FINAL HNL Appraisal Package 2 Pinn and Cannon Brook Initial Assessment Plus Document The Environment Agency March 2018 HNL Appraisal Package 2 Pinn and Cannon Brook IA plus document Quality information Prepared by Checked by Approved by Andy Mkandla Steve Edwards Fay Bull Engineer, Water Associate Director, Water Regional Director, Water Laura Irvine Graduate Engineer, Water Stacey Johnson Graduate Engineer, Water Revision History Revision Revision date Details Authorized Name Position Distribution List # Hard Copies PDF Required Association / Company Name Prepared for: The Environment Agency AECOM HNL Appraisal Package 2 Pinn and Cannon Brook IA plus document Prepared for: The Environment Agency Prepared by: Andy Mkandla Engineer E: [email protected] AECOM Infrastructure & Environment UK Limited Royal Court Basil Close Derbyshire Chesterfield S41 7SL UK T: +44 (1246) 209221 aecom.com © 2018 AECOM Infrastructure & Environment UK Limited. All Rights Reserved. This document has been prepared by AECOM Infrastructure & Environment UK Limited (“AECOM”) for sole use of our client (the “Client”) in accordance with generally accepted consultancy principles, the budget for fees and the terms of reference agreed between AECOM and the Client. Any information provided by third parties and referred to herein has not been checked or verified by AECOM, unless otherwise expressly stated in the document. No third party may rely upon this document without the prior and express written agreement of AECOM. Prepared for: The Environment Agency AECOM HNL Appraisal Package 2 Pinn and Cannon Brook IA plus document Table of Contents 1. Introduction .................................................................................................................................................. 6 1.1 Overview ........................................................................................................................................... 6 1.2 Objectives ......................................................................................................................................... 6 1.3 Key Documents ................................................................................................................................ 6 1.4 Supporting Documents ..................................................................................................................... 6 2. Site Background........................................................................................................................................... 7 2.1 Overview ........................................................................................................................................... 7 2.2 Project History .................................................................................................................................. 8 2.3 Existing Conditions ........................................................................................................................... 9 2.4 Flood History and Risk .................................................................................................................... 10 2.5 Constraints ..................................................................................................................................... 11 2.6 Existing Services ............................................................................................................................ 11 3. Process and Methodology ......................................................................................................................... 11 3.1 General ........................................................................................................................................... 11 3.2 Sizing of options ............................................................................................................................. 12 3.2.1 Flood Storage areas ....................................................................................................................... 12 3.2.2 Flood walls and embankments ....................................................................................................... 12 3.3 Economic analysis .......................................................................................................................... 12 3.3.1 Flood Storage ................................................................................................................................. 13 3.3.2 Flood Walls and Embankments ...................................................................................................... 15 4. Shortlisted Options..................................................................................................................................... 15 4.1 Do Nothing ...................................................................................................................................... 16 4.2 Do Minimum ................................................................................................................................... 16 4.3 Do Something ................................................................................................................................. 16 4.3.1 Initial Assessment 1 ........................................................................................................................ 17 4.3.1.1 Hatch End ....................................................................................................................................... 17 4.3.1.2 Pinner ............................................................................................................................................. 19 4.3.2 Initial Assessment 2 ........................................................................................................................ 20 4.3.2.1 East Ruislip ..................................................................................................................................... 20 4.3.3 Initial Assessment 3 ........................................................................................................................ 23 4.3.3.1 West Ruislip .................................................................................................................................... 23 4.3.4 Initial Assessment 4 ........................................................................................................................ 25 4.3.4.1 Ickenham ........................................................................................................................................ 25 4.3.5 Initial Assessment 5 ........................................................................................................................ 29 4.3.5.1 Hillingdon ........................................................................................................................................ 29 4.3.6 Initial Assessment 6 ........................................................................................................................ 31 4.3.6.1 Yiewsley.......................................................................................................................................... 31 5. Options and economic summary................................................................................................................ 33 6. Conclusions and recommendations ........................................................................................................... 35 6.1 Sensitivities ..................................................................................................................................... 35 6.2 Residual benefits ............................................................................................................................ 35 6.3 Key risks ......................................................................................................................................... 35 6.4 Option prioritisation ......................................................................................................................... 36 6.5 Funding, consultation and contributions ......................................................................................... 42 6.6 Modelling ........................................................................................................................................ 43 6.7 SuDS .............................................................................................................................................. 44 Appendix A : Options optimisation spreadsheet .................................................................................................... 45 Appendix B : Shortlisted options mapping ............................................................................................................. 46 Appendix C : Long list of options ........................................................................................................................... 47 Appendix D : Long list options mapping ................................................................................................................ 48 Prepared for: The Environment Agency AECOM HNL Appraisal Package 2 Pinn and Cannon Brook IA plus document Appendix E : Economics assumptions .................................................................................................................. 49 Appendix F : Excluded Options ............................................................................................................................