Witherley, Mythe Lane DETAILED FLOOD INVESTIGATION Investigation 2012-040 Ref.: Investigation: Witherley, Mythe Lane Date of 26/06/2012 Flooding: Revision FINAL David Smith Officer: R.JONES 5/11/14 Associates Senior Checked: E.KNIGHT 5/11/14 Technician Approved: J.MCGUINNESS Senior Engineer 5/11/14 D S A DAVID SMITH ASSOCIATESConsulting Structural & Civil Engineers 8 DUNCAN CLOSE MOULTON PARK NORTHAMPTON NN3 6WL TEL: (01604) 782620 FAX: (01604) 782629 email: [email protected] Web: www.dsagroup.co.uk Leicestershire County Council Environment and Transport Department, Technical Services Group, County Hall, Glenfield, Leicester. LE3 8RJ. PAGE CONTENTS NO. 1. Executive summary 2 2. Introduction 3 3. Site background 5 4. Flooding incident 6 5. Summary of impacts and findings 8 6. Responsibilities 9 7. Conclusions 11 8. Recommendations 12 9. Useful Links 14 Appendix A – Site Location Plan Appendix B – Existing Drainage System Appendix C – Flood Map for Surface Water (FMfSW) Appendix D – Flood Zones Map Appendix E – EA Standard Notice 1 1 EXECUTIVE SUMMARY 1.1 STATUTORY CONTEXT Section 19 of the Flood and Water Management Act 2010 (FWMA) states that, on becoming aware of a flood which meets certain predetermined criteria, the LLFA (Lead Local Flood Authority) must undertake a formal flood investigation in order to determine the relevant flood risk management authorities involved and which flood risk management actions have been (or should be) taken to mitigate future flood risk. Where an authority carries out an investigation, it must publish the results. This Flood Investigation Report (FIR) has been completed by David Smith Associates on behalf of Leicestershire County Council. It was deemed necessary to complete a formal investigation into the flood incident at Mythe Lane, Witherley, following the initial flooding on the 15 June 2012 and subsequent internal flooding of at least two properties on the 26th November 2012. 1.2 CAUSE OF FLOODING The flooding that occurred at Mythe Lane, Witherley is a result of the intense rainfall that fell over a short period of time. The inability of existing drainage systems to outfall effectively meant surface water flowed over ground to low points on Mythe Lane. 1.3 MAIN FINDINGS Existing drainage systems were unable to cope and outfall effectively to the Witherley Brook; this caused flooding of the highway and consequently restricted access to a number of properties. When the level of the Witherley Brook is below the highway drainage outfall the system does not cause flooding (normal conditions). When the highway outfall is below water level in the Witherley Brook, surface water food risk significantly increases. The level in the Witherley Brook is primarily governed by downstream levels in the River Anker as opposed to run-off from the Witherley catchment. 2 2 INTRODUCTION 2.1 LEAD LOCAL FLOOD AUTHORITY INVESTIGATION Section 19 of the FWMA states: (1) On becoming aware of a flood in its area, a lead local flood authority must, to the extent that it considers necessary or appropriate, investigate: (a) which risk management authorities have relevant flood risk management functions, and (b) whether each of those risk management authorities has exercised, or is proposing to exercise, those functions in response to the flood. (2) Where an authority carries out an investigation under subsection (1), it must - (a) publish the results of its investigation, and (b) notify any relevant risk management authorities 2.2 FLOOD INVESTIGATION CRITERIA A formal investigation will be carried out if one or more of the following occurs after a flooding event: • Loss of life or serious injury • Critical infrastructure flooded or nearly flooded from unknown or multiple sources • Internal property flooding from unknown or multiple sources In the following circumstances, discretion may be used to investigate a flooding incident: • A number of properties have been flooded or nearly flooded • Other infrastructure flooded • Repeated instances • Investigation requested • Risk to health (foul water) • Environmental or ecologically important site affected • Depth/area/velocity of flooding a cause for concern 2.3 RISK MANAGEMENT AUTHORITIES The following risk management authorities were identified as relevant to the flooding: • Leicestershire County Council – Lead local flood authority • Leicestershire County Council – Local highway authority • Environment Agency – Midlands Central Area • Hinckley and Bosworth Borough Council • Severn Trent Water 3 2.4 NON-RISK MANAGEMENT AUTHORITIES The following agencies are not prescribed as risk management authorities (RMAs). However, they have a significant involvement in flood risk management: • Leicester, Leicestershire and Rutland Local Resilience Forum 2.5 FLOODING INCIDENT An intense rainfall event was experienced in the locality of Witherley on the 26th November 2012, which led to water levels rising in main rivers and ordinary watercourse levels, causing flooding to occur. It was deemed necessary to complete a formal investigation into the flood incidents at Mythe Lane, Witherley on 26th November 2012. Access to at least three properties was affected and Mythe Lane was closed to traffic. 4 3 SITE BACKGROUND 3.1 LOCATION Witherley is situated in the south-west of Leicestershire in the borough of Hinckley-Bosworth, immediately east of Atherstone, and approximately 7 miles northwest of Hinckley. Mythe Lane is the main route in and out of the northeast part of Witherley (Appendix A Location Plan). The village is affectively encircled by watercourses to the west and east of the village. 5 4 FLOODING INCIDENT 4.1 PREVIOUS FLOODING INCIDENTS Leicestershire County Councils Highway Management System (HMS) has helped identify a number of incidents in which the carriageway and other areas have suffered from flooding. The following dates have been identified from the HMS system since 2010: Year Impact Three residential properties, gardens and access affected- 15/06/12 Mythe Lane closed to traffic. Three residential properties, gardens and access affected 26/06/12 Mythe Lane closed to traffic. Three residential properties, gardens and access affected- 26/11/12 Mythe Lane closed to traffic. 4.2 FLOOD INCIDENT An area of flooding occurred on Mythe Lane immediately north of the junction with Hall Lane and Atterton Lane. The carriageway was also flooded with standing water approximately 400 metres north of the village where an ordinary watercourse flows beneath Mythe Lane. During the period of heavy rainfall, flood water levels rose on Mythe Lane, this increase in levels presented a danger to pedestrians and vehicles. The flood water consisted of surface water flowing from higher ground, following contours along carriageways to the lowest areas of Mythe Lane. The bow wave from vehicles driving through the flood water increased the risk of internal flooding to adjacent properties. Some areas of land along the ordinary watercourse north of Witherley, including the rear gardens on Mythe Lane and Atterton Lane, were flooded by the high water level in the Witherley Brook. The main causes of flooding are considered to be: • Intense rainfall on the catchment. • High water levels in the Witherley Brook, The River Anker and the local ordinary watercourses. 6 4.3 RAINFALL ANALYSIS An intense rainfall event was experienced in the locality of Witherley on 26th November 2012. Approximately 33mm of rainfall is recorded to have fallen on this date. This was preceded by a total of 44mm of rainfall over three days, these levels of rainfall are considered to be substantial. Graph 1: Annual Rainfall, Market Bosworth 7 5. SUMMARY OF IMPACTS AND FINDINGS 5.1 IMPACTS People and property were affected by the flooding as a result of the significant rainfall. The County Council contacted affected persons to establish and quantify the physical effects of the flooding incident. It is understood from these conversations and County Council records that at least three properties flooded externally, and Mythe Lane was closed to traffic. 5.2 LOCAL DRAINAGE SYSTEM The investigation is focused within the catchment area containing the ordinary watercourse north of Witherley known locally as ‘Witherley Brook’. The catchment area extends to the north and east and drains predominantly agricultural land, and approximately half of the built up area of Witherley (North). 5.3 ORDINARY WATERCOURSE The riparian owned ordinary watercourse originates at three main locations near to Ratcliffe Culey, Atterton and Fenny Drayton. The three tributaries join north of Witherley. The single watercourse then flows northwest along Mythe Lane for approximately 600 metres before flowing into Witherley Brook. 5.4 MAIN RIVER The Witherley Brook runs adjacent to Mythe Lane in a northerly direction, the section of Mythe Lane closest to the village is protected from ‘fluvial’ flooding by bunding along the rivers banks. 5.5 HIGHWAY DRAINAGE SYSTEM A dedicated highway drainage system is present in Witherley which is responsible for draining surface water from the highway. There are three separate surface water drainage systems which discharge to two headwall outlets with flap valves on the ordinary watercourses immediately north of Witherley. These highway drainage systems are shown in appendix B. 5.6 SEVERN TRENT WATER A Severn Trent Water combined public sewerage system is recorded running north along the carriageway of Mythe Lane. This discharges to a pumping station immediately north of Witherley. The rising main from the pumping station is