Abstract:This study attempts to establish causes of historical variations in¯ood frequency and magnitude in a 119-year ood stage record at York, and assess its likely eects on¯uxes of ®ne sediment and associated base metal contaminants. Climatic controls of¯ooding are evaluated through analysis of synoptic¯ood generation, and large-scale land use changes are also considered in terms of their in¯uence on food frequency and magnitude and sediment availability. The late nineteenth century was characterized by low¯ood frequency and magnitude, but, as a consequence of upland metal mining, contaminant concentrations and downstream ®ne sediment delivery would have been high. A further decline in¯ood frequency and magnitude between 1904 and 1943, combined with the cessation of base metal mining, resulted in a decline of sediment and contaminant¯uxes. Between 1944 and 1968 increases in upland drainage, changes in agricultural practices and high¯ood frequency and magnitude resulted in signi®cantly enhanced sediment¯uxes. Rates of¯ux declined between 1969 and 1977 owing to extremely low¯ood frequencies and magnitudes. In the last two decades there have been a series of extreme magnitude¯oods, which through remobilization of mining-contaminated alluvium have resulted in high pollutant metal mining loads, probably approaching those of the late nineteenth century. In an historical context it appears that the LOIS monitoring programme has been undertaken in a period of extremely high ood frequency and magnitude in the UK, and signi®cantly enhanced sediment and contaminant metal¯ux. Analysis of synoptic¯ood generation suggests that any future increase in the frequency and vigour of cyclonic atmospheric circulations may result in a higher frequency of extreme¯oods and consequently increased sediment¯uxes.

No abstract

This study attempts to establish causes of historical variations in¯ood frequency and magnitude in a 119-year ood stage record at York, and assess its likely eects on¯uxes of ®ne sediment and associated base metal contaminants. Climatic controls of¯ooding are evaluated through analysis of synoptic¯ood generation, and large-scale land use changes are also considered in terms of their in¯uence on food frequency and magnitude and sediment availability. The late nineteenth century was characterized by low¯ood frequency and magnitude, but, as a consequence of upland metal mining, contaminant concentrations and downstream ®ne sediment delivery would have been high. A further decline in¯ood frequency and magnitude between 1904 and 1943, combined with the cessation of base metal mining, resulted in a decline of sediment and contaminant¯uxes. Between 1944 and 1968 increases in upland drainage, changes in agricultural practices and high¯ood frequency and magnitude resulted in signi®cantly enhanced sediment¯uxes. Rates of¯ux declined between 1969 and 1977 owing to extremely low¯ood frequencies and magnitudes. In the last two decades there have been a series of extreme magnitude¯oods, which through remobilization of mining-contaminated alluvium have resulted in high pollutant metal mining loads, probably approaching those of the late nineteenth century. In an historical context it appears that the LOIS monitoring programme has been undertaken in a period of extremely high ood frequency and magnitude in the UK, and signi®cantly enhanced sediment and contaminant metal¯ux. Analysis of synoptic¯ood generation suggests that any future increase in the frequency and vigour of cyclonic atmospheric circulations may result in a higher frequency of extreme¯oods and consequently increased sediment¯uxes.

This study presents a new analytical framework for combining historical flood data derived from sedimentological records with instrumental river flow data to increase the reliability of flood risk assessments. Historical flood records were established for two catchments through re‐analysis of sedimentological records; the Nant Cwm‐du, a small, steep upland catchment in the Cambrian Mountains of Wales, and a piedmont reach of the River Severn in mid Wales. The proposed framework is based on maximum likelihood and least‐square estimation methods in combination with a Generalised Logistic distribution; this enables the sedimentological data to be combined effectively with existing instrumental river flow data. The results from this study are compared to results obtained using existing industry standard methods based solely on instrumental data. The comparison shows that inclusion of sedimentological data can have an important impact on flood risk estimates, and that the methods are sensitive to assumptions made in the conversion of the sedimentological records into flood flow data. As current industry standard methods for flood risk analysis are known to be highly uncertain, the ability to include additional evidence of past flood events derived from sedimentological records as demonstrated in this study can have a significant impact on flood risk assessments.