M4 Corridor around Newport

PUBLIC INQUIRY QUESTION 

REFERENCE NO. :   PIQ/113 

RAISED BY: Dr Callaghan  DATE: 05/07/17 

RESPONDED BY: Matthew Jones   DATE: 06/07/17 

SUBJECT:  OBJ6919 Dr. Callaghan – Site Visit Questions and Answers 

  Dr Callaghan raised questions during a site visit with the inspectors on 05/06/17. 

   

  Response: 

Attached is the Welsh Governments response to the questions. 

 

 

A copy of this PIQ has been provided to Dr Callaghan.  

 

 

 

 

 

 

M4 INQUIRY  

Questions for Inquiry Re: Objections from Dr. Diana Callaghan,  Resident, Ty Mawr Lane, Marshfield 

 1. Flood Risk. 

 Dre‐Newydd Reen runs parallel to Ty Mawr Lane. The water run off from the new M4 section will collecting the lagoons and reed beds before discharge in to the Pwll Bargoed and Tyn y Brwyn reens, which then discharge into Dre‐Newydd reen. This large reen is classified as a waterway by Natural  Resources Wales,  and  it  acts  as  a  converging  reen  for  several  other minor  reens  and  ditches,  before  following  into  the  sea.  This  reen  flows  past  numerous properties  in the village of Marshfield, some of which are on the bank of the reen, before flowing out via Broadway reen to sea. During high tides, which cause tidal lock, the reen water flow can be observed as  ‘in  reverse’ as water back‐fills  the  reen. During periods of heavy rainfall, this creates significant pressure on the reen capacity.   Issue 1. The predicted volume of water predicted to flow into Dre‐Newydd reen from the 2 lagoons during a 1 in 100 storm event is 4.24 million gallons over several days. This volume of water is in addition to all existing water that runs off the current M4/A48M.   What is the net gain change in water volume when both the current and new M4 sections are operating, and in a 24 hour, 1 in 100 storm event?   How can it be predicted that the combined attenuated volume of current and new M4 run‐off water will be less than currently arrives at Dre‐Newyydd reen?   Issue 2:  Dre‐Newydd reen is experiencing flooding in its current state, with water overspilling the banks onto the road :  

a) Immediately adjacent and onto agricultural grazing land.  

b) Immediately opposite and onto the entry bridge for properties at the end of Ty Mawr Lane.  Approximately  10  years  ago  Up  to  a  foot  of  water  covered  the  road  and prevented access up the lane following a 3 period of continuous rain.   

c) The concern is that this period of rain is significantly longer than the 24 hour storm event used in the modelling and occurred when tidal lock prevented drainage of the reen, it flowed back and re‐filled Dre‐Newydd reen to the point of flooding.  

 What records exist of flooding in Ty Mawr Lane as a result of Dre‐Newydd reen over‐spilling the banks?  On this basis, how can Welsh Government support a development that deliberately adds to the volume and flow of water through a residential area?  

 

 

 Issue 3. During high tide, the water flow in Dre‐Newydd reen does not always remain static, but can reverse, flowing back towards the discharging reens creating a significant rise in water level in the reen.  How have you accounted for this effect in the modelling of water flow and flood risk in Ty Mawr Lane?   Issue 4.  Dre‐ Newyydd Reen is not identified in the Flood Risk Assessment documentation, yet it flows immediately past properties on Ty Mawr Lane. Natural Resources Wales has already highlighted the properties at the end of Ty Mawr Lane at risk of local flooding form the reen.   Given the sensitivity and impact of this area, why has there been no prior survey of this at‐risk area and why is is not included in the Flood Risk Assessment as an already identified flood risk area involving residential property?? 

  

Issue 5. There is a significant reen network in this area, it is therefore suggested that the Welsh Government should consider re‐routing the water outflow from WTA 1 and WTA 2 to reen that avoid residential areas using the shortest possible route to reach the sea.   Has this option already been given consideration and what is the justification for discharging the water into Dre‐Newydd Reen?  Issue 6. Visual maps published in the Flood Risk Assessment predict greater flooding in fields south of the new M4 immediately adjacent to Ty Mawr Lane. The map does not include the area at the west end of Ty Mawr Lane which has been established as already flooded and therefore does not specify whether this area will also experience greater flooding.  This issue needs to be addressed by reviewing the outcome of the modelling that predicted this flooding to include the at risk area at the end of the west end of Ty Mawr Lane.   

2. Parameters and Data for Modelling of Water Management   2.1 Discharge from the water treatment areas, based on the 1  in 100 storm event, will be released at the greenfield discharge rate of 3.5 litres/s/ha ‘over the storm and drain period’. The maximum of the range of storm durations included in the model is 24 hours.   What is the basis for selecting a 24 hour storm duration?  What measure  of  confidence  is  applied  that  a  24  hour  storm duration  is  unlikely  to  be exceeded? How can this be sufficient given the earlier evidence of flooding near properties following a 3 day period of rain? 

 

 

   

3. Further Action  Dr Patrick Ireland has visited the site and was unaware of prior flooding adjacent and within less than a twenty yards of properties on Ty Mawr Lane.  What further action will Welsh Government take in light of these objections? It is suggested that current documentation needs to be inclusive of the risks identified by the residents of Ty Mawr Lane and updated to include further modelling with more accurate predictions of possible flooding in this location.   Dr Diana Callaghan Tretio Ty Mawr Lane, Marshfield CF3 2YF  Qualifications: BSc Psychology (1st Class), University of South Wales PHD Psychology, University of South Wales 

File Note 06 July 2017

Prepared by: Mike Vaughan Subject Marshfield Council - Ty Mawr Lane Date Thursday, July 06, 2017

Questions raised by the objection

Dr Diana Callaghan informed a Statement of Evidence submitted by Marshfield Community Council (OBJ6902) in relation to the draft statutory Orders associated with the Welsh Government’s proposals for the M4 Corridor around Newport.

The Marshfield Community Council point of objection no. 32 to which Dr Callaghan informed has been supplemented by questions following a site visit on 5 July 2017. The Welsh Government has responded to (OBJ6902) and offers the following in further support

1. Flood Risk. Dre-Newydd Reen runs parallel to Ty Mawr Lane. The water run off from the new M4 section will collecting the lagoons and reed beds before discharge in to the Pwll Bargoed and Tyn y Brwyn reens, which then discharge into Dre-Newydd reen. This large reen is classified as a waterway by Natural Resources Wales, and it acts as a converging reen for several other minor reens and ditches, before following into the sea. This reen flows past numerous properties in the village of Marshfield, some of which are on the bank of the reen, before flowing out via Broadway reen to sea. During high tides, which cause tidal lock, the reen water flow can be observed as ‘in reverse’ as water back-fills the reen. During periods of heavy rainfall, this creates significant pressure on the reen capacity.

1.1. Issue 1. 1.1.1. The predicted volume of water predicted to flow into Dre-Newydd reen from the 2 lagoons during a 1

in 100 storm event is 4.24 million gallons over several days. This volume of water is in addition to all existing water that runs off the current M4/A48M.

1.1.2. What is the net gain change in water volume when both the current and new M4 sections are operating, and in a 24 hour, 1 in 100 storm event?

1.1.3. Issue 1a response. The change in land from a semi permeable rural environnment to paved impermeable carriageway will increase the volumetric runoff arising from a unit area. This volumetric runoff might be expected to rise towards 100% on completion of the scheme.

1.1.4. The design of the scheme, to include an attenuation lagoon and reedbed for each water treatment area, addresses the impact of increased runoff. The restrictions to peak runoff, as set by Natural Resources Wales have been demonstrated to limit flows into the receiving watercourses. The lagoons are designed to attenuate the 1 in 100 year flood with 30% allowance for climate change to a runoff rate of 3.5 l/s/ha, and each lagoon includes a measure of freeboard in its design. The risk of overtopping will be less and 1% per annum. This storage and attenuation will be achieved through use of a vortex flow regulator, which will pass forward the design flow, at 3.5 l/s/ha, at the specific design head.

1.1.5. Furthermore, the limit set also accounts for long term storage, such that any increase in volumetric discharge is released very slowly back to the environment. UK SuDS guidance as described in the 2015 CIRIA SuDS Manual (C753) describes the use of long term storage which is intended to provide a volumetric restraint, as opposed to flow control. This is intended to mimic the natural percolation of water through the soils. It is this volume that should be prevented from leaving the site (via infiltration or harvesting) or where this is not possible, controlled so that it discharges at very low

File Note 06 July 2017

rates that will have negligible impact on downstream flood risk. The guidance requires that only the greenfield runoff volume should be allowed to discharge at greenfield rates. However, the full guidance requires us to restrict all runoff to the maximum of either 2 l/s/ha or QBAR greenfield runoff.

1.1.6. Using the same guidance, the greenfield runoff rates for the Scheme are 3.7 l/s/ha for the 1 in 2 year storm, and 8.1 l/s/ha for the 1 in 100 year storm. Hence the design could either attenuate runoff during the 1 in 2 year event to 3.7 l/s/ha, and the 1 in 100 year event to 8.1 l/s/ha; or to attenuate all flows to 3.7 l/s/ha. In fact, the hydraulic design approach has been to attenuate the permitted pass-forward flow to a lower 3.5 l/s/ha. Hence, both flow and volumetric storage are suitably provided for.

1.1.7. How can it be predicted that the combined attenuated volume of current and new M4 run-off water will be less than currently arrives at Dre-Newyydd reen?

1.1.8. Issue 1b response. The original M4, (built circa 1967), did not have the benefit of an attenuated discharge. When the M4 was widened (circa 2008/2010), two attenuation ponds at Castleton were added. Attenuation was only provided for the M4 motorway: the A48M does not have the benefit of an attenuated discharge. The approximate catchments contributing to each pond, together with the pond locations are indicated on the figure below.

Figure 1 – existing M4 draiange

1.1.9. The majority of the M4 to the north of Castleton, (approximately 1.3km length), drains to attenuation pond A, (area shaded blue). The outfall from pond A, (blue arrows), discharges to a watercourse which flows to the east of the Bryn Ivor Care Home, (former Garden Centre). The watercourse is then culverted below the A48. The outfall of A48 culvert is located immediately to the east of WTA1. The watercourse then flows into the head of the Pwll Bargoed Reen.

1.1.10. To the west of Castleton, the M4 drains to Pond B. Pond B discharges into a small ditch/watercourse to the south of existing M4, (orange arrows). The route of the watercourse as it approaches the A48M is unclear. However ground contours would suggest that the ditch is culverted below the A48M and joins a watercourse which flows along the north eastern side of the village of Castleton. The stream flows around the west side of WTA1 to a confluence with the watercourse from Pond A at the head of the Pwll Bargoed Reen.

File Note 06 July 2017

1.1.11. The drainage from the A48 M on-slip and off slip does not have the benefit of any attenuation pond and discharges to the same watercourse as Pond A, (red arrows).

1.1.12. At the start of the KS3 process it was decided that due to the uncertainty with the capacity of the existing drainage system and the fact that pond A was space constrained by topography and provided no opportunity to install reed beds, we would provide a new drainage system that was capable of capturing and treating all flows up to a 100 year return period with an allowance for climate change. It was considered that the existing drainage system was not consistent with the water quality and flood risk objectives set at the start of the project.

1.1.13. As a result of the proposed junction improvements, parts of the A48M have now been subsumed into the drainage system for the M4CaN. All these works are provided with attenuation and treatment up to a 100 year return period with an allowance for climate change. The areas coloured red on the plan, approximately 1.7km of the A48(M), currently discharge to the Wentlooge Levels without the benefit of attenuation. These will be attenuated in the future should the M4CaN proceed in its current form. The consequence will be a net improvement in the flood risk situation.

1.1.14. It should also be noted that the attenuated rate of discharge applied to WTA1 is 3.5l/sec/ha. Whilst this rate of runoff may be appropriate for the Gwent levels, the catchment draining to WTA1 lies on the steeper slopes to the north. The existing unit rate of runoff will be much higher at this location. In the interest of making sure that we are not going to increase downstream flood risk we have adopted a precautionary approach and adopted the discharge rate of 3.5l/sec/ha for WTA1.

1.2. Issue 2: 1.2.1. Dre-Newydd reen is experiencing flooding in its current state, with water overspilling the banks onto

the road :

a) Immediately adjacent and onto agricultural grazing land.

b) Immediately opposite and onto the entry bridge for properties at the end of Ty Mawr Lane. Approximately 10 years ago Up to a foot of water covered the road and prevented access up the lane following a 3 period of continuous rain.

c) The concern is that this period of rain is significantly longer than the 24 hour storm event used in the modelling and occurred when tidal lock prevented drainage of the reen, it flowed back and re-filled Dre-Newydd reen to the point of flooding.

1.2.2. What records exist of flooding in Ty Mawr Lane as a result of Dre-Newydd reen over-spilling the banks?

1.2.3. Issue 2a response. It is notable that Drenewydd Reen enlarges as it heads west to Broadway (Marshfield). We have also identified from our site visits that the freeboard to Ty Mawr Lane reduces with proximity to Broadway so increasing the risk of flooding. Discussions with Natural Resources Wales have confirmed that flooding has occurred in this area, although no formal records of the event have been seen.

1.2.4. On this basis, how can Welsh Government support a development that deliberately adds to the volume and flow of water through a residential area?

1.2.5. Issue 2b response. The hydraulic modelling undertaken for the M4CaN has demonstrated that the proposed Scheme will not increase flood risk to Marshfield. The design principle applied to the drainage of the carriageways is such that all runoff, up to and including the 1 in 100 year storm with a 30% allowance for climate change, will be attenuated to a greenfield runoff rate, here set at 3.5l/s/ha.

File Note 06 July 2017

1.2.6. Our response to Issue 1b, above, refers to the additional, existing, carriageway from the A48(M) that is currently unattenuated and will be collected into the proposed drainage system. This change will limit what is currently a 100% runoff from 1.7km of carriageway into the receiving reens down to the greenfield rate, and provide betterment from that specific catchment.

1.2.7. The hydraulic modelling includes for three existing natural catchments draining from the north and discharging towards Ty Mawr Lane. These are described in the Flood Consequences Report (FCA) at Appendix 16.1 of the Volume 3 Environmental Statement (Document 2.3.2). The three catchments have a combined watershed of 270ha. The hydrological assessment evaluated a total peak flow arising from these catchments of 1100l/s in the 1 in 2 year event. This amounts to some 4.1l/s/ha. The hydrological assessment evaluated a total peak flow arising from the same three catchments of 3400l/s in the 1 in 100 year event. This amounts to some 12.6l/s/ha.

1.2.8. It is thus my opinion that existing greenfield rates are higher than those set here by NRW at 3.5l/s/ha. Further evidence is obtained from the UK SUDS tool which indicates the rates to increase from 3.7 l/s/ha at the QBAR event to 8.1 l/s/ha in the 1 in 100 year event. As such, the restrictions placed on discharge from the M4CaN will reduce the peak flows passing southwards to Drenewydd Reen and Ty Mawr Lane in comparison to those that can be expected under the existing conditions.

1.2.9. The combined storage within WTA1 and WTA2 amounts to some 19,500m3 (4,289,400 gallons). This receives water from a combined carriageway catchment of 20.8ha, and discharged into Pwll Bargoed Reen and Tyn-y-Brwyn Reen at a peak rate of 32.5l/s and 40.1l/s respectively. This is a combined peak flow of 72.6l/s. This data is contained in the Drainage Strategy Report at Appendix 2.2 in Volume 3 of the Environmental Statement (Document 2.3.2).

1.2.10. The same area (to be occupied by the Scheme), albeit mostly rural land, currently drains to the same receiving reen network, and flows towards Ty Mawr Lane. Under the existing conditions, that same area will contribute a peak flow of 72.8l/s to Drenewydd Reen, based on the greenfield runoff set by Natural Resources Wales. Under extreme weather conditions, the UK guidance suggests that this may rise to a peak flow of 168.5l/s. However, once the proposed M4CaN is developed, this flow will be limited to 72.6l/s – being marginally less flow. Hence the scheme offers an improvement in flood risk.

1.2.11. Pwll Bargoed Reen and Tyn-y-Brwyn Reen both discharge into Drenewydd Reen. As the flows have been attenuated to greenfield runoff the cumulative flows into Drenewydd Reen are unchanged from the existing case, as demonstrated by the hydraulic modelling.

1.2.12. This reduction in peak flow will reduce the risk of flooding beyond that credited today.

1.2.13. However, whilst any existing flooding will not be made worse, if there is a flood risk today then it is likely that there would also be in the future. The proposed Scheme is not intended to alleviate flooding and any improvements are a by-product of the design.

1.3. Issue 3. 1.3.1. During high tide, the water flow in Dre-Newydd reen does not always remain static, but can reverse,

flowing back towards the discharging reens creating a significant rise in water level in the reen.

1.3.2. How have you accounted for this effect in the modelling of water flow and flood risk in Ty Mawr Lane?

1.3.3. Issue 3 response. The development of the hydraulic model is described in my proof of evidence and in the Flood Consequences Report (FCA) at Appendix 16.1 of the Volume 3 Environmental Statement (Document 2.3.2).

1.3.4. Para 4.28 of my Proof reports, “The downstream boundary for the model was applied as a mean high water spring tide (MHWS) covering several days of tides representative of the state in the Severn Estuary at each outfall. The MHWS tidal curve was derived from work undertaken in the

File Note 06 July 2017

Severn Estuary Flood Risk Management Strategy (Document 17.2.16) and varies spatially with a peak level of 6.0m AOD at the mouth of the Rhymney and 6.8m AOD at the toll at Sudbrook.”

1.3.5. The hydraulic model incorporates several tide-time boundaries which are applied throughout the simulations. The model also includes the tidal outfalls, as described to us by Natural Resources Wales. The model then calculates, at every timestep, the ability to discharge against the tide, and permits a positive discharge or prevents it, with a varying discharge performance. This modelling technique is widely used for such problems and correctly represents the occurrence of tidelock in a fluvial system such as this. The hydraulic model does reflect the observation of reverse flow, which is the effect of the tidelock.

1.4. Issue 4 1.4.1. Dre- Newyydd Reen is not identified in the Flood Risk Assessment documentation, yet it flows

immediately past properties on Ty Mawr Lane. Natural Resources Wales has already highlighted the properties at the end of Ty Mawr Lane at risk of local flooding from the reen.

1.4.2. Given the sensitivity and impact of this area, why has there been no prior survey of this at-risk area and why is not included in the Flood Risk Assessment as an already identified flood risk area involving residential property??

1.4.3. Issue 4 response. The Ty Mawr Lane, and Marshfield village, are both included within the hydraulic model. This is further described in Issue 6 below. Survey using aerial based LiDAR data was applied to the hydraulic model. This is described in my Proof at Para 4.24.

1.4.4. The Flood Consequences Report (FCA) at Appendix 16.1 of the Volume 3 Environmental Statement (Document 2.3.2) is aimed at defining flood risk to and from the proposed Scheme. It is not intended to inform flood risk management across the wide Gwent Levels.

1.5. Issue 5. 1.5.1. There is a significant reen network in this area, it is therefore suggested that the Welsh Government

should consider re-routing the water outflow from WTA 1 and WTA 2 to reen that avoid residential areas using the shortest possible route to reach the sea.

1.5.2. Has this option already been given consideration and what is the justification for discharging the water into Dre-Newydd Reen?

1.5.3. Issue 5 response. The reen network forms a man made and managed grid of channels. These are drained by the main rive reens, which outfall through the major tidal gates. The route which will be found by any discharges, and indeed existing runoff, will take the path of least resistance to the sea. With the relatively significant conveyance capacity, and direct alignment to the coast, the Drenewydd Reen is the major carrier in this area.

1.5.4. An alternative drainage path, to a tidal outfall at Peterstone Gout, would be eastwards along Drenewydd Reen before turning south along Hawes Reen. However, the predominant flow path from Drenewydd Reen is westwards towards Marshfield and into Broadway Reen. This is evidenced by the placement of IDD control sluices in the reen network.

1.5.5. The discharge from both WTA1 and WTA2 will find their way into Drenewydd Reen regardless of subtle changes in there precise discharge point. Drenewydd Reen is a Natural Resources Wales main river. This watercourse flows west to Marshfield, before turning south and flowing alongside the highway knows as Broadway, as Broadway Reen. This then discharges to the Severn estuary via a second outfall at Peterstone Gout.

1.5.6. Discussions with Natural Resources Wales confirm this flow path from Drenewydd Reen to Broadway Reen as being the main flow path in the area. The Hawes Reen tends to drain into

File Note 06 July 2017

Drenewydd Reen, although can be operated under certain circumstances such that is flows the opposite way.

1.6. Issue 6. 1.6.1. Visual maps published in the Flood Risk Assessment predict greater flooding in fields south of the

new M4 immediately adjacent to Ty Mawr Lane. The map does not include the area at the west end of Ty Mawr Lane which has been established as already flooded and therefore does not specify whether this area will also experience greater flooding.

1.6.2. This issue needs to be addressed by reviewing the outcome of the modelling that predicted this flooding to include the at risk area at the end of the west end of Ty Mawr Lane.

1.6.3. Issue 6 response. The hydraulic model for the Wentlooge Levels is described in the Flood Consequences Report (FCA) at Appendix 16.1 of the Volume 3 Environmental Statement (Document 2.3.2).

1.6.4. This describes the model domains and hydrology and includes figures demonstrating that the hydraulic model does cover Marshfield, Ty Mawr Lane and Drenewydd Reen. Figure 6.6 from Appendix A.3 of the March 2016 FCA is included below.

Figure 2 – Area covered by hydraulic model – Wentlooge Levels

1.6.5. The flood map figures included in the FCA, figures 5.6 to figure 5.9 were cropped to cover the area pertinent to the M4CaN scheme, with Marshfield lying just off the west of the images and being outside of the immediate area of the proposed Scheme. These figures are reproduced overleaf, to indicate predicted flooding at Marshfield and Ty Mawr Lane.

Figure 3 - Baseline flood risk 1 in 100 year event with climate change

File Note 06 July 2017

Figure 4 - With-Scheme flood risk 1 in 100 year event with climate change

Figure 5 - Impact of M4CaN - difference in flood risk 1 in 100 year event with climate change

File Note 06 July 2017

File Note 06 July 2017

Figure 6 - Baseline flood risk 1 in 1000 year event

Figure 7 - With-Scheme flood risk 1 in 1000 year event

File Note 06 July 2017

Figure 8 - Impact of M4CaN - difference in flood risk 1 in 1000 year event

2. Parameters and Data for Modelling of Water Management 2.1.1. Discharge from the water treatment areas, based on the 1 in 100 storm event, will be released at

the greenfield discharge rate of 3.5 litres/s/ha ‘over the storm and drain period’. The maximum of the range of storm durations included in the model is 24 hours.

2.1.2. What is the basis for selecting a 24 hour storm duration?

2.1.3. Tests were undertaken in both the drainage and fluvial models to establish the worst case scenarios. As is usual for such modelling tests were specifically undertaken to evaluate the storm duration critical for both peak flow and peak storage.

2.1.4. UK hydrology is such that storms shorter than the critical duration provide more intense rainfall but that the whole catchment is not fully contributing to flows arriving at a point. Beyond the critical storm duration, whilst the full catchment is contributing, the intensity of the rainfall decreases and the runoff reduces. In the UK a 48 hour storm of a set return period will be less intense than a 24 hour storm of the same return period.

2.1.5. The tests were used to define the critical storm to provide peak flows in the Scheme’s drainage network; the critical storm to provide peak stored volume in the Scheme’s attenuation lagoons; and the critical storm to provide the worst flooding across the receiving watercourses.

2.1.6. My Proof, in section 4, describes how the attenuation lagoons have been sized for much longer duration storms than the channels and pipe networks. This is as expected because of the need to balance the inflow with the restricted outflow, and account for the volume held in storage. This is

File Note 06 July 2017

supported by the drainage modelling, which indicates a critical duration storm in excess of 24 hours for the lagoons. The fluvial modelling work identified that a 12hr storm generated the greatest depth of flooding across most of the levels. However, of note here is that both storms would be released from the proposed attenuation lagoons at the same rate, being based on the set greenfield runoff rate of 3.5l/s/ha.

2.1.7. What measure of confidence is applied that a 24 hour storm duration is unlikely to be exceeded?

2.1.8. UK hydrology is such that storms of all durations can occur. There is an established relationship between rainfall intensity, duration and frequency which reduces rainfall intensity with increasing duration for a fixed frequency. It is certain that storms of duration greater than 24hours will occur. Should those longer durations bring more intense rainfall, then the frequency of occurrence (likelihood or chance) of such storms is reduced – ie the return period increases.

2.1.9. The Scheme has been designed to cope with storm events up to the 1 in 100 year event with an allowance for climate change over 100 years into the future. The hydraulic modelling and assessment has demonstrated that the Scheme will not increase flood risk to property across the Gwent Levels and that any local impacts will be minor, and can be managed.

2.1.10. How can this be sufficient given the earlier evidence of flooding near properties following a 3 day period of rain?

2.1.11. As described above, under Issue 2, any existing flooding will not be made worse. If there are existing flood risks today then it is likely that there would also be in the future. The proposed Scheme is not intended to alleviate flooding and any improvements generated are a by-product of the design.

2.1.12. The Marshfield / Ty Mawr Lane area is known to be at flood risk – as identified on the NRW flood maps (DAM zone C1, and flood zone 3). This area is indicated to be at medium risk of flooding, being at from between a 1 in 30 year and 1 in 100 year return period event.

2.1.13. The observed 3-day (72 hour) event of a certain rainfall intensity would have had a certain frequency of occurrence. According to the depth-duration-frequency model in the UK’s Flood Estimation Handbook, a 72 hour rainfall event of 1 in 100 year return period at this location would have an average intensity of 2mm/hr. The same 1 in 100 year return period event for a 24 hour storm would precipitate a greater average of 4.6mm/hr. As the proposed drainage scheme is designed to cope with the worst case, being the large rainfall intensity and attenuate back to 3.5l/s/ha, longer duration events are adequately catered for.

File Note06 July 2017

Annex 1 (Insert Figure PLI-13i (Overview of Water Treatment Areas)