Use of multi-proxy flood records to improve estimates of flood risk: Lower River Tay, Scotland

Proxy flood records from sediment stacks in floodplain palaeochannels provide an opportunity to extend short instrumental records and thus improve current estimates of flood risk. The ‘Bloody Inches’ (a meander cutoff on the lower River Tay, Scotland) has been infilling with flood deposits since c. 1761. Agricultural flood embankments locally breach with flows > 850 m3 s− 1 (introducing silts into the palaeochannel) and extensively fail with flows > 1200 m3 s− 1 (which deposit sand). Repeated cores at the site (up to 1.4 m in depth) consistently reveal sand-rich flood units. In the upper section of the core, 137Cs dating enables these units to be correlated with clusters of floods in the post-1950 discharge record. Sand units in the lower part of the profile are correlated with major floods from 1814 onwards using a 210Pb-based chronology. These dates are independently corroborated by flood marks inscribed on Smeatonʹs Bridge in Perth, 15 km downstream. Estimates of historic high flows back to 1814 can also be recovered from these flood stage levels at Smeatonʹs Bridge. When combined with the 48-year-long flow record, these historic floods yield an augmented POT series extending back to 1847. Using a relatively high threshold of 1361 m3 s− 1 and the Generalised Pareto Distribution to model this POT series, estimates of the 50-, 100- and 200-year floods are 1875, 2250 and 2050 m3 s− 1, respectively. These results provide independent and robust confirmation of the flood risk at Perth using the standard statistical procedures advocated in the Flood Estimation Handbook. If successfully replicated at other sites, radiometric dating of sediment stacks in palaeochannels provides a new technique to further advance palaeohydrology and the recovery of historic floods.