4. Site Specific Assessments

4. Site specific assessments

4.1 Introduction

4.1.1.1 This chapter summarises the findings of the tidal and fluvial modelling and presents the evidence base for the Exception Test along with the recommendations for the following sites: • Tees Marshalling Yard (TMY) • Bowesfield North Phase 1 and Phase 2 • Boathouse Lane and Chandlers Wharf • Seal Sands (includes the Seal Sands, North Tees Pools and Port Clarence sites) • Haverton Hill (includes the Haverton Hill, Casebourne and Billingham Reach sites)

4.1.1.2 Figures have been produced for each site showing flood depth, flood extent and flood hazard for the different flood events. These figures are in Appendix A of this report (A1 to A31).

4.1.2 Flood Hazard

During the Level 2 modelling, hazard maps were produced directly within the modelling package (TUFLOW). The formula used for the hazard rating is shown below (d = depth, v = velocity and DF = debris factor).

4.1.2.1 Flood Hazard = d (v + 0.5) + DF

4.1.2.2 The flood hazards calculated are categorised and coloured in accordance with the latest recommendations2 as illustrated in Table 2.

Table 2 - Flood Hazard rating Flood Hazard Rating Hazard to People Colouring 0 No Hazard 0 to 0.75 Very Low Hazard 0.75 to 1.25 Dangerous for some 1.25 to 2.0 Dangerous for most Over 2.0 Dangerous for all

4.1.2.3 PPS25 provides no guidance as to which development type is permitted in certain hazard zones, although it is implied in the vulnerability classification of those developments.

2Gibbs, G., Surendran, S., Wade, S. and Udale-Calrke, H. (2008) Supplementary note on flood hazard ratings and thresholds for development planning and control purpose – Clarification of the Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2321/TR1

4.2 Tees Marshalling Yard 4.2.1 Site background

4.2.1.1 Figures showing the site boundary and the modelled flood extents, depths and hazard for the existing risk at Tees Marshalling Yard (TMY) can be seen in Appendix A, Figures A1 to A7. Flood extents and flood depths with the flood defences removed can be seen in Figures A8 to A11.

Site description

4.2.1.2 The main part of the TMY site occupies a narrow strip of land, approximately 2,500m long and up to 350m wide, parallel to the River Tees (see Figure A1 in Appendix A). The site sits astride the boundary between Stockton and Middlesbrough BC and the Tees Barrage is situated to the north of the site. Although this Level 2 SFRA is for Stockton BC, this assessment takes into account the part of TMY within Middlesbrough BC. To the south of the TMY site, the Teesside retail park, Aintree Oval and the Teesside Golf Course occupy a low-lying basin of approximately 122 hectares, which are currently at high risk of flooding (as reflected in the current Environment Agency Flood Zone maps).

4.2.1.3 For much of its length the TMY is protected from the River Tees by a broad strip of wide ground that is significantly higher (greater than 6m AOD) than the predicted extreme sea levels for the mouth of the Tees (see Table 1 in Chapter 2). However, there is a low area at around 4m AOD to the east of the TMY. Much of the remainder of the TMY lies between 3.50 and 4.50m AOD so is potentially at risk of flooding during extreme tides. High flood levels in the Tees estuary could potentially affect the TMY site from two sources: • Direct inundation from the Tees could occur across the low strip of ground between the A19 and the A1032 in the north-east of the site if estuary levels were to rise above approximately 4.5m AOD. This source is not expected to pose a significant present day risk but will need to be considered for future modelling scenarios when sea level rises of up to 1.0m could occur over the next 100 years. • Inundation via the Old River Tees watercourse that passes directly through the TMY site. The Old Tees discharges freely into the estuary and this, in turn, allows high tide levels to be passed relatively unhindered through the TMY along the Old River Tees. Tidal water flowing up the Old Tees was considered to be the main source of flooding to the TMY site during the 1953 event, in which flood levels (at the location of the barrage) were measured at 4.01m AOD. The fluvial flood risk posed by the Old River Tees is not expected to be high and following consultation with the client, the current study was limited to analysing the tidal flood risk. 4.2.2 Modelling

Modelling approach

4.2.2.1 The topography of the TMY and its surroundings are complicated. This implies that the nature of flooding would be dependent upon the flow paths of floodwater across the floodplain. Floodplain flows under these circumstances

are best modelled using 2D hydraulic modelling software. However, the limited conveyance that would result from modelling the Old River Tees using 2D software alone implies that this watercourse would be best represented by carving a 1D channel through the 2D domain. In view of the fact that no survey was available for modelling the Old River Tees (necessitating a very simplistic representation of this watercourse with simplified channel geometry and no structures), the Old River Tees was modelled using ESTRY. The final TMY model is therefore an ESTRY-TUFLOW (1D-2D) linked model.

Existing risk modelling results

4.2.2.2 The modelling produced the following results:

4.2.2.3 During a present day 1 in 200 year event only a negligible volume of water is predicted to exit the Old River Tees at the peak of the event. This event is therefore predicted to be close to the threshold of tidal overtopping for the Old Tees. No overtopping occurs from the main River Tees.

4.2.2.4 During a present day 1 in 1000 year event some floodwater is predicted to exit the Old River Tees within the Marshalling Yard close to the peak of the event. This flood water is generally modelled to flow directly away from the channel along the course of railway tracks or roads. However, only a small number of properties are modelled to be flooded during this event. Again no overtopping occurs from the main Tees.

4.2.2.5 During a future 1 in 200 year event with 100 years climate change (+cc), the flood risk is modelled to be significantly higher than during the equivalent present day event due to the increase in mean sea level that is predicted to occur in response to climate change. Overtopping of both banks of the Old River Tees is modelled to be the main source of flooding but flood water is also modelled to overtop the right bank of the main River Tees between the A19 and the A1032. Floodwater affects most of the small Teesside basin including the Marshalling Yard, Teesside Retail Park, Aintree Oval and the Teesside Golf Course. Peak flood depths across much of this area are typically between 1 and 2 metres. Modelled flow paths during the peak of the event are locally complex and can be seen to flow through underpasses in the railway embankment to inundate the golf course and retail park area. Flood Hazard across much of the basin is modelled to pose a “Danger for Most” people principally due to the high flood depths but locally, also due to high velocities along key floodplain flow routes.

4.2.2.6 Figures of the flood extent, hazard and depth for the existing situation can be seen in Figures A1 to A7 in Appendix A.

Developed site modelling results

4.2.2.7 The impact of fully raising the development has been modelled in order to see what impact this would have to the surrounding area. The following impacts were observed within the TMY model. • During a present day 1 in 200 year event no overtopping was modelled from either the Tees or Old River Tees and land raising across the TMY was modelled to have no impact on flood levels along the Old River Tees.

• During a present day 1 in 1000 year event, some floodwater was modelled to exit the Old River Tees immediately to the south of the TMY. However, the extent of flooding across the floodplain was less than under existing risk conditions. By looking at the model and the value of the floodplain (within the large tidal area) developing the TMY has the potential to increase levels by 30mm for the 1 in 1000 year event. This needs to be tested explicitly and the model configured to represent all floodplains in the tidal reach. In doing so, the impact of removing this floodplain is likely to show a lower order impact. • During a 1 in 200 year event +cc, the flood extent is modelled to be less than under existing risk conditions, principally because raising the TMY site removes this area from the area at risk of flooding. However, the modelled flood depths and hazard elsewhere in the modelled area are also lower than under existing conditions. This is because raising the TMY site prevents flow into the Teesside basin via the Marshalling Yard. Hence, by blocking flow routes through the Marshalling Yard flood risk is reduced to the adjacent Teesside basin. • The reduction in flood risk to the adjacent Teesside Basin may have a minor impact further afield. Firstly, flood levels at the upstream end of the Old River Tees are indicatively modelled to be higher (30mm - see comment in the second bullet) during the developed scenario. Secondly, development could have an adverse impact by raising flood levels on the River Tees as land raising prevents flood water entering the area between the A19 and the A1032. The maximum impact of development on River Tees flood levels was simplistically modelled (by adding a reservoir unit to the existing Lower Tees ISIS model) to be of the order of 18mm in the reach between the Barrage and Ayresome Wharf. This is a highly conservative value due to the simplistic modelling approach required to examine this impact and neither of these impacts is considered to represent a significant risk to areas elsewhere.

4.2.2.8 This modelling scenario has been used as one of the mitigation options and is labelled mitigation option 1. Two other mitigation options have been modelled. These are discussed in the next section.

4.2.2.9 Figures of the flood extents and flood depth (for the climate change flood event) for mitigation option 1 can be seen in Figures A12 and A13 in Appendix A.

4.2.3 Development implications and issues

Flood Zones and functional floodplain

4.2.3.1 The original SFRA completed in 2007 stated that over half of the TMY was in Flood Zone 3 and subject to flood depths of around 1m. The current Environment Agency Flood Zone maps (released in September 2009) show that approximately two thirds of the TMY site is within Flood Zone 3. Flood Zone 2 takes up a further section of the site. Figure 4 show the Flood Zone maps and the TMY site boundary.

Figure 4 - Flood Zone maps at TMY

OS Licence: © Crown Copyright 100023297 (2010). Key

Flood Zone 2 V.3.15

Flood Zone 3 V.3.15

4.2.3.2 The tidal modelling completed for this Level 2 SFRA has produced existing risk and undefended flood extents for the 1 in 200, 1 in 1000 and 1 in 200+ climate change year flood events.

4.2.3.3 The undefended flood extents can be seen in Figure A8 and in Figure 5 below. Undefended extents have been modelled so that a comparison can be made with the Environment Agency's Flood Zone maps (Figure 4) which are also produced without defences.

4.2.3.4 Undefended, modelled flood extents in Figure 5 shows that the TMY is not at risk from the 1 in 200 year event and only a small part is at risk from the 1 in 1000 year event. The majority of the site is flooded during the 1 in 200+cc event. These extents were produced using a 2D model and are very different to the Environment Agency Flood Zone map which shows the majority of the site is inundated during the 1 in 200 year event (Flood Zone 3).

Figure 5 - Undefended modelled extents at TMY (also see Figure A8 in Appendix A)

OS Licence: © Crown Copyright 100023297 (2010). Key

4.2.3.5 The 'existing risk' flood extents can be seen in Figure A1 in Appendix A. These include the flood defences on the Old River Tees and were also produced using a 2D model. These provide a more accurate graduation of the flood risk within the current Environment Agency Flood Zones specifically where the volume of inundation alongside and behind the tidal defences on the Old River Tees is more accurately calculated.

4.2.3.6 These extents are also different to the Flood Zones because tipping of earth alongside the Tees and general ground levels protect the majority of the marshalling yard from the 1 in 200 year event. The tidal defences alongside the Old River Tees protect the Teesside Retail Park. However, in large events and in particular events that factor in sea level rise, the defences are overtopped and flood extents similar to those provided by the Flood Zones are predicted.

4.2.3.7 The differences between the broad Flood Zones, which do not take into account defences, and the results of the existing risk modelling is that the actual and residual flood risks at TMY are not significant enough to rule this site out of being developed. There are various options that can be used to demonstrate that management of the residual risks can be achieved and the Exception Test passed.

4.2.3.8 A larger proportion of the site is at risk from the 1 in 1000 year event. The central area of the TMY is affected by this event. This assessment should include details on whether the site will be safe once developed and that the development will not increase flood risk elsewhere. These elements are summarised in the remaining sections.

4.2.3.9 These modelled flood extents will need to be reviewed by the Environment Agency before the conclusions made in this report can be finalised

Mitigation Options

4.2.3.10 A number of broad scale mitigation options were modelled for the TMY site. These were modelled to see if there were practical options available for delivering the site for residential development. This is necessary because although the modelling shows that the TMY is not at risk from the 1 in 200 and only a small part is at risk from the 1 in 1000 year events, it will be inundated by the 1 in 200+cc event. The flood depths from this event (see Figure A4 in Appendix A) indicates that mitigation measures should be possible. These options were also modelled to show that there are other possibilities apart from land raising available.

Mitigation Option 1 - land raising of the entire site

4.2.3.11 This option involved raising the whole of the site above the 1 in 200+cc level, this has been described in paragraph 4.2.2.7. The modelled flood extents and depths of this mitigation option can be seen in Figures A12 and A13 in Appendix A.

4.2.3.12 This option will require raising some part of the site by 2m, but more typically, by 1m, with additional foundation raising on top of this for freeboard. The areas where land raising will be required are defined by the 1 in 200 year +cc flood extent. The amount by which land will need to be raised can be seen by the corresponding flood depth map (i.e. the area at risk of 1m depths will need to be raised by 1m plus freeboard). This will require a significant volume to be raised, the cost of which will need to be factored into development appraisals of the site.

4.2.3.13 This removes the risk to the TMY site but the scale of land raising and impacts elsewhere suggested that other options could be considered to balance flood risk and urban form.

Mitigation Option 2 - raised defences on the Old River Tees

4.2.3.14 For this option raised defences were modelled either side of the Old River Tees (similar to those that are in place further upstream adjacent to the retail park).

4.2.3.15 The modelled flood extents and depths of this mitigation option can be seen in Figures A14 and A15 in Appendix A. This shows that the modelled embankments on the Old River Tees prevent flooding to the site from this watercourse up to the 1 in 200 year +cc change event. The embankments would need to be set at 5.1m AOD (plus freeboard - see glossary) for this level of protection. The cost of constructing and maintaining these defences would have to be paid for by the developer or funded through other means.

4.2.3.16 The underpasses within the railway embankment provide a flood flow route for the existing risk from the 1 in 1000 and 1 in 200+cc year events. These underpasses have been blocked for this mitigation option. This shows the railway embankment effectively acting as a flood defence for these extreme flood events. For the 1 in 200 year+cc event, flood depths would be in the region of 1m behind the railway embankment. This is a potential hazard, and if this mitigation option is chosen the risk should be considered / modelled in the FRA.

4.2.3.17 At the 1 in 200 year+cc tidal event, flooding to the site occurs from the main River Tees. Overtopping occurs at a low point in the east part of the TMY site (see Figure A14). This inundates the east part of the site (around a third of the site). Flood depths for this event are generally below 0.5m (see Figure A15). If this mitigation option were chosen, the development would need to manage this remaining flood risk. Due to the low depths, a combination of flood resilience and floor raising could reduce this risk.

Mitigation Option 3 - raised defences on the Old River Tees and part of the River Tees

4.2.3.18 This mitigation option includes raised defences on the Old River Tees and blocking the railway embankment underpasses (as in mitigation option 2). However, this option also includes blocking the flow route from the main River Tees by raising levels along a small stretch between the TMY and the River Tees.

4.2.3.19 The modelled flood extents and depths of this mitigation option can be seen in Figures A16 and A17 in Appendix A. This shows that flooding from all of the events (including the climate change event) is removed from the TMY site. However, as with Mitigation Option 2, the railway embankment on the south boundary of the TMY is acting as a flood defence, holding back 1m flood depths. The breach potential and hazard from this embankment should be modelled in an FRA if this option is selected.

Mitigation Option 4 - managed floor level system

4.2.3.20 This final option involves raising some areas within the TMY but allowing some parts of the site to flood during the 1 in 1000 and 1 in 200+cc year events. This option is effectively a compromise with Mitigation Option 1 (which proposes complete land raising).

4.2.3.21 The existing risk flood depth map for the 1 in 200 year event +cc (see Figure A4 in Appendix A) shows that flood depth varies across the site. Although the site is not at risk from the 1 in 200 year event and only partially at risk from the 1 in 1000 years event, the development will need to be designed up to the 1 in 200 year +cc event. Figure A4, shows the modelled flood depths that will need to be integrated into the site masterplan.

4.2.3.22 As the TMY is an old industrial site, it will probably need to be capped with fill to mitigate against contaminated land. Typically this capping could raise the site by 600mm. For parts of the site that are at risk of flood depths greater than 600mm, mounds could be raised beneath individual buildings, integrated into the foundations. This could raise the buildings by a further 300mm. For the parts of the site that are at risk of flooding from depths greater than 900mm (600mm + 300mm), flood resilient flood measures could be integrated into a further 600mm of building design (e.g. raised plug sockets and water resilient building materials). Any of the areas at risk of flooding greater than this (i.e. over 1.5m) should be left undeveloped (e.g. converted to green amenity space).

4.2.3.23 Figure 6 shows the different depths categories and therefore mitigation options required for the TMY.

• The areas in red should be kept free from flooding. • The areas in dark blue will require the first 600mm of the building to be designed with flood resilient measures. 300mm of threshold raising and 600mm of land capping fill will also be required. • The areas in light blue will require 300mm of threshold raising and 600mm of land capping fill. • The areas in green will able to manage flood risk using the 600mm of land capping fill. • These areas can be manipulated within the masterplan so that strategic open space can be utilized in a flood reduction capacity.

Figure 6 - 1 in 200 year+cc tidal depth areas at the TMY for Mitigation Option 4

Sustainability Appraisal Indicator 1 Are the flood risks (depths and hazards), prior to mitigation, significant enough to impact on urban design and could result in a risk of loss life to people using the site?

4.2.3.24 Limited flooding occurs from the Old River Tees at the 1 in 1000 year event with flood depths below 0.5m. The inception of significant flooding at the TYM occurs at the 1 in 200 year event+cc. The modelling showed that during this event, peak flood depths across much of this area are typically 1m (see Figure A4 in Appendix A). Flood Hazard across much of the basin is modelled to pose a “Danger for Most” people due to the high flood depths but also due to high velocities along key floodplain flow routes (see Figure A7 in Appendix A).

4.2.3.25 These 1-2m flood depths, in the south part of the TMY could pose a risk to people and will therefore need to be mitigated against. Three mitigation options have been modelled for TMY. These have been discussed in the previous section. Options 1, 2 and 3 should not significantly impact on urban design as the main aim is to prevent flooding occurring on the site rather than adapting the site to flooding. Option 4 will impact on urban design as a combination of land raising and flood resilience in different parts of the site is proposed.

Sustainability Appraisal Indicator 2 Can the development be made safe once developed and are residual risks acceptable? Access and Egress

4.2.3.26 The onset of significant flooding occurs at the 1 in 200 year +cc event, nearly all of the TMY site is shown to be at risk. The north part of the TMY is generally on higher land (>6m AOD) and not at risk of flooding. Safe access and egress routes can be made through this area. During this extreme event, people would be able to evacuate to the north (Tees Barrage Way) and west (Princeton Drive and Station Street). However, the access route will need to be free from flooding during the 1 in 200+cc event.

4.2.3.27 Mitigation option 1 for the TMY, raises the land above the 1 in 200 year +cc event (this is greater than the 1 in 1000 year event). As a result, the development and access routes would be free from flooding during the most extreme flood events.

4.2.3.28 Access and egress can still be made via the north (Tees Barrage Way) east (Newport Road) and west (Princeton Drive and Station Street). However, the basin (golf course and Teesside Retail Park), is flooded during the 1 in 200 year +cc event. This should therefore not be the emergency access and egress route for the development.

4.2.3.29 Mitigation option 2 leaves the east part of the site at risk of flooding and option 4 allows some parts of the site to flood. An access route that can be reached by all properties should therefore be designed in the west part of the site.

Mitigation option 3 removes flooding from all of the site. The normal access and egress routes can therefore be used if this option were to go ahead.

4.2.3.30 If mitigation option 2 is implemented, the development could potentially increase the strain on emergency services during the 1 in 200 year event +cc flood. In this flood event, with mitigation option 2, the east part of the site is at risk of up to 0.5m deep flooding. Some flood resilience measures are proposed for this option which should mitigate flood risk up to this event.

4.2.3.31 However, if ground levels are raised (mitigation option 1), residential development would not be expected to increase the strain on the emergency services as people will either stay in their homes or use the egress routes to the north that are available during 1 in 200 year +cc event.

Residual risks

4.2.3.32 There are no raised defences adjacent to the TMY and land raising (mitigation options 1) has the ability to elevate the area above all the flooding events. It is noted that the barrier mounds adjacent to TMY alongside the River Tees are not formal flood defences (see Section 2.1.3). However, this barrier will need to be assessed for their water retention properties and possibly a water cut off provided. Also it is acknowledged that capping of the probable contaminants on the site will require a reasonable level of ground raising.

4.2.3.33 Residual flood risk for mitigation option 1 is therefore not a significant enough issue for it to be considered as a design factor in the site layout.

4.2.3.34 For mitigation options 2 and 3, raised defences have been modelled along the Old River Tees. This potentially presents the residual risk of an embankment breach and the associated hazard this brings. However, a modern design of flood embankment should ensure that the risk of a breach will be minimal.

4.2.3.35 In addition, the underpasses on the railway embankment on the south boundary of the TMY have been blocked for options 2 and 3. This means the embankment is effectively being used as a flood defence. If mitigation options 2 or 3 are selected, the flood defence capabilities of the railway embankment should be investigated as there would be a risk of a breach during the 1 in 1000 and 1 in 200+cc year events. If this embankment is shown to be weak, it would need to be strengthened as a flood defence. If mitigation option 1 is implemented, the embankment could be lowered so that it does not block/overlook, the new developments.

4.2.3.36 Mitigation options 2 and 4 allow parts of the site to flood, but at shallow depths. This should not pose a flood hazard, but should be taken into account in a flood management plan, along with an emergency egress route.

Sustainability Appraisal Indicator 3 Is the increase in risk posed by climate change easily adapted to and/or can the development build in climate change resilience

4.2.3.37 The 1 in 200 year flood does not flood the site, but the 1 in 200 year plus 100 years climate change (+cc) completely inundates the whole of the site (see Figure A1). This event is also significantly greater then the 1 in 1000 year event. This is due to the expected increase in sea levels cause by climate change (~1m increase). This study has taken this event into account and the flood depths expected for this event (see Figure A4) can be managed through a number of different mitigation options. Climate change is therefore a major factor that needs to be built in now.

Sustainability Appraisal Indicator 4 Can flood risk to adjacent lands stay the same or be reduced as a result of the development and its mitigation measures.

4.2.3.38 As the site is at risk of tidal flooding, loss of floodplain is not considered to have a significant impact, compared with that usually associated with fluvial flood risk. As the area at risk from the 1 in 200 year event is so small (see Figure 5 and Figure A1 in Appendix A), if developed, the impact on others would be minimal (a maximum of 30mm). Alternatively, this area could be left undeveloped.

4.2.3.39 However, during the 1 in 200 year +cc event, flow routes are affected by the development. This occurs because if the TMY site were raised, this would prevent flow into the Teesside basin via the Marshalling Yard. This reduces flood risk to the Teesside basin which includes the Teesside Retail Park.

4.2.3.40 It can be summarised that development in the TMY will reduce flood risk to the basin to the south and that no flood storage compensation would be needed as the area at risk from the 1 in 200 year event could be left undeveloped. There is an impact at the upstream end of the Old River Tees for this larger event (1 in 200 year event +cc). The impact is predicted to be less than 30mm, however any increase in flood depth would need to be assessed in relation to the effects to property and access routes.

4.2.3.41 The impact of fully raising the site above the 1 in 200 year event +cc can be seen in Figure A12 (extents) and A13 (depths) in Appendix A.

Summary and proposed mitigation strategy

4.2.3.42 The current Environment Agency Flood Zone maps (version 3.15, released in September 2009) show that approximately two thirds of the TMY site is within Flood Zone 3. Flood Zone 2 takes up a further section of the site. Under PPS25, the Exception Test would need to be undertaken in order for this site to be allocated. The LPA should be able to show that the Sequential Test can be passed if there are no alternative areas in Flood Zone 1 or 2 available for housing. As TMY is a regeneration site, it will be assessed in the Sustainability Appraisal (SA) of the Regeneration DPD. Part of the SA includes the SA Flood Risk Indicators.

4.2.3.43 The hydraulic modelling completed for this study showed that the site is only at risk from the 1 in 1000 and 1 in 200+cc event. This is because the site is 31

surrounded by higher ground, which is believed to be mainly tipped spoil, which is broad enough not to be considered a flood defence that could fail.

4.2.3.44 The flood depths from these events can be managed by flood risk mitigation measures. Four mitigation options were modelled for this study, these are: • Mitigation option 1 - ground raising of the entire site above the 1 in 200 year+cc level. • Mitigation option 2 - raised defence on the Old River Tees and blocking the underpasses on the southern boundary railway embankment. • Mitigation option 3 - the same as 2 but raising the land between the TMY and the River Tees to prevent tidal overtopping via a low point in the east. • Mitigation option 4 - a small amount of land raising across the majority of the site with some additional threshold raising for individual buildings and flood resilience measures for some of the development.

4.2.3.45 These options are described in more detail earlier in this section. These options show that the site can be safely developed (part 'c' of the Exception Test).

4.2.3.46 The modelling has also shown that the development will not increase risk elsewhere for the 1 in 200 year event. For the lower probability events, a small increase in levels can be seen upstream. Overall development would decrease risk to the Teesside Retail Park, indicating that part c) of the Exception Test can be passed.

4.2.3.47 Option 4 would appear to have the least impact on others for the low probability events. But option 1 would overall, reduce risk to others.

Mitigation Summary Table Tees Mitigation Option Constraints Preferred Outcome Development Marshalling sequence and Yard indicative cost Mitigation Raising the entire site above the 1 in 200 year+cc flood Raising the entire site above the 1 in 200 Due to the expected Before Option 1 level. year+cc event will be a significant task as some levels of investment for development parts of the site would need to be raised by 2m. this regeneration site, goes ahead, the the need to raise the land will need to The Environment Agency do not require land for contaminated be raised and floodplain compensation for the TMY as the risk land capping and for some access is tidal. In addition, the TMY is only at risk of practical reasons roads flooding from the 1 in 1000 not the 1 in 200 year (roads and views), land constructed. flood raising is the preferred Mitigation Raised defences on the Old River Tees and blocking the Building defences on the Old River Tees option. This is most Option 4 would Option 2 underpasses in the railway embankment which follows potentially creates another hazard (breach of likley to look like option require the areas the southern boundary of the TMY. This effectively makes these defences). In addition, blocking the 4 (a compromised of lowest risk to the railway embankment a flood defence structure. underpasses converts the railway embankment option 1). Evidence be developed into a flood defence. The ability of this from regeneration plans first. Additional embankment to act as a flood defence will need at Reclamation Pond mitigation to be investigated. There will also be an (Seal Sands) indicates measures can additional cost with these options. that large scale land be introduced raising is possible and over time for the The standard of the properties may be that developers are remaining areas. compromised by being surrounded by the land willing to invest in this fill to the north, raised defences to the east and for the development of Costs - land railway embankment to the south. these sites. Options 2 raising = high. Mitigation The same as option 2 but raising the land between the The same as option 2 applies but there will be and 3 potentially Resilience Option 3 TMY and the River Tees to prevent tidal overtopping (at an additional cost of raising the land between present new hazards to measures the 1 in 200+cc event) via a low point in the east. the TMY and River Tees. the site (breaching of accommodated Mitigation Moderate amount of land capping (600mm) across the The urban design and yield of the site will be defences) and leave within Code for Option 4 majority of the site. Some threshold raising for individual affected by this option. There will also be the site surrounded by Sustainable buildings. Flood resilience measures for development will associated land rasing costs but not as much as embankments. Homes be required in other areas. the pervious three options. requirements 33

4.3 Bowesfield North Phase 1 and Phase 2

4.3.1.1 There are three development sites on the River Tees, Bowesfield North (Phase 1 and Phase 2), Boathouse Lane and Chandlers Wharf. Bowesfield North is upstream of Boathouse Lane and then Chandlers Wharf.

4.3.1.2 Bowesfield North has been assessed separately from the other two sites as it is further upstream. However, development here will have an impact on the downstream sites. These impacts are included in the text below.

4.3.1.3 Figures showing the site boundary and the modelled flood extents, depths and hazard can be seen in Appendix A, Figures A19 to A24.

4.3.2 Site background

Site description

4.3.2.1 The proposed development on the site of the former Bowesfield North industrial estate comprises of two large areas totalling 330,000m2 and two smaller fragments, 6800m2 and 5700m2. These sites are located on the southern boundary of Stockton, between the natural floodplain upstream of Stockton and the A66 road and rail bridges.

4.3.2.2 The minimum elevation of the Bowesfield North (Phase 1 and 2) is 3.2m AOD although most of the site lies above 6m above ordnance datum (AOD). Bowesfield North (Phase 1 and 2) falls to some extent within the Environment Agency's Flood Zone 2 and 3 maps (see Figure 7).

4.3.2.3 There are no formal flood defences along this section of the River Tees. The River Tees barrage, constructed in 1995 primarily to provide a range of environmental benefits to the tidal river Tees at Stockton, provides an element of flood protection from high sea levels. In general, the barrage was designed to maintain upstream flood levels within a narrow designated range.

4.3.2.4 Considering the low elevation above sea level, the proposed development site is potentially at risk of flooding during extreme tidal events, in addition to fluvial events.

4.3.3 Modelling

Modelling approach

4.3.3.1 The existing (1D) Tees ISIS model does not include a representation of the floodplains in the Bowesfield North (Phase 1 and 2) area. A key part of the modelling therefore involved how best to incorporate these floodplain areas into the model. A linked 1D-2D approach was chosen as this would be best able to model the expected complex floodplain flow routes in the vicinity of the

sites. While the 2D (TUFLOW) component of the model was entirely new, the ISIS component of the model was adapted from the existing risk model of the River Tees (DownDesign_def.DAT)3 supplied by the Environment Agency.

4.3.3.2 Modelling was undertaken to obtain flood outlines and flood depths to describe the existing risks. The following fluvial events were used: • The present day (i.e. without climate change added), 1 in 25 year fluvial event with a mean high water spring (MHWS) tidal boundary. • The present day, 1 in 100 year fluvial event with a MHWS tidal boundary. • The present day, 1 in 1000 year fluvial event with a MHWS tidal boundary. • A future, 1 in 100 year fluvial event with the effect of climate change added. To represent climate change, a 20% increase in flows was added to the 1 in 100 year fluvial flow and 900mm of sea level rise was added to the MHWS boundary. These conditions represent the predicted effects of climate change over 100 years time.

4.3.3.3 The lower sections of the Tees are controlled by the tidal barrage. Therefore, it is also appropriate to model a tidal flood event. Flood outlines and depth grids were produced at the proposed development sites for two extreme tidal events: • The present day, 1 in 200 year tidal event in conjunction with a 1 in 5 year fluvial event. • Future, 1 in 200 year tidal event with the effects of climate change added, in conjunction with a 1 in 5 year fluvial event (plus a 20% increase in the flow for climate change).

4.3.3.4 In areas where there is a combination of fluvial and tidal sources of flooding, it is usual to examine the modelling results to ascertain what the dominant flood scenario should be (see table 4.2). This dominant scenario should be used in the design of the site, but it should be noted, that climate change may alter the key flood risk mechanism.

Existing risk modelling results

4.3.3.5 The following section describes the modelling results for Bowesfield North. Before this, Table 3 shows a comparison of the modelled tidal and fluvial levels along with the tide levels for the Tees Estuary. Comparing these levels allows a conclusion to be made as to whether the site is fluvially or tidally dominated.

4.3.3.6 It can be concluded that Bowesfield North is fluvially dominated as the current and future fluvial event is greater than the current and future tidal events. Conversely, Boathouse Lane and Chandlers Wharf is tidally dominated as the current and future Tees Estuary levels are greater than the current and future fluvial levels.

3 This is a defended model of the lower reaches of the River Tees which resulted from a review of the full River Tees model carried out by JBA Consulting (2006s2143). 35

4.3.3.7 The 1 in 200 year tidal combined with the 1 in 5 year fluvial event shows what happens when the Tees Barrage is raised to hold back the 1 in 200 year tide level. The 1 in 5 year fluvial flow backs up, producing the flood levels shown in Table 4.2. Table 3 - Flood levels for the sites on the River Tees Tide levels at 1 in 200 tidal 1 in 100 year Outcome Tees mouth event fluvial flood (mAOD) combined with (mAOD) the 1 in 5 year fluvial flood (mAOD) Present Future Present Future Present Future day event day event day event Bowesfield 4.2 5.1 5.0 5.5 4.3 5.5 Fluvial North dominated Boathouse 4.2 5.1 4.8 5.4 3.9 4.8 Tidal Lane and dominated Chandlers Wharf

4.3.3.8 The hydraulic model results have provided the following conclusions (see Figures A18 to A21 for the flood extents and depths):

4.3.3.9 The Bowesfield North Phase 1 and 2 sites are not flooded during a 1 in 25 year fluvial event so should not be classified as functional floodplain.

4.3.3.10 The Bowesfield North Phase 1 site is flooded by direct overtopping of the Tees during a 1 in 100 year fluvial event with depths of up to 1m. However, the Phase 2 site is not flooded until the 1 in 1000 year event.

4.3.3.11 The 1 in 100 year modelled flood extent compares well to that of the Environment Agency's Flood Zone 3 Map upstream of Bowesfield North (Phase 1 and 2) but is smaller in the vicinity of the these sites (see Figure 7).

4.3.3.12 A future 1 in 100 year fluvial event (with sea level raised by 900mm to account for 100 years of climate change) is predicted to have a significantly larger flood extent than the present day fluvial 1 in 100 year event. This suggests that the Bowesfield North (Phase 1 and 2) development site is sensitive to a future increase in sea levels and fluvial flows (see Figure 8).

4.3.3.13 Flood depths within Bowesfield North (Phase 1 and 2) range from <0.1m to 2.2m for both the future fluvial and future tidal scenarios, although depths in the future tidal cycle can be as much as 0.5m higher than those of the future fluvial scenario at a particular point. The future tidal event has therefore been used in this assessment as a worse case scenario.

4.3.3.14 Hazards ratings range from 0.5 to 2.5 for both future scenarios ( see section 4.1.2 or a description of hazard ratings). 36

Developed site modelling results

4.3.3.15 Land raising across all of the proposed development sites to remove the flood risk from the future tidal 1 in 200 year event, increased river levels by 20mm in proximity to the Bowesfield North Phase 1 and 2. Modelling results for this event are subject to some uncertainty due to the extreme nature of the event and the indicative, precautionary modelling methods used. However, Environment Agency policy is that there should be no incremental loss of floodplain which will lead to an increase in downstream levels (no matter how small the increase).

4.3.3.16 By just developing at the proposed Bowesfield North (Phase 1 and 2) site (and not Boathouse Lane and Chandlers Wharf), there was a slight increase in the future fluvial 1 in 100 year extent at Chandlers Wharf, although the flood depth in this addition area is less than 10mm.

4.3.3.17 In addition, the modelling showed a slight increase in levels immediately upstream of the Bowesfield North (Phase 1 and 2) site, with increases of 30mm, if the site was raised.

4.3.3.18 The maximum flood levels at the proposed Bowesfield North (Phase 1 and 2) development sites (without freeboard – see glossary) are: • Future fluvial 1 in 100 year – 5.50m AOD • Future Tidal 1 in 200 year – 5.50m AOD

4.3.4 Development implications and issues

Flood Zones and functional floodplain

4.3.4.1 The original SFRA completed in 2007 stated that the majority of the Bowesfield North (Phase 1 and 2) site is in Flood Zones 1 and 2 but the river frontage was in 'candidate' Flood Zone 3b (functional Floodplain). The current Environment Agency Flood Zones are shown in Figure 7.

Figure 7 - Flood Zone maps at Bowesfield North (Phase 1 and 2)

Bowesfield North Phase 2

Bowesfield North Phase 1

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Flood Zone 2 V.3.15

Flood Zone 3 V.3.15

4.3.4.2 The river modelling completed for this Level 2 SFRA has provided flood extents for a number of different flood events. These are representative of the Environment Agency Flood Zones because there are no flood defences in place in this reach of the river. It is also worth noting that the previous modelling undertaken for the flood map is a bank to bank model, where the flood waters are constrained within the channel. The more detailed model includes these floodplain areas in a 2D representation, and as a result lowers flood levels within these sites.

4.3.4.3 The modelled extents are as follows and are shown in Figure 8 below and Figure 18 in Appendix A: • 1 in 25 year flood extent – representative of Flood Zone 3b (functional floodplain)

• 1 in 100 year flood extent – representative of Flood Zone 3a • 1 in 1000 year flood extent – representative of Flood Zone 2

4.3.4.4 A flood extent that takes into account climate change was also modelled for the 1 in 100 year event. The event is very similar to the 1 in 1000 year event (see Figure A18 for the climate change extent). This will be referred to as the ‘1 in 100 year event +cc'.

4.3.4.5 These modelled flood extents will need to be approved by the Environment Agency before the conclusions made in this report can be finalised.

Figure 8 - Modelled extents at Bowesfield North (Phase 1 and 2)also see Figure A18 in Appendix A

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4.3.4.6 This modelling shows that no part of the Bowesfield North (Phase 1 and 2) site is at risk from the 1 in 25 year event (Flood Zone 3b). This means that neither Phase 1 nor 2 is within the functional floodplain.

4.3.4.7 Part of the Phase 1 site, next to the Tees is at risk from the 1 in 100 year event (Flood Zone 3). PPS25 advises that these areas should only be developed in exceptional circumstances (Exception Test). If this area is to be developed, the Exception Test stipulates that the development should not increase flood risk elsewhere. Supporting this, the Environment Agency insists that compensatory flood storage is required for any development within Flood Zone 3 (1 in 100 / 200 year event). Due to the problems associated with finding and purchasing land for compensatory flood storage, this study advises against development here.

4.3.4.8 Within the master planning stage of the development it may be possible to design around the 1 in 100 year flood risk area leaving this area as open floodplain and amenity space. This area is relatively small and generally close to the Tees.

4.3.4.9 The north east part of the Phase 2 site is at risk from the 1 in 1000 year event (Flood Zone 2). PPS25 states that these areas are appropriate for residential development, subject to a Flood Risk Assessment and as long as no alternative lower risk sites can be found (Sequential Test). If developed, this area will require flood risk mitigation measures up to the 1 in 100+cc flood event (plus freeboard).

4.3.4.10 Part of the Bowesfield North (Phase 1 and 2) development is driven by a regeneration agenda. The approach taken in the SFRA is to bring forward regeneration sites to be assessed against the flood risk requirements outlined in part c) of the Exception Test, assuming planning justification can be found. However, where part of the site is greenfield, there may not be the planning justification to support passing the Sequential Test. Provision of housing in a Flood Zone 3 or even Flood Zone 2 area on a greenfield site will need to demonstrate that there are no alternative sites at lower risk.

4.3.4.11 The modelling results showed that flood depths range between <0.1m to 2.2m for the 1 in 100 year event +cc. The greatest depths are found in the Bowesfield North Phase 1 site (1-2.2m depths). This is the area which is at risk from the 1 in 100 year event. This SFRA recommends that this area is kept free from development. The remaining area is at risk from depths ranging from 0 to 1m from the 1 in 1000 year event. It should be relatively easy to manage flooding of these depths through land and floor raising so that they do not pose a significant risk to people.

4.3.4.12 The modelling results for the 1 in 100 year event +cc gave hazards ratings ranging from 0.5 to 2.5, with the majority in the 1.25-2.0 (danger for most) category. The 'danger for most' hazard rating is due to high velocities and depths that are expected from a large river in flood. Again, the area with the highest hazard rating is the area proposed to be kept free from development.

4.3.4.13 The remaining proposed development area (at risk from the 1 in 1000 year event) would not be at risk from this hazard if the land and floor raising put the development above the 1 in 100 year +cc flood level.

Sustainability Appraisal Indicator 2 Can the development be made safe once developed and are residual risks acceptable?

Access and Egress

4.3.4.14 Figure 8 and Figure A18 shows the maximum extent of the modelled flood events. The flood extents are confined to the east section of the Bowesfield North (Phase 1 and 2) site. The west part of Bowesfield North (Phase 1 and 2) is on higher ground and free from flooding.

4.3.4.15 Safe access and egress can therefore be made through the roads to the west (Westland Way and Bowesfield Lane).

4.3.4.16 As Bowesfield North (Phase 1 and 2) is a SHLAA allocation (residential development) on an industrial estate, there could potentially be an increase in the number of people requiring assistance from the emergency services. This would be in the area at risk from the 1 in 1000 year event. If this area is developed, mitigation measures are proposed which may include land and floor raising. This would remove the proposed residential properties from the areas at risk.

Residual risks

4.3.4.17 There are no raised defences for the Bowesfield North (Phase 1 and 2) proposed development site. If planning justification can be found, the proposed mitigation is to leave the area at risk from the 1 in 100 year event, free from development, but to design the remaining areas to the 1 in 100 year+cc event. There are no obvious residual risks associated with this option.

4.3.4.18 The flood risks used in design are fluvial, but there is also a residual risk of tidal flooding. The Tees Barrage can be raised to hold back the effects of a tidal flood (see section 2.2). The gates can be raised to 5m AOD, which means they can be raised above the 1 in 200 and 1 in 1000 year tidal flood event. However, the barrage has a limit and at the future 1 in 200 year event, there is a small amount of overtopping. This would have very little impact on upstream levels as only a small amount of overtopping, for a short period would occur.

4.3.4.19 The main residual risk is if the barrage fails or the gates are mistakenly left open. The chance of this occurring is low, the evidence for this is provided in section 2.2. The 1 in 200 year + cc tidal flood level is 5.1m AOD. If the barrage did fail during this tidal event, the areas of Bowesfield North Phase 1 and 2 below this level would flood. A 1 in 20 year fluvial flood was modelled with only one of the barrage gates left open. This was shown to increase river

levels by 20mm at Bowesfield. This residual risk is manageable within the freeboard allowance of the mitigation options.

Sustainability Appraisal Indicator 3 Is the increase in risk posed by climate change easily adapted to and/or can the development build in climate change resilience

4.3.4.20 The 1 in 100 year + climate change flood extent for Bowesfield North (Phase 1 and 2) is very similar to the current 1 in 1000 year event. This indicates than in 100 years time, the 1 in 1000 year event will become the 1 in 100 year event.

4.3.4.21 The flood depths for the climate change event are in the region of 0.5m. It should be possible to ground and floor raise the area at risk from 0.5m flood depths. This will remove the development from the 1 in 100 year+cc flood extent. There is an area within Phase 1 where flood depths are 1 to 2m, but this is to be left undeveloped (see Figure A21 in Appendix A).

Sustainability Appraisal Indicator 4 Can flood risk to adjacent lands stay the same or be reduced as a result of the development and its mitigation measures.

4.3.4.22 PPS25’s Practice Guide states that where development is proposed in flood risk areas, there may be a need to establish whether there is land available for compensatory flood storage, in order to ensure that overall flood risk does not increase. This means that development within the 1 in 100 year flood extent would require compensatory flood storage through the loss of floodplain.

4.3.4.23 It has been established in the SFRA that the incremental loss of floodplain along the Tees should be resisted and adequate compensation provided from areas not currently at risk in the 1 in 100 year event. This makes complete development of the site more difficult, as an area will need to be provided to compensate for loss of floodplain.

4.3.4.24 If compensatory flood storage can not be provided, development within the 1 in 100 year extent of Bowesfield North Phase 1, would increase flood risk downstream. There does not appear to be any land available for floodplain compensation, so it is recommended that the area covered by the 1 in 100 year flood extent is left undeveloped.

4.3.4.25 Bowesfield North Phase 2 is only at risk from the 1 in 1000 year event (Figure 8 and A18). Compensatory storage would not be required in this area, so this would not be a constraint to development. However, planning justification for development here (development in a greenfield flood risk area), could still be difficult.

Summary and proposed mitigation strategy Bowesfield North Phase 1

4.3.4.26 Figure 7 shows that around half of the Bowesfield Phase 1 site is within current Environment Agency Flood Zone 3. As residential development is proposed for this area, the Exception Test will need to be passed if this is to be allocated.

4.3.4.27 New hydraulic modelling has been completed for this SFRA. This shows that a much smaller section is at risk from the 1 in 100 year flood (equivalent to Flood Zone 3) as seen in Figure 8. A small section is also at risk from the 1 in 1000 year flood extent (Flood Zone 2).

4.3.4.28 It is proposed that the area at risk from the 1 in 100 year flood extent is retained as floodplain as the risk of flooding is high, the area is greenfield and it would be very difficult to find compensatory flood storage if this area were to be developed.

4.3.4.29 The low flood hazard and depths mean that the part of the site at risk from the 1 in 1000 year event can be developed safely though a combination of ground and floor raising. This mitigation strategy will raise the area at risk from the 1 in 1000 year flood above the 1 in 100 year+cc level.

4.3.4.30 The residual tidal risk is described under Bowesfield North Phase 2.

Bowesfield North Phase 2

4.3.4.31 Figure 7 shows that the west part of the Bowesfield Phase 2 site is within Flood Zone 3 and a further section is in Flood Zone 2. As residential development is proposed for this area, the Exception Test will need to be passed if this site is to be allocated.

4.3.4.32 The new modelling completed for this SFRA shows that Bowesfield Phase 2 is only at risk from the 1 in 1000 year flood and the 1 in 100 year +cc event. It is recommended that this area is also left undeveloped because: • The river frontage part of the proposed development area is greenfield • This river frontage area will be at greater risk of flooding in the future due to climate change • It will be difficult to show that the river frontage part of the site has passed the Sequential Test as there are likely to be other greenfield areas in Flood Zone 1 or other brownfield areas in Flood Zone 2.

4.3.4.33 This final point indicates that it is unlikely that planning justification can be found. However, the low flood depths and hazard in this area means that a combination of ground and flood raising will be able to raise any future development above the 1 in 100 year+cc flood level (which is greater than the 1 in 1000 year flood level).

4.3.4.34 For floor raising, finish floor levels should be set at 5.5m AOD plus freeboard or factor of safety. This level is based on the 1 in 100 year event +cc.

Compensatory floodplain storage would not be required here, as the Environment Agency stipulate that floodplain compensation only needs to be completed up to the 1 in 100 year event.

Mitigation Summary Table Site Mitigation Options Constraints Preferred Outcome Development sequence and indicative cost Bowesfield North The area at risk from the 1 in 100 year Not developing in the area at risk from Residential development on The areas within Flood Zone 1 Phase 1 flood should be retained as floodplain the 1 in 100 year flood would reduce the the site with floor raising and should be developed first. (see Figure 8). The remaining flood risk yield value of this site potentially making flood resilience in the area at Within the areas at flood risk, area can be mitigated up to the 1 in it less desirable to developers. risk from the 1 in 1000 year developing Phase 1 at the 100+cc event through a combination of flood following the Sequential same time as Phase 2 would floor raising and flood resilience. Developing in the 1 in 1000 year flood Test. The area at risk from the enable development ground risk area will require additional 1 in 100 year flood should be levels and access roads to tie investment for floor raising and flood retained as floodplain. in with each other. resilience building techniques. This is relatively small scale so is not a major Costs – a small section of floor constraint. raising and flood resilience = low As development within the 1 in 100 year extent is not proposed, compensatory storage will not be required. Bowesfield North Due to the relatively low flood depth, Not developing in the area at risk from The area at risk from the 1 in The areas within Flood Zone 1 Phase 2 flood hazard and flood extents, the the 1 in 1000 year flood would remove 1000 year event left should be developed first. section of the site at risk from the 1 in around a third of the developable area. undeveloped. 1000 year flood could be developed This area is likely to be the most If it can be shown that this site safely through floor raising and flood desirable as it is a water front location. can pass the ST, floor raising resilience. However, this area is and flood resilience will be greenfield and it will be difficult for it to Developing in the 1 in 1000 year area required. This should tie in pass the Sequential Test. would require extensive mitigation with the area at risk from the 1 measures. in 1000 year flood in Bowesfield Phase 1. If development is to go ahead in this area, it will be difficult to pass the Costs (if developed) – Sequential Test and the area at risk is extensive floor raising and greenfield. flood resilience = high

4.4 Boathouse Lane and Chandlers Wharf

4.4.1.1 Figures showing the site boundary and the modelled flood extents, depths and hazard can be seen in Appendix A, Figures A18 to A24.

4.4.2 Site background

Site description

4.4.2.1 Boathouse Lane and Chandlers Wharf can be split into a number of separate, smaller sites. Some of these have outline planning permission. Figure 9 shows the separate sites with the names that they will be referred to. It is important to consider the site as a whole so that land use and development layout can be influenced by the different levels of flood risk. However, in places, the sites will be referred to separately as some already have outline planning permission and also because it helps to describe where the different levels of flood risk are.

4.4.2.2 The proposed development sites at Boathouse Lane are located between the A66 road and rail bridges and Victoria Road bridge. The proposed development site at Chandlers Wharf is located downstream of Victoria Road bridge (see Figure 9).

4.4.2.3 Elevation of the proposed development sites at Boathouse Lane and Chandlers Wharf are between 3.5 and 4m AOD and the majority of the land lies below 5m AOD. The majority of these sites are contained within the Environment Agency's Flood Zone 3 (1 in 100 year) outline.

4.4.2.4 Considering their low elevation, the proposed development sites are potentially at risk of flooding during extreme tidal events, in addition to fluvial events.

Figure 9 – Boathouse Lane and Chandlers Wharf development areas

Chandlers Wharf

Boathouse Lane 1

Boathouse Lane 2

Boathouse Lane 3

Boathouse Lane 4

OS Licence: © Crown Copyright 100023297 (2010).

4.4.3 Modelling

Modelling approach

4.4.3.1 The modelling approach is the same as described in the Bowesfield section. The same figures (Figures A18 to A24) also cover these sites.

Existing risk modelling results

4.4.3.2 Hydraulic modelling of the River Tees was completed to understand the levels of flood risk at the Boathouse Lane and Chandlers Wharf sites. The following conclusions can be made from the modelling results.

4.4.3.3 The Boathouse Lane and Chandlers Wharf sites are not flooded during a 1 in 25 year fluvial event so, no part of the site should not be classified as functional floodplain. The Boathouse Lane 2 is flooded when the River Tees overtops its bank during the 1 in 100 year fluvial event. Parts of Boathouse Lane can flood to depths of up to 1m.

4.4.3.4 Flooding of the Chandlers Wharf site does not occur until the 1 in 1000 year event.

4.4.3.5 The modelled 1 in 100 flood extent is smaller than Flood Zone 3 within the Boathouse Lane and Chandlers Wharf development sites.

4.4.3.6 A future 1 in 100 year fluvial event (with sea level raised by 900mm to account for 100 years of climate change) has been modelled. This is predicted to have a significantly larger flood extent than the present day 1 in 100 year fluvial event. This suggests that the development sites are sensitive to future sea levels.

4.4.3.7 Flood depths within the proposed Boathouse Lane and Chandlers Wharf development sites range from <0.1m to 1.5m for the future fluvial scenario and 0.2-2m for the future tidal scenario. The mitigation strategy of this study has taken the conservative approach and is therefore based on the tidal event.

4.4.3.8 Hazard ratings range from 0.5 to 2.5 for both future scenarios.

Developed site modelling results

4.4.3.9 Scenarios have been modelled with the Chandlers Wharf and Boathouse Lane sites fully developed. This has been done so that it can be seen whether regeneration and development of these sites will have a negative impact on others.

4.4.3.10 Raising the proposed development sites, within the modelling has shown that the future 1 in 200 year tidal event increases water levels in the River Tees by: • 20mm in proximity to the Bowesfield North (Phase 1 and 2) • 40mm in proximity to Boathouse Lane and Chandler Wharf

4.4.3.11 However, modelling results for this event are subject to some uncertainty due to the extreme nature of the event and the way that existing floodplain has not been fully included in the model.

4.4.3.12 Raising the land across the proposed development sites (Bowesfield, Boathouse Lane and Chandlers Wharf) raised river levels, but this had little impact on the extent of flood risk outside of the proposed development sites. However, flood depths did increase elsewhere. Flood depths were modelled to increase by less than 50mm in some locations. As a result, compensation for the loss of floodplain will be a requirement if the areas at risk from the 1in 100 year event are developed.

4.4.3.13 In summary, the modelling has shown that raising ground levels on the proposed development sites only increases downstream flood levels by a small degree. However, one of the requirements of the Exception Test is that flood risk should not be increased to others and downstream flood risk should be reduced where possible. Consequently, the Environment Agency requires floodplain compensation where development is proposed in Flood Zone 3.

4.4.3.14 The parts of the sites that are at risk of flooding from the 1 in 1000 year event (and climate change events) should be developable with some simple flood risk mitigation measures.

4.4.3.15 Development within the 1 in 100 year extent will be more problematic as an area for floodplain compensation would be required. This is likely to be a constraint to development as there are no immediately available locations for compensatory floodplain storage. A floodplain storage option is further constrained by the flood depths expected at this site (up to 2m in places). Guidance provided in the Level 1 SFRA states that it is difficult to mitigate against flood depths above 1.5m (due to the volumes of infill required) and advises against it (see Section 1.1 of the Volume III SFRA). As a result, raising ground levels is not seen as a practical option for these sites.

4.4.4 Development implications and issues

Flood Zones and functional floodplain

4.4.4.1 The original SFRA completed in 2007 stated that 100% of the Boathouse Lane site was in Flood Zone 3a (Chandlers Wharf is a new site). The current Environment Agency Flood Zones (September 2009) are shown in Figure 10.

Figure 10 - Flood Zone maps at Boathouse Lane and Chandlers Wharf

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Flood Zone 2 V.3.15

Flood Zone 3 V.3.15

4.4.4.2 New modelled flood extents were generated for these sites (see Figure 11 below and Figure A18 in Appendix A).

4.4.4.3 This modelling shows that none of the sites area at risk from the 1 in 25 year event (Flood Zone 3b). This means that these sites are not within the functional floodplain.

4.4.4.4 Boathouse Lane 2 is at risk from the 1 in 100 year event (Flood Zone 3). PPS25 advises that residential development in these areas should only be developed in exceptional circumstances (Exception Test). Boathouse Lane 1

and 3 and parts of Boathouse Lane 4 and Chandlers Wharf are at risk from the 1 in 1000 year event (Flood Zone 2). PPS25 states that these areas are appropriate for residential development, subject to a Flood Risk Assessment and as long as no alternative lower risk sites can be found (Sequential Test).

4.4.4.5 Boathouse Lane 2 is on brownfield land so it could pass one element of the Exception Test. The following sections of this report will summarise whether it would pass part c) of the Exception Test (i.e. it will be safe without increasing flood risk elsewhere). The area at risk from the 1 in 100 year event will require mitigation measures up to the 1 in 100 year event +cc ( and with freeboard added) if it is to be developed.

Figure 11 - Modelled extents at Boathouse Lane and Chandlers Wharf (also see Figure A18 in Appendix A)

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4.4.4.6 The modelling results showed that flood depths range between <0.2m to 1.5m for the 1 in 100 year event +cc and 0.2-2m for the equivalent tidal event. The greatest depths are found in Boathouse Lane 2 (see Figure A21 in Appendix A). This is the area at risk from the 1 in 100 year event. The hazard rating for

this area is classed as ‘danger for most’. The preferred option if this area were to be developed is for first floor habitation only (allowing the first 2m to flood). This would significantly impact on urban design. Alternatively this area could be left as open green amenity space.

4.4.4.7 The remaining Boathouse Lane sites and Chandlers Wharf which are at risk from the 1 in 1000 year event, will experience depths more in the region of 0.5m. The hazard rating is classed as ‘danger for some’. The depth and hazard risk is similar to the 1 in 100 year+cc event. The development should manage flooding up the climate change event. Managing these flood depths (~0.5m) should be possible through ground and floor raising. This would not significantly impact on urban design.

Sustainability Appraisal Indicator 2 Can the development be made safe once developed and are residual risks acceptable?

Access and Egress

4.4.4.8 Figure 11 and A18 in Appendix A shows the maximum extent of the modelled flood events. This shows that large parts of the sites are at risk from the 1 in 1000 year event. This would prevent safe access and egress if left as it is.

4.4.4.9 The proposed mitigation strategy is to ground and floor raise in the 1 in 1000 year flood risk area and first floor development only in the 1 in 100 year flood area. This means that the finish floor levels will be set above the 1 in 100 year+cc flood level. Access routes should also be set above these levels.

4.4.4.10 This may be difficult for Boathouse Lane 2, where there will be no ground raising. It is therefore proposed that a pedestrian walkway is integrated in to the development, above the 1 in 100 year +cc flood level. This would lead to the higher ground on Bridge Road and Parkfield Road. This access route should be integrated into an emergency flood management plan. For the remaining Boathouse Lane sites and Chandlers Wharf, it should be possible to set access roads above the 1 in 100 year+cc flood level as some ground and floor raising is proposed here.

4.4.4.11 As the current proposal is residential development on an employment site, there could potentially be an increase in the number of people requiring assistance from the emergency services. If this area is developed, mitigation measures are proposed which would include land raising and raising drives and access roads. This would remove the proposed residential properties from the areas at risk and allow people to leave their properties without the help of the emergency services.

Residual risks

4.4.4.12 There are no raised defences for the Boathouse Lane and Chandlers Wharf development sites. The proposed mitigation strategy is to provide habitable floor levels above the climate change event. This will involve allowing the 54

ground floor area to flood in Boathouse Lane 2 and flood raising and flood resilience in the other sites. There is a residual risk here if this ground floor can still be accessed in times of flood e.g. if the ground floor area is used for garages. It should be possible to safely manage this residual risk through design and flood awareness through an emergency plan for the site.

4.4.4.13 Section 4.5 describes the Tees Barrage and the residual risks associated with it.

Sustainability Appraisal Indicator 3 Is the increase in risk posed by climate change easily adapted to and/or can the development build in climate change resilience?

4.4.4.14 The flood depths for the climate change event are in the region of 0.5m for Chandlers Wharf and Boathouse Lane 1, 3 and 4. The relatively shallow depths means raised ground and floor levels can be designed to put this development above the climate change flood level. Depths are between 1 and 2m for Boathouse Lane 2 (see Figure A21 in Appendix A). First floor only development (above the climate change level) is proposed for this area.

Sustainability Appraisal Indicator 4 Can flood risk to adjacent lands stay the same or be reduced as a result of the development and its mitigation measures.

4.4.4.15 PPS25’s Practice Guide states that where development is proposed in flood risk areas, there may be a need to establish whether there is land available for compensatory flood storage, in order to ensure that overall flood risk does not increase. This means that development within the 1 in 100 year extent would require compensatory flood storage through the loss of floodplain. However, the development should be designed to the 1 in 100 year +cc level (floor levels should be above this climate change level).

4.4.4.16 Development within the 1 in 100 year extent of Boathouse Lane 2 would increase flood risk to others. A location for compensatory flood storage would therefore need to be found.

4.4.4.17 From an initial review, there is no land available for this purpose. An alternative option would be first floor only habitation. This would require the ground floor to be open and flood up to the 1 in 100 year flood level (the first 2m of the development). Beyond this, floor levels would be raised up to the 1 in 100 year+cc level.

4.4.4.18 The remaining site areas at risk from the 1 in 1000 year event would need to be raised above the 1 in 100 year event +cc flood level. This will not increase flood risk downstream in the 1 in 100 year event and would therefore not require compensatory storage.

Summary and proposed mitigation strategy Chandlers Wharf

4.4.4.19 The Environment Agency's Flood Zone maps show that around half of this site is within Flood Zone 3 and a further section is in Flood Zone 2. For residential development, the Exception Test will need to be passed for this site to be allocated (according to PPS25).

4.4.4.20 New hydraulic modelling was undertaken for this study. This shows that the site is only at risk from the 1 in 1000 year flood. The low flood depths and hazard within this extent means that flood mitigation measures (ground and floor raising) can be designed to put the development above the 1 in 100 year+cc flood level. This should not significantly impact on yield values within the site but there will be an additional cost associated with it. As there is no part of the site is within the 1 in 100 year flood extent, compensatory flood storage will not be required.

Boathouse Lane

4.4.4.21 The Environment Agency's Flood Zone maps sow that around three quarters of the site is within Flood Zone 3, with a further, smaller section in Flood Zone 2. According to PPS25, the Exception Test should be passed if this site is to be allocated.

4.4.4.22 New hydraulic modelling completed for this site showed that a similar area as the Flood Zones is a risk of flooding, but predominantly from the 1 in 1000 year event. However, the Boathouse Lane 2 area is at risk from the 1 in 100 year flood (see Figure 11 and A18 in Appendix A). This more detailed modelling supports a revision to the Flood Zone map should be considered.

4.4.4.23 Similar to Chandlers Wharf, the flood depths within Boathouse Lane 1, 3 and 4 are low enough to be managed through ground and floor raising (up to the 1 in 100+cc flood level). However, Boathouse Lane 2 is at risk from greater depths and is at risk from the 1 in 100 year flood. This means that ground and floor raising would be problematic as compensatory flood storage would be required. It has been established in the SFRA that the incremental loss of floodplain should be resisted and adequate compensation provided from areas not currently at risk in the 1 in 100 year event. This makes complete development of the site more difficult, as an area will need to be provided to compensate for loss of floodplain if Boathouse Lane 2 is to be developed. Two options are therefore appropriate:

4.4.4.24 1) Boathouse Lane 2 (see Fig 9) is retained and not developed (or an equivalent area at the same level).

4.4.4.25 2) Development goes ahead but the mitigation measures should include some open space and first floor only habitation up to the 1in 100 year flood. Beyond this, flood resilience and floor raising should reduce the risk of flooding up to the 1 in 100 year+cc event. If this were undertaken, compensatory flood storage would not be required as the 1 in 100 year flood risk area would be allowed to flood. 56

4.4.4.26 If the access and egress along with floor raising can be designed, the development could go ahead safely, without increasing risk elsewhere and therefore passing part c) of the Exception Test.

Mitigation Summary Table Site Mitigation Options Constraints Preferred Outcome Development sequence and indicative cost Boathouse The hydraulic modelling shows that these sites are at There will be an additional Development of the Residential could be developed on these Lane 1, 3, risk from the 1 in 1000 year flood. For the 1 in 100 cost by requiring mitigation sites with floor raising sites. For Boathouse Lane 2, first floor only 4 and year +cc flood, depths are generally below 0.5m measures to remove the and flood resilience up habitation is one of the options proposed Chandlers (although depths up to 1m could be expected in parts risk from the sites up to the to the 1 in 100 year+cc (see below). This means that flats rather Wharf. of Boathouse Lane 3 and Chandlers Wharf). This 1 in 100 year+cc event. flood event. than houses may need to be developed. means a combination of floor raising and flood Raising the access roads This site separates Boathouse Lane 1 and resilient mitigation measures should remove the risk to this level will also be an 3. The developer should be made aware of from these sites up to the climate change event. additional cost. These the different development types required in requirements could make this area. A pedestrian access and egress An access and egress roads, raised above the the site less desirable to route for Boathouse Lane 2 may be climate change event will be possible due to the low developers. required through Boathouse Lane 3 and 1. flood depths and the proposed floor raising of the Boathouse Lane 4 and Chandlers Wharf development. Floodplain compensation could be developed independently. will not be required as these areas are at risk from Costs - Floor raising, flood resilience and the 1 in 1000 year flood, raised access roads = medium. not the 1 in 100 year flood. Boathouse The new hydraulic modelling shows that this is the Retaining this area for Leaving the ground Boathouse Lane 2 may require a different Lane 2 only site within the 1 in 100 year flood extent. This green space would floor open to flood. This development type (possibly flats) as first means that if the site were to be fully developed, significantly reduce the area could be used for floor only habitation is proposed. This will floodplain compensation would be required. Flood yield value of this land. car parks or amenity need to be integrated into the adjacent depths for the climate change event can be up to 2m Compensatory flood space. Flats may need development plans. Raised pedestrian in this area. These depths will need to be mitigated storage would be a difficult to be developed in this walkways may also be required through against. to find and a significant area in order to obtain Boathouse Lane 1 and 3. cost for the developer. First sufficient housing The options for this site are: floor habitation would numbers. Raised Costs - First floor only development, floor 1) Retain this area for green/amenity space. reduce the yield value and pedestrian walkways raising, flood resilience and raised 2) Raise ground levels and provide compensatory may reduce the rather than roads would pedestrian walkway = high. flood storage. attractiveness of the be required through the 3) Leave the ground floor (first 2m) open so that this development. surrounding sites. area can flood and have first floor only habitation.

4.5 Seal Sands and Haverton Hill

4.5.1.1 The remaining two areas described in this Level 2 SFRA are at risk of tidal flooding. A tidal model has been produced to predict the extent and severity of flooding here. The site background and modelling descriptions are similar to both areas (Haverton Hill and Seal Sands) so will be described together. The Haverton Hill sites will be described separately from the Seal Sands sites in the 'Development implications and issues' section.

4.5.1.2 Figures showing the site boundary and the modelled flood extents, depths and hazard can be seen in Appendix A, Figures A25 to A31.

4.5.2 Site background

Site description

4.5.2.1 The proposed employment sites considered in this study are distributed along the north bank of the River Tees from the Billingham Beck outfall to the mouth of the Tees. These sites are: • Port of Clarence complex - a number of small sites lying above 4m AOD • North Tees Pools complex – Within this complex this is Dormans Pool which encompasses a Site of Special Scientific Interest (SSSI) pond and marsh area, with land as low as 1.5m AOD. There is also Reclamation Pond which is also low with elevations as low as 1.5m AOD, however, planning permission has been granted to raise to land here to 6m AOD. The remaining proposed sites within this complex generally lie above 5m AOD. • Seal Sands complex at mouth of the Tees - elevations are generally between 4.5 and 5.5m AOD, with some isolated areas as low as 2.5m AOD (Seal Sands 9). • Billingham Reach – number of small sites (totally <0.1k m2), generally lying above 3m AOD, • Casebourne – approximately 0.06k m2 between 2-5m AOD • Haverton Hill – a number of small sites totalling approximately 0.18k m2, lying above 3.5m AOD See Figures 12 and 13 below for the location of these sites.

Figure 12 – Seal Sands development areas

Seal Sands complex

Reclamation Pond

Dormans Pool

North Tees Pools complex

Port Clarence complex

OS Licence: © Crown Copyright 100023297 (2010). Key

Figure 13 – Haverton Hill development areas

Haverton Hill 3 Haverton Hill 2 Casebourne

Haverton Hill 1

Billingham Reach

OS Licence: © Crown Copyright 100023297 (2010).

4.5.2.2 There are official raised defences, unofficial defunct embankments, features that hold back flood waters (railway embankment) and raised river frontages. These can reduce the risk of flooding but also have the ability to increase flood risk. These official and unofficial defences are described in sections 2.1.1 and 2.1.2.

4.5.3 Modelling

Modelling approach

4.5.3.1 The nature of any flooding resulting from extreme sea levels is likely to be critically dependent upon the flow paths followed by floodwater across low- lying areas. This situation is best modelled using 2D hydraulic modelling software and in order to capture key flood flow routes across the floodplain a standalone TUFLOW (2D) model was created.

4.5.3.2 TUFLOW was configured to record water level, depth, velocity and hazard maps of the floodplain at half hour intervals during each model run. Hazard

was configured to the latest DEFRA Guidance on modelling flood hazard4 by using the conservative approach with regard to debris factors. Depth and hazard grids were exported from the TUFLOW results at a definition of 4m and flood outlines were generated from the TUFLOW peak depth grids at the same resolution.

4.5.3.3 More details on the modelling approach and set up can be seen in the modelling technical notes. These have been submitted to the Environment Agency for review and can be obtained upon request.

4.5.3.4 The following extreme sea level events were modelled: • Present day, Tidal 1 in 200 year event (return period 1 in 200 years). • Present day, Tidal 1 in 1000 year event (return period 1 in 1000 years). • Future, 1 in 200 year Tidal event with adjustment for climate change in 100 years time.

Existing risk modelling results

4.5.3.5 The model results demonstrate the following elements of flood risk under existing conditions: • All proposed development sites within the modelled area fall within the future tidal 1 in 200 year event outline with the exception of a number of proposed sites at Port Clarence. Peak future flood depths across much of the area vary widely, but can reach 3.5m deep within a number of proposed development sites. Hazard rating across much of the basin is modelled to pose a “Danger for Most” people. A small area stemming from the left bank of Greatham Creek depicted as ‘Danger for All’ may be attributed to model instability, and this is not considered to influence the results for the areas under investigation. • Modelled future tidal 1 in 200 year peak water levels are typically below 5.13m AOD. The maximum peak water levels within the study area are evident across the central low lying basin between Port Clarence (to the south), A1185 (to the north) and A178 (to the east). Levels are slightly lower (4.25 - 4.5m AOD) for the low lying area between the A178 and the oil refinery. • The present tidal 1 in 1000 year event modelled outline is smaller than that of the Environment Agency's Flood Zone 2 outline in the north east section of the study area. The majority of proposed development sites at Seal Sands (2, 3 5, 6, 7, 8 and 10) fall within Flood Zone 2 but not the modelled extent for the present day 1 in 1000 year event. Along the western and southern sections of the study area the flood outlines are much more comparable. • Flood water first enters the modelled area from Greatham Creek, both upstream and downstream of the bridge and then soon after from a point just upstream of Billingham Reach site 3. Later, additional entry routes at Port of Clarence and the oil refinery in the east of the modelled area occur.

4 SUPPLEMENTARY NOTE ON FLOOD HAZARD RATINGS AND THRESHOLDS FOR DEVELOPMENT PLANNING AND CONTROL PURPOSE – Clarification of the Table 13.1 of FD2320/TR2 and Figure 3.2 of FD2321/TR1. (http://randd.defra.gov.uk/Document.aspx?Document=FD2321_7400_PR.pdf). 62

However, in all scenarios the route from Greatham Creek is the dominant source of flood water throughout the study area; with a flow route south that extends to the Port of Clarence filling the central low lying basin and a route east towards the proposed development sites at Seal Sands.

Developed site modelling results

4.5.3.6 The impact of fully raising the proposed development sites above the future tidal 1 in 200 year event flood levels were modelled to have the following impacts: • There was little impact on the extent of the flood risk outside of the proposed development sites. • There were negligible impacts on peak flood depths across most of the study area but some localised increases, specifically in the vicinity of the oil refinery to the east of the Reclamation Pond site. These increases can be as high as 60cm and are thought to be a result of the proposed development blocking a low lying storage area encompassed in the Reclamation Pond site, which does not fill to capacity in the existing risk scenario.

4.5.4 Seal Sands development implications and issues

Flood Zones and functional floodplain

4.5.4.1 The sites discussed in this section include the Seal Sands complex, Tees Pool complex and Port Clarence complex (see Figure 12 for these areas). Water compatible land use is proposed for the Port Clarence complex. Employment Land use is proposed for Tees Pool and Seal Sands.

4.5.4.2 The current Environment Agency Flood Zone maps show that these sites are affected but not fully covered by Flood Zone 3. Flood Zone 2 extends a little further into the sites (Figure 14). However, the sites are fully surrounded and would be cut off by Flood Zone 3. This is because the employment sites are situated on higher ground which may have been filled in the past.

Figure 14 - Flood Zone map at Seal Sands

OS Licence: © Crown Copyright 100023297 (2010). Key

Flood Zone 2 V.3.15

Flood Zone 3 V.3.15

4.5.4.3 The tidal modelling completed for this Level 2 SFRA has provided flood extents for a number of different flooding events with the formal tidal flood defences removed. The extents can be seen in Figure 15 and Figure A25 in Appendix A. The flood events have been modelled without flood defences so a comparison can be made with the Environment Agency's Flood Zone maps. As more detailed modelling techniques have been used (TUFLOW) these extents should be more accurate than the Flood Zone maps. However, the modelling outputs will need to be verified by the Environment Agency before the conclusions made in this report can be finalised.

Figure 15 - Modelled extents at Seal Sands (also see Figure A25 in Appendix A)

OS Licence: © Crown Copyright 100023297 (2010). Key

4.5.4.4 The new modelling shows a large reduction in the flood extent compared to the Flood Zone maps. The 1 in 200 year event only affects the Dormans Pool site (see Figure 15 and Figure A25 in Appendix A). This is a SSSI designation and will not be developed. The rest of the Seal Sands employment allocations are outside of the areas at risk. The 1000 year event increases in extent by a small degree, inundating the Reclamation Pond site.

4.5.4.5 According to PPS25, these employment allocations should be acceptable. However, issues such as the climate change flood extent, flood depth and hazard, access and egress and mitigation measures are important constraining factors and are discussed below.

4.5.4.6 The Port Clarence Complex (see Figure 12) is only at risk from the 1 in 200 year+cc event (see Figure 15). A small part of this site is at risk from this event and the flood depths are in the region of 0.5m. As water compatible land use is proposed here, this should not affect urban design. The only issue is access and egress (which is discussed later).

4.5.4.7 Much of the North Tees Pools Complex (see Figure 12) is not at risk of flooding from any of the events modelled. However, for the 1 in 200 year event, the Dormans Pool site is flooded to depths of around 2.5m. During the 1 in 1000 year event, this depth increases to around 3m. In addition, for this event, the Reclamation Pond site is flooded to around 0.5m. The climate change extent increases the depth of flooding at Dormans Pool to around 3.5m, while Reclamation Pond floods to around 3m. These two areas are comprised of large ponds and marshy land. For Reclamation Pond, there is a tipping point, where the higher land on its east boundary overtops and the area fills to significant depths.

4.5.4.8 These flood events would be expected to affect urban design due to the significant depths expected. However, large scale land raising is planned for Reclamation Pond before development takes place. Although this amount of land raising appears excessive, a developer submitted a planning request in 2008 to raise the Reclamation Pond to over 6m AOD. This will raise the land to well above the 1 in 1000 year tide level with 100 years climate change added (the 1 in 1000 year +cc tide level is 5.3m AOD). This effectively removes any flood depth and hazard risk to this area which has been classed as the most vulnerable to flooding. Some of the existing ground levels are at 2m AOD, so the land will be raised by 4m in places.

4.5.4.9 Although the Dormans Pool site is included in Stockton BC's proposed allocations, it will not be developed as it is a SSSI wetland designation (see

Figure 16). The area south of Dormas Pool is to be developed and is subject to over 2m flood depths, but based on the planning permission for Reclamation Pond, land raising will remove the risk from this site.

4.5.4.10 The remaining sites at the Seal Sands complex are at risk from the 1 in 200 year +cc event but are flooded to much lower depths (around 0.5m). The flood hazard here is generally ‘danger for some’, which means only the most vulnerable people would be at risk. The proposed employment development here should be designed to manage flooding up to the climate change event. As the depths are in the region of 0.5m, land raising is not necessary. A combination of raised floor levels and flood resilience measures should mitigate these sites against the 1 in 200 year +cc flood event. This will impact on urban design but not significantly.

Sustainability Appraisal Indicator 2 Can the development be made safe once developed and are residual risks acceptable? Access and Egress

4.5.4.11 The allocations are fully surrounded by flood water during the 1 in 200 year flood event, with the River Tees to the east, north and south and the flooded land to the west. This presents an emergency access and egress issue. Fortunately, the allocations are on higher ground and not at significant risk of flooding for the 1 in 200 and 1 in 1000 year events. However, during the 1 in 200 year event +cc, the sites are affected by greater extents and depths. As described in Sustainability Indicator 1, mitigation measures will be put in place to remove flood risk from these sites for this event. This means that the people working at these employment sites will be able to stay within the large flood free area until the water subsides. However, emergency access and egress may still be required if the flood water blocking the access route stays for a long period.

4.5.4.12 The oil refinery complex currently has an emergency access road, on the north boundary with the Tees Estuary. This could be the primary emergency access and egress route for Port Clarence, North Tees Pools and Seal Sands. This road would need to be raised above the 1 in 200 year +cc flood level. This road could then link into the A178 and the A1185 which would also need to be raised. The modelling currently shows that these roads are flooded to depths between 2 and 3m for the climate change event.

4.5.4.13 Within the proposed development area, roads leading to this primary route would also need to be raised above the climate change level. This should not be difficult as flood depths within the sites are in the region of 0.5m (if land the land raising Reclamation Pond takes place).

4.5.4.14 The costs and practicalities of raising the primary access road above the climate change flood event will be a constraint. However, any works to improve access to the employment sites in an extreme event, will also benefit the oil refinery. It is vital that the oil refinery is kept operational at all times, even during an extreme flood event. The Tees Tidal Strategy affirms this by 67

proposing to raise the emergency access road to the oil refinery (this is the same as the primary access route, proposed in this study).

4.5.4.15 As the area is at tidal risk and for employment land use, flood warnings and evacuation would not be as significant an issue as for residential development in fluvial risk locations. Tidal flooding is easier to predict and people can be sent home or prevented from coming to work if an extreme event is predicted (rather than the need for reception centres for residential development).

4.5.4.16 However, a skeleton staff would still be required for the safe operation of the oil refinery. If the primary access road could not be raised above the 1 in 200 year +cc flood level, provisions could be made for people to stay on the site (which is on higher ground) during this extreme flood event.

4.5.4.17 This is an issue for the Stockton BC emergency planning team and should tie into any emergency plans for the oil refinery.

Residual risks

4.5.4.18 The proposed mitigation strategy is to raise ground and floor levels at Reclamation Pond and south of Dormas Pool above the 1 in 200 year +cc event. Raised floor levels and flood resilience measures are proposed for the Seal Sands complex. There are no obvious residual risks associated with this mitigation strategy.

4.5.4.19 However, the primary flood pathway is from Greatham Creek (a tidal creek directly connected to the Tees Estuary) overtopping and then flooding to the east. Greatham Creek has raised flood defences which prevent regular inundation of the area immediately to the south. However, these defences have been known to breach and are predicted to fail at approximately the 1 in 10 year event. The Tees Tidal Strategy recommends improving these defences so that they will not fail until after the 1 in 50 year event. The employment sites are to the east of these defences, so they would not be impacted by the hazard of a defence breach.

4.5.4.20 There is another, older embankment directly to the west of the allocations (The Long Drag). This embankment varies in height (between 2 and 5m) and the Environment Agency’s flood defence database states that this embankment offers no flood defence benefits. It is recommended that the sites to the east of this embankment are raised to the 1 in 200 year +cc flood level. This would put the sites above this embankment, removing any possible breach hazard. Alternatively, the embankment could be removed as it offers no flood defence benefits.

Sustainability Appraisal Indicator 3 Is the increase in risk posed by climate change easily adapted to and/or can the development build in climate change resilience?

4.5.4.21 The current 1 in 200 year event only floods a small part of the North Tees Pools complex (see Figure A25). The main flood risk to the sites comes from 68

the 1 in 200 year+cc event (the 1 in 200 year event in 100 years time). This assessment has therefore focussed on how flooding from this future event can be managed.

4.5.4.22 Flood Risk Sustainability Indicators 1 and 2 describe how development can take place while taking into account the climate change event.

Sustainability Appraisal Indicator 4 Can flood risk to adjacent lands stay the same or be reduced as a result of the development and its mitigation measures.

4.5.4.23 As the sites are at risk of tidal flooding, loss of floodplain is not considered to have a significant impact, compared with that usually associated with fluvial flood risk. The Environment Agency does not normally require compensatory flood storage for the loss of floodplain in tidal areas.

4.5.4.24 However, as part of this area is defended, the loss of tidal floodplain may be an issue due to overtopping of the tidal defences. During the 1 in 200 year event, flooding occurs from two places, the defended Greatham Creek and the undefended Tees Estuary to the south. As flooding also occurs from an undefended location, and the defences are expected to breach and not overtop, the area at risk will not act as a basin i.e. displacement of tidal floodwaters would not occur.

4.5.4.25 The modelling did show some localised increases, specifically in the vicinity of the oil refinery to the east of the Reclamation Pond. This is thought to be the result of the proposed development blocking a low lying storage area encompassed in the Reclamation Pond site, which does not fill to capacity in the existing risk scenario.

Summary and proposed mitigation strategy

4.5.4.26 Figure 14 shows that parts of the employment sites within the Seal Sands complex and North Tees Pools complex are within Flood Zone 3. Flood Zone 2 extends slightly further into these sites. According to PPS25, these sites can be allocated subject to a Flood Risk Assessment (FRA). Before this, the Sequential Test should be applied in the manner set out in PPS25.

4.5.4.27 This study has shown that these sites can be developed safely without increasing risk to others. The tidal modelling removed the defences as they are predicted to breach at very low return period events. The actual risks are significantly lower than the simplistic methods used in the construction of the Flood Zones. The modelled extents show that only two areas were significantly flooded. These are the Dormans Pool and Reclamation Pond. Dormans Pool will not be developed as it is an internationally designated wetland (SSSI). The Reclamation Pond has planning permission for ground raising up to 6m AOD. This means that the highest risk areas (according to the modelling) will either not be developed or raised well, above the extreme tide levels. The remaining areas at risk only flood to relatively shallow depths and floor raising along with flood resilient measures can remove these areas 69

from flood risk. Water compatible land use (e.g. ship building) is proposed for the Port Clarence sites, although flood risk should not be a constraint to the development itself, safe access and egress will still need to be considered.

4.5.4.28 The proposed access and egress route, in the event of a flood blocking the existing route to the west, is described in Flood Risk Sustainability Indicator 2. The Tees Tidal Strategy supports maintaining this development area into the future as there is an existing oil refinery complex here. Part of the strategy includes maintaining an emergency access road to the north of Seal Sands. Access roads throughout all of the sites can lead to this primary route in the north. As the flooding is relatively shallow within the sites, this can easily be achieved. Development of these employment sites will be dependant on emergency planning issues. As the oil refinery is a vital asset that needs to be operational at all times, it is likely that development of these sites can be safely developed along side the existing uses.

4.5.4.29 When proposing mitigation for development at Seal Sands, consideration must be given to the surrounding environmental designations. In and around the proposed development allocations are national and international wetland designations. It is advised that Natural England and the Environment Agency (FRB team) are made aware of the proposals at an early stage, so that any environmental constrains can be identified. Figure 16 below shows the SSSI and Ramsar sites along with the proposed development allocations. The 1 in 200 year +cc flood extent, shows the areas where ground raising may be required (see Figure 15 and A25 in Appendix A for this extent).

Figure 16 - SSSI and Ramsar sites at Seal Sands

SFRA 2009 Allocations

Mitigation Summary Table Sites (see Mitigation Options Constraints Preferred Development sequence and Figure 12) Outcome indicative cost Seal Sands These sites are only at risk from the 1 in 200 year+cc event. There will be a cost associated with Employment An emergency route will need to complex During this event, flood depths of around 0.5m could be raising floor levels and flood resilient land use could be planned which connects Port expected. These depths can be mitigated by raising floor building techniques. be allocated Clarence, North Tees and Seal levels and integrating flood resilient measures. The with the Sands with the primary raised emergency access road to the north will need to be raised Apart from one proposed access outlined road to the north. above the 1 in 200 year+cc level. This will be the primary road, these sites would be cut off by mitigation access and egress route for all the sites (see below) in the at least the 1 in 200 year flood event. measures. Costs - floor raising, flood event of a tidal flood blocking the normal routes. resilience and raised primary Floodplain compensation will not be access and egress route = required as the risk is tidal. medium North Tees The sites around Dormans Pool and Reclamation Pond are Apart from one proposed access The ground raising would need Pools at risk of flooding from the 1 in 200 year and 1 in 1000 year road, these sites would be cut off by to take place before these sites complex event. Dormans Pool will not be developed upon but land at least the 1 in 200 year flood event. are developed. and floor raising above the 1 in 200 year+cc event will be required for the other areas. Roads leading to the primary The amount of land raising required is Costs -significant ground raising, access and egress route will also need to be raised above significant (by 3m in places). This is a floor raising, flood resilience and this flood event. constraint but as indicated by a recent raised roads leading to the planning application for ground raising primary road = high. (at Reclamation Pond) it has already been seen as necessary and possible by developers. Port These sites are free from flooding apart from a small section Apart from one proposed access Costs - raised roads leading to Clarence during the 1in 200 year+cc event. As water compatible land road, these sites would be cut off by the primary road = medium. complex use is proposed here (e.g. ship building), mitigation at least the 1 in 200 year flood event. measures will not be appropriate. Irrespective of this, flood depths are shallow and flooding would not occur until the 1in 200 year+cc event. Similarly to North Tees, a flood free route northwards to the primary emergency access road (north of Seal Sands) will be required. As flooding is shallow and limited in extent, this should be achievable.

4.5.5 Haverton Hill area development implications and issues

Flood Zones and functional floodplain

4.5.5.1 The assessment below includes the Haverton Hill sites, Casebourne site and Billingham Reach site (see Figure 13 for the location of these sites).

4.5.5.2 The current Environment Agency Flood Zone maps show that all of the sites (apart form parts of the Haverton Hill allocations) are entirely within Flood Zone 3. The Haverton Hill sites are shown to be partially within Flood Zones 2 and 3. The Environment Agency's Flood Zone maps show flood extents with flood defences removed. The Flood Zones can be seen in Figure 17 below.

4.5.5.3 The Haverton Hill sites have been safeguarded in the Core Strategy as being priority locations for river based / port developments. This means they will be classed as water compatible under PPS25. The Billingham Reach and Casebourne sites have been allocated as employment land use (less vulnerable under PPS25).

Figure 17 - Flood Zone map at Haverton Hill, Casbourne and Billingham Reach

Flood Zone 2 V.3.15

Flood Zone 3 V.3.15

4.5.5.4 The tidal modelling completed for this Level 2 SFRA has provided flood extents for a number of different flooding events with the formal tidal flood 73

defences removed. The extents can be seen in Figure 18 and Figure A25 in Appendix A. The flood events have been modelled without flood defences so a comparison can be made with the Environment Agency's Flood Zone maps. As more detailed modelling techniques have been used (TUFLOW) these extents should be more accurate than the Flood Zone maps. However, the modelling outputs will need to be verified by the Environment Agency before the conclusions made in this report can be finalised.

4.5.5.5 The 1 in 200 year flood event shows Haverton Hill 1 fully flooded with Haverton Hill 2 and 3 partially flooded. The Casebourne site is not flooded during the 1 in 200 year event. The 1000 year event floods all the sites apart from Haverton Hill 2 and 3 which are still only partially flooded. Figure 18 - Modelled extents at Haverton Hill, Casbourne and Billingham Reach (also see Figure A25 in Appendix A)

4.5.5.6 For the 1 in 200 year event, parts of the sites flood to between 0 to 1m. The hazard rating for this event is classed as ‘danger for some’. The flood depths are relatively shallow and the slower velocities associated with tidal flooding would not present a significant hazard to people.

4.5.5.7 For the 200 year +cc flood event, flood depths are more in the region of 2m, apart from the Casebourne site which floods to around 3m. This only slightly increases the hazard rating due to the low flood velocities. But this rating now falls into the category of ‘danger for most’. However, as the Casebourne site floods to greater depths, the hazard rating is ‘danger for all’, this includes danger to the emergency services. It can be concluded that the flood risks at this site are significant. The level of flood hazard during the climate change event could result in the loss of life and the scale of flood risk means that there will be an impact on urban design.

Sustainability Appraisal Indicator 2 Can the development be made safe once developed and are residual risks acceptable? Mitigation, Access and Egress

4.5.5.8 Some of the sites (Haverton Hill 1, 2 and 3) are water compatible as the intended use is shipbuilding / repairs. It is accepted that these sites will be subject to flooding although site safety will still need to be considered. A flood warning and evacuation plan will therefore be a key part of the mitigation strategy for Haverton Hill 1, 2 and 3. This will include identifying safe access and egress routes and use of the tidal flood warning service to evacuate or stay off the site. Access and egress routes are discussed later in this section.

4.5.5.9 The remaining sites (particularly Casebourne) could flood to depths that are considered unmanageable. However, the significant levels of investment in the area means that a combination of ground raising and flood resilience measures will be able to manage the risk from the sites. The sites should be raised up to the 1 in 200 year level (4.2 mAOD). As the Casbourne site is as low as 2m AOD in places, the ground will need to be raised by 2m in these areas to put it above the 1 in 200 year flood level. Ground raising will be more typically in the region of 1m for most of the sites though. The 1 in 200 year +cc level is 5.1m AOD. Flood resilience building measures such as water resistant materials, raised plug sockets etc should be implemented up to this level. The mitigation summary table outlines the mitigation requirements for each site.

4.5.5.10 Access and egress routes need to be above the 1 in 200 year +cc flood level. As seen in Figure 18 and Figure A25 in Appendix A, the majority of the sites are below this level. The proposed mitigation strategy is to land raise the sites above the 1 in 200 year flood level. The access roads will need to be raised 1m higher than this to be above the climate change level. It may not be 75

possible to raise access roads by this amount due to the limited space and gentle gradients required for vehicles. If an access road can not be raised by a further 1m, pedestrian walkways could be, allowing safe access and egress during this extreme event.

4.5.5.11 Once access roads or walkways have been raised for the Billingham Reach and Casebourne sites, links can be made with the A1046, which is on higher ground. The remaining sites (Haverton Hill), are backed by a railway embankment. On the other side of this embankment is Port Clarance which has flooded in the past and is shown to flood at greater depths then the employment sites. The photo (Figure 3) from the draft Tees Tidal Strategy shows Port Clarence to the left and the railway embankment to the right during the 1953 tidal flooding event. The embankment does prevent the flood water that floods Port Clarence, inundating the Haverton Hill sites but it is not a formal flood defence. Flooding also occurs at the Haverton Hill sites directly from the Tees. The railway embankment is higher than the 1 in 200 year +cc level so this could be used as an emergency pedestrian walkway for the Haverton Hill sites. The Tees Tidal Strategy's preferred option is to improve this embankment so that it acts as a flood defence. This is an active railway line and consultation with Network Rail will need to take place for this is taken any further.

Residual risks

4.5.5.12 The proposed mitigation strategy is to raise ground and floor levels above the 1 in 200 flood level and implement flood resilience measures up to the climate change level for the Casebourne and Billingham Reach sites. There are no obvious residual risks associated with this mitigation strategy.

4.5.5.13 For the Haverton Hill 1, 2 and 3 sites, the proposed land use is ship building which is classed as water compatible in PPS25. People working on these sites will be at risk of flooding during a tidal flood event. An evacuation plan should be established to mitigate this risk which should fully integrate the flood warnings available for this area i.e. once a flood warning has been received, nobody should enter the site and the remaining people should leave immediately.

4.5.5.14 There is a potential residual risk to the Haverton Hill sites as a railway embankment separates these sites from Part Clarence. Flooding to Port Clarence occurs due to breaching from Greatham Creek and the flood depths at Port Clarence are greater then those at the Haverton Hill sites. If this embankment were to breach, there could be another inundation to the Haverton Hill sites. However, the difference in depths either side of the embankment means that the flood hazard (velocity and depth) would not be significant.

4.5.5.15 In addition, the preferred option for the Tees Tidal Strategy is to improve the flood defence standard that this embankment offers. This SFRA also proposes widening this embankment for a high level emergency access road. Both of these measures would increase the strength of the embankment and along with the proposed ground raising, difference between the development

and the top of the embankment would be low, further reducing the risk and impact of a breach. The detailed FRA should still consider this as a residual risk though.

Sustainability Appraisal Indicator 3 Is the increase in risk posed by climate change easily adapted to and/or can the development build in climate change resilience

4.5.5.16 Climate change will increase the flood extent and depth to these sites (compare Figure A26 to A28 in Appendix A). This shows that all the sites will be inundated (rather than part of the sites), depths will be increased and access and egress will be made difficult. The mitigation strategy is to land raise the Casebourne and Billingham reach sites to above the 1 in 200 year +cc level. Access and egress to the Haverton Hill 1, 2 and 3 sites will be raised above the 1 in 200 year +cc level.

Sustainability Appraisal Indicator 4 Can flood risk to adjacent lands stay the same or be reduced as a result of the development and its mitigation measures.

4.5.5.17 As the site is at risk of tidal flooding directly from the undefended estuary, loss of floodplain is not considered to have a significant impact, compared with that usually associated with fluvial flood risk. Flood risk to adjacent land will therefore be small.

Site Summary

4.5.5.18 All of the sites are at least partially within Flood Zone 3. Developing these sites for employment or water compatible use is appropriate according to PPS25. The Exception Test does not apply to these sites. A FRA will still be required to show that the sites will be safe once developed and will not increase flood risk else where.

4.5.5.19 The Environment Agency requires new developments and their access and egress routes to be dry during the climate change flood event (1 in 200 year flood +cc). The climate change tidal flood event level is 5.1m AOD.

4.5.5.20 This means that although the development is acceptable according to PPS25, mitigation measures will be required to ensure the development and access roads do not flood during the climate change event. The Haverton Hill 1, 2 and 3 sites have been set aside for river based port developments. These are classed as water compatible in PPS25 so flood risk to these sites will need to be managed to reduce financial losses. However, access and egress routes will still need to be clear for the climate change scenarios.

4.5.5.21 The tidal modelling has shown that flood depths at the sites for this climate change scenario range between 0.5m and 3m. Guidance provided in Volume III of the SFRA (Table1-1) advises that for flood depths over 1.5m mitigation measures will be difficult to implement. This could be a constraint, but the 77

regeneration drive in this area means that measures such as land raising could still be feasible even at this scale. Land raising is the preferred mitigation option for these sites, compensatory flood storage would not be required as the flood risk is tidal.

4.5.5.22 Although some of these sites fail sustainability appraisal flood risk indicator 1, implementing mitigation measures means that the risks can be managed. The mitigation measures also mean the remaining indicators will not be negative. The mitigation strategy is summarised below.

Mitigation Summary Table Site Mitigation Options Constraints Preferred Outcome Development sequence and indicative cost Haverton Hill 1 Water compatible development is The pedestrian walkways would need Water compatible As there is no direct mitigation proposed, mitigation will not be required investment. But in order for this site to be development can go ahead other than a raised pedestrian for this land use. Pedestrian walkways used for port developments (e.g. ship with a pedestrian access route walkway. Development of will need to be raised by between 1 and repairs), the site would need to stay at alongside the railway these sites could go ahead 2m. The possibility of tying these in with the existing level (i.e. no land raising of embankment. before the Casebourne and the existing railway embankment the site itself). Billingham sites. should be investigated. Haverton Hill 2 Same as Haverton Hill 1 but depths are lower, more in the region of 0.5 to 1m. Costs – raised pedestrian Haverton Hill 3 Same as Haverton Hill 1 but depths are lower, more in the region of 0.5 to 1m. walkway = medium. Casebourne Employment use is proposed for this Compensatory storage would not be Employment land use Significant land raising is site. This is a low lying site with flood required as it is tidal. Although the development can go ahead required but the site is depths above 2m. Land raising by a amount of land raising is significant, the with ground raising above the separated from Billingham so maximum of 2m (more generally 1m) regeneration drive in this area means 1 in 200 year event and flood this would not need to happen would raise the site above the 1 in 200 investment should be there (see existing resilience to the 1 in 200 year at the same time. year event. Flood resilient measures planning permission at Seal Sands). +cc event (5.1m AOD). should be implemented up to the 1 in Costs – significant land raising 200 year +cc event (another 1m). An of the entire site and raised access road or pedestrian walk way access road/walkway = high. would need to be raised by another 1m above the newly raised ground. Billingham For employment land use to go ahead Same as the Casebourne site, but the Same as the Casebourne site. Land raising is required but the Reach on this site, land raising by around 1m amount of land raising would be less due site is separated from would be required to put the site above to lower depths. Casebourne so this would not the 1 in 200 year event. Flood need to happen at the same resilience measures should be time. implemented up to the future event (5.1m AOD). An access road or Costs – land raising of the pedestrian walk way would need to be entire site and raised access raised by another 1m above the newly road/walkway = high. raised ground.

5. Flood Risk Assessments and Surface Water

5.1 Flood Risk Assessment requirements for the Level 2 sites

5.1.1.1 In accordance with current planning guidance, the planning process ensures only appropriate development in areas vulnerable to flooding. This includes adopting a precautionary approach to decisions based on estimates of the present and future impact of flood risks.

5.1.1.2 Whilst the Level 1 SFRA focuses on delivering a strategic assessment of flood risk within Stockton BC, this Level 2 SFRA has gone one step further in investigating flood risk in more detail at specfic allocations. This Level 2 SFRA has outlined which sites could be developed safely (and pass part c of the Exception Test) and what mitigation measures will be required to do this. There is still a need for a site specific flood risk assessment (FRA) to resolve detail and integrate the results into the masterplan and urban design.

5.1.1.3 General FRA guidance for developers has been supplied within the Stockton BC Level 1 SFRA, which must be referred to (see Chapter 3 of Volume III). Elements of the FRA guidance are outlined below: • Appropriate land use in flood risk areas • Undefended areas – flood risk mitigation • Defended areas • Overtopping • Breaching • Public Safety and rapid inundation • Feasibility of flood risk mitigation

5.1.1.4 Using the information supplied by both levels of SFRA there are a number of key flood risk issues which should be included in a FRA for the developments identified in this Level 2 SFRA.

5.1.2 Flood risk from the River Tees

5.1.2.1 A combined 1D/2D model has been produced for the River Tees adjacent to the Bowesfield North (Phase 1 and 2), Boathouse Lane and Chandlers Wharf sites. When a planning application for these sites is submitted, a detailed site specific Flood Risk Assessment (FRA) should be completed for each site providing the details outlined above.

5.1.2.2 For these development sites, more detailed 2D floodplain modelling may be required. This would provide further details on flow pathways and flood extents.

5.1.2.3 In addition, the FRA should show that the proposed mitigation measures are technically viable i.e. • If it is recommended that an access road is raised, is there enough space within the site to provide vertical and horizontal alignments?. • Can first floor only development be integrated into the design without reducing the floodplain storage volume?

5.1.2.4 Any FRA should provide full details and drawings to show that the mitigation measures can be implemented.

5.1.2.5 The climate change flood levels for the development sites are provided below. These are the highest levels modelled. Development mitigation measures should be built up to this event. For example, finished floor levels or flood resilience measures should be set at these levels (plus freeboard). It is recommended that the fluvial scenario is used as the tidal scenario has been combined with a 1 in 5 year fluvial flood, which makes it a much lower probability than the fluvial scenario.

5.1.2.6 The modelled extreme flood levels at the proposed Bowesfield North (Phase 1 and 2) development sites are: • 1 in 100 year+cc fluvial flood level – 5.50m AOD • 1 in 200+cc year tidal flood level (combined with a 1 in 5 year fluvial flood) – 5.50m AOD

5.1.2.7 Maximum flood levels at the proposed Boathouse Lane and Chandlers Wharf development sites are: • 1 in 100 year+cc fluvial flood level – 4.80m AOD • 1 in 200+cc year tidal flood level (combined with a 1 in 5 year fluvial flood) – 5.40m AOD

5.1.2.8 If compensatory floodplain storage is decided as the preferred option (in order to allow the development in Flood Zone 3 to go ahead) this must be considered as part of a planning application or consent application (if needed) since they will affect the final appearance of a development, particularly regarding landscaping. The compensation proposal will therefore form part of the flood risk assessment and be included in application drawings of the site layout.

5.1.2.9 It should be noted that compensation works won’t increase the land available for development on a site, they’ll merely reconfigure it for more convenient use. In order to increase the land available on site, further land off site is required.

5.1.2.10 If flood storage is proposed, the Environment Agency states that developers must ensure there’ll be no loss of flood flow or flood storage capacity for floods up to the severity of the 1 in 100 year fluvial flood as a result of their development. The level for flood plain compensation is the replacement of the volume lost from the flood plain through development with new flood plain volume by reducing nearby ground levels. The compensatory volume must be at the same level (within reasonable working limits) as the lost storage.

5.1.3 Flood risk from the Old River Tees

5.1.3.1 A combined 1D/2D model was developed for the Old River Tees to ascertain the risk to the Tees Marshalling Yard (TMY). This included 2D modelling of the River Tees overtopping the tipped land separating the Tees from the TMY. The 1D element consisted of carving the channel of the Old River Tees through the 2D model domain. This 1D channel element was simplified as there was no topographic survey of the Old River Tees available. For a site specific FRA, the Environment Agency may require that a survey of the Old

River Tees is undertaken and integrated into the existing model developed for this SFRA. This would give a more accurate representation of areas at risk of flooding.

5.1.3.2 There is a broad strip of higher ground separating the River Tees from the TMY which is believed to be mainly tipped spoil. This was considered broad enough not to be considered a flood defence. This should be confirmed during an FRA so that it can be established that there is no breach hazard from this higher ground. In additional one of the mitigation options is to raise part of this land to stop inundation of the TMY from the River Tees during the 1 in 200+cc event.

5.1.3.3 If mitigation options 2 or 3 are selected, the risk of a breach from raised defences on the Old River Tees should be assessed. In addition, a breach assessment of the railway embankment (southern boundary of TMY) should be undertaken if the underpasses are blocked which means the embankment will be used as a flood defence.

5.1.3.4 As with the River Tees sites, the viability of the mitigation measures should be confirmed through design and costing should be noted that for these sites to be developed, significant land raising is proposed. If defences are chosen as the preferred mitigation option, then the risk from a breach in the new defences should be assessed.

5.1.3.5 For the FRA, finish floor levels should be placed at 5.07m AOD plus an appropriate freeboard to protect the development against future events (up to 2107).

5.1.4 Flood risk from the Tees Estuary

5.1.4.1 A 2D tidal model was produced for the sites at risk from the Tees Estuary (Seal Sands sites and the Haverton Hill sites). This model and the outputs should be used to help produce a detailed FRA, covering the items listed in section 5.1.

5.1.4.2 The mitigation strategy is for large scale ground raising. Again, the economic and technical practicalities of ground raising should be confirmed by the developer prior to the completion of a detailed FRA. An outline plan should also be drawn up at an early stage to confirm that access roads and developments can be raised to the levels stated in this document (e.g. is there enough space for the required driveway gradient).

5.1.4.3 For the Haverton Hill sites, part of the mitigation strategy is to tie in the access roads with the existing railway embankment. Discussions with Network Rail will need to be undertaken to confirm that this can be undertaken. The Environment Agency may also require a breach assessment of the railway embankment. However, if ground raising is undertaken, this would put the development site above the land at the other side of the railway embankment. A similar assessment may be required for disused flood defence embankment (The Long Drag) to the west of the Seal Sands sites. Again, ground raising would reduce the breach hazard eliminating the need for an assessment.

5.1.4.4 For the site specific FRA, finish floor levels should be set at 5.13m AOD plus an appropriate freeboard to protect the development against future events (up to 2107).

5.1.4.5 The Greatham Creek tidal flood defence embankments are the primary flood pathway to the Seal Sands sites. The Tees Tidal Strategy preferred option is to improve these flood defences as they currently provide a low standard of protection. If this development goes ahead in advance of any flood defence improvements, developer contributions may be required as the improvements would benefit the new development (even if the Seal Sands ground was raised, the access and egress route would still be at risk of flooding).

5.1.4.6 The Environment Agency does not normally require compensation in coastal locations as the volume of flood plain displaced by the development will be transferred to the sea. However, if the area at risk is protected by flood defences, tidal flood water may become trapped behind breached or overtopped defences. Displacement of floodwaters from new development may be an issue in this case. The SFRA modelling showed that there was a localised increase in flood depths east of the Reclamation Pond site when the development was added. This should be looked at in more detail during the FRA to see if the development is increasing risk by displacing enclosed, tidal floodwaters.

5.1.4.7 A major element of the FRA for the sites at Seal Sands will be emergency planning, specifically emergency access. This is discussed in section 5.1.4 below.

5.1.5 Emergency Planning

5.1.5.1 Appropriate emergency planning must be incorporated in any flood risk assessments. Emergency planning can be a crucial tool in reducing the residual risk to both people and to lesser degree property. Current flood response plans must be considered if development is going to place a greater number of people in areas of high risk whether the actual risk can be managed or not.

5.1.5.2 The following summarises the emergency planning issues for the sites assessed in this Level 2 SFRA:

• Bowesfield, Boathouse Lane and Chandlers Wharf - to tie in with the proposed mitigation measures, emergency access routes would be raised allowing people within this residential area to leave the flooded area safely. There would therefore not be an increased strain on the emergency services. However an emergency plan will still be required. • Tees Marshalling Yard - to tie in with the proposed mitigation measures, raised access roads to higher ground would allow people within this residential area to leave the flooded area safely. However, the access route to the south would be cut off. There would not be an increased strain on the emergency services. • Haverton Hill - Mitigation measures proposed access roads that tie in with the Port Clarence Railway embankment. This would provide safe access and egress in the event of en extreme tidal flood. In addition, as the allocations

are employment, the risk to people is less than the residential sites. There would not be an increased strain on the emergency service for these sites. • Seal Sands - Mitigation measures proposed include raised access roads. However, due to the extent and depth of flooding, it is still possible for this area to be cut off during an extreme flood. This could put an increased strain on emergency service. More details on this area are provided below.

5.1.5.3 Further consultation with Stockton BCs emergency planning department will be required for the Seal Sands allocations. Flood risk to the existing development within this area (oil refinery) and the future development should be factored into any emergency plans. The plans should take account of the potential for the Greatham Creek defences to breach, which would cut off access and egress to Seal Sands. Raised access roads are proposed for the oil refinery (Tees Tidal Strategy) and the development allocations. It should be noted that the access roads will need to be raised by significant amounts in some places.

5.1.5.4 The emergency plans should also include provisions to allow the oil refinery and new developments to continue operating should an extreme tidal flood event occur.

5.2 Surface Water 5.2.1 SUDS

5.2.1.1 This section provides a strategic summary of the applicability of SUDS techniques in Stockton BC. This is a broad scale assessment and should therefore not be used for assessing individual sites but it should be used for strategic planning.

5.2.1.2 Table 5 shows the soil types, the expected ground conditions from this soil type and the SUDS techniques that will be possible with these ground conditions.

5.2.1.3 The SUDS techniques are categorised as storage (i.e. water stored on site and then slowly released) or infiltration (i.e. where surface water is allowed to infiltrate into the ground). Infiltration SUDS require ground conditions that allow the infiltration of surface water through the ground. Clay rich soils and areas with a high water table will not be suitable for infiltration SUDS. Table 4 shows the infiltration and storage SUDS techniques.

5.2.1.4 For this broad assessment, the soils data used is a simplified 1:250,000 soils dataset, derived from the more detailed National Soil Map. This is Cranfield University data and is available online. The drift geology data was obtained in GIS format from the British Geological Survey.

Table 4 - Suitability of SUDS Techniques SUDS Technique Infiltration Storage Green Roofs U V Permeable Paving V U Rainwater U V Harvesting Swales V V Detention Basins V V Ponds U V Wetlands U V Source: PPS25 Practice Guide

Table 5 - Strategic SUDS Applicability Area Soils and drift Ground conditions SUDS Implications Seal Sands area Drift - raised marine deposits of sand and These soils are naturally wet and have SUDS infiltration techniques will not be gravel. Soil - loamy and clayey soils of naturally high groundwater levels. possible, only SUDS storage techniques or coastal flats. underground storage basins.

Port Clarence area Drift - raised marine deposits of sand and These soils are naturally wet and have SUDS infiltration techniques will not be gravel. Soil - loamy and clayey soils of naturally high groundwater levels. possible, only SUDS storage techniques or coastal flats. underground storage basins.

Haverton Hill area Drift - raised marine deposits of sand and These soils are naturally wet and have SUDS infiltration techniques will not be (including Billingham gravel. Soil - loamy and clayey soils of naturally high groundwater levels. possible, only SUDS storage techniques or Reach) coastal flats. underground storage basins.

Area to the north of the Drift - raised marine deposits of sand and These soils are naturally wet and have SUDS infiltration techniques will not be Tees Barrage including gravel. Soil - loamy and clayey soils of naturally high groundwater levels. possible, only SUDS storage techniques or North Shore and Malleable coastal flats. underground storage basins. Industrial Estate

Area to the south of the Drift - raised marine deposits of sand and These soils are naturally wet and have SUDS infiltration techniques will not be Tees Barrage including gravel. Soil - loamy and clayey soils of naturally high groundwater levels. possible, only SUDS storage techniques or Tees Marshalling Yard and coastal flats. underground storage basins. the University.

The green area around Loamy and clayey floodplain soils. These soils are naturally wet and have SUDS infiltration techniques will not be Billingham Beck naturally high groundwater levels. possible, only SUDS storage techniques or underground storage basins.

Areas adjacent to the river Drift - clay, silt and sand alluvium. Soil - Freely draining soils absorb rainfall The soil type indicates that it should be suited Tees including Boathouse freely draining floodplain soils. readily and allow it to drain through to to infiltration SUDS systems. However, site Lane and Bowesfield underlying layers.. specific knowledge shows that the low lying areas will not be suited to infiltration due river and tide levels. Norton and Wolvston area Loamy and clayey soils and drift. This soil/drift type leads to slightly Infiltration systems should be possible, they in the north part of Stockton impeded drainage. These soils form a could be impeded after prolonged heavy BC. tight, compact, deep subsoil that rainfall though. impedes downward water movement. After heavy rainfall, particularly during the winter, the subsoil becomes waterlogged. All the remaining areas of Drift - till. Soils - Slowly permeable This type of soil leads to impeded Infiltration type SUDS may not be possible Stockton BC seasonally wet slightly acid but base-rich drainage. In soils with impeded due to the soil type. This will restrict SUDS to loamy and clayey soils drainage winter waterlogging results in storage techniques or underground storage very wet ground conditions. basins.

5.2.2 Critical drainage areas and Surface Water Management Plans

5.2.2.1 The Level 1 SFRA for Stockton BC identified 'candidate' critical drainage areas (CDAs). Critical drainage areas come in many shapes and sizes and drivers for characterising an area as a CDA come from severe existing flooding problems from either overland routes or piped drainage systems, or proposed intensity of development and its impact on runoff, or an area where multiple sources of flooding are linked.

5.2.2.2 Draft CDAs were achieved by obtaining the Environment Agency's 'Areas Naturally Vulnerable to Surface Water Flooding Maps' and data on historic surface water flooding problems. These two datasets were overlain to see if there were and locations that were strongly correlated and could be highlighted as 'candidate' CDAs. These locations were then sent to Northumbrian Water (NWL) for consultation. NWL have detailed sewer flooding data and local knowledge which will enable them to confirm whether the locations identified in the Level 1 SFRA are in fact critical drainage areas. However, some of the candidate areas may be removed for the following reasons: • NWL have completed a scheme to deal with the problem. • NWL have programmed a scheme to deal with the problem in the near future. • Through their site specific knowledge and more detailed data sources, NWL have confirmed that there the location is not a CDA.

5.2.2.3 The candidate CDAs identified in the Level 1 SFRA were: • Billingham • Seal Sands • Lustrum Beck

5.2.2.4 Following consultation on the draft Level 2 SFRA, another CDA was recommended for inclusion by the Environment Agency and a local councillor. The area recommended is in Yarm, at Yarm High Street, near to the River Tees. In 2005, the Tees river level rose to the top of the flood defence wall. This resulted in the surface water drainage system backing up on Yarm High Street. As a result, flooding occurred, inundating a number of properties. As this area is in Flood Zone 3, it would not normally be considered a CDA. However one of the purposes of CDAs is to identify areas, if developed, that could have a critical effect elsewhere. It is thought that development on higher ground in Yarm could increase the severity and frequency of this event. As a result, Yarm has been included as a CDA.

5.2.2.5 The conclusions of the above consultation and recommendations are summarised below. However, feedback from NWL is still required.

CDA Recommendation Lustrum Lustrum Beck has been identified as a Critical Drainage Area Beck (CDA), due to a combined surface water and fluvial flood risk and will probably require a Surface Water Management Plan (SWMP). A SWMP should look in detail at drainage assets and local flood risk and assess feasible options for reducing risk. This may include a drainage strategy for the collection of development sites to identify areas suitable for SUDS and where surface water flow paths could be opened up in new development. A green infrastructure workshop was undertaken for Lustrum Beck. This is summarised in Volume III of the Level 1 SFRA and recommends various catchment, source control options. Billingham The area of Billingham highlighted in Volume II of the Level 1 SFRA, has a high coverage of surface water flood risk, which ties in (to some extent) with historic flooding records. When feedback from NWL is received, it should be confirmed whether there is an existing risk here, from multiple drainage sources that should be studied in a SWMP. Seal Sands Due to being flat, low lying and next to the Tees Estuary, the Seal Sands and North Tees Pools area has high surface water flood risk coverage. This risk will not be from surface water flow pathways and there will not be a risk of deep flooding, due to the topography of the land here. When development of this area takes place consultation with land owners should be undertaken and opportunities for a strategic surface water management solution should be sought. This could be covered in a SWMP. Yarm High river levels in the Tees cause backing up of the surface water drainage system around Yarm High Street. Development on higher ground in Yarm could increase runoff and exacerbate the problem. This location is recommended as a CDA due to the existing drainage system problems and the potential for this risk to increase.

5.2.2.6 Details on why Surface Water Management Plans (SWMP) are needed can be found in Section 6.5 of Volume II of the Level 1 SFRA. When the SWMP for Stockton BC is commissioned, it should focus on the locations identified above.

5.2.2.7 Until a SWMP has been completed, integrated drainage solutions should be prepared for larger sites or areas that fall within the identified CDAs. Where major flow paths have been identified these should be considered in the master planning of the site and the sequential placement of development.

5.2.3 Surface water assessments in FRAs

5.2.3.1 If a site is in Flood Zone 1 and over 1 hectare, or any size site in 2 or 3, a flood risk assessment is required which will look at surface water drainage5.

5 Paragraph 26 and E9 of Planning Policy Statement 25: Development and Flood Risk, Communities and Local Government, December 2006

The FRA accompanying the planning application should show how surface water management is functioning on the site at present and how it is to be undertaken in the new development.

5.2.3.2 All sewers that will subsequently be adopted by the sewerage undertaker must be designed and built in accordance with the requirements of Sewers for Adoption, Edition 6 (WRc 2006). This document provides guidance on suitable return periods for use in the design of sewerage systems for various development types. In general terms, sewers should be designed to ensure that no flooding occurs above ground level for events with a return-period of 30 years.

5.2.3.3 For events with a return-period in excess of 30 years, surface flooding of open spaces such as landscaped areas or car parks is acceptable for short periods, but the layout and landscaping of the site should aim to route water away from any vulnerable property, and avoid creating hazards to access and egress routes.

5.2.3.4 No flooding of property should occur as a result of a one in 100 year storm event (including an appropriate allowance for climate change).

5.2.3.5 Off-site impacts should not increase flood risk elsewhere. For the range of annual flow rate probabilities up to and including the 1 in 100 year event, including an appropriate allowance for climate change, the developed rate of run-off into a watercourse, or other receiving water body, should be no greater than the existing rate of run-off for the same event. Run-off from previously- developed sites should be compared with existing rates, not greenfield rates for the site before it was developed. Developers are, however, strongly encouraged to reduce runoff rates from previously-developed sites as much as is reasonably practicable.

5.2.3.6 The Level 1 SFRA identified a number of areas where surface water flooding would be an important factor to considerer when allocations come up for development.

5.2.3.7 Table 6 below shows the larger proposed development sites that have a high percentage of surface water flooding vulnerability. Some of these sites are discussed below.

Table 6 - Sites with a large percentage area vulnerable to surface water flooding Surface Water Vulnerability Low Intermediate High Vulnerability Vulnerability Vulnerability Area Area Area Area Site ID Name % % % (ha) (ha) (ha) (ha) Durham Tees 91 Valley Airport 70.4 24.1 34.1% 8.3 11.7% 0.0 0.0% 9 Seal Sands 2 61.5 19.9 32.3% 4.4 7.2% 0.0 0.0% 32 Seal Sands 6 30.9 9.5 30.9% 0.5 1.5% 0.0 0.0% North Tees Pools 1 (Rec 15 Pond) 64.8 18.5 28.6% 1.6 2.5% 0.0 0.0% 11 Seal Sands 4 39.9 10.2 25.6% 2.7 6.7% 0.0 0.0% Tees Marshalling 46 Yard West 31.1 7.6 24.5% 1.3 4.1% 0.1 0.2% North Tees Pools 5 (Dormans 108 Pool) 154.4 35.9 23.3% 8.9 5.7% 0.2 0.2% North Tees 16 Pools 2 31.6 7.2 22.6% 2.2 7.0% 0.1 0.4% 58 Wynyard 2 33.4 7.2 21.6% 3.6 10.7% 0.7 2.0%

5.2.3.8 It should be noted that the Areas Vulnerable to Surface Water Flooding maps provided for this SFRA were only produced as a strategic assessment, and whilst they help identify potential depths of flooding, drainage routes and ponded areas, they should not be used for site-specific flood risk assessments. They do not preclude the need for more detailed drainage assessment during site specific FRAs rather they identify the potential problems and where greater detail is needed.

5.2.3.9 Sites of particular concern are those situated on immediate flow paths, sites with large surface water vulnerability coverage and large greenfield sites which could increase flood risk downstream. Flat floodplains adjacent to rivers and estuaries are generally susceptible to many forms of flooding, and protection from the fluvial or tidal source does not necessarily reduce or remove the other sources. Surface water flooding and management for such sites should be considered during master planning stage.

5.2.3.10 The Tees Marshalling Yard and Seal Sands were two allocation areas identified having large percentage areas vulnerable to surface water flooding. However, there are no major surface water flow pathways and these are generally low lying areas vulnerable to surface water pooling.

5.2.3.11 The proposed mitigation measures for these sites are to raise the ground above the extreme tidal flood levels. This would allow modifications in the

topography to be made in order to manage surface water runoff, using the land gradient rather then relying on surface water sewer systems. In addition, a new surface water sewer system will need to be designed for the development which, in addition to the land raising, would reduce the surface water flood risk.

5.2.3.12 Whilst it is possible for these sites to control the surface water originating on the development site, it has to be noted that where developments are located over identified flow paths, development proposals must also incorporate their impact on surface water originating further a field into their mitigation plans. As the Borough of Stockton is generally flat and low lying, there are not many major surface water flow pathways through potential development sites. Two examples are: St Michael’s School Sites (SHLAA) and Wynyard 2 (employment sites).

5.2.3.13 The above should be linked to or feed into Surface Water Management Plans (SWMP) when it is developed. An integrated approach to controlled surface water drainage can also lead to a more efficient and reliable surface water management system as it enable a wider variety of potential flood mitigation options to be used.

Appendices A. Figures

A1 Flood extents for the existing risk at Tees Marshalling Yard A2 Maximum flood depth for existing risk in the 1 in 200 year tidal flood at Tees Marshalling Yard A3 Maximum flood depth for existing risk in the 1 in 1000 year tidal flood at Tees Marshalling Yard A4 Maximum flood depth for existing risk in the 1 in 200 year tidal flood +100 years climate change at Tees A5 Maximum flood hazard for existing risk to people for the 1 in 200 year tidal flood at Tees Marshalling Yard A6 Maximum flood hazard for existing risk to people for the 1 in 1000 year tidal flood at Tees Marshalling Yard A7 Maximum flood hazard for existing risk to people for the 1 in 200 year tidal flood +100 years climate A8 Flood extents for Tees Marshalling Yard with all flood defences removed A9 Maximum flood depth for the 1 in 200 year tidal flood at Tees Marshalling Yard with all flood defences removed A10 Maximum flood depth for the 1 in 1000 year tidal flood at Tees Marshalling Yard with all flood defences removed A11 Maximum flood depth for the 1 in 200 year tidal flood +100 years climate change at Tees Marshalling Yard with all flood defences removed A12 Impact on flood extents with mitigation option 1 A13 Impact on flood depth with mitigation option 1 A14 Impact on flood extents with mitigation option 2 A15 Impact on flood depth with mitigation option 2 A16 Impact on flood extents with mitigation option 3 A17 Impact on flood depth with mitigation option 3 A18 Flood extents for the existing risk at the Bowesfield and Boathouse Lane A19 Maximum flood depth for existing risk in the 1 in 100 year fluvial flood at the Bowesfield and Boathouse Lane sites. A20 Maximum flood depth for existing risk in the 1 in 1000 year fluvial flood at the Bowesfield and Boathouse Lane sites. A21 Maximum flood depth for existing risk in the 1 in 200 year tidal flood +100 years climate change at the Bowesfield and Boathouse Lane sites. A22 Maximum flood hazard for existing risk to people for the 1 in 100 year fluvial flood at Bowesfield and Boathouse Lane sites. A23 Maximum flood hazard for existing risk to people for the 1 in 1000 year fluvial flood at Bowesfield and Boathouse Lane sites. A24 Maximum flood hazard for existing risk to people for the 1 in 200 year tidal flood +100 years climate change at the Bowesfield and Boathouse Lane sites A25 Undefended flood extents for the Seal Sands and Haverton Hill Sites A26 Undefended maximum flood depth for the 1 in 200 year tidal flood at Seal Sands and Haverton Hill Sites A27 Undefended maximum flood depth for the 1 in 1000 year tidal flood at Seal Sands and Haverton Hill Sites A28 Undefended maximum flood depth for the 1 in 200 year tidal flood +100 years climate change at Seal Sands and Haverton Hill Sites A29 Undefended maximum flood hazard to people for the 1 in 200 year tidal flood at Seal Sands and Haverton Hill Sites A30 Undefended maximum flood hazard to people for the 1 in 1000 year tidal flood at Seal Sands and Haverton Hill Sites A31 Undefended maximum flood hazard to people for the 1 in 200 year tidal flood +100 years climate change at Seal Sands and Haverton

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