The 'Muckle Spate of 1829'- reconstruction of a catastrophic flood on the River Findhorn, Scottish Highlands

Abstract

On August 3rd 1829 north-east Scotland recorded the most severe catastrophic flood in modern UK history. Sir Thomas Dick Lauder’s "An account of the great floods of August 1829 in the province of Moray and adjoining districts" (1830) provides a detailed eyewitness account of this event, which can used to reconstruct the flood. The flood was generated by a slow moving depression in the Moray Firth that produced an unstable northerly airflow over the NE Grampian Mountains. Reliable precipitation records are not available for the centre of the storm, but 95 mm in 24 hours was recorded at Huntly. The River Findhorn, one of the most severely affected drainage basins, was subject to detailed analysis by Lauder including the reporting of numerous flood levels on bridges and within bedrock gorges. Reconstruction of estimated flood flows at five of these sites has been undertaken using Manning’s equation. Moving steadily downstream, peak flows of 474m3s-1 (322.2 km2), 1042 m3s-1 (515.4 km2), 1322 m3s-1 (568.1 km2) and 1484 m3s-1 (599.6 km2) are reported on the main stem of the Findhorn with 451 m3s-1 (171.9 km2) on a major tributary. These values lie just within the limit when plotted in relation to Acreman’s (1989) envelope curve for catastrophic floods within the UK. The same river was also subject to a catastrophic flood in August 1970 when a peak flow of 2406 m3s-1 was recorded at Forres (782 km2). This is recognised as the largest gauged flow for a river in the UK. Lauder’s account clearly demonstrates that the 1829 flood reached higher stages than that in 1970, however the gauged flow for 1970 is larger than the reconstructed peak flow for 1829. Our reconstruction thus raises doubt as to the credibility of what is claimed to be the largest gauged flood in the UK