LONG WHATTON & DISEWORTH FLOOD RISK MITIGATION & RESILIENCE STUDY Final Model Report AUGUST 2020 CONTACTS SIMON AINLEY Project Manager dd +01752 689006 Arcadis. e [email protected] 34 York Way London N1 9AB Copyright © 2018 Arcadis. All rights reserved. arcadis.com VERSION CONTROL Version Date Author Checker Approver Changes 01 09/06/2020 S Ainley J Sourbutts N McClung First Issue 02 16/07/2020 S Ainley J Sourbutts N McClung Second Issue 03 25/08/2020 S Ainley J Sourbutts N McClung Second Issue This report dated 25 August 2020 has been prepared for Leicestershire County Council (the “Client”) in accordance with the terms and conditions of appointment (the “Appointment”) between the Client and Arcadis UK (“Arcadis”) for the purposes specified in the Appointment. For avoidance of doubt, no other person(s) may use or rely upon this report or its contents, and Arcadis accepts no responsibility for any such use or reliance thereon by any other third party. ii CONTENTS VERSION CONTROL .......................................................................................................... II 1 SUMMARY ................................................................................................................ 1 2 PROJECT SCOPE & CONTEXT ............................................................................... 2 2.1 Previous Investigations ....................................................................................................................... 2 2.2 Scope of this Project ............................................................................................................................ 2 2.3 Project Objective .................................................................................................................................. 3 2.4 Data & Information ................................................................................................................................ 3 3 MODEL BUILD .......................................................................................................... 4 3.1 Software ................................................................................................................................................. 4 3.2 Modelling Approach ............................................................................................................................. 4 3.3 Feature Conceptualisation................................................................................................................... 4 3.4 Catchment Topography ....................................................................................................................... 9 3.5 Modelling Assumptions & Limitations ............................................................................................. 14 4 HYDROLOGY & RAINFALL ................................................................................... 15 4.1 Catchment Runoff ............................................................................................................................... 15 4.2 Rainfall Events .................................................................................................................................... 17 5 MODEL VALIDATION ............................................................................................. 19 5.1 Historical Flooding ............................................................................................................................. 19 5.2 EA Surface Water Flood Mapping ..................................................................................................... 22 6 FLOOD RISK ASSESSMENT ................................................................................. 24 6.1 Flooding Wetspots ............................................................................................................................. 24 6.3 East Midlands Airport ......................................................................................................................... 33 7 OPTIONS APPRAISAL ........................................................................................... 37 7.1 Flood Mitigation Strategy ................................................................................................................... 37 7.2 Option Long-list .................................................................................................................................. 37 7.3 Option Short-listing ............................................................................................................................ 41 7.4 Promoted Options Evaluation ........................................................................................................... 44 8 ECONOMIC EVALUATION ..................................................................................... 53 8.1 Overview .............................................................................................................................................. 53 iii 8.2 Scheme Costing .................................................................................................................................. 53 8.3 Flood Benefits ..................................................................................................................................... 55 8.4 FCERM Funding .................................................................................................................................. 56 9 FURTHER MITIGATION & CATCHMENT RESILIENCE ........................................ 57 9.1 Monitoring & Community Preparedness .......................................................................................... 57 9.2 Adaptation & Re-purposing Strategies ............................................................................................ 58 9.3 Natural Flood Management................................................................................................................ 59 10 CONCLUSIONS ...................................................................................................... 61 10.1 Flood Risk............................................................................................................................................ 61 10.2 Influence of the East Midlands Airport ............................................................................................. 61 10.3 Options Feasibility .............................................................................................................................. 61 10.4 Economic Viability .............................................................................................................................. 61 ..................................................................................................................... 62 Data & Information Register .......................................................................................................................... 62 ..................................................................................................................... 64 Watercourse Structures Register ................................................................................................................. 64 ..................................................................................................................... 71 2D Roughness Values .................................................................................................................................... 71 ..................................................................................................................... 73 Flood Risk Mapping ........................................................................................................................................ 73 ..................................................................................................................... 88 Infiltration Parameters.................................................................................................................................... 88 ..................................................................................................................... 91 Diseworth – Property Level Resilience, Proposed Schedule..................................................................... 91 ..................................................................................................................... 95 East Midlands Airport Ponds Active Discharge Control – Technical Overview ...................................... 95 iv FIGURES Figure 1 – Example of Property Roof Runoff in the Model ................................................................................ 6 Figure 2 – Typical Connectivity of Highway Drainage System .......................................................................... 6 Figure 3 – East Midlands Airport Surface Water Model Subcatchments & Drainage Network ......................... 7 Figure 4 – East Midlands Airport Ponds Layout and Interconnectivity (schematic) .......................................... 8 Figure 5 – East Midlands Airport Ponds Operating Regime (Schematic) ......................................................... 9 Figure 6 – Demonstration of 2D Mesh Elements and Levels .......................................................................... 10 Figure 7 – Examples of Topographic 2D Meshing Detail ................................................................................ 10 Figure 8 – Topographic Adjustments to Buildings and Roads .......................................................................