About Professor Simon Haslett

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Cardiff, Wales, United Kingdom
Professor of Physical Geography and Pro Vice-Chancellor at the University of Wales and the University of Wales Trinity Saint David.

Monday, 9 January 2012

Flood history in a South Wales valley

The final report of research that I have been undertaking since 2002 was published in 2011 by the Geological Society of America. The final report brought together new data and data previously presented in a series of articles I published in the Monmouthshire Antiquary (Proceedings of the Monmouthshire Antiquarian Association).


The floodplain deposits laid down in the Usk River system in Monmouthshire yield clues about the role of climate and human activity in shaping the landscape of Wales since the Ice Age, and gives insights into the impact of both sea-level rise and the development of agriculture. The study also reveals that the potential impact of lowland river flooding has increased through the centuries and is greater now than it has ever been.

Using a range of geological, geographical and archaeological techniques, the floodplains of the Usk river system in Monmouthshire in southeast Wales were investigated. Floodplain deposits are laid down when a river floods with at least one layer of sediment usually being deposited every year; these annual deposits record environmental changes very much like tree rings. The Usk River valley was selected because of the thick red deposits (see photo) that have accumulated there and the fact that the River Usk is a tributary of the Severn Estuary; an important global feature reputedly with the second highest tidal range in the World.

The research shows that the floodplains came into existence around 6500 years ago during the Stone Age. This was a time when the melting of the Ice Age glaciers and ice sheets had raised sea level rapidly to flood the South Wales coast. Before this time, the rivers flowed fast through this region on their journey to the distant sea, but the fast rising and encroaching sea ponded back the rivers forcing them to flood. The first sign of this was waterlogging of the Ice Age landscape and the formation of peat many miles inland. Such a scenario of surface waterlogging and increased river flooding may happen again in the future, not necessarily because of climate change, but if a barrage is built across the Severn Estuary to harness tidal energy experts have said that this would cause an instantaneous rise in sea level of around three metres.



The imprint of the rise of agriculture in Wales can also be seen in the floodplain record. The research shows that there has been a tenfold increase in floodplain deposits since before the nineteenth century. This increase is likely due to continued deforestation in the hills of the river catchment, and also due to an increase in ploughing through changing land use. One consequence of this tenfold increase is that towns that lie within the river valleys have become exposed to increased potential impacts of flooding as deposits are laid down. For example, when the Roman’s built the fort of Burrium on a spur of glacial gravel in the Usk valley in the first century AD, the fort lay around one and half metres further above the floodplain than the modern town of Usk does that now occupies the site. Year on year, there has been less space to accommodate floodwaters since Roman times, more so due to agriculture, so that the potential impact of flooding has increased and the town has flooded badly several times during the twentieth century.

Although this study currently focuses on one river valley, the results suggest that it is generally representative of rivers draining into the Severn Estuary. Further work is planned, especially to collect more sediment ages from dating archaeological finds and also through the radiocarbon dating of peat and other organic remains within the floodplain deposits.

Bibliography

Haslett, S. K., 2011. Holocene sedimentation in a pericoastal river system (South Wales, UK): relationship to sea level, human activity, and coastal sediment flux. In: Brown, A. G., Basell, L. S. and Butzer, K. W. (eds), Geoarchaeology, Climate Change, and Sustainability. Geological Society of America Special Paper 476, pp. 93-103.

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