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Early on Friday 8 June 2012 an unseasonably deep depression drifted northeast across Wales, with a ‘wrap-around’ occluded front slowly rotating around it. The combination of the depression, frontal zone, strong westerly winds in its wake with attendant coastal convergence, and orographic uplift, delivered heavy and persistent rain for several hours over central Wales. This led to serious flooding in many areas, particularly in and around Aberyst-wyth (Ceredigion), whilst the River Leri, north of Aberystwyth, overtopped its banks. The cumulative radar (Figure 1) suggests that over 125mm of rain may have fallen over the Cambrian Mountains in just 24 hours.
Aberystwyth is a small town with a population of roughly 16 000. It is positioned at the bottom of a valley at the confluence of two rivers, the Ystwyth and the Rheidol, close to the Irish Sea by Cardigan Bay; the steep hill behind the town marks the start of the Cambrian Mountains. Its topography and position on the west coast of Wales makes it susceptible to strong winds and outbreaks of rain, particularly in a westerly air flow. The closest weather station to Aberystwyth in the official UKMO synoptic network is situated at Trawsgoed, just 13.5km to the southeast (Figure 2). Trawsgoed is also positioned in a valley, next to the river Ystwyth. The weather station here has been operating since 1983, and the average rainfall in June is 75.8mm; between 0600 utc on 8 June and 0600 utc on 9 June, 81mm was measured, and there were also gusts of up to 50mph. The Environmental Agency Wales (EA) has raingauges positioned in and around Ceredigion, particularly in areas prone to flooding, such as close to river channels or in floodplains. They reported a (short-period) record daily rainfall total at the River Clarach gauge, just north of Aberystwyth, whilst on the River Rheidol 198mm of rain fell at Nant-y-moch and 184mm at Dinas, both in 36 hours. At Pwll Peiran on the River Ysytwyth 148mm was recorded in 36 hours, the highest total there since December 1964.
The heavy rain across the Cambrian Mountains on 8 June quickly flowed down the mountain sides towards the valleys and rivers. The areas that were worst affected by the flooding were just north of Aberystwyth, within the valleys and coastal areas. Settlements that experienced the most severe flooding were Talybont, Dolybont, Borth, Capel Bangor, Llanbadarn Fawr, Penrhyncoch, Llanfarion, Clarach and Goginan, as illustrated in Figure 2.
Aberystwyth is a popular holiday resort with a number of caravan sites nearby. On 9 June flooding forced the evacuation of many of these, and three people received treatment for minor flood-related injuries; over 200 caravans were flooded in and around Aberystwyth. One hundred and twenty properties within Ceredigion were flooded. Dyfed-Powys Police reported the closure of at least ten major roads as a result of the flooding, debris and damaged bridges, and the U1086 from Talybont to Nantymoch was closed because of a landslide. However, by 10 June things had significantly improved with water levels beginning to drop; only two roads remained closed and the emergency services were working to pump away excess water.
Within Aberystwyth, one area that was severely affected by the flooding was the industrial estate and retail park, Parc-y-Llyn. This was built in 1994, and although it is within the floodplain of the River Rheidol it was not thought to be at risk. However, based on the current planning guidelines TAN 15: Development and Flood Risk (Welsh Government, 2013), which provide technical guidance to supplement the Planning Policy in Wales in relation to development and flooding, it is unlikely the development would have been allowed today. Parc-y-Llyn is occupied by both residential and retail buildings as well as the Aberystwyth University playing fields (Figure 3). Fortunately, the flood plain had been taken into consideration for the recent development of housing in the area; the houses were built on raised ground and were not affected by the flooding – unlike the retail park, where up to 1.5m of flood water was recorded in some areas, causing shops to shut for days after the event. The Morrison and B & Q stores were severely flooded as was the local doctor's surgery, the Ystywth Medical Group, where the water caused significant damage to consulting rooms, the reception area and the office, as well as irreversible damage to all electronic equipment on the ground floor. The surgery provides clinical care for 9000 patients, but due to the flooding this was not available for several days. Aberystwyth University gave the doctors access to one of its buildings and the surgery operated from there until it reopened on 4 March 2013. The flooding was exacerbated because the high tide in Aberystwyth harbour around midday on 9 June prevented the excess discharge from the River Rheidol from draining into the sea.
As noted in Weather log (Eden, 2012), June 2012 was a relentlessly cyclonic month over the UK, with the lowest mean sea-level pressure at the 55°N 05°W gridpoint in June in a record going back 140 years. Over England and Wales, it was the equal wettest June since records began in 1766 and the coldest June since 1991.
Figure 4 shows the progress from 7 June to 9 June of the deep depression (lowest central pressure was about 980mbar early on the 8th) that brought the heavy rain and strong winds described in this article. The worst of the weather in mid Wales was on the Friday (8th); on Saturday afternoon it became brighter and less windy as the depression moved away. The heavy rain had been forecast several days in advance, and on 7 June the Met Office issued ‘yellow alerts’ for Ceredigion for Thursday night and Friday morning, whilst the EA issued flood alerts for upper Teifi (north Ceredigion), lower Teifi (mid Ceredigion) and flood warnings for Aberystwyth, Llanbadarn and -Parc-y-Llan (all within the River Rheidol catchment area) and Rhydyfelin and Llanfarian in the River Ysytwyth catchment area.
The heaviest rain fell over the Cambrian Mountains as a result of orographic enhancement, a phenomenon whereby rainfall is intensified in the presence of high ground due to moist air being forced to rise and cool, increasing the ‘efficiency’ of the deluge. Orographic enhancement can be observed frequently across UK uplands, such as the Scottish Highlands, Yorkshire Dales and Welsh mountains.
On 8 June, as the depression and occlusion moved across Wales with their associated persistent rain, the strong westerly wind enhanced the orographic uplift, as is illustrated by the cumulative precipitation radar images (Figure 5) which show that up to 30mm of rain fell in the Cambrian -mountains whilst surrounding areas at a lower altitude received only 5–10mm. This was an unusual example of orographic enhancement, because more normally in Wales the heaviest rain falls over Snowdonia. Sibley (2005) recorded a sequence of such events in north Wales in February 2004 when 417mm of rain and snow fell across Snowdonia in six days, whereas there was less than 75mm in low-lying and coastal areas. Sibley (2010) also studied orographic enhancement over Cumbria, southwest Scotland and north Wales in November 2009 when a large depression was situated to the northwest of the UK and strong southwesterly winds from a long sea-track pushed moist air across north Wales; the heaviest rain again fell across Snowdonia. In this 2012 event the heaviest rain did not fall over Snowdonia but over the Cambrian mountains because of the particular track of the depression through the centre of the country on this occasion.
The lack of rain in east Wales and Shropshire, directly downwind of the orographic enhancement, is notable in Figure 5, and is also illustrated by the rainfall totals shown in Figures 6 and 7. In contrast to the 53mm of rain recorded at Trawsgoed between 0600 and 1800 utc on 8 June, parts of east Wales had less than 1mm of rainfall, whilst between 1800 on 8 June and 0600 utc on 9 June 28mm of rain fell over the Cambrian Mountains and less than a millimetre in Telford and Kidderminster.
Peak recorded flow along the River Rheidol in Aberystwyth was 227m3s–1 at 0945 utc on 9 June, which hydrological records suggest has a return period of 200 years. The defences are only built for a 1-in-100 year event, which may be part of the reason for such a severe flood in Aberystwyth. Since the flooding, concerns have been aired about contamination from floodwater. Mines in the surrounding areas, many of which have not been used since the beginning of World War One, have left toxic metals such as lead, zinc and cadmium in river banks and nearby areas; the worry is that the floodwater may have eroded the surrounding areas, washing the toxins downstream. There are fears the toxins could pollute land and pose a risk to animals and crops. Professor Mark Macklin of Aberystwyth University is leading a team to survey the catchment areas of the Rivers Leri, Rheidol and Ysytwyth to determine if the toxins pose a threat.
An exceptional rainfall event affected Aberyst-wyth and the surrounding area on 8 June 2012, causing extensive flooding. Oro-graphic enhancement, more normally associated with rainfall over Snowdonia, was a key factor in bringing over 100mm of rain in 24 hours to some upland localities whilst there were much lower totals in many low-lying districts.
The author gratefully acknowledges the support of MeteoGroup UK which provided the data and maps used in this study, particularly Julian Mayes for his support. She would also like to thank Ceredigion County Council (2012) for making the Flood Investigation Report publicly available.