Flood-induced embankment failures on the River Tay: implications of climatically induced hydrological change in Scotland

Agricultural flood embankment failure frequency within the Tay drainage bas in in Scotland is explored by examination of breach data (228 breaches in t otal) collected during an eight-year period in which a large number of high discharge flood events (with return periods of up to 120 years) occurred. The data illustrates that overtopping is the main mechanism of failure, tha t certain reaches and specific locations are particularly vulnerable to fai lure, and that a near-linear increase in number and total length of flood e mbankment failures occurs with percentage increase in flood peak discharge. Non-hydrological factors contributing to flood embankment instability incl ude construction over former river channels, location on the outside of mea nder bends and "honeycombing" of embankments as a result of rabbit burrowin g. Hydrological data suggests an increase in the frequency of high magnitud e flood events since 1988 in the drainage basin and an identifiable spatial variability; catchments draining more westerly regions exhibit the greates t shift in pattern. The significance of these shifts and future possible ch anges in flood magnitude and frequency for flood embankment failure risk ar e quantified; 5% increases in flood magnitude result in increases in the nu mber of flood embankment failures of between 13 and 26%. Similarly a 5% dec rease in flood peak discharges leads to decreases of between 11 and 25%. Mo re generally it is shown that land next to rivers with flood embankments is sensitive to modest changes in flood generating conditions.