Effect of grain shape and porosity on coastal internal erosion and cavity collapse under continued waves, rainfalls and overtopping

Coastal internal erosion, cavity formation and collapse are influenced by the development and decrease in suction, that is, negative pore water pressure relative to atmospheric air pressure, which depends on sand grain size. Here, a series of prototype-scale experiments was conducted to examine the effect of sand grain shape and associated porosity on the sequential processes of internal erosion, cavity formation, expansion, collapse and enlargement of collapse under continued actions of waves, rainfalls, and overtopping. This was made by testing grounds with identical grain size distributions, yet, having substantially different grain shapes and porosities. A sequence of incident waves was imposed, followed by severe continued actions of rainfalls or overtopping. The results demonstrate that the sand grain shape and associated porosity substantially affected the resistance and stability to collapse in coastal grounds. Indeed, the critical levels of forcing that led to collapse in higher-grain sphericity and lower-porosity sands were 2.6 times higher in rainfall intensity, and 8.6 times higher in total rainfall, and 2.7 times higher in overtopping cycles, compared with lower-grain sphericity and higher-porosity sands. The cavity sizes became less than half in higher-grain sphericity and lower-porosity sands. The air-entry suction formula that takes account of the influence of the different sand porosities and effective grain sizes can account for the observed distinct characteristics of the resistance and stability to the coastal internal erosion and cavity collapse under continued waves, rainfalls and overtopping.