PM_5561_121914120159
I
• -
"
TREGARON TRADING SERVICES LTD
FLOOD CONSEQUENCES ASSESSMENT
ABERCOED QUARRY near TREGARON
BRIAN KILLINGWORTH LTD
CGA&T/CSC
DFS&H f
esc
17. DEC 201~
REF/CYF
JULY 2014
~PM
5561 121914120201
Tregaron Quarry
Flood Consequences Assessment
CONTENTS
1.0
Introduction
3
2.0
Existing Flood Defences
5
3.0
Extent of Flooding
6
4.0
Suggested Design Features
8
5.0
Conclusions
9
APPENDICES
Appendix 1
Location Plan and Positions of Sections
Appendix 2
TAN 15 Development Advice Map (DAM)
Appendix 3
Hydraulic Modelling by Black and Veatch July 2014
•
•
Appendix 4
Hydrology report
Modelling report
Topographical survey data
•
Cross Sections all Afon Teif showing extreme water levels from
modelling report (BV 2014).
Appendix 5
Predicted Extreme Flood Outlines
Appendix 6
Sections for Re-profil ing quarry
Appendix 7
Potential site layout
Appendix 8
Photographs
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Tregaron Quarry
Flood Consequences Assessment
Document issue details:
BKL project no.
180/14
Client's reference no.
Version no.
Issue date
Issue status
1
July 14
Final Report
Principal Author
Principal Checker
Distribution
Principal Reviewer
B Killingworth
B Killingworth
In producing this report, BKL has relied upon information provided by others. The completeness or accuracy of
this information is not guaranteed by BKL.
COVER PHOTOGRAPH
Abercoed Quarry showing present storage of re-cycling materials and plant used in the
process.
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Tregaron Quarry
Flood Consequences Assessment
1.0
INTRODUCTION
1.1
Tregaron Trading Services Ltd, (TIS Ltd) has commissioned Brian Killingworth Ltd (BKL)
to carry out a Flood Consequences Assessment (FCA) in support of an application for
planning consent for the construction of a building at Abercoed Quarry. Abercoed Quarry is
situated on the left bank of the Afon Teifi floodplain at Abercoed near Tregaron.
1.2
The requirement for the FCA has been identified following consultation with Natural
Resources Wales (NRW). A location plan showing the site is provided as Appendix I of this
FCA. The precise siting of the building within the quarry has yet to be finalised.
The results of this FCA will be key to the final site selection.
1.3
Abercoed Quarry currently benefits from a planning permission to restore the quarry
area by the spreading of inert waste. Due to changes in the way inert type waste materials
are dealt with and the drive to increase recycling rates. TIS Ltd now wish to apply for
planning permission to lise the quarry as an inert and soil waste treatment facility. This
facility will be able to recycle wastes such as soils. construction and demol ition waste and
uncontaminated. excavated materials into secondary aggregates. top and sub-soils. TIS Ltd
proposes to bring inert construction and demolition type wastes. including top and sub soils.
to the site for recycling. These materials will be stockpiled on site until there is sufficient to
screen and/or crush in order to recycle the materials into secondary aggregates and soils for
reuse within the construction and landscaping sectors.
1.4
There is currently a Portacabin and weighbridge on site but planning consent is sought for a
building on sile to replace the Portacabin, This building would provide storage for some of
the waste processing products such as top soils and also provide a storage area for any
unsuitable wastes which may be removed from the incoming waste materials such as plastic.
wood or any unauthorised wastes. The building would enable plant maintenance activities to
take place under cover, and the building would also include an office to provide improved
health and welfare facilities for site staff. It is proposed that a building of approximately 60ft
(18.3m) x 40ft (12.2m) is constructed along with a lean-to section of 20ft (6.1 m) x 60ft
(18.3m). The lean-to area would accommodate the plant and equipment used in the recycling process.
1.5
The site will be surfaced with hardcore which will be maintained in an unrutted state and will
be fit for purpose. As the site will only store inert wastes and soils outside the proposed
building there will be no contaminated surface water run off from the site. Surface water will
enter the existing drainage lagoon system at the site which served the quarry. This drainage
system discharges to the river downstream of the quarry via the natural reed bed system in
the neighbouring field. TIS Ltd have an arrangement with NRW to regularly sample this
discharge point.
1.6
The proposed site will be required to operate under an Environmental Permit issued by
NRW.
1.7
Technical Advice Note 15 (TAN 15) was issued in July 2004 by the Welsh Assembly
Government to provide technical guidance and supplement the policy set out in Planning
Policy Wales 2002 (PPW) in relation to development and flooding. TAN 15 provides a
framework within which risks arising from both river and coastal flooding can be assessed.
The general approach of PPW, supported by the TAN. is to advise caution in respect of new
development in areas at high risk of flooding by setting out a precautionary framework to
guide planning decisions.
1.7
The operation of the precautionary framework is governed by a Development Advice Map
containing three zones A, B and C. with Zone C sub-divided further into C I and C2.
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Flood Consequences Assessment
Table 1.1 - Zone Descriptions and Uses within the Precautionary Framework
Description of Zone
Considered to be at little or no risk of
fluvial or tidal/coastal flooding
A
Areas known to have been flooded in the
past evidenced by sedimentary deposits.
B
Based on Environment Agency extreme
flood outl ine. equal to or greater than
0.1% - I In 1000 year (river. tidal or
coastal)
Areas of the floodplain which are
developed and served by significant
infrastructure including flood defences.
without
Areas of the floodplain
significant flood defence infrastructure.
C
CI
C2
Use within the precautionary framework
Used to indicate that justification test is not
applicable and no need to consider flood risk
further
Used as part of a precautionary approach to
indicate where site levels should be checked
against the extreme (0.1 %) flood level. If site
levels are greater than the flood levels used to
define adjacent extreme flood outline there is no
need to consider flood risk further.
Used to indicate that flooding issues should be
considered as an integral part of decision making
by the appl ication of the justification test.
including assessment of consequences.
Used to indicate that development can take place
subject to appl ication of justification test.
including acceptability of consequences.
Used to indicate that only less vulnerable
development should be considered subject to
application of justification
test. including
Emergency
acceptabil ity of consequences.
vulnerable
development
services and highly
should not be considered.
1.8
The site is undefended and designated as being within Zone C2 on the DAM covering the
area. A copy of this flood map. forms Appendix 2 to this FCA.
1.9
The proposed development is classified within TAN 15 as being less vulnerable. The
development is clearly justifiable by reason of the extant planning consent for the
reinstatement of the Quarry.
CGA&T/CSC
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July 2014
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~.
BLACK & VEATCH
Document no.
lIem
Component
Sheet Rev. no.
120838 I 0370 I 01 I
2
A
016
Project name:
Assistance to B.T.Killingworth
Document name:
Tregaron Quarry - Hydrology
Ltd
CONTENTS LIST
1.
INTRODUCTION
3
2.
CATCHMENTDESCRIPTORS
3
3.
FEHSTATISTICALMETHOD
3
4.
RAINFALL RUNOFFMETHOD
5
5.
RESULTS & DISCUSSION
6
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File name: 0370_Tregaron_FCA_Hydrology.docx
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Flood Consequences Assessment
3.0
EXTENT OF FLOODING
3.1
The Extreme Flood Outline (EFO) shown on the DAM is based on a combination of
historical flood data (if available) or computer models from which Flood Zone maps are
produced. Within the TAN, paragraph 4.3 acknowledges that "while robust for triggering the
application of the tests (for both forward planning and decision making), at the present time it
is inevitable that information will be improved and refined over time." In this instance, the
modelling work carried out as part of this FCA has demonstrated that the EFO based on the I
in 1,000 year event as shown on the DAM (Appendix 2) is accurate in identifying a flood risk
to part of the floor of the Quarry.
3.2
The flow estimates for both the I in 100 (I %) and the I in 1000 (0.1 (je) return period events
for the Afon Teifi were prepared using the Flood Estimation handbook (FEH) with the
hydraulic modelling carried out using software package ISIS v3.6. The results of the model
showing the I in 100, I in 100 + 20% climate change allowance and I 111 tOOO year
conditions are given III Appendix 3 of this Assessment which also includes a fuller
description of the hydrology and the modelling processes applied.
3.3
Flood levels upstream of the disused railway indicate the extent of the Afon Teifi flood plain.
With regard to the Quarry, Cross Section 3 passes through the centre of the site in the
general location of the proposed building. The flood levels shown on the section indicate
that under extreme I in 100 year return period events the left bank of the Afon Teifi will
overtop and floodwater will enter the site up to a level of 153.05m AOD. Once the I in 100
year flow is increased by 20% to allow for climate change, this level increases by 250mlll to
153.30m AOD and increases further to 153.96111under I III 1000 year conditions. The
positions of the cross sections upon which the hydraulic modelling is based plus the actual
sections showing the flood levels derived from the model are provided as Appendix 4.
3.4
As part of the original planning application for the site, in 2000 an Environmental Impact
Assessment (ElA) was prepared on behalf of TIS Ltd by ExCAL Ltd. Part of this work
included a topographical survey. Unfortunately copies of this survey only exist in a pdf
format, which prevents a true scale being applied to the drawings, however, copies of the
drawing provide a good basis for demonstrating the flood outlines from the various extreme
flood events, since they contain a number of spot levels within the Quarry site. Appendix 5
to this Assessment comprises a copy of this drawing showing the predicted flood extents.
3.5
The ExCAL survey was carried out to an arbitrary level datum. However, it has been
possible to correlate both the ExCAL survey and the recent river survey carried out as part of
the preparation of this FCA. The figure of 64.4lm added to the levels shown 011 the ExCAL
survey converts these levels to Ordnance Datum (00) to enable the flood contours shown in
Appendix 5 to be plottecl. For convenience. the flood levels provided for Cross Section 3
within Appendix 3 are given below to both the Arbitrary Datum (AD) and Ordnance Datum
(00).
PeakWater Level m
Model
Node
'.
1 in 100
Survey
1 in 100
1 in 1000
annual probabilitv
Section
annual probabllltv
TEIFI_0790
C53
"
annual probabilitv
+ climate change
AD
OD
AD
OD
AD
OD
88.64
153.05
88.89
153.30
89.55
153.96
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Flood Consequences Assessment
4.0
SUGGESTED DESIGN FEATURES
4.1
The approximate extent of the consented area of re-profiliug is also shown on a further copy
of the ExCAL survey (Appendix 7) together with a potential layout for the ongoing working
of the site and a suggested position for the proposed building.
4.2
Returning to the ExCAL profiles, section AA shows a higher existing band of material
peaking at some 105m along the horizontal scale. The crest level of this material is
approximately 91.0111(AD) or 155.41111(00). The present site entrance roadway finishes at
a level of 89.5m (AD) or 153.91111(00). it is therefore proposed to continue this roadway at
a level of say 90.0m between the existing road and this band of raised material before
looping it back to the existing entrance. The road would then form a flood bund against the
Afon Teifi. Given the small remaining volume of storage within the quarry floor which
would be lost by this construction. it is suggested that the impact of any subsequent increase
in flood level over the remaining area of the Teifi floodplain would be insignificant.
However, if there were concerns, then culverts through the proposed highway extension,
could provide a link between the river and the interior quarry floor, while the quarry
remained operational as a re-cycling plant.
4.3
The building would be constructed within the loop formed by the roadways at a level at or
slightly higher than the road surface to avoid surface water entering the building. The
weighbridge would be re-located to form part of the roadway loop.
4.4
The interior of the quarry floor would be utilised in the interim operational period to store
materials etc. The material presently stored outside the line of the bund formed by the
extended roadway could be moved inside behind the bund. This would provide additional
floodplain storage and also prevent any risk of potential spillage into the river.
4.5
The ground floor of the building and the roadways would be a solid form of construction.
probably concrete.
4.6
All electrical sockets should be mounted a minimum of 500111111
above the Finished Floor
Level (FFL) of the building.
CGA&T/CSC
DFS&f-l/CSC
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Tregaron Quarry
Flood ConsequencesAssessment
3.6
It is worth noting that the I in 100 year return period flood level does not reach the floor of
the quarry. Only when levels increaseas a result of applying climate change to the I in 100
year flow or when the I in 1000 year condition is considered, does the flood outl ine cover
part of the existing floor of the quarry.
3.7
The ExCAL survey was originally prepared to provide an indication of fill profiles and
quantities of inert material required to satisfy these profiles. These profiles in the form of
sections AA and BB and a plan showing the location of thesesectionsthrough the quarry are
included as Appendix 6 to this Assessment. Only section AA applies in terms of the extent
of flooding. On Section AA, which has been preparedfrom the survey and is therefore also
to AD, the flood levels for the I in 100 year, I in 100year + 20% and I in 1,000year return
periods have been plotted. This confirms that only the I ill 100 year + 20% and I in 1000
year flood eventsactually enter the floor of the quarry and that even under these severe
conditions, the depth of flooding is lessthan 0.5m. Further, the extent of the re-profil ing, for
which planning consent has been given, covers part of this flooded area demonstrating a
small reduction in available floodplain once the re-profiling is completed.
CGA&T/CSC
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Flood Consequences Assessment
5.0
CONCLUSIONS
5.1
The following conclusions may therefore be deduced with regard to the development sile:
•
As far as can be determined from the EA W Floodzone plan on their web-site. the
Flood Zone Map is correct at this location in indicating that there is a flood risk to
the floor of the quarry.
•
Part of the present floodplain identified within the floor of the quarry wi II be lost
once the quarry is reinstated in accordance with the existing planning consent.
•
A suggested means of ongoing operation of the quarry by constructing roadways
wh ich would act as flood bunds against the Afon Teifi, would mean a further small
loss of available floodplain.
However. neither the loss of floodplain related to the
existing planning consent. nor the further small loss associated with the suggested
roadway construction outlined above. are considered sufficient to create a significant
increase in flood depth ove!' the remnlnlng extensive Aron Teifl floodplatn.
•
The further loss of floodplain could be offset
by the movement of mnrerials presently
stored within the identified floodplain to the quarry floor behind the bunds created
by the roadways or if it was considered necessary by the construction of cul verts
beneath the roadway bunds to ensure the availability of the quarry floor to
accommodate extreme flood events.
"
The proposed building is classified within TAN 15 as Industrial which has an
indicative maximum depth of flooding of Im. Even without the suggestedprotection
of the roadway bunds, the buildlng could be sited virtually anywhere within the
quarry floor which has an average level of 153.30111AOD. with Il Finished Floor
Level (FFL) of say 153,50m. some 70111111 above the I in 100 yeo!' plus climate
change level of 153.43m AOO. The small area of floodplain lost by the construction
(18.3 x 18.3m
335m2) could be readily compensated by re-profiling elsewhere
within the site.
=
6.2
•
The floor of the residential area of the ground floor needs to be of solid construction.
•
Electrical sockets need to be mounted at least 500mm above floor level. And the
remaining services, especially the foul drainage, need to be designed to take account
of the overland flow passing the property under extreme conditions.
Providing the previous measures are incorporated within the redevelopment proposals. this
FCA has demonstrated that when the proposed development proceeds there will be:
"
Minimal risk to life
•
Minimal potential damage to property
•
Minimal
impact of the proposed development on flood risk generally
CGA&T/CSC
DES&H/CSC
17, DEC 2014
REF/CYF
July 2014
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PM-5561- 121914120213
Appendix 3
Hydraulic Modelling by Black and Veatch July 2014
3.1
3.2
Hydrology report
Modelling report
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BLACK & VEATCH
Document no.
Component
120838 I
Item
Sheet
0370 I 01 I
1
Rev. no.
A
ot6
Project name:
Assistance to B.T.Killingworth Ltd
Document name:
Tregaron Quarry _ Hydrology
Project component: Hydrology for Tregaron Quarry FCA
Document description: Hydrology calculations for Afon Teifi at Tregaron Quarry in mid-Wales
Summary of revision history:
Rev.
Issue date
A
23 Jul14
Description of revision
Initial version
Preparer
Checker
Reviewer
Approver
TSP
BTK
TPMP
TPMP
The full revision history of the document should normally be recorded by using the attributes in Cygnet or ProjectWise.
If there are several reviewers (such as for different disciplines), enter the details in successive rows.
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File name: 0370_Tregaron_FCA_Hydrology.docx
© Black & Veatch Limited
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Date printed: 29 July 2014
A310
Form FA310/17
r--I
PM 5561 121914120215
Project
~.
BLACK & VEATCH
Document no.
lIem
Component
Sheet Rev. no.
120838 I 0370 I 01 I
2
A
016
Project name:
Assistance to B.T.Killingworth
Document name:
Tregaron Quarry - Hydrology
Ltd
CONTENTS LIST
1.
INTRODUCTION
3
2.
CATCHMENTDESCRIPTORS
3
3.
FEHSTATISTICALMETHOD
3
4.
RAINFALL RUNOFFMETHOD
5
5.
RESULTS & DISCUSSION
6
CGA&T/CSC
DES&H/CSC
17 DEC 2014
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File name: 0370_Tregaron_FCA_Hydrology.docx
© Black & Veatch Umited
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(W07)
Date printed: 29 July 2014
A310
Foon FA310/17
PM 5561 121914120217
ID,
Project
BLACK & VEATCH
Document no.
120838
Component
Item
I 0370 I 01 I
Sheet
3
Rev. no.
A
ot6
Project name:
Assistance to B.T.Killingworth Ltd
Document name:
Tregaron Quarry - Hydrology
--~--~--~~------------------------------------------~
TREGARON QUARRY
FLOOD CONSEQUENCE ASSESSMENT
HYDROLOGY
1.
INTRODUCTION
As part of a Flood ConsequenceAssessment (FeA), design flows have been calculated for The Aton
Teifi as it flows past Tregaron Quarry. The site is located to 2km to the south-west of Tregaron in midWales. The Afon Teifi is a large catchment, with an area of around 950km2at its outfall to the sea.The
subject site is located close to the headwaters of the river in the Cambrian Mountains. The FCAis
required because the owner wishes to build a warehouse within the old quarry to store re-cycling
materials. Both FEHstatistical and rainfall runoff methods have been used in this assessment.
2.
CATCHMENT DESCRIPTORS
Where the Afon Teifi flows past Tregaron Quarry, it has a catchment area of 164km2. The headwaters
are located in the Cambrian Mountains and contain some small reservoirs, which are of little influence
on the hydrology of the catchment further downstream. Catchment descriptors are shown below in
Table 2.1.
Table 2.1- Catchment Descriptors
Catchment Descriptor
Aton Teiti
GRID REFERENCE
AREA
ALTBAR
ASPBAR
ASPVAR
BFIHOST
DPLBAR
DPSBAR
FARL
LOP
PROPWET
SAAR
SPRHOST
URBEXT2000
3.
SN 6680058150
164.39
291
264
0.12
0.435
14.99
110
0.984
27.3
0.65
1451
43.63
0.0022
FEH STATISTICAL METHOD
WINFAP-FEH3 was used to derive a pooling group tor the watercourse using the HiFlows-UKdataset
(v3.3.2). Only those stations listed as suitable for pooling were used and is shown in Table 3.1. The
pooling group was indicated as being stronelv.heteraeenenus (H2 =5.231 and was therefore reviewed.
CGA&T/CSC
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File name: 0370_Tregaron_FCA_l-Iydrology.docx
© Black & Veatch Limited
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(W07)
REF/CYF
------------_._-
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BLACK & VEATCH
Sheet Rev. no.
4
A
016
Project name:
Assistance to B.T.KilIingworth Ltd
Document name:
Tregaron Quarry _ Hydrology
The Ribble @ Arnford (71011) was found to have a number of issues.The weir structure suffers from
structural movement due to a geological fault and suffers from weed growth during the summer
months, both factors which could affect gauging; but more critically there is storage just upstream of
the gauging station, in the form of long Preston Ings,and this is believed to result in the unusually low
growth curve for this stationl.
A sensitivity test was carried out to show what impact removing this site had on the growth curve. The
resulting heterogeneity of the pooling group was reduced (H2 = 3.83). Using the GeneralisedExtreme
Value (GEV) distribution (recommended distribution by WINFAP-FEHv3), shows the growth curve
factor for the 1 in 100 annual probability flood to increase from 2.03 to 2.14 (an urbanisation factor is
not applied due to rural nature of catchment).
Table 3.1- Default Pooling Group for Afon Teifi at Tregaron Quarry
Reference
Station Name
Nith @ HallBridge
Luce @ Airyhemming
Urr @ Oalbeattie
Ystwyth @ Pont Llolwyn
Lune @ LunesBridge
Ayr @ Catrine
Ribble @ Arnford
Kent @ Victoria Bridge
Fowey @ Restormel
Wye @ Odol Farm
LugarWater @ langholm
Kinnel Water @ Bridgemuir
Kent @ Sedgwick
Blackwater @ Oerrymeen Br.
TOTAL
Note: Site 71011 removed from final pool group
79003
81003
81001
63001
72015
83003
71011
73012
48011
55026
83004
78005
73005
203022
Length of Record
(years)
47
40
43
51
33
36
42
33
24
43
34
27
44
21
Area
(km2)
155.76
170.87
197.07
174.54
140.83
167.21
203.22
183.23
167.21
172.17
203.7
229.26
212.19
183.49
Distance
0.163
0.212
0.326
0.338
0.389
0.403
0.405
0.435
0.452
0.459
0.471
0.477
0.49
0.498
518
QMED was calculated from catchment descriptors using the revised QMED equation. Within the Teifi
catchment, there are two gauging stations; both listed as suitable for QMED estimates, which might
then be used to improve the estimate of QMED for the subject site. These are both located
downstream of Tregaron and have substantially larger catchment areas:
•
•
62002 Teifi @ lIanfair: catchment area = 518km1 (~320%larger)
62001 Teifi @ Glanteifi: catchment area = 897km1 (~550%larger)
The indication from these catchments is that QMED is over estimated from catchment descriptors and
that an adjustment factor ofO.6 should be applied. This creates a significant reduction in QMED;even
accounting for the distance between catchment centroids by using a moderation term, as shown in
Table 3.2.
The neighbouring catchment to the north of the Ystwyth is also gauged (63001 Ysthwyth @ Pont
CGA&T/CSC
I http://www.ceh.ac.uk/data/nrfa/data/peakflow.htmI?7101
File name: 0370_Tregaron_FCA_Hydrology.docx
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....
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!lem
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Sheel
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Document name:
Tregaron Quarry - Hydrology
Llowyn) and the site is included in the pooling group. The catchment descriptors for the site show
clear similarities to the Teifi catchment at Tregaron quarry and the catchment centroids are closer.
This donor catchment indicates an under estimate in QMED and that an adjustment factor of 1.11
should be applied, which would result in a slight increase in QMED once the moderation term is
applied, as shown in Table3.2.
Table 3.2 - Estimates of QMED
Possible Donor Sites
Parameter
62002
62001
63001
QMEDobs (m'/s)
126.5
193.7
93.9
QMEDcd (m3/s)
210.1
311.5
84.6
Factor
0.60
0.62
1.11
15.6
27.6
10.0
81.9
96.7
.'
Distance of Centroids
(km)
QMEDadj (m3 Is)
78.3
J
*QMEDfrom catchment descriptorsfor subject site = 92.9 m Is
The estimates in QMED clearly vary quite considerably. The two sites within the same catchment
indicate that QMED from catchment descriptors is over estimating flows significantly. However,
although these two sites are in the same catchment, there is a large difference in catchment area and
the intervening area could have significant hydrological influence that is not representative of the
subject site located much closer to the headwaters.
The neighbouring catchment of the Ystwyth indicates that QMEDis slightly underestimated. The site is
included in the pooling group for the subject site and in terms of geographical location and size is a
better representative of the subject site. A QMED estimate of 96.7 m3fs will therefore be used. Design
flows calculated from the statistical method are given in Table 5.1.
4.
RAINFALL RUNOFFMETHOD
Designflows have been calculated using the Revitalised Flood Hydrograph (ReFH)method. The critical
storm duration was calculated as 8.25 hours, assuming a data interval of 0.25 hours and rounding the
storm duration to nearest odd integer of the data interval. The design flows calculated by this rainfall
runoff method are given in Table 5.1. The ReFHmethod is now seen as superseding the original FEH
rainfall-runoff method with only a few exceptions; there were no exceptional circumstances for this
catchment.
CGA&T/CSC
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17 DEC 2014
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File name: 0370_Tregaron_FCA_Hydrology.doex
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Iil.
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Document no.
BlACK&VEA TCH
I 0370 I 01 I
A
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016
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Assistance
Document name
Tregaron
5.
120838
Rev.no.
sneet
Item
Componenl
to B.T.KilIingworth
Ltd
Quarry - Hydrology
RESUL TS & DISCUSSION
Design flows derived using the FEH statistical and rainfall runoff methods for the Afon Teifi are shown
in Ta ble 5.1.
Table 5.1 - Design Flow Results the Afon Teifi at Tregaron Quarry
Return Period
(years)
QMEO
100
1000
Peak Flow (mS Is) - Statistical (GEV)
Default Pooling Group
Adjusted Pooling Group
96.7
96.7
206.5
188.6
204.5
226.0
Peak Flow (mJ/s)
- ReFH
103.3
246.0
421.0
The design flow for the 1 in 100 annual probability flow from the ReFH method is at least 13% higher
than that derived from the statistical method, which is significantly higher. The FEH does state that
beca use the statistical method has been developed using a much larger database of flood events and
mor e directly calibrated to reproduce flood frequency for the UK, it is often preferable to the rainfallruno ff approach. However it is recognized that a precautionary approach is often taken when
cons idering flood risk, in which case the ReFH approach would be used.
method (with a few
Whe n the ReFH method was introduced to replace the FEH rainfall-runoff
exce ptions), it had been calibrated using flood frequency curves derived from pooling analysis and for
this reason tended to provide peak flows more consistent with the FEH statistical method. This is not
produces a similar result to the ReFH method'.
the case here and indeed the FEH rainfall-method
Based on this, the implication is that the ReFH method is possibly over estimating peak flows for this
catc hment and we would recommend that the GEV statistical approach is used. The adjusted pooling
grou p results should be used as the Ribble @ Arnford is shown not to be representative of the Teifi
catc hment. This increases the peak flow estimates in comparison to the default pooling group.
In ad dition, to avoid underestimating the 1 in 1000 annual probability flow, but retain continuity, we
reco mmend using the ratio between the 1 in 1000 and 1 in 1000 annual probability flows from the
ReFH method (1.71) and applying that to the 1 in 100 annual probability flow from the statistical
met hod (206.5 m3js). This gives a 1 in 1000 annual probability flow of 353 ml/s. The adopted design
flow s are given in Table 5.2.
Table 5.2 - Adopted Design Flows
Return Period (years)
Peak Flow (m3/s)
100
100 + climate change
1000
206.5
247.8
353
_.
..
CGA&T/CSC
DES&H/CSC
11 DEC 2014
2 FEHrainfall run off 1 in 100 annual probability peak flow = 242 m3/s
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Tregaron FCA- Hydraulic Modelling
Project component: Hydraulic Modelling for Tregaron FCA
Document description: Short report detailing modelling of the Aton Teifi, which presents flood risk to
a proposed development within Tregaron Quarry
Summary of revision history:
Rev.
A
Issue date
23 Jul14
Preparer
Description of revision
Initial version
TSP
Checker
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Approver
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Rev. no.
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Document name:
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CONTENTS LIST
1.
INTRODUCTION
3
2.
MODEL BUILD
3
3.
RESULTS
4
4.
SENSITIVITYANALYSIS
5
5.
CONCLUSION
7
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Rev. no.
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to B.T.Killingworth
Ltd
Document name:
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CGA&T/CSC
DES8:H / esc
PANDY MILL FLOOD CONSEQUENCE ASSESSMENT
HYDRAULIC MODELLING
1.
17 DEC 2014
REF/CYF
INTRODUCTION
Hydraulic modelling of a short reach of the Afon Teifi, where it flows past Tregaron Quarry (located
2km south-west of the town of Tregaron in mid-Wales), has been carried out as part of a Flood
ConsequencesAssessment(FCA).The FCAis required becausethe owner wishes to build a warehouse
in the old quarry to store re-cycled materials. The modelling will be used to assessthe flood risk to the
development site from the 1 in 100 annual probability flood, the 1 in 100 annual probability flood with
climate change and the 1 in 1000 annual probability flood.
2.
MODELBUILD
A 990m long reach of the Aton Teifi was modelled as an unsteady-state model in ISISv3.6. The model
starts around 200m upstream of the quarry and includes no structures until it reaches the disused
railway embankment at the downstream end. The proposed development will be located within the
old quarry works, south west of Tregaron, on the left-bank side of the Aton Teifi.
Channel
The model has been built using 4 surveyed sections, which were supplied on behalf of Brian
Killingworth Ltd, by Jim Wright Surveying (July 2014). The Afon Teifi channel is IS-20m wide at banktop and has a wide floodplain between ISO - 300m with well-defined steep sides.Although the model
can show glass-walling along the right edge of the floodplain, the steep valley sides meant that
extending the sections further would make little difference to modelled water levels.
Roughness
The watercourse winds its way along the valley. It has a gravel bed and also contains a significant
amount of weed. In a few places the banks are lined by trees and bushes; but are generally an
extension of the floodplain, which is formed from well cropped grassfrom grazing. Roughnessvalues
using Manning's 'n' have been applied as shown in Table 2.l. Sensitivity of the model to roughnesshas
been tested; refer to Section 4.
Table 2.1 - Roughness Values
Description
Bed - winding channel, gravel bed and weed
Banks-lined with trees and bushes
Banks/Floodplain - well cropped grassfrom grazing
Manning's 'n'
0.045
0.060
0.035
Structures
Along this reach of river, there are no structures in the channel. At the downstream end of the
modelled reach, a disused railway embankment crosses the floodplain and impedes the flow. The
bridge deck across the Teifi has long since been removed, but the abutments remain, leaving a 17m
wide span for the channel to passthrough. Around 10m further along the embankment (on the rightbank side) there is an underpass structure, which is arch - shaped but no more than 4m in width
(springing level not surveyed and has been estimated from a photograph).
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of7
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Document name:
Tregaron rCA _ Hydraulic Modelling
----~----------~--------------------------------------~
Although there is no bridge deck across the Teifi, the obstruction caused by the railway embankment
that would force flow through the opening means that it would act as a bridge. This has been
modelled using an ARCHbridge unit (node: TEfFI_OOOOb),
with the soffit and springing level set well
above any possible flood level. The underpass has also been included in the same bridge unit, as in
large floods the underpasswould convey flow.
Boundary Conditions
The model has been run using hydrographs generated by the ReFHunit, which has been scaled to fit
the peak of the design flow from the FEHstatistical method (see 120838/0370/01 Tregaron_ FCA_
Hydrology for details). Table 2.2 gives the peak flow for the modelled events. Climate change is
represented by increasingthe flow by 20%.
Table 2.2 - Peak Flow for Modelled Events
Peak Flow (m~/s)
Return Period(years)
100
100 + climate change
1---------1000
206.5
247.8
353
The watercourse is as an average gradient of 1 in 350 along the modelled reach and based on the
1:25,000 as map appears to have a similar gradient downstream. The river section at the railway
embankment has been copied to 300m downstream of the embankment and lowered by 0.86m based
on the 1 in 350 gradient. Only the channel part of the section has been retained and the floodplain
then coarsely defined using 1:25,000 as map contours. A NCDBYboundary has been used with the 1
in 350 gradient applied. The sensitivity of the model to the downstream boundary has also been
tested; refer to Section 4.
3.
RESULTS
The model was run for the 1 in 100 annual probability flood, 1 in 100 annual probability flood with
climate change and the 1 in 1000 annual probability flood. The resulting peak water levels are given in
Table 3.1. A long-section of peak water levels is shown in Figure 3.1. The results show that the
embankment across the floodplain is having a significant impact in increasingwater level upstream of
it. The head loss across the embankment in the 1 in 100 annual probability flood is nearly 1.2m,
increasing to 1.35m when climate change is included. The result is quite a flat water surface profile
upstream of the embankment.
Model node TEIFf_0790will be used to depict the peak water level and floodplain extent in the area of
the quarry where a warehouse is proposed.. This gives a peak water level of 153.0Sm00 for the 1 in
100 annual probability flood, increasing by 2S0mm when climate change is included. These levels will
be used to determine the extent of the floodplain within the old quarry and thus where the
warehouse can be built. The 1 in 1000 annual probability flood increases the peak water level to
153.96m Oü, which would result in flooding of the warehouse, but this is an acceptable consequence.
CGA&T/CSC
DFS~,H J r"'r::.C
File name: 0370_Tregaron_FCA_Modelling.docx
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..
Date printed: 29 July 2014
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PM 5561 121914120229
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~,
Document no.
BLACK & VEATCH
Component
120838 I
Item
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0370 I 02 I
Rev. no.
5
A
of7
Project name:
Assistance to B.T.KiIlingworth Ltd
Document name:
Tregaron
FCA -
Hydraulic Modelling
Table 3.1- Peak Water levels
Peak Water Level (m LD)
Model
Node
Survey
Section
CS4
CS3
CS2
CS1
TEIFI 0990
TEIFI 0790
TEIFI 0404
TEIFI OOOOu
Figure 3.1 -long-section
1 In 100
annual probability
1 in 100
annual probability
+ climate change
1 in 1000
annual probability
153.23
153.43
154.02
153.05
153.30
153.96
152.85
153.17
153.90
152.77
153.11
153.84
showing Peak Water levels along Afon Teifi
154
153.8
153.6
153.4
153.2
153
152.8
152.6
152.4
152.2
152
_.'
.--.i->:
149.4
149.2
149
148.8
148.6
148.4
ü:
NO<IeLabel
-
4.
10000R
-
IOOYR+CC- IOOVR
ü:
W
w
I-
0 Bed
I-
••. Lefl BaIlk
•.
Right
ü:
W
I-
Bank I
SENSITIVITY ANALYSIS
Because there is no data against which to calibrate the model, a sensitivity test of the model is
necessary to give information on the possible error range and whether this is significant. The following
sensitivity tests were carried out on the 1 in 100 annual probability flood model:
•
•
•
Change peak flow by +/-20%.
Change Manning's 'n' by +/-10%.
Change water level in downstream boundary by +O.5m.
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Date printed: 29 Juil 2014
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PormFA 10/17
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ID.
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BLACK & VEATCH
Document no.
Component
Item
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Rev. no.
A
120838 I 0370 I 02 I 6
of7
Project name:
Assistance to B.T.KilHngworth Ltd
Document name:
Tregaron
FeA _ Hydraulic
Modelling
The sensitivity model results are given in Tables 4.1 - 4.3. The 20% increase in flow results in an
increase in peak w ater level 0.34m immediately upstream of the embankment, reducing to O.20m
increaseat the quarry. A reduction in flows of 20% hasa greater impact, with water levels reduced by
0.43m at the embankment and by 0.27m at the quarry. Peakwater levels are lesssensitive to changes
in roughness, with water levels varying around +/-0.10m depending on whether roughness is
increased or decreased. Increasing the downstream boundary level by O.5m shows some effect along
the whole modelled reach, but upstream of the embankment the increase in peak water level is less
than 0.10m.
In summary, the model displays an expected level of sensitivity to roughness and the downstream
boundary condition . The model is more sensitive to changes in flow and demonstrates a variation in
peak water level of at least O.25m on the quarry site. Although equally sensitive to an increase or
decreasein flow, it is recommended that the warehouse development is situated within the quarry as
conservatively as possible to take account of this uncertainty.
Table 4.1 - Model Sensitivity Resultsto Flow
1 in 100 annual
Model Node
probability
PWL
(m LD)
20% increasein
flowPWL
(m LD)
Difference
20% decrease
(m)
in flow PWL
(m LD)
Difference
(m)
TElFI_0990
153.23
153.43
0.20
153.05
-0.18
TEIFI_0790
153.05
153.30
0.25
152.78
-0.27
TEIFI_0404
152.85
153.17
0.32
152.45
-0.41
TEIFI_OOOOu
152.77
153.11
0.34
152.34
-0.43
TEIFI_OODOd
151.58
151.76
0.18
151.37
-0.21
TEIFI_-0300
150.84
151.02
0.18
150.62
-0.21
Table 4.2 - Model Sensitivity Resultsto Roughness
1in 100 annual
Model Node
probability
PWL
(m LD)
10% increasein
roughnessPWL
(m LD)
10% decrease
Difference
(ml
in roughness .,'
PWL
(m LD)
Difference
(m)
TElFI_0990
153.23
153.30
0.07
153.16
-0.07
TEIFI_0790
153.05
153.13
0.08
152.95
-0.10
TEIFI_0404
152.85
152.94
0.09
152.72
-0.13
TEIFI_OODOu
152.77
152.87
0.10
152.65
-0.12
TEIFI_OOOOd
151.58
151.66
0.09
151.51
-0.07
TEIFI_-0300
150.84
150.94
0.10
150.75
-0.09
CGA&T/CSC
DES&H/CSC
17 DEC 2014
File name: 0370_Tregaron_FCA_Modelling.docx
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Date printed: 29July2014
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Form FA310/17
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Project
~.
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Document no. 120838 I 0370 I 02 I
7
Rev. no.
A
of7
Project name:
Assistance to B.T.Killingworth Ltd
Document name:
Tregaron FCA - Hydraulic Modelling
Table 4.3 - Model Sensitivity Resultsto Downstream Boundary
,
!.
D/bdy levels increased
by 0.5m(m LO)
Diffèrence
(m)
153.23
153.25
am
TEIFI 0790
153.05
153.09
0.04
TEIFI 0404
152.85
152.91
0.06
TEIFI OOOOu
152.77
152.85
0.08
TEIFI OOOOd
151.58
151.69
0.11
TEIFI -0300
150.84
151.34
0.50
Mof:iel Node
TEIFI 0990
5.
1 in 100 annual
probability
PWL
(mOD)
-c
F'
'"
,
CONCLUSION
An unsteady-state model of a short reach of the Afon Teifi was built to assessthe flood risk to a
proposed warehouse development within the old quarry, 2km to the south-west of Tregaron. The
model has been used to determine a peak water level in the quarry so that the warehouse can be
situated outside the floodplain. The model gives a peak water level of 153.05m 00 for the 1 in 100
annual probability flood, increasing to 153.30m OD when climate change is accounted for. The model
is shown to be reasonably sensitive to flow, so it is recommended that the warehouse is positioned
conservatively, as far away from the floodplain as possible. The 1 in 1000 annual probability flood
increases water levels by nearly a further 700mm, which will result in the flooding of the proposed
warehouse.
CGA&T/CSC
DES&H/CSC
17 DEC 2014
REF/CYF
File name: 0370_Tregaron_FCA_Modelling.docx
© Black & Veatch Limited
?/13
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Date printed: 29 July 2014
A310
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PM-5561 - 121914120233
Appendix 4
Topographical survey data
Cross Sections on Afon Teifi showing extreme water levels
from modelling report (BV 2014).
CGA&T/CSC
DES&H / esc
17, DEC 2014
REF/CYF
PM 5561 121914120235
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Photo 1
Disusedrailway embankment, main Afon Teifi Bridgewith the deck removed and the
stock underpass at the rhs of the picture
Photo 2
Typical floodplain upstream of the disused railway embankment
CGA&T/CSC
DES&H/rSC
17. DEC 2014
REF/CYF
PM 5561 121914120252
Photo 3
Left bank floodplain at CS 2
Photo 4
Right bank floodplain at CS2
CGA&T/CSC
DES&H I esc
17 DEC 2014
REF/CYF
PM 5561 121914120253
Photo 5
Left bank floodplain - quarry at CS3. Cover picture.
Photo 6
Right bank floodplain at CS3
CGA&T/CSC
DES&H / esc
11. DEC 2014
REF/CYF
~PM
- 5561 - 121914120255
Photo 7
Left bank floodplain up to road at CS4
Photo 8
Right bank floodplain at CS4
CGA&T/CSC
esc
17 DEC 2014
DES&H I
REF/CYF
This report was prepared by :
BRIAN KILLINGWORTH
CGA&T/CSC
DES&H /
esc
17DEC 2014
REF/CYF
LTD
Brooklands
Cemlyn Park
Penmaenmawr
Conwy
LL346BP
Tel: 01492622401
e-mail: [email protected]