Fleet Section 19 Report March 2018
Total Page：16
File Type：pdf, Size：1020Kb
Hampshire County Council Flood Investigation Report Ba Fleet Section 19 Flood Investigation Report Flood Investigation Report Document Information Document Information Document 2018-01-30/HA/R.J504706.01 Reference Document Revision - Report Status Final Draft Date 30 January 2018 Author VW Checked SC/CM Approved FM Economy, Transport and Environment Department The Castle Winchester Hampshire SO23 8UD Tel: 0845 603 5638 Fax: 01962 847055 www.hants.gov.uk No part of this document may be copied or reproduced by any means without prior written permission from HCC. Any advice, opinions or recommendations within this document; should be read and relied upon only in the context of this document as a whole, do not in any way purport to include any manner of legal advice or opinion, are based on the information made available to HCC at the date of this document and on current UK standards, codes, technology and construction practises as at the date of this document. No liability is accepted for any use of this document other than for the purpose for which it was originally prepared and provided. Hampshire County Council cannot accept responsibility for any use or reliance on the contents from this report by any third party. Maps are reproduced from Ordnance Survey material with the permission of Ordnance Survey on behalf of the Controller of Her Majesty’s Stationery Office © Crown Copyright. Unauthorised reproduction infringes Crown Copyright and may lead to prosecution or civil proceedings. HCC 100019180 [2011]. _____________________________________________________________ Fleet Flood Investigation Report Page 2 of 21 Executive Summary This Flood Investigation Report has been compiled by Hampshire County Council (HCC) under their responsibilities as Lead Local Flood Authority (LLFA) for Hampshire and the duty to investigate significant flood events as defined under the Flood and Water Management Act 2010. This report provides a summary of: the extent and consequences of the flooding in August 2015 in Fleet, Hampshire; actions undertaken or proposed by each of the identified authorities; information relating to improvement works or general maintenance; and any further works required and engagement with landowners where required. This report has been prepared to advise the risk management authorities (RMAs) of the circumstances leading to the flooding that took place in August 2015, to inform the relevant RMA’s flood risk management functions, and to guide the development of any future mitigation schemes in the Fleet area. This report has been undertaken as a high level Section 19 investigation and does not include detailed assessment and modelling. The report is based on records of internal property flooding and road closure information from a variety of sources. While every reasonable effort has been made to validate the information, the nature of the data and the methods of collation, mean that it may not include every occurrence of flooding. Hampshire County Council, as Lead Local Flood Authority, disclaims any responsibility for the accuracy or comprehensiveness of the information supplied, and accepts no liability for any indirect, consequential, or incidental damages or losses arising from use of the information. _____________________________________________________________ Fleet Flood Investigation Report Page 3 of 21 1. Introduction 1.1 Requirement for Investigations 1.1.1 Hampshire County Council, as Lead Local Flood Authority, has a responsibility to record and report flood incidents, as detailed in Section 19 of Part 3 of the Flood and Water Management Act 2010 (reproduced below). PART 1 FLOOD AND COASTAL EROSION RISK MANAGEMENT 3. Supplemental powers and duties Section 19: Local authorities: investigations On becoming aware of a flood in its area, a lead local flood authority must, to the extent that it considers it necessary or appropriate, investigate – which risk management authorities have relevant flood risk management functions, and whether each of those risk management authorities has exercised, or is proposing to exercise, those functions in response to flood. Where an authority carries out an investigation under subsection 1) it must – publish the results of its investigations; and notify any relevant risk management authorities. 1.1.2 Hampshire County Council’s criteria for undertaking a Section 19 investigation requires that internal flooding of more than 20 properties has taken place during one weather event, from one flooding source in one local area. 1.1.3 This report was undertaken to investigate the flooding incident in August 2015 at a number of locations across Fleet, Hampshire as more than 20 properties in total were affected internally by flood water within this area. 1.1.4 This report aims to provide details of the flooding incident that occurred with a review of the roles of all risk management authorities having a responsibility for flooding at the identified flood site. The report also provides an overview of any works undertaken or proposed at the site in order to mitigate against flood risk, and any recommendations for further works or investigations in order to reduce the risk of flooding. 1.2 Site location and description 1.2.1 Fleet is located within Hart District Council and is one of the most urbanised areas of the district. It is within the Loddon River catchment with a _____________________________________________________________ Fleet Flood Investigation Report Page 4 of 21 number of sub-catchments across the area. During the August 2015 flooding event, there were a number of individual sites recorded as having flooded across the district of Hart. The largest concentration of property flooding was identified to be within the Fleet area which is covered by two sub-catchments namely Fleet Brook and Hart (Crondall to Elvetham). These sub-catchments are identified in Figure 1 below. Fleet Figure 1: Fleet Site Location Plan 1.2.2 Fleet is primarily an urban area although, as shown below, it is surrounded by rural and agricultural areas. This has a key bearing on how surface water flows are managed as well as on the key flood mechanisms. _____________________________________________________________ Fleet Flood Investigation Report Page 5 of 21 Figure 2: Land Classifications of the Fleet area 1.2.3 Differing land categories can have a substantial impact on flood mechanisms. Urbanised areas tend to have a high proportion of impermeable surfaces and surface water is usually managed via underground pipes and similar drainage features. The impermeable surfaces lead to a high concentration of water flowing into pipes and blockages that can cause short term but significant flooding. Rural and agricultural areas tend to allow water to infiltrate into the soil more easily, as well as having more vegetation which slows the flow of water. These areas tend to be managed through the use of ditches which also slow water flow. The land classifications in the Fleet area, as taken from the National Receptors Database, are indicated in Figure 2 above. It must be noted that this is not a detailed representation of the land use of individual land parcels but an overview of the typical uses over the wider area. _____________________________________________________________ Fleet Flood Investigation Report Page 6 of 21 2. Flooding Incident Triggering Investigation 2.1 In August 2015, there was significant rainfall across Hampshire and a number of incidents of flooding were reported to Hampshire County Council and Hart District Council. At this time, the Crondall weather station recorded over two thirds of the month’s rainfall in the space of 1 hour. 2.1.2 It has been confirmed that although there were significant levels of rainfall, the Crondall Direct Alarm, installed to alert residents of potential flood waters, was not triggered. The river level rose by 0.44m however this was still substantially below the trigger level for the alarm. 2.1.3 There was a high concentration of flooding incidents in the Fleet area and 29 properties reported internal flooding with 34 others reporting external flooding (gardens / garages etc). 2.1.4 The flooding in this area was reported to be a mix of surface water, foul water and flooding from watercourses. It is apparent that the volume and intensity of rainfall in such a short space of time overwhelmed the drainage systems and watercourses across the area. A number of residents requested sandbags from Hart District Council but the speed of the flooding meant that it was not always possible to deliver these prior to the flooding occurring. 2.1.5 Although this is largely an urbanised area, there are numerous watercourses directly adjacent to properties and the majority of those affected in August 2015 event were significantly impacted by these watercourses. 2.2 Hart (Crondall to Elvetham) 2.2.1 Within the Hart sub-catchment, the majority of flooding occurred in the direct vicinity of the Sandy Lane Ditch and its tributaries with the key areas affected with internal property flooding as indicated in Figure 3 below. 2.2.2 The key areas affected were: Tavistock Road Basingbourne Road Longdown Greenways 2.2.3 From the information reported, there appeared to be issues with blockages of the watercourse culverts and blockages in the foul and surface water systems. In addition, the drainage networks were reported to be overflowing indicating that the amount of rainfall experienced overwhelmed the drainage systems. _____________________________________________________________ Fleet Flood Investigation Report Page 7 of 21 Figure 3: Recorded Flooding incidents in Hart (Crondall to Elvetham) catchment 2.2.4 Although blockages of the different drainage systems were identified, without more detailed information about the exact location and nature of the blockages, it is not possible to accurately identify the relevant organisations responsible. However, the text in Section 5 gives general information about the different areas of responsibility. 2.3 Fleet Brook 2.3.1 Within this sub-catchment, there was a greater distribution of flooding but again, the majority occurred in the direct vicinity of watercourses.