Cardiff Local Development Plan
2006-2026
City of Cardiff Council Response
Action Point 5 of Hearing Session 5
April 2015
Strategic Planning
City of Cardiff Council
County Hall
Atlantic Wharf
Cardiff CF10 4UW
City of Cardiff Council
Cardiff Local Development Plan 2006 – 2026
Statement in response to Action Point 5 from Hearing Session 5:
Allocated Sites (1) Strategic Sites A, B & H
Introduction
1.
An Action Point has arisen from Hearing Session 5: Allocated Sites (1)
Strategic Sites A, B, & H where the Inspectors have asked Council,
Developer and Natural Resources Wales (NRW) to prepare a Statement of
Common Ground/Position Statement for Strategic Site H in relation to
fluvial and tidal flooding concerns and how they can be mitigated, including
showing how sufficient funding can be provided to undertake defence
works. This follows on from Action Point 4 of Hearing Session 5 which
asked the Council to justify the allocation of Strategic Site H in line with
the requirements of Planning Policy Wales, the sequential tests in TAN15
and TAN23.
2.
The Action point is worded as follows:
Action Point 5, Hearing Session 5: Council, developer and Natural
Resources
Wales
to
agree
a
Statement
of
Common
Ground/position statement in relation to fluvial and tidal flooding
concerns and how they could be mitigated.
Also to show that
sufficient funding could be provided to undertake defence
improvements (in line with requirements in TAN 15 Appendix 1).
Council Response
3.
The latest flooding Development Advice Maps (DAMs) issued by the Welsh
Government in January 2015 to accompany TAN15: Development and
Flood Risk include the allocation at Strategic Site H within flood risk zone
C1. These are areas of the floodplain which are developed and served by
significant infrastructure, including flood defences and in accordance with
the precautionary framework for flood risk set out in TAN15 are used to
indicate that development can take place subject to application of the
justification test, including the acceptability of consequences. TAN15
defines commercial development as less vulnerable development in terms
of the justification test and states that such development should only be
permitted in zone C1 if it is determined by the planning authority to be
justified in that location. Paragraph 6.2 of TAN15 states that development
can only be justified if it can be demonstrated that:
i.
,
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
1
ii
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,
iii.
It concurs with the aims of PPW and meets the definition of
previously developed land (PPW fig 2.1); and.
iv.
The potential consequences of a flooding event for the particular
type of development have been considered, and in terms of the
criteria contained in sections 5 and 7 and appendix 1 found to be
acceptable.
4.
The Councils response to Action Point 4 of 5 sets out a justification for the
site in relation to criteria (i), (ii) and (iii) of paragraph 6.2 and purpose of
this Statement is to specifically respond to criteria (iv) of paragraph 6.2
relating to the acceptability of the potential consequences of flooding.
5.
As part of the evidence base for the Plan the Council have undertaken a
comprehensive Strategic Flood Consequence Assessment (SFCA) which
included an assessment of this site against the criteria set out in Sections
5 and 7 and Appendix 1 of TAN15. The study progressed through to Stage
3 for this site (see documents EBF.05 and EBF.06) and investigated
options for both tidal and fluvial flood risk mitigation for the site and
identified associated costs for each of the options.
6.
The study assessed the site in three parts north and south of the Railway
(106LGEM) and an additional smaller area north of the Railway (24LGRM).
This concluded that through, raising site levels and providing
compensatory flood storage that approximately 80% of the site area north
of the railway and 73% of the site south of the railway was developable in
line with TAN15. It also found that 100% of the additional smaller area
north of the Railway (24LGRM) was developable in line with TAN15.
7.
In terms of the cost of mitigation works to achieve these developable areas
the study identified £2.5 million for works to the seawall at Sluice Farm
(also known as Tabb’s Gout) and land raising costs for the two sites north
of the railway line of £10.65 million and an additional £7.89 million for
raising the site south of the railway. In addition to this there would be
additional costs associated with constructing compensatory flood storage
areas.
8.
The SFCA has therefore demonstrated that fluvial and tidal flood risk can
be mitigated and that a large portion of the site would be developable in
line with the requirements of TAN15. In this respect it is important to note
that the Statement of Common Ground (EDP.008) agreed between the
2
developer and the Council now only proposes development north of the
railway, with the area south of the railway being utilised for compensatory
flood storage facilities and ecological mitigation.
9.
The Statement of Common Ground also set out principles in order to
protect the value of the Gwent Levels SSSI and the reen system within the
site. These include ensuring that the development maintains a minimum
buffer of up to 12.5 metres from main reens and up to 7 m from field
ditches and if the infilling of any main reen or field ditch proves to be
unavoidable at the application stage a commitment that it would be
realigned (with at least an equivalent capacity) around the perimeter of
the development or a compensatory length of ditch should be provided
elsewhere within the site.
10.
In order to provide further clarity on proposed compensatory storage
facilities referred to in the SFCA reports, Atkins have provided a Technical
Note (see Appendix 1) setting out the principle of a multi cell system to
provide compensatory fluvial flood storage to mitigate for ground raising
within the site.
11.
In addition, to this the developers flood risk consultants, Waterman, have
produced a schematic diagram of the proposed ecological and flood
mitigation reens south of the railway (Appendix 2). In response to issues
raised by NRW, Waterman have confirmed that:
•
The new reens will be connected to the existing reens (over their full
depth and width) and in this respect quantity and quality of water
within the new reens will be the same as that in the existing. As the
existing reens support the special features of the SSSI, it follows that
the new reens would be just as effective.
•
Detailed discussion around the level of the berm and gradient of the
sides within the new reens would be advanced at the planning
application stage.
•
As the new reens will be connected to the existing reens (over their full
depth and width) there will be complete hydraulic connectivity. In this
case, the flow of water into the newly created reens would be the same
as the existing reens. The fluctuation of water levels would also be the
same.
•
Flow within the existing reens could be defined as greenfield runoff and
as such flow into the newly created reens could be defined as the
same. Any discharges from future developed land to the north of the
railway would be restricted to greenfield rates.
•
The newly created reens will hold water permanently in the same that
the existing reens currently do.
3
12.
•
The management of the mitigation reens will be discussed in detail at
the planning application stage, however as with the existing reens
there would be the need for limited sections of culverting/bridging to
allow access for maintenance. The total length of new open channel
reens (minus the narrow culverted sections) would be relied upon for
mitigation and it is in this way that the required number and length of
new reens will be determined.
•
The proposed additional/new reens to the south of the railway are a
direct replacement (equivalent length or greater) for the loss of any
existing reens to the north of the railway. This (in combination with the
multi cell system that has been proposed as part of the mitigation
measures to the north of the railway) will either replace or increase
flood storage as a result of the loss generated by the development to
the north of the railway. Thereby offsetting any loss as a result of the
development.
In relation to funding of the mitigation measures outlined above it is
important to note that the defence works of £2.5 million identified in the
study for the seawall at Sluice Farm (also known as Tabb’s Gout) are now
underway and are fully funded and committed. The cost of land raising
and compensatory flood storage will be significantly reduced through the
proposal to develop only north of the railway and it is considered that
development and related flood infrastructure could proceed on a phased
basis so development costs are phased over time.
NRW Response
Flood Risks Management Matters
13.
NRW agrees with the principles illustrated in Atkins Technical Note “Multi
Cell Compensatory Storage for Ground Raising,” (Revision 1.0, dated 13th
February 2015). It appears from the graph (Figure 2) that the
height/storage relationship does not exceed the existing baseline
condition, that is, the post development profile (with flood cells) creates
additional storage at given flood elevations within each cell.
14.
NRW also appreciates that the proposals highlighted are schematic and will
be developed further, which is reliant on how much additional site area
could be reclaimed. This will be assessed within a more detailed site
specific Flood Consequence Assessment and submitted in support of a
planning application. We therefore agree matters pertaining to flood risk
management for LDP purposes only.
Gwent Levels: Rumney and Peterstone Site of Special Scientific Interest
(SSSI)
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15.
We note that clarification has been provided in the outline mitigation
strategy, which has a focus on reen management. Our view remains,
however, that Strategic Site H should not be allocated in the LDP. The
allocation is located entirely within the Gwent Levels: Rumney and
Peterstone SSSI and will result in loss of the SSSI area. NRW’s view, based
on experience over the past 20 years of other large scale developments
within the Gwent Levels suite of SSSIs is that such developments cannot
be accommodated in such a way as to both conserve and enhance the
SSSI. The allocation is therefore not in accordance with Planning Policy
Wales (PPW) with a presumption against development in the SSSI
(paragraph 5.5.8).
Council’s Conclusion
16.
In view of the findings of the SFCA for Strategic Site H and the details on
flood mitigation and compensatory measures and funding sources set out
above the Council consider that fluvial and tidal flood risk can be mitigated
to enable the development to comply with TAN15 and also protect the
value of the Gwent Levels SSSI and no changes to the Plan are therefore
considered necessary. Given the response from NRW they do not agree
with the conclusion that the value of the Gwent Levels SSSI is protected.
5
Technical Note
Project:
Strategic Site H
Cardiff LDP
To:
NRW
City of Cardiff Council
Subject:
Multi Cell Compensatory
Storage for ground raising
From:
Huw Richards
Job number: 5109602
Document ref: DG-012
Revision
Purpose description
Originated
Checked
Reviewed
Authorised
Date
Rev 1.0
Issue
DMH
DW
HR
HR
13/02/15
Background
This note is intended to address a query raised by NRW in relation to the ability of a multi cell flood storage
system to mitigate the loss of fluvial flood storage which would result from raising parts of Strategic Site H of
the Cardiff LDP. The concern is described on page 4 of NRW letter dated 5th January 2015, referenced as
Cardiff Local Development Plan 2006-20026 Hearing Session 5 Allocated Sites(1) – KP2 Strategic Site H –
South of St Mellons Business Park.
Strategic Site H comprises sites 24LGRM and 106LGEM which have been assessed as part of the SFCA.
Please note that site H now only considers land to be north of the main Fishguard – Paddington railway line.
The query relates to the area of the site which is developable in compliance with TAN15.
Tidal flood risk
Tidal flood risk can be mitigated by strategic improvements to the Wentloog sea defences as described in
the SFCA. Tidal flood risk is not considered further in this technical note.
Fluvial flood risk
It will be fluvial flood risk issues which dictate how much of the site can be developed.
Site 24LGRM
Site 24LGRM essentially remains flood free during a 0.1% (1 in 1000) annual chance fluvial event. Currently
flooding is only predicted along a low lying corridor adjacent to the Faendre reen.
Based on the modelling results, development of nearly all the Site 24LGRM would be possible (assuming
tidal risk is mitigated through by strategic improvements of the Wentloog sea defences). Maintaining a 12m
wide buffer to the Faendre reen will retain the existing flood storage within the reach.
Site 106LGEM
Detailed modelling indicates that approximately 60% of the site 106LGEM remains flood free during a 0.1%
(1 in 1000) annual chance event. It is therefore considered that at least 60% of the site can be developed
without any mitigation with respect to loss of fluvial flood plain storage.
It is anticipated that some re-distribution of flood plain may be necessary to consolidate developable land.
This should not be an issue provided that level for level flood storage is not reduced.
Overall flood conveyance to the south will need to be maintained through the provision of the existing or an
equivalent reen system.
DG012_1_Technical note_Multi Cell Flood Storage Concept LDP Strategic Site H.docx
The Stage 3 SFCA identified that approximately 80% of the site 106LGEM was potentially developable. This
would require mitigation for flood storage to be undertaken within land currently contained within the
predicted 0.1% chance flood extents.
Concept of the multi cell storage system – potential to achieve compensatory storage for ground raising
To achieve compensatory storage for any raising of ground levels It would not be acceptable to simply ‘dig a
hole’ within the flood plain as this would fill up at an early stage within a flood event. However, it would be
possible to mitigate loss of storage by adopting a series of compartmentalised storage cells.
The individual storage cells would be designed such that they each inundate at different flood stage. This
will ensure that the existing height- storage relationship is not compromised.
The section below illustrates how this could be achieved.
Typically the height storage relationship would be changed as shown on the graph. Provided that the
modified (post development) height/storage relationship lies below the existing baseline condition, the
volume of flood storage at any given flood elevation would not be reduced.
It is estimated that 50 % of the site which lies within the 0.1% fluvial flood extents could be ‘reclaimed’
without compromising the existing flood storage relationship. However this will depend on the flood depths
and the existing ground water regime. The issue of how much additional site area could be reclaimed would
be dealt with as part of a detailed site specific FCA.
It is concluded that between 60% and up to 80% of site 106LGEM could be developed with a suitable flood
mitigation scheme. This would be confirmed by a site specific FCA when details of the proposed
development are available.
DG012_1_Technical note_Multi Cell Flood Storage Concept LDP Strategic Site H.docx
Figure 1 - Typical Section through a Storage Cell Mitigation Scheme.
Flood level
Pre development
ground level
Ground raised
Ground level
Storage lost
Storage gained
Figure 2 - Height Storage Relationship – Existing Baseline Situation and Post Development with Flood Cells
DG012_1_Technical note_Multi Cell Flood Storage Concept LDP Strategic Site H.docx
DG012_1_Technical note_Multi Cell Flood Storage Concept LDP Strategic Site H.docx