4. Site Specific Assessments
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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 19 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 20 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. 21 • 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.