Of the 10 check dams and weirs constructed across four sites, three required maintenance. These three are substantial earth and log weirs with rock armouring at Site 14. This site is situated low in a valley where velocities and volumes of flow are high. Two lots of relatively minor but timely maintenance have been done at this site to maintain the integrity of the structures while they‘ settle’ into the landscape.
Stability achieved in catchments
Remediation works in the catchments of gullies had a high level of success. Of the 14 sites, 11 catchments were stable after 4-5 years, two catchments were mostly stable and one was deemed unstable( Figure 18). The unstable catchment is at Site 19; a small site not prioritised for maintenance; described in the maintenance section above.
12
10
8
6
4
2
0
11
Stable
2
Mostly stable
0
Partly stable
1
Active / unstable
Figure 18. The level of stability achieved in the treated catchments of small-scale gullies 4 – 5 years after the inital treatment. Assessed December 2024; sites where only the gully was treated are excluded.
Land condition assessments( LCAT) offer insight into the catchments’ progress towards achieving stable soils that support resilient, diverse vegetation. These have been displayed graphically, separated by soil type.
Figure 19 displays three years of LCATs scores from treated catchments with Sodosols; duplex soils with highly erodible subsoil. All these sites are within the Scottville area. Some of the catchments had soils that graded into Chromosols; erodible duplex soils that tend to slake but not disperse. Site 13 is one of these where there was dispersive duplex soil at the gully and the lower parts of the catchment and slaking duplex soil in the higher parts of the catchment. Site 13 is the only site with Sodosol soil that was deemed to be‘ mostly stable’( Figure 18). The other five sites represented in Figure 19 achieved a‘ stable” rating( Figure 18).
In Figure 19 there are a broad range of LCAT scores with some sites reaching similar conditions to the well vegetated and stable control sites. Land conditions tended to rise and then fall over the three years. Drivers of reduced land condition were similar across the sites: Dominant pasture species, soil erosion and, less commonly, dominant pasture density. Common pasture species are Sabi grass( Urochloa mosambicensis), Indian couch( Bothriochloa pertusa) and Seca stylo( Stylosanthes scabra). These are the dominant species at Site 8 shown in Figure 20. At several sites Seca stylo increased in density during the three years of monitoring while the sown grass species seemed to decline. This was observed at Site 10( Figure 20). Factors causing this may include a decline in fertility following the initial disturbance and grazing pressure.
The consistency of the land condition scores for the catchments of the four small-scale gullies in Vertosol( Downs country) soils are shown in Figure 21. The four sites are classified as stable in Figure 18. The consistent scores suggest that the interventions had little effect on land condition, and this reflects field observations over the three years. The treatment at all sites involved installing banks to slow runoff and divert it away from gully heads. Therefore, the intervention did not concentrate on pasture renovation. However, the sites with banks designed to hold water changed the vegetation in the vicinity and while they prevented runoff from entering gully heads, the banks did not appear to improve the land
30 GULLY MAINTENANCE AND MONITORING TECHNICAL REPORT