The hydrogeomorphic influences on alluvial gully erosion along the Mitchell River fluvial megafan

Abstract

Hydrogeomorphic processes influencing alluvial gully erosion were evaluated at multiple spatial and temporal scales across the Mitchell River fluvial megafan in tropical Queensland, Australia. Longitudinal changes in floodplain inundation were quantified using river gauge data, local stage recorders and HEC-RAS modelling based on LiDAR topographic data. Intra-and interannual gully scarp retreat rates were measured using daily time-lapse photographs and annual GPS surveys. Erosion was analysed in response to different water sources and associated erosion processes across the floodplain perirheic zone, including direct rainfall, infiltration-excess runoff, soil-water seepage, river backwater and overbank flood inundation. The frequency of river flood inundation of alluvial gullies changed longitudinally according to river incision and confinement. Near the top of the megafan, flood water was contained within the macrochannel up to the 100-year recurrence interval, but river backwater still partially inundated adjacent gullies eroding into Pleistocene alluvium. In downstream Holocene floodplains, inundation of alluvial gullies occurred beyond the 2-to 5-year recurrence interval and contributed significantly to total annual erosion. However, most gully scarp retreat at all sites was driven by direct rainfall and infiltration-excess runoff, with the 24-h rainfall total being the most predictive variable. The remaining variability can be explained by seasonal vegetative conditions, complex cycles of soil wetting and drying, tension crack development, near-surface pore-water pressure, soil block undermining from spalling and overland flow, and soil property heterogeneity. Implications for grazing management impacts on soil surface and perennial grass conditions include effects on direct rainfall erosion, water infiltration, runoff volume, water concentration along tracks, and the resistance of highly dispersible soils to gully initiation or propagation under intense tropical rainfall.

Working Knowledge: characterising collective indigenous, scientific, and local knowledge about the ecology, hydrology and geomorphology of Oriners Station, Cape York Peninsula, Australia

“…The first two methods were the focus for generating local knowledge from former and present day workers at Oriners Station, and the field visits and literature surveys were important in generating additional scientific information. Shellberg's previous hydro-geomorphic research (Shellberg et al 2010(Shellberg et al , 2013Shellberg 2011) about the lower Mitchell River catchment were augmented with additional scientific analyses of the Oriners Station landscape from local and regional datasets (Barber et al 2012). Formal scientific sampling of Oriners Station was not part of the research scope and was not undertaken.…”

Rapid Cycles of Episodic Adjustment: Understanding the Holocene fluvial archive of the Daintree River of Northeastern Australia

“…Brooks et al (2014aBrooks et al ( , 2014b and Caitcheon et al (2012) have questioned the significance of hill slope erosion in tropical catchments, suggesting bank erosion (Caitcheon et al, 2012) is more important. Similarly, Wasson et al (2002), Brooks et al (2009) and Shellberg et al (2013) have suggested that gully erosion is also a significant source of sediment. However, the contribution of floodplains as a sediment source, particularly when and if they experience substantial or complete stripping, has received little attention for catchments draining to the GBR lagoon.…”

Sediment production and yield from an alluvial gully in northern Queensland, Australia

“…Daily delineation of index scarp locations from oblique photographs demonstrated that the total scarp retreat was the cumulative sum of numerous and variable daily changes over each wet season, most often as discrete failures of overhanging soil blocks. Both direct rainfall and infiltration‐excess runoff dominated scarp retreat at WPGC2a, in contrast to groundwater seepage and pore water pressure (Shellberg et al ., ). Shallow overland flow from the low‐gradient vegetated floodplain above the scarp was relatively clear compared to the turbid water runoff following erosion on the scarp face, indicating that ungullied parts of the catchment are not major sources of sediment.…”

“…The gully study catchment (WPGC2a) is located in remote northern Queensland, Australia (–16°28'16" , 143°46'36" ; Shellberg et al ., ). WPGC2a is one alluvial gully of thousands delineated and described by Brooks et al .…”