Essex County Council Flood Investigation Report

Transcription

1 Essex County Council Stock City of Chelmsford Rev Date Details Author Checked and Approved By 01 February 2015 Draft report for stakeholder consultation Ed Clarke Flood Investigation Engineer Lucy Shepherd Lead Local Flood Authority Manager 02 February 2015 Final revisions based on consulation response Ed Clarke Flood Investigation Engineer Lucy Shepherd Lead Local Flood Authority Manager

2 Introduction Purpose and Requirements of the Essex County Council as the LLFA has a responsibility to record and report flood incidents as detailed within Section 19 of the FWMA 2010: Section 19 (1) On becoming aware of a flood in its areas, 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 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. Essex County Council has established criteria for Section 19 Flood Investigation Reports as follows: The internal flooding* of a property on more than one occasion OR The internal flooding* of five or more properties in a single event AND An ambiguity surrounding the source or responsibility of a flood incident. *Internal flooding does not include the flooding of gardens and garages; only properties where internal flooding is above threshold level. 1

3 Site Information Site Location Mill Road & High Street, Stock (Grid Ref: ,198906) Figure 1 Indicative location of Flood Investigation Area (Base Map: Ordnance Survey, 2014). Flood Risk Figure 2 Updated Flood Map for Surface Water (ufmfsw), (Environment Agency, 2013). 2

4 As the updated Flood Map for Surface Water (ufmfsw) shown in Figure 2 demonstrates, the Flood Investigation area exhibits a high level of flood risk, with significant areas predicted to be at risk during a 1 in 30 year return period rainfall event (3.3% probability of occurring any given year). If the ufmfsw is viewed at a wider scale as shown in Appendix D, it becomes evident that the village is located on a natural drainage path towards Stock Brook, which is classified as a Main River and is a tributary of the River Wid. The area which is at the highest level of risk based on the model appears to be the centre of the village, however it is worth noting that due to the lack of specific local factors incorporated within the model this should only be viewed as an approximate indication of flood risk and is not suitable for identifying individual properties. Flood History Severe flooding affected more than five properties internally during heavy rainfall on 20 th July 2014, although exact numbers may be considerably higher than this. Evidence from affected residents suggested that floodwaters reached 2 feet in depth and a number of properties suffered extensive damage. In addition to this, reports from residents and other Risk Management Authorities suggest that an ongoing flooding issue has been present at the junction of Mill Road and High Street, although whether this has previously resulted in internal flooding to property is currently unclear. The approximate area affected by flooding is shown in Figure 3, based on photographic and anecdotal evidence. This area is indicative only and should not be used as a representation of properties experiencing internal flooding. Photos taken during the event on 20 th July 2014 are shown in Figures 4 and 5, and Appendix A. Figure 3 Indicative flood area during the events [Not to be used as indication of individual properties experiencing internal flooding]. (Base Map: Ordnance Survey, 2014). 3

5 Figure 4 Flooding in Stock on 20 th July 2014 (BBC, 2014; Photo by Samuel Myers). Figure 5 Flooding in Stock on 20 th July 2014 (Photo supplied by resident, 2014). 4

6 Drainage System Figure 6 Indicative Drainage Layout (Ordnance Survey, 2014). The key aspects of the drainage system are demonstrated in Figure 6, based on information from Digdat and site inspections. As is shown, the most significant component of the drainage system in the immediate area is an AW sewer running from the north east to south west. The AW infrastructure maps did not specify a diameter of the sewer and requests for information have been unsuccessful. As such the capacity of the system is currently unclear. A number of highway gullies drain into the sewer and are shown on the map, the concentration of the gullies around the junction of the High Street and Mill Lane is likely a result of the ongoing surface water issues in the area mentioned previously. Beyond the southern edge of the map, the sewer continues south west as shown in Appendix B, before outfalling into Stock Brook. There are discrepancies between the AW maps and site observations, so it is unclear at which point the AW sewer outfalls, however inspection revealed a large diameter pipe marked by a black dashed line on the map. 5

7 Possible Causes Rainfall/Antecedent Moisture Conditions The main event in question occurred at the end of a very warm and dry period of weather. Whilst the land was not saturated it is possible that the fields in the surrounding areas would have been very dry and the top layer of soil baked, causing the initial rainfall to be transferred as surface runoff and increasing the speed at which the rainwater drained from the area. The average monthly rainfall from the two nearest Met Office weather stations shows 52.5mm and 45.3mm for Stanford-le-Hope and Writtle respectively. As summer rainfall is very variable, the spread of data shows that 20% of the recorded values used to calculate the monthly average rainfall for July are higher than 67.8mm and 69.4mm for Stanford-le-Hope and Writtle respectively. The Met Office have stated that they expect these monthly values to be exceeded once every 5 years on average (20% of all months). The rainfall event was a very substantial convectional summer storm. During the period from 14:00-17:00 a local rainfall gauge at Hanningfield recorded 66mm rainfall, with the heaviest intensity showing 60mm falling within one hour, between 14:30-15:30. On comparison with the monthly average values for July it is evident even when using the upper values provided by the Met Office that almost the entire of the average rainfall for July fell within 3 hours. Approximately 91% of the rainfall recorded during the storm fell in the period between 14:30-15:30, with the highest intensity showing 18.2mm/per 15mins. Highway/Surface Water Drainage System This data suggests that the volume and intensity of rainfall falling during the event would have likely been substantially greater than the design capacity of the drainage system in the local area causing the system to be overwhelmed and flooded. Photographic evidence and resident reports suggested that highway gullies in the local area were surcharging and not able to remove the water from the road surface. Given the intensity of rainfall it is likely that this was due to the overwhelming of the design capacity of the infrastructure rather than any specific blockages, however drainage concerns had previously been reported by residents to the northern limit of the flood area shown in Figure 3. During inspection by Essex Highways in May 2014, a blocked gully was identified as being in need of maintenance, with subsequent clearance undertaken in Autumn It is unclear at this stage whether the blockage contributed to the flooding on 20 th July It is possible that it may have exacerbated the issue in the immediate area, however the intensity of the rainfall means that it would likely have been overwhelmed even if clear. The main surface water drainage route through the village is an Anglian Water surface water sewer of unknown 6

8 Culvert Conditions Open Watercourse Conditions System at Capacity diameter, shown in Figure 6 running from north to south. Reports and evidence from local residents suggested that there had been issues with this sewer in the past and contact with Anglian Water established that works have been done previously, the latest being in Requests for details on the works and any subsequent inspection before or following the flooding on 20 th July 2014 have been unsuccessful. The outfall of the system marked on Anglian Water sewer infrastructure maps appeared to be buried on initial inspection (Photo C3, Appendix C), however investigation downstream revealed an additional concrete culvert section which outfalls into Stock Brook. As this is not marked on the Anglian Water map it is unclear whether this is part of the adopted sewer system or whether it would influence the performance of the main sewer. The culvert section to which the Anglian Water sewer outfalls was found to be flowing freely on inspection meaning that a significant blockage upstream was unlikely. It was however evident at certain points that the culvert had become uncovered and that sections of the pipe had split from each other (see Photo C1, Appendix C). Whilst the culvert was damaged, this allows water to escape at periods of high flow and would not have contributed to the flooding. The culvert was installed under an open watercourse section, which at the time of inspection was flowing at a reasonable velocity, likely draining the adjacent land. A deep section to the north, immediately south of where the outfall of the Anglian Water system is located was completely dry at the time of inspection (Photo C2, Appendix C). It is possible that the presence of this culvert, particularly if it is of a smaller diameter than the Anglian Water sewer, may have provided a constriction on flow. If this is proven to be the case, it may be prudent to consider the reinstating of the open watercourse to provide additional capacity. There is currently no specific evidence to suggest that the condition of any open watercourse in the local area was a primary factor during the flood events, however it is possible that the historic culverting of open watercourses has increased flood risk in the area. It is probable that the intensity of rainfall previously mentioned overwhelmed the design capacity of the entire surface water system in the area, however due to the lack of information received in response to requests made of Anglian Water it is not currently possible to ascertain whether it is feasible that significant improvements could be made. 7

9 Responsibilities and Recommendations Lead Local Flood Authority To ensure that the owners of land on which a culvert, watercourse or drainage system is present are aware of their responsibility to keep the feature clear and functioning effectively. Facilitate sharing of information and collaboration between RMAs and the local community. Consider using enforcement powers under Section 25 of the Land Drainage Act 1991 should landowners fail to maintain watercourses effectively. Record and inspect any significant drainage features identified on the site as part of the Flood Risk Asset Register required under Section 21 of the Flood and Water Management Act Consider undertaking further study into mitigation options, including working with landowners to reinstate open watercourse sections if beneficial, and investigate the potential for funding for flood alleviation works. Essex Highways Consider use of powers under Section 100 of The Highways Act 1980 to prevent surface water flowing onto the public highway and/or to properly drain the highway. Continue to work in partnership with other RMAs, providing information and comments and funding when appropriate and to support hydraulic modelling work, the recommendations of which should address/consider the flood risk on the public highway. Inspect and clear highway drainage in the area on a regular basis to reduce flood risk. Consider improvements to the highway drainage system, either by installing additional drainage infrastructure or improving the capacity of existing infrastructure. Anglian Water Check and clear the adopted sections of sewer where necessary. Comply with duty to cooperate with relevant RMAs under Section 13 of the FWMA Provide information on the capacity of the main sewer in the area to the LLFA. Chelmsford City Support LLFA in raising awareness of riparian landowner responsibilities. Council Continue to share information held on drainage layouts with all RMAs. Riparian Landowners Ensure that watercourses or culverts on, or adjacent to, their land are kept clear and free flowing. Provide information to the LLFA on surface water drainage systems which may contribute to/from the infrastructure identified in this report. Residents/Business Take measures to protect themselves and their property when flooding is imminent. Owners Document and photograph flood incidents where possible, report flooding to CCC or the LLFA. 8

10 Conclusion We have investigated which Risk Management Authorities have relevant Flood Risk Management Functions in accordance with the FWMA as part of this study. Those RMA s and relevant functions are referenced above within the recommendations section. It is the conclusion of this study that the flood event on 20 th July 2014 was most likely due to a combination of the occurrence of an intense and unprecedented rainfall event, drainage infrastructure which was unable to cope with the volume and intensity of rainfall, and the high level of residual flood risk in the area. The volume of rainfall experienced over the afternoon of the 20 th July was in excess of the Met Office average rainfall for the entire of July and this, particularly the extreme intensity during the one hour period between 14:30-15:30 during which over 90% of the day s rainfall fell, would have overwhelmed the design capacity of the drainage system in the area. As demonstrated in Figure 2, the flood area exhibits a high level of residual flood risk with the junction of Mill Road and High Street predicted to be at risk in a 1 in 30 year event or greater (3.3% probability of occurring in any given year). The junction is also at relative low point within the immediate area which encourages water to gather before flowing south towards the River Wid. It is likely that this was previously the beginning of a natural flow path or possibly watercourse which forms a tributary of the river and which has been historically culverted. The boundary lines of the properties to the south of the junction (observable in Figure 6) support the historic existence of an open watercourse. Where open watercourses have been converted to culverted sections, this causes a significant loss of capacity and a constriction on the flow from upstream, increasing flood risk in the upstream area. It is probable that the installation of the culvert occurred many years ago however, so it is unlikely that a practical or feasible solution exists in reinstating the open watercourse. Given the location of the flood area relatively early in the flow path it is unlikely that a solution such as upstream attenuation would provide any benefit. The diameter of the main Anglian Water sewer is unknown, but if the private culvert section to which the sewer outfalls is of a lesser diameter it is possible that this may provide a further restriction on the system and increase flood risk upstream. It is unclear at this stage the reasons as to why the culvert was installed under a significant sized open watercourse, however it is recommended that the feasibility of reinstating the full watercourse as an outfall to the Anglian Water sewer is investigated and its effects evaluated. As no feasible option to mitigate the flooding issue is immediately discernible, it is recommended that further detailed hydrological study is undertaken to identify and evaluate options for reducing the flood risk to properties in the local area. These conclusions are based on the information currently available and may change subject to future works or subsequent investigations. 9

11 Acronyms AW Anglian Water CCC Chelmsford City Council EA Environment Agency ECC Essex County Council EH Essex Highways FIR FWMA Flood and Water Management Act 2010 LDA Land Drainage Act LLFA Lead Local Flood Authority LHA Local Highway Authority RMA Risk Management Authority Glossary of Terms Term Culvert Main River Ordinary Watercourse Surface Water Definition Covered channel/pipeline All watercourses shown as such on the statutory main river maps held by the Agency and DEFRA or Welsh Office, as appropriate. All rivers, streams, ditches, drains, cuts, dykes, sluices,sewers (other than public sewers) and other passages through which water flows that are not designated as main rivers. Rainwater which is on the surface of the ground (whether or not it is moving), and has not entered a watercourse, drainage system or public sewer. Useful Contacts and Links Essex County Council Highways Incident Line Flood Investigation Engineer All calls may be charged (24hrs) (Mon-Fri, 9am - 5pm) Legislation Highways Act 1980: Water Resources Act 1991: Land Drainage Act 1991: EA - Living on the Edge a guide to the rights and responsibilities of riverside occupation: EA - Prepare your Property for Flooding: Reducing flood damage; flood protection products and services ECC Flood and Water Management in Essex: National Flood Forum Blue Pages: Advice and contacts for flood protection products Six Steps to Flood Resilience: Step-by-step guidance and advice for property owners interested in Property Level Protection 10

12 Appendix A Photo from 20 th July 2014 (Provided by resident November 2014). 11

13 Appendix B Wider Drainage Network A B Base Map: Ordnance Survey (2014) 12

14 Appendix C Photos from site visit (December 2014) Photo C1 Taken at Point B looking north. Photo C2 Taken at Point A looking south. Photo C3 Taken at Point A looking north. 13

15 Appendix D Extended area of the Updated Flood Map for Surface Water UFMfSW: EA, 2013; Base Map: Ordnance Survey (2014) 14