Forestry Commission
GLENCOE LOCHAN
REPORT ON AN INSPECTION UNDER RESERVOIRS ACT 1975,
SECTION 10(2) OF THE ACT
DRAFT
May 2014
Inspecting Engineer
Prof A K Hughes
Atkins Limited
Epsom Gateway
2 Ashley Avenue
Epsom
Surrey
KT18 5AL
Atkins
DOCUMENT HISTORY RECORD
Doc No.
Title:
Report on an Inspection under Reservoirs Act 1975, Section 10
Glencoe Lochan
Rev
Date
Description/Reason for Issue
Orig
Ckd
Appd
R01
22.05.2014
Draft 1 issued for review
AKH
AKH
AKH
A01
28.05.2014
Draft issued to clients
AKH
AKH
AKH
106735574
Draft May 2014
Reservoirs Act 1975
Glencoe Lochan
CONTENTS
Page
1.
NAME AND SITUATION OF RESERVOIR
3
2.
NAME AND ADDRESS OF ENGINEER MAKING THE REPORT
3
3.
PANEL OF WHICH THE ENGINEER IS A MEMBER
3
4.
NAME AND ADDRESS OF OWNER AND/OR UNDERTAKER
3
5.
NAME AND ADDRESS OF ENFORCEMENT AUTHORITY
4
6.
DATE OF INSPECTION
4
7.
NAME AND ADDRESS OF SUPERVISING ENGINEER
4
8.
8.1
8.2
8.3
8.4
CERTIFICATES, REPORTS OF PREVIOUS INSPECTIONS AND OTHER ITEMS OF
INFORMATION WHICH WERE PROVIDED TO THE ENGINEER
Certificates
Reports
Supervising Engineers Statements
Other Information including Drawings
4
4
5
5
5
9.
9.1
9.2
9.3
9.4
9.5
9.6
9.7
9.8
9.9
9.10
9.11
9.12
GENERAL DESCRIPTION
Description of Reservoir
Geology of the Site
Catchment
Dam Details
Details of Modifications, Remedial Works and History
Overflow
Inlet and Outlet Pipework and Valving Arrangements
Scour
Instrumentation
Method of Recording Water Levels
Access
Valley Downstream of the Dam
5
5
6
6
6
6
6
7
7
7
7
7
7
10.
GENERAL DESCRIPTION OF THE INSPECTION MADE AND THE CONDITIONS
FOUND
General
Catchment
Dam
Overflow Structure and Channels
Valve Shaft and Tunnel
Inlet and Outlet Pipework and Valves
Seepages/Drainage Flows
Settlement and Movement
Instrumentation
Method of Recording Water Levels
Access for Maintenance and Emergency
Control of Inflow from Direct and Indirect Catchments
Movement of Surrounding Land which Might Affect the Stability of the Reservoir
10.1
10.2
10.3
10.4
10.5
10.6
10.7
10.8
10.9
10.10
10.11
10.12
10.13
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8
8
8
9
11
11
11
11
11
12
12
12
12
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Reservoirs Act 1975
Glencoe Lochan
10.14
Area Downstream of the Dam
11.
ADEQUACY AND CONDITION OF WASTE WEIR AND OVERFLOW AND
CHANNELS IN CONNECTION THEREWITH
Flood Assessment
11.1.1
Categorisation
11.1.2
Freeboard Details, etc
11.1.3
Flood Flow Capacity Assessment
11.1.4
Summary of Flood Study Analysis
Alterations to Overflow Sill
Any Alteration in Level to which the Water may be Stored
Efficiency of Scour Pipe and Other Means of Lowering the Water in and Controlling
the Inflow to the Reservoir
11.1
11.2
11.3
11.4
12
12
12
12
13
13
13
14
14
14
12.
SEISMIC RISK
14
13.
SUPERVISION PROVIDED BY THE OWNER/UNDERTAKER
15
14.
CORRECTNESS OF PARTICULARS IN THE PRESCRIBED FORM OF RECORD
REQUIRED TO BE KEPT UNDER SECTION 11 OF THE ACT
15
15.
15.1
15.2
15.3
15.4
15.5
15.6
15.7
FINDINGS AND RECOMMENDATIONS OF THE ENGINEER
Findings
Recommendations as to Measures to be taken in the Interests of Safety under
Section 10(6) of the Act
Other Measures Recommended to be taken but not requiring Supervision by a
Qualified Civil Engineer within the Meaning of the Act
Measures Recommended in the Interests of Improving Monitoring and Supervision
under Section 11 of the Act
Matters of Safety of Personnel
Matters to be watched by the Supervising Engineer in accordance with Section 10(4)
of the Act
Recommendations as to the Date of the Next Inspection
16
16
17
17
17
18
18
18
APPENDICES
Appendix A
Appendix B
Appendix C
Appendix D
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Recommendations from Previous Report and Status
Site Location Plan
Photographs
Spillway Coordinates
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1.
NAME AND SITUATION OF RESERVOIR
The reservoir is known as GLENCOE LOCHAN, situated about 0.75 km north of
Glencoe in the Parish of Lismore and Appin in the Lochaber District of the Highland
Region of Scotland.
National Grid Reference NN 105 596 1ies within the site.
2.
NAME AND ADDRESS OF ENGINEER MAKING THE REPORT
Prof Andrew K Hughes BSc PhD DMS CEng FICE FCIWEM MIMgt
Atkins Limited
Woodcote Grove
Ashley Road
Epsom
Surrey
KT18 5BW
Office Tel: 01372 756287
Home Tel: 01737 360162
Mobile Tel: 07834 506339
3.
PANEL OF WHICH THE ENGINEER IS A MEMBER
All Reservoirs Panel (appointed to the Panel until 31 May 2016).
4.
NAME AND ADDRESS OF OWNER AND/OR UNDERTAKER
Forestry Commission Scotland
1 Highlander Way
Inverness Business Park
Inverness
IV2 7GB
Contact: Alan Dickerson
Office Tel: 0131 445 6976
Home Tel: 01721 724570
Mobile Tel: 0771 275 1042
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5.
NAME AND ADDRESS OF ENFORCEMENT AUTHORITY
Soon to be replaced by
SEPA Erskine Court
Castle Business Park
Stirling
FK9 4TR
Transport Services
The Highland Council
Glenurquhart Road
Inverness
1V3 5NX
6.
DATE OF INSPECTION
7th May 2014.
7.
NAME AND ADDRESS OF SUPERVISING ENGINEER
Mr Alan Dickerson
Forestry Commission
Northern Research Station
Roslin
Midlothian
EH25 9SY
Office Tel: 0131 445 6976
Home Tel: 01721 724570
Mobile Tel: 0771 275 1042
8.
CERTIFICATES, REPORTS OF PREVIOUS INSPECTIONS AND
OTHER ITEMS OF INFORMATION WHICH WERE PROVIDED TO THE
ENGINEER
8.1
Certificates
(1)
(2)
(3)
(4)
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No Certificates relating to construction; built before the ambit of the Reservoirs
(Safety Provisions) Act, 1930 & Reservoirs Act 1975
Inspecting Engineer’s Certificate under Section 2 of the Reservoirs Act 1930
(Safety Provisions) by Mr A.C Allen dated 30 June 1989
Inspecting Engineer’s Certificate under Section 10(5) of the Reservoirs Act
1975 by Mr H.J. Gudge dated 30 October 1994
Inspecting Engineers Certificate under Section 10(5) of the Reservoirs Act 1975
by Dr A.K. Hughes dated 21st September 2004.
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Glencoe Lochan
8.2
Reports
1)
2)
3)
8.3
Inspecting Engineer’s Report under Section 10 of the Reservoirs Act 1975 by
dated
Inspecting Engineers Report under Section 2 of the Reservoirs (Safety
Provisions) Act 1930 by Mr A.C. Allen dated 30 June 1989.
Inspecting Engineers Report under Section 10(5) of the Reservoirs Act 1975 by Dr
A.K. Hughes dated 21st September 2004.
Supervising Engineers Statements
(1) Supervising Engineer’s Statement by Mr I.D.M. Hampson dated 1st April 2004
(2) Supervising Engineer’s Statement by Mr I.D.M. Hampson dated 5th September
2003
(3) Supervising Engineer’s Statement by Mr A. Dickerson dated 18th December 2013.
8.4
Other Information including Drawings
(1) Prescribed Form of Record Relating to the Reservoir
(2) On Site Plan
(3) Flood Plan – On Site data 7th February 2014.
9.
GENERAL DESCRIPTION
9.1
Description of Reservoir
The reservoir is retained by three earthfill embankments.
The main dam consists of an embankment approximately 7 metres high and 50 metres
long although there is also a vertical concrete wall about 0.7 metres thick on the
upstream face.
The other two embankments are essentially low bunds of up to 2 metres in height
augmented by gabions on the crest.
The reservoir has a stated capacity of 73,000 cubic metres and a surface area of 38,000
square metres when full to its top water level.
The reservoir appears to have been constructed by about 1895. The reservoir is used for
fishing.
The lochan is essentially a recreational project and together with the smaller ponds
downstream, a garden landscaping feature. It probably supplied water to the large
house immediately downstream and legend has it that it was built for a North American
princess.
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It was built by Lord Strathcona.
9.2
Geology of the Site
The geology of the site consists generally of gneisses and other metamorphosed rocks.
There is little soil cover to a very hilly/mountainous area.
9.3
Catchment
The catchment has an area of only some 0.22 square kilometres.
The reservoir sits on a three way watershed with a very small catchment. On the north
side the land rises for some 100 metres before falling over 250 metres to Loch Leven.
On the south side the ground rises to the Pap of Glencoe.
9.4
Dam Details
All embankments form part of the footpaths system in the area.
The downstream face of the main dam has a slope of about 1 in 4.
The vertical concrete wall some 0.7 metres thick at the top of the upstream slope has
been augmented by a row of boulders between 0.3 and 0.45 metres high, bedded and
jointed in cement mortar.
The other two 'subsidiary' embankments consist primarily of low earth embankments
heightened and levelled by the addition of some stone filled gabions.
The footpaths were improved in 2011 and 2012.
The fourth spillway is formed by the removal of 2 gabions. This spillway operated in
2007/8. The level is about 180 mm above that of the main spillway.
9.5
Details of Modifications, Remedial Works and History
Attempts have been made to seal leakages in May 1994 and May 1996 and the spillway
was repaired in June 1997 to stop leakages.
At various times drainage grips have been dug in the area downstream of the dam, mitre
and on the shoulder.
9.6
Overflow
There are four spillways at this reservoir.
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The "main" spillway cascades down the face of the main embankment in an open
channel 1.55 metres wide controlled by a broad crested weir.
The second outlet is a piped culvert 400mm in diameter under the forest road to the
right of the main embankment.
The third and lowest outlet is in the north west embankment consisting of a rectangular
culvert approximately 400 mm wide and 300 mm high with walls formed from gabions.
The fourth overflow is formed through the gabion wall on the north embankment.
9.7
Inlet and Outlet Pipework and Valving Arrangements
None known.
9.8
Scour
There is a scour facility which seems to consist of what appears to be a 200/250 mm
pipe controlled by a headstock in the middle of the main embankment dam.
The scour is said to be 150 mm in diameter.
9.9
Instrumentation
There is no instrumentation on or in the embankments.
9.10
Method of Recording Water Levels
Water levels are recorded by reference to a gauge board attached to a post near the main
embankment.
9.11
Access
Access to the reservoir is via approximately 500 metres of surfaced road from the public
highway. The reservoir is close to a Forestry Commission car park. The reservoir is
encircled by a forest road.
9.12
Valley Downstream of the Dam
The valley from the north west dam runs in a north westerly direction to join Loch
Leven.
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The valley from the north east dam runs to the north east again to join Loch Leven
whilst the valley from the 'southern' dam runs through a series of pools and ponds
adjacent to the hospital before joining the river just to the north of Glencoe village.
10.
GENERAL DESCRIPTION OF THE INSPECTION MADE AND THE
CONDITIONS FOUND
10.1
General
I inspected the site on 7th May 2014. I was accompanied by Mr Alan Dickerson,
Supervising Engineer, Mr Graeme White, trainee Supervising Engineer and Kirsty
Mann, LFD all from the Forestry Commission.
The weather at the time of inspection was fine, dry and sunny albeit with some showers
and in the preceding two week period had been changeable with some rain, heavy at
times.
The water level at the time of the inspection was such that water was passing over the
'lowest' main spillway to a depth of about 35 mm.
10.2
Catchment
The catchment was seen to be very small and limited in size and the site was seen to be
the junction of 3 watersheds - two to Loch Leven and the other to the River at Glencoe.
10.3
Dam
The upstream face of the 'main' embankment could not been seen but it was surmounted
by a low stone 'wall' set on a concrete plinth. It is likely to have an upstream slope of
about 1 in 3.
The crest of the main embankment carries a stone path with a small wooden footbridge
providing access across the spillway channel.
The concrete plinth mentioned above carries stones in a 'castellated' format. The crest
had good line and level. The wall and stone had good line and level indicating little
movement.
The downstream face of the main embankment is ‘wet’ in places with moss growth,
there were two very soft areas, one on the left mitre near the base and a second on the
right mitre about half way down.
The downstream face of the main embankment is rather uneven as a result of trees and
ferns which had grown up in the past and boulders. A lot of work has been done to
improve the face but it is still rather uneven but does not change and is in a satisfactory
condition.
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The mitres had been cleared and there was a flow in both mitres. I recommend the
mitre drains be kept clear.
The crests of banks No 2 (north east) and No 3 (north west) also carry a stoned path but
each has a stone filled gabion 'wave wall'. Both crests have a satisfactory line and level.
The north east embankment was inspected. Much of the upstream face could not be
seen as it was under water but the water line was straight and in general conditions
seemed satisfactory.
The crest carried a footpath and a gabion wave wall. The gabions have suffered some
deterioration and damage due from visitors. They will require replacement in the years
to come but will probably last another 10 years.
The downstream face is rather irregular because of the ferns and peat etc. but it has not
altered as far as I can see and useful work has been undertaken to clear the face. There
is one tree on the embankment near the centre and the crest. This is not a problem at the
moment but I recommend the tree on the north east embankment be managed by
pollarding and trimming to ensure it does not become too big or out of balance.
On the north embankment again little of the upstream face could be seen but the
upstream face and water line appeared to be satisfactory with no signs of distress.
The crest carries a footpath and a gabion wall as a wave wall. The crest and wall
appeared to be in a satisfactory condition with good line and level. Again the gabions
show some signs of distress and damage caused by visitors but they will probably last
10 years. A number of gabions in the centre have been removed to form a fourth
spillway.
The downstream face is grassed and was seen to have good grass cover and a
satisfactory line and level.
The downstream face of each embankment had been recently 'cut', but it is clear that the
vegetation on the faces is variable. However, the faces of the embankments were seen
to be generally in a satisfactory condition and have satisfactory line and level. I
recommend that the vegetation on the downstream face of the embankments is
regularly cut, at least once a year, to enable inspection/examination and promote a
good grass cover.
10.4
Overflow Structure and Channels
The 'main' spillway No 1 is situated on the main embankment. This appears to be a
random stone chute down the face with a broad crest weir approach. The condition was
difficult to assess because of the vegetation and the mode of construction. However, the
Supervising Engineer informs me that it functions adequately. There is a small 'gap' in
the channel between the weir and the chute. I recommend the gap in the spillway
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floor be filled with a flexible filler. A floating screen would not cause a problem as
long as it is maintained.
There is some undercutting in the spillway channel about 6 metres down from the weir.
I recommend in the short term repairs to the spillway be repaired where undercut
and damaged. The edges are poorly defined and can allow water to leave the channel.
I recommend in the short term the sides of the main spillway be improved.
In a ‘large event’ it is likely that the spillway will fail and water will leave the channel
and erode the embankment which could cause failure. Thus I recommend that the
spillway be rebuilt and provided with sufficient capacity to ensure the
embankment is protected – this work should be carried out within the next 3-5
years.
The No 2 spillway exists as a pipe just off the right hand end of the main embankment.
This appears to be in a satisfactory condition.
The second spillway consists of a pipe discharging into a channel which runs around
the right hand end of the main embankment and then into the ponds downstream. This
pipe was clear as was the channel. The last section of pipe appears to have moved
slightly.
At spillway No 3, the spillway is a culvert through the crest formed with gabions giving
a culvert with a width of about 400 mm and a height of about 300 mm. The condition
appeared to be satisfactory although the Supervising Engineer notes that the gabions are
broken in places. I recommend that the spillway is watched for signs of further
deterioration and it be planned to be replaced in the next 3-5 years.
The channel downstream has been lined with stone but this is not likely to stay in place
in a severe storm. I recommend the areas downstream of all spillways be
watched/inspected for damage after severe storms which cause them to operate.
A timber weir has been installed to ensure the main spillway, spills first. The weir
appeared to be in a satisfactory condition.
The No 4 spillway exists on the north east embankment. It was formed by removal of
the gabions in the centre of the crest where the embankment as highest. The crest is
significantly higher than top water level and it is likely the main dam will overtop
before this spillway operates. The fourth spillway is formed by the removal of 2
gabions. This spillway operated in 2007/8. The level is about 180 mm above that of
the main spillway. There is some erosion at the entrance to the spillway. I recommend
repairs are undertaken to the 4th spillway. Thus I recommend the spillway be
removed to the left hand end of the dam where the spillway should be made as
large as possible at say +125 mm above top water level – this should be carried out
within the next year.
At the outlet in the north west corner the gabions either side of the opening are failing
with some of the contents falling out. I understand the situation is slowly deteriorating.
There is some wave erosion at the entrance.
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10.5
Valve Shaft and Tunnel
Not applicable.
10.6
Inlet and Outlet Pipework and Valves
The scour was operated with ease in my presence and the flow left to run clear which
occurred within a very short period. The spindle was modified in 2009 to limit the
travel of the spindle and stop it being wound right out. Valve operating records show
that the valve was operated in 1994, 1997, 1998, 2000, 2003 and annually since then
with the last operations on 26th July 2013 and 16th January 2014.
I recommend that all valves are operated over their full range, against reservoir
head, at least once a year and a record made of their operation.
I recommend that a full scale scour test is carried out at least once a year and the
water left to run until the flow runs clear and is free from sediment.
10.7
Seepages/Drainage Flows
On the main embankment the mitres are quite damp with reed growth indicating areas
of wetness. In addition drainage grips have been dug, particularly on the right mitre to
collect seepages. These were damp or had standing water in them at the time of
inspection. These seepages appear to have existed for some time. At the toe of the
embankment at the end of the scour culvert on the light hand side there was a significant
flow of water. This water is said to be long-standing and to have been witnessed for
many years and was noted by Mr Gudge.
There were two very wet spots on the dam and the flow at the toe.
If the seepage/leakage remains constant with no increase of flow or increase in turbidity
then this is acceptable but clearly if the situation changes this would be of concern.
There were no seepage flows apparent on the other two embankments.
I recommend that the seepage flows be watched for increased flow or increased
turbidity at least at monthly intervals.
10.8
Settlement and Movement
There are no signs of settlement or significant movement.
10.9
Instrumentation
Not applicable.
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10.10
Method of Recording Water Levels
Water levels are recorded by a board on the upstream right hand wall of the main
spillway (2005).
The method of recording water levels is considered to be satisfactory. I recommend
water levels are recorded at least monthly and at times of major storms/outflows.
10.11
Access for Maintenance and Emergency
Access is easy from the public highway via 500 metres of surfaced road.
I consider that access for minor maintenance and emergency to be adequate.
10.12
Control of Inflow from Direct and Indirect Catchments
The inflow from the very direct catchment cannot be controlled.
There is no indirect catchment.
10.13
Movement of Surrounding Land which Might Affect the Stability of the
Reservoir
I saw no signs of movement of the surrounding land which might affect the stability of
the reservoir.
10.14
Area Downstream of the Dam
The area immediately downstream of the main dam is a fairly steep heavily wooded
valley in which there are a number of ornamental ponds but also buildings associated
with the hospital. The stream enters the river close to the village of Glencoe before
entering Loch Leven.
The secondary dams both discharge to fairly steep, uninhabited valleys which run
through heavy woodland before crossing the B863 and entering Loch Leven.
11.
ADEQUACY AND CONDITION OF WASTE WEIR AND OVERFLOW AND
CHANNELS IN CONNECTION THEREWITH
11.1
Flood Assessment
11.1.1
Categorisation
The previous Inspecting Engineer classified the reservoir as Category D. After
consideration of the publication ‘Floods and Reservoir Safety: An Engineering Guide’
published by the Institution of Civil Engineers in 1996, I consider that the dam falls
within Category C, a dam where a breach could endanger lives in a community, but
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where overtopping could be permitted. Under this categorisation the design flood is the
1,000 year flood where overtopping can be allowed.
11.1.2
Freeboard Details, etc
Freeboard is of the order of 0.6 metres.
Annual average rainfall is quoted as 2,300 mm and the fetch 450 metres from the north
east to south west although that line is interrupted by islands so I have used a fetch of
250 metres towards the north west.
11.1.3
Flood Flow Capacity Assessment
The average annual rainfall on the catchment is 2,300 mm as derived from the various
maps included with the Flood Studies Report published in 1975. Analysis of the flood
flows was carried out using the rapid assessment method with the document 'Floods and
Reservoir Safety: An Engineering Guide'.
The design flood (PMF) inflow into the reservoir was found to be 10.8 cumecs. When
routed through the reservoir, the outflow is very similar because of the limited reservoir
size. The appropriate coefficient for the overflow weir was assessed as 1.7.
These calculated flood flows and resultant flood rises differ from the results quoted in
the previous inspection report in view of a revision of the assessment methods and
equations since that time.
11.1.4
Summary of Flood Study Analysis
Design Flood Inflow
Design Flood Outflow
Static Flood Rise
Wave Surcharge
Total Surcharge
Available Freeboard to crest
(cumecs)
(cumecs)
(metres)
(metres)
(metres)
(metres)
-
10,000 year
1,000 year
5.4
5.3
1.2
0.34
1.54
0.5 - 0.6
3.2
3.1
0.94
0.34
1.28
I have assessed the PMF as 10.8 cumecs. Therefore the 10,000 year event is of the
order of 5.4 cumecs and the 1000 year event is about 3.2 cumecs.
If one calculated the flood rise for the PMF storm it is of the order of 1.97 metres whilst
that for the 10,000 year storm is 1.2 metres.
The wave run up has been calculated as 0.34 metres which is of course less than the
recommended minimum of 0.6 metres in the Engineering Guide. However the site is
completely surrounded by trees and is on the side of a hill making the site very sheltered
and so I think it appropriate to use the calculated value of wave run up. Wave run up is
not relevant of course for the overtopping event.
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Even with the concession with a freeboard of some 500-600 mm the spillway capacity
is inadequate. Additional spillway capacity has been provided since the last inspection.
The most cost effective way of providing this capacity was to allow part of No 2 dam
(the dam in the north east) to overtop as this has sufficient grass cover and is the lowest
dam in the series. I suggested removal of some 2 gabions allowing for side slope of the
gabion wall would suffice and meet the recommendation to be carried out and this was
done.
I consider spillway capacity is adequate.
11.2
Alterations to Overflow Sill
There do not appear to have been any alterations to the overflow sill since the time of
the last inspection.
11.3
Any Alteration in Level to which the Water may be Stored
I consider that the water may continue to be stored up to top water level.
11.4
Efficiency of Scour Pipe and Other Means of Lowering the Water in and
Controlling the Inflow to the Reservoir
The catchment is very small.
The 'scour' seems to operate very efficiently. However, the capacity is small and it is
likely to take a considerable amount of time (as much as 60 days) to drop the reservoir
level. Thus I recommended an on site plan be written and this has been done however,
more work is required. I consider the scour to be satisfactory as a means of lowering
the water in the reservoir.
Access is good which would mean it would be easy to install separate pumping. There
are no means of controlling inflow to the reservoir.
12.
SEISMIC RISK
Consideration of the publication ‘An Engineering Guide to Seismic Risk to Dams in the
United Kingdom’ published by BRE in 1991 results in the dam being placed in
Category I based on the following classification factors:Capacity Classification Factor
Height Classification Factor
Evacuation Requirements Classification Factor
Potential Downstream Damage Classification Factor
Total Classification Factor
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0
2
4
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The dam lies within Zone A; a zone with a relatively high number of events especially
large ones. The Guide states that under this classification “it may be sufficient to
examine the embankment and any ancillary works for any feature which would be
particularly susceptible to damage by earthquake ….seismic analysis would only be
undertaken where such an initial appraisal defines features which are a serious cause for
concern.
The risk of an escape of water is likely only to be associated with sufficient deformation
of the downstream slope and I do not consider that any significant deformation would
occur. I am also mindful of the comments in Section 7.10 of the guide where it notes It
is unlikely that earthquakes will cause major damage to well built dams on stable
foundations in Britain. International experience has indicated that virtually any well
built embankment dam can withstand moderate earthquake shaking with peak
accelerations up to 0.2g with no detrimental effects. The probability of the occurrence
of an earthquake of significantly greater severity at any UK dam is relatively low …. I
consider that the dam is in a good condition on the basis of obvious visual evidence and
the monitoring records, and I do not consider that the seismic risks are sufficient to pose
a significant risk to the dam.
I do not consider a seismic safety evaluation is required at this time.
13.
SUPERVISION PROVIDED BY THE OWNER/UNDERTAKER
The Supervising Engineer visits the site at least once a year but usually visits more
frequently than this. The area is heavily frequented by the public. Forestry
Commission staff visit the site frequently.
I recommend that Forestry Commission staff visit the site at least once a month
and walk around all three embankments looking for signs of distress.
In this situation I consider the level of supervision to be adequate.
14.
CORRECTNESS OF PARTICULARS IN THE PRESCRIBED FORM OF
RECORD REQUIRED TO BE KEPT UNDER SECTION 11 OF THE ACT
I examined the Prescribed Form of Record Relating to the Reservoir for the period 1st
March 1993 until 17th December 2013. The maximum water level recorded was 60 mm
on 17th December 2013and the minimum level was – 6.7 metres on 9th August 1999.
I have the following comments on the various parts of the Form as follows:
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Part 1
I recommend water levels are recorded monthly.
Part 2
No comment
Part 3
Amendments are required as a result of this inspection.
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Part 4
Additions are required as a result of this inspection.
Part 5
No comment
Part 6
No comment
Part 7
No comment
Part 8
No comment
Part 9
No comment
Part 10
No comment
Part 11
No comment
15.
FINDINGS AND RECOMMENDATIONS OF THE ENGINEER
15.1
Findings
My findings as a result of the inspection are that:-
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(i)
the dam falls within Category A as defined by the publication Floods &
Reservoir Safety (third edition);
(ii)
the dam falls within Category I as defined by the publication An Engineering
Guide to Seismic Risk to Dams in the United Kingdom and the Application
Note;
(iii)
the reservoir/dam is adequately maintained and generally is in a satisfactory
condition;
(iv)
the overflow arrangements are now adequate and satisfactory to pass the
required design flood outflow;
(v)
the margin between the top of the dam and wave wall and the overflow level is
more than recommended in Floods and Reservoir Safety,
(vi)
there are effective means of lowering the reservoir level;
(vii)
no movement of the surrounding land has been noted which might affect the
stability of the reservoir;
(viii)
the Undertakers are complying with their obligations under Section 11 of the
Act.
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(ix)
15.2
I have reviewed the previous report and can confirm that any previous
recommendations in the interests of safety have been carried out or are no
longer required
Recommendations as to Measures to be taken in the Interests of Safety
under Section 10(6) of the Act
I recommend that:(i)
(ii)
(iii)
(iv)
15.3
in the short term repairs to the spillway be repaired where undercut and damaged
in the short term the sides of the main spillway be improved
the spillway be rebuilt and provided with sufficient capacity to ensure the
embankment is protected – this work should be carried out within the next 3-5
years
the spillway be removed to the left hand end of the north dam where the
spillway should be made as large as possible at say +125 mm above top water
level – this should be carried out within the next year
Other Measures Recommended to be taken but not requiring Supervision
by a Qualified Civil Engineer within the Meaning of the Act
I recommend that:(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
15.4
the mitre drains be kept clear
the vegetation on the downstream face of the embankments is regularly cut, at
least once a year, to enable inspection/examination and promote a good grass
cover
the tree on the north east embankment be managed by pollarding and trimming
to ensure it does not become too big or out of balance
the gap in the spillway floor be filled with a flexible filler
the spillway is watched for signs of further deterioration and it be planned to be
replaced in the next 3-5 years
the areas downstream of all spillways be watched/inspected for damage after
severe storms which cause them to operate
repairs are undertaken to the 4th spillway
Measures Recommended in the Interests of Improving Monitoring and
Supervision under Section 11 of the Act
I recommend that:
(i)
(ii)
(iii)
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the seepage flows be watched for increased flow or increased turbidity at least at
monthly intervals
water levels are recorded at least monthly and at times of major storms/outflows
all valves are operated over their full range, against reservoir head, at least once
a year and a record made of their operation
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(iv)
(v)
15.5
the areas downstream of all spillways be watched/inspected for damage after
severe storms which cause them to operate
Forestry Commission staff visit the site at least once a month and walk around
all three embankments looking for signs of distress
Matters of Safety of Personnel
None.
The comments with respect to Health and Safety relate only to those elements of Health
and Safety associated with the owner and his staff and any other individuals associated
with the safety of the dam. They relate only to issues noted during the inspection and
should not be considered to be exhaustive or complete. The comments in no way relate
to issues associated with others and in particular members of the general public or
those using facilities at or visitors to the site.
15.6
Matters to be watched by the Supervising Engineer in accordance with
Section 10(4) of the Act
I recommend that the Supervising Engineer visits the site at least once a year and pays
attention to any leakage, seepage or settlement and in particular ensures that:-
15.7
(i)
The Prescribed Form of Record is complete.
(ii)
the valve is operated over its full range.
(iii)
the scour test is carried out.
(iv)
the flows at the toe is watched.
(v)
the spillway and approach is kept free of debris
Recommendations as to the Date of the Next Inspection
The next inspection by an Inspecting Engineer under Section 10(2) of the Act should be
undertaken within 10 years of this inspection, i.e. before 7th May 2024.
Dated this ............... day of ....................................... Signed ...........................................................
Member of Panel AR
as constituted under the Reservoirs Act 1975
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This inspection report results from a visual inspection of the reservoir’s condition on the date of
the inspection. No liability can be accepted in respect of any defects not visually apparent or that
arise subsequent to the date of the visit. It is important that the Undertaker or their agents, reports
as soon as possible any change in the condition of the reservoir to the Supervising Engineer.
Appointment to All Reservoirs Panel until 31 May 2016.
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APPENDIX A
Recommendations from Previous Report and
Status
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RECOMMENDATIONS FROM PREVIOUS REPORT AND STATUS
(i)
the screen on the secondary overflow be removed,
or at least moved back into the reservoir.
Done
(ii)
the sandbags in the secondary spillway channel be
removed.
Done
(iii)
the screen on the third spillway be removed, or at
least moved back into the reservoir.
Done
(iv)
additional spillway capacity be provided at the site
for the design storm.
Done
(v)
inconsistencies in Part1 of the Form are put right
and the other changes noted – Part 14 included.
Not complete
(vi)
the vegetation on the downstream face of the
embankments is regularly cut, at least once a year,
to enable inspection/examination and promote a
good grass cover.
Done
(vii)
the gap in the spillway floor be filled with a flexible
filler.
Done
(viii) the spillway is watched for signs of further
deterioration.
Done
(ix)
when the valve is operated it is left open until the
flow runs clear to remove any silt accumulated
around the pipes.
Done
(x)
the 'exit' pond of the significant flow at the toe be
opened up and any modifications made to allow the
volume of flow and the turbidity of the flow to be
assessed.
Done
(xi)
the valve is operated over its full range at least once
a year and a record made of its operation.
Done
(xii)
the valve/penstock spindle be extended to ensure
that it always remains
Done
(xiii) Next inspection by 12th May 2014
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Done
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APPENDIX B
Site Location Plan
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APPENDIX C
Photographs
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