Careful study needed if Wales is to make good use of floodplains says Simon Haslett, Professor of Physical Geography and Associate Pro Vice-Chancellor of the University of Wales in his article at Wales Online. Certainly for residents of flooded areas it may be the first time in living memory that they have suffered an inundation, but that doesn’t mean that floods have not occurred in these areas in the past.
After floods devastated parts of Mid Wales earlier this month, Professor Simon Haslett argues that the potential impact of flooding in these Welsh valleys is now greater than it has been for centuries
This June might turn out to be the wettest on record, but the recent floods in river valleys across Wales raise questions as to whether these events are exceptional or to be expected from time to time.
Certainly for residents of flooded areas it may be the first time in living memory that they have suffered an inundation, but that doesn’t mean that floods have not occurred in these areas in the past.
Not unsurprisingly, many of the areas flooded recently are river valleys and their flood plains which, as the name suggests, are landscapes that have been created by flooding, and so are prone to flooding, even if not frequently.
Flooding only becomes a hazard if people try to occupy land with a flood history or interfere with river systems in some other way. If flood plains are avoided then the potential risk is greatly reduced. Planning has allowed such flood areas to be built upon, but what is the risk and will the risk increase with climate change?
An alarming example of human interference in a river system was highlighted two weeks ago with the evacuation of the village of Pennal in Gwynedd on June 10 after it was threatened by a dam-burst. A month-and-half of rain that fell in 24 hours threatened to break down a wall of rock waste in a disused quarry. If it had failed it could have sent a wall of water crashing down the valley.
Looking at historical events in Wales, the evacuation of Pennal was a very sensible decision. On November 2, 1925, 16 people died in the North Wales village of Dolgarrog when a series of reservoir dams burst after weeks of heavy rain and poor dam construction. Thousands of gallons of water poured down the Porth Llwyd River valley, picking up debris and transporting large boulders as it went.
Houses were swept away and lives were lost.
So the risk from a dam burst is very real.
Investigating longer term flooding in river valleys is, however, quite challenging compared to recent events for which good written records exist. One strategy is to examine floodplain deposits. These are laid down when a river floods with at least one layer of sediment usually being deposited every year. These annual deposits record floods and environmental changes, very much like tree rings.
Recently, I have been involved in a study that has succeeded in yielding some remarkable insights into the flooding history of the Usk River Valley in Monmouthshire, helping to understand potential future flooding risks. The deposits laid down here are also providing clues about the role of climate, sea level and human activity in shaping the landscape of Wales since the Ice Age.
The Usk valley was selected for the study 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. Published by the Geological Society of America , the study used a range of geological, geographical and archaeological techniques.
The research reveals that the flood plains came into existence around 6,500 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.
Such a scenario of surface waterlogging and increased river flooding may happen again in the future perhaps because of climate change, but also perhaps if a barrage is built across the Severn Estuary to harness tidal energy.
Experts have said that a barrage would cause an instantaneous rise in sea level of about three metres.
The imprint of the rise of agriculture in Wales can also be seen in the floodplain record. Results show that there has been a 10-fold increase in floodplain deposits since before the 19th century.
This increase is likely due to continued deforestation in the hills of the river catchment, an increase in ploughing through changing land use, and perhaps due to climate change.
One consequence of this 10-fold increase is that towns that lie within the river valleys would suffer greater impacts of flooding as deposits are laid down. For example, when the Romans 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 does the modern town of Usk 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 20th century. In Usk, substantial flood defences were constructed to protect the town following the last major flood in 1979.
Although this study focuses on one river valley, the results suggest that it is generally representative of rivers draining into the Severn Estuary, and perhaps elsewhere. 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 in the floodplain deposits.
It is clear however, that the potential impact of flooding in these Welsh valleys is now greater than it has been for centuries, and the flooding experienced this summer, once analysed, may add to the weight of this evidence. This evidence should be considered carefully in forecasting future flood risk and in planning sustainable use of Welsh floodplains and river valleys.