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Technical Memoranda 31
Riley SJ and East TJ
Supervising Scientist, 1990
ISSN 0810-9532
ISBN 0 644 15498 4
The Guidelines to The Code of Practice on the Management of Radioactive Wastes from the Mining and Milling of Radioactive Ores (1982) specify structural lives for rehabilitated tailings structures of the order of 1000 years, ie the design is in a geological time frame and not a secular one. There are large volumes of material involved in the structures at Ranger and the areas of disturbance are extensive e.g. approximately 5 km2 for the final rehabilitation landforms.
The major agent of erosion of the rehabilitation structures will be water, either as concentrated or distributed flow across the surface of the structure or as soilwater and groundwater flow. Estimated peak discharge for the 100 year event from the batter slopes is 0.5 to 5 L/s/(m.width). Peak discharge from the surface of the proposed rehabilitation structure (~ 5 km2) to Magela Creek is calculated to be approximately 70 cumecs. The potential for drainage network development with extensive rilling and gullying is high. Rapid weathering of the chlorite rich rocks, which break down into expansive clay smectite, will significantly alter the infiltration capacity and erodibility of the surface.
It is not possible to rely on the results of monitoring under existing climates as these may not be typical of future climatic regimes. The only cost-effective method of testing long-term stability under different climatic scenarios is by the use of computer modeling of the erosional characteristics of rehabilitation structures.
It is proposed to use simulated rainfall and concentrated surface flow to study the erodibility characteristics of the surface materials and to assess the impact on erosion rates of different surface materials, slope geometries (gradients, shapes and lengths) and ground covers (vegetation and rock material). Artificial rainfall, produced by rainfall simulators, has an advantage over natural rainfall in that experimental conditions concerning rainfall intensity, duration and energy can be controlled. Concentrated flow will be produced by discharging water through a flume siting on the slopes. The experiment will enable existing hydrological and erosion models to be tested and will allow new models to be developed (if there is a need).