Seasonal quantification of coastal processes and cliff erosion on fine sediment shorelines in a cold temperate climate, north shore of the St. Lawrence maritime estuary, Quebec

A quantification of coastal erosion processes on a clay cliff in a cold temperate region was conducted. This study was based on a network of markers that were measured on a monthly basis from 1998 to 2003. During that period, the average retreat rate of the cliff was 1.5 m/y. Our results demonstrate that weathering is a more significant cliff retreat factor than hydrodynamic processes on fine sediment shorelines. This statement opposes conventional understanding. In fact, 65% of the annual cliff retreat took place through the winter season when the waves could not reach the foot of the cliff because of an ice foot. This erosion is caused by cryogenic processes in winter, particularly through freeze-thaw cycles, whereas desiccation and wave undercutting contributed respectively for 20% and 15% of the total annual retreat. The field measurements conducted before and after major storms, especially on October 29, 2000, illustrated that wave undercutting was negligible for the clay cliff. These results do not corroborate with previous studies showing that cliff erosion is mostly controlled by wave undercutting with negligible winter erosion. In a context of global warming, the intensity of cryogenic processes can become more important due to milder winters, an increase in the number of freeze-thaw cycles, and the reduction of the ice foot and snow cover (especially on south-facing cliffs directly exposed to solar radiation). This study demonstrates that the evaluation of sensitivity of coastal systems to climatic change should not be done just for sea-level rise and increased storminess, but also for other climatic parameters. Future research should also take into account approaches combining the studies of marine and terrestrial erosion processes.