Posted on 10 January 2012
Professor Simon Haslett examining the red floodplain deposits of the Usk Valley, Monmouthshire.
The potential impact of flooding is now greater in a Welsh river valley than it has been for centuries - that is the finding of a study carried out by University of Wales Scientist, Professor Simon Haslett.
The study, which took 10 years to complete, has succeeded in yielding some astonishing insights into the impact of sea-level rise and the development of agriculture on the Usk River Valley in Monmouthshire.
According to Professor Haslett, deposits laid down in the Usk River Valley are also yielding clues about the role of climate and human activity in shaping the landscape of Wales since the Ice Age.
Published by the Geological Society of America, Professor Haslett’s research has focused on the study of deposits laid down by rivers in floodplains in southeast Wales, using a range of geological, geographical and archaeological techniques in his investigation. He said:
“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 floodplains of the Usk river system in Monmouthshire in Southeast Wales, were the focus of the study. They were selected because of the thick red deposits that have accumulated there and the fact that the River Usk flows into the Severn Estuary – reputed to have the second highest tidal range in the World.
The study reveals 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 held 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.
Professor Haslett added:
“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” says Professor Haslett “is that towns that lie within the river valleys would suffer greater 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 higher 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. In Usk, substantial flood defences were constructed to protect the town following the last major flood in 1979.”
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.
Simon Haslett is Professor of Physical Geography and Associate Pro Vice-Chancellor of the University of Wales, and Dean of STEM at the University of Wales Faculty.
Notes to Editors:
Professor Haslett’s blog is available at http://profsimonhaslett.blogspot.com/2012/01/flood-history-in-south-wales-valley.html
To access Professor Haslett’s Geological Society of America paper http://specialpapers.gsapubs.org/content/476/93.short