construction phase ( e . g . sediment traps )
▪ Where terrain reforming of vertical and near-vertical surfaces is being undertaken , it may be necessary to establish experimentally what is the appropriate stable slope batter for the given soils
▪ Hardening of key slope components
▪ Hydrological reconfiguration and associated drainage management needs to be undertaken , i . e . providing the means by which the gully network may drain without instigating further erosion . Drainage diversion should only be undertaken where flow can be safely diverted without creating a new erosion problem elsewhere .
▪ Soil import or on-site soil building to provide material to cap unstable subsoils and provide a sustainable growth medium
▪ Revegetation and ongoing maintenance ( The need for ongoing maintenance was strongly emphasised by a number of experienced practitioners at the workshop . It was considered unlikely that a successful treatment could be implemented with a one off treatment .) Identify a source of soil ameliorants , particularly organic material but also gypsum and nitrogenous fertilisers , and plan for their appropriate transport to site and application . The volume of material required would appear to justify the establishment of green-waste collection services in the main population centres .
- Locate and transport to site the necessary engineering consumables ( rock , geofabric , concrete , permeation grouts etc .) and have experienced installers available on site .
• There is an urgent need to undertake a first pass mapping and categorisation exercise to identify the most active gullies and describe the range of material characteristics encountered at these sites . Of particular concern is the proportion of very fine silt and clay ( less than about 16um ), as this is the component that is transported to the most distant reefs , and the degree to which the subsoil undergoes dispersion and or slaking . Sodic subsoils will require expert assessment as structures ( e . g . simple rock chutes ) and approaches used elsewhere ( e . g . battering back to a standard gradient , or applying gypsum to just the first 10 cm of exposed subsoil ) will likely fail due to intense tunnelling .
• Despite the clear need for intervention of the sort described above , extensive applied research is urgently required to determine the precise cost-effectiveness of different combinations of treatment options across the range of gully types in large field trials . This research will need to include evaluation of :
-The best way to manage the risks associated with the first wet season following intervention . Is it better to invest upfront in irrigation and fertiliser application to ensure a cover crop is well established before October , or might it be better to establish a type of storm insurance fund , where a proportion of all project budgets is diverted into a pool that is accessible by other projects in the event of destructive first rains . -The best way to manage / rehabilitate the range of gully-channel connections , as distinct from the gully areas more generally . This may involve numerical and physical modelling of the various options available for reconfiguring this crucial nexus ( e . g . a hardened slot canyon compared with a large open swale that will alter the hydrodynamics of the trunk stream ). -Rules of thumb that relate material characteristics to likely success of treatment options . Can we predict a stable slope angle based on assessment of soil characteristics and optimal selection of types of organic matter for topsoil design , or will a time-intensive ( 1 year plus ) slope angle stability test be required in each and every case . Likewise , can we identify the effective depth of gypsum incorporation based on standard low cost soil tests ? -What is the best way to monitor current and future export rates ? How do the methods likely to be available and applied in a broader program compare to results obtained by more accurate and precise methods based on ground-based
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