The role of groundwater in forming theatre-headed valleys in unconsolidated sediments and bedrock

Groundwater has been implicated as an important geomorphic agent in valley development. In unconsolidated sand and gravel, field observations and numerical simulations have shown that groundwater can lead to the formation of theatre-headed valleys. However, the temporal scale at which these valleys form and the influence of geologic heterogeneities on valley evolution remain poorly constrained. In bedrock, the development of valleys by groundwater remains controversial, and other processes (e.g. plunge-pool erosion, megaflooding) have been proposed to explain such landforms.

Here I will present two case studies, from the Canterbury coastline of New Zealand (unconsolidated sediments) and the Maltese Islands (limestone), to address these knowledge gaps.

In the first case study (New Zealand), I integrate field observations, luminescence dating, multi-temporal UAV and satellite data, time-domain electromagnetic data and slope stability modelling to show that theatre-headed valley erosion by groundwater in sand and gravel is an episodic process that occurs once every 227 d on average, when rainfall intensities exceed 40 mm/d. Valleys can be elongated at rates of 30 m/d via the formation of alcoves and tunnels by groundwater seepage, followed by retrogressive slope failure due to undermining and a decrease in shear strength. Valley location is determined by the occurrence of hydraulically conductive zones, such as relict braided river channels, and sand lenses.

In the second case study (Maltese Islands), I combine field and remote sensing observations from Gnejna Valley with numerical modelling to demonstrate that groundwater seepage is the key driver of theatre-headed valley formation in jointed limestones over clays. Erosion takes place via widening of joints and fractures by fluid pressure and dissolution, and creeping of the underlying clay layer, both of which lead to slope failure at the valley head and its upslope retreat. The talus is removed by creep and sliding on the valley bed. The location and width of the valley are determined by the location of a master fault and the extent of the damage zone. An exponential decrease of seepage away from the master fault gives rise to a theatre-shaped head.