Geomorphological observations as aids to the design of coast protection works on a part of the Dee estuary

Abstract

Summary A section of the southeast shore of the Dee estuary is occupied by unstable cliffs formed of a complex succession of glacial deposits. Design of remedial works has necessitated an investigation of the nature and balance of geomorphological processes operating. Between 1973 and the present, a balance over a year has generally been achieved between failure of a clayey flow till in the upper cliff and erosion of the resulting colluvium from the toe. During exceptional storms, colluvium and in situ cohesive and non-cohesive materials are eroded, producing a rapid and larger-scale response at the crown of the slope. During winter, high pore water pressures are generated in perched water tables within the succession. Recent stabilization works at the toe in one downstream section appear to have caused severe toe erosion and slope instability over a distance of approximately 73 m of cliff line immediately upstream of the works. Geomorphological studies have also indicated appropriate angles for naturally stable slopes within this mixed succession. Grading or backfilling of the toe to achieve these specified slope angles, and toe protection with or without drainage, have been undertaken or are planned.