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Cardiff Strategic Flood Consequence Assessment - Stage 3 Assessment Seawall Road Proposed Gypsy and Traveller Site Cardiff Council

23 May 2014

Cardiff Strategic Flood Consequence Assessment - Stage 3 Assessment Seawall Road Proposed Gypsy and Traveller Site

Atkins Cardiff SFCA - Stage 3 Assessment Seawall Road Site | Version 1.0 | 12 May 2014 |

5109602

Notice

This document and its contents have been prepared and are intended solely for Cardiff Council’s information and use in relation to Seawall Road, Pengam Green proposed development site.

Atkins Limited assumes no responsibility to any other party in respect of or arising out of or in connection with this document and/or its contents.

Document history

Job number: 5109602 Document ref: 5109602/69/DG/001

Revision Purpose description Originated Checked Reviewed Authorised Date

Rev 1-D1 Internal review DW KIO DMH 25/02/14

Rev 1 Revised format Inc of NRW review comment

- Inc SMP policy

- Inc cost estimates

KIO DMH 14/04/14

1 Issue to client KIO DMH HR HR 12/05/14

2 Minor edits following client review

KIO DMH HR KIO 23/05/14



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Table of contents

Chapter Pages

Executive summary 1

1. Introduction 4 1.1. Previous assessments 5 1.2. Existing flood defence infrastructure 6

2. Methodology 7 2.1. Development lifetime 7 2.2. Hydraulic Modelling 8

3. Results 11 3.1. Development Lifetime to 2085 (71 years) - 0.5% chance event 11 3.2. Development Lifetime to 2110 (96 years) - 0.5% chance event 12 3.3. All development lifetimes - Existing (2010) 0.1 % chance event 16 3.4. Coastal Erosion impact on flood risk 18 3.5. Cost estimates 19

4. Summary and Conclusions 20 4.1. Erosion risk to the area adjacent to the site 21 4.2. Cost estimates 21 4.3. Summary 21

Tables Table 3-1 Indicative erosion protection costing for Rover Way area 19 Table 4-1 Summary of risks and mitigation required by time horizon 20

Figures Figure 1.1 Proposed site extent 1 Figure 1.1 Proposed site extent. 4 Figure 1.2 Location of LDP site 19LBU in relation to the 0.5% (1 in 200) chance flood extents for time

horizons 2010, 2085 and 2110* 5 Figure 2.1 Comparison of effect of NRW River Rhymney defence improvements. SFCA and the updated

model results – 0.5% (1 in 200) chance event, 2110. 9 Figure 3.1 Flood Depths - 0.5% (1 in 200) chance event, 2085 11 Figure 3.2 Flood Depths - 0.5% (1 in 200) chance event, 2110 12 Figure 3.3 Area to be lowered to mitigate raising of the site 13 Figure 3.4 Flood Depths – with site raised plus mitigation - 0.5% (1 in 200) chance event, 2110 14 Figure 3.5 Flood Velocities with site raised plus mitigation - 0.5% (1 in 200) chance event, 2110 14 Figure 3.9 Flood Depths - 0.1% (1 in 1000) annual chance event, 2010. 17 Figure 3.10 Flood Velocities - 0.1% (1 in 1000) chance event, 2010. 18



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Executive summary

Introduction and purpose This report summarises the work undertaken to examine the flood risk with regard to the feasibility of developing a proposed Gypsy and Traveller Site comprising 65 pitches at Seawall Road, Pengam Green.

The assessment has been undertaken in accordance with the guidance contained in Planning Policy Wales TAN15: Development and Flood Risk. This provides guidance to local planning authorities (LPAs) regarding the assessment of flood risk when they are preparing their Local Development Plan (LDP).

Figure 1.1 Proposed site extent

Planning Policy Guidance (Flood Risk)

The site would need to comply with the guidance contained in Technical Advice Note 15: Development and Flood Risk (TAN15). The site lies within Flood Zone C2, (Active flood plain, as indicated on the development advice maps (DAM’s))

Section 6 of TAN15 requires that development defined as highly vulnerable (which includes any residential dwellings) should not be permitted within Zone C2. However Planning Authorities in Wales have been permitting highly vulnerable development to be located within Zone C2, provided that the site can be justified under Section 6 of TAN15 and that the consequences of flooding are acceptable.

It should be noted that the TAN15 designation would appear to take no account of the existing flood defence infrastructure in the area. If these defences were taken into account, along with consideration of the existing infrastructure, it could be considered that the site and surrounding area should be considered Zone C1, (Areas of the floodplain which are developed and served by significant infrastructure, including flood defences). TAN 15 permits development in Zone C1 subject to justification under section 6 and acceptable consequences of flooding.



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It should also be noted that Welsh Office Circular No: 07/12 which came into effect on 30th July 2012,

requires any development of more than 10 dwellings in Zone C2 to be referred to Welsh Ministers for determination.

If the local planning authority were minded to grant planning permission for the site, then under the guidance outlined above they would need to refer the application to Welsh Ministers. The development may be called in for determination. It would be advisable to canvas Welsh Government on their attitude towards the site prior to investing any sums of money on enabling the development.

Existing Flood and Coastal Erosion Risks The assessment takes account of programmed improvement works to be completed by Natural Resources Wales in 2014. A base year of 2010, to be consistent with analysis completed for the supporting Strategic Flood Consequence Assessment, has been used in the assessment and referenced for the time horizons considered for climate change over the potential development lifetimes. This would need revisiting at the time of planning submission for the site.

The site is classified to be within TAN15 Zone C2, and thus not considered to be benefiting from significant flood defence infrastructure. However, this does not reflect the benefit provided by both the defences along coastline and the flood defences along the right-bank of the River Rhymney.

The assessment has determined that the site is currently flood free for a 0.5% (1 in 200) annual chance tidal flood event. The general area surrounding the site is currently at risk from a 0.1% (1 in 1000) annual chance tidal flood event overtopping the flood defences along the River Rhymney. It is predicted that approximately two-thirds of the site would be at inundated during this event.

The effects of climate change and sea level rise over 75 years to 2085 result in the wider area around the site including the access and egress routes to the site being inundated by a 0.5% (1 in 200) chance event flood. The majority of the site would however remain flood free.

Over a 100 year development lifetime to 2110, the site will become completely inundated by a 0.5% (1 in 200) chance event. This is caused by overtopping from the River Rhymney and a predicted breach of the coastal defence due to wave overtopping. Flood depths and velocities over the site and surrounding areas exceed the permissible values contained within current planning policy guidance (TAN15).

A section of the coastal defences at Rover Way is also considered to be potentially at risk of failure due to erosion rates on the coastal frontage. This is separate to the likelihood of a breach developing due to overtopping in extreme tide events. The risk of a failure of this section of the defence due to erosion is considered to be high within the development lifetime.

Summary and recommendations Following assessment of the flood risk conditions, the Seawall Road site could be developed in compliance with TAN15 if a 75 year development lifetime (to 2085) is considered acceptable by the local planning authority, and the area was considered to be benefitting from significant flood defence infrastructure.

Mitigation measures would be required to manage flood risk to the site and the access/egress routes.

Strategic works in line with the policy in the Shoreline Management Plan to protect existing properties in the wider area and the site will also be required to mitigate the risk of erosion of the existing coastal defences. Indicative cost estimates for capital investment to reduce the erosion risk are in the order of £3M. The possible transfer of the existing Rover Way Gypsy and Traveller Site to the proposed Seawall Road site could facilitate the works to the coastal defences.

It should be noted that any FCA assessments provided in support of a planning application for the proposed site layouts would need to account for the appropriate development lifetime at the time of the formal submission. NRW may require further modelling to account for climate change beyond 2085 tested for this current report.



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If the Council are minded to proceed with the development of the Gipsy and Traveller site, there is a risk that the application would be called in for determination by Welsh Ministers. It is recommended that the Council canvas Welsh Government on their attitude towards the site prior to investing any sums of money on enabling the development



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1. Introduction The purpose of this report is to summarise the work undertaken to examine the flood risk to a proposed Gypsy and Traveller Site comprising 65 pitches at Seawall Road, Pengam Green.

The assessment has been undertaken in accordance with the guidance contained in Planning Policy Wales TAN15: Development and Flood Risk. This provides guidance to local planning authorities (LPAs) regarding the assessment of flood risk when they are preparing their Local Development Plan (LDP).

The site lies in Zone C2 on the Development Advice Map (DAM) which supports TAN15. This zone is based on the Natural Resources Wales (NRW) extreme flood outline, and is used to define areas which are at risk of flooding during a 0.1% (1 in 100) flood event and which do not benefit from significant flood defence infrastructure. Further consideration of this allocation is provided in Section 2.1.1.

Assessment of the flood risk to the wider area including the development site has previously been undertaken as part of the Cardiff Strategic Flood Consequences Assessment

1 (SFCA). These assessments

have taken account of the benefit to the management of flood risk to the area provided by the coastal defences which are not accounted for within the TAN15 mapping. The results of this and other pertinent assessments of flood risk and coastal erosion carried out for the area are summarised below.

Additional assessments undertaken to assess the flood risks for the specific proposal for the Gypsy and Traveller Site are described in Sections 2 & 3. The overall conclusions for the site are presented in Section 4.

Figure 1.1 Proposed site extent.

(See Figure 1.2 for location in relation to the wider area).

1 Cardiff SFCA, Phase 2 Part 1 Update, Extend Development to 2110, Areas A, H and I, Atkins Limited on behalf of Cardiff Council,

August 2011, ref 5097656-DG-017



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1.1. Previous assessments

1.1.1. Cardiff Strategic Flood Consequences Assessment The site lies within Area I of the wider SFCA Stage 2 assessment and forms part of site ID number 19LBU. Paragraph 6.3.4 of the report sets out the strategic development context for the wider LDP assigned development area

The most significant source of flooding to the site results from overland flow routes from the north and east due to overtopping of the right bank of the River Rhymney. The coastal frontage adjacent to the existing Gypsy and Traveller Site on Rover Way is also identified as being subject to potential breach during extreme tide levels and wave overtopping.

The SFCA indicated that only a small proportion of the wider Candidate Site could be developed in line with TAN15 guidance. This is due to the predicted wide extents of flooding over the site during a 0.5% (1 in 200) chance flood event within the likely development lifetime. Indicative acceptance criteria contained within TAN15 Appendix A1.14 require that developments are flood free for a flood event of this chance.

Information from other Atkins assessments has also been used to inform this assessment, namely;

Cardiff Council, Coastal Erosion Risk Assessment - An overview assessment of coastal erosion risks along Cardiff Council’s coastline, Atkins ref 5113153/70/DG/007

Rover Way Site Specific Assessment - Review of potential flood and erosion risks to Rover Way Traveller Site. Atkins ref: 5117999/70/DG/001

Figure 1.2 Location of LDP site 19LBU in relation to the 0.5% (1 in 200) chance flood extents for time horizons 2010, 2085 and 2110*

*Note: flood extents from SFCA 2011. Situation pre-NRW improvement works to River Rhymney defences.

Proposed Seawall Road site



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1.1.2. Shoreline Management Plan Ensuring the integrity and condition of the Rover Way frontage has strategic, as well as local, implications which have an impact on the assessment of the Seawall Road site.

The Shoreline Management Plans (SMP) set out the objectives and strategy for the future management of the shoreline around the UK. The Severn Estuary SMP covers the shoreline around Cardiff, from Lavernock Point up the estuary to Haw Bridge in Gloucester and along the north Somerset coast to Brean Down.

The SMP is further divided into frontages or policy areas. The shoreline along Rover Way is located within Policy Unit: CAR 2 – Western end of Cardiff Bay Barrage to mouth of River Rhymney at Rover Way

The preferred policy along this frontage is to “Hold the line”, with recommendation that this policy manages the risk of impacts from flooding and erosion to industrial and residential areas of Cardiff.

The SMP defines this policy as follows;

Hold the Line (HTL) by maintaining or changing the standard of protection. The standard of protection could increase or decrease. The intent of this policy is to maintain the current position of the shoreline. This does not necessarily mean that the existing defences would be maintained in exactly the same form as they are at present. There may be a need to adjust the local alignment in the future or to replace or add to structures e.g. constructing cross shore or shore-linked structures, such as groynes or breakwaters, may be one approach adopted under this policy in specific cases. The policy sets the intent to maintain the current position of the coast in an appropriate manner, which will differ depending on the specific local issues. The standard of protection could increase, decrease or stay the same.

Recommendations within the SMP of the scale of works needed to achieve this advise that;

earth embankments will require repair and maintenance during the short-term

some sections may need to be replaced.

The effect of coastal erosion on the flood defences at Rover Way has been previously assessed. This is introduced in the following section and the implications for the Seawall Road site are discussed further in Section 3.4.

1.2. Existing flood defence infrastructure The site is at risk from flooding during extreme events due to overtopping of the coastal defences and the tidal defences along the lower reaches of the River Rhymney. These defences are predicted to overtop during an existing 0.5% (1 in 200) annual chance tidal event.

The probability of breach of the defences is included in the SFCA tidal inundation modelling. A key assumption has been made that the defences are maintained in their current condition over the assessment period.

An erosion assessment of the Rover Way shoreline frontage was completed as part of the separate coastal erosion study reviewing the coastal erosion risk for Cardiff

2. This assessment identified that, based on

potential erosion rates (0 to 1m per year), failure of the existing coastal defences would be likely by 2033 unless intervention actions are put in place.

The management of this risk of erosion and impacts on the coastal defences is discussed in Section 3.4 below.

2 Cardiff Council - Coastal Erosion Risk Assessment, Atkins, July 2013, ref 5113153/70/DG/007



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2. Methodology

2.1. Development lifetime A 100 year development lifetime has been considered in line with the principles set out in the LDP and supporting SFCA for proposed residential developments (defined as “highly vulnerable development in Section 5 TAN15). For comparison purposes, a potential 75 year development lifetime has also been assessed. This is the recommended development lifetime more usually considered for potential commercial developments (categorised as less vulnerable development Section 5 TAN15).

A 75 year development lifetime for a Gypsy and Traveller Site could be considered acceptable, when consideration is given to the transient nature of the site’s intended occupants, i.e. gypsy and travellers,

The risks over both the 75 year and 100 year time horizons are considered in this report.

The Planning Authority would have to give further consideration to the acceptability or otherwise of the development lifetime.

2.1.1. TAN 15 Acceptance Criteria Any assessment would be subject to comparison against the guidance provided in Technical Advice Note 15: Development and Flood Risk (TAN15).

Section 6 of TAN15 requires that development defined as highly vulnerable (which includes any residential development) should not be permitted within Zone C2. However it should be noted that in the past Planning Authorities in Wales have been permitting highly vulnerable development to be located within Zone C2, provided that the site can be justified under Section 6 of TAN15 and that the consequences of flooding are acceptable.

Here it is assumed that Cardiff Council considers the proposed Gypsy and Traveller Site to be justified as part of a strategy to sustain an existing settlement, that the development concurs with the aims of Planning Policy Wales (PPW) and that it meets the definition of previously developed land.

It should also be noted that the TAN15 designation (Zone C2) would appear to take no account of the benefit provided by the existing coastal defence infrastructure and the flood defence embankments that are situated along the River Rhymney. If these defences were taken into account, along with consideration of the existing infrastructure, it could be considered that the site and surrounding area should be considered Zone C1 (Areas of the floodplain which are developed and are served by significant infrastructure, including flood defences).

Finally, the reader’s attention is drawn to the requirements of Welsh Office Circular No: 07/12. This circular and direction sets out requirements for local planning authorities to refer certain planning applications to the Welsh Ministers. This is to allow them to decide whether they wish to call in the application for their

determination. This requirement came into effect on 30th July 2012. It requires local planning authorities to

refer applications for emergency services or highly vulnerable development, where the whole of the land where the development is proposed to be located is within flood zone C2, (as shown on the development advice maps (DAM’s)). In cases consisting of residential development, the threshold for requiring notification is set at 10 or more dwellings. If the local planning authority were minded to grant planning permission for the site, then under the guidance outlined above they would need to refer the application to Welsh Ministers. The development may be called in for determination. It would be advisable to canvas Welsh Government on their attitude towards the site prior to investing any sums of money on enabling the development.



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2.1.2. Flood Consequences The potential consequences of flooding to the proposed development site are assessed below with reference to the indicative acceptance criteria contained in TAN15. This guidance sets out the considered acceptable level of flood risk to a development, summarised as follows;

A development should be flood free during its development lifetime for a 1% annual chance fluvial and 0.5% annual chance tidal flood event

A development may be subject to an allowable impact of flooding during the 0.1% annual chance event in accordance to the indicative acceptance criteria set out in TAN15 Appendix A1.15

Initial discussion with Natural Resources Wales has highlighted a number of issues to be addressed with regard to managing flood risk associate with the proposed development.

Clarity is required on the maintenance and management regimes to be applied:

to the coastal defences to prevent breach due to overtopping over a development lifetime

for any localised measures to manage the risk of coastal erosion over a development lifetime.

There is no existing plan in place to consider any strategic mitigation measures to address the flood risk to the wider existing developed area around the proposed site. No such strategic measures could therefore be considered to facilitate the development with regard to the identified flood risk to the development site.

2.1.3. Development Lifetime and climate change The likely impacts of climate change on the assessment will depend on the development lifetime considered appropriate. The longer the lifetime considered, the greater will be the impacts on flood risk. Two time horizons have been considered for this assessment:

75 years and 100 years.

The periods to 2085 and 2110 have been taken to approximate to the two periods, taking a base year of 2010 consistent with the overall SFCA which allows this assessment to make best use of existing analysis already undertaken for the SFCA. These time horizons currently represent development lifetimes of 71 and 96 years respectively for assessments carried out today and are considered appropriate to provide an indication of the likelihood of managing the order of flood risk to the site.

Sea level rise

The greatest impact of climate change in relation to flood risk to the site is sea level rise. Based on current projected rates of sea level rise accepted by Welsh Government in line with UK Defra and Environment Agency research, sea levels are estimated to increase by approximately 1m over a 100 year period.

The overtopping of defences and the probability of breach of the coastal defences would increase slightly for the additional years of climate change if the baseline year is updated to current day. It would need to be confirmed with NRW at the time of submission of a planning application and supporting FCA as to the extent of any revised modelling to update the assessment to the appropriate base year (i.e. the year of submission).

2.2. Hydraulic Modelling The model developed as part of the SFCA Stage 2 assessment for the area between the Rhymney estuary and Cardiff Bay has been used to inform the study. The model was previously accepted by the Environment Agency/Natural Resources Wales as part of the SFCA assessments.

The methodology to assess the feasibility of the Seawall Road site was agreed with Natural Resources Wales as part of this project (meeting held 30 January 2014). In summary this included;

Updating the SFCA Stage 2 model to run using the most recent version of the TUFLOW modelling software



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Revise baseline / existing conditions to include the NRW capital works which are to be completed this year to improve the standard of service alongside Rhymney River Bridge Road Industrial Estate. These will provide a top of wall level of 8.779mAOD (providing protection from a 1:200 (0.5%) year tidal flood event with 50 years sea level rise allowance).

The potential breach in the existing coastal defences to the south-west of the site is included within the modelling assessment. This approach is consistent with that in the SFCA.

The model has been used to test and review mitigation options. The model results inform the assessment as to whether the flood risks associated with the development can be managed in compliance with TAN15 guidance. i.e. considering both the risk to the occupants of the development whilst ensuring no adverse impact on flood risk elsewhere.

2.2.1. Model boundaries and inflows The inflow boundaries to the SFCA hydraulic model have not been updated as part of the assessment.

As described above, the tide levels and breach assessment of the coastal defence are based on sea level rise to future time horizons of 2085 and 2110.

2.2.2. Updated SFCA Baseline model results The SFCA (Area I) hydraulic model has been updated to include the NRW programmed capital works on the River Rhymney (following discussion with NRW). These works result in revised predicted flood extents adjacent to the site as a result of a reduced volume of overland flood waters from the north.

Figure 2.1 provides a comparison in flood extents for the future scenario to 2110 as presented in the SFCA of August 2011 and the updated model. This shows the reduced extent of flooding as a result of the River Rhymney defence works compared to the pre-works situation for a 0.5% chance tidal flood event in 2110.

Figure 2.1 Comparison of effect of NRW River Rhymney defence improvements. SFCA and the updated model results – 0.5% (1 in 200) chance event, 2110.



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The results of the updated hydraulic modelling for assessment of the Seawall Road site against TAN15 criteria are presented in the following section.



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3. Results The assessment has considered two time horizons for the potential development lifetime:

to 2085, reflecting a 75-year development lifetime.

to 2110, reflecting a 100-year development lifetime.

The results and impacts on the site for the 0.5% (1 in 200) annual chance flood events in 2085 and 2110 and the existing (2010) 0.1% (1 in 1000)

3 annual chance event are discussed below.

All results assume that the coastal defences provide a continued standard of service against overtopping over the development lifetime and are not subject to loss due to erosion. However, there is an identified risk due to erosion of the seaward side of the coastal defence at Rover Way. The impact of this risk is covered further in Section 3.4 below.

3.1. Development Lifetime to 2085 (71 years) - 0.5% chance event

3.1.1. Baseline Situation The revised baseline result which includes the NRW’s improvements to river defences at Rhymney River Bridge Road Industrial Estate, shows that during a 0.5% (1 in 200) annual chance event in 2085, the majority of the site would be flood free (as indicated Figure 3.1 below). The existing access to the site would be inundated during this event.

Figure 3.1 Flood Depths - 0.5% (1 in 200) chance event, 2085

3 Note; at the time of writing, NRW do not recommend applying climate change to the 0.1% chance flood events. It is understood from

discussions with NRW as part of this current assessment that this may change, and is currently the subject of discussion with Welsh Government.



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Flood free access to the site during a 0.5% annual chance event is required over the development lifetime in order to comply with TAN15 indicative guidance (A1.12).

To allow the whole site to be developed in line with TAN15 guidance, some raising of the proposed site would be required around the southern and western boundaries. If the southern and western areas were to be raised, mitigation for the loss of floodplain storage would be required elsewhere although this should only involve relatively minor works. Alternatively, the extents of the site footprint would need to be reduced to only consider the flood free areas.

3.2. Development Lifetime to 2110 (96 years) - 0.5% chance event

3.2.1. Baseline Situation The figures below show the predevelopment (existing) scenario during a 0.5% (1 in 200) annual chance tidal event in 2110 accounting for climate change.

The site is subject to significant flooding during the event, with approximately 50% of the site is flooded to depths in excess of 1m (as indicated in Figure 3.2 below). The access to the site along Seawall Road is also inundated to depths in excess of 1m over a length of 300m.

Over the development lifetime to 2110 the site cannot be considered to comply with A1.14 or A1.12 of TAN15.

Figure 3.2 Flood Depths - 0.5% (1 in 200) chance event, 2110



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3.2.2. Mitigation Works - Site Raising To achieve a development lifetime to 2110, mitigation works would be required to reduce the flood risk to the site.

Raising the site to above the 0.5% (1 in 200) annual chance flood level in 2110 would ensure that the development site would meet the threshold frequency set out in A1.14 of TAN15. However, the resulting loss of flood plain storage would need to be mitigated to ensure no adverse impact on flood risk elsewhere.

Such mitigation of the potential loss of storage would be possible by lowering the area to the east of the site by 0.5m as (indicated on Figure 3.3). This adjacent area is identified within the same larger LDP candidate site 19LBU (see Figure 1.2 above) previously assessed in SFCA 2011 Report.

Figure 3.3 Area to be lowered to mitigate raising of the site

Model results indicate that this lowering is sufficient to mitigate the loss of storage and ensure that no detriment results to the surrounding areas from the raising of the proposed Gypsy and Traveller Site (model results are presented in Figure 3.4).

However, access to the site would remain an issue with high flood depths and velocities over the potential access/ egress routes for the site (see Figure 3.4 and Figure 3.5 below).

Ground levels lowered by 0.5m

Proposed site



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Figure 3.4 Flood Depths – with site raised plus mitigation - 0.5% (1 in 200) chance event, 2110

Figure 3.5 Flood Velocities with site raised plus mitigation - 0.5% (1 in 200) chance event, 2110



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3.2.3. Mitigation Works – Prevent breach of existing defences Under existing conditions a breach of the coastal defences near the existing Rover Way Gypsy and Traveller Site would occur during a 0.5% (1 in 200) chance event by 2110 due to the amount of tidal overtopping.

We have investigated the impact on flood depths and velocities along the access routes to the site if this breach was prevented. This could be achieved, for example, through strengthening or re-positioning the defence.

By removing the likelihood of this breach occurring, flood levels adjacent to the site are reduced by approximately 0.6m. However, flood depths and velocities along the access route would still exceed 1.0m and 0.45m/s respectively, (as presented in Figure 3.6 and ). Escape and evacuation routes cannot therefore be considered to be operational under all conditions, as required by A1.12 of TAN15.

Access to the site would still not meet the TAN15 acceptance criteria with the defence reinforcement in place.

Figure 3.6 Flood Depths with site raised and No breach - 0.5% (1 in 200) chance event, 2110



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Figure 3.7 Flood Velocities with site raised and No breach - 0.5% (1 in 200) chance event, 2110

3.3. All development lifetimes - Existing (2010) 0.1 % chance event Appendix A1.15 of TAN15 indicates limits of depths and velocities which are considered to be acceptable in the event of a 0.1% (1 in 1000) annual chance event. Current NRW guidance suggests that the sensitivity to climate change of an event of this low probability does not need to be applied as part of an FCA.

It is predicted that approximately two-thirds of the site would be at inundated during an existing (2010) 0.1% (1 in 1000) annual chance flood event (Figure 3.8). The depths of flooding would remain below 0.6m over the majority of the site and so within the indicative acceptance criteria for residential development given in A1.1.5 of TAN15. However, approximately 10% of the site area would flood to depths in excess of 0.6m, but below 1.0m. These areas would need to be raised accordingly. It is considered likely that the impacts of raising a proportion of the site could be mitigated by lowering some of the higher levels on site.

Flood depths along the access road to the site along Seawall Road would also be above 0.6m for an approximate length of 100m.

To mitigate the risk for access and egress to the site the following could be considered.

this area of highway would need to raised locally and the resulting loss of floodplain storage mitigated accordingly, or

access and egress routes for the site could be managed through an effective flood warning and evacuation system.

Flood velocities within the site and along the access road are within the indicative acceptance criteria set out in TAN15 during the 0.1% (1 in 1000) annual chance event (Figure 3.9).



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Figure 3.8 Flood Depths - 0.1% (1 in 1000) annual chance event, 2010.



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Figure 3.9 Flood Velocities - 0.1% (1 in 1000) chance event, 2010.

3.4. Coastal Erosion impact on flood risk As noted in section 1.1.2 the existing coastal defences are at potential risk of erosion from the seaward side. A previous review of potential erosion risks to Rover Way Gypsy and Traveller Site identified a failure of a section of the coastal defence by 2033 unless intervention actions are put in place. Whilst the timeframe is uncertain, it would be a reasonable assumption that within the development lifetime this erosion risk would cause failure of existing embankments.

Should this risk be realised, the overall area including the proposed development site will be inundated by a 0.5% (1 in 200) annual chance tidal event from the time of failure.

The risk of failure of the coastal defences at Rover Way due to erosion is considered to be high within the development lifetime. Works will be required to reduce the risk of failure of the defences. This risk due to erosion of the seaward side of the coastal defences is distinct from the risk of a breach of the defences due to overtopping in extreme tide events, referred to in Section 1.1.1.



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3.5. Cost estimates

Mitigation – site raising The cost of raising the site and providing the mitigation works to lower the adjacent area will vary depending on the source of fill material. This may be available as excess material from another site in the local area, and thus the cost is related mainly to haulage of the material to the site.

Similarly, disposal of any excavated materials will need similar consideration, with related testing to determine its suitability for reuse on other sites.

Mitigation - coastal erosion The Rover Way area is predicted to be at a high level of erosion risk with associated significant impacts on flood risk to the wider hinterland area. Maintaining this coastal alignment is also consistent with the adopted “Hold the line” policy set out in the SMP. Costing of the potential engineering interventions is given in Table 3-1. Indicative cost estimates suggest a capital investment in the order of £2-3M is required to prevent the erosion of the Rover Way frontage. This could be achieved by either:

Reinforcement of the coastal defences

Constructing a new defence along a route set back from the current alignment.

These works will have a benefit to the wider area including existing residential and commercial properties and will not simply facilitate development of Rover Way. Responsibility with regards maintenance and management regimes for the defences would also need to be determined and set out as part of any ongoing and long-term commitment.

These costs are a scale of magnitude greater than those estimated for the raising of the development area if required to realise a 100-year development life time.

Table 3-1 Indicative erosion protection costing for Rover Way area

Option Short term

(2013-2026) cash cost (£k)

Medium to long term

(2026 to 2110) cash cost (£k)

Whole life cash cost (£k)

Discounted (Present value) whole life cost

(£k)

Monitoring 25 169 195 54

Reconstruction of the gabion basket system*

1,854 335 2,189 1,913

Rock armouring of the seaward face

3,732 335 4,066 3,790

Construction of revetment system

2,681 335 3,016 2,740

Taken from “Cardiff Council - Coastal Erosion Risk Assessment”, Atkins, July 2013, ref 5113153/70/DG/007

* It is noted that the cost for construction of a gabion basket system is included in the presented options. This indicates the possible

cost for replacement of the existing failed erosion protection assets to the Rover Way frontage. It also provides a comparison against the other options presented. Gabion basket systems are however not considered a standard method of protection against significant wave action and erosion as would be expected along this stretch of coastline.



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4. Summary and Conclusions The assessment has identified the flood risk to the Seawall Road Site over the likely lifetime of development considering both 75 year and 100 year development lifetimes. The risks and likelihood of mitigation are summarised below. These assume that improvements to the River Rhymney flood defences, programmed for construction in 2014 are undertaken by Natural Resources Wales.

The following table provides a summary of the flood risk to the site, assessed against criteria defined in TAN15 guidance. The assessment has been undertaken assuming the area is re- classified as Zone C1, recognising the benefits from existing significant infrastructure (as described in section 2.1.1 above).

Table 4-1 Summary of risks and mitigation required by time horizon

Time horizon

Event probability (Return period)

Existing/ Baseline situation

TAN 15 compliance

With mitigation TAN 15 compliance with mitigation

Current day - (2010 assessment)

0.5% or 1:200 annual chance

(T200)

Not remodelled but SFCA analysis indicates would be flood free

A1.14 N/A A1.14


(T1000)

site – approximately 10% of the site area will flood to depths in excess of 0.6m but to less than 1.0m.

A1.15 site - passes TAN15 A1.15

A1.15

access – depths and velocity exceed criteria

A1.12 access – mitigation actions through alternative route and warning procedures can be put in place

A1.12

75 - development lifetime (2085 assessment)


(T200)

site – majority of site is flood free

A1.14 But only subject to coastal defence improvements required to mitigate for erosion failure

site – localised site raising / reduced development extent

A1.14 with coastal defence improvements

access – main access road would be flooded, less than 0.3m depth

A1.12 access – mitigation actions through alternative route and warning procedures can be put in place

A1.12 with coastal defence improvements

coastal erosion – foreseeable loss of coastal defences due to erosion resulting in the site being affected by all tidal flood events

coastal erosion – improvements works to the coastal defences are required to ensure their integrity



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Time horizon

Event probability (Return period)

Existing/ Baseline situation

TAN 15 compliance

With mitigation TAN 15 compliance with mitigation

100 - development lifetime (2110 assessment)


(T200)

The site would be inundated over its entire area, with depths exceeding 1m of a large proportion of the site.

A1.14 site – site raising with compensation ground lowering of adjacent site

with coastal defence improvements

Access and egress routes would also be flooded to depths exceeding 1m.

A1.12 access – affected by floodwaters all around the site limiting opportunity to for safe access under all conditions

A1.12

coastal erosion – foreseeable loss of coastal defences due to erosion resulting in the site being affected by all flood events

coastal erosion – improvements works to the coastal defences are required as described above

4.1. Erosion risk to the area adjacent to the site There is a risk to the wider area from the failure of the existing coastal defence infrastructure due to erosion which needs to be addressed. If potential erosion rates are realised and the existing embankment fails, then the site and the surrounding area will be at greater risk such that they will be affected by the 0.5% (1 in 200) chance event from then on. Although not modelled as part of this assessment, it is likely that the chance of flooding would increase significantly if erosion is allowed to proceed unabated. The loss of this frontage has a strategic impact on the local area irrespective of the intended development at Seawall Road.

Through suitable development and planning conditions it may be possible to sequence the development of the proposed Seawall Road site, and re-development of the existing Gypsy and Traveller Site at Rover Way. This could facilitate works to reduce the erosion risk to the coastal defences and potentially provide wider amenity and access improvements to the adjacent coastal path and coastal frontage.

4.2. Cost estimates Indicative cost estimates for capital investment to reduce the erosion risk to the wider Rover Way frontage indicate in the order of £3M investment is required.

4.3. Summary The flood risk to the site and the wider area is subject to works being undertaken by NRW to the existing flood defences at River Way. The Seawall Road site could be developed with acceptable flood consequences, if a 75 year development lifetime (to 2085) is considered acceptable by the local planning authority and that they are content to accept highly vulnerable development in an area designated as Zone C2.

Mitigation measures recommended above would be required to manage flood risk to the site and the access/egress routes. If these mitigation works were accepted, and the works undertaken to ensure the integrity of the existing coastal defences, the site could be developed to provide an indicative 75-year (to 2085) development lifetime.

It should be noted that any final FCA assessments against the proposed site layouts would need to account for the appropriate development lifetime at the time of the formal submission of any development planning application.



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If the Council are minded to proceed with the development of the Gipsy and Traveller site, there is a risk that the application would be called in for determination by Welsh Ministers. It is recommended that the Council canvas Welsh Government on their attitude towards the site prior to investing any sums of money on enabling the development.

© Atkins Ltd except where stated otherwise. The Atkins logo, ‘Carbon Critical Design’ and the strapline ‘Plan Design Enable’ are trademarks of Atkins Ltd.

Kevin Owen Atkins West Glamorgan House 12 Orchard Street Swansea SA1 5AD

[email protected]

seawall road proposed gypsy and traveller site · developing a proposed gypsy and traveller site...

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