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Part of: Flooding

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Key Facts

  • In the floods of 2012 (beginning in April), the Environment Agency reported that over 4000 properties in England and Wales had flooded by the end of August. 
  • In December 2013 and January 2014 Broadmead Lane Industrial Estate, Keynsham flooded.
  • Apart from the 2012-13 floods, there have been approximately eight floods in Chew Magna and Chew Stoke since 1960.
  • In the 2012-13 floods, five separate floods were identified that triggered flood warnings in Chew Magna and Chew Stoke.
  • At the time of the 2012 floods, 69 properties in Chew Magna had formal flood protection in place as part of the Property Level Protection (PLP).
  • 31 properties in Chew Magna were confirmed to have flooded in the 2012 September flood and 17 in the November floods.
  • 12 properties in Chew Stoke were confirmed to have flooded internally in the September and November 2012 floods.

What does the data say?

Flooding in Bath and North East Somerset 1

A Bath & North East Somerset regional Surface Water Management Plan was prepared in 2015 and identified that Bath, Keynsham, Whitchurch, Chew Magna, Chew Stoke, West Harptree, Midsomer Norton and Radstock are the locations in Bath & North East Somerset which have suffered the most flooding.

Figure 1: Overview of the interactive map of B&NES flood incidents that took place between 2009-2014, taken from the 2015 Surface Water Management Plan2

During 2013-2014, Wessex water identified 44 occurrences of sewer flooding, including those attributable to surface water flooding, in B&NES.3

Broadmead Lane Industrial Estate, Keynsham Flood December 2013 and January 20144

In December 2013 and January 2014 Broadmead Lane Industrial Estate, Keynsham flooded.

Broadmead Land Industrial Estate is situated approximately 1km North of Keynsham and lies adjacent to the River Avon. The Industrial Estate is within the functional floodplain of the River Avon and is described by the Environment Agency as being at ‘High risk’, having a greater than1 in 30 chance of flooding each year.

The industrial Estate has been affected by fluvial flooding over many years and there are reports that flooding events have been more frequent in recent years.

In June 2014, B&NES Council produced a report about the flooding that occurred in December 2013 and January 2014. The main findings of this report were:

  • Flooding of the Industrial Estate occurred as a consequence of the River Avon exceeding bank-full capacity
  • The Industrial Estate became inundated by flood plain water ponding on surrounding land as opposed to direct bank overtopping;
  • The access road to the site became impassable, resulting in a high level of risk to people and properties in the Industrial Estate;
  • Flood warnings were issued to the Industrial Estate units, however the Christmas holidays meant that response by property owners / occupiers was limited.

Flooding in Chew Magna and Chew Stoke  5 6

Both Chew Magna and Chew Stoke are located near Chew Valley Lake to the south of Bristol in North East Somerset and close to the northern edge of the Mendip Hills.

The steep nature of the catchments and predominance of low permeability soils and bedrock leads leading to a rapid response to rainfall make this area particularly susceptible to flooding.

There are two major watercourses flowing into Chew Magna: the Winford Brook and is a tributary of the River Chew. Both catchments are markedly different in terms of scale and response. Both are also influenced to varying extents by on-line reservoirs.

The Winford Brook drains a small catchment of approximately 20km2 and has steep topography which leads to a rapid response to rainfall, with water levels rising quickly in the village. Chew Magna Reservoir is located on the Winford Brook upstream of the village. This is a small reservoir with a volume of about 70,000m3 and at high flow would fill from empty in around an hour. Despite the fact that the Winford Brook catchment is approximately one-third of the size of the River Chew catchment, it is steeper and more rapidly responding than the River Chew catchment. Consequently, it poses the highest flood risk, of the two watercourses.

The River Chew, in contrast, drains a larger catchment of about 71km2. The Chew Valley Lake dominates the catchment, located approximately 2km upstream of Chew Magna and with a volume of about 20,000,000m3 is nearly 300 times larger than Chew Magna Reservoir. This results in the response of the River Chew to rainfall being less rapid than the Winford Brook.

The Chew Stoke Stream, classified as a Main River, flows through Chew Stoke and joins the River Chew approximately 1km downstream of the village. Chew Stoke Stream drains a small catchment of approximately 10km2 and has steep topography which leads to a rapid response to rainfall, with water levels rising quickly in the village. There are no reservoirs within the Chew Stoke catchment. 

Historic floods 7

Apart from the 2012-13 floods, there have been approximately eight floods in Chew Magna and Chew Stoke since 1960. Three of these floods occurred in 1999 and 2000, but the main flooding occurred in July 1968. The Chew Valley and surrounding catchments were devastated when a summer storm centred over the area and caused widespread destruction and flooding. In only 18 hours 175mm of rain fell, double the area's average rainfall for the whole of July. Soils in the area were already waterlogged after previous rainfall.

It was in Chew Stoke that the record rainfall occurred. Unlike previous floods, where the water soon receded, the floodwaters continued to rise. They reached the top of the petrol pumps at the garage next to the stream and flooded through the Post Office. In some low-lying areas of the village, water quickly rose to the level of the upstairs windows. In Chew Magna, 88 properties were flooded, many up to 2m deep including a church hall, village school and fire station.

2010 and 2012-13 floods 8 9

Across most of the UK, 2010-2012 was remarkable in climatic terms, characterised by exceptional extremes and departures from typical seasonal rainfall patterns. 2011 was the second warmest year on record (from 1910) for the UK. The drought intensified through the winter of 2011/12 and parts of England recorded their lowest 18-month rainfall (for periods ending in March) in at least 100 years.

Early April 2012 saw a significant change in the weather patterns. April rainfall totals were the highest on record across most of the country, the wettest April for at least 230 years in England and Wales. As a consequence the focus switched rapidly from drought stress to flood risk. Cyclonic weather patterns continued, and rainfall over the April-July period for England and Wales was the highest since the records began in 1766.

The subsequent persistence of near-saturated soil conditions made most rivers very responsive to the summer deluges and very unusually, allowed sustained recharge to most aquifers. In April runoff rates increased rapidly in impermeable catchments and by the final week, flood alerts were widespread. The EA reported that over 4000 properties in England and Wales had suffered fluvial or flash flooding by the end of August. The unsettled conditions continued into the autumn. 

Bath and North East Somerset and the 2012-13 floods10

Following one of the wettest years on record in 2012, as with other areas of the UK Bath and North East Somerset, experienced a series of floods. Between Sunday 23rd September 2012 and Monday 24th September 2012 many rain gauges received over 70mm of rain, particularly over the North Somerset area.

The Chew Valley and the villages of Chew Magna and Chew Stoke in particular were badly affected. During the period between September 2012 and January 2013 five separate floods were identified that triggered flood warnings, two of which resulted in significant property flooding during September 2012 and again in November 2012, with at least 35 properties being affected on a number of occasions.

The floods also caused dangerous conditions across the Pilgrim's Way ford in Chew Stoke. A number of drivers attempted to cross this ford at various times but had to be rescued from their vehicles. Tragically during the November 2012 floods one person died when their vehicle was washed off the ford approximately 120m downstream.

The properties affected by the 2012-13 floods

The EA provided information on flooded properties during the September and November 2012 floods. This information has come from resident questionnaires and EA flood reconnaissance.

A number of properties were significantly affected, with reports of internal flood depths reaching 400mm or more, many affected properties have since undergone considerable renovation following the floods, with residents forced to secure temporary accommodation, causing significant distress and upheaval for families.

Chew Magna

September 2012 event

November 2012 events (s)

Total no. of properties with Property Level Protection (PLP)

69

69

Total number of properties confirmed as flooded

31

17

Total confirmed as not flooded

9

4

Total unconfirmed

29

48

Table 1: Number of properties confirmed as flooded and not flooded in  Chew Magna in the 2012 September and November floods 11

The Fire Station in Chew Magna was also affected by the 2012 floods it was reported to have experienced 25mm of flooding internally.

Table 2 summarises, from the best available data the property flooding experienced in Chew Stoke during September and November 2012. These predominantly relate to flooding from fluvial sources, but records from flood surgery identify additional surface water flooding incidents, though precise numbers are not yet confirmed.

Chew Stoke

Recorded property flooding

Internal property flooding

12*

Property flooding confined to the gardens, surrounds and outbuildings

2

Property flooding, not internal, just at access points (e.g. first floor flats)

3

Table 2: Properties surveyed for the PLP scheme that experienced flooding in Chew Stoke during September and November 2012. (*Further assessment required to confirm precise numbers affected by surface water flood – 5 estimated) 12

Flooding on 24th September 2012

The largest rainfall depth was recorded for the 24th September 2012 flood, but soils in the catchments were not saturated at this time which is likely to have reduced the severity of the flood.

This flood was the most severe in terms of depth on the Winford Brook.  The flood warning for Winford Brook was issued with only a 49 minute lead time, giving very little response time for residents.

chew_magna_winford_brook_sept_2012_flood_map

Figure 2: Chew Magna Winford Brook catchment approximate flood extent on 24th September 2012  13

In both September and November 2012, the most significant flood depths, velocities and hazards to people from the Winford Brook were experienced at the junction of Streamside and Butham Lane, and continuing along The Batch.

chew_magna_river_chew_sept_2012_flood_map

Figure 3 Chew Magna River Chew catchment approximate flood extent on 24th September 2012 14

The greatest flood depths in the River Chew catchment were along Dumpers Lane, specifically the low points north of Tunbridge Close where depths in excess of 1m were reported.

Flooding on 21st-25th November 2012

During this period there were three distinct floods, The rainfall depth for these November floods was less than for the September flood. However the duration over which this rain fell was much shorter, hence the intensity of the rainfall was significantly greater. Also, unlike the September flood, the catchments were already saturated and consequently the rainfall runoff was likely to have been quicker and to have reached the watercourses more rapidly. The lag between the rainfall and the peak flow at the Chew Magna Reservoir gauge was extremely short, at less than 30 minutes.

Internal flood depths reached up to 530mm in the November 2012 flood, causing major damage to internal fixtures, fittings, fabric and furniture to a number of properties.

Flood sources

Fluvial – Investigations have revealed that the main source of flood risk to Chew Magna and Chew Stoke is fluvial from the Winford Brook, the River Chew, and the Chew Stoke Stream combined with flows in the Ordinary Watercourse that pass down Pilgrim's Way. This flooding occurs as a result of:

  • water flowing out of banks across floodplain areas;
  • overtopping at low spots along the banks; and
  • water backing-up in surface water drains.

Ordinary watercourses – Flooding caused by the ordinary watercourses in the area is believed to have affected many properties, and it was reported that inadequate capacity and blockages in these watercourses contributed to this flooding.

Surface water - is reported to have affected properties within Chew Magna and Chew Stoke during the 2012 floods. This was due to roads acting as pathways, conveying floodwater during rapid rainfall events. It was also caused by the intense rain falling on saturated ground, leading to rapid surface water runoff from nearby agricultural fields, running towards roads.

Groundwater - The area has a mixed geological character, dominated by marls, mudstones and secondary limestone bands which would generally be classified having low potential for groundwater flooding. However due to the impact of the wettest year on record the borehole at Felton experienced a sharp rise and very high groundwater levels and some property flooding occurred due to groundwater rising up through the property floors. This latter phenomenon reflects locally poor drainage characteristics entering the sub-structure of the property, rather than rising groundwater from deep aquifers.

Reservoirs - Due to the prolonged and exceedingly wet conditions, both the small Chew Magna Reservoir and the much larger Chew Valley Lake were overflowing continuously from around April 2012 onward. The wettest year on record caused both to fill rapidly from what were in fact significantly low levels due to the developing drought up to April 2012.

It should be noted that where there are a combination of flooding sources, it is often difficult to separate or ascribe the impacts from each source.

What does the community say?

Residents of Chew Magna and Chew Stoke  15 16

Following the 2012 floods, the Council and the EA held workshops and drop-in surgeries for residents to discuss and share their experiences, and thoughts on possible solutions to local flooding problems.  Detailed information and feedback from residents was also obtained by the EA, as part of the property-level protection (PLP) scheme.

Some of the key issues raised by the residents were:

  • There is some confusion and misunderstanding of the EA flood warning codes. Many residents were concerned at receiving so many 'warnings', although it is possible that this is a misconception of flood alerts.
  • Need for greater clearance and maintenance of highways, paths, gullies and ditches.
  • Insufficient ditches and drainage aids such as hedges.
  • Concern that something should be done to reduce the risk of someone getting swept off the Irish Ford again.