Hampshire County Council Flood Investigation Report St Mary Bourne Flood Investigation Flood Investigation Report Document Information Document Information Document Reference 2012-12-14/HCC/1101 Document Revision - Report Status DRAFT Author S Reghif Checked Tim Lawton – Head of Highways (West) Approved Hampshire County Council 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; a) should be read and relied upon only in the context of this document as a whole, b) do not in any way purport to include any manner of legal advice or opinion, c) 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]. 1 Introduction Requirement for Investigations Hampshire County Council (HCC), within its role 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. 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 – 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 flood. Where an authority carries out an investigation under subsection 1) it must – a) publish the results of its investigations; and b) notify any relevant risk management authorities. This investigation was undertaken to look at the foul water issues in St Mary Bourne, thought to be caused by groundwater infiltration, because the impacts to people and their health were deemed significant, to trigger an investigation in line with the County Council’s guidelines. This report aims to provide details of the incident that occurred, information on the flood history at this location as well as identifying whether each Risk Management Authority has suitably exercised its relevant functions, powers and responsibilities in their response to this incident. It also includes an overview of any works undertaken or proposed in order to mitigate against future flood risk, and makes recommendations for further works or investigations. 2 Site Location and Description The village of St Mary Bourne is located to the North East of Andover within the borough of Basingstoke and Deane as shown in Figure 1. The village is the largest within the Parish of St Mary Bourne which also includes the villages of Stoke and Binley. Figure 1 – Location Plan St Mary Bourne is a rural and historic area with over 70 listed buildings and the largest concentration of thatched buildings in Basingstoke & Deane borough. This area also contains a large area of agricultural land and water meadows including the Vitacress site to the south of the village. The village is located over a chalk aquifer and as such can suffer from groundwater flooding when the water table rises under certain rainfall conditions, normally during the winter and spring months. As can be seen in Figure 2, St Mary Bourne is located to the southern edge of the Bourne Rivulet River Catchment (as defined by the Environment Agency) and is the conduit for the majority of water from the catchment as a whole before it reaches the River Test. The Bourne Rivulet, also known as the River Swift, is a seasonal groundwater river and is classed as a ‘main river’ meaning that it falls under the Environment Agency’s remit in terms of flood risk management and responsibility. Figure 2 – Bourne Rivulet Catchment Area 3 Incident triggering investigation In December 2012, Hampshire County Council was contacted by several residents who were unable to use washing or toilet facilities in their properties due to the foul sewer being backed up and the risk of internal foul flooding. In the opinion of these residents, this problem was not being given sufficient importance by the water company responsible for managing flooding from sewers (Southern Water). This perception was compounded by an alleged lack of communication with local residents by the water company. Although this was considered to be primarily an issue for Southern Water to deal with Hampshire County Council decided to investigate this issue in view of the combination of different contributory flood factors and the number of different flood risk management bodies involved. The aim of this investigation was to identify what has, is and could be done to assist the residents and alleviate the risk of foul flooding. A Hampshire County Council representative visited the site on 14 December 2012 and spoke to a number of residents about the issues as well as the Southern Water representatives who were on site dealing with the problems in the village at this time. St Mary Bourne village was not suffering from surface water flooding at this point. There was very little ponding on the highway and no evidence of significant flooding although the surrounding fields were noticeably waterlogged. The Bourne Rivulet was in full flow and although the actual depth was not recorded, there did not appear to be an imminent risk of it overtopping. However, the level of the water appeared to be at a very similar level to the highway which illustrates the height of the water table and corroborated the assumption that the issues were related to groundwater. Southern Water staff were active in the village at the time of this site visit and had identified that three properties were affected. One resident, reported that the Southern Water sewer had backfilled all of the lateral pipes, forcing them to hire a chemical toilet as no other facilities were functioning. This issue had occurred earlier in the year, for three weeks in August 2012 as well as the current incident which started on 2 December 2012 and was on-going for a considerable period. The resident had been in the property for seven years and, while aware of the potential flood risk, had never known the issues to start so early in the year. Southern Water had arranged for tankers to remove water from the sewer at this point as well as initiating overpumping directly into the Bourne at two locations as shown on Figure 3. It was reported that a neighbouring property was suffering the same issues, and the effects were exacerbated by the presence of young children at this property. The third property in the line was not directly affected at that time and the occupants were able to use their bathroom facilities. However, this property is the furthest upstream of the line of three linked properties. The sewerage network links the three properties, each to the next before entering the main sewer. Therefore the problems being experienced at the most downstream of the three properties, were compounded by both of the upstream properties. It is worth noting that under new legislation, Southern Water is now responsible for sewerage pipes that serve more than one property. Southern Water’s operational staff were on site co-ordinating the tankers and were undertaking water quality tests on the foul water to determine the level of pollutants in the system. These tests and visual inspections showed the water to be very clear and with extremely low levels of pollutants which emphasised that this was primarily a groundwater infiltration issue. This low level of pollutant meant that Southern Water were allowed to ‘overpump’ into the Bourne Rivulet without causing a detrimental affect on the watercourse and its quality. The over-pumping also included filtering out any remaining pollutants before the extracted water was released into the Bourne. The filtered out pollutant was disposed of at an approved facility to ensure the contaminates did not enter the Bourne. The water quality testing will continue for the duration of the overpumping as required by the Environment Agency. Figure 3 – Overpumping Locations 4 History of Flooding There is a long history of flooding related problems in this area but it is widely accepted that the root cause is high groundwater levels and the infiltration of this groundwater into the foul water sewer. Although above ground flooding has occurred, the issue causing the most concern is the inability of the foul water sewer to function effectively in periods of high groundwater. A number of reports, surveys and investigations have been carried out and it is accepted that there is a substantial amount of groundwater entering the sewers which is preventing some properties from being able to discharge any of their foul water into the system as well as the risk of the sewer backing up into properties. Much of the information below has been obtained from the 2005 Halcrow Report into the groundwater flooding that occurred in 2000/01, St Mary Bourne Parish Council and a local resident.