SNH Commissioned Report: Historical Survey of the River … · COMMISSIONED REPORT Summary....

C O M M I S S I O N E D R E P O R T

For further information on this report please contact:

East Highland Area Office

Scottish Natural Heritage

Fodderty Way, Dingwall Business Park

Dingwall IV15 9XB

Tel: 01349 865333

This report should be quoted as:

Mountain Environments (2000). Historical Survey of the River Conon.

Scottish Natural Heritage Commissioned Report F00PA40.

This report or any part of it should not be reproduced without the permission of Scottish Natural Heritage

which will not be unreasonably withheld. The views expressed by the author(s) of this report should not be

taken as the views and policies of Scottish Natural Heritage.

© Scottish Natural Heritage 2002.

Historical Survey of the

River Conon

Report No. F00PA40

Historical Survey of the

River Conon

Report No. F00PA40

Abstract

Keywords: Conon, floodplain habitats, hydro-schemes, floodbanks, hydrology, floods

i) The floodplain of the River Conon is a residual alluvial woodland site, which has been

incorporated into the Wet Woods LIFE Project run by the Caledonian Partnership. The river

catchment has undergone numerous artificial modifications affecting the naturalness of the

river and its floodplain. These modifications include the construction of upstream reservoirs

for hydro-power generation, floodbanks to prevent flooding of the riparian areas and

drainage of the floodplain and surrounding hills for improved arable farming, grazing and

commercial forestry.

ii) Flow data from the River Conon catchment were collated to describe any hydrological

changes which occurred over the period of record. Since 1829 flood events causing

inundation of the Conon floodplain occurred approximately once every 10 years both prior to

and following the construction of the hydroelectric scheme. This suggests that the

hydroelectric scheme has not altered the frequency of the higher magnitude floods however

the increase in flood frequency in the 1980s and 1990s is not evident in the flood chronology

and could be attributable to the hydroelectric scheme.

iii) The flood embankments confine the mean annual flood on the Conon however the largest

flood events still inundate the floodplain. If the flood banks were removed then the Conon

floodplain would be subjected to more frequent flooding.

iv) Changes in the habitat mosaic below Dunglass Island are dominated by the expansion of

woodland closely related to the river, either in strips along the river bank or on the stable

sediment deposits forming islands.

v) From the information collected in this study it is clear that there are likely to have been

several significant changes in the hydrological regime of the River Conon and the adjacent

floodplain. These changes include:

C O M M I S S I O N E D R E P O R T

Summary

Historical Survey of the River Conon

• The Blackwater total flow is significantly reduced due to water transfers, this will affect the

river habitat at Contin Island which will be drier throughout the year;

• The Conon total flow at Moy Bridge is likely to be enhanced by the water transfer from the

Orrin, this will increase the wetness of the habitats between Moy Bridge and the Orrin

confluence;

• The Conon and Blackwater summer flood flows are reduced, this will result in the river and

river-estuary habitats being drier in summer;

• The major winter floods on the Conon and Blackwater are unchanged and so the resulting

inundation of all habitats will be unaffected;

• The medium flood events on the Blackwater are unchanged and so the inundation of Contin

Island will be unaffected;

• The medium events on the Conon are attenuated with a reduced peak flow hence all river

habitats below Moy Bridge will be drier.

Acknowledgements

Mountain Environments would like to thank the landowners for their help during the study and for

permission to work on their land. Similarly we would like to thank the hydrologists in the SEPA

Dingwall office for their comments and the data on flood levels and river flows. We would also like to

thank Suzanne Hay for her assistance in the production of the maps and illustrations.

All copied maps reproduced with permission from the Ordnance Survey under copyright licence

reference AL 100033117.

For further information on this project contact:

East Highland Area Office, Scottish Natural Heritage

Fodderty Way, Dingwall Business Park, Dingwall IV15 9XB. Tel: 01349 865333

For further information on the SNH Research & Technical Support Programme contact:

The Co-ordination Group, Advisory Services, 2 Anderson Place, Edinburgh. Tel: 0131 446 2400


Contents

1. Background to the study 1

2. Catchment characteristics 2

2.1 General character 2

2.2 Character of the upper catchments 4

2.2.1 Orrin and Meig 4

2.2.2 Bran and Blackwater 5

2.3 Character of the floodplain corridor 6

3. Developments within the catchment 12

3.1 Deforestation 12

3.2 Hydro-electric scheme 12

3.3 Floodbanks and drainage of the floodplain 14

4. Hydrological assessment 16

4.1 Historical flood chronology of the Conon 16

4.2 Hydrological characteristics of the Conon: post Hydroelectric Scheme 24

4.3 Flood regime of the Conon: post flood embankment construction 27

4.4 Effects of the Hydroelectric Scheme and flood embankments: summary 28

5. Changes in floodplain habitats 30

5.1 Floodplain habitat mosaic 30

5.2 Changes in the hydrology of the floodplain habitats 31

6. Future management options 35

References 37

Appendix 1: Channel changes at the Conon-Blackwater and

Conon-Orrin confluences 39

Map 1 Conon Floodplain Survey, Moy Island – River Channel Present 40

Map 2 Conon Floodplain Survey, Moy Island – River Channel 1873 41

Map 3 Conon Floodplain Survey, Moy Island – River Channel 1904 42

Map 4 Conon Floodplain Survey, Conon-Orrin Confluence –

River Channel Present 43


River Channel 1873 44



Map 7 Conon Floodplain Survey, Dunglass Island –

River Channel Present 46






Appendix 2: Results of the floodbank survey 49

Figure A.1 Floodbank assessment: Clashuile to Moy Island 49

Figure A.2 Floodbank assessment: Moy Island to Dunglass Island 50

Appendix 3: Changes in floodplain habitat mosaics below

Dunglass Island 51

Habitat Mosaics 1946 52

Habitat Mosaics 1977 53

Figures

Figure 2.1 Map of Scotland showing the location of the River Conon catchment 2

Figure 2.2 Map of the River Conon catchment 3

Figure 2.3 Long profile of Strathconon along the Meig and Conon 4

Figure 2.4 Cross section of the Meig catchment at Glenmeannie 5

Figure 2.5 Cross section of the Conon catchment at Moy Bridge including the

Orrin catchment to the south 7

Figure 2.6 Cross section of the Conon catchment at Dunglass Island 8

Figure 2.7 Locations of the surveyed cross sections 9

Figure 2.8 Cross section of the Conon floodplain at Fairburn 9

Figure 2.9 Cross section of the Conon floodplain at Moy Bridge 10

Figure 2.10 Cross section of the Conon floodplain below Moy Island 10

Figure 3.1 Sketch map of the Conon hydro schemes 13

Figure 3.2 Assessment criteria and checklist for the flood bank survey 15

Figure 4.1 Conon catchment monthly average runoff 25

Figure 4.2 Flow regimes from the Conon at Moy Bridge and Meig at Glenmeannie 26

Figure 4.3 Predicted flood inundation with floodbanks present 27

Figure 4.4 Predicted flood inundation with no floodbanks 28

Tables

Table 4.1 Annual maximum flood series on the Conon at Moy Bridge 21

Table 4.2 Historical flood chronology for the River Conon 23

Table 4.3 River flow gauging stations in the Conon catchment 24

Table 6.1 Summary of the likely impacts on floodplain habitats 33


1

1. Background to the study

The floodplain of the River Conon is a residual alluvial woodland site, which has been incorporated

into the Wet Woods LIFE Project run by the Caledonian Partnership. Despite the River Conon being

a candidate Special Area of Conservation it has undergone numerous artificial modifications

affecting the naturalness of the river and its floodplain. These modifications include the construction

of upstream reservoirs for hydro-power generation, bunds to prevent flooding of the riparian areas

and drainage of the floodplain and surrounding hills for improved arable farming, grazing and

commercial forestry.

The extensive modifications to the river channel and the surrounding lands have affected the natural

vegetation communities and altered the hydrological regime of the River Conon. Many of the

floodplain habitats have been affected with the natural vegetation either removed during the

expansion of enclosed farmland or replaced by non-native species. Changes in the flood regime of

the river have had a direct impact on floodplain habitats with the frequency and duration of flooding

reduced by the hydro-schemes and bund systems. In addition, modifications to the drainage of the

surrounding hillslopes have affected the lateral flow of water into the floodplain and drainage has

been greatly modified to compensate for the construction of the floodbanks.

The overall vision of the River Conon Wet Woods LIFE Project is to understand the context in which

the floodplain woods originally developed and to use this information to restore, as far as possible,

the natural hydrology of the river system and hence the woodlands themselves. The Project aims to

achieve this by gathering information about the historical changes in land use and river engineering

and interpreting this information to develop a strategy for restoring the floodplain habitats. In May

2000 Scottish Natural Heritage, on behalf of the Project, commissioned Mountain Environments to

carry out an historical survey of the River Conon. The specific objectives of the survey were to:

1. Provide an overall description of the present state and evolution of the riparian habitat

mosaics and river dynamics of the River Conon and its floodplain;

2. Provide a detailed assessment of the historical trends of the geographical expansion or

reduction of the riparian woodlands.

The survey was carried out during June and July 2000; this report covers the results of that survey.


2

2. Catchment characteristics

2.1 General character

The River Conon catchment is located in the Northern Highlands of Scotland (Figs.2.1 and 2.2) and

could be described as a typical Scottish highland glen with rugged mountains in the headwater areas

and glaciated valleys containing active gravel bed rivers feeding into a lowland floodplain. The long

profile of Strathconon along the rivers Meig and Conon (Fig.2.3) indicates three contrasting sections,

the steep upper section down to Loch Beannacharain, the moderate gradient of the middle section with

the steep fall below Loch Meig and the gentle gradient from Torr Achilty down to the estuary.

The bedrock geology of the upper catchments comprises granite and schist with the lower

catchment being mostly sandstone. This contrast of hard and soft rocks is reflected in the general

topography of the catchment with the high, steep sloped mountains in the west contrasting with the

low, gentle hills and floodplain in the east. The topography and landforms of the catchment have

also been modified by past glaciations. In the core mountain area the valleys are straight and deeply

incised with typical glacial features such as hanging valleys, truncated spurs and moraines. In the

lower catchment there are extensive deposition features on the valley sides and along the coastal belt.

The climate of the catchment is greatly modified by the mountain core with high precipitation and

cold temperatures in the west and lower precipitation and warmer temperatures in the east. Due to

the high rainfall, steep slopes and thin soils in the mountains, the headwater catchments have a

rapid surface runoff with the burns and rivers being very responsive to rainfall. Base-flows are

generally low because of both the hard rocks and thin soils which reduce the sub-surface storage

capacity. River flows in winter and spring are enhanced by snowmelt which affects both flood flows

and base-flows. The rivers in the lowland floodplain are dependent on sources in the mountain core

Figure 2.1 Map of Scotland showing the location of the River Conon catchment


3

with relatively little input from lower valley sources due to the low rainfall and low topography.

Eight artificial reservoirs exist in the upper areas of the catchment (Section 3.2) which significantly

modify the downstream river flows (Section 4.2).

The natural land cover of the catchment would originally have been woodland, grassland and marsh

on the floodplain, extensive deer forest on the valley sides up to an altitude of some 450m with

heaths and moorland at the higher altitudes (Scottish Natural Heritage, undated). The importance of

the deer forests in the region is reflected in the original forest names still retained on contemporary

maps e.g. Strathconon Forest, Glencarron and Glenuig Forest, Cabaan Forest, and Strathgarve

Forest. Over past centuries much of the natural vegetation has been lost as a consequence of

agricultural development and the demand for fuel. The present day ground cover includes moorland

and bare-rock at high altitudes, small areas of non-native commercial forests and rough grazing at

medium altitudes, and improved pasture and arable land in the lower valley and floodplain.

There are no major towns in the valley although there are many small rural settlements in the upper

catchment, such as Strathconon, and several larger semi-urban settlements in the lower catchment,

such as Conon Bridge. Most of the settlements now have new houses reflecting the modern

demands for accommodation and the popularity of the area in residential terms. The valley of the

Conon is also the eastern end of two major communication routes joining the west and east coasts

of Scotland. A major road and railway link the coasts using Strathbran (the Bran catchment) while a

road uses the higher altitude route in upper Strathgarve (the Blackwater catchment).

Figure 2.2 Sketch map of the River Conon catchment


4

2.2 Character of the upper catchments

The headwater areas of the Conon catchment can be divided into four sub-catchments: the deep

and rugged southern catchments of the Orrin and Meig and the more rounded northern catchments

of the Bran and Blackwater (Fig.2.2).

2.2.1 Orrin and Meig

The two southern catchments, Orrin and Meig, are similar in physical character being deep, narrow

glaciated valleys with steep, rocky slopes. A cross section of the Meig (Fig.2.4) shows the

topography to be steep sided with a narrow valley floor. The two catchments contrast in the extent of

human influence, the Orrin has no settlements in its upper parts but contains a major hydro-power

reservoir while the Meig contains a small loch, two relatively small reservoirs, numerous farms,

houses and estate buildings.

The upper areas of both catchments have thin soils and extensive bare rock on the hillslopes with

small areas of peat in ridge top hollows. This type of ground cover combines with the high rainfall

(>2000mm) in these headwater areas to give rapid surface runoff and highly responsive burns. The

main human activities in these areas are stalking and hillwalking although neither will significantly

affect the runoff. Some hill roads have been constructed to give easier access into the mountains

and small patches of native woodland exist, mainly in gullies.

Within the relatively sheltered glens the conditions are more suitable for agricultural developments

and commercial forestry. Most of the lower ground in the Meig is enclosed with the fields used for

winter grazing and summer silage production. On the lower slopes, above the floodplain of the Meig,

there has been some planting of non-native trees for commercial forestry. A public road exists in the

Meig enabling access into the core mountain area. The upper Orrin shows a significant contrast to

the developed Meig with no enclosed ground and no roads or houses.

Figure 2.3 Long profile of Strathconon along the rivers Meig and Conon


5

On the floodplain of the Meig there is some artificial drainage, mainly in the form of a single large

open channel at the base of the valley side while on the lower slopes there has been minor drainage

associated with the forest plantations. This drainage in the Meig is unlikely to have affected the river

flows lower down the catchment.

Both the Orrin and Meig have high sediment loads, including fine material transported in suspension

and coarse sediment transported as bedload. There are extensive depositional features in the river

channels, which are likely to be highly dynamic due to the responsive nature of the rivers and the

large amount of material available from the numerous source areas. The sediment in both rivers

would have originally been transported down to the floodplain however with the construction of the

hydro-electric dams, a high proportion of the load will now be deposited in the reservoirs.

2.2.2 Bran and Blackwater

The two northern catchments, Bran and Blackwater, are similar in character being broad valleys with

relatively gentle slopes and extensive soil covers. In contrast to the southern catchments the Bran

and Blackwater have a lower relief with deeper soils and less areas of exposed rock. Although they

have approximately the same precipitation as the southern catchments the surface runoff will be

less rapid and the river flow regimes less responsive because of the additional soil storages. The

land cover in the northern catchments is mostly rough grazing and moorland with several patches of

plantation forestry. It is unlikely that the small areas of forestry have any significant influence on the

river flow regimes even though the trees extend onto the floodplains and in some cases to the

edges of the river channels. Below the confluence of the Bran and Blackwater the valley narrows

with the river tumbling down a steep rocky channel, the Rogie Falls, before reaching the Conon

floodplain.

Figure 2.4 Cross section of the Meig catchment at Glenmeannie


6

The most significant influence on the flows in the burns and rivers is the infra-structure related to the

hydro schemes (Section 3.2). There are extensive aqueducts and tunnels in both catchments

which intercept many burns and divert their flows into the hydro-system. A proportion of the output

from Loch Vaich is released as compensation flow for the upper Blackwater while the total output

from Loch Glascarnoch is diverted into the Bran for hydropower generation. The river channel below

Loch Glascarnoch is dry during summer conditions but spill-waters from the reservoir generate river

flow during the winter. The coarse sediments in the Blackwater above Garve reflect the influence of

the hydro-schemes in terms of flood flows, sediment storage and mobility of the channel bed. The

lack of pebble size material in the channel indicates the trapping of sediments in the reservoirs and,

unlike the Orrin and Meig, there is a lack of sediment input from tributaries to compensate for the

reservoir storage. The growth of vegetation on unconsolidated sediment deposits in the channel,

again unlike the Orrin and Meig, indicates the lack of mobility during the spring and summer

probably due to the reservoirs altering the flood flow regimes in these seasons (Section 4.2).

2.3 Character of the floodplain corridor

The floodplain corridor down to Conon Bridge comprises the broad floodplain, the valley sides and the

two major river confluences. The floodplain is used for intensive arable farming growing mainly cereal

crops and also for grazing of dairy cattle. Flood banks were constructed in the 19th century to protect

this agricultural land (Section 3.3). There are several roads running along the valley but only two

crossing the river, one in the upstream part of the floodplain at Moy and the other at the downstream

end at Conon Bridge. There are several villages including Marybank, Contin Bridge, Maryburgh and

Conon Bridge and many individual houses. The fisheries interests on the river are highly acclaimed

and very important to the local economy. The river channel and banks are maintained by the

landowners and structures such as croys and weirs have been built to create sediment spawning

banks and deep-water pools. It is understood that the weirs at Dunglass Island were originally built to

trap migrating fish, which were then removed using nets; this is no longer practiced.

The cross section of the valley at Moy Bridge (Fig.2.5) shows the wide flood plain at this point and

the steep gradient of the northern valley side in contrast to the gentle gradient on the southern side

leading over into the Orrin catchment. Fig.2.5 also shows the steeper gradient of the southern valley

side of the Orrin and also the contrast in elevation, some 40m, between the Conon at Moy Bridge

and the Orrin due south of Moy. Unusually the floodplain corridor becomes slightly more constricted

down the valley with the floodplain below Dunglass Island being some 450m wide and the north and

south valley sides both moderately steep (Fig.2.6).

The floodplain of the Conon has been modified over geological time by a complex system of

geomorphic processes. The influence of human activities only covers the past two centuries and has

only made superficial changes to the natural structure of the floodplain. The complexity of the

system is due to the main stem of Strathconon being joined in the floodplain area by the Blackwater

and Orrin tributary catchments. The natural system, before the construction of the reservoirs and

flood banks, would have been the River Meig/Conon as the major artery feeding a highly responsive

gravel bed river into the piedmont zone. The river would have rapidly dispersed its energy, forming a

meandering channel with frequent flood flows overtopping the channel banks. Associated with

the loss of energy would have been the deposition of large quantities of the river’s sediment load in

the channel and over the floodplain. Siltation within the channel and over the floodplain adjacent to

the channel would have encouraged frequent channel switching leaving remnants of the previous

channels on the floodplain.


7

Figure 2.5 Cross section of the Conon catchment at Moy Bridge including the Orrin catchment to

the south

As part of the study a series of micro-topographical surveys were undertaken across the Conon

floodplain to understand its structure and to develop a floodplain hydraulic model (Section 4.3). Four

cross sections were surveyed taking readings of the surface elevation at 10m intervals and to a

height discrimination of 1mm. The locations of the four cross sections are shown in Fig.2.7. Cross

sections 2 at Fairburn (Fig.2.8), 3 at Moy Bridge (Fig.2.9) and 4 below Moy Island (Fig.2.10) all show

similar micro-topographical features with a surface gradient sloping away from the river to the edge

of the floodplain. The sections at Fairburn and the right (looking downstream) floodplain below Moy

Island both had a decrease in elevation of some 1m while at Moy Bridge the decrease was some

2m. This is due to historical floods overtopping the river channel and spilling their high sediment

loads laterally across the floodplain. The highest deposition would have been adjacent to the river

channel building up this ground more than away from the channel. At the two downstream sections

the elevation of the water surface at the time of the survey (dry weather levels) were similar to the

ground surface elevations at the edges of the floodplain. At the upstream section the water level

was substantially below the ground surface elevation. Therefore it appears that the channel at Moy

Bridge and Moy Island is becoming unstable due to the agradation of sediment in the channel.

The three floodplain cross sections also show features super-imposed on the general slope away

from the river. These are depositional features, asymmetrical in shape with the steeper slope on the

river side and a gentler gradient “tail” on the landward side. On the section below Moy Island these

features were found on both the left and right floodplains and on both sides the steeper slope was

on the river side. These features were again formed by the sediment ladened floodwaters spilling

out of the channels and forming dune type of features in different flood conditions

The floodplain cross sections therefore contributed to the understanding of the development of the

floodplain. It is recommended that additional cross sectional surveys are carried out further

downriver in a future study.


8

The two major river confluences affect the flow down the Conon, the channel form and the development

of the floodplain. As was previously mentioned, the flow regime of the Orrin, Meig and the Blackwater are

contrasting because of the runoff conditions in the headwater catchment areas and the storage within

the upstream reservoirs. At the confluences of the Conon-Blackwater and the Conon-Orrin the major

hydrological impacts will be when the flood peaks coincide. This is likely to produce the largest floods

downstream of the confluences and lead to the greatest inundation of the floodplain. It is difficult to

estimate the frequency of these occurrences because of the lack of river flow data below the Torr

Achilty reservoir on the Conon and on the River Orrin. It is therefore recommended that data from the

Scottish Environment Protection Agency (SEPA) level only station below Torr Achilty and the gauging

station at Contin Bridge are analysed to characterise the flood regimes and to determine the frequency

of coincident flood peaks at the Moy Bridge gauging station. It is also recommended that a level only

station is installed on the lower Orrin to similarly analyse flood peaks below the Conon-Orrin confluence.

Within the River Conon channel there are two major sediment deposits (islands) downstream of river

confluences which are now stabilised by vegetation. Moy Island downstream of the Conon-Blackwater

confluence and Dunglass Island downstream of the Conon-Orrin confluence are both typical of the

lower reaches of a gravel bed river where a tributary joins the main river and deposits its coarse

sediment load. This can be seen in many other gravel bed rivers throughout Scotland. In addition to

forming island habitats they also act as controls on the upstream water levels by forming a barrier or

“leaky plug” in the river. The location of these islands is usually on the reach immediately downstream

of a confluence, e.g. Moy Island. Dunglass Island is however some distance downstream of the Conon-

Orrin confluence and so does not appear to conform to the usual pattern. The reason is due to the

historical shift upstream of the confluence leaving behind the associated island. Historical 1:10000

maps of the area were obtained from the National Library for 1876 and 1904 and more recent maps,

dated 1971, supplied by SNH. Details of the river channels and deposition features were taken from

the historical maps and plotted on the contemporary maps (Appendix 1). It can be seen from the

diagrams in Appendix 1 that very little has changed since 1876 hence it can be assumed that the major

channel switching occurred prior to this date.

Figure 2.6 Cross section of the Conon catchment at Dunglass Island


9

Figure 2.8 Cross section of the Conon floodplain at Fairburn

Figure 2.7 Locations of the surveyed cross sections


10

Figure 2.9 Cross section of the Conon floodplain at Moy Bridge

Figure 2.10 Cross section of the Conon floodplain below Moy Island


11

The historical maps also show that the channel of the River Orrin turns abruptly left just as the Orrin

enters the floodplain of the Conon (Appendix 1). An extrapolation of the line of the Orrin upstream of

this break would put its confluence just upstream of Dunglass Island, i.e. where it could have been

more expected. The River Orrin is naturally a moderately steep gravel bed river which is abruptly

changed when it meets the floodplain of the River Conon. The elevation of the Orrin channel relative

to the River Conon channel was previously shown to be some 40m higher (Fig.2.5) hence the

gradient of the Orrin down to the confluence with the Conon will be much greater than the Conon.

Large amounts of sediment will therefore be transported down the steeper Orrin and, throughout

historical times, will have been deposited and dispersed over the floodplain of the Conon. This has

formed a typical cone or fan shaped feature over which the river would have flowed. This is a

feature which is seen in other Scottish gravel bed rivers but in the case of the Orrin the river has

built up such a large cone of sediment that it has become unstable and switched course to the left

edge of the cone, moving the confluence with the Conon upstream of its previous position near

Dunglass Island.

The effect of the Orrin fan on the floodplain off the Conon will have been considerable. A textbook

example of a floodplain would show the topography widening out downstream. In the case of the

Conon the floodplain is restricted by the Orrin fan to the land around the Conon-Blackwater

confluence and then glacial deposits in the coastal area confine the river and restrict the floodplain

to a narrow strip down to Conon Bridge and Maryburgh.


12

3. Developments within the catchment

3.1 Deforestation

Removal of the native forests is likely to have altered the hydrology of the upper catchments which

could have changed river flows and flooding of the lowland floodplain. The native forests would have

been open structures creating efficient interceptors of rain, snow and mist. The potential evaporation

in these exposed areas would have been significant and so the likely evaporation losses would have

been greater than at present with the moorland ground cover. The effect on river flows down the

catchment would have been a reduced volume of runoff, particularly in the summer season. Storm

runoff and flood flows in the early autumn period could also have been reduced as the drier soils

wetted up after a dry summer period however it is unlikely that runoff from winter rainfall events has

changed significantly. Winter snow storage and snow melt could have been affected by the loss of

forest cover with the trees protecting the snow pack until a major warm front crossed the area

causing rapid snow melt, enhancing the downstream flood.

The other possible effect of removal of the forest is in accelerated soil erosion. The original tree

canopy would have provided protection from rainfall impact and the roots would have helped to bind

the soils together. Removal of the tree cover would have accelerated the erosion processes and

also reduced the soil water storage. This could have caused more rapid runoff and greater

downstream flood peaks.

The combined downstream effects of the deforestation in the headwater areas are therefore likely to

be complex and it is difficult to quantify the effects especially when other possible scenarios, such

as changing snowfall patterns or seasonality of flooding, are also considered.

3.2 Hydro-electric scheme

The upper Conon hydroelectric scheme involves a large and complex system of water transfers,

storage, control and power generation (Payne, 1988). The scheme is likely to exert a considerable

effect on the downstream floodplain.

The Conon scheme includes eight dams, nine tunnels, numerous diversions and six power stations

with a total generating capacity of over 100mW (Fig.3.1). It also includes an unusual cascade

development with the water, which passes through the Mossford, Grundie Bridge and Achanalt

power stations channelled into Loch Luichart, through the Luichart power station into Loch

Achonachie and through the Torr Achilty power station. Torr Achilty receives all of the water involved

in the Conon scheme.

Construction of the Fannich part of the scheme was carried out in the period 1946 to 1951. It involved

the construction of numerous aqueducts and tunnels to increase the flow into Loch Fannich, the

raising of Loch Fannich and the transfer of water to the 24mW Grundie Bridge power station. The

construction to transfer water from the loch involved a complex sloping tunnel driven from the

hillside above the Grundie Bridge power station some 30m below the Loch. Above the power station

the water is conveyed by a single steel pipe which branches to feed two turbines in the power

station.


13

The Glascarnoch-Luichart-Torr Achilty part of the scheme was constructed in the period 1951 to 1957.

It involved the construction of two dams, the Glascarnoch, a concrete gravity and earthfill dam, and the

Vaich, an earthfill with concrete core dam. This created two artificial lochs whose natural catchments

were supplemented by water transfers and in turn the water from Loch Vaich was transferred into Loch

Glascarnoch. An 8km long tunnel then took the water to the 24mW Mossford power station on Loch

Luichart. A small barrage was built on the River Bran at the eastern end of Loch Achanalt and the

water passed through the 2.4mW Achanalt power station. Water from the three power stations,

Grundie Bridge, Mossford and Achanalt, was discharged into Loch Luichart, which was created by the

construction of a mass gravity dam. A further artificial loch was created by the construction of the

concrete gravity, buttress and earthfill Meig dam and water was transferred from Loch Meig into Loch

Luichart. Below Loch Luichart the water was passed through the 34mW Luichart power station. Finally

a mass gravity dam and 15mW power station were constructed on the Conon at Torr Achilty creating

Loch Achonachie. The principal function of the Torr Achilty dam was to even out the flow of water

downstream, and as such is the only dam fitted with floodgates.

Figure 3.1 Sketch map of the Conon hydro scheme


14

Construction of the Orrin part of the scheme was carried out in the period 1955 to 1961. It involved

the construction of a mass gravity dam some 15km upstream of the Conon-Orrin confluence. The

dam wall was some 300m long and created an artificial loch 8km in length. Water from the loch is

transferred through a 5km long concrete lined tunnel and steel pipeline to the 18mW Orrin power

station and discharged into Loch Achonachie. The Orrin dam also included four fish passes which

were also incorporated into the Torr Achilty, Luichart, Meig and Achanalt constructions. It is said

(Payne, 1988) that some of these fish passes have extended the migratory range of salmon in the

Conon catchment.

3.3 Flood banks and drainage of the flood plain

The system of flood banks throughout the catchment includes small-localised protection in the upper

catchments, a short length between Loch Meig and Loch Achonachie, an extensive length on the

Conon below Torr Achilty and on the Blackwater below Contin. The floodbanks were constructed in

1836 to protect Comrie farm but were raised and extended in 1856 to protect the Arcan Flats

(Fairburn Estates pers. comm., North of Scotland Hydro-electric Board, 1989). The banks are still in

generally good condition with a covering of vegetation, which helps to protect and stabilise them.

As part of this project a survey of the flood banks was carried out to gather information about their

condition and to map any potential problem sites. The survey was carried out by walking down the

banks and a check list, in the form of a simple flow chart (Fig.3.2), used to assess their condition.

The result of the survey (Appendix 2) shows that the floodbanks are in good condition with no

breaches and no places in need of repair. All the floodbanks along the south-side of the Conon

(Arcan Flats) and along Dunglas Island were given a Grade A classification. Only the banks

protecting the land upstream of the Conon-Blackwater confluence were given a Grade B, reflecting

their less stable condition in places.

As a consequence of the construction of the flood banks the natural drainage of the floodplain would

have been affected. Although there are few lateral burns flowing off the valley sides there is

significant seepage of water through the sandy soils and into the floodplain. The flood banks

effectively prevent drainage of this water into the river and so a deep drain was constructed in 1836

through the right flood plain from below Wester Balloan, near Marybank, parallel to the main river

and down to join the Conon just below the Orrin confluence. It is mostly a surface drain but as it

approaches the River Orrin and the cone of sediment, it enters a pipe, which takes the water under

the Orrin and into the Conon. This drain was crossed by two floodplain cross sections at Moy Bridge

and below Moy Island (Figs. 2.9 and 2.10). These surveys showed that the drain was 1.2m deep,

which was also below the water surface in the river at both points in the low flow conditions. The

drain is reported by the local farmers to be free flowing and hence clear of any obstructions.

In other parts of the floodplain, such as the meander downstream of Moy Island and near the

Cragget Woods, small burns drain over the flood plain and into the Conon. It is reported that during

flood flows these burns are unable to enter the river and so back up causing localised flooding.


15

Figure 3.2 Assessment criteria and checklist for the flood bank survey

Is there a flood bank present

No Grade

D

Yes

Is the flood bank complete No Grade

without breaches C

Yes

Is the flood bank

established with:

(a) bank material, i.e.

consolidated or loose

No

Grade

(b) vegetation, i.e.

complete or incomplete

No

B

Yes

Grade

A


16

4. Hydrological assessment

Flow data from the River Conon catchment were collated to describe any hydrological changes

which occurred over the period of record and to relate changes to either natural hydro-climatic

variability or the effects of human influences. A number of sources were utilised to obtain

information relating to river flow and flood regimes. SEPA operates several river flow gauging

stations in the Conon catchment, data for these stations were obtained from 1976. Historical flood

and flow information prior to 1976 was available in the form of previous reports, anecdotal records

and newspaper articles; these were accessed through the Chronology of British Hydrological Event

(Law et al., 1998) and from the National Library of Scotland.

While this study is not concerned with future changes which may affect the flood patterns it should

be noted that climatic change scenarios for the northern UK, developed by the Climate Change

Impacts Research Group (1996), indicate an increased autumn rainfall of up to 15-25% by the

2050s (Arnell, 1996). This suggests increasing flood flows in Scotland and there is already some

evidence of changes with exceptionally high winter flows during the 1980s and 1990s on some rivers

in the Highlands (Black, 1993).

4.1 Historical flood chronology of the Conon

One of the earliest records of flooding on the River Conon is for the flood of 1829. This event

received most attention for the flood on the River Spey (Robert H Cuthbertson & Partners, 1990).

Analysis of the flood, estimated the return period for the River Spey to be between 500 and 1000

years (Werrity and Acreman, 1985). For the Conon, the flood of 3rd-4th August 1829 is noted as:

‘On the estate of Moy, 23 houses were totally destroyed, and as many families

obliged to bivouac in the fields. One farm of 140 annual rent was completely ruined,

the river running through the centre of it’ (Anon, 1829).

Flood embankments were constructed on the Conon in 1836 to protect Comrie Farm (Fairburn

Estates pers. comm., North of Scotland Hydro-electric Board, 1989). The embankments were no

doubt constructed in response to the 1829 flood and also due to the demands for utilising the Conon

floodplain for agriculture purposes.

Following the establishment of the flood embankments major flooding occurred across much of

Scotland on the 29th September 1852. Reports of the flood noted:

‘The North of Scotland suffered much from the same storm, and its accompaniment

of heavy rain. The rivers and brooks were greatly flooded; bridges, building and farm-

stock were swept away’ (Anon, 1853).

Whether the flood embankments were successful in protecting the floodplain from inundation is

uncertain. However it is documented is that following the 1852 flood the Conon flood embankments

were raised in 1856 and extended downstream to protect the Arcan Flats, providing additional

protection to the agricultural lands (Fairburn Estates pers. comm., North of Scotland Hydro-electric

Board, 1989).


17

The new flood embankments were overtopped during a series of flood events between 1870 and 1900.

Floods on 26th September 1872, 1st January 1874, 20th October 1874 and 29th January 1892 caused

flood inundation of the Conon floodplain. In particular the flood of 1892 caused much damage to

buildings, livestock and crops. In turn the floods have been recorded by a rainfall observer and by

Nairne (1895).

‘…Floods in Ross-shire, the rainfall of 27th being 1.38 inches and of 28th 1.16 inches;

at Strathconon rivers ran over their banks’ – rainfall observer on the flood of

26th September 1872 (Anon, 1873).

‘Very heavy rain, all surrounding country flooded’ – rainfall observer on the flood of

1st January 1874 (Anon, 1875).

‘1.37 inches of rain, country flooded’ – rainfall observer on the flood of 20th October

1874 (Anon, 1875).

‘In Mid-Ross all the streams, rivers and lochs were swollen to a tremendous height,

and a great amount of damage was done to roads, railways and house and

agricultural property… A large portion of the Contin district was inundated by the

overflow of the Blackwater. The manse and glebe, and the churchyard, which

practically form an island on the stream, as well as several of the houses in the

village and neighbourhood, were beset with 4 or 5 feet of water, and great alarm was

felt as to the safety of the people. On Friday night, three men on horseback

attempted to over the stream by one of the small bridges in the vicinity of the island,

which were almost covered by water. The rush of the flood, however, was too strong

for the horses to keep their feet, and they stumbled into the river, and were carried

down some distance, the riders barely saving themselves by clinging on to some

shrubbery… Conon valley was also submerged by the floods to a considerable

extent, and a great amount of damage sustained. Several stockyards in the Fairburn

district were swept away, and a flock of sheep was carried by the force of the river

down as far as Dingwall, where the most of them were rescued by the fishermen –

Nairne on the flood of 29th January 1892 (Nairne, 1895).

Nairne’s documentary of the 1892 flood provides some detailed description of the role of flood

embankments in the Conon floodplain. He explains the role of the newly built railway embankment in

protecting the village of Conon, where in previous floods, probably either 1872 or 1874, the village

was flooded:

‘At Conon, the lower part of the village was covered with 4 or 5 feet of water…Conon

village, in former times, used to be frequently flooded, but since the railway was

made it has been protected by the Conon embankment between the station and the

bridge. About 25 years ago, a house on the south side of the embankment was

washed away, but the village was fully protected by the bank.’


18

The description of the 1892 flood continues with an explanation of how the village of Conon was flooded:

‘The east side of the river from the bridge downwards belongs to Seaforth, as the

original course passed further east than it now does, and the farm of Islandmore on

the Conon side therefore belongs to the Brahan estate and the parish of Fodderty.

Along this side, there was an embankment, and further down, a sluice to keep back

the sea from an older channel. When the flood was at its height it was also a spring

tide at high water, and the sluice broke, with the result that the sea rushed up and

considerably increased the danger to life and property. The result was that the bank

gave way immediately below the road bridge, and the water flowed along the Conon

Hotel, and, as we have said, flooded the lower part of the village, to a depth of over

four feet. It entered every house below the Post Office except two, and caused great

consternation…A considerable amount of water also oozed through the railway bank,

but the rear of the houses on that side of the village was comparatively dry, and three

cows and two horses were taken through a dwelling house from the flooded street to

the higher ground in rear. The people were taken very much by surprise, as they did

not expect the flood to come from below the bridge, and they did not, therefore, take

all the precautions they might.’

Finally, Nairne described the role of the flood around Dunglass Island, where the majority of the

floodwaters switched from the Conon channel to the Dunglass channel for a time:

‘Port of Dunglass farm, about 100 acres, consists of Dunglass Island in the river and

the embankment here broke, with the result that over twenty acres was covered with

a thick layer of gravel that renders it unfit for further tillage. The Conon channel used

to be the larger of the two but a gravel bank was thrown across above the Islands,

and the greatest part of the river, for a time flowed through the Dunglass channel.

The diversion of the river caused enormous damage to salmon ova, as the breeding

banks were left dry, and something like a million ova practically became useless.’

Major flooding throughout the north of Scotland occurred in 1903, only 11 years following the 1892

flood on the Conon. Many rivers in the northern Highlands experienced major flooding on the

30th March 1903 following prolonged heavy rainfall. Again, reports on the flooding prove

inconclusive as to whether the Conon floodplain suffered inundation. Newspaper reports state:

‘The heavy rains last week, and particularly Friday, the strong westerly gale, have

caused much flooding and damage in the north of Scotland, rivers have been swollen

to a height seldom known, but though the volume of water in all the rivers is almost

as large as in the floods eleven years ago (1892), the destructive effects, so far as

has yet been learned, have not been anything like so great.’ (Anon, 1903).

Although flooding was experienced on several occasions elsewhere in northern Scotland over the

next thirty years, 1932 was the next documented flood event for the Conon. On January 19th 1932,

heavy rainfall resulted in flooding in the Strathconon area and in particular the Conon floodplain. The

Inverness Courier noted:


19

‘A storm of wind, accompanied by torrential rain, visited the district on Thursday, and

continued until the weekend. All the low-lying ground was flooded, and on Thursday

evening the flooding blocked the road between Muir of Ord and Strathconon, and cars

and travellers had to return to the Muir for the night. Marybank has also felt the brunt of

the storm, some houses flooded, and the road covered with water.’ (Anon, 1932).

In Scotland at this time, much debate was centred on the possibility of introducing hydropower

schemes to the Highlands. One such scheme that was suggested for development was in the Conon

catchment. In a paper by Newlands (1908), he attempts to describe the hydrological characteristics of

the Conon catchment and how the catchment would be suited for hydropower development. He states:

‘In the absence of information regarding rainfall it is sometimes possible to arrive at the

discharge from a catchment area, by observation and measurement of the flow of the

river draining that area. This, like a study of rainfall, requires long-continued

observation, and reliable figures are not easily got. I have however, been able to get

records of the flow of certain rivers in this country, and have reduced them to a

constant discharge in cubic feet per minute per square mile of catchment area…The

recorded flow of the Conon is probably characteristic of that from most of our highland

rivers, but for the purpose of this paper is must be pointed out that the ratio of the

ordinary discharge of the Conon is to its flood discharge as 1 is to 27.2, showing that a

very much increased ordinary flow could be got if the flood water, or part of it, were

impounded and brought under control, and this again applies to all out northern rivers.’

In Newlands’ paper he does not present how the figures were derived for the River Conon and it is

difficult to compare these figures with contemporary data due to the effects of the additional

catchwaters since Newlands’ work. However the present ratio of mean flow to the mean annual flood is

approximately 1 to 7.5 and the ratio of the 95th percentile flow to the mean annual flood is 1 to 38.

Newlands continues by comparing his calculations with some flow estimates performed for the Ness:

‘On the weir at Dochgarroch, Mr. Bateman, C.E., estimates the ordinary dry weather

discharge of the river at 250 cubic feet per second. During the flood of 1834 he

estimates the discharge of 29,000 cubic feet per second, and in the flood of 1849 –

which carried away the stone bridge – 52,000 cubic feet per second. His estimate of

250 cubic feet per second, as the ordinary summer discharge almost agrees with that

for the Conon, taking into account the relative size of the watersheds, and is equal to

about 20 cubic feet per minute per square mile of catchment area.’

Given Newlands’ catchment area for the Conon of 399 square miles it is possible to work out what

Newlands estimated the flood discharge to be for the Conon. Assuming Newlands earlier

understanding of the ordinary discharge was equivalent to his later ordinary summer discharge and

using his ratio of 1 to 27.2 – his flood discharge for the Conon would be approximately 102 cubic

metres per second. This estimate is significantly lower than the present mean annual flood recorded

on the Conon of 351 cubic metres per second. Although additional catchwaters have been

introduced through the hydroelectric scheme, it would seem unrealistic to associate such an

increase in the expected mean annual flood discharge to the additional catchment, indicating

Newlands’ figures to be underestimated.


20

The Conon hydroelectric scheme was constructed between 1946 and 1961. During this period major

flooding was experienced in the Conon catchment. The Inverness Courier details the flood of

23rd November 1947:

‘The stormy conditions which ushered in the thaw on Wednesday night have

continued in the north generally, heavy rain and high winds experienced in many

districts, with the resultant flooding of many roads and fields…in Ross-shire the rapid

thaw also caused widespread damage, the rivers Orrin and Pefferey overflowing their

banks in parts. Substantial losses have been sustained by flockmasters, whose

sheep were trapped by rising waters. One Ross-shire farmer, it is understood, lost as

many as thirty sheep.’ (Anon, 1947)

The flood of 1947 was clearly severe enough to cause flooding on the Orrin and resultant damage to

the adjacent lands. However, no mention was made to flooding on the Conon which suggests that

the flooding was restricted to the Orrin tributary of the Conon only. This could be explained by either

the storm and snow-melt being concentrated on the Orrin catchment, or that the construction works

and raising of Loch Fannich resulted in much of the floodwaters being stored within the Loch or that

in the nature of newspaper reporting of such incidences, only the most news-worthy events were

recorded.

Flooding did not occur in the Conon floodplain again until the 1960s when two major flood events

occurred in the space of four years. The first flood of the two occurred on February 12th 1962 when

the Conon overflowed the flood embankments resulting in widespread floodplain inundation, as

described in the Inverness Courier article at the time:

‘The most exciting and alarming incident in the north took place yesterday, when,

after being marooned for several hours on the rooftops of three lorries, seven men

were rescued by boat after the river Conon had burst its banks near Marybank, on

the Muir-of-Ord-Garve road…the river which is spanned by a bridge about 50 yards

in length at that point has swollen to almost a mile wide, and many sheep and cattle

belonging to farms on its banks were drowned…Mr. James Forbes, manager of

Comrie, near Contin, had 14 young calves drowned in their steading when the river

Conon overflowed…he added that it was the first flooding he had seen in the area for

many years.’ (Anon, 1962)

The next flood to affect the Conon occurred only four years later in 1966. By then river flow

measurement was being undertaken by the Scottish Hydro-electric Board on the River Conon at

Moy Bridge. The station was in a different location to the present Moy Bridge station and some

doubt exists as to the accuracy of the data from this site (SEPA pers. comm.). The station was

assigned a grade B in the Flood Studies Report (NERC, 1975) indicating some doubt as to the stage

to flow rating. A flood occurred on the 17th December 1966, when the river gauging station recorded

a peak flow of 1076 m3 s

-1 (Anon, 1993). This flow was not exceeded while measurements

continued at the original station and, as the annual maximum flood data from the present Moy

Bridge station show (Table 4.1), have also not been exceeded in the period 1982 to 1999.


21

Table 4.1 Annual maximum flood series on the Conon at Moy Bridge

Date Flood Peak

(m3 s

-1)

6 February 1989 703.9

2 January 1992 617.0

31 December 1983 556.4

11 March 1990 507.0

16 January 1993 491.6

6 December 1999 411.0

30 January 1982 408.2

1 January 1984 358.8

1 March 1997 351.5

27 February 1995 318.7

6 March 1994 276.8

28 December 1986 264.9

31 January 1985 239.9

17 October 1991 238.2

6 November 1996 228.4

9 October 1988 201.3

11 February 1998 190.8

21 January 1987 173.2

As Table 4.1 shows, the highest recorded flood in the period 1982 to 1999 was on the 6th February 1989.

Several reports were produced following this flood, one suggesting that the rainfall associated

with the peak flow had a 1 in 100 year return period (Law, 1989). Another more substantial report

(North of Scotland Hydro-electric Board, 1989) investigated the 1989 floods and the role of the hydro

schemes in the area. This report stated:

‘During January and the early part of February 1989, high rainfall in the north and west

of Scotland led to widespread flood damage. The monthly rainfall for January was

exceptional and in some locations the amount recorded is only expected to occur every

few hundred years. The two days of the 5th and 6th February were no less exceptional

and included the highest two day rainfall ever recorded in the UK. Most of the damage

was experienced during the storm in early February, although a few locations suffered

more in mid January.’ (North of Scotland Hydro-electric Board, 1989).

The North of Scotland Hydro-electric Board (NSHEB) report describes the flood management

operational procedures for the Conon scheme:

‘The management procedures incorporate positive measures which result in

significant reductions in peak flows and quantities of water discharged down rivers

with hydro developments; to operate main storage reservoirs with the objective of

keeping storage available so that, when heavy rain occurs, the water running off the

catchments is captured in reservoirs without spilling. In this way the Board can

provide substantial flood relief for downstream interests; to operate flood management

procedures which ensure, wherever possible, that peak river flows are less than the

corresponding peak flows would have been before the schemes were constructed; to


22

anticipate floods and then to turn those catchments, which are normally diverted from

outside the natural basin of a scheme, back into their original water courses. By this

action the amount of water flowing into a scheme is reduced during floods.’

The report also gives an indication of the frequency of inundation in the previous 100 years. This

confirmed the occurrence of the flood events in the Conon in 1892, 1932 and 1947 and clarified that

inundation occurred in 1903 and 1916 when newspaper articles proved inconclusive. It also

introduced four new events which were not previously recorded (1922, 1928, 1936 and 1950):

‘The flood banks which have proved to be most vulnerable in times of flood are the

ones built…to protect Comrie Farm and the ones…to protect the Arcan Flats. In the

February floods these banks were breached in a number of places…it has been

suggested that the frequency of flooding in the Conon valley has increased since the

Hydro Scheme was built…Historical evidence does not support this suggestion. It is

well known that the flood banks at Arcan were breached in 1892 and 1922. However in

the period of 60 years between the 1892 flood and the early 1950s, when the scheme

was being constructed, there were many other notable events and on nine occasions

the district experienced extensive flooding, viz 1892, 1903, 1916, 1922, 1928, 1932,

1936, 1947 and 1950. The amount of damage sustained in these earlier floods is

difficult to determine in detail but it is very clear that on most occasions low lying land,

particularly in the Scatwell/Comrie area and in the Arcan Flats, was extensively

inundated by either breaching or overtopping of the flood banks. Since the scheme was

brought into operation, these areas have been inundated and flood banks breached on

four occasions in a period of nearly 40 years viz, 1962, 1966, 1983 and 1989.’

The NSHEB report presents results from a model established to assess how the Conon catchment

would have responded to the flood if the river were in its natural undeveloped state. No details are

presented as to the model structure and only their conclusions are presented; however the study

was validated by an independent audit, commissioned by NSHEB. The results and general findings

of the study were:

‘It was established that the peak recorded flow of 7400 cusecs – for the Blackwater –

would have been about 34% higher without the scheme…the integrity of the flood

banks protecting Arcan Flats depends on the combined flows of the rivers Conon and

Blackwater. The results from the computer model show that the overall effect of the

scheme was beneficial and that without the scheme the combined flow would have

been 13% higher than it actually was.’

Since the flooding in 1989, one more incidence of flood plain inundation has occurred. The flood of 2nd

January 1992 was recorded by the Moy Bridge river-gauging station; the peak flow being recorded at

617 m3 s

-1. This flood is the second highest for the period 1982 to 1999, second only to the 1989 flood.

The history of flooding of the River Conon is summarised in Table 4.2. The table plots the floods in

chronological order, listing the year of occurrence (column C), source of flood record (column D),

highlighting when the flood banks were constructed (column E), when the hydroelectric scheme was

introduced (column F) and highlighting the period of SEPA flood record (column G).


23

Table 4.2 Historical flood chronology for the River Conon

A B C D E F G

Date of Conon Flood Source of Flood Record Flood Hydro SEPA

Banks Scheme Record

1800

1810

1820

Aug 1829 Inverness Courier

1830

Comrie

1840

1850 Sep 1852 Annual Register

Arcan

1860

1870 Sep 1872 British Rainfall

Jan 1874, Oct 1874 British Rainfall

1880

1890 Jan 1892 Nairne, NSHEB

1900 Mar 1903 Inverness Courier, NSHEB

1910

1916 NSHEB

1920 1922 NSHEB

1928 NSHEB

1930 Jan 1932 Inverness Courier, NSHEB

1940

Jan 1947 Inverness Courier, NSHEB

1950 1950 NSHEB

1960 Feb 1962 Inverness Courier, NSHEB

Dec 1966 Institute of Hydrology, NSHEB

1970

1980 Dec 1983 SEPA, NSHEB 3rd

Feb 1989 SEPA, NSHEB 1st

1990 Jan 1992 SEPA 2nd

2000


24

4.2 Hydrological characteristics of the Conon: post hydroelectric scheme

SEPA monitors river flows at four locations in the Conon catchment, Table 4.3. As mentioned in the

previous section, the North of Scotland Hydro-electric Board originally operated the Moy Bridge

river-gauging station. The station was taken over by the Highland River Purification Board in 1976

and later by SEPA.

Table 4.3 River flow gauging stations in the Conon catchment

River Station Grid Reference Catchment Area Start of Record

Conon Moy Bridge NH482547 961.8 1 Jan 1976

Blackwater Contin NH455563 336.1 1 Sep 1981

Meig Glenmeannie NH286528 120.5 1 Nov 1985

Bran Dosmucheran NH205602 116.0 1 Nov 1989

The River Conon at Moy Bridge is downstream of the Conon catchment hydro scheme and is affected

by the operation of this scheme. The Moy Bridge station is described in the National Water Archive as:

‘The catchment at this point is enhanced by 20% by inter-basin transfers from the

catchments of the River Orrin, Ewe, Broom and Carron for power generation.

Extensive volumes of surface storage are controlled for power generation with the

hydrograph at Moy Bridge being dominated by the influence of Torr Achilty power

station.’ (Anon, 1993)

A short distance upstream of the Moy Bridge gauge, the river flow is monitored on the Blackwater at

Contin. Again the catchment at this point is influenced by the hydro scheme and is described as:

‘Within the Blackwater catchment runoff from 50% of the natural catchment along

with inter-basin transfers from the rivers Broom and Carron amounting to 20% of the

natural catchment bypass the station for power generation and discharge to Loch

Luichart. Storages in Loch Vaich and Loch Glascarnoch are controlled for power

generation.’ (Anon, 1993)

River flows are monitored on two rivers, Meig and Bran, further upstream within the Conon

catchment. Both gauging stations monitor flows which are not influenced by the hydro schemes. The

characteristics of the Meig at Glenmeannie, one of these stations, are:

‘There are no significant artificial influences in the catchment thereby providing a

useful indication of the natural runoff. The only significant surface storage is Loch

Beannacharain through which 70% of the catchment drains.’ (Anon, 1993)


25

The location of gauging stations above and below the hydro-scheme presented the opportunity to

determine the influence of the scheme on river flows. Section 4.1 demonstrated the effects of the

hydro scheme on the major floods on the Conon, this section concentrates on the catchment runoff

and the flow regimes.

The flow records for all four river-gauging stations in the Conon catchment were analysed to

determine the monthly average runoff for each station. The results of this analysis are shown in

Fig. 4.1, with the data presented as cubic metres per second per square kilometre of catchment area.

The results in Fig.4.1 show that:

a. The monthly average runoff for the Blackwater at Contin is significantly less when compared

to the natural river regimes recorded at Glenmeannie and Dosmucheran. This reflects the

inter-basin water transfers from the Blackwater catchment into the hydro schemes;

b. The monthly average runoff for the Conon at Moy Bridge is very similar to the two natural

catchments from April to December. However the first three months of the year illustrate a

lower runoff for the Conon when compared to the natural catchment systems.

Figure 4.1 Conon catchment monthly average runoff


26

The river flow patterns below the hydro schemes can also be analysed by comparison of the

hydrograph records (Fig.4.2). The hydrograph for the Meig at Glenmeannie shows a very responsive

reaction to rainfall events while the Conon at Moy Bridge shows little variation in flow due to the

dampening effect of the reservoir storage on the hydrograph.

Figure 4.2 Flow regimes from the Conon at Moy Bridge and Meig at Glenmeannie


27

4.3 Flood regime of the Conon: post-flood embankment construction

From the flood chronology (Section 4.1) it appears that the establishment of the Conon floodplain

embankments has not prevented major flooding and inundation of the floodplain. However, the more

frequent floods, described by the mean annual flood, which may have caused inundation in the pre-

flood embankment era, now do not result in floodplain inundation. To demonstrate this, a hydraulic

model was constructed to show the effects of the embankments on the mean annual flood.

The model used was a one-dimensional backwater type model known as HEC-RAS (US Army

Corps, 1997). The model was constructed using river cross sectional surveys at points along the

River Conon (Section 2.3). Hydrological data from Moy Bridge gauging station were entered into the

model for the mean annual flood, the model was then calibrated using the observed levels at Moy

Bridge gauging station for the mean annual flood.

Outputs from the model are shown in Figs. 4.3 and 4.4. The model illustrates that the flood

embankments confine the mean annual flood within the main channel (Fig.4.3). However, if the

embankments are removed from the model, the same flood would inundate the floodplain. Hence,

prior to the floodbanks being constructed the floodplain would have been subjected to more frequent

flooding than occurs at present.

Figure 4.3 Predicted flood inundation with floodbanks present


28

4.4 Effects of the hydroelectric scheme and flood embankments: summary

The previous sections have described the flow regimes of the rivers and have analysed the results

to determine whether the construction of hydropower dams and flood embankments altered

inundation of the Conon floodplain. The following summarises the results:

• A historical flood chronology for the Conon has been collated dating back to the earliest

record of flooding on the Conon in 1829. It can be summarised that events causing

inundation of the Conon floodplain occurred approximately once every 10 years both prior to

and following the construction of the hydroelectric scheme. This suggests that the

hydroelectric scheme has not altered the frequency of the higher magnitude floods and

contradicts the NSHEB (1989) report which suggested that the scheme had reduced the

frequency of flooding. However the increase in flood frequency in the 1980s and 1990s

which has been reported (Black, 1993 and Rowling, 1989) is not evident in the flood

chronology for the Conon. This could be attributable to the hydroelectric scheme.

• It is difficult to determine the effect of the hydroelectric scheme on the magnitude of flooding

in the Conon. Newlands’ river flow and flood calculations would have been useful to compare

with post-hydro scheme river flow measurements. However, the accuracy of Newlands’

figures is uncertain. The NSHEB report presents results of a modelling exercise for the 1989

flood event. The report suggests that the magnitude of the 1989 flood in the Conon would

Figure 4.4 Predicted flood inundation with no floodbanks


29

have been 13% higher without the hydroelectric scheme. Indeed the operating rules of

NSHEB (now Scottish and Southern Energy) attempt to minimise the flood risk through

utilising reservoir storage for floodwaters and releasing waters prior to any potential

prolonged rainfall.

• In addition to the floods, the hydroelectric scheme has a major impact on the river flows of

the Conon and Blackwater. Gauged river flow records for the Blackwater suggest significantly

reduced runoff and reduced variability due to the storage and inter-basin water transfers.

• The effects of the flood embankments have been shown to confine the mean annual flood

on the Conon. If the flood banks were removed then the Conon floodplain would be

subjected to more frequent flooding.


30

5. Changes in floodplain habitats

5.1 Floodplain habitat mosaic

Evidence for the changes in the floodplain habitats of the River Conon was obtained from a series of

aerial photographs covering the river below Dunglass Island. Aerial coverage of the complete

floodplain was only available for 2000. Other information about the floodplain habitats was available

from a ground survey of woodland vegetation carried out in 1995 (Tidswell, 1995).

The series of aerial photographs covering the lower floodplain includes the following years: 1946,

1954, 1962, 1967, 1977, 1988 and 2000. From these photographs the changes in habitats over the

past 54 years was determined. Each photograph was scanned and five different habitat types

classified:

1. Water

2. Un-vegetated river sediment deposits

3. Dense woodland

4. Sparse woodland or scrub

5. Agricultural or urban

The habitat classification for the floodplain between Dunglass Island and the estuary is shown in

Appendix 3. A similar classification, on a larger scale, was done for the 2000 image of Dunglass

Island. Appendix 3 also includes the results from the 1995 woodland survey.

From the series of aerial photographs it is evident that the changes in the habitat mosaic below

Dunglass Island are dominated by the expansion of woodland. The patches of woodland are closely

related to the river, either in strips along the river bank or on the stable sediment deposits forming islands.

The expanded woodland habitats have mainly replaced the un-vegetated river sediment deposits while

the agricultural and urban land adjacent to the river has limited expansion away from the river.

The woodland survey, carried out by Tidswell in 1995, concentrated on the northern side of the

floodplain. In the report on the survey, Tidswell comments that much of the woodland is associated

with certain physiological features including banks of river shingle, former river meanders, and river

terraces. The woodland on Contin Island is dominated by alder but variations in micro-topography

show that on the better drained ground alder and birch form a canopy above tussocky grasses and

mosses while on the poorly drained ground there is smooth grassy ground vegetation or

rushes/sedges below a canopy of larger multi-stemmed alder. At Easter Moy the woodland

represents a fragmented residue of oak dominated vegetation on sloping ground and alder swamp

woodland on the level ground and in the relict river channels. The woodland on the margins of

Dunglass Island and the adjacent floodplain is largely scrubby alder while on the islands close to the

mouth of the River Conon the woodland is open alder.

From the information collected in this study it is clear that there are likely to have been several

significant changes in the hydrological regime of the River Conon and the adjacent floodplain. These

changes will have affected the floodplain habitats, including the floodplain woods.


31

The floodplain habitats, where remnants of the woods exist, can be divided into three groups in

terms of the hydrological controls and the likely management options:

• River habitats – including mid-channel islands and the immediate riparian zone, often

between the river and the flood bank. These are habitats where the wetness of the ground is

due to both natural channel overtopping during river floods and subsurface seepage from the

river;

• Floodplain habitats – areas away from the immediate riparian zone and often behind the

flood banks, including relict channel features and the insides of meanders. These are

habitats which are now rarely flooded by the main river but can be flooded by small burns or

drains and also have a high water table due to lateral seepage of water from the valley sides;

• River-estuary habitats – areas within the inter-tidal zone where the river channel bifurcates

as it meets the estuary. These are habitats which receive frequent flooding from the river as

it meets the estuary, in particular if the river flood is coincident with a high tide.

Because of their different hydrological controls the options for restoring these groups of habitats

would involve different management interventions. The practicalities in some cases are also limited

because of the extensive developments, which have taken place on the floodplain.

5.2 Changes in the hydrology of the floodplain habitats

The likely effects of historical deforestation on hillslope runoff characteristics can be described with

some confidence however there is more speculation in describing the effects on floodplain habitats.

Deforestation is likely to have increased headwater river flows and sediment loads and the storm

runoff could have become more rapid. It is therefore possible that the deforestation increased the

downstream river flows and the frequency of flooding. This would have also increased the frequency

of inundation over all three types of floodplain habitats and the seepage of water into the river and

river-estuary habitats. As there is unlikely to be extensive reforestation of upper Strathconon in the

coming decades, this might be of benefit to the floodplain habitats. A summary of the likely effects of

deforestation on the three different habitats is shown in Table 6.1.

The effects of the upstream hydro schemes were described in Chapter 4 by comparing gauged

catchments with different influences. The schemes were shown to have caused significant changes

to the river flow regimes with the following likely effects on the habitats:

• The Blackwater total flow is significantly reduced due to water transfers – this will affect the

river habitat at Contin Island which is likely to receive less seepage from the river and hence

this habitat will be drier throughout the year;

• The change in the Conon total flows at Moy Bridge could not be quantified however it will not

be affected by the Meig and Blackwater transfers but it is likely to be enhanced by the water

transfer from the Orrin – this will increase the seepage from the river in the habitats between

Moy Bridge and the Orrin confluence hence this could cause these habitats to be wetter;


32

• The Conon and Blackwater summer flood flows are reduced because of water storage within

the reservoirs – this will affect all river and river-estuary habitats by reducing the frequency

of inundation and probably also the seepage from the river hence these habitats will be drier

in summer;

• The major winter floods on the Conon and Blackwater are unchanged and so the resulting

inundation of all habitats will be unaffected;

• The medium flood events on the Blackwater are unchanged and so the inundation of Contin

Island will be unaffected;

• The medium events on the Conon are attenuated with a reduced peak flow affecting all river

habitats below Moy Bridge hence the habitats will be drier;

• As neither the Orrin nor the Conon below the Orrin are gauged then the effects of the Orrin

water transfer is not included in most of this analysis.

A summary of the likely effects of the hydro-schemes on the three different habitats is shown in

Table 6.1.

A history of the flood banks was given in Chapter 3 and the hydrological evidence presented in

Chapter 4 showed that the construction significantly reduced floodplain inundation by moderate

floods but the large floods still caused extensive flooding. The topographic survey detailed in

Chapter 2 also showed that, without the flood banks, if overtopping of the natural channel occurred,

there would be a flow of water away from the river because of the gradient of the ground surface.

Drains in the floodplain were installed following the construction of the flood banks. One main drain in

particular was mentioned in Chapter 2 as being significant in affecting the wetness of the floodplain

habitats. Other drains have been constructed to take laterally draining burns through the floodplain and

into the river. Without these drains, the water table throughout the floodplain would be raised

expanding the areas of suitable habitats but causing the wetting up of many agricultural fields.

The flood banks and drains are likely to have the following impacts on the floodplain habitats:

• Floodplain habitats will only be flooded by the river in rare events hence habitats will be

much drier in most years;

• River habitats will be inundated to greater water depths and for longer durations because

flood flows will be confined to the channel between the flood banks rather than dispersing

over the floodplain. This effect could become more severe downstream and so would affect

Dunglass Island much more than Contin Island;

• River-estuary habitats could also be severely affected by the concentrated flood flows;

• Drains throughout the floodplain have lowered the water table hence all floodplain habitats

will be drier.


33

A summary of the likely effects of the flood banks and drains on the three different habitats are

shown in Table 6.1.

Table 6.1 Summary of the likely impacts on floodplain habitats

Habitat

River Results of deforestation:

Blackwater Moy-Orrin Dunglass

Flood-

plain

River-

estuary

Increased flows ➚ ➚ ➚ ➝ ➚

Increased flooding ➚ ➚ ➚ ➚ ➚

Habitat

River Results of hydro-schemes:


Flood-

plain

River-

estuary

Blackwater flow reduced ➘ ➝ ➝ ➝ ➝

Conon flow increased ➝ ➚ ➝ ➝ ➝

Summer floods reduced ➘ ➘ ➘ ➝ ➘

Major winter floods unchanged ➝ ➝ ➝ ➝ ➝

Blackwater medium winter floods

unchanged

➝ ➝ ➝ ➝ ➝

Conon medium winter floods

reduced

➝ ➘ ➘ ➝ ➘

Habitat

River Results of flood banks and drains:


Flood-

plain

River-

estuary

Flooding over floodplain reduced ➝ ➝ ➝ ➘ ➝

Flood levels between flood banks

increased

➝ ➚ ➚ ➝ ➚

Flood levels downstream increased ➝ ➝ ➝ ➝ ➚

Drains reduce the water table over

the floodplain

➝ ➝ ➝ ➘ ➝

Key:

➚ Increased wetness

➘ Decreased wetness

➝ No change


34

In summary of the previous sections the following hydrological changes are likely to have taken

place at each habitat:

• The wetness of the Blackwater river habitat (Contin Island) was increased by the

deforestation but later decreased by the hydro schemes;

• The wetness of the Moy-Orrin river habitat was increased by the deforestation, increased by

the flood banks but decreased by the hydro-schemes;

• The wetness of the Dunglass river habitat was increased by the deforestation, increased by

the flood banks but decreased by the hydro-schemes;

• The wetness of the floodplain was increased by the deforestation but decreased by the flood

banks and drains;

• The wetness of the river-estuary habitat was increased by the deforestation, increased by

the flood banks but decreased by the hydro schemes.

The history of change is therefore a complex sequence of events at each site.


35

6. Future Management Options

The suggested approach to future management of the floodplain habitats is to first prioritise those

habitats where there would be most benefit for least disturbance and expense. Four significant

catchment management issues have been discussed in this report: deforestation, hydropower, flood

banks and drainage. Their potential for alternative management is discussed below.

The deforestation of the headwater catchments has had a significant effect on all floodplain habitats.

Although there are current initiatives to re-establish some native forests in other parts of Scotland

these schemes are unlikely to be extensive in the headwaters of Strathconon. Also as the

deforestation was considered to benefit the floodplain sites then afforestation is not considered as a

practical or desirable management option.

The hydro-power schemes have had a significant effect on the river habitats, particularly by reducing

the flood flows and also the total flow in the Blackwater. The options for releasing more water from

the reservoirs are limited as the only outflow, which is controllable, is the Torr Achilty. Water

transfers between the other reservoirs could be managed in alternative ways however this could

compromise the generating capacity of the whole Conon scheme. It is suggested that this

management option is not considered at present until other options have been further investigated.

The flood banks have had a significant effect on flooding in all habitats. River and river-estuary

habitats are likely to have had increases in inundation due to the confinement of floods between the

banks while the floodplain habitats have seen a major reduction in inundation events. The flood

banks offer significant protection to the valuable agricultural lands and their wholesale removal

would cause frequent devastation and disruption throughout the area. Other solutions could be

considered such as the installation of sluices or flap valves however there would still remain a high

risk of extensive damage. The only area where flood bank removal would not be a high risk is over

Dunglass Island. The banks only exist on the upstream section of the island and their removal would

increase the frequency of flooding of the immediate riparian zone, the relict river channel in the

centre of the island and, in major events, over the whole island. As there are no burns or drains on

the island this is the only option if this site is to be restored. Against this option are the fisheries

interests as the island and its banks provide an important facility for fishermen in this area. This may

be resolved by only removing those sections of the flood banks not used by the fishermen so

allowing the passage of flood waters while retaining them as a fishing facility.

Drainage of the central part of the floodplain has had a significant effect on the water table and

hence the floodplain habitats. The remnant wet woodland habitats, most notably the relict ox bow

lake to the south of Moy Island, indicate that the water table even during the summer season needs

to be close to the surface. The topographic surveys across the floodplain, which included the main

drains and the ox bow lake, indicated that the elevation difference between the ground surface of

the floodplain and the water table was some 100cm. To raise the water table by this amount all of

the artificial drains would have to be blocked. It is unlikely that this would be acceptable to the

landowners however a partial blockage of the drain could benefit some lower lying ground while still

being acceptable to the landowners. Therefore in the floodplain south of the river a slight raising of

the water table by a partial blocking of the main drain should be enough to enhance the areas, which

already exhibit wet conditions. This would also raise the water table in the relict ox bow lake.


36

Alterations to the drainage in those isolated sections of floodplain, including the inside of the

meander downstream of Moy Island and the areas between the meander and the Orrin confluence,

could raise the water table in these areas. To enhance these habitats the channels of the small

burns and drains passing through the areas should be constricted so that a large portion of the flow

seeps into the soils within these sites. This should raise the water table and increase the wetness of

the ground. The advantage of these sites is that is that they are isolated from the rest of the

floodplain and so an increased wetness would not impact a larger area of agricultural land.

The effectiveness of the alternative management methods should be monitored at each stage and

any necessary adjustments made. It is strongly recommended that monitoring of the water table in

all priority locations be implemented as soon as possible. This would not only check the state of the

habitat but it would also provide information for the landowners on the changes to the drainage of

their fields.

As a conclusion to this study the authors recommend the following prioritisation for the sites:

• Sections between Moy Island and the Orrin confluence – alterations to the drains;

• Dunglass Island – partial removal of flood banks;

• Ox bow lake and other low lying areas in the central floodplain – partial blockage of the main

drain;

• Moy Island, Conon-Blackwater confluence and the Conon-Orrin confluence – alteration of

the flow regime;

• Section downstream of Conon Bridge – alteration of the flow regime;

• Contin Island – alteration of the flow regime.


37

References

Anon. (1829) The Inverness Courier, Wednesday August 12th 1829.

Anon. (1853) Annual Register of the year 1852. Chronicle Section.

Published F. and J. Rivington, London.

Anon. (1873) British Rainfall for 1872.

Anon. (1875) British Rainfall for 1874.

Anon. (1903) The Inverness Courier, Tuesday February 3rd 1903.

Anon. (1932) The Inverness Courier, Tuesday January 19th 1932.

Anon. (1947) The Inverness Courier, Tuesday November 25th 1947.

Anon. (1962) The Inverness Courier, Tuesday February 13th 1962.

Anon. (1993) Hydrological Data United Kingdom: Hydrometric Register

and Statistics 1986-90. Institute of Hydrology, Wallingford.

Arnell, N. (1996) Global Warming, River Flows and Water Resources.

Wiley, Chichester.

Black, A. R. (1993) Climate change and its effects on Highland river flows.

The Effects of Global Warming on the Highlands of

Scotland. Proceedings from a seminar, Kingussie,

May 1993.

Climate Change Impacts Research Review of the Potential Effects of Climate Change in the

Group (1996) United Kingdom. London: HMSO.

Law, R. (1989) An Investigation of the Heavy Rainfall of 5-6th February

1989, in the Ness, Beauly and Conon Catchments.

A report to the North of Scotland Hydro-Electric Board,

Institute of Hydrology, Wallingford.

Law, F., Black, A. R., Scarrott, R. M. J. Chronology of British Hydrological Events.

and Millar, J. B. (1998) http://www.dundee.ac.uk/geography/chbe/

Nairne, D. (1895) Memorable Floods in the Highlands during the

Nineteenth Century…Part Fifth.

NERC (1975) Flood Studies Report. Natural Environment Research

Council, London, 5 Vols. (Reprinted with 16

supplementary reports, Institute of Hydrology, 1993).


38

Newlands, A. (1908) The possibilities of power from highland lochs and rivers

– part II. The Inverness Courier, November 27 1908, pg 3.

North of Scotland Hydro-electric Board Final Report of Floods in the Northern Hydro Schemes,

(1989) January/February 1989. NSHEB.

Payne, P.L. (1988) The Hydro. Aberdeen University Press, Aberdeen.

Robert H Cuthbertson & Partners (1990) Flooding in Badenoch and Strathspey. Report to the

Highland Regional Council, Robert H. Cuthbertson and

Partners.

Rowling, P. (1989) Rainfall variation and some implications for flooding in

the Allan catchment, Central Scotland. Weather, 44,

No.4.

Scottish Natural Heritage (undated) The Natural Heritage of Scotland: an overview.

SNH Publications and Design.

Tidswell, R. J., (1995) A survey of the woodland vegetation on the River Conon

islands, Ross and Cromerty. Summary document for

Brahan Estate & Contin Island prepared by S Cohen, SNH.

US Army Corps (1997) HEC-RAS User Manual. US Army Corps of Engineers.

Werrity, A. and Acreman, M. C. (1985) The flood hazard in Scotland. In: Harrison, S. J.

Climatic Hazards in Scotland. Geo books, Norwich.


39

Appendix 1: Channel changes at the Conon-Blackwater and

Conon-Orrin confluences

The following maps show the channel positions and shapes of the key features in the River Conon.

The selected locations are:

a. Conon-Blackwater confluence and Moy Island;

b. Conon-Orin confluence;

c. Dunglass Island.

For each location the positions of the channels and shape of the features are firstly highlighted on

the “present day” map and then the positions as shown on two historical maps (1873 and 1904) are

overlain on the present day map.


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Map 1 Conon Floodplain Survey, Moy Island – River Channel Present


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Map 2 Conon Floodplain Survey, Moy Island – River Channel 1873


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Map 3 Conon Floodplain Survey, Moy Island – River Channel 1904


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Map 4 Conon Floodplain Survey, Conon-Orrin Confluence – River Channel Present


44

Map 5 Conon Floodplain Survey, Conon-Orrin Confluence – River Channel 1873


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Map 6 Conon Floodplain Survey, Conon-Orrin Confluence – River Channel 1904


46

Map 7 Conon Floodplain Survey, Dunglass Island – River Channel Present


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Map 8 Conon Floodplain Survey, Dunglass Island – River Channel 1873


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Map 9 Conon Floodplain Survey, Dunglass Island – River Channel 1904


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Appendix 2: Results of the floodbank survey

Figure A.1 Floodbank assessment: Clashuile to Moy Island

Key to flood bank assessment


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Figure A.2 Floodbank assessment: Moy Island to Dunglass Island


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Appendix 3: Changes in floodplain habitat mosaics below Dunglass Island

The habitat mosaic below Dunglass Island was mapped from aerial photographs. The following

maps show the extent of each classified area for the years 1946 and 1977.


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Conon Floodplain Survey Habitat Mosaics – Aerial Photographs 1946


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Conon Floodplain Survey Habitat Mosaics – Aerial Photographs 1977

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