Monitoring gully erosion in Great Barrier Reef catchments
Gullies are landscape erosional features impacting large areas globally. Sediment eroded from gullies is known to result in agricultural production losses and degradation of water quality which subsequently impacts the aquatic and marine environments as well as damaging infrastructure such as roads.
In Queensland, Australia, the most notable impact of gullies is in Great Barrier Reef (GBR) catchment areas where gullies account for the dominant source of sediment in waterways flowing to the GBR and in the GBR lagoon. Research has shown that gullies contribute to the degradation of the Reef’s water quality and impact important seagrass communities along the coast which are essential to marine processes and as a breeding ground and food source for many reef species including dugongs and turtles. The widespread occurrence of gullies in some GBR catchments is also an issue for agricultural productivity and presents a significant challenge to land managers trying to balance gully prevention and restoration activities with agricultural sustainability and water quality outcomes for the GBR. The management of these erosional features is complex and requires knowledge of their spatial and temporal characteristics at a range of scales to assess the suitability/effectiveness of prevention and remediation efforts. Terrestrial laser scanning (TLS) provides an innovative tool for acquiring rapid and repeatable information on gully morphology for sites typically <10 ha.
The Queensland Department of Environment and Science’s Remote Sensing Centre, in partnership with the Joint Remote Sensing Research Program and CSIRO have been researching and developing operational gully monitoring methods and processing systems using RIEGL’s scanners for a number of years. The RIEGL instruments have consistently met the scientific standards to accurately and precisely monitor the complexities of gullies and the dynamic changes in gullies over time, while efficiently delivering the data for post-processing, for this scientific program.
The acquisition process to survey gully morphology initially involves RIEGL’s automated reflector search function to locate reflector targets over survey reference marks. This allows accurate registration of multi-temporal datasets. Multiple scans are then registered together using reflectors as tie points and more recently using reflectorless workflows. Multiple scan locations are selected by the operator to ensure a high point density and also to minimise the occlusion of the ground surface. Earlier work by the team utilised the vz400 scanner but this has recently been supplemented with the addition of a vz400i with its improved features significantly increasing the speed and density of collected data. Automated registration and MTA processing has been performed either on the fly or using RiSCAN Pro and stored within the new .rdbx format for direct input to our python based workflow. A series of standardised products have been developed, including: interpolated Digital Elevation Models (DEMs), elevation change over time or DEMs of Difference (DoDs)(refer to Figure 3), gridded maximum return height and volumetric change (erosion and deposition).
It is anticipated that this information will help inform investment in gully management in the GBR and other sensitive regions of Queensland. It is also hoped that this work can inform approaches for scaling up to broader spatial areas, either statistically or through integration with other airborne or satellite based sensors. This work has been published in international journals, government reports, and in trials to understand the effectiveness of gully remediation efforts.
Figure 1: Cylindrically projected image showing the returned reflectance from a TLS vz400 scan.
Figure 2: Scanner in action at a gully within the GBR catchment: (left) RIEGL VZ400 angled using a tilt mount and (right) a colour coded point cloud derived from an integrated DSLR camera.
Figure 3: TLS derived DEM sequence from 2016 to 2019 demonstrating erosional change for a gully located in the GBR catchment. This work was funded by the Queensland Government and the Australian Government’s National Environmental Science Program Tropical Water Quality Hub (Projects 2.14 and 5.9).
Goodwin, N.R. Armston, J., Muir, J. and Stiller, I. (2017). Monitoring gully change: A comparison of airborne and terrestrial laser scanning using a case study from Aratula, Queensland, Australia. Geomorphology, 282, 195-208.
Goodwin, N.R. Armston, J., Stiller, I., and Muir, J. (2016). Assessing the repeatability of terrestrial laser scanning for monitoring gully topography: A case study from Aratula, Queensland, Australia. Geomorphology, 262, 24-36.