Quantifying sediment transport from eroding gullies using LiDAR

Gully erosion contributes greatly to sedimentation rates and soil loss in a number of environments globally. Gullies incise drainage lines, increase connectivity within the landscape and facilitate the transportation of sediment and nutrients from upland areas— contributing to excessive sedimentation and nutrient loading in drinking water reservoirs. The extent and increase in gully erosion has traditionally been measured through methods such as aerial photography and cross-sectional surveys. New methods such as high resolution topographic surveys provide the opportunity to measure geomorphic change at levels of detail not previously seen. This study sought to use LiDAR to quantify the response of two gully networks within the Southern Tablelands of New South Wales to a large rainfall event. High resolution Digital Elevation Models (DEMs) were constructed from LiDAR datasets and differenced to find areas of change using Geomorphic Change Detection software. This enabled the volume of sediment lost from the study sites due to the rainfall event to be determined, which was then used to estimate potential volumes of nitrogen and phosphorus exported. Average gully slope, drainage area, aspect and stream order were all assessed as potential morphological controls on the location and intensity of gully erosion at the study sites. Both study sites were net erosional during the study period, estimated to have exported thousands of m3 of sediment (13,835 ± 3,945 m3 at Arthursleigh and 2,855 ± 1,587 m3 at Dixons Ck) and associated concentrations of nitrogen and phosphorus (between 3 – 12 t N and 0.47 – 1.7 t P at Arthursleigh and 1 – 4 t N and 0.28 – 0.63 t P at Dixons Ck). The areal change at both sites was small and spatially variable but erosion occurred primarily on gully walls and floors. Morphological controls were deemed to have no influence on erosion at either study site. This study suggests that LiDAR is a useful tool for quantifying change in gully extent while also providing insight into potential nutrient outputs. It is also suggested that morphological controls such as average slope and drainage area are not the sole determinants for the location and intensity of erosion, with other potential influences such as the rainfall event being considered