DISCUSSION
LIMITATIONS OF THE DATA
This study had a small number of sample sites( 24) in a relatively small region of Queensland and most were in highly erodible soils. In addition, sites were only monitored for three years during relatively moderate climatic conditions. With regards to cost estimates and cost effectiveness calculations in this report, there is some uncertainty in the sediment reduction estimates and construction costs only included direct materials and labour. Cost estimates did not include those associated with site identification, surveying, design, compliance with legislation, project management, contracting and monitoring. Therefore, caution should be exercised when applying any of the conclusions or recommendations in this discussion as every erosion site is unique and methods of estimating sediment loss are still being refined.
LARGE-SCALE SITES
Although there are only five large-scale gully remediation sites some useful analysis is possible because the sites are similar across important variables commonly accepted to drive success or failure. These are:
• Climate— the sites are in the same regional area, and since construction there has been average yearly rainfall totals. However, it is likely that high intensity events have occurred,
• Soil type— all sites have Sodosols, fragile soils with dispersive and slaking subsoils, Site 1 seemed to be more prone to tunnel erosion.
• Implementation— all sites were supervised by professionals, design plans were followed, and maintenance was timely.
• Focus on improving land condition- All sites were fenced, cattle were excluded for two or more growing seasons, and any grazing was for a moderate term( one week to two months).
The discussion of the large-scale sites will address four key questions:
1. How long did it take for the large-scale sites to stabilise?
2. What were the primary causes of reduced success / soil stability at the sites and how can these be remedied?
3. What changes to planning and design are recommended to improve large-scale gully remediation?
4. What maintenance schedules and maintenance costs should be embedded in the remediation plan?
How long did it take for the large-scale sites to stabilise?
In the absence of major weather or climate events and with timely maintenance, an intensive landforming, amphitheatre-like reshaping treatment on gullies in erosive, Sodosol soils( Sites 2, 3 and 4) may stabilise with high pasture cover within four years. Sites with tunnel erosion such as Site 1 are likely to take longer to achieve stability and will rely on continued yearly monitoring and timely maintenance. All sites are vulnerable to damage during extreme events. Although sites were considered stable or mostly stable this is determined from surface appearance and above ground vegetation condition. Monitoring in this project did not investigate how soils were developing towards stability at depth, nor the depth of the roots of the vegetation present. Hence, monitoring and maintenance may be required at all sites for many years, particularly after extreme rainfall events. Landloch Pty Ltd( 2024) advises that, with the impacts of climate variability and grazing, the recovery period for soil and vegetation in incised gullies following large-scale remediation like Sites 1-4 in the BBB is likely to be relatively long( 5-20 years).
A high level of vegetation cover is essential to stabilise highly disturbed, reshaped gullies. As expected for a cropping-like revegetation approach, most of the increases in vegetation cover took place in the first two years following
40 GULLY MAINTENANCE AND MONITORING TECHNICAL REPORT