The Maltings, Cardiff
Flood Consequence Assessment (FCA)
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+ 33 Cardiff Road, Taff's Well, CARDIFF, CF15 7RB ( 029 2081 3385
* [email protected]
www.earthsciencepartnership.com
The Maltings, Cardiff
Flood Consequence Assessment (FCA)
Prepared for:
Morgan Sindall Construction
& Infrastructure ltd
Unit 5
Cae Gwyrdd
Greenmeadow Springs Business Park
Tongwynlais
Cardiff
CF15 7AB
Document Reference: ESP.5998b.2384
Revision
Status
01
Final
Date
October
2015
Checked by
D Bettosi
BSc, MSc, FGS
Written and Approved by
G Sommerwill
BSc, MSc, CGeol, FGS, RoGEP Specialist
Signature:
Notes:
N.B.
1. This report has been checked and approved in accordance with Earth Science Partnership's
accredited (ISO 9001) Quality Management System.
Once issued this document is Uncontrolled, for the latest version and/or to confirm you have authorisation to use it please contact the
Earth Science Partnership at [email protected] or by telephone at 029 2081 3385.
This document has been optimised for double sided printing and therefore may produce some blank pages when printed single sided.
Report General Notes
1)
Earth Science Partnership (ESP) believes that providing information about limitations is essential to help clients
identify and therefore manage their risks. These risks can be mitigated through further investigation or research,
but they cannot be eliminated.
2)
This report may not be used for any purpose other than that for which it was commissioned.
3)
We have taken reasonable efforts to ensure the accuracy and reliability of the information presented herein. Some
of this information has been obtained from third party sources and they are referenced as appropriate.
Nevertheless, no guarantee is provide on the authenticity or reliability of this information.
4)
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accordance with terms of the commission. The report shall not be relied upon or transferred to other parties without
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report, plan or document may be reproduced, published or adapted without their consent. However, complete
copies of this report may be made and distributed by the Client as an expedient in dealing with matters related to its
commission.
Flood Consequence Assessment (FCA)
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Contents
1
Introduction and Objectives
1
2
Site Description and Context
3
2.1
Location, Description and Topography ...................................................................... 3
2.2
Proposed Development and Environmental Setting .................................................. 3
2.3
Mode of Flooding: Surface Water & Existing Drainage .............................................. 5
2.4
Mode of Flooding: Fluvial Flooding ............................................................................ 5
2.5
Mode of Flooding: Sea-level / Tidal ........................................................................... 5
3
Tan 15 Assessment
6
3.1
Justifying the Location ............................................................................................... 6
3.2
Assessment of Flooding Consequences .................................................................... 7
3.3
Flood Mitigation Measures ........................................................................................ 8
4
Key Conclusions, Considerations and Recommendations
10
Enclosures
Figure 1
Site Location Plan
Figure 2
Site Layout Plan (Ref: Cardiff Council – EDUC1069 – Aug15)
Figure 3
Topographic Survey and (October 2016)
Figure 4
Proposed Site Layout
Appendix A
Welsh Assembly Government TAN15 Development Advice Map
Appendix B
Correspondence from Natural Resources Wales and Cardiff Council
General Notes (FCA)
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1
Introduction and Objectives
The Earth Science Partnership Ltd (ESP), Consulting Engineers, Geologists and
Environmental Scientists, were instructed by Morgan Sindall Construction &
Infrastructure Ltd to undertake a flood consequence assessment for a possible school
redevelopment at The Maltings in the South of Cardiff.
The Development Advice Map, produced by the Welsh Assembly Government, identifies
the majority of the site as existing within Zone B; defined as an area, known to have
been flooded in the past as evidenced by sedimentary deposits. A small portion of the
site in the north east is classified as Zone C2, defined as being an area of the floodplain
without significant flood defence infrastructure.
The actual proposed development footprint falls within Zone B, with access roads
extending eastward toward Zone C2.
Technical Advice Note 15 Development and Flood Risk, National Assembly for Wales
2004 (TAN 15) states that in Zone B, a precautionary approach should be used to check
site levels against the 0.1% flood event. If site levels are greater than flood levels, there
is no need to consider flood risk further.
In Zone C2, TAN15, states that only less vulnerable development should be considered
subject to the application of a justification test, including acceptability of consequences.
This report is intended to document information relevant to the justification test and to
assess and determine the acceptability of the consequences.
This assessment has been undertaken in general accordance with TAN 15, using
available information on flood and ground levels to address the relevant requirements of
the TAN15 guidance.
During extreme events, this site, as for many other sites in south-west Cardiff, may be
affected by flooding and may not directly match the suggested acceptance criteria setout within TAN15. For similar sites assessed previously, it has been agreed with Natural
Resources Wales and Cardiff Council that for extreme events, the detailed requirements
of TAN15 are not strictly applicable for these areas. Instead it is deemed appropriate
that reasonably practicable measures are implemented to mitigate the potential impacts
at extreme events.
This report represents the findings of the brief relating to the proposed end use as
detailed in the text. Other physical, contamination and environmental considerations are
outside the scope of this assessment. The brief did not require an assessment of the
implications for any other end use, nor is the report a comprehensive site
characterisation and should not be construed as such. Should an alternative end use be
considered, the findings of the assessment should be re-examined relating to this use.
This assessment is based on our own investigations and on detailed enquiries with
Natural Resources Wales (NRW) who have provided information relevant to the study.
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The scenarios discussed in this report to illustrate potential flood events are not
predictions of what will actually happen. They are presented to assist the developer and
statutory authorities to take account of flooding issues when considering the proposed
development – as required by TAN 15.
This report represents the findings and opinions of experienced practitioners, working
within a brief agreed with the client. Earth Science Partnership does not provide legal
advice and the advice of lawyers may also be required.
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2
2.1
Site Description and Context
Location, Description and Topography
The site is located between East Tyndall Street and Lewis Road in east Cardiff. The
National Grid Reference of the centre of the site is (ST) 319836, 176095 and the
postcode is CF24 5NE. A Site Location Plan with approximate site boundary is
presented as Figure 1.
The site is predominantly public open space with grass surfacing with a number of
mature trees present at the boundaries and through the central portion, with a small
area of hardstanding present in the south east portion, adjacent to small football
fields.
The north east portion has been developed with landscaping and footpaths
comprising a public garden type feature, but available plans identify a Gas Governor
in this area which is also recorded on the topographic survey provided by the client.
The very north portion contains a circular depression within the landscaping and a
disused play area is present in the north central portion.
A topographic survey provided by the client in September 2015 indicates ground
levels of around 9.0mAOD in the north east portion rising to between 9.1mAOD and
9.5mAOD in the south and central portions. A stockpile at the east boundary is
recorded at between 9.3mAOD and 10.7mAOD. The west area of the site is
identified as between 10mAOD and 11.8mAOD. The existing site layout is presented
as Figure 2 and the topographic survey as Figure 3.
2.2
Proposed Development and Environmental Setting
We understand that the development is to comprise a school and that ground levels are
not intended to be significantly altered (currently 9.5-11mAOD). It is reasonably
assumed that the site will be re-levelled to around 9.7mAOD or higher.
Development Advice Maps issued by the Welsh Government (Appendix A) indicate that
the development footprint falls within Zone B, with an access road identified as within
Zone C2. This development use as a school (Public Building) is classified as “highly
vulnerable” by TAN15.
The site is situated within the historic Cardiff dock area, and the Severn Estuary exists
approximately 1.5km to the south. The nearest major river is the River Taff, located 2km
to the east.
Selected photographs of the site are provided below.
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Insert 1: Central Portion Looking West
Insert 2: West Portion at Higher Elevation
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2.3
Mode of Flooding: Surface Water & Existing Drainage
Information has been requested from Cardiff Council regarding the predicted
performance of local drainage infrastructure during flood events. At the time of writing
this information is yet to be received and this study will be updated should any pertinent
information be presented within their response.
The “Risk of Flooding from Surface Water” maps on the Environment Agency website
(October 2015) indicate some potential localised general issues in the wider area and at
the lower ground in the east part of the site. It is understood that where possible, a
Sustainable Drainage Strategy will be adopted for the scheme.
2.4
Mode of Flooding: Fluvial Flooding
From the information provided on the NRW website, and their data response provided in
September 2015 (ATI-08104a Appendix B), the site is not indicated to be at risk from
Fluvial Flooding.
2.5
Mode of Flooding: Sea-level / Tidal
NRW describe the derivation of the tidal flood data as follows:
“The current tidal flood data in this area was updated by NRW in 2013. This study uses
sea level nodes within the Severn Estuary, based on a set of extreme sea levels
generated by the EA in 2011 for current day (2008). The levels were projected in-land
over a digital terrain model to produce depth and elevation grids as well as tidal
mapped outlines for both the 0.5% (1 in 200) AEP (annual exceedance probability) and
the 0.1% (1 in 1000) AEP; including climate change and upper confidence intervals (+/95%).”
In February 2011, extreme sea levels used in this model were superseded by a
nationally consistent set of extreme sea levels. These levels were derived using a tidal
model calibrated to UK tidal gauge data. The model output is provided for node locations
spaced at approximately 2km. 95% confidence bounds for these values were also
derived using the confidence intervals for each node location. The extreme sea levels
comprise still water level including storm surge, however they do not account for local
wave action. The baseline estimations are for the year 2008, so climate change is
calculated relative to this year, for example add 10.5mm for the year 2011.
Table 1: Design Sea Levels (max elevation)
2015
Elevation
(mAOD)
2115
0.5% (1in200)
0.1% (1in1000)
0.5% (1in200)
8.5
9.1
9.6
It is understood that climate change effects are not normally applied to the extreme
event (0.1%), as the accumulation of assumptions makes their accuracy unsuitable for
reasonable decision making.
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3
3.1
Tan 15 Assessment
Justifying the Location
The site is recorded to be located within Zone B; defined as an area, known to have
been flooded in the past as evidenced by sedimentary deposits. A small portion of the
site in the north east is classified as Zone C2, defined as being an area of the floodplain
without significant flood defence infrastructure.
The actual proposed development footprint falls within Zone B, with access roads
extending eastward toward Zone C2.
In Zone C2, TAN15, states that only less vulnerable development should be considered
subject to the application of a justification test, including acceptability of consequences.
As the only part of the site located within Zone C2 is an access road, it is considered this
is an acceptable use.
In terms of the wider use of Zone C, the following requirements are necessary in line
with TAN15:
1. Its location in Zone C is necessary to assist, or be part of, a local authority
regeneration initiative or a local authority strategy required to sustain an existing
settlement; or
2. Its location in Zone C is necessary to contribute to key employment objectives
supported by the local authority, and other key partners, to sustain an existing
settlement or region; and
3. It concurs with the aims of Planning Policy Wales 2002 and meets the definition of
previously developed land; and
4. The potential consequences of a flooding event for the particular type of development
have been considered, and found to be acceptable.
The site satisfies the first test above and is known to be previously developed land (test
three). The requirements of test four, as described in Planning Policy Wales (PPW) and in
Section 7.0 of TAN15, are discussed further and addressed in the text below.
Specifically they relate to the following:
As stated in TAN15, Section 7.3, where development is justified the assessment can be
used to establish whether suitable mitigation measures can be incorporated within the
design to ensure that development is as safe as possible and there is:
· Minimal risk to life;
· Minimal disruption to people living in and working in the area;
· Minimal potential damage to property;
· Minimal impact of the proposed development on flood risk generally; and,
· Minimal disruption to natural heritage.
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With regard to bullets 4 and 5, given that the development replaces existing property, it
will have little/no discernible affect on current flood risks in the vicinity, and there will be
no disruption to natural heritage. The other items are discussed in greater detail,
including comparison against the guidance of TAN15 listed in Table 2 and Table 3, in
Section 3.2, below.
Table 2: Indicative guidance of TAN15 – Section A1.14 (Target for no flooding)
Type of Development Threshold Frequency (Years)
Residential
Fluvial
Tide
1%
0.5%
(Most Appropriate)
Table 3: Indicative guidance of TAN15 – Section A1.15 (Suggested tolerable flood
conditions)
Type of
development
Maximum depth
of flooding (mm)
Property
Maximum rate of rise
of floodwaters (m/hr)
Maximum speed of
inundation of flood risk
area (hrs)
Access
Residential
(Most
Appropriate)
3.2
600
600
Maximum velocity of
floodwaters (m/sec)
Property
Access
0.1
4
0.15
0.3
Assessment of Flooding Consequences
3.2.1 Data Sources and Consideration of Flood Defences
The indicative flood conditions have been assessed using NRW data obtained in
September 2015 (ATI-08104a), re-produced in Appendix B.
It is robustly assumed that owing to the significant resources applied by NRW in
developing the flood prediction model for Cardiff that further modelling and assessment
would not provide a better understanding of conditions, or justifiable amendment of the
data provided. Further refinement and slight changes to the significant depths predicted
for extreme events would not alter the approach adopted for assessing and managing
flood risk at the site.
From the information provided on the NRW website, and their data response provided in
September 2015 (ATI-08104a Appendix B), the site is not indicated to be at risk from
Fluvial Flooding and tidal risks are discussed below.
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3.2.2 0.5% Probability Event (Q200 / 1in200 probability in any one year) +CC
The information provided by NRW for the 0.5% probability event, including climate
change, show the site and immediate surrounding area is shown to be flood free at this
event. We understand that the preliminary design requires the site to be re-levelled/reprofiled to around 9.7mAOD or higher, the TAN15 requirements for the 1%+cc flood
event are considered to be met.
3.2.3 0.1% Probability Event (Q1000 / 1in1000 probability in any one year)
The information provided by NRW for the 0.1% probability event, including climate
change, show the site and immediate surrounding area is shown to be flood free at this
event. We understand that the preliminary design requires the site to be re-levelled/reprofiled to around 9.7mAOD or higher, the TAN15 requirements for the 0.1% flood event
are considered to be met.
Whilst predicted flood depths for the surrounding roads and land areas may exceed the
TAN15 suggested tolerable conditions for the 0.1% flood event, the alteration of ground
levels outside the development footprint is beyond the control of the client.
3.3
Flood Mitigation Measures
During extreme flood events the wider area surrounding the site will be affected by
flooding. The floodwaters will rise at relatively slow rates and move relatively slowly –
more broad ponding; however, peak depths are relatively significant for surrounding land
and access. It is considered that the predicted flood scenario and peak flood conditions
during extreme events can be appropriately managed by the implementation of a
number of nominal mitigation measures discussed below:
· It is likely that the Met Office / media will be able to provide significant advance
warning of such extreme rainfall/tide, possibly as much as a number of days or
weeks.
· With the time afforded by the advance warning above, school administrators will
have the opportunity to close the school to pupils, and move valuable property to
higher levels of the building, or away from the property if necessary.
· The predicted depths, rate of rise, speed of inundation, and velocities are likely
to satisfy the TAN15 suggested tolerable conditions for more extreme events.
· Whilst predicted flood depths for the surrounding roads and land areas exceed
the TAN15 suggested tolerable conditions for the 0.1% flood event, the
alteration of ground levels outside the development footprint is beyond the
control of the client. NRW data maps indicate that flood free access can be
obtained along East Tyndall Street to the north, providing dry access to major
routes at the west.
Due to the risk being posed by extreme tidal events, it is robustly assumed that the
defences implemented in Cardiff including the Cardiff Bay Barrage are suitable for use
and the alteration of ground levels outside the development footprint is beyond the
control of the client.
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In addition, we understand that Cardiff Council are aware of the inherent flood risks for
south and west Cardiff during extreme events, and are discussing/preparing a wider
flood management/evacuation policy that could be implemented by the authority. This
will include the contribution and co-operation of key stakeholders including emergency
services, which would further reduce flood risks for the development.
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4
Key Conclusions, Considerations and Recommendations
Based on the above recommendations, the following table summarises the satisfaction
of the worst case TAN15 requirements (access road within Zone C2).
Table 4: Summary of TAN15 Requirements
TAN15 Statement
Satisfies
Requirements
Its location in Zone C is necessary to assist, or be part of, a local
authority regeneration initiative or a local authority strategy required to
sustain an existing settlement.
ü
Its location in Zone C is necessary to contribute to key employment
objectives supported by the local authority, and other key partners, to
sustain an existing settlement or region.
n/a
It concurs with the aims of Planning Policy Wales 2002 and meets the
definition of previously developed land.
ü
The potential consequences of a flooding event for the particular type of
development have been considered, and found to be acceptable.
ü
The actual footprint of external areas and buildings is wholly located within Zone B.
The TAN15 requirements for the 1%+cc flood event and the more extreme event (0.1%)
are considered to be met. Whilst predicted flood depths for the surrounding roads and
land areas may exceed the TAN15 suggested tolerable conditions for the 0.1% flood
event, the alteration of ground levels outside the development footprint is beyond the
control of the client.
Should any areas of the site be proposed below 9.7mAOD, further assessment will be
required to assess the impact of the 0.1% event.
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FIGURES
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Appendix A
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Welsh Assembly Government TAN15 Development
Advice Map
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Appendix B
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Correspondence from Natural Resources Wales
and Cardiff Council
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