Monday 30th November 2015
Blog post 9 of 10 about the geomorphology of Wales. Click on images to view larger version in a separate window. Parallel blog in Welsh at http://hywelgriffiths.blogspot.co.uk/
Reason 9. The Earth’s landscapes are becoming more hazardous. Both global environmental change and human activities are increasing the magnitude and frequency of geomorphological hazards, which occur wherever and whenever land surface stability is affected and adverse socio-economic impacts are experienced.
Throughout Welsh history, various geomorphological processes – many of them related to extreme events – have represented hazards to local communities. Being located in the temperate mid latitudes far from tectonic plate boundaries, in recent millennia Wales has been largely unaffected by the various hazards posed by extreme events such as glacial outburst floods, tsunami and volcanic eruptions, while earthquakes are a relatively infrequent occurrence (Reason 2). By contrast, given frequent intense and/or prolonged rainfall, a lengthy coastline that is exposed to Atlantic swells, and much steeply sloping terrain, extreme events such as river and coastal flooding, rockfalls, and landslides are common hazards. While such events may result from entirely natural causes, human factors can enhance their severity, possibly increasing the damage to infrastructure and/or the loss of human lives.
The events that took place in the small village of Dolgarrog, Conway, provide a case in point. Following a period of heavy rainfall that had started in mid October, on the evening of 2nd November 1925, the dam wall of the Llyn Eigiau reservoir in the Carnedd Mountains was breached. The breach released water that flowed downstream and then overtopped and breached the dam wall on the Coedty reservoir. Collective failure of the two dams caused a flood that continued along the Afon Porth Llwyd, rapidly cascading down its steep escarpment course towards Dolgarrog. The water and many thousands of tonnes of transported debris inundated part of the village, forming a fan with imbricated (stacked) boulders up to several metres in diameter on the western margin of the River Conwy floodplain (Photo: Stephen Tooth). 16 people lost their lives in the disaster, a figure that would have been much higher had many villagers not been watching a film in the local theatre (Source: Fearnsides, W.G. and Wilcockson, W.H. 1928. A topographical study of the flood-swept course of the Porth Llwyd above Dolgarrog. The Geographical Journal, v.72, pp.401-416). In 2004, a memorial trail was created through the boulder fan, and this serves as a sobering reminder of the lasting impacts that such extreme events can have on local communities.
On 21st October 2016, the 50th anniversary of the disaster in Aberfan, Merthyr Tydfil, will also provide pause for reflection. In this instance, a toxic combination of several days of heavy rainfall and negligent management practices contributed to failure of a local colliery spoil tip on the side of Mynydd Merthyr, liberating over 150 000 m3 of water-saturated debris. Some of the debris was re-deposited on the lower slopes of the mountain, but some 40 000 m3 continued as a viscous flow more than 10 m deep, rapidly inundating parts of the village, including the classrooms at Pantglas Junior School. 144 people, the majority of them schoolchildren, died in the disaster (Source: https://en.wikipedia.org/wiki/Aberfan_disaster; http://www.nuffield.ox.ac.uk/politics/aberfan/home.htm).
Around Wales’s coastline, extreme coastal storms represent some of the biggest geomorphological hazards. Recently, this was illustrated to dramatic effect along the seafront in Aberystwyth, Ceredigion, which was subject to a succession of high tides and high-energy wave events in the winter of 2013/14. In the most extreme event (early January 2014), the ground floors and basements of many seafront properties were flooded, paving slabs were eroded, and large volumes of sand and gravel were deposited across the promenade, roads and car parks (Photos: Stephen Tooth). While many parts of the seawall survived unscathed, in at least one place, the wall was breached, and erosion of the backfill lead to subsidence and partial collapse of a seafront shelter (for an animated 3D laser scan of the damaged shelter, see https://www.youtube.com/watch?v=8S9YFmPNTHI). As a Grade II-listed structure, the building material was salvaged and the shelter later reconstructed, and other parts of the seafront were quickly cleaned up and repaired. While such events are dramatic, and had never before been witnessed by many local residents, they are certainly not unprecedented, for significant damage to Aberystwyth’s seafront also had been caused by extreme storms in January 1938 and October 1927, as well as in earlier decades. Moreover, despite much of the media’s appetite, it may never be possible to attribute any individual extreme event to the impacts of global climate change. But in a world with rapidly rising sea levels, warmer average air temperatures and a more unstable atmosphere, such coastal storms may provide insights into the types of geomorphological hazards that in future may become more common along the Welsh coastline.
Did You Know? According to a Welsh government-sponsored report, 1 in 9 people in Wales live in properties that are at risk of flooding from rivers or the sea. In total, this represents 357 000 people and 220 000 properties (Environment Agency Wales, 2010. Future Flooding in Wales: Possible Long-Term Investment Scenarios. Available at: http://webarchive.nationalarchives.gov.uk/20140328084622/http://www.environment-agency.gov.uk/static/documents/Research/Flooding_in_Wales_Flood_defences_ENGLISH_V5.pdf). Many of these properties are located in towns and cities in the low-lying, densely populated coastal areas of north Wales and the post-industrial valleys and lowlands of the southeast, having been constructed when regulations on building in flood-prone areas were lax. With projections of future increases in inland and coastal flood frequency and magnitude, the number of flood-prone properties is likely to increase, with flood management likely to become one of Wales’s most pressing environmental management problems. Some of the impacts of extreme floods are obvious, such as the inundation and damage or destruction of property and infrastructure. Other impacts are less obvious but may be just as severe. For example, in the headwaters of many Welsh river catchments, historical mining activities have led to significant concentrations of heavy metals in floodplain sediments. During extreme floods, many of these sediments are re-mobilised by bank erosion, and then transported downstream and re-deposited on lower-elevation floodplains. This can result in widespread pollution of agricultural land and even domestic gardens. For example, levels of lead, zinc and cadmium locally have been found to be above recognised guideline values, and may pose significant risks to the health of grazing animals (Foulds, S.A., Brewer, P.A., Macklin, M.G., Haresign, W., Betson, R.E. and Rassner, S.M.E. 2014. Flood-related contamination in catchments affected by historical metal mining: an unexpected and emerging hazard of climate change. Science of the Total Environment, v.476, pp.165-180). Such studies show that despite the end of most metal mining over a century ago, its legacy continues to affect the Welsh population. Due to the largely invisible nature, however, this particular geomorphological hazard is often overlooked.