ENVIRONMENT Harworth Group Gateway 180, Bradholme Farm (Site ref. 160) Flood Risk Summary for Local Plan Examination

Oct 2020

Document Number: BGT-BWB-ZZ-XX-RP-YE-0006_FRTN BWB Reference: NTW_2654_FRTN_4

Revision Date of Issue Status Author: Checked: Approved:

P03 20/10/20 S1 Iqbal Rassool Graham Littlewood Iqbal Rassool

Notice

All comments and proposals contained in this report, including any conclusions, are based on information available to BWB Consulting during investigations. The conclusions drawn by BWB Consulting could therefore differ if the information is found to be inaccurate or misleading. BWB Consulting accepts no liability should this be the case, nor if additional information exists or becomes available.

BWB Consulting Limited accepts no responsibility for any use of or reliance on the contents of this document by any third party. No part of this document shall be copied or reproduced in any form without the prior written permission of BWB

Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

1. INTRODUCTION

Instruction

1.1 As part of the current Doncaster Local Plan Examination, the Inspector requested summary information from the Environment Agency in advance of the Hearing session dedicated to Flood Risk matters (14th Oct 2020), to better understand flood risk conditions at the potential employment sites. A summary note was submitted by the Environment Agency, but the Inspector has requested that the Council expand on this summary information to provide more comprehensive comparison between the sites, making use of any supplementary site specific assessment information.

1.2 This document has been produced by BWB Consulting Ltd on behalf of Harworth Estates and presents our observations and comments to the Environment Agency’s Flood Risk summary submission, specifically in relation to Site 160, proposed Gateway 180 development at Bradholme Farm. To assist the Inspector and the Council with collating this information within a wider summary table we have retained the same format of table headings to that within the Environment Agency’s document.

1.3 Our comments within this summary are informed by the following:

1.3.1 Formal pre-application consultation meetings with the Environment Agency (8th June 2018) and Drainage Board (Feb 2020), 1.3.2 Review of the updated baseline model data received from the Environment Agency for the 2018 River Torne Hazard Mapping Study and the 2016 Upper model (received Nov 2019). 1.3.3 Supplementary assessment work identifying the flood zones to be inconsistent with the latest modelling data at site 160 and resulting in the submission of a formal flood zone challenge to the Environment Agency (submitted 24th June 2020 and attached as Appendix A). We understand that this is still pending, but the output provided in the Environment Agency’s summary information to the EIP similarly confirms a lack of correlation between the indicative Flood Zones and the latest model predictions.

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

2. SUPPLEMENTARY SUMMARY INFORMATION

2.1 The following table provides point by point commentary and supplementary findings associated site 160 with respect to the Environment Agency’s own summary issued as part of the Doncaster Local Plan Examination.

Parameter Environment Agency BWB Comments comments

Availability Refer to Doncaster Local We would concur with the Environment Agency’s view that the most up to date models and Authority Area Model covering the area relevant to Site 160 and other potential employment sites in the suitability of Update note. M18/M180 corridor are the following: model data - Upper Humber Model 2016 (which includes the River Don) - River Torne Hazard Mapping Study 2018 (which includes the South Soak Drain) - Tidal Trent Model 2013 - Isle of Axholme Flood Risk Management Strategy 2014.

From pre-application consultations with the Environment Agency and Isle of Axholme Drainage Board it was identified that relatively speaking flood risk from the Trent and Drainage Board network are negligible sources and that flood risk to the site should be based on the findings of the Humber and Torne models.

We would regard the Upper Humber and Torne models as being of a suitable level of detail to form the basis of a Level 2 SFRA for locations in M18/M180 corridor.

Flood Zone Approximately 93% flood The first part of the Environment Agency’s comments appear to refer to the relative areas zone 3, 7% flood zone 2 of Flood Zone 2 and 3 from the published Flood Maps for Planning (extract of this is shown

(Approximately 5-10% of in Figure 1 below). In terms of the flood zone information in the public domain, the relative the site is also shown to be areas for FZ2 & FZ3 is a statement of fact. within Flood Zone 3b the Functional Floodplain of the South Soak Drain).

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

Figure 1 – Published Flood Zones

However, based on our own reviews of the latest models and the Environment Agency’s own model predictions, we would consider the flood zones to be unrepresentative of flood risk in this location and the Environment Agency comment associated with the potential for a small amount of flood zone 3b on the site is inaccurate.

We would concur with the Environment Agency’s summary note that the most representative model for flood risk at site 160 is the 2018 River Torne Model. This model has been reviewed for use as the representative tool for undertaking a site specific assessments in this location and in reviewing bank levels and site specific topographical survey information the flood predictions shown in Figure 2 have been produced.

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

Figure 2 – Updated predictions from River Torne model

Consultations with Isle of Axholme Drainage Board has established that the published flood zones are a derivation of theoretical risk from the Trent (circa 14km away) combined with the unlikely scenario of a complete failure of the pumped systems for the whole of the Isle of Axholme area. Therefore, not regarded as realistic reflection of risk but more an unlikely residual risk to inform development levels.

Due to the complete lack of correlation between the flood zones shown in figure 1 and latest modelling predictions shown in Figure 2, a formal Flood Zone challenge has been submitted to the Environment Agency to alter the flood zone designations to a more realistic profile (Appendix A). Based on what we consider to be a more realistic flood zone designation in line with figure 2 the proportion of flood zones are as follows:

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

FZ1 (outside 1 in 1000 year prediction) = 63% FZ2 (between 1 in 100 year and 1 in 1000 year predictions) = 15% FZ3a (within 1 in 100 year prediction) = 22%

It should be noted that FZ3b is not separately identified in the published flood zones. As per the flood zone definitions in Table 1 of PPG for FZ3b “Local planning authorities should identify in their Strategic Flood Risk Assessments areas of functional floodplain”. The current Level 1 SFRA does not show the presence of FZ3b in this area and therefore the Environment Agency’s comment about FZ3b under this section titled “Flood Zones” is misleading as it is not defined by a SFRA. That said from our own analysis using topographical and bank level information for the site, no functional floodplain within the site. The functional floodplain predicted to the north of the site will remain as such and would not form part of any planning application.

In an Area No We would concur that the currently published Flood Zones do not show this area to be Benefiting benefiting from defences. However the Environment Agency also provide in their summary from to the EIP, output from the Upper Humber Breach model (2016) which suggest flood risk to Defences parts of the site during a breach of the flood defences on the river Don circa 2.5km away. This again places significant doubt on the accuracy and relevance of the published flood

zones as the model output clearly suggests that parts of the site are benefiting from the River Don defences. We would also highlight concerns over this particular parameter in the Environment Agency’s summary table as being a yes or no response and that being interpreted as being positive or negative respectively. The proximity to defences is a hugely important supplementary consideration to make under this parameter as the consequential risk to life is much more significant in a breach event closer to the defence. This consideration is featured prominently in the Strategic Flood Risk Assessments for Goole, Hull and many other Level 2 SFRAs across the country, by restricting any form of development within the “Rapid Inundation Zone” and “Danger for All” hazard area, which are generally in areas closest to defences.

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

Therefore, under this parameter and the methodology employed in other Boroughs, we would regard Site 160 to be in a more appropriate location some 2.5km away from the defences compared with say Site 001 which is immediately adjacent to the defences and likely to be in the Rapid Inundation Zone.

Flood Events The EA holds no records of From our own investigations and consultations we concur with this statement that there previous flood events at are no records or anecdotal suggestions of previous flooding at the site. this site

Covered by Yes The Environment Agency’s confirmation of Flood Warning is correct. To expand further, Flood information has been obtained from the Environment Agency and the site has the Warning following warning codes on the EA Flood Warning Service: Area  “123FWB058” which is “River Don and Dutch River from to Rawcliffe Bridge”. There are no instances of a flood warning being raised for this site.  “123FWT057” which is “River Don at Moorends and Thorne”. There are no instances

of a flood warning being raised here.  “034WAB421” which is “Tidal Trent wider floodplain from Gainsborough to the Humber confluence”. There are no instances of a flood alert being raised for this site.

Although the location is included within the warning system there is no record of flood alerts/warnings being raised for the site, which highlights the relative low risk of site 160 compared with other areas in the catchment.

Site Specific Flood Risk Assessment

Depth on 1% (100yr) Breach This relates to output from the 2016 Upper Humber Breach model and is the best available Site Between 0m -2m Map information to understand residual risk in the area. Subject to a site being outside the Below “Rapid Inundation Zone” and “Danger to All hazards” areas, we would regard this residual risk information to solely inform site mitigation and finished levels not to inform a Sequential

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

Test. Figure 3 below is the depth map output provided in the Environment Agency summary for a breach location on the River Don located some 2.5km to the north-west.

Figure 3 – Depth Map for 100 year Breach

Because of the significant distance from the breach to the site and relatively coarse nature of the strategic model, it is likely that breach model predictions are likely to be more precautionary as you move further away from the breach location. However, using this information as a worst case it still provides reassurance that there is significant areas of high ground within the site that allows for cut and fill balance to deliver the development without relying on unsustainable importation of material to raise plot levels.

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

As seen in Figure 4 below, proposed development masterplan for site 160, this layout facilitates a cut and fill balance of material on site whist maintaining adequate floor levels to prevent internal flooding, with significant landscaped corridors along the watercourses and a net gain of compensatory floodplain storage to reduce flood risk elsewhere.

Figure 4 – Proposed Masterplan for Site 160 Hazard on 1% (100yr)+30% Climate With respect to the 100 year +30% event this comes from the River Torne 2018 model and site Change corresponds well to our flood predictions shown in Figure 2. Figure 5 below is the hazard Mostly ‘Danger to most’ map provided in the Environment Agency summary note:

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

Figure 5 – River Torne Model Flood Hazard

As can be seen in this map, the Environment Agency’s statement that the hazard is “Mostly Danger to Most” is fundamentally incorrect. The vast majority of the site has no flood hazard designation at all (less than Low Hazard). Therefore, for this particular event we should conclude that the hazard for the site is mostly zero.

The red band running through the centre of the site should be disregarded as this is the watercourse alignment and flood hazard should only apply to out-of-bank flood areas.

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

1% (100yr) Breach This hazard output is again extracted from the 2016 Upper Humber model for a breach on Mostly ‘Danger to most’ the River Don 2.5km away. As per the comments made under the Depth section, this is likely to be a precautionary residual risk event away from areas of Rapid Inundation and Danger to All, and so should solely informs site parameters and not a Sequential Test. Figure 6 below is the hazard map provided in the Environment Agency summary note:

Figure 6 – Breach Model Flood Hazard

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

The EA comment states this to be “Mostly Danger to Most”. Our estimation is that this output shows over 50% of the site to be “Danger Most” so confirm the statement to be factually correct.

Again, as a further point of clarification there are narrow bands of red “Danger to All” within the site shown in Figure 5 but these should be disregarded as a mapping error as flood hazard mapping should not include the watercourses themselves but just reflect out- of-bank flooding, and it is evident that these red corridors are the watercourse channels which will naturally carry a deeper and higher velocity flow.

Velocity on 1% (100yr) Breach It should be noted with the exception of the higher velocities of 0.3-1m/s in the site Between 0-1m/s watercourse channels running through the site, the vast majority of the at risk areas during this breach event are at very low velocities of 0-0.3m/s. This is due to the significant distance from the breach location dissipating much of the energy of the breach which would in-turn mean the speed of on-set from a breach occurring to the site flooding would be a number of hours as opposed to the sites in closer proximity to the defences.

This is an important consideration in flood management for any site in a flood warning area as the longer the speed of onset the more time is available for individual sites to implement emergency measures around access and egress. Figure 6 below is the output on velocities provided in the Environment Agency’s summary note.

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

Figure 6 – Breach Model Velocities

Sources of N/A To summarise sources of extent of flood risk at site 160: Flood Risk 1) South Soak Drain and tributaries – Fluvial flood risk extents shown Figures 2 & 5 2) River Don – EA breach model suggest residual risk of moderate hazard to parts of site in the event of a breach of the defences. 3) Isle of Axholme drainage network – pre-app feedback confirm a residual risk in the event of failure of pumped systems coinciding with tidal flooding, which is likely to prolong the period of flooding rather than the extent.

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

3. SUMMARY

3.1 To assist the Council and Inspector with the collation of comparable flood summary information for site 160, Section 2 provides a point by point commentary and supplementary detail to the Environment Agency’s summary note submitted to the Examination. This gives greater clarity around the sources and limitation of the data used.

3.2 Our findings are that the flood zones for this location are misleading and are subject to challenge. Based on the latest modelling evidence we would estimate the following proportions of flood zone areas within the site:

FZ1 (outside 1 in 1000 year prediction) = 63% FZ2 (between 1 in 100 year and 1 in 1000 year predictions) = 15% FZ3a (within 1 in 100 year prediction) = 22%

3.3 It is understood that the collation of this information is to inform the Inspectors understanding of flood risk at the various employment sites within high risk areas. Although this information may form the basis of a useful aid memoir to the Examination, we do not regard this as a suitable substitute for a Strategic Flood Risk Assessment undertaken to a level of detail proportionate to the level of risk of the sites being allocated.

3.4 The information in the Environment Agency’s summary note and within this supplementary summary for site 160, serve to demonstrate that flood risk is highly complex in the Borough and much more complex than could be interpreted from the Flood Zones. It also highlights that the level of risk at site 160 is contradictory to that suggested in the Flood Zones. In our opinion, the decision to rely solely on the content of the Doncaster Level 1 SFRA (2015), which is largely based on Flood Zone designations, does not demonstrate a proportionality of detail of assessment we would expect for the allocation of sites within such complex high risk areas.

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Site 160 Flood Risk Summary, Oct 2020 BGT-BWB-ZZ-XX-RP-YE-0006_FRTN

APPENDIX A

Site 160 - Flood Zone Challenge Letter

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