Rehabilitation Performance Metrics (via remote sensing)
For many years mine rehabilitation practitioners have argued the need to address issues with respect to rehabilitation monitoring and performance. One of the common arguments is that ground based monitoring methods used to evaluate the performance of rehabilitation are limited in their ability to effectively measure attributes at an appropriate range of spatial and temporal scales. A further argument has been to include more rehabilitation monitoring at the landscape scale, rather than at the species scale.
Better data for better outcomes
In order to help address this issue, Astron has developed Rehabilitation Performance Metrics (RPM) that can be assessed at the landscape scale using remotely sensed data, including high precision data collected from our own Bramor rTK UAS, data from a company owned UAV or data from a manned aerial or satellite pass. UAV data is particularly useful due to the ultra-high resolution imagery from which we can derive very accurate measurements.
High resolution remote sensing offers rehabilitation practitioners the potential to measure rehabilitation performance effectively over large spatial scales that are typical to many mine rehabilitation projects.
Being solutions driven, our strength is in collecting high quality data and turning it into actionable information, allowing rehabilitation practitioners to make informed and timely decisions.
Compliance driven performance metrics
The Department of Mines and Petroleum (DMP) have identified that the key closure objectives for rehabilitated mines to be:
- physically safe for humans and animals
- geotechnically stable
- geochemically non-polluting/non-contaminating
- capable of sustaining an agreed post-mining land use.
To this end, we’ve focused on developing rehabilitation metrics that can be assessed at the landscape scale using remote sensed data in the key areas of landform geometry, landform stability and vegetation.
Monitoring and assessment of these rehabilitation metrics at the landscape scale ensures that the performance of a whole landform is understood and the need for management activities can be identified and directed to strategic areas if required. Remote sensed data can form the basis for tracking of rehabilitation performance against completion criteria over the life of a landform and can be utilised to identify high risk areas early in the landform construction process when remedial actions can be completed at the lowest cost.
Remote sensed data can be used to monitor and assess any landform geometry metrics that are of interest and can be tailored to site specific requirements.
Monitoring of landform geometry metrics throughout the landform construction and primary rehabilitation earthworks stages of the rehabilitation process allows you to:
- assess compliance with approved landform designs
- track progress against mine plans and schedules
- provide updated data for entry into the annual Mine Rehabilitation Fund calculation
- identify areas at higher risk of erosion before they become a problem.
Landform geometry metrics that are useful to most mine sites and are commonly linked to approval conditions or completion criteria include:
- landform height and footprint
- batter height and angle
- berm width and angle
- crest bund height and width.
Rehabilitation Performance Metrics can also be used to assess whether abandonment bunds comply with the Department of Industry and Resources (now Department of Mines and Petroleum) Safety Bund Walls Around Abandoned Open Pit Mines Guideline (DoIR 1997) with respect to bund height and width.
Example metrics for batter angle and abandonment bund height are shown below.
Batter Angle: Batter angle is the angle in degrees of the slope that runs from the toe to the crest of a batter. The batter angle metric can be used after the completion of primary earthworks to identify areas where the constructed landform is out of compliance with the landform design. This helps detect areas at higher risk of erosion before they become a problem.
Abandonment Bund Height: Abandonment bund height is the vertical distance between ground level and the top of the bund. Assessment of abandonment bund height allows for the assessment of compliance with regulatory guidelines and calculation of the volume of waste rock required to make incomplete bunds compliant.
The monitoring and assessment of landform stability metrics can be tailored to site specific risks and completion criteria, particularly where quantitative targets have been set. Monitoring and assessment of erosion has two key purposes:
- to guide management actions (if required)
- to show progress towards achieving landform stability.
During the early stages of rehabilitation, monitoring of gully location, spacing, depth and width can be used to determine if and where remedial activities are required. As the whole landform can be assessed any problem areas can be identified and understood. Repeat imagery captures result in a time series of landform data which allows:
- comprehensive calculation of total landform change from which areas and volumes of erosion and deposition can be identified and visualised
- determination of whether gullies are still active or have stabilised
- identification of areas where loss of soil material is likely to be impacting on vegetation establishment.
Rehabilitation monitoring can also assess actual performance against modelled performance, such as SIBERIA or WEPP modelling commonly undertaken as part of landform design.
Example metrics for gully depth and gully volume are shown below.
Gully Depth: Gully depth is the vertical depth from the ground surface of an erosion feature. During early stages of rehabilitation, monitoring gully depth is useful to determine whether remedial actions are required to prevent erosion making a landform unstable. Repeat monitoring can determine whether gullies are still active or have stabilised.
Gully Volume: Gully volume is the calculated volume of material lost due to gully formation (expressed per unit area). Gully volume identifies areas of landform instability and where loss of soil material is likely to be impacting on vegetation and habitat establishment.
For most mine sites, establishment of self-sustaining native vegetation is a key objective for rehabilitation. Completion criteria are often set based on level of vegetation cover (or density) and species richness in comparison with an appropriate analogue vegetation community. Vegetation metrics can also be used as an indicator of fauna habitat.
Assessment of vegetation cover over a whole landform identifies any areas of poor vegetation establishment that may require remedial activities. As assessment is conducted at the landscape scale, the level of patchiness and heterogeneity in vegetation cover in the rehabilitation can be checked against that occurring naturally in the analogue vegetation to determine if areas of low vegetation cover are significant. Repeat imagery capture allows tracking of rehabilitation performance and assessment against completion criteria.
Vegetation strata can be used as an indicator of fauna habitat. Assessment of vegetation height can be used to determine if all vegetation strata are returning and occur in similar percentages to an analogue vegetation community.
Assessment of vegetation health can be used to identify areas where vegetation stress is occurring which may require remedial activity. Temporal analysis and comparison with analogue vegetation can determine whether rehabilitation is responding to seasonal and environmental changes in a similar way to the surrounding environment.
Examples of metrics for vegetation height and vegetation health are shown below.
Vegetation Height: Vegetation height is a measure of the vertical height of the canopy. Vegetation height can be used to classify vegetation into strata groups for comparison against analogue sites and in determining the heterogeneity of vegetation structure. With repeat measures the change in vegetation height can be used to as an indicator of vegetation dynamics.
Vegetation Health: Vegetation health is measured using the Normalised Difference Vegetation Index (NDVI) which provides a relative metric of vegetation condition. This metric can be used to identify areas of vegetation stress that may be caused by acidic and/or metalliferous drainage or soils with poor physical or chemical properties.
We are here to help
The above examples are just a few of the Rehabilitation Performance Metrics that we have developed and our remote sensing and rehabilitation teams are continually adding to the list. Whether you are looking for a complete rehabilitation monitoring approach or a solution to a specific rehabilitation related question, the dedicated team at Astron are here to help.