PROCESS-RESPONSE COASTAL BLUFF RECESSION MODEL, APPLICATION TO HOLDERNESS COAST (UK)
Authors: Castedo Ruiz Ricardo , Paredes Bartolomé Carlos , Murphy William , Lawrence James 
Published: December 2012

Coastal erosion and coastal instability, threatens property, businesses and life. Because of the great concentration of natural resources in coastal zones, it is imperative that coastal change is well understood to allow for effective management and, where necessary, engineering intervention. The development of cliff erosion predictive models is mainly limited to geomorphological data because of the complex interactions between coupled processes acting in time and space result in large-scale variations. There have been few reliable process-response models of cliff recession published. These have been based on functional relationships between the dominant physical processes covering the shoreface, beach and cliff or bluff, avoiding the geotechnical retreat mechanisms and behaviour within the cliff. Under this procedure, the resulting simulations of cliffs of differing behaviour can produce identical annual retreat characteristics despite the potential responses to a changing environment being unequal.

According to this inconvenience, a new process-response recession model has been developed. It incorporates the behavioural characteristics of coastal rock cliffs, the geology of which is dominated by overconsolidated clays and an associated protective talus wedge. This work brings together classic geotechnical stability analyses for rotational landslides called "Swedish† or "Fellenius† Method of slices among with collapse stability analysis for toppling mass failure. Once cliff material has fallen (landslide or toppling failure), colluvium formation at cliff foot is incorporated into the model using three different procedures, according to the geomechanical features of the weathered debris material. The first procedure is based on the friction angle for weathered materials; the second one is based on the angle of reach for landslides and the last one is based on field observations. The irregular recession cyclic pattern continues with the colluvium wedge erosion and the cliff profile steepening before a new failure occurs. Unlike previous models, the calibration parameter is restricted to account for hydrodynamic uncertainties from marine action and not includes the material strength. This new model introduces geotechnical parameters to evaluate the rock mass instability and failure through a safety factor, including RMR, cohesion, friction angle, uniaxial compressive strength of soil or rock mass of each geological component within the cliff, groundwater, etc. Among the capabilities of the model, it provides precise and stable responses to some of the inherent uncertainties in cliff or bluff recession processes, including those morphodynamics caused by different failure mechanisms, such as colluvium generation, groundwater influence, erosive tidal cycles and the cliff behaviour under changing climate conditions (i.e. sea level rise).

In this work, the presented model is validated through profile evolution assessment at various locations of Holderness Coast, UK. The model reflects that the coastal oversteeping is a key factor of instability in coastal cliffs, accompanied by cliff height that determines the failure mechanism present in the study area. Higher sea levels would remove the previous failed material at the base of the cliff more quickly and cause the slope to become unstable more frequently. Sea level rise during this century produce an increasing erosion rates over soft cliffs. Additionally, higher groundwater level also produces an increase in the number and size of the slope failure. The results represent an important step-forward in linking material properties to the processes of cliff recession and the subsequent long-term response under changing environmental conditions.