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3.2.10.6 Summary of Water and Sediment Processes at Oriners Lagoon
The size and volume of Oriners, its importance for residential purposes and the complexity of
the controls on water and sediment inputs and outputs, meant that a conceptual model was
created to visualize the major factors and their interactions (see Figure 86 in Part 4). The
main independent controller on the water volume of Oriners is the climatic input of rainfall
and resultant runoff. Rainfall can vary annually, seasonally and daily (Figure 44; Figure 45;
Figure 46) and influence rainfall-runoff patterns and vegetation cover, with the latter in turn
mediating runoff from rainfall. Land use impacts such as grazing, fire, weed invasions and
roads can also influence vegetation cover, which in turn mitigate water and sediment runoff
volumes in response to rainfall. The main inputs of water for Oriners Lagoon are from the
Eight Mile Creek anabranches (Figure 64; Figure 66), small tributaries and gullies off the
Holroyd Plain, and combined baseflow and groundwater (GW) input. The main sediment
sources to Oriners Lagoon are also Eight Mile Creek anabranches (Figure 68; Figure 69), but
sediment sources from gully and road erosion are increasing due to local land use impacts
(Figure 69ef; see Section 3.2.11 on gully erosion below).
Downstream factors also influence the water volume and storage of Oriners Lagoon by
controlling the outputs of water. Evaporation and transpiration (combined as
evapotranspiration, ET) are key natural processes that slowly draw down the water volume in
the lagoon over the dry season. Evaporation likely dominates total ET, but the transpiration
of lagoon water by trees into the atmosphere could also be locally significant. Additional
pumping withdrawals are made by the human use of Oriners station. At present these
withdrawals are minuscule, but could increase in the future with more development. The
height of the outlet sill of the lagoon (Figure 70) is the key factor that controls the total water
storage volume of the reservoir, in addition to the volume of floodwater and baseflow output.
Different types of vegetation growing on the sill, in addition to LWD, can influence the sill
roughness and stability. Native perennial grass is likely being a better stabilizer of soil across
the entire sill, rather than trees that funnel water into discrete scour locations. Human land
use of the local area around the lagoon outlet can have significant impacts on the erosion or
deposition of sediment on the sill, specifically roads, gullying, and vegetation changes
(Figure 70), which again in turn influence lagoon water volume. A big unknown is the degree
that the sill height influences the flushing of sediment from the lagoon during floods, such as
sand bedload. A lower sill height could allow for easier passage of sand, but a low sill high
could also reduce water depth, turbulence and shear stress in the lagoon during flood, which
promote local scour of the lagoon bottom.
3.2.11 Gully erosion
3.2.11.1 Background on gully erosion
Gully erosion is the process by which running water cuts new unstable channels into erodible
soils and regolith. Gullies are larger than rills, which can be ploughed or easily crossed, but
smaller than streams, creeks, arroyos, or river channels. Gullying causes severe land
degradation, is a major component of contemporary sediment budgets, and is a major source
of sediment pollution to aquatic ecosystems. Two types of gully erosion are typically found in
Australia, including Oriners and Sefton Stations: 1) hillslope or colluvial gully erosion, and 2)
alluvial or floodplain gully erosion (Brooks, Shellberg et al. 2009).
Hillslope gullies are generally located in headwater settings, where they erode into colluvium
(or alluvium mixtures), saprolite, weak sedimentary rock, or other weathered rock, and have
also been defined as valley-side or valley-head gullies. Their length is much typically greater
than their width and their erosion mechanism is typically overland flow in which excess shear
stress exceeds resisting forces (Brooks, Shellberg et al. 2009).
Alluvial gullies are incisional features entrenched into floodplain or terrace alluvium typically
not previously incised since initial deposition. They are often located adjacent to rivers or
floodplain water bodies in the Mitchell-Alice catchment (Brooks, Shellberg et al. 2009;
Shellberg 2011; Shellberg, Brooks et al. 2012) . They are locally called “breakaways”, or
Working Knowledge at Oriners Station, Cape York
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