Intense, short-duration rainfall led to surface water flooding in the Cornish village of Coverack on 18 July 2017. Around 50 properties were flooded; road access was limited during the flooding and remains compromised.
On 18 July 2017, a torrent of water and debris tore through the Cornish coastal village of Coverack, affecting about 50 properties. The media interest, eyewitness accounts and snapshots of physical data continue to be released, but the immediate reaction is that this is "another Boscastle". Whilst there are some similarities, mainly the short intense rainfall and relative proximity of these two Cornish villages, there is a fundamental difference: the Boscastle floods were a result of a river system unable to cope with the volume of water feeding into it from its steep upstream catchment, whilst the Coverack floods were a surface water event. They occurred when intense rainfall on higher ground to the west of Coverack caused rapid overland runoff above and into the village, overwhelming drains, ditches and a couple of small becks, running off down the roads and leading to surface water flooding.
Thunderstorms in southern England brought torrential rain, large hail and flash flooding on 18 July 2017. Emergency services responded to flash flooding in Kent, Berkshire and Cornwall, where helicopters were used to carry out flood rescues.
Coverack Timeline, 18 July 2017
|15:40||First phone calls report the flooding|
|17:00||Rain subsides but the water levels build up|
|17:20||A major incident is declared|
Initial, unverified estimates suggest that around 100mm of rainfall fell in 3 hours. The Met Office rainfall radar suggests peak rates of 16-32mm per hour for 3-4 hours.
On 18 July, the Met Office issued the following warning: “Thundery showers are expected to push north across southern parts of the UK through Tuesday evening. Although many places won’t see these showers, there is a chance of localised flooding of homes, businesses and susceptible roads. Frequent lightning may be an additional hazard with possible disruption to power networks. Similarly, but very locally, hail may cause impacts.”
The heavy rain was extremely localised. Information held at 19 July suggests that none of the main weather stations in the area recorded significant rainfall levels.
Figure 1: Satellite image of lightning on 18 July 2017 13:15 BST. Source: Met Office.
Figure 2: Screenshot of the rainfall radar. Source: Met Office.
On 18 July, torrents of water over 1m deep swept through the village after a period of heavy rain. Flood water and debris made village access roads impassable.
Coverack is situated in a very small river basin and currently, there is no river gauge in or around the village. Analysis of river levels recorded by gauges in the area corroborates the view of a localised flash-flood event. The Manaccan flow gauge north of Coverack did show elevated levels, but only bordering the “flooding is possible” threshold. The Loe Pool gauge north-east of Coverack and those upstream of Loe Pool showed no change of water level during the storm, as the heavy rainfall did not affect this area.
Figure 3: Map of Coverack river basin. Source: Environment Agency.
Figure 4: Screenshot showing the EA telemetric gauges near Coverack. Source: gaugemap.co.uk.
A direct rainfall flood event
Most eye witness accounts and social media posts show torrents of water flooding down the small but main roads (B3294 and Polcoverack Lane) that enter the northern part of the village, and dramatic cascades of water over the sea wall onto the beach (Figure 5). Images and videos suggest that the two small streams running parallel to the roads were overwhelmed during the deluge, exacerbating surface water flooding and possibly lifting some of the reported large boulders out onto the road in the process. Many might consider the small channels as a source of the problem and question their capacity, especially at pinch points such as culverts and bridges where it appears they overtopped. However, they remain only part of the problem, as their upstream contributing areas are very small (2km2 compared to the 20km2 Boscastle river catchment, itself considered small). During the Coverack event, the streams acted as drainage routes, similar to the drains running down the side of the roads. All were overwhelmed simply because the rainfall was so intense.
Because its catchment is so small, Coverack has not been identified as at risk from rapid river response flooding. This emphasises the importance of direct rainfall runoff flooding and also the challenges of modelling very localised events.
Figure 5: Floodwater flowing down a road in Coverack. Source: BBC News.
How common is this for Cornwall?
Despite their differences, intense flooding of the types witnessed in Coverack and Boscastle is not uncommon in south-western counties. The geographical nature of this area plays its role; close proximity to the sea provides an abundant source of moisture, winds converge along the coast, and the elevated topography generates uplift and orographically enhanced rainfall. This may make the counties of Devon and Cornwall more susceptible to flooding from local intense rainfall events than other inland areas.
Following the 122mm in 2 hours that led to the Boscastle floods and the 184mm of rainfall recorded in Cornwall in August 2014 (with no notable resultant flooding), the Met Office shared some historical rainfall totals (see Table 1). Interestingly, the rainfall that fell in Camelford, Martinstown and Lynmouth, all during the 1950s, led to newsworthy and damaging flooding. Although these totals are for 24-hour periods, the 1950s events were reported to be short duration storms, similar to that of 18 July 2017.
|July 1955||Martinstown, Dorset||279.4mm|
|June 1917||Bruton, Somerset||242.8mm|
|August 1924||Cannington, Somerset||238.8mm|
|August 1952||Lynmouth, Devon||229.5mm|
|June 1957||Camelford, Cornwall||203.2mm|
Table 1: Record 24-hour rainfall totals. Source: Met Office.
The rainfall estimate from Coverack (around 100mm over 3 hours) may not appear to be extraordinary for this area. However, the localised nature of events like this makes it harder to obtain a true rainfall total; the values measured by the nearest rain gauge, even one close by, may differ substantially from the actual local rainfall. Rainfall radar often provides a valuable insight but is not without issues, including higher levels of uncertainly. This can make short intense rainfall events and their resultant flooding hard to predict.
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