cardiff sfca stage 3 preliminary site assessments area a sites · the wentloog levels comprise an...

Cardiff SFCA Stage 3 Preliminary Site Assessments Area A Sites Cardiff County Council

April 2012

Cardiff SFCA Stage 3 Preliminary Site Assessments

Atkins Stage 3 Preliminary Site Assessments | Version 2.0 | 11 April 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 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 35 pages including the cover.

Document history

Job number: 5109602 Document ref: Stage 3 Preliminary Site Assessments

Revision Purpose description Originated Checked Reviewed Authorised Date

1-D1 Internal Draft CRC DMH 07-03-12

1 Draft issue to client CRC DMH HR HR 21-03-12

2 Final issue to client CRC DMH HR HR 11-04-12



Table of contents

Chapter Pages

1. Introduction 6 1.1. Background 6 1.2. Stage 3 assessment objectives 7

2. Sub Area A – Wentloog Levels 8 2.1. Sub Area A candidate sites 8

3. Approach & Assessment Methodologies 9 3.1. Previous studies and data 9 3.2. Site data collection 9 3.3. Tidal flood risk assessment - Hydraulic modelling 10 3.4. Fluvial flood risk 10 3.5. Flood risk from other sources 11 3.6. Mitigation measures and cost estimates 11

4. Existing Flood Risks by Site 12 4.1. Study Site 22LGRM 12 4.2. Study Site 21LGR 15 4.3. Study Site 24LGRM 16 4.4. Study Site 2LGRM 17 4.5. Study Site 106LGEM 20

5. Tidal Flood Risk Mitigation Options and Costs 22 5.1. Reducing risk to the candidate sites 22 5.2. Mitigating potential adverse impacts of development 22 5.3. Modelling results 23

6. Summary and Recommendations 27 6.1. Tidal flood risk 27 6.2. Fluvial flood risk 28 6.3. Flood risk from other sources 28

7. Conclusions 29

Appendices 30

Appendix A. Environment Agency Guidance – Stage 3 Assessment 31 A.1. Extract from Environment Agency guidance to local authorities 31

Appendix B. Drawings 33 B.1. Site location Plan 33

Drawing No. 5109602/RCF/001 – Wentloog Tidal Area Overview 33 B.2. Updated Model Results 33

Drawing No. 5109602/RCF/100 – 0.5% (1 in 200) annual chance tidal breach event extents. Current and future. 33

Drawing No. 5109602/RCF/101 – 0.1% (1 in 1000) annual chance tidal breach event. Predicted Maximum depths, 2010. 33

Drawing No. 5109602/RCF/102 – 0.1% (1 in 1000) annual chance tidal breach event. Predicted Velocities at Maximum depths, 2010. 33

Drawing No. 5109602/RCF/103 – 0.5% (1 in 1000) annual chance tidal event – overtopping only. Extents to 2110. 33

Appendix C. Hydrogeological Screening Note 34



Tables Table 2.1 - Sub Area A Sites ............................................................................................................................. 8 Table 5.1 – Options considered for mitigating raised site levels ..................................................................... 23 Table 5.2 – Options to mitigate site raising through to 2110 ........................................................................... 25

Figures Figure 1-1 – Phased approach to Cardiff SFCA ................................................................................................ 7 Figure 4-1 Site 22 LGRM - Maximum depth, current 0.1% (1 in 1000) annual chance event ........................ 13 Figure 4-2 Site 22 LGRM - Velocity at maximum depth, current 0.1% (1 in 1000) chance event ................... 13 Figure 4-3 Site 22LGRM - 0.5% (1 in 200) annual chance tidal breach event extents for 2085 and 2110. .... 13 Figure 4-4 Site 22LGRM Overtopping only event – tide level equivalent to 0.5% (1 in 200) breach event in 2110 ................................................................................................................................................................. 13 Figure 4-5 Site 21LGR extent of 0.5% (1 in 200) annual chance breach event, 2110 .................................... 15 Figure 4-6 Site 24LGRM - 0.5% (1 in 200) annual chance tidal breach extents, 2085 and 2110 ................... 17 Figure 4-7 Site 24LGRM - 0.5% (1 in 200) annual chance tidal overtopping only event extents (no breach), 2110 ................................................................................................................................................................. 17 Figure 4-8 Site 2LGRM Maximum depth current 0.1% (1 in 1000) annual chance tidal breach event ........... 18 Figure 4-9 Site 2LGRM Velocity at maximum depth - current 0.1% (1 in 1000) annual chance tidal breach event ................................................................................................................................................................ 18 Figure 4-10 Site 2LGRM 0.5% (1 in 200) annual chance tidal breach extents, 2085 and 2110 ..................... 19 Figure 4-11 Site 2LGRM 0.5% (1 in 200) annual chance tidal event (no breach), 2110 ................................ 19 Figure 4-12 Site 106LGEM Maximum depth – current 0.1% (1 in 1000) annual chance event ...................... 21 Figure 4-13 Site 106LGEM Velocity at maximum depth – current 0.1% (1 in 1000) chance event ................ 21 Figure 4-14 Site 106LGEM 0.5% (1 in 200) annual chance tidal breach extents, 2085 and 2110 ................. 21 Figure 4-15 Site 106LGEM 0.5% (1 in 200) annual chance tidal event (no breach), 2110 ............................. 21

Atkins Stage 3 Preliminary Site Assessments | Version 2.0 | 11 April 2012 | 5109602 6

1. Introduction

Cardiff Council has appointed Atkins Ltd to undertake Strategic Flood Consequences Assessment (SFCA) Stage 3 assessments 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 within the development lifetime taking, into account predicted climate change impacts.

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 which 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 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 principal flood risk at these sites has been identified as tidal. The scope of this assessment was determined to tie in with the LDP programme.

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.


Figure 1-1 Phased approach to Cardiff SFCA

1.2. Stage 3 assessment objectives The overarching aim of this Stage 3 assessment is to demonstrate the viability of including a number of Sub Area A candidate sites within the LDP. To this end, the objectives of the study are to:

Determine the feasibility of mitigating the tidal flood risk (i.e. principal flood risk) at the sites of interest;

Where feasible, develop high-level mitigation options (site-specific or strategic) and preliminary cost estimates;

Provide an assessment of residual flood risks from other sources and confidence in those risks based on available data; and

Identify where additional data or modelling is required to quantify residual flood risk and demonstrate the feasibility of mitigation.

The level of detail of this assessment is subject to a number of limitations including the scale of the tidal modelling and the availability of data for assessing risk from other sources. This assessment is also restricted by the wider LDP programme agreed with the Welsh Government which requires the Council to prepare a Preferred Strategy for consultation in November 2012.


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 (Drawing 5109602/RCF/001). 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 breach of the existing tidal defences. Risks from direct surface water runoff and fluvial flooding of the reen system were not fully characterised within the phases of the SFCA completed to date. If the tidal flood risk can be mitigated, the residual risks from these sources will need to be characterised further.

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 candidates sites are listed in Table 2.1. Improvements to the 2D model since completion of Phase 2 (see Section 3.3) have resulted in an increased predicted flood risk to Site 21LGR. This site has therefore been included within this Stage 3 assessment.

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. Only the northern portion has been included in this assessment. The southern portion was considered unsuitable in Phase 2.

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.


3. Approach & Assessment Methodologies

It was agreed with Cardiff County Council that this preliminary Stage 3 assessment would address specific issues at each of the identified sites to the greatest extent possible, recognizing the given time limitation as described in Section 1.2. The methods of assessment are described in the following sections.

3.1. Previous studies and data We have reviewed the SFCA Phase 2 analysis to inform this assessment and have also obtained copies of other relevant report.

We have reviewed the 1992 HR Wallingford Wentloog Levels study6. The report provides an assessment of

flood risk from the reens for different development scenarios based on the results of hydraulic modelling of the reen system. The model included details of the weir structures (sluices), bridges, outfalls and other major controls (minor structures were not explicitly represented). Minor field ditches were excluded because penning levels were considered high enough such that this secondary storage volume would be taken up prior to a flood event. Mean High Water Springs was used as the downstream boundary condition with high water set to correspond with the arrival of the runoff peak at the tidal outfall. Operational practices were taken into account by setting sluices at 1992 winter penning levels. We have obtained a copy of the Caldicot and Wentlooge Levels IDB Water Level Management Plan which contains current penning levels for comparison.

The 1992 study also assessed the geotechnical stability of the existing tidal defence embankments. Twenty-two boreholes were collected, showing that the strength of the substrate decreases with depth below the embankment. A stability analysis concluded that factor of safety values exceed recommended minimum values, indicating that geotechnical failure is unlikely.

More recently, a strategic level assessment7 carried out for the Severn Estuary on behalf of the Environment

Agency identified a risk of breach along certain sections of the defence caused by overtopping and the potential for erosion of the back face. The assessment outlines strategic level options for the management of flood risk in the Severn Estuary, including the Wentloog Levels. It identifies a preferred strategy that involves short-term works to improve the standard of protection and longer term plans to maintain that standard of protection taking into account predicted sea level rise. The potential breach locations considered in the current study are consistent with the susceptible sub reaches defined in the Severn Estuary Strategy. The report provides estimates of costs associated with defence improvements which we have utilised in this study.

3.2. Site data collection A site visit was undertaken on 25

th January 2012 for the purpose of gaining a better understanding of the

reen system affecting the fluvial flood risk at the candidate sites. We met representatives from the IDB on site to discuss operations procedures and obtained a copy of their Water Level Management Plan which sets out winter and summer penning levels for the sluices. We also visited and collected data on the sizes of as many of the culverts through the railway embankment as we could access with the IDB‟s assistance. Safe access to many of the culverts was impractical where the headwall and outfalls were located on land owned by Network Rail. Collected data, including direct measurements where possible, were used to update the existing hydraulic model for the area.

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

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


3.3. Tidal flood risk assessment - Hydraulic modelling The following improvements were made to the existing TUFLOW model for this Stage 3 assessment.

Re-generation of the base DEM of the model using the latest available LiDAR data; the western area of the model was recently re-flown at a 0.5m resolution.

Inclusion of additional railway culverts within model. Updated and improved information on locations and dimensions was provided by Network Rail and verified during our site visit within the limits of visual inspection. The invert levels of the culverts were estimated based on LiDAR data and the size of the culvert.

An additional breach location at Peterstone Gout was incorporated.

A filtering error in the LiDAR was corrected. This had previously caused a footbridge across Pil-du Reen to be represented as a blockage, preventing flow along the reen.

The effects of the model improvements on predictions of flooding at each site are described in the site-specific sections in Section 4.

The model was used to assess the baseline tidal flood risk for the current situation and including the impacts of climate change through 2110. We tested various options for mitigating flood risk at the sites and the subsequent loss of flood storage if inundation of the sites is prevented.

In the course of this assessment the Environment Agency confirmed that improvement works to the coastal defence at Sluice Farm (SR1-2) are on its list of planned works in the medium term. This length of defence is particularly susceptible to overtopping and breach. Although the works are anticipated to be carried out within the next five years, the plans are subject to Welsh Government sign off and funding. Given that funding has not yet been secured, we have taken two approaches with regard to works at Sluice Farm. We have considered the inclusion of works at Sluice Farm as mitigation for proposed development and have also considered whether alternative mitigation options would be effective should the works at Sluice Farm be carried out by the government prior to development of the sites of interest.

3.4. Fluvial flood risk Hydrological analyses were carried out to estimate rainfall depths and determine surface water runoff from the reen catchments. No information was available on the local urban drainage systems; therefore, catchment areas were estimated based on LiDAR data.

Rainfall depths and runoff hydrographs were generated for the catchments draining to the proposed development sites using Revitalised Flood Hydrograph (ReFH) methodology and software

8. Although ReFH

is most appropriate for rural catchments, it was used to make a rapid initial assessment of rainfall and runoff for this study given the programme constraints. For two of the five catchments, additional analyses will be required which are discussed in the site-specific sections in Section 4.

Rainfall depths generated for 2, 25- and 49-hr storms were compared with similar hydrological data from the 1992 HR Wallingford assessment and conclusions drawn with regard to the potential flood risk from the reens, taking into account the potential effects of changes in winter penning levels.

The predicted 1% (1 in 100) annual chance event rainfall totals are approximately 10% lower than those used by HR Wallingford to model a 1% (1 in 100) annual chance flood event in the reen system. Based on this outcome, it is considered reasonable to use the 1992 assessment as a basis for assessment fluvial flood risk from the reens in the first instance. We have provided recommendations for further studies which would need to be undertaken to confirm this risk.

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

Centre for Ecology and Hydrology.


3.5. Flood risk from other sources A hydrogeological desk study was undertaken to investigate the groundwater flood risk implications associated with development at the proposed sites. The study encompassed:

Review of available information;

Assessment of potential groundwater flood risks; and

Comment on further work to reduce uncertainty in understanding the risks.

A technical note was produced and is attached in Appendix C. Site-specific findings are briefly summarised in Section 4.

3.6. Mitigation measures and cost estimates We have estimated costs for undertaking capital works required to mitigate flood risk at the proposed development sites. These estimates are based on experience of similar schemes and site-specific information developed as part of the 2011 Severn Estuary Flood Risk Management Strategy and this study.

For options involving works to prevent breach, we have assumed that the mechanism of failure is overtopping and erosion of the back face of the embankment. For tidal defences, it is reasonable to assume that the velocity of the flow along the back face of the embankment during combined tidal and wave overtopping would be sufficiently high to require the installation of a flexible concrete revetment system. A sheet pile toe and cut-off would also be necessary to prevent undermining of the embankment from the reen running along the landward toe of the structure. For the defence along the east bank of the River Rhymney, a lower-cost grass reinforcement option is considered suitable, although a cut-off would also be required. The costs provided cover detailed design, environmental consultation and construction. We have excluded maintenance costs on the basis that they are unlikely to change from existing maintenance costs and thus are already known.


4. Existing Flood Risks by Site

The flood risk to each of the sites is summarised in this section based on the currently available information for all sources of flooding. Where the existing information is not sufficiently detailed to characterise the risk from any of the sources, this commented on.

Further works required to understand the flood risk from these sources are suggested where these are key to assessing the likelihood of successful mitigation of the overall flood risk.

4.1. Study Site 22LGRM This previously undeveloped land is known as Trowbridge Mawr.

4.1.1. Tidal flood risk The following is a summary of the improved 2D modelling results shown on Drawings 5109602/RCF/100 through 5109602/RCF/103.

Current situation

The site is predicted to be flood free during a 0.5% (1 in 200) annual chance tidal event.

During a 0.1% (1 in 1000) annual chance tidal event, approximately 50% of the site would be flooded. The eastern part of the site and localised areas of high ground would remain flood free. Depths and velocities would generally remain within TAN15 A1.15 indicative acceptance criteria. (see Figure 4-1and Figure 4-2).

Previous modelling results indicated that only a small area of southern extent of site would be inundated during a 0.1% (1 in 1000) annual chance tidal event. This difference is the result of model improvements particularly additional culvert information for the railway embankment.

Considering climate change and sea level rise allowances to 2085

During a 0.5% (1 in 200) annual chance tidal event, flooding is predicted in the western two-thirds of the site by 2085 with the exception of one small area of high ground near the middle of the site which would remain flood free. The eastern third of the site where the ground is higher than surrounding levels would also remain flood free. These results are similar to the results from previous modelling.


The site topography is such that the predicted flood extent for a 0.5% (1 in 200) annual chance event in 2110 is similar to that described for 2085. This result is consistent with previous modelling.

The topography of the site is such that the predicted flood extent would be similar for the case of overtopping only/ no breach of the defences (see Figure 4-3 and Figure 4-4). Depths of flooding, however, would be approximately 0.4m less.

In both scenarios, the eastern third of the site would remain flood free and an access/ egress route would be available from the north.

Mitigation of the tidal risk through to 2110 may be possible subject to further investigation to confirm the feasibility of raising the existing defence along the River Rhymney east bank (see Section 5).


Figure 4-1 Site 22 LGRM - Maximum depth, current 0.1% (1 in 1000) annual chance event

Figure 4-2 Site 22 LGRM - Velocity at maximum depth, current 0.1% (1 in 1000) chance event

Key Key

Blue - <0.6m Light blue <0.15m/s

Yellow - >0.6m - <1.0m Dark blue >0.15m/s - <0.3m/s

Red - >1.0m Yellow 0.3m/s – 0.45m/s

Red >0.45m/s

Figure 4-3 Site 22LGRM - 0.5% (1 in 200) annual chance tidal breach event extents for 2085 and 2110.

Figure 4-4 Site 22LGRM Overtopping only event – tide level equivalent to 0.5% (1 in 200) breach event in 2110

Key Key

Orange Tidal breach extent 2085 Brown Extent of tidal overtopping only Space

Brown Tidal breach extent 2110


4.1.2. Fluvial flood risk Results from previous modelling of the reen system

7 suggest that the banks of Nant Rhosog Fawr in the

southern half of the site would be overtopped during a present day 1% (1 in 100) annual chance event assuming development of the site and no change in ground levels. Limited risk is also anticipated from Rhosog Fach Brook which runs along the western edge of the site.

If this site is to be taken forward, the fluvial flood risk would need to be characterised in more detail. Based on these preliminary results, we would recommend further investigation of the fluvial flood risk to the site, including an assessment of risk assuming the realisation of climate change impacts over the lifetime of the development. Repeating the 1992 assessment of the entire Wentloog Levels would be expensive, take a considerable amount of time and produce more detail than required for the limited land area to be included within the LDP. We therefore recommend that a new model be created that includes Rhosog Fach, Rhosog Fawr West and Rhosog Fawr Branch Reens. Creation of the model would require the following tasks:

Collection of new topographic survey of Rhosog Fach, Rhosog Fawr West and Rhosog Fawr Branch Reens, including survey of any culverts through the railway embankment;

Review of sewer information for existing development to confirm the extent of the urban catchment areas draining to the reens of interest;

Additional hydrological analysis to refine the estimate of runoff from the urbanised catchment; and

Determination of the runoff distribution between Rhosog Fawr and Rhosog Fach. This may be best investigated using sensitivity testing.

Once the fluvial flood risk is known, the model could be used to identify any adverse impacts of development and test potential mitigation measures.

Based on the existing limited information, it should be assumed that a portion of the site area would need to be set aside as flood storage in extreme fluvial events. The extent of the loss of developable land would be subject to more detailed assessment of the fluvial risk. In summary, the present understanding of the fluvial flood risk is not considered sufficient to form a basis for removing this site from the LDP.

4.1.3. Flood risk from other sources Data reviewed for this study suggest that the groundwater table is near to the ground surface. As such, it is likely to be sensitive to changes in local drainage associated with development. The risk of flooding from groundwater itself is anticipated to be low at this site. However, surface water flooding may be an issue following rainfall events given the shallow groundwater table.

The use of SuDS should be considered to maintain the current water table conditions. Soakaways are considered unlikely to be appropriate because of the low ground permeability and the potential to raise the already high groundwater table.

The potential exists that breaking of ground for foundations or other could intercept confined (artesian) groundwater causing localised flooding. The probability is considered low at this site; however, a site-specific investigation consisting of exploration holes would need to be carried out to confirm this.

Further details on the geology of the site, implications for groundwater and recommendations can be found

in the full technical note attached within the appendices.


4.2. Study Site 21LGR This study site is known as Areas 9-12, St Mellons and is situated north of the railway embankment.


Current situation



No flooding is anticipated at the site during a 0.5% (1 in 200) annual chance tidal event in 2085.


Approximately 75% of the site would be flooded during a 0.5% (1 in 200) annual chance tidal event in 2110 (see Figure 4-5).

Previous modelling indicated that the site would be flood free during this event.

The site is predicted to be flood free in the event of overtopping only (i.e. no breach).

Mitigation of the tidal risk through to 2110 is possible; potential options and costs are given in Section 5.

Figure 4-5 Site 21LGR extent of 0.5% (1 in 200) annual chance breach event, 2110

Key

Orange – Tidal breach extent 2085

Brown – Tidal breach extent 2110

4.2.2. Fluvial flood risk The main watercourse affecting this site is Pil-du Reen which is fed by numerous smaller ditches and drains to St Mellons Pond. Previous reen modelling results

7 indicate that water levels are within 150mm of

overtopping the banks of Pil-du Reen during a present day 1% (1 in 100) annual chance flood event assuming development of the site. Winter penning levels downstream of Pil-du Reen have remained consistent or have fallen since the 1992 assessment was undertaken; therefore, it is likely that the current flood risk is unchanged or improved.

The fluvial flood risk will need to be characterised in more detail for a Stage 3 assessment. We recommend that further study be undertaken to confirm the fluvial flood risk to the site and assess the risk to 2110. As explained in Section 4.1.2, the time and expense in modelling the entire Wentloog Levels would be out of proportion to the scale the proposed development. Furthermore, it would not be possible to complete the


work within the LDP programme. We therefore recommend that a new smaller reduced model of the St Mellons Pond catchment be created. Creation of the model would require the following tasks:

Collection of new topographic survey of the local reen network, including survey of any culverts under roads and through the railway embankment;

Review of sewer information for existing development to confirm the urban catchment areas draining to the reens of interest; and

Additional hydrological analyses to refine the estimate of runoff from the urbanised catchments. Hydrographs already produced for this study are likely to be appropriate for the predominantly rural catchments in the eastern part of the study area.

Once the fluvial flood risk is known, the model could be used to identify any adverse impacts of development and test potential mitigation measures.

4.2.3. Flood risk from other sources The conclusions for this site are similar to those for Site 22LGR described in Section 4.1.3.

Further details on the geology of the site, implications for groundwater and recommendations can be found in the full technical note attached within the appendices.

4.3. Study Site 24LGRM This site, at the east to Cypress Drive, lies within TAN15 Zone C1.


Current situation



Localised flooding in low-lying areas in the south and east is predicted during a 0.5% (1 in 200) annual chance tidal event in 2085. The proportion of the site that would be inundated is less than 3% (see Figure 4-6)

This is a slight difference from previous results which predicted the site to remain flood free during this event.


Approximately 50% of the site would be flooded during a 0.5% (1 in 200) annual chance tidal event in 2110 as indicated on Figure 4.6.

Flooding would be concentrated in the low-lying areas of the site in the south and east, particularly along Fendre Reen.

Previous SFCA model results indicated a similar flood extent.

The site is predicted to be flood free in the event of a 0.5% (1 in 200) annual chance tidal event in 2110 that causes overtopping of the existing defences but does not breach them (see Figure 4-7).

Mitigation of the tidal risk through to 2110 is considered to be possible. Potential options and costs are given in Section 5.


Figure 4-6 Site 24LGRM - 0.5% (1 in 200) annual chance tidal breach extents, 2085 and 2110

Figure 4-7 Site 24LGRM - 0.5% (1 in 200) annual chance tidal overtopping only event extents (no breach), 2110

Key

Orange - Tidal breach extent 2085

Brown – Tidal breach extent 2110

Key

Brown -Extent of tidal overtopping only

4.3.2. Fluvial flood risk Results from the previous reen modelling study

7 indicate that the capacity of Faendre Reen, which runs

along the eastern boundary of the site, is sufficient to contain flow from a present day 1% (1 in 100) annual chance flood event. Development of the site itself was not considered. As noted in the report, however, topographical survey data for Faendre Reen is not considered to be very reliable. Furthermore, winter penning levels in this area have changed since the 1992 modelling study. Considering these uncertainties, it is recommended that the current flood risk be re-assessed. A modelling study of the St Mellons Pond catchment should be undertaken as described in Section 4.2.2.



4.4. Study Site 2LGRM Site 2LGRM extends north to south around the eastern boundary of the existing St Mellons Business Park. The entire site lies within TAN15 Zone C1. This site encompasses a main area of land for development and a road providing access which runs 400m south of the main site area. The southern end of the main site area is low lying (below 4.8mAOD) compared with general ground elevation within the Wentloog Levels. Ground levels in the northern part of the main site area are above 6.5mAOD. Little Greenlane Reen marks the eastern boundary of the site.



Current situation


Approximately 30% of the site, including the existing access road, would be flooded during a 0.1% (1 in 1000) annual chance tidal event. Depths are generally less than 0.6m and velocities less than 0.15m/s (see Figure 4-8 and Figure 4-9, below)

Previous results indicated that this site would be flood free during a current 0.1% (1 in 1000) annual chance flood event. This has changed as the result of efforts to improve predictions of sources of tidal risk and the effects of the railway embankment including its culverts on the conveyance of tidal flood waters.

Figure 4-8 Site 2LGRM Maximum depth current 0.1% (1 in 1000) annual chance tidal breach event

Figure 4-9 Site 2LGRM Velocity at maximum depth - current 0.1% (1 in 1000) annual chance tidal breach event

Key Key

Blue - <0.6m Light Blue <0.15m/s Yellow - >0.6m - <1.0m Dark Blue >0.15m/s - <0.3m/s Red - >1.0m Yellow 0.3m/s – 0.45m/s Red >0.45m/s


Approximately 50% of the site would be flooded during a 0.5% (1 in 200) annual chance tidal event in 2085.

Previous results indicated that this figure would be closer to 25%.


Approximately 60% of the site would be flooded during a 0.5% (1 in 200) annual chance tidal event in 2110. The northern part of the site would remain flood free (see Figure 4-11 Site 2LGRM 0.5% (1 in 200) annual chance tidal event (no breach), 2110).

These results are similar to previous results for this event.

In the event of overtopping only (i.e. no breach), the flooded extent would be less - approximately 30% of the site (see Figure 4-11).

In both scenarios, flood depths on Heol Las would be sufficiently high to cut off access to the site via this route. An alternative access/ egress route from the north or west would need to be provided.



Figure 4-10 Site 2LGRM 0.5% (1 in 200) annual chance tidal breach extents, 2085 and 2110

Figure 4-11 Site 2LGRM 0.5% (1 in 200) annual chance tidal event (no breach), 2110

Key Key

Orange Tidal breach extent 2085 Brown Extent of tidal overtopping only

Brown Tidal breach extent 2110Space Holder

4.4.2. Fluvial flood risk The 1992 HR Wallingford report refers to an incident of flooding along Little Greenlane Reen on 10

th March

1981. The storm event that produced the flooding was analysed and considered to have an approximately 10% (1 in 10) annual chance of occurrence. Model results

7 for Little Greenlane Reen, however, do not

indicate overtopping during a present day 1% (1 in 100) annual chance flood. HR Wallingford state in their assessment that the location of the flooding was outside the limits of the model extent; therefore, it cannot be used to test the model accuracy. Other sources of modelling uncertainty exist for this site. The topographical data for Little Greenland Reen were not considered very reliable, nor did the model explicitly represent the downstream culvert under the access road leading to the site. In addition, winter penning levels in this area have changed since the 1992 assessment was completed.

We consider that there to be a low level of confidence in the results of the 1992 fluvial modelling results for this site. Based on site observations it is likely that the southern low lying portion of the site may be subject to flooding during extreme events. Further detailed assessment is required to characterise the fluvial risk. We recommend a localised modelling assessment to determine the fluvial flood risk along Little Greenlane Reen as described in Section 4.2.2.

4.4.3. Flood risk from other sources The conclusions for this site with regard to risk of groundwater flooding and SuDS are similar to those for Site 22LGR described in Section 4.1.3.

The geology of this site is such that breaking ground for foundations or other may intercept confined (artesian) groundwater and cause localised flooding. A site-specific investigation consisting of exploration holes should be carried out to confirm this risk.

Further details on the geology of the site, implications for groundwater and recommendations can be found

in the full technical note attached within the appendices.


4.5. Study Site 106LGEM This site is composed of two parts, one north and one south of the railway embankment. Both parts of the site are low lying compared with the rest of the Wentloog Levels and both lie within TAN15 Zone C1. It is considered unlikely that options to reduce the risks to the southern portion of the site would be viable, as it is considered unlikely that miigation of the adverse impact on flood risk elsewhere could be mitigated.

This assessment has, therefore, focused on the portion of Site 106LGEM situated north of the railway embankment.


Current situation


The extent of the 0.1%(1 in 1000) chance event has increased to 70 % of the site area as the result of improvements made to the model, particularly improved representation of the culverts through the railway embankment. However, the flood depths would not be expected to exceed 0.6m and velocities would remain below 0.15m/s (see Figure 4-12 and Figure 4-13). These are within the TAN15 A1.15 indicative acceptance criteria. The eastern part of the site would remain flood free as would localised areas of high ground.


The majority of the site would be flooded during a 0.5% (1 in 200) annual chance tidal event in 2085.

Previous results indicated that the extent of inundation would be approximately 25%.

Access via Fortan Road to the north of the site is predicted to remain flood free.


The entire site would be flooded during a 0.5% (1 in 200) annual chance tidal event in 2110.

In the event of overtopping only (i.e. no breach), approximately 80% of the site would be flooded.

Fortan Road to the north of the site is predicted to remain flood free.



Figure 4-12 Site 106LGEM Maximum depth – current 0.1% (1 in 1000) annual chance event

Figure 4-13 Site 106LGEM Velocity at maximum depth – current 0.1% (1 in 1000) chance event

Figure 4-14 Site 106LGEM 0.5% (1 in 200) annual chance tidal breach extents, 2085 and 2110

Figure 4-15 Site 106LGEM 0.5% (1 in 200) annual chance tidal event (no breach), 2110

Key Key

Orange Tidal breach extent 2085 Brown Extent of tidal overtopping only

Brown Tidal breach extent 2110

4.5.2. Fluvial flood risk The 1992 modelling study

7 suggests that the site would be flood free during a present day 1% (1 in 100)

annual chance event assuming development of the site. As described for Site 24LGRM above, the topographical information for this area was not considered reliable. Furthermore, winter penning levels in this area have changed since 1992. In light of this, we recommend that a modelling study be carried out to more accurately assess the fluvial flood risk through to 2110. The proposed study is described in Section 6.2.




5. Tidal Flood Risk Mitigation Options and Costs

5.1. Reducing risk to the candidate sites Possible options for excluding tidal flood water from the proposed development sites have been considered. These include:

raising site levels; and

installing structures to prevent back flow of tidal water via the culverts through the railway embankment.

However, the modelling indicates that the railway embankment is overtopped in the western and eastern parts of the modelled area during future 0.5% (1 in 200) annual chance tidal breach events. Therefore, the options to mitigate the risks to the site have focussed on raising ground levels within the sites. The total area which may be raised within each site is limited by the access requirements to the reens. The Caldicot & Wentlooge Levels IDB requires a 7m-wide buffer at the top of each bank of the major reens for maintenance access; this natural floodplain width would also serve to convey flood flows. An increased buffer width of 12m is required on both banks of reens within the SSSI (Sites 21LGR, 24LGRM, 2LGRM and 106LGEM). Buffers along the major reens shown on Drawing 5109602/RCF/001 have been taken account of in calculating the volumes of fill required.

Costs were developed for raising site levels above the predicted 0.5% (1 in 200) annual chance event level in 2110 with mitigation in place.

The alternatives of defending the railway from overtopping and preventing backflow via the culverts would require extensive works on land outside of the developer‟s or Cardiff Council‟s control. These alternatives are therefore considered unlikely to be viable and have not been explored further.

5.2. Mitigating potential adverse impacts of development Raising site levels would result in a loss of floodplain storage. To mitigate the loss of storage, options have been considered which would reduce the volumes of tidal water entering the Wentloog Levels during future 0.5% (1 in 200) annual chance tidal flood events.

The options considered have focussed on works to improve the standard of protection for localised lengths of the coastal defence.

The options include reinforcing different sections of the coastal defences (including the defence on the left bank of the River Rhymney) to effectively reduce the probability of breach towards zero during tidal overtopping events. However, some sites may be flooded by overtopping only events (i.e. if no breach occurs) under existing conditions. Without some raising of the defence levels, reinforcement alone would not mitigate the loss of storage resulting from elevating ground levels of those sites affected by overtopping only events. The 2D inundation model was run in an overtopping/ no breach mode and the results presented on Drawing 5109602/RCF/103. This identifies those sites that would need defences to be raised in order to mitigate for the loss of storage volumes caused by elevating the sites.

The lengths of coastal defence that have been considered for reinforcement only or for raising and reinforcement are listed in Table 5.1 below, together with the anticipated nature of the works required. Works are required to lengths of the order of several hundred metres. The exact extent of works necessary would be subject to further detailed investigation.


Table 5.1 – Options considered for mitigating raised site levels

Embankment location Options considered

Summary of feasibility

River Rhymney (SR1-RREB)

440m (south of railway embankment)

Prevent breach – reinforcement

The back face of the embankment south of the railway would be reinforced. Upon initial consideration, reinforced grass may provide sufficient protection given predicted depths and velocities of overtopping. Additional detailed assessment would be required to confirm.

Raise embankment The existing embankment south of the railway would be raised to reduce overtopping and the back face reinforced with geotextile to prevent breach. Potential works are constrained spatially by the river to the west and a reen running along the landward toe of the embankment. Where required heights of raising exceed what would be feasible at the site, overtopping may be reduced by constructing a new defence to the east of the existing defence alignment. It is noted that in both cases the existing railway embankment would form a low spot in the defence.

Little Wharf (SR1-0)

740m


The back face of the embankment would be reinforced with a flexible concrete revetment system.

Prevent breach – polder development

The site was previously identified as a candidate for polder field construction together with foreshore replenishment and Spartina planting

9; subsequent study

indicated that foreshore replenishment would be prohibitively expensive (Paul Canning, Atkins, pers. comm.); the effectiveness of polder construction alone is unknown; option excluded at this stage.

Raise embankment The embankment would be raised by 0.6m to reduce overtopping and the back face reinforced with geotextile to prevent breach.

Sluice Farm (SR1-2)

740m


The current EA proposal includes raising; therefore, reinforcement alone was not considered.

Raise embankment The current EA proposal is to raise the embankment by 0.6m. The back face of the embankment would be reinforced with geotextile to prevent breach.

Modelling of mitigation options

The hydraulic model was run to test the effectiveness of various combinations of options considered likely to be viable, with sites raised within the 2D domain of the model. The results grids for maximum depth and velocity for each scenario tested were compared with the baseline, unmitigated situation for the 0.5% (1 in 200) annual chance event in 2110. Difference grids were generated to identify changes in depths and velocities across the Wentloog Levels.

5.3. Modelling results

5.3.1. Testing of measures to mitigate loss of flood storage The results of the testing of various mitigation options and the costs of those options are summarised in Table 5.2. For all mitigation options presented, comparisons were made between the model results (depths and velocities) for the existing undeveloped situation and the mitigated post-development scenarios.

9 WS Atkins, January 2004. Gwent Levels Foreshore Management Plan Phase III, Final Report.


As shown in Table 5.2, it would be possible to mitigate the loss of storage resulting from elevating ground levels at Sites 21LGR, 24LGRM, 106LGEM and 2LGRM. Mitigation for increasing ground levels at Site 22LGRM may also be possible subject to further investigation of the feasibility of raising the defence along the east bank of the River Rhymney (SR1-RREB) south of the railway.

5.3.2. Discussion of results For the options listed in Table 5.2, comparisons of the flood depths show either no increase or small decreases compared with those for the existing undeveloped scenario. The corresponding changes in velocities show no adverse impact over the majority of the area of the Wentloog Levels. Some areas of small increases in velocity are indicated. However, these correspond to areas where flood levels are reduced in the mitigated post-development case. The overall risk to these areas is therefore considered to be marginally reduced.

For the case of Site 22LGRM further explanation of the model results are given below.

In the event of breach of the tidal defences, then mitigation for raising the majority of the flooded portion of Site 22LGRM could be achieved. Preventing breach at River Rhymney east bank (SR1-RREB) and Little Wharf (SR1-0) would reduce the volume of tidal water entering the floodplain sufficiently to compensate for the loss of flood storage. A 40,000m

2 portion of the western part of the site would need to be maintained at

current ground levels and allowed to flood in order to prevent adverse impacts to the west of the site.

However, when the case of overtopping only (i.e. no breach) is considered, the adverse effects caused by raising the flooded portion of Site 22LGRM would require a reduction of overtopping from the River Rhymney. We have investigated whether raising the existing defence south of the railway would reduce overtopping volumes and provide adequate mitigation. Model results suggest that to provide adequate mitigation, the defence would need to be raised above the level of the railway embankment. During extreme events, water would continue to overtop the railway embankment.


Table 5.2 – Options to mitigate site raising through to 2110

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

Estimated cost 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

a (m

3)

Cost of raising site (£k)

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


6.31 0.85 12,000 358

24LGRM – Land east of Cypress Drive

6.31-6.32 0.38 14,400 433

106LGEM – Cardiff Parkway

6.31-6.32 0.75 308,900 9,268

21LGR – Land at Areas 9-12 St Mellons

6.32 0.23 31,500 946

Raise embankment at Sluice Farm (SR1-2)

Prevent breach at Sluice Farm

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 4,600

(2,500 plus 2,100)

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

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

Maximum flood depths everywhere reduced with greatest benefit in the west



Assumes it would be feasible to raise the existing defence along its current alignment

Further investigation would be necessary to determine the minimum required defence height at SR1-RREB and confirm site raising volumes


6.10 0.74 9,900 300

SR1-RREB – 440m 24LGRM – Land east of Cypress Drive

6.12 0.30 8,500 260


6.10-6.12 0.62 255,000 7,650


6.11 0.22 18,000 540

22LGRM – Trowbridge Mawr

6.68 0.91 173,800 5,200

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.

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





6.02 0.70 9,100 274


6.02-6.03 0.26 5,300 160


6.02-6.03 0.54 223,100 6,693


6.03 0.23 12,800 385

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

Raise embankment at Little Wharf (SR1-0)


Raise site levels

SR1-RREB – 440m 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




5.99 0.69 8,900 266

SR1-0 – 740m 24LGRM – Land east of Cypress Drive

5.99-6.02 0.26 4,900 148


5.99-6.02 0.53 217,400 6,522


6.02 0.23 12,300 369


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

Estimated cost 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

a (m

3)

Cost of raising site (£k)

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 2,900 Assumes EA has carried out proposed works at Sluice Farm (SR1-2) (i.e. no developer contribution)



Assumes that relocation of the defence would be required in order to provide the necessary height increase; further investigation would be necessary to confirm this

Cost excludes compensation or land purchase costs



6.10 0.74 9,900 300


6.12 0.30 8,500 260


6.10-6.12 0.62 255,000 7,650


6.11 0.22 18,000 540

22LGRM – Trowbridge Mawr

6.68 0.91 173,800 5,200

aVolumes 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.


6. Summary and Recommendations

6.1. Tidal flood risk The risk from the tidal source of flooding could be managed for four of the sites assessed, namely Sites 2LGRM, 24LGRM, 106LGEM and 21LGR, if site ground levels are raised and adequate mitigation for lost storage is in place. It is possible that the risks associated with development of Site 22LGRM could also be managed. However, further investigation is required to clarify the technical viability of the potential mitigation measures.

Sites 2LGRM, 24LGRM, 106LGEM and 21LGR

The results given in Table 5.2 suggest that the preferred mitigation option for the four sites would be to raise and reinforce the coastal defence embankment at Sluice Farm (SR1-2) to reduce overtopping and effectively prevent breach. It would need to be confirmed if a developer contribution to the Environment Agency‟s planned works at Sluice Farm would be considered as mitigation.

Alternatively the option to raise the coastal embankment and prevent breach at Little Wharf (SR1-0) could be considered. Preventing breach at the River Rhymney (SR1-RREB) in addition to at Little Wharf would have little added benefit and a much higher cost than completing the works at Little Wharf alone.

Site 22LGRM

Our analyses indicate that further measures would be required to mitigate the adverse impacts of raising Site 22LGRM above the 0.5% (1 in 200) chance tidal event level to 2110. Considering breach events only, the options described above in relation to the other four sites would mitigate for elevating ground levels over the majority of Site 22LGRM, providing an area of approximately 40,000m

2 in the western part of the site is left at

existing levels. However, for the case of an overtopping only event (i.e. no breach), raising of the River Rhymney defences would also be required.

Raising the existing River Rhymney defence (SR1-RREB) south of the railway could reduce overtopping volumes sufficiently to mitigate elevating the majority of Site 22LGRM. The final defence height and alignment would depend on whether the works at Sluice Farm (SR1-2) are also considered as mitigation. Regardless, further investigation would be necessary to determine the required final defence height and the technical feasibility of the works.

Ground levels in the eastern one-third of Site 22LGRM are higher than surrounding levels. This portion of the site may be developed without raising site levels should mitigation for the rest of the site not be pursued.

All sites

Wherever development occurs, undeveloped buffer strips must be left at the top of the right and left banks of the reens maintained by the Environment Agency and the IDB. Buffers must be 12m for sites within the SSSI (Sites 21LGR, 24LGRM, 2LGRM and 106LGEM) and 7m elsewhere.

For detailed site-specific assessments, the tidal modelling should be refined at the site scale. The current model is based on a 10m grid resolution which may be too coarse to capture the finer detail of the system of reens and ditches through the area. Increasing the model resolution would provide better representation of these features. To minimise the associated increase in model run times, the resolution of some other (less key) parts of the modelled area could be reduced. Site-specific modelling would enable refinement of site raising volumes.

To support more detailed modelling, survey would be required of the reen system and culverts within and adjacent to the areas of interest. We also recommend a detailed review of the LiDAR data to ensure that structures such as bridges were filtered correctly with changes to be made as necessary.


6.2. Fluvial flood risk The fluvial flood risk to the sites over the lifetime of development cannot be defined with confidence based on the available data. We consider that modelling of parts of the reen system will be necessary for those sites for which tidal risk can be mitigated. The general approach has been discussed and agreed in principle with the Environment Agency at a meeting on 27

th February 2012. Two catchments north of the railway line

will need to be modelled. These would incorporate the following systems of reens:

Catchment 1 (Site 22LGRM): Rhosog Fach, Rhosog Fawr West and Rhosog Fawr Branch Reens

Catchment 2 (Sites 21LGR, 24LGRM,106LGEM, 2LGRM): Pil-du, Hendre, Walwyn Place, Fen Eliffant, Faendre, Green Lane Branch, Little Greenlane, Greenlane, Ty-Ffynon, Railway and Tarwick Reens and St Mellons Pond

As a first approximation, the downstream boundary conditions would be taken as the level of bankfull flow in the reens south of the railway embankment: Rhosog Fawr for Catchment 1 in the west and Greenlane, Railway and Tarwick Reens for Catchment 2 in the east. This assumption is based on the concept that flow overtopping the banks of these reens would tend to spread out over a wide floodplain, limiting the maximum flood levels at the model boundary. Sensitivity testing of this assumption would need to be undertaken to test the impact on modelled flood levels at the candidate sites.

It is anticipated that a 1D-2D TUFLOW or similar model would be used to represent the extent of flooding. The extent of hydraulic model development and the hydrological assessment would to a large degree be informed by the initial model build and sensitivity tests.

The following data would be necessary for the development of the models:

Collection of new topographic survey of all the reens of interest, including survey of any culverts under roads and through the railway embankment;

Review of sewer information for existing development to confirm catchment extents in urbanised areas that may drain to the reens of interest;

Additional hydrological analysis to refine rainfall and runoff the estimates urbanised catchments; and Determination of the runoff distribution between branching reens. This may be best investigated using sensitivity testing.

6.3. Flood risk from other sources Data reviewed for this study suggest that the groundwater table within the sites of interest is near to the ground surface. As such, the groundwater table is likely to be sensitive to changes in local drainage associated with development. The risk of flooding from groundwater itself is anticipated to be low at the sites; however, surface water flooding may be an issue following rainfall events given the shallow groundwater table.

The use of SuDS should be considered to maintain the current water table conditions. Soakaways are considered unlikely to be appropriate because of the low ground permeability and the potential to raise the already high groundwater table.

It is possible that breaking of ground, for example for foundations, could intercept confined (artesian) groundwater causing localised flooding. Site-specific investigations consisting of exploration holes would need to be carried out to confirm this risk at all five of the sites assessed above..

Site-specific investigations should include the following:

Exploration holes (trial pits or boreholes) to ascertain the site-specific geology;

Installation of boreholes to monitor groundwater levels in the various strata; and

Depending on the results of the above two investigations, borehole pumping tests to determine aquifer properties.

These tasks would provide the basis for a hydrogeological assessment report that would identify site-specific groundwater flood risks with more certainty and advise on potential mitigation measures where significant risk is identified. The investigations could be incorporated into other site investigation undertaken for geotechnical and/or environmental reasons.


7. Conclusions

Based on the preliminary Stage 3 assessments, it is likely 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. The residual risk from fluvial flooding would require further characterisation and definition provided by hydraulic modelling of parts of the reen system local to the candidate sites.

The cost of mitigation works to the coastal defences to facilitate development of these four sites is estimated to be of the order of £2.5M. In addition to this the costs of raising all four sites would be of the order of £11M, with Site 106LGEM contributing over £9M of this total.

It may also be feasible to mitigate the tidal flood risk to the majority of Site 22LGRM in line with TAN15; an area of approximately 40,000m

2 in the western part of the site would have to be left at existing levels.

Additional investigation is needed to determine the scope, technical viability and cost of raising a 440m length of the River Rhymney defences south of the railway. The fluvial risk to this site would also require further characterisation through modelling of the local reen system.

The cost of works to the River Rhymney defences, which would be in addition to the above costs required to mitigate development of the other four sites, is estimated to be of the order of £2.1M. The cost of raising Site 22LGRM is estimated to be £5.2M.

The mitigation options described have been discussed in principle with Environment Agency officers. However, the acceptance by the Environment Agency of these options is subject to submission of the details described in this report and any supporting information that may be requested.

Appendices


Appendix A. Environment Agency Guidance – Stage 3 Assessment

A.1. Extract from Environment Agency guidance to local authorities

Stage 3: Test site suitability

Purpose

To confirm that the flood risk to any proposed site(s) can be managed to an acceptable level and that the site itself will not exacerbate flooding elsewhere over the lifetime of the development.

Managing flood risk can have a significant impact on the design, cost and viability of developments. LPAs should satisfy themselves that the flood risk management measures associated with a potential allocation are feasible and practicable.

Testing .suitability

Section10.6 of TAN15 states that.

“Where the local planning authority wishes to allocate a site, and can justify such an allocation, the local planning authority will need to undertake a broad level assessment of the consequences of flooding occurring on that site, in consultation with the Environment Agency. This assessment should demonstrate that the consequences of flooding have been understood and are capable of being managed in an acceptable way. Where such local information has been produced then this should be reflected in the plan.”

Therefore, LPAs need to demonstrate that the development is capable of meeting TAN15 requirements, set out in Section 7 and Appendix 1, over its lifetime. To do this, they need to demonstrate that:

the development can provide a safe and secure living and/or working environment throughout its life, taking into account climate change;

existing and proposed flood risk management systems are structurally adequate, particularly under extreme overtopping conditions, and taking account of the effects of climate change;

the costs of providing all new/approved flood mitigation measures, including defences, can be secured;

the costs of future maintenance for all new/approved / existing flood mitigation measures relied upon, including defences, can be secured;

the site can be provided with effective flood warnings;

off site escape/evacuation routes are operational under all conditions;

flooding is not increased elsewhere and is ideally reduced;

See Extent of flooding below for event details.


Results

LPAs will understand whether the flood risks can be acceptably managed for the proposed allocations.

It should also considerably reduce the time and resources required for any subsequent FCAs carried out at the planning application stage.


Appendix B. Drawings

B.1. Site location Plan

Drawing No. 5109602/RCF/001 – Wentloog Tidal Area Overview

B.2. Updated Model Results

Drawing No. 5109602/RCF/100 – 0.5% (1 in 200) annual chance tidal breach event extents. Current and future.

Drawing No. 5109602/RCF/101 – 0.1% (1 in 1000) annual chance tidal breach event. Predicted Maximum depths, 2010.

Drawing No. 5109602/RCF/102 – 0.1% (1 in 1000) annual chance tidal breach event. Predicted Velocities at Maximum depths, 2010.

Drawing No. 5109602/RCF/103 – 0.5% (1 in 1000) annual chance tidal event – overtopping only. Extents to 2110.


Appendix C. Hydrogeological Screening Note

© 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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