quality concentrations at the treatment site were not statistically different to prior treatment . Therefore , using the statistical approach to estimate effectiveness , then in 2020-21 , the value was zero . Over the two years combined , this equates to ~ 0.5 or ~ 50 % effectiveness for this site .
● Although the hillslope diversion banks appeared to have reduced the concentrations and amount of runoff compared to the control in 2020-21 , it is likely that much of the sediment is coming from the gully channel walls as the gully starts to widen ( rather than lengthen ). Further years of monitoring at this site will continue to provide useful and important insights for this type of gully management over time .
● The Mt Pleasant site implemented a landscape rehydration project with a concurrent aim to reduce the erosive force of runoff within the treatment gully . This project appears to have had some success ; however , the changes between the treatment and control site are small as the property was already in relatively good condition compared to other sites . When properties have good vegetation metrics and relatively mature and inactive gully / channel systems , rehabilitation is not worth doing for water quality benefits alone . There is negligible improvement in water quality and sediment savings relative to other more active gully sites , however , there may be other benefits including increased ecological habitat and increased soil moisture .
● Interestingly , the Mt Pleasant treatment site had the lowest sediment concentrations , but highest sediment loads of any site in 2020-21 . This is because it has a large catchment area relative to other sites in this study , highlighting that the volume of water or discharge is a key driver of sediment delivery in these systems .
● The larger scale rehabilitation works at Mt Wickham have resulted in statistically improved vegetation metrics on the hillslope and within the gully after just two wet seasons . These changes improved further after a third wet season . There are also statistically significant improvements in TSS concentrations following the major earth works at the treatment site . Sediment loads have also declined following treatment , relative to the un-treated control . Given the site has statistically improved water quality and sediment yields following treatment , the effectiveness value was calculated as ~ 0.85 for this treatment after two years , and 0.91 after three years .
● The larger scale rehabilitation works at Glen Bowen have shown early signs of success with significantly improved cover and biomass on the treatment site . Visually , the water leaving the treatment site is relatively clear and considerably less turbid than the control site which reached sediment concentrations as high as 68,000 mg / L . The vegetation will need to be carefully managed at the treatment site due to the low rainfall during the establishment phase .
● High gully head-cut erosion rates can be rapidly ‘ switched off ’ using hillslope diversion banks that effectively starve the gully of runoff and reduce soil loss , but their effectiveness may be limited in above average rain seasons or during extreme weather events . These approaches may also transfer the primary source of erosion from the headcut to the gully walls . Therefore , such approaches should be combined with vegetation stabilisation within the channel .
● Alluvial gullies can be rehabilitated using re-shaping and re-vegetation in relatively short time frames (~ 12-24 month ) given sufficient rainfall . Engineering treatments ( e . g ., earth works and rock chutes etc .) are likely to be important for stabilising the vegetation at these sites in the long term . These alluvial gullies have high sediment concentrations , and very high rehabilitation costs . Ongoing maintenance costs for these engineering works may also be significant .
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