Rapid and Episodic Groundwater Erosion of Coastal Gullies Controlled by Rainfall Intensity and Substrate Variability: Insights from the Canterbury coast (New Zealand)

Abstract:

Our understanding of gully evolution by groundwater erosion predominantly relies on experiments and numerical simulations. As a result, the temporal scale of gully evolution and the influence of geological heterogeneity on their formation are poorly constrained. In this study we address these gaps in knowledge along the Canterbury coast of the South Island (New Zealand) by integrating field observations, optically stimulated luminescence dating, multi-temporal Unmanned Aerial Vehicle and satellite data, time-domain electromagnetic data, and slope stability modelling. Gully erosion is a key process shaping the sandy gravel cliffs of the Canterbury coastline. It is an episodic process associated to groundwater flow that occurs when rainfall intensities exceed 40 mm per day (about 1.6 times per year). The majority of the gullies in a study area located south-east of Ashburton has undergone erosion, predominantly by elongation, during the last 11 years. The two largest gullies, on the other hand, have not been eroded in the last 2 ka. Gullies form at rates of up to 30 m per day via two processes: the formation of alcoves and tunnels by groundwater seepage, followed by retrogressive slope failure due to undermining and a decrease in shear strength driven by excess pore pressure development. Gullies are located at hydraulically-conductive zones, such as relic braided river channels and possibly tunnels, and sand lenses exposed across sandy gravel cliff.