vegetation cover or farming practice are likely to have a minimal effect on the 1 in 200 year
flood flow for the catchment.
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Flood defence walls and embankments along the north bank of the river
The most cost effective and sustainable option was found to be a combination of flood
embankments and walls along the north bank of the River South Esk. The proposed flood
defence scheme for Brechin comprised the following:
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A grassed earthen bund around the leisure centre in the Inch;
An earthen embankment between the Inch and the upstream end of River Street;
A sandstone masonry reinforced concrete wall between the upstream end of River Street
and Brechin Bridge, shown in Figure 6;
A mixture of earthen embankment and reinforced concrete wall between Brechin Bridge
and the downstream end of the caravan park.
In terms of sustainability this option was found to have:

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No effect on downstream and upstream flood risk;
A minimal effect on the terrestrial and aquatic environment;
The lowest capital, operation and maintenance costs.
The main disadvantage of this option is that to protect Brechin against the 1 in 200 year flood
the defences will need to be at least 1.7 m high which will significantly reduce the amenity
value and visual appeal of the area. In order to mitigate this impact two options are currently
being investigated. The first is to provide raised walkways and viewing platforms as shown
in View 1 of Figure 5. The second is to use a partial height wall with demountable barriers
placed on top, thus lessening the visual impact of the scheme, shown in View 2 of Figure 6.
However, the River South Esk catchment is “flashy” in nature and the lead time which is
available to deploy the demountable defences is limited to around four to five hours. Studies
are currently being undertaken to assess how this lead time can be increased in order to
reduce the chance of demountable defences not being put in place in time.
View 1
Figure 6
Full height flood defence
View 2
Partial height flood defence
with demountable barrier
Views of possible flood defence options for River Street, Brechin
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Geomorphological modelling of the River South Esk
Background
In the context of the geomorphology of the River South Esk there were two specific aspects
that needed to be addressed with respect to the preferred flood mitigation option:
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
The long and short term impact of the proposed flood defences on the geomorphology;
The potential for an increase in general scour to take place in the area of Brechin Bridge
as a result of the implementation of flood defences.
The River South Esk is a steep, gravel bed river. Through Brechin the river is not as steep as
further upstream in the catchment, but it is still a high energy river which transports relatively
large quantities of gravel. The bed of the river through Brechin consists of coarse gravel with
a d50 sediment size of the order of 50 mm to 80 mm. The movement of gravel within such a
system is normally episodic with gravel movement only taking place during a few flood flows
each year. For the rest of the year it is likely that the shear stress applied by the flow on the
bed of the river is too small to move the sediment.
Sediment transport modelling
To assess the impact of the proposed flood mitigation option an ISIS sediment transport
model was set up. This model used the output from the ISIS hydraulic model, a sediment
transport equation and the equation of sediment continuity to predict sediment transport rates
along the river, erosion and deposition and bed level changes throughout the river system.
The ISIS sediment model was run for both a long term flow regime and a flood event. The
long term flow regime was derived from 20 years of records of mean daily flows recorded at
the Brechin flow gauge. The 1 in 200 year design hydrograph was used to represent the flood
event.
The sediment transport model of the River South Esk was calibrated by running it with
observed historical flows and adjusted so that it represented existing conditions. The
adjustments consisted of modifying the quantity of sediment that enters the modelled reach at
the upstream end. This was adjusted until the upstream bed level was approximately constant
over the 20 year period.
Results of the sediment transport modelling
The numerical morphological model was used to simulate the river with the proposed flood
defences in place using the observed 20 year mean daily flow series and a 1 in 200 year
design hydrograph. The sediment modelling indicated the following:
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In the long term the quantity of sediment passing through the Brechin reach of the River
Sourth Esk would be approximately the same under both existing conditions and with the
proposed flood defences in place;
There would be no significant aggradation or degradation of the river bed over a period of
20 years as a result of implementing flood defences in Brechin;
The proposed flood defences would have a minimal impact on the general scour that
would occur during a 1 in 200 year flood at Brechin Bridge.
CONCLUSIONS
The following conclusions can be made:

The ISIS hydraulic model proved to be a useful tool for evaluating a range of flood
mitigation options;
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
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It is important to evaluate both structural and non-structural flood mitigation measures at
a catchment level. However, in some cases, such as Brechin, the most sustainable and
cost effect flood mitigation measure is to implement structural measures such as flood
defences rather than non-structural measures;
Geomorphological modelling of the River South Esk has indicated that the construction
of the flood defence scheme will have a negligible effect on the long term geomorphology
of the river and a minimal effect on general scour at Brechin Bridge.
ACKNOWLEDGEMENTS
I would like to thank George Gray and Derek Davidson of Angus Council, as well as various
employees of SEPA, for their help and support whilst undertaking the various studies detailed
in this paper.
REFERENCES
1. ANGUS COUNCIL, DIRECTOR OF ROADS (2003) Flooding events of
October/November/December 2002 Report No. 115/03 23 January 2003
2. ASHFAQ, A. & WEBSTER, P. (2002) Evaluation of the FEH rainfall-runoff method for
catchments in the UK, CIWEM Journal Vol. 16, No. 3, pp223-228, 2002
3. DEFRA/ENVIRONMENT AGENCY (2004) Project FD2114: review of the impacts of
rural land use and management on flood generation. Part A: Impact study report
4. DEFRA/ENVIRONMENT AGENCY (2006) Sustainable flood and coastal erosion risk
management Final draft technical report 1 (forthcoming)
5. HR WALLINGFORD LTD (2004) Brechin flood alleviation scheme – Hydraulic
modelling and morphological modelling final report EX4924 Rev 1.0 March 2004
6. HR WALLINGFORD LTD (2003) Brechin flood alleviation scheme - Hydrological
assessment – EX4911 December 2003
7. RICHARDSON, E.V. & DAVIS, S.R. (1995) Evaluating scour at bridges, Report No.
FHWA-IP-90-017, Hydraulic Engineering Circular No. 18 (HEC-18), Third Edition,
Office of Technology Applications, HTA-22, Federal Highways Administration, U.S.
Department of Transportation, Washington, D.C., U.S.A., November, pp204
8. SCOTTISH EXECUTIVE NATIONAL TECHNICAL ADVISORY GROUP ON
FLOODING ISSUES (2004) Sustainable flood management sub-group report 23
http://www.scotland.gov.uk/Topics/Environment/Water/Flooding/16919/15683 (accessed
2 March 2006)
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