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Cardiff SFCA Addendum to Stage 3 Preliminary Site Assessments - Area A Cardiff County Council October 2012

Cardiff SFCA Addendum to Stage 3 Preliminary Site Assessments - Area A

Atkins Cardiff SFCA - October 2012 -5109602

Notice

This document and its contents have been prepared and are intended solely for Cardiff County Council’s information and use in relation to preliminary Stage 3 assessment of fluvial flood risk to selected sites.

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

This document has 24 pages including the cover.

Document history

Job number: 5109602 Document ref: 5109602/DG008

Revision Purpose description Originated Checked Reviewed Authorised Date

1-D1 Internal draft CRC DMH/KIO

1 Draft for client comment CRC KIO HR HR 10/08/12

2 Client Comments Incorporated

DMH KIO HR HR 19/10/12


Atkins Cardiff SFCA - October 2012 -5109602

Table of contents

Chapter Pages

1. Introduction 4 1.1. Background 4 1.2. Stage 3 addendum objectives 6

2. Sub Area A – Wentloog Levels 7 2.1. Sub Area A candidate sites 7

3. Approach & Assessment Methodologies 8 3.1. River Rhymney defences 8 3.2. Fluvial flood risk 9

4. Existing Flood Risks by Site 14

5. Flood Risk Mitigation Options and Costs 14 5.1. River Rhymney defence 14 5.2. Feasibility and costs 14 5.3. Fluvial flood risk 17 5.4. Summary of fluvial flood risk 18

6. Summary and Recommendations 19 6.1. Mitigation of risk to Sites 2LGRM, 24LGRM, 106LGEM and 21LGR 19 6.2. Mitigation of risk to Site 22LGRM 19 6.3. Summary of developable areas by Site 19

Appendices 21

Appendix A. Reen System Hydrological Assessment 22

Appendix B. Drawings 23 B.1. Updated Fluvial Model Results 23

Tables Table 2.1 - Sub Area A Sites 7 Table 3.1 – Mitigation options tested 9 Table 3.2– Catchments for 1D/2D modelling 9 Table 5.1 - Options to mitigate site raising through to 2110 15 Table 5.2 – Summary of development potential accounting for fluvial flood risk 18

Figures Figure 1-1 Phased approach to Cardiff SFCA 5 Figure 3-1 Main (pink) and sub (green) drainage areas within Catchment 1 10 Figure 3-2 Main (pink) and sub (green) drainage areas within Catchment 2 11 Figure 3-3 Environment Agency 0.1% (1 in 1000) annual chance direct rainfall flood extents 13 Figure 3-4 Initial water levels used as downstream boundary conditions for the current assessment 13


Atkins Cardiff SFCA Stage 3 Area A - Addendum | Version 2.0 | October 19 2012 4

1. Introduction

Cardiff Council has appointed Atkins Ltd to undertake Strategic Flood Consequences Assessment (SFCA) Stage 3 studies for a number of candidate sites included within the draft Cardiff Local Development Plan (LDP). The selected sites were identified in earlier phases of the SFCA to be at significant risk of flooding due to tidal inundation within the development lifetime taking into account predicted climate change impacts.

This addendum report builds on, and should be read in conjunction with, the SFCA Stage 3 Preliminary Assessment Report. This addendum report assesses the fluvial flood risk and feasibility of raising the existing defences along the east bank of the River Rhymney as part of the mitigation approach for the acceptance of candidate sites to be included within the LDP.

1.1. Background The following quoted background information has been taken from the July 2011 Cardiff SFCA Phase 2 Part 2 report

1 to provide a brief explanation of the motivation for assessing flood risk to development:

“As part of the planning process, under the Planning and Compulsory Purchase Act 2004, each planning authority in Wales is required to prepare a development plan for its area. Development plans provide strategic guidance for development, detail site specific policies and identify proposals for development.

The Welsh Assembly Government’s Planning Policy Wales (PPW) provides the planning policy framework under which the Local Planning Authorities (LPAs) must prepare their LDPs. The LDP must encourage patterns of development that are economically, socially and environmentally sustainable. As part of this approach, PPW highlights the need to reduce flood risk by avoiding development within high risk areas, as opposed to the previous approach of flood defence and mitigation of the consequences of flooding. The guidance requires LPAs to consider the catchment as a whole and take a strategic approach to managing flood risk.

PPW is supplemented by a number of Technical Advice Notes (TANs). TAN15: Development and Flood Risk provides guidance to LPAs regarding assessment of flood risk when they are preparing their LDPs. TAN15 seeks to guide planning decisions so that new development is directed away from areas that are considered to be at high risk of flooding. Where development within a high risk area is being considered, TAN15 outlines justification tests that must be met for specific development types to proceed.”

The Cardiff SFCA has been to date carried out following a phased approach. A Phase 1 scoping assessment was issued to the Environment Agency Wales for comment in April 2009

2. This resulted in an

agreed approach to the study and defined sub areas encompassing the candidate sites and some existing allocations. Phasing of the SFCA is illustrated in Figure 1-1.

Based on the results of the Phase 2 assessment3,4,5

, it was determined that an additional study would be required at some sites to demonstrate whether the flood risk could be managed to an appropriate level and the sites deemed suitable for inclusion within the LDP. Cardiff Council commissioned Atkins to undertake a preliminary Phase 3 (Stage 3) assessment to include selected sites within Sub Area A; the report was submitted in April 2012

6.

The most significant source of flooding to Sub Area A sites is tidal inundation resulting from overtopping and potential breach of the existing tidal defences. Potential mitigation options were identified as part of the preliminary Stage3 assessment. The proposed options comprised raising the candidate sites and compensating for the resulting loss of flood storage. The compensation would be achieved through reinforcement and raising of the tidal defences to reduce the quantities of water overtopping the existing defences. Mitigation for candidate sites in the west could be achieved by raising defences along the east bank of the River Rhymney. However, insufficient data existed to determine the feasibility of raising these existing flood defences.

In addition there was insufficient data to assess the fluvial flood risk to the sites from the area’s reen network.

1 Atkins, July 2011, Cardiff SFCA Phase 2 Part 2 Final Report. 2 Atkins, April 2009, Cardiff SFCA Report on Phase 1 (Scoping Study).

3 Atkins, November 2009, Cardiff SFCA Phase 2 Report.

4 Atkins, February 2010, Cardiff SFCA Addendum – Area A and Area I Revision Based on 2D Tidal Inundation Modelling.

5 Atkins, November 2011, Cardiff SFCA Phase 2 Part 1 Update, Extend Development Lifetime to 2110, Areas A, G, H and I, Final

Report. 6 Atkins, April 2012, Cardiff SFCA Stage 3 Preliminary Site Assessments Area A Sites



At the request of Cardiff Council, Atkins has assessed the feasibility of raising the River Rhymney defence and determined the required height by which it should be raised. This was achieved through additional modelling of the tidal flooding scenarios. Atkins has also carried out a modelling study to examine the fluvial flood risk to the sites and any impacts of site development. The results of the studies are documented in this addendum.

Figure 1-1 Phased approach to Cardiff SFCA



1.2. Stage 3 addendum objectives The aim of this addendum is to provide additional information required for determining the viability of including a number of Sub Area A candidate sites within the LDP. To this end, the objectives of this study are to:

Assess the feasibility of raising the existing defence along the east bank of the River Rhymney;

Determine the height by which the defences need to be raised to provide adequate mitigation for proposed raising of the site levels;

Assess the fluvial flood risk posed to the sites by the major reens flowing through and adjacent to the sites;

Assess any adverse impacts of site raising;

Considering the above, determine the feasibility of mitigating the fluvial flood risk;

Where applicable, develop further mitigation options.



2. Sub Area A – Wentloog Levels

Sub Area A covers part of the area of the Wentloog Levels extending from the Rhymney River in the west to St Mellons Business Park in the east. The Wentloog Levels comprise an area of historically reclaimed land. As part of the reclamation, an extensive network of ditches (reens) was constructed to drain water out to sea. This network is currently managed by the Caldicot & Wentlooge Levels Internal Drainage Board (IDB). Land use within the area includes wide swathes of agricultural land with urban and suburban areas in the north. Much of Sub Area A is located within the Gwent Levels – Rumney and Peterstone Site of Special Scientific Interest (SSSI). The main London to South Wales railway line runs west-east across the Levels.

The most significant source of flooding to Sub Area A sites is tidal inundation resulting from overtopping and potential breach of the existing tidal defences. It has been determined in the preliminary assessment completed for Area A that this risk can be mitigated by raising site levels with compensatory works to the coastal defences

7. However, risk associated with fluvial flooding of the reen system has not yet been fully

characterised and so is addressed in more detail in this addendum report.

2.1. Sub Area A candidate sites This section summarises the sites identified by Cardiff Council for Stage 3 assessments based on the advice given by the Environment Agency in their letter dated 24

th November 2011.

The selected candidate sites are listed in Table 2-1.

Table 2-1 - Sub Area A Sites

Site name Proposed development type Additional details

22LGRM - Trowbridge Mawr Residential/ Industrial Currently, the eastern third of the site is developable.

21LGR – Land at areas 9-12, St Mellons

Residential

24LGRM – Land East of Cypress Drive

Residential/ Mixed Use

2LGRM – Land at St Mellons Business Park

Residential/ Mixed Use

106LGEM – Cardiff Parkway Retail/ Industrial One site area north and one south of the railway embankment. The southern portion was considered unsuitable in Phase 2 but has been included in the current assessment.

2.1.1. Environmental designations at Sub Area A candidate sites Sites 24LGRM, 2LGRM and 106LGEM and the majority of Site 21LGR are situated within the limits of the SSSI. Site 22LGRM lies outside the designation.

7 Atkins, April 2012, Cardiff SFCA Stage 3 Preliminary Site Assessments Area A Sites



3. Approach & Assessment Methodologies

As noted above, coastal and tidal flood risk aspects were addressed within the preliminary SFCA Stage 3 assessment for Area A. The following sets out the adopted approach for assessing the residual risk posed by the fluvial flood risk from the River Rhymney and the separate Wentloog Levels drainage reen network.

3.1. River Rhymney defences

3.1.1. Topographical survey New topographical survey was collected in June 2012 of the existing defence along the east bank of the river south of the railway and of the eastern riverbank north of the railway. Examination of the survey revealed that defence levels south of the railway are lower by up to 0.5m than those determined from LiDAR and used previously in modelling tidal flooding. Surveyed top of bank levels north of the railway are also lower than suggested by LiDAR.

3.1.2. Tidal Model update We have updated the existing 1D/2D tidal inundation model with the above topographical data and have re-run the baseline scenarios and the mitigation options presented in Table 5.2 of the preliminary SFCA Stage 3 assessment for Area A.

We have also investigated whether it would be possible to mitigate the development of Site 106LGEM south of the railway. Mitigation was previously considered unlikely given the area’s location and low-lying topography. However, model results from the Stage 3 assessment suggest that the effects of raising the tidal defences (reduction in the volume of tidal water entering the area) may be significant enough to offset the impacts of raising site levels (i.e. loss of flood storage area) south of the rail line.

Raising heights for the coastal defences at Sluice Farm and Little Wharf are based on proposals included in the Severn Estuary Flood Risk Management Strategy (SEFRMS)

8. The SEFRMS also identifies the River

Rhymney defences as being improved after 2030. In a letter dated 17 May 2012 (ref: SE/2010/112913/OR-03/AE1-L01), Environment Agency Wales has stated

9 that it would be working with Cardiff Council to raise

the defence, but that the work would take place outside the Local Development Plan timeframe of 2026. The defence at Sluice Farm has been raised by an average of 0.6m in the modelling for this assessment in looking forwards over the development lifetime.

Table 3-1 summarises the mitigation options we have tested using the updated tidal model.

8 Atkins and ABPmer, September 2011, Severn Estuary Flood Risk Management Strategy Strategic Options Assessment Report

9 Environment Agency Wales letter dated 17 May 2012 (ref: SE/2010/112913/OR-03/AE1-L01),



Table 3-1 – Mitigation of tidal flooding - options tested

Option Works to enable development Mitigation option description

1 Raise site levels

Would not mitigate for raising Site 22LGRM

Raise Sluice Farm (SR1-2) by 0.6m and prevent breach

Developer contribution required

2 Raise site levels

40,000m2 of western part of Site 22LGRM left unraised

Raise Sluice Farm (SR1-2) by 0.6m and prevent breach

Raise River Rhymney east bank (SR1-RREB) south of the railway by an average of 0.8m and prevent breach

Developer contribution to raising Sluice Farm (SR1-2) required

3 Raise site levels

Would not mitigate for raising Site 22LGRM

Assume Sluice Farm (SR1-2) has been raised

Raise Little Wharf (SR1-0) by 0.6m and prevent breach

4 Raise site levels

not mitigate for raising Site 22LGRM


Raise Little Wharf (SR1-0) by 0.6m and prevent breach

Prevent breach at River Rhymney east bank (SR1-RREB)

5 Raise site levels



Raise River Rhymney east bank (SR1-RREB) by an average of 0.8m and prevent breach

All options were tested for effectiveness during scenarios involving:

1. breaches of the tidal defences 2. overtopping only of the tidal defences.

3.2. Fluvial flood risk

3.2.1. Reen system and catchment areas For the purposes of this study, the area was divided into two catchments as described in the following table.

Table 3-2– Catchments for 1D/2D modelling

Reen catchment Included reens/ water bodies Affected sites

Catchment 1 Rhosog Fach Reen

Rhosog Fawr West Reen

Rhosog Fawr Branch Reen

22LGRM

Catchment 2 Pil-du Reen

Hendre Reen

Walwyn Place Reen

Fen Eliffant Reen

St Mellon’s Pond

Railway Reen

Tarwick Reen

Faendre Reen

Ty-Ffynon Reen

Greenlane Reen

Green Lane Branch Reen

Little Greenlane Reen

21LGR

24LGRM

106LGEM

2LGRM



The contributing areas to each reen (Figure 3-1 and Figure 3-2) were delineated initially based on LiDAR and refined based on sewer records held by Cardiff County Council.

Figure 3-1 Main (pink) and sub (green) drainage areas within Catchment 1



Figure 3-2 Main (pink) and sub (green) drainage areas within Catchment 2

3.2.2. Hydrology Inflow hydrographs for 1% (1 in 100) and 0.1% (1 in 1000) annual chance events are required for assessing fluvial flood risk in line with TAN15

10 guidance. Hydrographs were generated for a range of storm durations

in order to investigate both limitations on conveyance and storage, and generation methodologies were selected based on the nature of the catchments (i.e. rural or urban). We have attached the details of the hydrological assessment within Appendix A.

10 Planning Policy Wales Technical Advice Note 15: Development and Flood Risk, Welsh Assembly Government, July 2004



3.2.3. Model development A one dimensional (1D)/ two dimensional (2D) hydrodynamic model of the two catchments was developed using newly acquired topographical data and hydrology. The model is an ESTRY/TUFLOW model which is both 1D and 2D to the north of the Cardiff/Newport railway but only 2D to the south of the railway.

The new topographical survey was collected in late May 2012. The survey included cross sections of the reens of interest as well as structures such as culverts and sluices. Due to there being little change in the cross sectional geometry along the reens, surveyed cross sections were requested at a greater than normal spacing with the model calculating interpolated sections to provide an assessment that was numerically stable.

Additional information was received from the IDB which provided the location of their maintained sluice structures as well as the penning levels for the different reens during both the summer and winter months. The model utilised the higher summer penning levels to set the elevation of weirs included within the hydraulic model to mimic the controlling effect of the sluices.

To the north of the Cardiff/Newport railway line, varying initial water levels were set within the model to reflect the levels being maintained by the sluices along the different reens. The initial conditions were also applied to the 2D model domain to inundate the ditches to the penning levels that are to be expected over the summer months.

To the south of the Cardiff/Newport railway line, initial water levels (IWL) were set to an elevation above the top of bank based on flood volumes required to inundate the area. To provide a sensibility check, the IWL were compared to flood extents being predicted by a direct rainfall model provided by the Environment Agency. The IWL set were similar to those of the 0.1% (1 in 1000) annual chance event direct rainfall event as shown by Figure 3-3 and 3-4 below.

The models were run using the critical storm duration of 6.5 hours. Other durations tested included a 2.5hr storm and a 12.5hr storm. The peak flow predicted by a 2.5hr critical storm was greater than that of a 6.5hr event but the overall volume of the hydrograph was less. The 6.5hr storm predicted greater flood extents. The 12.5hr storm duration again had a lower peak flow but a greater volume associated with the hydrograph than the 6.5hr event. However, over a period of 12.5 hours it is expected that a proportion of the inflow volume would be discharged through the outfall structures and into the Bristol Channel which would free up storage area downstream of the railway line. It is for this reason that the final model is based on the 6.5hr storm duration. It has been assumed that for the duration of the 6.5hr hydrograph, the outfalls would be tide locked. Consequently, no water leaves the model domain downstream of the railway line during the simulation period.



Figure 3-3 Environment Agency 0.1% (1 in 1000) annual chance direct rainfall flood extents

Figure 3-4 Initial water levels used as downstream boundary conditions for the current assessment



4. Existing Flood Risks by Site

The assessments provided in the preliminary Area A Stage 3 report remain valid. The main flood risk to the sites is due to tidal inundation.

If undeveloped, the sites are at risk of fluvial flooding, either from the River Rhymney or the reens system. The following section details the potential impact of the mitigation options described in Table 3-1.

5. Flood Risk Mitigation Options and Costs

5.1. River Rhymney defence

5.1.1. Model results The results of the updated tidal modelling confirm that Options 1, 3 and 4 of Table 3-1 would be effective in mitigating for the loss of storage that would occur if Sites 21LGR, 2LGRM, 24LGRM and 106LGEM (including south of the railway) were raised for development. Flood depths elsewhere in the Wentloog Levels would be reduced by in the order of 0.1 to 0.2m when compared with the baseline. Localised increases in velocity would occur as a result of lowered depths, but the overall risk when taking into account effects on depth and velocity is considered reduced relative to the baseline.

Options 2 and 5 were found to be effective in mitigating for the loss of storage caused by raising and developing all sites, including Site 22LGRM.

5.2. Feasibility and costs The preliminary assessment highlighted the need to determine the technical feasibility of raising the defence at the east bank of the River Rhymney (see Section 6.1). Upon further investigation, raising of the defence in its current location is considered feasible but would likely require relocation of the reen running along the toe on the landward side of the embankment. This would arise from the need to widen the base of the structure. The costs for options involving raising the defence have been updated to reflect the additional scope.

The scopes and costs of the various options provided in the preliminary assessment (Table 5.2, Stage 3 report) have been revised and updated based on the latest model results and are presented in Table 5-1. Model improvements have led to changes in the predicted site raising levels and volumes.

Note. It is likely that the area of each site which may be developed in line with TAN15 will be limited by the fluvial flood risk. See sections 5.4 and 6.3.



Table 5-1 - Options to mitigate site raising through to 2110

Mitigation option Reference

Mitigation option Total estimated length of defence reinforced or/and raised

Estimated costb

of mitigation works to coastal defence (£k)

Details Sites for which mitigation for loss of storage can be achieved

Raised level above the 0.5% event level by 2110 (mAOD)

Average height of raising (m)

Volume of fill required (m

3)

a

Cost of raising site (£k)

b

Option SFCA-A1 Raise embankment at Sluice Farm (SR1-2)

Prevent breach at SR1-2

Raise site levels

SR1-2 – 740m 2,500 Assumes developer contribution toward proposed EA works at Sluice Farm (SR1-2)

Maximum flood depths everywhere reduced with greatest benefit in the east

In some areas where maximum flood depths are reduced, peak velocities are increased

New access/ egress route from north would be required at Site 2LGRM

This option would not mitigate for raising Site 22LGRM

This option is considered the preferred option for mitigating raising of the other sites


6.24 0.87 12,000 360

24LGRM – Land east of Cypress Drive

6.24 0.32 11,000 330

106LGEM – Cardiff Parkway (north of railway)

6.24 1.03 422,000 12,660

106LGEM – Cardiff Parkway (south of railway)

6.25 1.25 361,000 9,480

21LGR – Land at Areas 9-12 St Mellons

6.24 0.27 30,000 900

22LGRM – Trowbridge Mawr This option would not mitigate for raising Site 22LGRM

Option SFCA-A2 Raise embankment at Sluice Farm (SR1-2)


Raise embankment south of the railway line at River Rhymney east bank (SR1-RREB)

Prevent breach at SR1-RREB

Raise site levels

SR1-2 – 740m

SR1-RREB – 440m

4,600

(2,500 plus 2,100)

Assumes developer contribution toward proposed EA works at Sluice Farm (SR1-2)


Maximum flood depths everywhere reduced with greatest benefit in the west



Defences would be raised along their current alignments

The reen running along the landward toe of SR1-RREB would need to be relocated to the east


6.14 0.79 11,000 330


6.13 0.28 7,000 240


6.13 0.92 379,000 11,370


6.14 1.15 290,000 8,700


6.13 0.24 20,000 600

22LGRM – Trowbridge Mawr 6.71 0.93 175,000 5,250

Options below assume that the Environment Agency’s proposed works at Sluice (SR1-2) are complete and so are effectively compared to a revised baseline situation.

Option SFCA-A3 Raise embankment at Little Wharf (SR1-0)


Raise site levels

SR1-0 – 740m 2,700 Assumes EA has carried out proposed works at Sluice Farm (SR1-2) (i.e. no developer contribution)

Maximum flood depths everywhere reduced with greatest benefit in the west when compared with a new baseline assuming the works at SR1-2 are complete





6.06 0.73 10,000 300


6.07 0.27 6,000 180


6.07 0.86 353,000 10,590


6.07 1.08 272,000 8,160


6.06 0.24 15,000 450




Mitigation option Reference

Mitigation option Total estimated length of defence reinforced or/and raised

Estimated costb

of mitigation works to coastal defence (£k)

Details Sites for which mitigation for loss of storage can be achieved

Raised level above the 0.5% event level by 2110 (mAOD)

Average height of raising (m)

Volume of fill required (m

3)

a

Cost of raising site (£k)

b

Option SFCA-A4 Prevent breach at River Rhymney east bank (SR1-RREB)

Raise embankment at Little Wharf (SR1-0)


Raise site levels

SR1-RREB – 440m

SR1-0 – 740m

3,700

(1,000 plus 2,700)

Assumes EA has carried out proposed works at Sluice Farm (SR1-2) (i.e. no developer contribution)



Further investigation would be required to confirm that reinforced grass would provide adequate protection at SR1-RREB




6.03 0.71 10,000 300


6.06 0.27 6,000 180


6.06 0.85 349,000 10,470


6.04 1.04 263,000 7,890


6.06 0.24 15,000 450


Option SFCA-A5 Raise embankment south of the railway line at River Rhymney east bank (SR1-RREB)

Prevent breach at SR1-RREB

Raise site levels

SR1-RREB – 440m

2,100 Assumes EA has carried out proposed works at Sluice Farm (SR1-2) (i.e. no developer contribution)





The reen running along the landward toe of SR1-RREB would need to be relocated to the east


6.14 0.79 11,000 330


6.13 0.28 7,000 240


6.13 0.92 379,000 11,370


6.14 1.15 290,000 8,700


6.13 0.24 20,000 600

22LGRM – Trowbridge Mawr 6.71 0.93 175,000 5,250

a Volumes for Sites 2LGRM, 21LGR, 24LGRM and 106LGEM exclude 12m wide buffers on each bank of the major reens through or adjacent to the sites. Volumes for Site 22LGR exclude 7m buffers.

b Costs exclude compensation or land purchase costs



5.3. Fluvial flood risk The following sections describe the existing flood risk at each of the sites and discuss the extents of developable areas. The areas required as buffers around the reens have not been taken into account when calculating developable areas. This is in order to avoid double counting at those sites where compensatory flood storage may be situated adjacent to the reens.

5.3.1. Study Site 22LGRM It was determined through modelling that fluvial flooding of this site is particularly sensitive to the choice of downstream boundary conditions. When an appropriately conservative approach is adopted (i.e. bankfull flow in the channels downstream of the railway) the western 60% of the site is subject to fluvial flooding. The high ground in the east would remain flood free during a 1% (1 in 100) annual change event including climate change and a current 0.1% (1 in 1000) annual chance event. The results suggest that the majority of the site functions as flood storage to the benefit of existing development downstream of the railway embankment.

Development of the high ground in the east could proceed without issue providing a flood-free access/ egress route is maintained. Development elsewhere on site would require the raising of ground levels over parts of the site with the provision of compensatory flood storage to mitigate for the loss of storage volume. It is approximated that one-third of the flooded area would be developable as a result. In summary, an estimated 50% of the total site area may be developed in line with TAN15 (noting that the tidal risk will also need to be mitigated).

The details of the storage requirements should be determined as part of a site-specific FCA with consideration given to a multi-cell system. A buffer width of 7m on either side of Rhosog Fach and Rhosog Fawr reens should be retained to allow for maintenance access.

5.3.2. Study Site 21LGR Model results indicate that the site is flood free during a 1% (1 in 100) annual chance event including for climate change. During a 0.1% (1 in 1000) annual chance event, flooding along the downstream portions of the minor ditches through the site is predicted, although flood depths would remain below 600mm.

On the basis of these findings, development of almost the entire site would be possible, noting that buffer areas of 12m should be reserved along both banks of Hendre Reen and Pil Du Reens.

5.3.3. Study Site 24LGRM This site is predicted to be flood free during a 1% (1 in 100) annual chance event through to 2110 with the exception of a 300m

2 low-lying area along an existing ditch on the eastern site boundary. Predicted flood

extents during a 0.1% (1 in 1000) annual chance event are similar.

Based on these results, development of nearly the entire site would be possible (assuming the tidal risk is mitigated, see Section 5.2). An approximately 300m

2 low-lying compensatory flood storage area should be

incorporated into the design of the site to prevent the loss of flood storage during fluvial events. This should be confirmed and the details determined as part of a site-specific FCA. A 12m buffer along Fendre Reen should be reserved for maintenance access.

5.3.4. Study Site 2LGRM Model results suggest that the southern part of the site (approximately 25% of the total site area), which is relatively low lying, would be inundated during a 1% (1 in 100) annual chance event including climate change. This includes the narrow portion of the site currently providing access from the south. The same area would be flooding during a current 0.1% (1 in 1000) annual chance event.

The results indicate that 75% of the site could be developed without raising site levels (assuming the tidal risk is mitigated, see Section 5.2). Alternatively, site levels could be raised to maximise the developable site area. Allowing for compensatory flood storage on site using a multi-cell storage system the area of development could increase to approximately 85%. The option of offsite storage could also be considered to allow development of the entire site. This should be confirmed and the details of the compensatory storage determined as part of a site-specific FCA. A 12m buffer along Little Green Lane Reen should be provided for maintenance access. As a result of flooding on Fortran Road and the buffer requirement, a new access/ egress route from the north would be required.



5.3.5. Study Site 106LGEM

5.3.5.1. Site 106LGEM north of the railway

Approximately 30% of the northern portion of Site 106LGEM is predicted to flood during a 1% (1 in 100) annual chance event including climate change. The main flooded areas are the north-eastern and south-western corners of the site. Elsewhere, flooding is predicted along the existing drainage ditches crossing the site.

Flood depths during a current 0.1% (1 in 1000) annual chance event are typically 50mm deeper and the flood extents are slightly greater, covering 40% of the site. The existing access route from the north is predicted to remain flood free. Flood depths are below 600mm over the entire site during the 0.1% chance event. These are on average below 250mm.

Based on the above results, raising site levels and providing compensatory flood storage would allow for the development of approximately 80% of the site area north of the rail line (assuming the tidal risk is mitigated, see Section 5.2). Details of the flood storage arrangements to ensure level for level compensation, for example through design of a multi-cell system, should be developed as part of a site-specific FCA. Buffers 12m in width should be maintained clear along both banks of the major reens bordering and crossing the site: Greenlane Reen, Fendre Reen, Ty-Ffynon Reen and Railway Reen.

5.3.5.2. Site 106LGEM south of the railway

Flooding is predicted over approximately 30% of the site area during a 1% (1 in 100) annual chance event including climate change.

The flood extents during a 0.1% (1 in 1000) annual chance event are slightly more extensive, covering a third of the site. Flooding is concentrated along the eastern boundary of the site and along existing reens and drainage ditches. The depths of flooding are below 600mm, typically between 150 to 200mm.

Based on the results, approximately 73% of the site is likely to be developable in line with TAN15 (assuming the tidal risk is mitigated, see Section 5.2). Through raising site levels and providing compensatory flood storage it may be possible to increase the developable site area to nearer 80%. As for the northern part of the site, details of the flood storage arrangements to achieve level for level compensation, for example through the design of a multi-cell system, should be clarified as part of a site-specific FCA. Flood-free access to the site from the east would be required. In addition, a buffer 12m wide would be necessary on either side of Railway Reen which flows through the middle of the site.

5.4. Summary of fluvial flood risk A summary of the proportions of the sites that may be raised and developed is provided in the following table.

Table 5-2 – Summary of development potential accounting for fluvial flood risk

Site Level of fluvial flood risk Developable proportion of site

22LGRM Western 60% of site at risk of fluvial flooding 50%

21LGR Limited fluvial flood risk 100%

24LGRM Limited fluvial flood risk Approx. 100%

2LGRM Southern 25% of site at risk of fluvial flooding 85%

106LGEM (north of railway)

30% of site inundated at 1% (1 in 100) inc. climate change event

40% site flood at current 0.1% (1 in 1000). Depths typically less than 250mm.

Likely that 80% of site area developable, subject to confirmation as part of detailed development of mitigation measures

106LGEM (south of railway)

25% of site inundated at 1% (1 in 100) inc. climate change event

33% of site flood at current 0.1% (1 in 1000). Depths typically less than 200mm.

73%

Potentially increasing to 80%, subject to detailed assessment of flood storage measures.



6. Summary and Recommendations

The results of the tidal portion of this assessment support the recommendations made in the preliminary Area A Stage 3 Report. The recommendations presented below have been refined based on the results of the fluvial flood risk assessment.

6.1. Mitigation of risk to Sites 2LGRM, 24LGRM, 106LGEM and 21LGR

Our assessment suggests that the tidal flood risk to four of the five sites (Sites 2LGRM, 24LGRM, 106LGEM and 21LGR) assessed could be managed in line with TAN15 guidance by raising the sites and providing a compensatory reduction in flooding to the wider area by raising the defence at Sluice Farm.

The cost of mitigation works at Sluice Farm to facilitate development of these sites would be of the order of £2.5M. In addition, costs would be incurred for raising site levels and constructing compensatory flood storage facilities to mitigate any off-site increase in fluvial flood risk.

Taking into account the requirement for compensatory flood storage, the cost of raising all four sites would total approximately £19M (compared with £24M if the entire site areas outside of the reen buffers were raised). This total includes the cost of raising the portion of Site 106LGEM located south of the railway, which was excluded from the estimate in our previous report. The costs of constructing compensatory flood storage areas, including any compensation costs for off-site facilities, would be in addition to these figures.

6.2. Mitigation of risk to Site 22LGRM Our investigation indicates that it would be feasible to mitigate the tidal and fluvial flood risk to approximately 50% of Site 22LGRM in line with TAN15. Raising the River Rhymney defence by an average of 0.8m would be necessary to mitigate the tidal flood risk and is considered technically feasible. The reen running along the landward toe of the defence embankment would have to be relocated further east to make room for widening the base of this embankment.

The cost of mitigation works to the River Rhymney defence, which would be in addition to the above costs for the other four sites, would be approximately £2.1M. The cost of raising 50% of Site 22LGRM would be of the order of £3M. The cost of constructing compensatory flood storage areas would also be in addition to the above figures.

6.3. Summary of developable areas by Site Separate estimates of the proportion of the developable area of each site have been derived based on mitigation of the two main sources of flooding, fluvial and tidal.

In general, for the sites considered, it is the fluvial risk which limits the proportion of each site which may be developed. The areas of each site which are developable in line with TAN15 are summarised in Table 6-1 below.

Table 6-1 Summary of Estimated Developable area – tidal and fluvial risks

Site Developable proportion of sitea

22LGRM 50%

21LGR 100%

24LGRM Approx. 100%

2LGRM 85%

106LGEM (north of railway) Approximately 80%, subject to confirmation as part of development of detailed fluvial mitigation measures.

106LGEM (south of railway) 70%. Potentially increasing to 80%, subject to confirmation as part of development of detailed fluvial mitigation measures.

a The proportion of the site that may be raised to allow for mitigation of tidal and fluvial flood risk while providing for compensatory flood

storage. Developable areas do not account for exclusion of mandatory buffers along reens.

All sources of flood risk will need to be managed to acceptable levels in line with TAN15 guidance as part of any site specific FCAs.

Appendices



Appendix A. Reen System Hydrological Assessment

Cardiff SFCA Hydrological Assessment.docx

Cardiff SFCA Hydrological Assessment

Project: Cardiff SFCA To: Candice Constantine, Atkins

Subject: Hydrological Assessment From: Clare Grout, Atkins

Date: June 2012 cc: Dominic Harries, Atkins

1. Introduction Atkins Limited was appointed by Cardiff Council in January 2009 to undertake a Strategic Flood Consequence Assessment (SFCA) for Cardiff. The SFCA will form part of the evidence base for the Local Development Plan (LDP) and facilitate a risk-based approach to development planning in Cardiff with respect to flooding. A number of candidate sites for the LPD were identified and based on the results of the initial stages of assessment, it was determined that additional study would be required at some sites to demonstrate whether flood risk can be managed to an appropriate level and the sites are suitable for inclusion within the LDP.

Area A covers the area of the Wentloog Levels extending from the Rhymney River in the west to St Mellons Business Park in the east. The Wentloog Levels comprise an area of historically reclaimed land. As part of the reclamation, an extensive network of ditches (reens) was constructed to drain water out to sea. This network is currently managed by the Caldicot & Wentlooge Levels Internal Drainage Board (IDB).

The most significant source of flooding to Area A sites is tidal inundation resulting from overtopping and potential breach of the existing tidal defences. There is however also the risk of fluvial flooding from the reen system. The 1992 HR Wallingford Wentloog Levels study

1 provides an assessment of flood risk from the

reens for different development scenarios. Further hydrological analysis was undertaken as part of the SFCA preliminary stage 3 assessment, but at the time no information was available on the local urban drainage systems and recommendations were made for further studies to confirm the fluvial flood risk from the reens. This is the reason for this hydrological assessment, which will provided inflow hydrographs for the hydraulic modelling of two catchments of interest as follows:

Rhosog Fach, Rhosog Fawr West and Rhosog Fawr Branch Reens in the west (referred to as Rhosog).

Pil-du, Hendre, Walwyn Place, Fen Eliffant, Faendre, Green Lane Branch, Little Greenlane, Greenlane, Ty-Ffynon, Railway and Tarwick Reens and St Mellons Pond in the east.

For the purposes of this hydrological assessment, these areas have been divided into four main catchments: Rhosog in the west and Pil_du Reen, Faendre Reen and Greenlane Reen in the east.

2. Methodology A similar methodology was followed for all of the catchments requiring hydrological assessment. This can be summarised as follows:

Obtain catchment boundaries and descriptors form the FEH-CD-ROM.

Where necessary, use the area-weighting method to derive the FEH catchment descriptors for interstation sub-catchments. The area-weighting method is not appropriate for DPLBAR and therefore this descriptor was calculated using the FEH DPLBAR equation.

Review the catchment boundaries using LiDAR, mapping of the Reen network, local knowledge and drainage plans from the Local Authority.

Produce catchment boundaries for all sub-catchments requiring hydrological assessment.

Derive catchment descriptors for all sub-catchments.

Review catchment descriptors to determine the most appropriate method for hydrological assessment. In line with Environment Agency guidance, for catchments classified as being up to moderately urbanised

1 HR Wallingford, June 1992, Wentloog Levels Project Study Report Volume 1: Test, Tables and Figures.



(URBEXT1990<0.125 or URBEXT2000<0.150), the Revitalised Flood Hydrograph (ReFH) methodology2 is

recommended for derivation of flood flow hydrographs. The ReFH method should not however be used for heavily or very heavily urbanised catchments (0.125<URBEXT1990<0.500 or 0.150<URBEXT2000<0.600). For such catchments, it is recommended to use the FEH Rainfall-Runoff method.

Undertake hydrological assessment using the preferred method to derive hydrographs for the 1 in 100 annual chance event and the 1 in 1000 annual chance event, with storm durations of 2.5 hours (critical duration for urban catchments), 6.5 hours (critical duration for rural catchments), 12.5 hours and 25.5 hours with a 30 minute timestep. The summer rainfall profile will be used for all sub-catchments. This is because a summer profile is most appropriate for urban catchments, which make up a majority of the area to be modelling.

Undertake a FEH statistical analysis including the derivation of growth curves for four FEH catchment pooling groups and QMED calculations for each sub-catchment. Make an urban adjustment to the QMED value and growth curve where appropriate.

Scale the critical duration hydrograph to the FEH statistical peak for each sub-catchment. Use the same scaling factor to adjust the hydrographs generated using other storm durations. These hydrographs will form the inflow to the hydraulic model.

The subsequent sections provide more detail about the methodology for each of the catchment areas. Further detail on the hydrological assessment can also be found in the ‘Cardiff FEH Calculation Record’ which should be read alongside this technical note.

2.1. Rhosog

Catchments

Three FEH catchments were derived using the FEH-CD-ROM for the Rhosog area. These were as follows:

Rhosog Fach catchment (0.51km2) draining part of the Rumney residential area.

Rhosog Fawr Upper catchment (0.89km2) draining a larger part of the Rumney residential area including

the catchment above.

Rhosog Fawr whole catchment (1.74km2) draining to Wentloog Avenue and included the two catchments

above.

The area-weighting method was used to generate the FEH catchment descriptors for the upper and lower interstation catchments. The resulting three FEH catchments and their areas are listed in Table 1.

The catchment boundaries were subsequently revised. The resulting four revised catchments, their areas and an indication of the nature of boundary revision is also listed in Table 1 and displayed of Figure 1.

Table 1 – Rhosog Catchment Conceptualisation

FEH Catchment Revised Catchment Boundary Revisions

Name Area (km2) Name Area (km

2)

Rhosog Fach 0.51 Rhosog Fach 0.35 North eastern catchment boundary amended.

Rhosog Fawr Upper Interstation

0.38 Rhosog Fawr Sub-Catchment 2

0.57 Eastern and western boundaries extended.

Rhosog Fawr Lower Interstation

0.85

Rhosog Fawr Sub-Catchment 1

0.15 Area draining to Pil_du Reen identified and excluded.

Rhosog Fawr Sub- 0.24 Catchment south of the

2 Kjeldson, T.R., 2007, Flood Estimation Handbook Supplementary Report No. 1, The Revitalised FSR/FEH Rainfall-Runoff Method,

Centre for Ecology and Hydrology.



Catchment 3 railway line excluded.

Figure 1 – Rhosog Catchment Conceptualisation

Hydrological Assessment

The hydrological assessment for Rhosog Fach and Rhosog Fawr Sub-Catchment 2 used the descriptors from the FEH catchment listed in Table 1, but with the revised catchment areas. A single assessment was undertaken for the lower catchment area with the Rhosog Fawr Lower Interstation descriptors but a revised area of 0.39km

2. Resulting flows were then scaled by area to provide separate inflows for the Rhosog Fawr

Sub-Catchments 1 and 3.

All three catchments are classified as very heavily urbanised with an URBEXT2000 value in excess of 0.15. The FEH Rainfall-Runoff method was therefore used to obtain the hydrograph, with the resulting flow scaled to fit the flood peak derived using the FEH statistical method.

2.2. Pil_du Reen

Catchments

Three FEH catchments were derived using the FEH-CD-ROM for the Pil-du Reen area. These were as follows:

Pil_du Upper Sub-Catchment (0.54km2) draining the Trowbridge residential area and outfalling to the

main Reen channel along Hendre Road.

Pil-du Upper Catchment (0.94km2) draining a larger Trowbridge residential area (including the sub-

catchment) to the main Reen channel along Hendre Road.



Pil_du Whole Catchment (1.94km2) to St Mellons Pond (including the upper catchment).

The area-weighting method was used to generate the FEH catchment descriptors for the upper and lower interstation catchments. The resulting three FEH catchments and their areas are listed in Table 2.


Table 2 – Pil_du Reen Catchment Conceptualisation



2)

Pil_du Upper Interstation 0.40 Pil_du Sub-Catchment 1 0.72 Extended south-west to include Cardiff College.

Pil_du Upper Sub 0.54 Pil_du Sub-Catchment 2 0.56 Extended north, but eastern boundary shifted west.

Pil_du Lower Interstation 1.00

Pil_du Sub-Catchment 3 0.71 Area south of railway excluded.

Hendre Sub-Catchment 0.15 Catchment extended to east to include the whole of the lake.

Figure 2 – Pil_du Reen Catchment Conceptualisation




The hydrological assessment for Pil_du Sub-Catchment 1 and 2 used the descriptors from the FEH catchment listed in Table 2, but with the revised catchment areas. A single assessment was undertaken for the lower catchment area with the Pil_du Lower Interstation descriptors but a revised area of 0.86km

2.

Resulting flows were then scaled by area to provide separate inflows for the Pil_du Sub-Catchment 3 and the Hendre Lake Sub-Catchment.

All three catchments are classified as (very) heavily urbanised, with URBEXT2000 values in excess of 0.15. The FEH Rainfall-Runoff method was therefore used to obtain the hydrographs, with the resulting flows scaled to fit the flood peak derived using the FEH statistical method.

2.3. Faendre Reen

Catchments

Three FEH catchments were derived using the FEH-CD-ROM for the Faendre area. These were as follows:

Faendre Catchment 1 (1.43km2) draining the predominantly rural area north and east and Cypress Drive

and outfalling to the main Reen channel at Fortran Road.

Faendre Catchment 2 (1.56km2) with the main Reen channel draining the urban residential area of St

Mellons and outfalling at Cypress Drive.

Faendre and Pil-du Reen whole catchment (6.01km2) including the two sub-catchments above, the

whole Pil_du Reen catchment and the (inter-station) area to the north and south of St Mellons Business Park.

The area-weighting method was used to generate the FEH catchment descriptors for this lower interstation catchment. The resulting three FEH catchments and their areas are listed in Table 3.


Table 3 – Faendre Reen Catchment Conceptualisation



2)

Faendre 1 1.43 Faendre Sub-Catchment 2 1.62 Minor revisions to southern boundary.

Faendre 2 1.56 Faendre Sub-Catchment 3 1.53 Minor revisions to northern boundary.

Faendre Lower Interstation

1.08

Faendre Sub-Catchment 1 0.20

Catchment boundary reflects that of the St Mellons Business Park. Northern FEH area excluded as this drains to the Little Greenlane Reen.

Faendre Sub-Catchment 4 0.34 Areas draining to Little Greenlane Reen and Pil_du Reen excluded.



Figure 3 – Faendre Reen Catchment Conceptualisation


The hydrological assessment for Faendre Sub-Catchment 2 and 3 used the descriptors from the FEH catchment listed in Table 3, but with the revised catchment areas. A single assessment was undertaken for the lower catchment area with the Faendre Lower Interstation descriptors but a revised area of 0.54km

2.

Resulting flows were then scaled by area to provide separate inflows for the Pil_du Sub-Catchment 1 and 4.

Sub-Catchments 1, 2 and 4 are not classified as heavily urbanised, with URBEXT2000 values less than 0.15. The ReFH method was therefore suitable to use to obtain the hydrographs, which will then be scaled to fit the FEH statistical flood peak. Sub-Catchment 3 however is classified as very heavily urbanised with an URBEXT2000 value in excess of 0.15. The FEH Rainfall-Runoff method was therefore used to obtain the hydrograph, with the resulting flow scaled to fit the flood peak derived using the FEH statistical method.



2.4. Greenlane Reen

Catchments

Two FEH catchments were derived using the FEH-CD-ROM for the Greenlane Reen area. These were as follows:

Little Greenlane Reen Catchment (0.59km2) draining the rural area comprising the St. Mellons golf

courses.

Faendre, Pil-du and Greenlane Reen whole catchment (6.28km2) including the whole Pil_du Reen

catchment, the whole Faendre catchment and the (inter-station) area draining into the Greenlane Reen to the east and south of St Mellons Business Park.

The area-weighting method was used to generate the FEH catchment descriptors for this lower interstation catchment. The resulting two FEH catchments and their areas are listed in Table 4.

The catchment boundaries were subsequently revised. The resulting two revised catchments, their areas and an indication of the nature of boundary revision is also listed in Table 4 and displayed of Figure 4.

Table 4 – Greenlane Reen Catchment Conceptualisation



2)

Little Greenlane 0.59 Greenlane Sub-Catchment 1 0.85 Revisions to northern and western boundary.

Greenlane Reen Interstation

0.27 Greenlane Sub-Catchment 2 0.43

Figure 4 – Greenlane Reen Catchment Conceptualisation




The hydrological assessment for Greenlane Sub-Catchments 1 and 2 used the descriptors from the FEH catchments listed in Table 4, but with the revised catchment areas. Both Sub-Catchments are not classified as heavily urbanised, with URBEXT2000 values less than 0.15. The ReFH method was therefore suitable to use to obtain the hydrographs, which will then be scaled to the FEH statistical flood peak.

2.5. Summary Table 5 provides a summary of the hydrological assessment method adopted for each sub-catchment.

Table 5 – Hydrological Assessment Method

Catchment Methodology

Rhosog Fach

FEH RR and Statistical

FEH catchment descriptors.

Rhosog Fawr

Sub-Catchment 1 FEH Interstation catchment descriptors and hydrograph scaled from larger catchment.

Sub-Catchment 2 FEH Interstation catchment descriptors.

Sub-Catchment 3 FEH Interstation catchment descriptors and hydrograph scaled from larger catchment.

Pil_du Reen

Sub-Catchment 1

FEH RR and Statistical

FEH Interstation catchment descriptors.

Sub-Catchment 2 FEH catchment descriptors.

Sub-Catchment 3 FEH Interstation catchment descriptors and hydrograph scaled from larger catchment. Hendre

Faendre Reen

Sub-Catchment 1 ReFH and statistical

FEH Interstation catchment descriptors and hydrograph scaled from larger catchment.

Sub-Catchment 2 FEH catchment descriptors.

Sub-Catchment 3 FEH RR and

Statistical FEH catchment descriptors.

Sub-Catchment 4 ReFH and statistical

FEH Interstation catchment descriptors and hydrograph scaled from larger catchment.

Greenlane Reen Sub-Catchment 1 ReFH and

statistical

FEH catchment descriptors.

Sub-Catchment 2 FEH Interstation catchment descriptors.



3. Results Hydrographs have been generated for all 14 sub-catchment inflow locations for the 1 in 100 and 1 in 1000 annual chance events and for storm durations of 2.5 hours, 6.5 hours, 12.5 hours and 25.5 hours. These are suitable to use as inflows to the hydraulic modelling. Table 6 provides a summary of the peak flows for the two critical storm durations for all the sub-catchments.

Table 6 – Hydrological Assessment Results

Catchment

Flood Peak (m3/s)

2.5 hour Storm Duration 6.5 hour Storm Duration

1 in 100 1 in 1000 1 in 100 1 in 1000

Rhosog Fach 0.55 1.03 0.46 0.80

Rhosog Fawr 1 0.19 0.36 0.18 0.31

Rhosog Fawr 2 0.89 1.67 0.69 1.22

Rhosog Fawr 3 0.31 0.58 0.28 0.50

Pil_du 1 0.80 1.43 0.75 1.26

Pil_du 2 0.86 1.53 0.76 1.28

Pil_du 3 0.68 1.29 0.79 1.41

Hendre 0.14 0.27 0.17 0.30

Faendre 1 0.21 0.37 0.25 0.42

Faendre 2 1.68 2.09 2.00 3.38

Faendre 3 2.32 4.13 2.14 3.60

Faendre 4 0.36 0.63 0.42 0.71

Greenlane 1 0.91 1.65 1.06 1.86

Greenlane 2 0.34 0.61 0.40 0.70



Appendix B. Drawings

B.1. Updated Fluvial Model Results Drawing No. 5109602/RCF/110 – Area A – 1% (1 in 100) Annual Chance Fluvial Flood

Extents for 2110

Drawing No. 5109602/RCF/111 – Area A - 0.1% (1 in 1000) annual chance Fluvial Flood Event Predicted Maximum Depths for 2010

Drawing No. 5109602/RCF/112 – Area A – 0.1% (1 in 1000) Annual Chance Fluvial Flood Event Predicted Velocities at Maximum Depths for 2010

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

Huw Richards Atkins Limited West Glamorgan House 12 Orchard Street Swansea SA1 5AD

Email: [email protected] Telephone: +44 (0) 1792 641172 Fax: +44 (0) 1792 472019

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