FLOOD RISK ASSESSMENT, PERTAINING TO
THE PROPOSED SMARTSTAGE
DEVELOPMENT ADJACENT TO THE FORMER
DEESIDE TITANIUM SITE, DEESIDE,
FLINTSHIRE
FLOOD RISK ASSESSMENT, PERTAINING TO
THE PROPOSED SMARTSTAGE DEVELOPMENT
ADJACENT TO THE FORMER DEESIDE TITANIUM
SITE, DEESIDE, FLINTSHIRE
Prepared By:
Opus International Consultants (UK) Ltd
G N Taylor
Engineer
Reviewed By:
W J Grindley
Divisional Director
© Opus International Consultants (UK) Ltd 2009
Burton House
Rossett
Nr Wrexham LL12 0HY
Telephone:
Facsimile:
01244 571 444
01244 570 272
Date:
Reference:
Status:
RC5304
Draft
Flood Risk Assessment, Pertaining to the Proposed Smartstage Development
CONTENTS
1
EXECUTIVE SUMMARY ....................................................................................................... 1
2
INTRODUCTION ................................................................................................................... 2
3
EXISTING SITE ..................................................................................................................... 3
4
FLOOD ZONE CLASSIFICATION ......................................................................................... 6
5
FLUVIAL AND TIDAL FLOOD RISK ..................................................................................... 8
5.1 River Dee & Dee Estuary: ............................................................................................ 8
5.2 Shotwick Brook:............................................................................................................ 8
5.3 Unnamed Watercourses:............................................................................................ 10
5.4 Effects of Climate Change:......................................................................................... 10
6
FLOOD MITIGATION AND SURFACE WATER DRAINAGE STRATEGY........................... 12
7
Access and Egress ............................................................................................................ 13
8
CONCLUSIONS AND RECOMMENDATIONS..................................................................... 14
APPENDIX A
APPENDIX B
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Flood Risk Assessment, Pertaining to the Proposed Smartstage Development
1
EXECUTIVE SUMMARY
This Flood Risk Assessment (FRA) is compliant with the requirements set out in Technical
Advice Note 15 – Development and Flood Risk (TAN15). The FRA has been produced on
behalf of Legat Owen, the land agents for the Smartstage development, in respect of a
planning application for the proposed industrial development (B1, B2 & B8 Units) on land
adjacent to the Former Deeside Titanium Site on the Deeside Industrial Park, which is
allocated within the Unitary Development Plan for this class of development (approximate
grid reference: SJ 331500 371659).
The site is situated approximately 2km north of the River Dee and 2km west of the
Dee Estuary and lies within a Zone C1 flood risk area (an area of floodplain which is
developed and served by significant infrastructure, including flood defences).
Evaluating the risk of flooding, from all potential sources, has concluded that the risk from
tidal inundation should a breach of the defences along the River Dee or Dee Estuary
occur poses the greatest risk to the development site.
However, it is the landowners
intention to raise the site levels to 7.1m AOD to match his adjoining property, the former
Deeside Titanium site, with finished floor levels for the proposed building units set at
7.35m AOD which is above the 1 in 1000 year tidal levels and offers a far greater level of
protection than required to satisfy the requirements of TAN15.
A surface water drainage strategy has been included within this FRA which demonstrates
that a piped system with associated attenuation lagoons can be provided to attenuate
flows to the green field run off rate of 33l/s, which has been agreed with the Environment
Agency Wales.
The application of SuDS has been investigated and the potential application of features
are provided on the attached surface water drainage strategy drawing showing the
locations of the attenuation lagoons providing volumes of storage up to the 1 in 100 year +
20%. The potential application of soakaways will be investigated further during detailed
design and once a source of fill material has been agreed for the site.
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2
INTRODUCTION
This Flood Risk Assessment (FRA) is compliant with the requirements set out in Technical
Advice Note 15 – Development and Flood Risk (TAN15). The FRA has been produced on
behalf of Legat Owen, the land agents for the Smartstage Development, in respect of a
planning application for the proposed industrial development (B1, B2 & B8 units) on land
adjacent to the former Deeside Titanium Site on the Deeside Industrial Park.
Site Name:
Land adjacent to the former Deeside Titanium site
NGR:
SJ 331500 371659
Site Area (Ha):
7.8
Development Type:
Industrial
EA Development Control Area:
Environment Agency Wales
Local Planning Authority:
Flintshire County Council
The report is based upon the following information:
(i)
Site Layout Plan
(ii)
Topographical Survey
(iii)
Flood Levels by Environment Agency Wales (EAW)
(iv)
Report pertaining to the extension of the Deeside Industrial Park (DIP) carried by
Veryards & Partners, prior to the reclamation and development of the DIP
(v)
Site visits undertaken by Opus International Consultants (OIC)
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3
EXISTING SITE
The proposed development site is located within the northwest corner of the Deeside
Industrial Park adjacent to the former Deeside Titanium site, as shown in Figure 1.1.
Figure 3.0 – Site Location Plan
The land is currently designated as ‘green field’ but is listed within the Unitary
Development Plan (UDP) for this particular class of development.
Drawing No. FCA (50) 01 within Appendix A shows the existing site Areas A & B including
surveyed topographical data carried out in June 2004. The level information shows that
Area A is relatively flat with levels between from 3.9m AOD and 4.6m AOD over its 450m
length. Area B is again relatively flat with levels predominately in the order of 6.75m AOD
over its area.
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A watercourse, constructed to serve the Deeside Industrial Park, runs along part of the
eastern boundary and the full length of the southern boundary, and is designated ‘Main
River’ due to its connection with Shotwick Brook and the attenuation lagoons at the
water meadows.
Another watercourse, not classified as ‘Main River’, runs in a north to south direction,
bisects Areas A & B and connects to the aforementioned watercourse, via a 750mm
diameter culvert.
In 1982 Veryard & Partners (now Opus International Consultants (UK) Ltd) were
commissioned by the Welsh Development Agency to prepare a Development Plan for the
extension of the Deeside Industrial Park, which culminated in the production of a report
titled ‘The Extension of the Deeside Industrial Park’.
The report recommended raising the general level of the site with the finished level
contained from 6.4m AOD at the centre to 5.5m AOD towards the east and south sides.
Land levels were subsequently raised by hydraulically placing sand fills, sourced from the
White Sands area in the Dee Estuary (extracts of the report relevant to this FRA are
provided in Appendix B).
The area of the proposed development site was privately owned by Deeside Titanium.
Consequently, the site levels were not raised during the Grant aided hydraulic land fill
exercise. However, the proposed development site formed part of Shotton – Deeside
development zone and as such was set aside for future filling and development.
It is stated within the report that the Deeside Industrial Park would require raising to a
minimum level of 5.5m AOD to provide adequate freeboard above the normal maximum
water level in the water meadows and associated watercourses / canals.
The River Dee lies approximately 2 kilometres to the south and the adjacent land is
protected by local flood defences. Further flood defence is provided to the area by an
earth bank known as ‘Broken Bank’, which lies approximately 2 kilometres to the west, and
runs north to south from Burton Point to Summers Jetty on the River Dee. Broken Bank
lies at a level of approximately 7.9m AOD. The Wrexham to Bidston Railway Line (which
was constructed at a level of approximately 8.0m AOD) acts as a secondary line of flood
defence to the west of the site.
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4
FLOOD ZONE CLASSIFICATION
As shown in Figures 4.0 and 4.1 the site is considered to be at risk of flooding, as
determined by the TAN15 Development Advice Maps (DAM) for the area. The maps are
based upon the output of flood zone mapping work undertaken by the Environment
Agency Wales (EAW).
Figure 4.0
Figure 4.1
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The TAN15 DAM shows that parts of the site lie within the extreme flood outline, and
within a Zone C1 flood risk area. Zone C1 is defined by TAN15 as an area of floodplain
which is developed and served by significant infrastructure, including flood defences.
This report will assess the potential risk of flooding to the proposed development and, if
necessary, recommend any mitigation measures that may be required in order to meet the
requirements of TAN15.
Sources of Flood Risk
This table below the potential sources of flood risk to the site:
Flood Source
Description
Presence
Fluvial
In Flood Zone C1
Tidal
In Flood Zone C1
Canals
X
Groundwater
Presence of groundwater encountered on site.
Sewers
Existing sewer network within Tenth Avenue.
Reservoirs
Reservoirs located approx 0.7km to the north.
Pluvial Run-off
Risk from pluvial runoff on site to be mitigated.
Development Drainage
Potential to increase flood risk to wider catchment if
not mitigated.
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5
FLUVIAL AND TIDAL FLOOD RISK
5.1
River Dee & Dee Estuary:
The site is located within the catchment of the River Dee and Dee Estuary with the DAM
indicating that the area is protected by defences up to the 0.5% probability event (1 in 200
year return period). The flood risk is primarily due to a failure or breach of these defences
causing tidal inundation from the River Dee or Dee Estuary. Flowing consultation with the
EAW, it has been confirmed that tidal levels for the area are 6.89 and 7.18 for the 0.5%
probability event and the 0.1% probability event (1 in 1000 year return period)
respectively.
Should a breach occur at the Broken Bank embankment then flow, to the east, towards
the development site would be impeded by the embankments of the A548 and the Shotton
to Neston railway line.
Similarly, a breach to the north bank defences along the River Dee, downstream of the
Hawarden railway bridge would again be impeded by the same railway embankment.
In the event of a breach occurring upstream of the Hawarden railway bridge flows would
be impeded by the embankments of the A550 and the Chester to Shotton Railway line.
In consequence of the above, it could be argued that, as a result of the existing tidal
defences being in place, together with the further protection offered by the infrastructure
embankments associated with the highway and railway embankments, should the tidal
defences fail, the risk of the development site flooding to the levels as indicated by the
EAW would appear reduced.
5.2
Shotwick Brook:
Shotwick Brook rises in the Willaston Area and flows down from the uplands, crossing a
flat plain before finally discharging to the River Dee in the vicinity of Hawarden railway
bridge. The watercourse is designated ‘Main River’ by the EA. Adjacent to Shotwick
Brook there are three water meadows, which were constructed to serve as attenuation
lagoons for the Deeside Industrial Park development. The water meadows are set at a
higher level than the bed level of Shotwick Brook and, under normal conditions, are not
connected with watercourse. The outfall from Shotwick Brook to the River Dee is affected
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by tide levels in the estuary. At high water the tidelock period lasts for about four hours
during which time no discharge from the brook is possible.
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However, as soon as water levels in the brook exceed 3m AOD the water meadows begin
to fill providing additional storage to Shotwick Brook during ‘tide’ lock’ situations when the
River Dee is influenced by the incoming tide.
5.3
Unnamed Watercourses:
The watercourses associated with the Deeside Industrial Park, such as the ones adjacent
to the development site, form a series of open channels of flat gradients (similar to canals)
constructed to serve as a means of conveying surface water run-off generated by the
development, via the water meadow attenuation lagoons, to Shotwick Brook and;
therefore, are also influenced during tidelock periods.
Within the Veryard & Partners’ report it was determined that water levels within the water
meadows and associated watercourses / open channels should not rise above 4.2m AOD
during normal storm conditions and as such the site levels of the Deeside Industrial Park
would require raising to a minimum level of 5.5m AOD to provide adequate freeboard
above the normal maximum water level in the water meadows and associated
watercourses / open channels.
Taking into consideration the tidal influence of the River Dee on Shotwick Brook the 1 in
200 year tidal event will pose the greatest risk to the development site; therefore, the risks
posed from other sources of flooding are negated and, although considered, will not be
discussed further within this report.
5.4
Effects of Climate Change:
In accordance with Section E, Appendix 1 of TAN 15 an assessment of the likely impact of
long term sustainability of the development, in considering the effects of climate change
will make.
Current guidance on the effects of climate change summarised by Appendix 2 of TAN 15,
recommends that consideration of tidal levels be given through the provision of an
increase in flood levels by 250mm to 300mm over the design life of a scheme. As stated
previously the 1 in 200 year tidal flood level is predicted to be 6.89m AOD, therefore, in
consideration of climate change a final level of 7.19m AOD may be expected. As the site
levels will be raised above the expected level created by climate change the effects on the
site are negligible.
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Assessing the effects of climate change on rainfall, suggests that flows resulting from
increased rainfall over catchments may lead to a 20% increase in flows throughout
catchments, therefore, the 1 in 100 year attenuation volume provided will be increased by
20% to cater for the effects of climate change.
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6
FLOOD MITIGATION AND SURFACE WATER DRAINAGE STRATEGY
The greatest potential flood risk to development site is from a breach of the defences to
the River Dee or Dee Estuary.
It is the landowners intention to raise the site levels to 7.1m AOD to match his adjoining
property, the former Deeside Titanium site, with finished floor levels for the proposed
building units set at 7.35m AOD which is above the 1 in 1000 year tidal levels and offers a
far greater level of protection than required to satisfy the requirements of TAN15.
The existing green field site will be developed for industrial units including estate roads,
car parking and service yard areas. The site access will be off Tenth Avenue and
approximately 75% of the green field site will become impermeable in its developed state.
The surface water run-off from the development will be conveyed within conventional
gravity sewers and discharged to the local watercourses at a rate of 33l/s which has been
agreed with the EAW. On-site attenuation will be provided in the form of open channelled
storage lagoons and will provide protection up to the critical 1 in 100 year + 20% storm
event. None of the existing sewers in DIP2 are adopted consequently the sewers will not
be offered for adoption at this time but will be designed and constructed in accordance
with Sewers for adoption 6th Edition to facilitate future adoption at such time DIP2 sewers
are adopted as public sewers.
It is proposed that the none ‘Main River’ watercourse will be diverted around the northern
boundary of the site and connected into the head of the watercourse classified as
‘Main River’. Initial discussions regarding this proposal have taken place with the EAW.
The EAW have indicated that they are not minded to object to this proposal, as long as
the loss of storage within the watercourse is minimised. It is; therefore, proposed to widen
the watercourse bed width over its diverted length from 3.5m to 6.5m to compensate
against the reduced length and maintain the ‘status quo’ of the storage volume. The
necessary details will be submitted to EAW to obtain their written consent of the proposed
works.
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7
Access and Egress
The primary access and egress to serve the proposed development will be via Tenth
Avenue within the Deeside Industrial Park which leads to the A548.
The level of Tenth Avenue is in the order of 5.5m AOD, which is common for the Deeside
Industrial Park.
As the Deeside Industrial Park currently lies within an Environment Agency flood zone
warning area we would recommend that each unit on the proposed site signs up to the
Environment Agency’s Floodline Warnings Direct, to allow them to be warned about any
incidents and to evacuate site.
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8
CONCLUSIONS AND RECOMMENDATIONS
This FRA demonstrates that the development of the proposed site can safely be carried
out without conflicting with the requirements of TAN15, subject to the following:
•
Ground levels of the site are raised and finished floor levels are set at a minimum
of 6.9m AOD (1 in 200 year tidal flood level).
•
Surface Water run-off from the development will be limited to ‘green field’ run-off
rates agreed with the EAW and appropriate on-site attenuation up to the 1 in 100
year standard will be provided.
The requirements of TAN15 have been satisfied as follows:
•
The development site is listed within the UDP for industrial use and therefore
complies with the with justification test (points i or ii and iii).
•
The potential consequences of a flooding event for the development have been
considered and this FRA report shows that the consequences will be managed
down to an acceptable level for the type and extent of development proposed.
•
Mitigation measures can be incorporated within the design to ensure that the
development is as safe as possible and there is:
•
•
•
•
•
Minimal risk to life.
Minimal disruption to people living and working in the area.
Minimal potential of damage to property.
Minimal impact from the development on flood risk generally.
Minimal disruption to natural heritage.
Other TAN15 considerations are:
•
Where adopted plans contain allocations, then it is considered reasonable for
Planning Authorities to require development proposals to plan for the consequence
of flooding
•
Sustainable drainage systems will be incorporated in the development proposals in
order to control surface water as near to the source as possible. To this end,
further consideration will be given to SuDS solutions when the fill material has been
identified.
•
The proposals conform to the requirements for industrial development.
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APPENDIX A
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APPENDIX B
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