05 October 2015
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Compiled by Perth & Kinross Council, the Scottish Environment Protection Agency (SEPA) and Scottish Natural Heritage (SNH), the report has been sent to Alyth Community Council, and is now being shared with the wider local community.
The co-ordinated investigation has confirmed that extreme rainfall and high flows within the Alyth Burn were the primary cause of the flooding.
Following an in-depth analysis of the flood conditions, SEPA has concluded that the intensity of rainfall across the Alyth catchment on the morning of 17 July was enough to cause a significant flood event to occur. Analysis of rainfall data has shown that approximately 55 to 60 mm fell over a six hour period, with the highest intensity occurring between 4:00 am to 8:00 am, when the flooding started. This resulted in a peak flow in the Alyth Burn estimated by SEPA to be around 40 m3/s. This is a rare event with an estimated 0.5% annual probability (also called a 1 in 200 year flood).
Despite community concerns about the impact of fallen timber from the Den of Alyth, the timber and woody debris observed within the watercourse are typical features of floods of such magnitude, particularly when there are sections of woodland along the river banks. This debris was a very visible consequence of the high flows in the watercourse. While the collapsed footbridges, timber and woody debris may have locally influenced flow patterns, it was not the cause of the flooding. Furthermore, the investigations suggest that flooding would still have occurred even if the collapsed footbridges and debris had not been present.
The debris consisted of a mixture of uprooted trees and branches, sawn and cut timber, telegraph poles, a skip and other general waste material. The uprooted trees came from eroded river banks upstream of Alyth. This was caused by high flows in the watercourse during the flood event and is therefore very difficult to predict or prevent. Other sawn and cut timber was also observed but the sources of this material remain unclear and this could have arisen from several locations within the catchment area.
In response to concerns over the possible contribution by beavers to the scale of flooding, SNH contracted the Royal Zoological Society of Scotland (RZSS) to investigate. All beaver dams located along the stream network feeding into the Alyth Burn above the Den of Alyth were found to be intact with only minor damage to a few which beavers had repaired with mud. There was no sign of current beaver activity in the Den itself. While much of the flood borne timber had been debarked and could appear to have been associated with beaver felling, close inspection found an absence of characteristic beaver tooth marks on the wood. This indicated that the loss of bark had been caused by abrasion against rocks and trees rather than by beavers. RZSS concluded that beavers made little or no significant contribution to the flood.
Leader of Perth & Kinross Council, Councillor Ian Miller said:
I would like to take this opportunity to thank local residents for all the photographs, videos, eyewitness accounts, information on flow paths and completed questionnaires they provided which enabled the Council and its partnership agencies to complete the detailed investigations which were necessary to compile the report.
"It is only through improving our understanding of the causes and impacts of the flooding that we will be able to manage it in future. The conclusions from the joint report will provide a basis to develop potential measures to mitigate the impact of any further severe flooding. The finalised flood report is now available on the Council's website.“
Pascal Lardet, SEPA Flood Unit Manager, said:
Our joint investigations have shown that this event seems to have been the largest flood in Alyth over the last 140 years and was triggered by a very intense period of rain in the early morning of 17 July. The flows resulting from the sheer volume of rainfall not only exceeded the capacity of the Alyth Burn, but also caused significant erosion to the banks of the watercourse throughout the catchment, with trees and other debris washed down the river.
"The joint report provides a valuable starting point from which to develop sustainable measures to mitigate the impact of severe flooding in the future.”
David Bale, SNH Area Manager for Tayside & Grampian said:
We were pleased to contribute to the joint investigation. The objective approach has been essential in coming to an understanding of the complexity of the flood’s cause and effects. It has also enabled us to answer many of the questions posed by those who have been affected by such a traumatic event. We now look forward to continuing to work with a range of partners to identify long term ways of enabling the catchment to become more resilient to any such severe rainfall events in the future."
Notes to editors
SNH's news release on the flooding investigation - http://snh.presscentre.com/News-Releases/SNH-findings-on-Alyth-flooding-and-beavers-1da.aspx
To view the Combined Report please see the following link:
Alyth Flood - Combined Report (Final)