Mechanisms of floodbank failure during large flood events on the rivers Tay and Earn, Scotland

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Abstract
This paper describes flood embankment breaches on the Tay and Earn river systems, Scotland, during a 100-year return period flood in January 1993. The location of flood embankment breaches are compared with failures mapped during another large flood event in February 1990 and other historical information on flood embankment instability. Breaches were identified using colour aerial photography taken 10 days after the January 1993 flood event and a detailed field survey of every flood embankment breach was subsequently undertaken. In total, 116 breaches resulted from the 1993 flood event, most as a result of overtopping, and extensive floodplain damage was associated with a number of the failures. Specific sections of the Tay and Earn river systems demonstrated a high incidence of flood embankment breaching. Along these sections, embankments on the outside of bends and overlying old river courses are highly vulnerable to failure. Embankments bordering tributary streams are also vulnerable to failure as a result of being perpendicular to the direction of flow across the floodplain during inundation. A number of failures, however, cannot be attributed directly to their position relative to the river or former river courses. In these cases local factors, such as scour around fence-posts or poor flood embankment construction are probably dominant. Given the repeated breaching of embankments during flood events, changing agricultural land-use practices, and possible changes in flood magnitudes and frequencies, a number of recommendations concerning flood embankment management on the Tay and Earn river systems are proposed.