Plan banks that slow down rather than retain water
If constructing earthen banks in the catchments of a gully, build them to slowly drain runoff, not to retain runoff. These structures tend to be called diversion banks on grazing land and contour banks on cropping land. Diversion banks and contour banks are designed to intercept runoff before it concentrates and starts to cause erosion, and then safely channel it into a more stable area( Carey et al., 2015, Ch. 8 p. 6). They need to have a low slope on the channel( 0.1 per cent- 0.4 per cent) and not hold water behind them, as opposed to a dam bank or pondage bank.
Dams, retention( rehydration) banks and other structures that hold water are not likely to suit catchments above gullies because:
• Many soils at gullies in the BBB, when wet, are prone to tunnel erosion that leads to bank failure. In this study, well-constructed banks failed at three sites, two of which had land crabs in the vicinity that are suspected of initiating the tunnel( see Figure 32), and the other occurred at a soil change where a cracking clay met a sandy loam.
• Retained water must be released at the outlet with minimal erosive force. This requires a well-designed, properly constructed, and regularly maintained bywash( for dams) or spreader( for banks). Achieving this can be particularly challenging in dispersive or slaking soils.
An inland freshwater crab species, likely Austrothelphusa transversa, was found at Sites 10 and 24 near earthen walls that had held water but failed during the wet season due to tunnel erosion. While not conclusive, there is a reasonable possibility that crab burrows contributed to these failures, particularly at Site 10, where the soil is sodic( Sodosol).
These crabs are adapted to life in ephemeral waterholes across northern Australia. They dig burrows into the sides of channels and banks to reach the water table, where they aestivate( remain dormant) until the wet season( Waltham et al., 2014, pp. 1 – 2). Individual crabs can create burrows up to one metre deep in dense clay soils along freshwater rivers, creeks, drainage channels, swamps, pools, and farm dams( Queensland Museum, n. d.). During dormancy, they seal their burrows with earth to retain moisture, allowing them to survive dry conditions. Females also care for their young during this time, keeping them in a thin film of water beneath their abdomens. When the rains arrive, the burrows rehydrate, and the crabs emerge, releasing their fully developed young into the water. In prolonged droughts, the juveniles can remain in suspended development for up to six months, waiting for suitable conditions( Queensland Museum, n. d.).
At Site 10, 11 crab holes were observed in October 2023, along with two dead females, within a 3-metre stretch of the earthen wall, precisely where the wall had previously failed, both before and after gully remediation works. In 2024, two additional burrows were found at the base of the same wall, around the highest level reached by dam water.
It is believed that these burrows, being long, sloped downward, and potentially reaching up to one metre in depth as described by Waltham et al.( 2014, p. 2), could have initiated piping in the sodic soil when the dam water level was high, ultimately contributing to wall failure.
Figure 32. Additional information— Site 10 Freshwater land crabs may have undermined remediation efforts
NQ DRY TROPICS 49