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 basin in Scotland is explored by examination of breach data (228 breaches in total) 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, that certain reaches and specific locations are particularly vulnerable to failure, and that a near-linear increase in number and total length of flood embankment failures occurs with percentage increase in flood peak discharge. Non-hydrological factors contributing to flood embankment instability include construction over former river channels, location on the outside of meander bends and "honeycombing" of embankments as a result of rabbit burrowing. Hydrological data suggests an increase in the frequency of high magnitude flood events since 1988 in the drainage basin and an identifiable spatial variability; catchments draining more westerly regions exhibit the greatest shift in pattern. The significance of these shifts and future possible changes in flood magnitude and frequency for flood embankment failure risk are quantified; 5% increases in flood magnitude result in increases in the number of flood embankment failures of between 13 and 26%. Similarly a 5% decrease in flood peak discharges leads to decreases of between 11 and 25%. More generally it is shown that land next to rivers with flood embankments is sensitive to modest changes in flood generating conditions.