Flake failure: a new mass-movement mechanism affecting peat beds eroded intertidally, Severn Estuary, southwest Britain

Flake failures as observed on the Welsh bank of the Severn Estuary (SW Britain) are large (≤10 m) but thin (≤1 m), flake-shaped masses that became detached along subhorizontal, concave-up fractures from Holocene peat beds exposed intertidally on cliff-bound ledges. The failure surfaces are marked by patterns of radially divergent hackles and concentric hesitation lines that indicate brittle fracture of the peat in response to concentrated and probably impulsive forces applied to the edge of a bed exposed on the cliff. Mechanical theory indicates that the impulse was applied roughly horizontally, but in some cases with a significant upward bending moment. The failures are linked to storm events, suggesting that large waves breaking close to or against the upper part of the cliff provided the destructive force. The spatio-temporal frequency of flake failure is unknown, but the effect of the particular events studied was to release of the order of 90 m3 km−1 of peat from the edge of the ledge.