FLOOD RISK AND SURFACE WATER DRAINAGE ASSESSMENT HILLHOUSE RESTORATION SITE, OFF JAMESON ROAD, THORNTON CLEVELEYS ON BEHALF OF NPL ESTATES

Transcription

1 FLOOD RISK AND SURFACE WATER DRAINAGE ASSESSMENT HILLHOUSE RESTORATION SITE, OFF JAMESON ROAD, THORNTON CLEVELEYS ON BEHALF OF NPL ESTATES Integra Consulting Engineers Limited NS / 2543 Fountain House Fountain Street Manchester M2 2EE April 2011

2 CONTENTS 1. INTRODUCTION 2. STANDARDS AND LIMITATIONS 3. CURRENT SITUATION 3.1 Site Location and Description 3.2 Hydrology and Flooding 3.3 Probability of Flooding 3.4 Climate Change 3.5 Existing Site Surface Water Drainage 4. PROPOSED DEVELOPMENT 4.1 Development Proposals 4.2 Surface Water Drainage Proposals 4.3 Construction 5. CONCLUSIONS 5.1 Flooding 5.2 Site Surface Water Drainage 5.3 Flood Risk Management Measures 5.4 Residual Risk 2

3 APPENDIX Appendix 1 Site Location Plan Appendix 2 Existing Topographical Survey Appendix 3 Proposed Site Development Plan 3

4 1. INTRODUCTION 1.1 Integra Consulting have been commissioned by NPL Estates to undertake a Flood Risk and Surface Water Drainage Assessment (FRA) for Hillhouse Restoration site off Jameson Road, Thornton, Cleveleys. This document is submitted in support of the detailed planning application for the restoration works proposed at the application site. 1.2 The objective of the FRA was to identify potential flooding issues and any consequent implications on the proposed development. Both existing and proposed surface water drainage for the site development are also considered in detail. The assessment has been undertaken in accordance with the pro-forma guidance contained in Planning Policy Statement 25 Practice Guide The Assessment of Flood Risk dated June 2008 and Annex E of Planning Policy Statement 25 paragraph E It is noted that Planning Policy Statement 25 Practice Guide The Assessment of Flood Risk states that the content of an FRA should always be appropriate to the scale and nature of the development. 1.4 Consultations have been undertaken with the Environment Agency (EA), Wyre BC and United Utilities (UU) - the results of these discussions are reflected in this document. 4

5 2. STANDARDS AND LIMITATIONS This report has been prepared solely for use by NPL Estates. It shall not be relied upon or transferred to any other party without the prior written authorisation of Integra Consulting. The findings and opinions in the report are based on information derived from a variety of different sources. Integra Consulting do not accept any liability for the accuracy or otherwise of any information provided by third parties. It should be noted that some of the aspects considered in this study may be subject to change with time. Therefore, if the development is delayed or postponed, consideration may need to be given to reviewing such issues to confirm that no changes have taken place, either at the site or within relevant legislation. To date we have consulted with the Environment Agency, Wyre BC and United Utilities. Further consultations are likely as the scheme progresses to detailed design stage. It should be noted that even where responses have been received from Regulators, these could be subject to change at a later date. 5

6 3. CURRENT SITUATION 3.1 Site Location and Description The site covers an area of approximately 4.3 hectares and is located to the north / north east of Fleetwood Road, Cala Gran Caravan Park and the former Fleetwood to Poulton le Fylde railway line and directly south of Jameson Road landfill site. Jameson Road lies to the north west of the application site with the Wyre Estuary to the east as shown on the site location plan in Appendix 1. The site has previously operated as a landfill and it is now proposed to undertake restoration works including remediation / reclamation of the existing site lagoons and the introduction a series of ditches within the site to provide and encourage ecological habitat. It is proposed to raise the current site levels significantly through the formation of a series of earth mounds in order to create a new soft landscaped topography. Following liaison with the EA, it is confirmed that the site lies in Flood Zone 1 but, on the basis that the site is over 1 hectare in area, it is necessary to provide a Flood Risk Assessment to accompany the detailed planning application for the proposed site restoration works. 3.2 Hydrology and Flooding Following liaison with the EA, it has been confirmed that the site lies in Flood Zone 1. On this basis, no detailed flood analysis is appropriate for the application site. It is noted that the application site is classified as more vulnerable in accordance with Table D2 in PPS25 on the basis of the previous site use as a landfill. 3.3 Probability of Flooding With reference to the relevant Environment Agency flood map, the site lies in Flood Zone 1 which comprises land assessed as having a less than 1 in 1000 annual probability of river or sea flooding in any year (<0.1%). From discussions with United Utilities and Wyre Borough Council, there are no historical records of flooding from sewers, highway drainage, overland flow or groundwater at the site. Following liaison with the EA, it has been confirmed that the site lies in Flood Zone 1 and no detailed flood analysis is therefore required for the application site. 6

7 3.4 Climate Change On the basis that the application site is to remain fully soft landscaped following restoration works, greenfield post development surface water discharge rates will apply and there is therefore no requirement for site attenuation of surface water in the post-restoration works situation. 3.5 Existing Site Surface Water Drainage The existing landfill site consists of Dry Tip Numbers 1 to 4 inclusive together with Lagoon Numbers 1 and 2. From a surface water drainage perspective, the two lagoons discharge surface runoff to the Wyre Estuary in the north eastern and south eastern corners of the site via a series of on site drainage ditches. Surface water from the four dry tips currently drains through infiltration. 7

8 4. PROPOSED DEVELOPMENT 4.1 Development Proposals The proposed application covers a detailed planning application for restoration of the former Hillhouse Landfill site. The application site is classified as more vulnerable in Table D2 of PPS25 due to its current landfill status. The proposed development is situated in Flood Zone 1 and there is therefore no requirement for any detailed flood analysis / modelling. 4.2 Surface Water Drainage Proposals The issue of surface water drainage to the post restoration site has been considered with reference to the hierarchy of surface water disposal as noted in the Building Regulations H3: i) Sustainable urban drainage systems (SUDS) ii) Discharge of surface water off site direct to watercourse iii) Discharge to adopted sewer Using the above hierarchy, the potential for using soakaways at the site was considered in relation to the nature of the existing soils. A study of the relevant BGS geological maps indicates that the site is underlain by landfill materials overlying relatively impermeable Tidal Flat clays and silts over mudstone bedrock. Furthermore, from a study of local BGS boreholes, a relatively high groundwater table is anticipated at the application site. SUDS are made up of one or more structures built to manage surface water runoff. They are used in conjunction with good management of the site to prevent flooding and pollution. There are four general methods of control: Filter strips and swales Filter drains and permeable surfaces Infiltration devices Basins and ponds 8

9 However, on the basis of the potential for soil contamination associated with the previous landfill site use, the use of post restoration surface water drainage soakaway systems is considered inappropriate. The Wyre Estuary is located at the north east corner of the application site. Discharge of surface water off the site into this watercourse is clearly feasible and will ensure that all surface water drainage will take place and is controlled within the site boundaries. Discharge of surface water from the post restoration re-modelled soft landscaped site surface will also act to ensure that there will be no off site flood flows during extreme weather conditions that may affect third party land ownerships. The re-modelling of the site topography will involve the formation of a series of raised soft landscaped mounds which typically fall at a gradient of circa 1 in 25 in order to encourage surface run off and prevent ground percolation. It is proposed that a series of land drains are constructed within the application site in order to collect surface run off from the re-modelled landscape which will discharge to the Wyre Estuary at the north eastern corner of the site. 4.3 Construction The final drainage system will be confirmed during the final drainage detailed design process. This system will be designed in order to prevent any existing surface water discharging from the site to the public sewers that service the area. 9

10 5. CONCLUSIONS 5.1 Flooding From discussions with United Utilities and Wyre Borough Council, there are no historical records of flooding from sewers, highway drainage, overland flow or groundwater at the site. Following liaison with the EA, it has been confirmed that the site lies in Flood Zone 1 and no detailed flood analysis is therefore required for the application site. The Environment Agency has stipulated that there are to be no off site surface water flood routes generated by the development during an enhanced 1 in 100 year storm. 5.2 Site Surface Water Drainage The issue of surface water drainage to the proposed development has been considered with reference to the hierarchy of surface water disposal as noted in the Building Regulations H3. SUDS infiltration systems are not considered likely to be appropriate for the development site, principally due to the potential presence of soil contamination associated with the previous landfill site use. The Wyre Estuary is located at the north eastern corner of the application site. Discharge of surface water off the site into this watercourse is clearly feasible and will ensure that all surface water drainage will take place and is controlled within the site boundaries. Discharge of site surface water from the re-modelled soft landscaped surface will also act to ensure that there will be no off site flood flows during extreme weather conditions that may affect third party land ownerships. It is proposed that a series of land drains are constructed within the application site at the base of the earth mounds in order to collect surface run off from the re-modelled site the land drains will then be connected to a collector drain prior to discharging to the Wyre Estuary at the north eastern corner of the site. 10

11 5.3 Flood Risk Management Measures There will be a site management Health and Safety document prepared in respect of the site. 5.4 Residual Risk There will be no off site overland flood flows generated by the proposed development. The proposed site restoration works will provide a positive impact on local flora and fauna that depend on the local watercourses for their survival. 11

12 APPENDIX 12

13 APPENDIX 1 SITE LOCATION PLAN 13

14 APPENDIX 2 EXISTING TOPOGRAPHICAL SURVEY 14

15 APPENDIX 3 PROPOSED SITE DEVELOPMENT PLAN 15

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