Groundwater erosion of coastal gullies along the Canterbury coast (New Zealand): a rapid and episodic process controlled by rainfall intensity and substrate variability

Abstract

Abstract. Gully formation has been associated to groundwater seepage in unconsolidated sand- to gravel-sized sediments. Our understanding of gully evolution by groundwater seepage mostly relies on experiments and numerical simulations, and these rarely take into consideration contrasts in lithology and permeability. In addition, process-based observations and detailed instrumental analyses are rare. As a result, we have a poor understanding of the temporal scale of gully formation by groundwater seepage and the influence of geological heterogeneity on their formation. This is particularly the case for coastal gullies, where the role of groundwater in their formation and evolution has rarely been assessed. We address these knowledge gaps along the Canterbury coast of the South Island (New Zealand) by integrating field observations, luminescence dating, multi-temporal unoccupied aerial vehicle and satellite data, time domain electromagnetic data and slope stability modelling. We show that gully formation is a key process shaping the sandy gravel cliffs of the Canterbury coastline. It is an episodic process associated to groundwater flow that occurs once every 227 d on average, when rainfall intensities exceed 40 mm d−1. The majority of the gullies in a study area southeast (SE) of Ashburton have undergone erosion, predominantly by elongation, during the last 11 years, with the most recent episode occurring 3 years ago. Gullies longer than 200 m are relict features formed by higher groundwater flow and surface erosion > 2 ka ago. Gullies can form at rates of up to 30 m d−1 via two processes, namely 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. The location of gullies is determined by the occurrence of hydraulically conductive zones, such as relict braided river channels and possibly tunnels, and of sand lenses exposed across sandy gravel cliffs. We also show that the gully planform shape is generally geometrically similar at consecutive stages of evolution. These outcomes will facilitate the reconstruction and prediction of a prevalent erosive process and overlooked geohazard along the Canterbury coastline.

How Can the Morphometric Characteristics and Failure Conditions of a Historic Gully Caused by Intense Rainfall Be Reconstructed?

“…Headward retreats in the formation of gullies is a well-known process, which according to some is related to landslides, especially in the case of intense rainfall, as observed for example in volcanic soil in Ethiopia [16,17,75], in saprolites of various lithologies in Madagascar [13,76,77] or in fluvio-deltaic deposits as in New Zealand [78]. Nevertheless, these processes are often related to internal erosion [13,18,52,76,79,80].…”

An experimental study of drainage network development by surface and subsurface flow in low-gradient landscapes

“…Seepage erosion has been studied at different spatial scales as a form of channel development. At large scales, seepage erosion has been attributed as the primary driver of channel development for drainage networks in unconsolidated materials (Coelho Netto et al, 1988;Micallef et al, 2021;Pillans, 1985;Schumm and Phillips, 1986;Uchupi and Oldale, 1994), and in bedrock in places like the Colorado Plateau (Howard, 1988;Laity and Malin, 1985) and Florida Panhandle (Schumm et al, 1995). The channel heads of these networks are often described as "amphitheater-shaped" due to the distinctive high relief headwalls that form when seepage erosion undermines channel headwalls and causes mass wasting (Laity and Malin, 1985), although this morphology may arise from any curvature-driven mechanical process (Petroff et al, 2018).…”

“…The degree to which drainage networks develop by overland flow or seepage erosion depends on a number of factors including substrate, rainfall rate, and relief. Field studies in unconsolidated sands and gravels have found that groundwater seepage can play an important role in channel development and formation (Coelho Netto et al, 1988;Dunne, 1990;Lapotre and Lamb, 2018;Micallef et al, 2021;Pillans, 1985;Schumm et al, 1995;Schumm and Phillips, 1986;Uchupi and Oldale, 1994). As…”

“…In order for channel heads to erode by seepage erosion, there must be enough precipitation that the infiltrating fraction can provide the discharge needed to overcome cohesion holding particles in place. Field studies by Micallef et al (2021) in a series of coastal gullies in New Zealand, for example, found a rainfall threshold of 40 mm/day necessary for seepage erosion to occur. Experimental studies find that the velocity of exfiltrating groundwater also must be high enough to remove the eroded particles deposited at the base of slopes, setting up the headwall for continued erosion (Abrams et al, 2009;Howard and McLane, 1988;Onda, 1994).…”

The Role of Fluid Seepage in the Erosion of Mesozoic Carbonate Escarpments

“…In subaerial settings, fluid seepage has been shown to unambiguously lead to the formation of box canyons only in unconsolidated sand to gravel sized sediments (Lapotre & Lamb, 2018; Lobkovsky et al., 2007; Micallef et al., 2021; Schumm et al., 1995). The efficacy of fluid seepage as an erosive agent in bedrock remains disputed, however, and there is no unambiguous example of a bedrock box canyon formed by seepage erosion (Lamb et al., 2006).…”