Proposed Re-development, At 321 London Road, Wyberton. Flood Risk Assessment - Revised

Transcription

1 Proposed Re-development, At 321 London Road, Wyberton Flood Risk Assessment - Revised Prepared By: C Mason C.Eng M.I.C.E May 2015

2 Contents Page No. 1. Introduction 2 2. Site Description 2 3. Flood Hazard 3 4. Probability 4 5. Climate Change 4 6. Detailed Proposals 5 7. Residual Risks 6 8. Off-site Impacts 7 9. Flood Risk Management Conclusions 8 Appendices Appendix A Exception Test Appendix B Emergency Response Strategy Figures 1. Location Plan 2. Site Survey 3. Breach Locations 4. Layout Plan 5. Building Plans 6. Hazard Plans Note: Data relating to flood risk supplied (mainly) by the Environment Agency and reproduced under licence No. CCN/2015/38844 This revised FRA takes into account comments made by the EA on the FRA dated March

3 1.0 Introduction 1.1 The owner of No. 321 London Road, Wyberton wishes to demolish the existing, derelict buildings on the site, and replace them with 4 new residential properties. 1.2 Jenny McIntee (Architectural Practice) has been instructed to prepare the required plans for submission of a planning application. As part of the application, a Flood Risk Assessment (FRA) is required, as the site is in Flood Zone 3 i.e. an area which would have a high degree of flood risk, if no flood defences existed. 1.3 This FRA has been prepared by C. Mason C Eng M.I.C.E. It takes account of the requirements of the National Planning Policy Framework (NPPF) and the accompanying Technical Guide. It relies on data provided by the Environment Agency under Licence No. CCN/2015/38844 and it refers to data in Boston District Councils Strategic Flood Risk Assessment (BDC SFRA). Reference is also made to data supplied by Black Sluice Internal Drainage Board (BSIDB). 2.0 Site Description 2.1 The site location is shown on Figure 1- Location Plan. The Post Code is PE21 7AU and the Grid Reference , The existing buildings on the site include previous residential properties and an old Smithy, together with sheds and outbuildings. The site is situated in a wellestablished residential area, with access available from both London Road and Ralphs Lane. 2.3 Ground levels shown on a Topographical Survey carried out by Axis Surveys Ltd, vary between 2.90m OD and 3.42m OD, with an average level of 3.20m OD (See Figure 2 Site Survey). 2.4 The current land usage would be classed as more vulnerable in Table 2 of the NPPF Technical Guide. The proposed use for residential purposes will remain classified as more vulnerable. 2.5 The EA Flood Map shows that the site is in Flood Zone 3 (a), and this is confirmed by maps in the Boston District Council s SFRA. This indicates that the site is in an area which could be affected by flooding from the sea by a flood that has a 0.5% (1 in 200) or greater chance of happening each year, or from a fluvial river flood that has a 1% (1 in 100) or greater chance of happening each year. This assumes that no flood defences are present. 2.6 More vulnerable classes of development are acceptable in Flood Zone 3 (a), subject to application of the Sequential and Exception Tests. As regards the Sequential Test, the re-development of this brownfield site can only take place on the site. Hence, consideration of alternative sites is irrelevant. The Exception Test, as defined in the NPPF, requires 2 criteria to be met. These are considered in Appendix A Exception Test. 2

4 2.7 Much of the existing site is covered by impermeable surfaces. Re-development may, marginally increase the total equivalent impermeable area (see Section 6 Detailed Proposals). 3.0 Flood Hazard 3.1 The following potential sources of flooding were identified :- Tidal - The Haven Fluvial - South Forty Foot Drain Arterial Drains in Black Sluice IDB area Public sewers Highway drains On site drainage systems 3.2 Tidal flooding could occur by overtopping of the defences, or as a result of the defences failing (i.e. breaching). The defences nearest to London Road consist of hard floodwalls (brick/concrete) with typical crest levels around 6.10m ODN. The Environment Agency s year 2006 Tidal Model predicts a 0.5% tide of 5.93m ODN. Hence, there is a small freeboard against overtopping. The structural condition of the defences is good and failure by breaching is unlikely. Modelling of possible breaches has been carried out for the E.A. Locations are shown in Fig Rising tide levels in the future will increase the possibility of overtopping and breaching. These factors are considered in Section 5 Climate Change and Section 7 Residual Risks. 3.4 The 1%, plus climate change, water level in the South Forty Foot has been modelled at 2.88m ODN. This is considerably higher than the normal operating levels, but still almost half a metre below ground levels at the site. Hence, this potential source of flooding can be discounted. 3.5 The site is located near the boundaries of several Black Sluice IDB catchment boundaries. However, it is believed that the area drains into the Chain Bridge Catchment. Typical water levels in the arterial drains are:- Wyberton Town Drain 1.56m ODN, Old London Road (B1397) 1.61m ODN, and Wyberton by Pass (A16) 1.63m ODN. At Wyberton Marsh P St, operating levels are 2.12m ODN and in Frampton Town Drain 2.55m ODN. All of these levels are well below the average ground level on the site of 3.20m ODN. Hence, the risk of flooding from arterial drainage systems is minimal. 3.6 Anglian Water sewer plans show a public foul sewer flowing eastwards in Ralphs Lane (150mm V.C). This connects into a 225mm V.C sewer on the east side of London Road, flowing northwards (see Figure 3 Anglian Water Sewers). Surcharging of these sewers, resulting from blockages or hydraulic overloading, could lead to flooding on or around the site. However, the sewers are over 2.0m deep, and any resultant flooding is likely to be shallow. It would be quickly dealt with by Anglian Water to avoid flooding in properties. 3.7 No surface water sewers are shown on AWs Plans, but there are gullies in both Ralphs Lane and London Road. There is also a minor watercourse shown to the 3

5 south side of No. 333 London Road. If the localised road drainage system became overloaded, or gullies became blocked, then localised flooding could occur which may impact on the site. Depths would be shallow, with flows following the directions of falls in the road profiles (these appear to be to the south and west, although the whole area is relatively flat). 3.8 On site surface water drainage systems will depend on the permeability of the ground, with preference being given to the use of soakaways. Foul drainage will be connected to AW public sewers. Flooding could occur if pipelines become blocked, but this is a residual risk which can be mitigated by regular inspection and maintenance. Surface water pipe sizes will take into account run-off from a 1 in 100 year, 6 hour storm, with an allowance for climate change. 4.0 Probability 4.1 The Boston DC SFRA shows the relative probability of flooding, from both tidal and fluvial sources, as LOW, allowing for the presence of flood defences (i.e. between 1% and 0.1% per annum). 4.2 The probability of surface water flooding in the locality is shown on EA Maps as LOW (i.e. between 1 in 100 (1%) to 1 in 1000 (0.1%)). However, this seems lower than usual and a figure of between 1 in 30 and 1 in 100 years would seem more realistic i.e. 3.33% to 1% particularly taking into account the localised highway drainage systems. 4.3 The probability of flooding from other sources i.e. arterial drains and underground water is negligible. 5.0 Climate Change 5.1 The NPPF Technical Guidance document provides information to enable potential impacts of climate change to the taken into account. 5.2 Table 4 contains recommended contingency allowances for net sea level rises for periods up to 2115 and Table 5 gives recommended national precautionary sensitivity ranges for peak rainfall intensities, peak river flows, off shore wind speeds and wave heights. 5.3 To the year 2115, these factors are quoted as:- Peak river flow +20% Tide level increase above 1.0m Peak rainfall intensity +30% 5.4 There is continuing debate regarding the future predicted impacts of climate change, particularly on the long term increases in sea level rise. However, if tidal levels in the Haven did rise by over 1.0m, then considerable improvements would be required to the sea defences, to maintain current standards of protection. 5.5 The Environment Agency, working with other organisations (including Port Authorities) is seeking to ensure that Shore-line Management Plans (SMPs) and Catchment Flood Management Plans (CFMPs) are complied with. These will ensure 4

6 that appropriate standards of protection are retained where it is economically viable to do so. 5.6 The River Witham CFMP, indicates that the Environment Agency will adopt a Flood Risk Management Policy involving improvements at Boston, which allows for the impacts of climate change. Hence, the site is likely to remain at similar or improved standards of Tidal Defence, as those which currently exist, into the foreseeable future. 6.0 Detailed Proposals 6.1 The proposed re-development is shown on Figure 4 Layout Plans, and Figure 5 Building Plans. 6.2 Roof water will be taken to soakaways, constructed in compliance with the Building Regs. Pathways will be a gravel or similar permeable materials, and foul sewage will be connected to the main public sewers. Access to the bungalow will be from Ralphs Lane, and access to the 3 houses from London Road. 6.3 The areas of the various elements are:- Overall site area 1280m 2 Roof areas 396m 2 Driveways 140m 2 (permeable materials) Landscaping and gardens 744m m 2 Total This gives a future equivalent impermeable area of:- 396 x 0.9 = x 0.3 = x 0.1 = m 2 say 445m 2 The existing site has less roof area, but more hard surfaces concrete paths and tarmac areas. The equivalent impermeable area is:- Roof areas 263m 2 x 0.9 = Hard Surfaces 144m 2 x 0.7 = Grassed areas 873m 2 x 0.1 = m 2 The increased equivalent impermeable area is only approximately 21m 2. Hence, there will be no significant increase in surface water run-off from the site. Soakaways will be designed to accept run-off in a 1 in 100 year, 6 hour storm plus the impacts of climate change. It flows had to be balanced to pre-existing run-off rates, before discharge, a total storage volume of approximately 18m 3 has been calculated assuming a rainfall event of 150mm in 6 hours. Taking into account the individual roof areas, storage volumes of 5.75m 3, 4.50m 3 (twice) and 3.25m 3 would be needed for the bungalow, the 2, 4 bedroom houses and the 3 bedroom house. If necessary, these storage volumes could be achieved by the use of geo-cellular crates. However, the strong preference is for soakaways designed in accordance with BRE Digest 365, and the Building Regulations. 5

7 6.4 The ground floor levels will be raised by 650mm. This is maximum amount that can be achieved within the constraints of the site dimensions whilst keeping the gradients of ramps reasonable. The finished floor levels of no lower than 3.85m ODN should be above any potential flood level. However, a degree of flood resistance and flood resilience will be included in the design of the buildings. This issue is considered further in Section 9 Flood Risk Management. 6.5 Foul sewers will be connected to the existing public sewers, at locations approved by Anglian Water. 7.0 Residual Risks 7.1 The main residual risks which could impact on the site are overtopping or failure of the Haven defences. Others, which would have lesser consequences are blockages or system failures in the public sewerage systems. 7.2 The Environment Agency have supplied data relating to hazardous conditions arising from overtopping and breaching of the Haven defences. This computer generated 2D, Hazard Mapping is reproduced n Figure 6 Hazard Maps 7.3 From the Hazard Maps the following data has been extracted:- Breach Year Annual Chance % (1 in 200) % (1 in 1000) % (1 in 200) % (1 in 1000) Overtopping % (1 in 200) % (1 in 1000) Max. Velocity (m/s) Max. Depth (m) Max. Hazard Low Low Danger for Most Danger for Most NIL NIL NIL NIL NIL NIL Note: The site is not affected by overtopping for present day (2006) scenarios, or for future (2115) scenarios. Hence, the only residual risk is if a breach occurs. The probability of this occurring is very low. The defences are in good condition, and they are inspected regularly. 7.4 At times of flood dry access/egress in the area would not be possible. However, even if a breach occurred, there would be periods of up to 8 hours or so in which flood water would not be issuing from the river (as tides fall below the level of the gap in the defences). During these periods it is reasonable to assume that Emergency Service vehicles would be able to safely operate along London Road 6

8 (hazard ratings would be significantly reduced from peak values, as flood water spread out and reduced in both depth and velocity). 7.5 Blockages and failures in the public sewerage systems would be quickly dealt with by Anglian Water and consequential flooding would be temporary and probably shallow. 8.0 Off-site Impacts 8.1 No adverse off-site impacts are envisaged as a result of this re-development. 9.0 Flood Risk Management 9.1 The property will be connected to the Environment Agency s Flood Warning Direct system, and an Emergency Response Plan will be drawn up and implemented, similar to the example given in Appendix B. 9.2 The final list of flood resistant/resilience measures should be agreed between the owner (developer), the architects and the Council s Building Inspectors. Appropriate selection of items will help to keep house insurance premiums down. Useful advice can be found in the publication Homeowners Guide to Flood Resilience A living Document, and in the publication Improving the Flood Performance of New Buildings Flood Resilient Construction (produced by the Department of Communities and Local Government). 9.3 Consideration should be given to:- The use of barriers for doors and windows Installation of electric ring mains at first floor level with drops to ground floor sockets and switches. Placing electrical sockets at least 1.0m above ground floor level. Using non-return values on foul sewerage systems. The use of water resistant materials for floors (e.g. 150mm concrete slab with polythene membrane at least 1200 garage). Floor finishes of either sand/cement screeds, ceramic tiles or stone tiles. The use of PVC or ceramic tiles for skirting boards. The use of cement/lime render for walls (1:6:1 cement:sand:lime). Self-sealing covers on airbricks (if applicable). Effective seals at service entry points. At depths predicted on the Hazard Maps, for 2115, the Environment Agency prefer floor levels to be raised by 1.0m, and flood resilience to be incorporated 300mm above predicted flood level. This is not possible on this site, because of relative levels of adjacent properties. Hence, a compromise is suggested where floor levels are raised 650mm above average ground levels, barriers are provided to doors, for a further 650mm, and flood resilience included inside the buildings to a height of at least 1.0m above floor levels. 7

9 10.0 Conclusions 10.1 As a result of this revised FRA the following conclusions have been reached:- The site is in Flood Zone 3 (a) as shown by the EAs Maps and maps in the SFRA. The current use of the land is classified as more vulnerable and the replacement buildings will have the same classification. More vulnerable uses are acceptable in Flood Zone 3 (a) subject to the Sequential and Exception Tests being satisfied. The re-development of these derelict buildings can only be achieved on this site. Hence, the sequential test is not applicable in this case. The Exception Test is shown to be satisfied, by this assessment (see Appendix A). The only significant risk of flooding at this site is from the tidal Haven, and this is related to a very low probability of a breach occurring. Overtopping of the Haven defences does not appear to cause flooding at this site, even in the long term (see EA Hazard Maps) Flood resistant/resilience measures can be included in the design of the new building, to mitigate any residual flood risks. Floor levels will be raised by 650mm, and barriers used on doors. Flood resilience measures will be at least 1.0m above floor levels (see Section 9.3) The use of an Emergency Response Plan will help to increase awareness, and improve safety for residents The re-development of this brownfield site would improve the environment and would help to avoid potential anti-social uses of the derelict buildings. Note. The Environment Agency are promoting and progressing a major project for the construction of a Tidal Barrier which will reduce tidal flood risk in Boston Overall, it is concluded that with appropriate mitigation, the re-development of this site for residential purposes can be achieved safely, without increasing flood risk elsewhere. The site has previously had an outline planning application approved for 4 properties (B/05/0326 approved on 18 th July 2005). Hence, the general principles of re-developing the site were accepted, at that time. 8

10 Appendix A Exception Test A1. The NPPF requires a sequential approach to be adopted which seeks to avoid locating development in relatively high flood risk areas, if possible. However, it is accepted that, where an individual proposes to re-develop their property in an existing flood risk area, the consideration of alternative sites is not likely to be a realistic option. A2. As residential development has been selected for replacement building then the exception test must be applied. For the test to be passed, 2 criteria must be satisfied:- The development must provide wider sustainability benefits to the community that outweigh flood risk etc. Residential developments are needed in Boston, and this brownfield site is becoming derelict, with the strong possibility of being used for anti-social purposes. A FRA must demonstrate that the development will be safe, without increasing flood risk elsewhere, and where possible reduce flood risk overall This FRA demonstrates that the site can be developed safely without increasing flood risk elsewhere. It also shows that mitigation measures can reduce the potential consequences of flooding, by raising floor levels, by introducing resilient construction and by improving the awareness of flooding impacts and self-help measures. 9

11 Appendix B Emergency Response Strategy B1. In situations such as those which exist at London Road i.e. where there is a relatively LOW probability of flooding but potential for SIGNIFICANT consequences, flood warning and emergency response procedures are particularly important. The Emergency Response Strategy for the London Road site will be based on 3 factors:- Increasing awareness Ensuring adequate preparedness Ensuring co-ordinated action B2. Residents will be made aware of:- The nature and extent of potential flooding (low probability/significant consequences) The Environment Agency s Flood Warning Service (get to know the Flood Warning Codes, and the need to register with Floodline) The importance of self-help measures (the need for a Flood Plan actions before and during flooding) Actions to be taken in severe events (protect what you can, but evacuate when told, act on advice of emergency services) An information pack will be supplied containing details of all the above items, with telephone contact numbers, guidance notes, and check lists (as per advice provided by the Environment Agency, Lincolnshire County Council and Boston Borough Council). B3. To ensure adequate preparedness all occupiers will be encouraged to register for the Environment Agency s Flood Warning Service Floodline Warnings Direct). B4 When Flood Warning messages are received all residents should take actions defined in their information packs. These will include, inter alia:- Moving as much as possible upstairs to minimise flood damage. Locating and securing emergency supplies Turning off gas, electricity and water supplies Listing to the local radio station for updates and/or calling Floodline. B5. If an event leads to the issuing of a Severe Flood Warning then residents should listen to, and act on the advice of the emergency services (and of Flood Wardens if any are operating in the locality). 10

12 B6. Following exercise Watermark (in March 2011), various recommendations were made which will influence how major flooding incidents are dealt with, in the future. Flood Wardens in various localities will be kept aware of the developing processes involving multi-agency co-ordination, and they could arrange for essential, key facts to be communicated to all residents (possibly by presentations made by Council or Environment Agency Officers). B7. The main aims of the Emergency Response Strategy are:- To avoid loss of life To save important documents (e.g. Birth/marriage certificates, insurance policies etc.) To protect whatever can be done safely (e.g. electrical goods, TV etc.) To minimise property damage (Note: The process should take into account the needs of children and vulnerable people, and special needs of ethnic groups). 11

13 Figures 1. Location Plan 12

14 2. Site Survey 13

15 3. Breach Locations 14

16 4. Layout Plan 15

17 5. Building Plans Plot 1 Plot 2 16

18 Plot 3 Plot 4 17

19 6. Hazard Plans 18

20 19