Understanding drivers of earthflow dynamics and sediment delivery

Earthflows occur throughout New Zealand’s soft-rock hill country and may play an important role in catchment sediment dynamics and related water quality issues by providing a persistent source of sediment to the channel network. However, earthflows are not well accounted for in catchment sediment models. A better understanding of their drivers, movement dynamics, and sediment load contributions is needed to improve existing models, and to forecast their trajectories under projected climate change.

We present findings from four years of monitoring an 80,000 m² earthflow in the Haunui research catchment, New Zealand. A range of complementary proximal sensing technologies have been deployed to provide data on sub-daily to multi-year movement rates, and annual volumetric change. A continuously operating GNSS receiver (cGNSS), located in the toe of the earthflow, records at 30-second intervals and is differentially corrected against a nearby reference station to produce centimetre-scale measurements of toe movement. Piezometers have been installed to provide a continuous timeseries of pore water pressure near the failure surface, and are complemented by continuous meteorological and stream flow data from a nearby monitoring station, allowing phases of earthflow movement to be linked with hydrological drivers. The high-frequency data are supplemented by a network of ~30 monitoring pegs distributed across the earthflow and routinely surveyed with rtk-dGNSS to provide a spatial representation of annual movement rates. Repeat UAV-based Structure-from-Motion (SfM) surveys provide high-resolution annual measurements of volumetric change via morphological budgeting.

We present insights into the movement dynamics and sediment load contribution of the earthflow across a range of hydrological conditions, from summer low flows to storm events. Movement rates up to 0.25 m day^-1and 7.5 m yr^-1 have been recorded, with average annual movement rates of 4-6 m across the earthflow. Phases of movement have coincided with seasonal fluctuations in groundwater levels, with continuous movement occurring from June to November (winter and spring) when groundwater levels remain elevated. Accelerations in movement during these months are associated with periods of higher magnitude rainfall and stream flow. Cessation of movement occurs as the earthflow body begins to dry in December, and the earthflow remains mostly stable until June when groundwater levels become elevated again.

Morphological budgeting has revealed erosion in the earthflow head has been largely offset by deposition in the toe, with gross volumetric change of 34,000 m³ and net change of 4,000 m³. The estimated annual sediment contribution to the stream network has varied from 4% to 32% of the catchment sediment load. These data indicate groundwater level is an important parameter for modelling earthflow dynamics, and may be critical in forecasting earthflow response to climate change. Cycles of deposition and erosion of material at the earthflow-channel interface complicate the link between on-slope phases of movement and contribution to stream sediment loads. Simple box models may therefore inadequately represent sediment delivery from earthflows over shorter timescales.