Damage Criteria and Practical Solutions for Protecting River Channels

The coal industry is seeking to better understand the nature of mining induced subsidence impacts on river systems and practical ways that these impacts can be managed without unnecessarily sterilising valuable coal resources. To further this understanding, the industry, through the Australian Coal Association Research Program (ACARP) has supported Project C12016 "Damage Criteria and Practical Solutions for Protecting River Channels". The project aims to investigate the nature of the disturbances that occur in the base of river channels, the potential impacts of these disturbances on water flow paths as well as the range of practical strategies available for the assessment, mitigation and remediation of these impacts. This report presents the outcomes of Project C12016.

Our research indicates that there are essentially three practical strategies for controlling the impacts of mining induced subsidence on river channel rockbars. One is to provide space within the rock mass in the base of the river channel to accommodate the horizontal movements that occur as a result of mining subsidence so as to minimise rock fracturing. The second is to fill the mining induced fracture network once it forms, either with natural materials such as sand or water (compensation flow) or with man-made materials such as grout or polyurethane resin. The third is not to mine within close proximity of river channels, although the offsets required may be significant depending on the circumstances.

The suitability of remediation techniques depends upon each site's particular circumstances with the base flow in the river channel being one of the most important factors. In watercourses where the base flow is low or intermittent, artificial flow is considered the most practical method for restoring permanent surface flow if required. In river channels where the base flow is greater, remediation strategies using various fill materials have been demonstrated to be practical and are considered likely to be effective in maintaining surface flow in the long term. In channels where the section of river channel affected by subsidence is controlled by a downstream structure not affected by subsidence, there is no need for remediation of river bed fracturing to maintain surface flow because the hydraulic gradient is above the bed level and the fracture network within the base of the river channel remains flooded.

The option to protect river channels by not mining under them sterilises coal resources that have significant value to the community and this value needs to be considered in balancing the costs and benefits. Equally there is a medium to long term environmental cost associated with mining impacts on river channels that needs to be taken into account. There are considered to be practical remediation and mitigation strategies available for river channels where the flow is greater than about 1 ML/day and the value of the coal resource is sufficient to allow these strategies to be implemented.

Strategies for circumstances where subsidence movements cause a flow path to develop from the surface to the mine have not been considered in this study, but have been discussed in detail in ACARP Project Report C13013.