5 Understanding flood risk in MDC One of the key purposes of this SFRA is to identify the influential local flood risk issues and to summarise recorded local flood incidents and predicted flood risk to the area. Flood risk can arise from a variety of different sources, as described within this section. Often however, flooding originates from a combination of courses as flood mechanisms are integrated. 5.1 How flood risk is assessed A flood is now formally defined in the Flood and Water Management Act (2010) as “any case where land not normally covered by water becomes covered by water”. The Act also states that a flood, as defined above, can be caused by: • Heavy rainfall • A river overflowing, or its banks being breached • A dam overflowing or being breaches • Tidal waters • Groundwater • Anything else (including a combination of factors) In the context of the FWMA (2010) a flood does not include: • A flood from any part of a sewerage system, unless wholly or partly caused by an increase in the volume of rainwater (including snow and other precipitation) entering or otherwise affecting the system • A flood caused by a burst water main (within the meaning given by section 219 of the Water Industry Act 1991) The FWMA (2010) states that flood risk “means a risk in respect of flood”, where risk is “assessed and expressed (as for insurance and scientific purposes) as combination of the probability of the occurrence with its potential consequences”. Thus, it is possible to define and express flood risk as: Consequences Flood Flood Receptor Receptor = Probability X Hazard X presence X Vulnerability Risk Magnitude 5.2 Historical flooding Mendip District has a history of documented flood events with the main sources being from fluvial flooding, with surface water flooding also significant. There are small areas of documented reservoir flooding and sewer flooding too. Several different data sources have been used to understand the historic flood risk in Mendip District, including from the previous SFRA, as well as the EA, SCC, Wessex Water and Mendip District flood incident databases. Events from the flood incident databases are outlined in Table 5-1, Table 5-2 and Table 5-3. 5.2.1 Previously reported flood incidents in the Mendip District Level 1 SFRA (2008) Fluvial flooding The areas around Glastonbury and Street were recorded in the 2008 SFRA as being affected the most from fluvial flooding, referring to the EA’s Recorded Flood Outlines 30 Dataset. Other incidents of fluvial flooding were recorded along the River Frome at Frome, particularly alongside Rodden Road on the right bank. This resulted in the Devon & Somerset Fire and Rescue Service being called out almost 20 times between 2005 and 2008 to help assist with the removal of water from properties. Shepton Mallet is also a location of known flood risk with incidents of flooding of the River Sheppey. Many historic flooding incidents have been recorded at Shepton Mallet, involving 14 Fire & Rescue Service call-outs between 2005-2008. The SFRA also noted that in North Wootton the River Redlake regularly floods following extreme rainfall. This has previously resulted in difficulties in crossing the bridges in Stock Lane and Northtown. Older incidents of historic fluvial flooding have been recorded in the 2008 SFRA under the historic flooding section. These records have been obtained from Parish Councillors, SCC and drainage boards, and combined with the EA Flood Reconnaissance Information System (FRIS). Particularly large events are noted below: 9th-25th August 1954 The event involved an overflowing of the River Brue on the 9th and 13th of August resulting in serious flooding in Butt Moor and areas of Barton Moor. This overflowing also caused the South Drain to overflow below Sharpham Bridge causing flooding in Street Heath, Walton Heath and Sharpham. The flooded areas were not completely free of water until the 25th August. 10th July 1968: During this event, nine properties flooded in and around Mells due to the overflowing of the Mells Stream. The degree of property flooding varied greatly with ceiling to floor flooding affecting some properties. In addition to this, five properties were also flooded in Nunney and an unknown number flooded in Frome. 19th August 2006: Intense rainfall on the 19th August resulted in flooding to Oakhill High Street. Roads received much of the runoff that overwhelmed and blocked highway drainage systems, with several properties suffering internal flooding. EA rain gauge measurements estimate a return period of 1 in 200 years for this event. Other significant historic fluvial events recorded in the 2008 SFRA include the 11th January 2008 and 28th August 1960. Full details of these events can be found in section 3.3 of the MDC Level 1 SFRA (2008). Groundwater flooding The EA flood incident dataset (FRIS) used in the previous SFRA (2008) identified two locations of ground water flooding; one at Glastonbury and one at King’s Sedgemoor to the north of Henley. No other incidents of groundwater flooding were highlighted during the data collection exercise. Surface water flooding The SFRA highlighted that there have been several serious floods in Westbury-sub- Mendip recorded, all caused by surface water runoff. Stoke St. Michael Parish has similar surface water flooding problems, with drains regularly blocked. 31 In addition to this, numerous surface water flooding incidents have been reported in Nunney as far back as 1960, with more recent and frequent events occurring from the early 2000s onwards. Sewer flooding The previous SFRA (2008) states that the EA flood incident dataset (FRIS) did not contain any records of sewer flooding at the time of writing. Therefore, the primary source of data used to inform historical sewer flooding was supplied by Wessex Water. The majority of historic sewer flooding recorded by Wessex Water was located in Frome, with the information provided only representative of the situation as of November 2007. It was noted in the report that Wessex Water were currently undertaking extensive work to eliminate the majority of foul sewage flooding incidents by 2010, therefore they would expect to see most, or all of the locations removed by 2010. Reservoir flooding Whilst there is an acknowledgement of the location of reservoirs in and around the MDC area and the level of flood risk posed by reservoirs in the previous Level 1 SFRA, there is no mention of any recorded reservoir flood incidents. 5.2.2 Previous documents and reports describing flood incidents SCC’s website provides a summary of previous flood investigation reports. Reports relevant to the study area include: • Parsonage Lane, The Street, Church Lane, Broadway and Greenditch Close, Chilcompton Flood Investigation Report 2016 • Croscombe, Shepton Mallet Flood Investigation Report 2012 A summary of these are provided below: Parsonage Lane, The Street, Church Lane, Broadway and Greenditch Close, Chilcompton flooding incident June 2016 During the incident on the 17th June surface run-off water from the catchment area travelled via the B3139 to the lowest point on The Street, Chilcompton. Heavy rainfall was witnessed in the 6 days prior to the event followed by 44.6mm rainfall between 15:00 and 17:00 on the 17th June. The consequential high volumes of surface water run-off from the large catchment, increased the water volume in the River Somer. This resulted in the river overtopping its banks opposite a public footpath known as The Pitching. The large volume of water also overwhelmed the highway drainage system. Probable causes of the flooding were due to exceptional levels of rainfall at almost 10 times greater than the average daily rainfall for June. Existing drainage systems and watercourses were unable to cope with the high volume of water in the short duration of the event. Further actions and investigations were undertaken and a large obstruction in the watercourse was moved. Previous flooding evidence and historical data identifies that the properties suffering internal flooding are in a Flood Zone 3 area. Therefore, these properties have a high probability of flooding identified by the EA’s Flood Map for Planning. This investigation concluded that the highway drainage system had been maintained appropriately and was only a contributory factor. The system was overwhelmed by an increased volume of water outside the highways design capabilities and curtilage. The LLFA’s Civil Contingencies Unit worked with the Parish Council and community to put in place a Flood Action Plan. Inspections of the watercourse showed no areas of 32 concern related to the ordinary watercourse maintenance. There was however one small section that required the removal of a build-up of stones and debris caused by the flood event. It was found that there may be an opportunity for the LLFA to carry out Property Flood Resilience (PFR) assessments of the properties that suffered internal flooding. Croscombe, Shepton Mallet flooding incident July 2012 On the 11th July, intense rainfall led to large amounts of surface water runoff from the steep catchment to the south of the village of Croscombe. The runoff flowed down roads and accumulated on Long Street (A371), where it was unable to enter the river. This affected eight properties. Most of this surface water flowed down Old Street Lane and Jack’s Lane. In addition, there were also reports of overflow from foul manholes located close to Townend Bridge. Probable causes of the flooding were the high volumes of surface water runoff from locally intense rainfall, with flooding the most severe experienced in the last 20 years. The steepness of the catchment resulted in the highway drainage either being bypassed or blocked with debris washed down from upstream. SCC are due to undertake an appraisal of alleviation options for mitigating with surface water flood risk in consultation with local stakeholders.