Investigation of coastal recession along 272km of the Pembrokeshire Coast National Park has identified erosion processes, mechanisms and parameters. Map evidence has established spatial variability in long term (66 years) retreat rates between the various rock sections.The northern section between Cardigan and St. David's Head was stable with an erosion rate of only 0. 5cm/yr for the mainly Ordovician igneous and sedimentary outcrops. High erosion rates were shown in the Coal Measure series rocks which outcrop in the west at St. Bride's Bay (18cm/yr) and in the east near Amroth (12. 6cm/yr). Negligible erosion was found in the Carboniferous limestone while the Old Red Sandstone exhibited varying rates from zero to 21.Ocm/yr. Mean high water mark (MHWM), mean low water mark (MLWM) and cliffline position were found to be inadequate indicators of recession.Field sites were selected to represent the different lithologies of Old Red Sandstone, Carboniferous limestone and Coal Measure series that comprise some 70% of the total rock exposures. Field and laboratory values for uniaxial compressive strength were comparable. Joint roughness coefficient (JRC) obtained from tilting tests suggested that standard equations were more applicable to situations of low roughness rock with low normal strength.Failure modes for cliffs of the western Coal Measures were mainly translational, for those of the eastern outcrop block sliding. Old Red Sandstone cliffs mainly exhibited plane sliding, Carboniferous limestone block failure. Low discontinuity shear strength favoured translational failure. High dip angles >75* were favourable to toppling, medium dip angles 45*-75" to plane slides and low dip angles, 45' to translational slides. Where applicable numerical modelling safely predicted failure modes for the various rock types.
|Date of Award||1992|