About Alluvial Gully Erosion
Alluvial gully erosion is both a natural and human land use accelerated erosion process. These alluvial gullies or ‘breakaways’ initiate on steep river and creek banks along river frontages and erode into river terraces and elevated floodplains with highly erodible soils (Plate 2; Plate 3). River incision over geologic time (base level), dispersive or sodic soils (high exchangeable sodium on clay particles), and intense monsoon rainfall and flooding are natural factors priming the landscape for gully erosion. Since the margins of terraces and elevated floodplains are only infrequently inundated or backwatered by flood water, erosion from direct rainfall and overland water runoff from subtle terrace/floodplain slopes dominates gully scarp retreat.
Sediment dating in alluvial gullies has shown that gully erosion rates have increased greater than 10 times since European settlement in some locations. Historic aerial photographs also document increased gully erosion after land use change. Modern gullies have eroded into older floodplain hollows and drainage channels that were earlier phases of gully erosion, as well as steep river banks. The recent accelerated phase of gullying can be linked to the introduction of cattle that congregate along river frontages, reduction of perennial grass cover, concentration of water along cattle tracks (pads), and an increase in water runoff into gullies (Plate 4), as well as intense late-dry season fires, roads, fence lines, agricultural clearing, and infrastructure development.
Sediment budget research in the Normanby catchment estimated that 37% (1,148,200 t/yr) of fine sediment (<63 µm) entering the river system comes from alluvial and colluvial gullies. Independent sediment tracing data suggests that >87% of fine sediment inputs (<10 µm) originate from sub-surface sediment sources (channel bank erosion, gully erosion, etc.), with hillslope surface sources only a minor contributor. The mapped area of active, bare-earth, alluvial gully erosion viewable by aerial photographs in the Normanby catchment is > 1000 ha. It has been estimated that >10,000 ha of gullies exist in the Normanby once the masking of trees is removed using Light Detection and Ranging (LiDAR) topographic data. These alluvial gullies are mostly concentrated on dispersible or sodic soils along terraces and elevated floodplains of river frontage areas, where cattle grazing is focused.