Use of historic water level records for re-assessing flood frequency:case study of the Spey catchment

Flood levels are available for four river flow gauging station sites in the Spey catchment, northern Scotland, for the major floods of 1768 and 1829. Field survey and Manning's equation were used to estimate peak discharge for each of the historic flood levels, to assist with the assessment of flood frequency. Standard UK methods encourage the use of historical information, but uptake presently appears to be limited. Compared with pooling and single site analyses, graphical re-assessments of frequency incorporating estimated historical flows represented increases in the 100-year flood of between 15 and 116%, illustrating that historical information may lead to major shifts in estimated frequency. These results are preferred over pooling results particularly because of the mismatch in the seasonality of major floods on the lower Spey compared with other sites available for pooling. Practitioners are urged to make use of historical flood information, subject to quality assessment, wherever possible.