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Soil armouring, sediment yield, and acid rock drainage from steep slopes under high intensity rainfall in the West Coast of New Zealand

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan

Citation:  2007 ASAE Annual Meeting  072053.(doi:10.13031/2013.23215)
Authors:   Thomas A Cochrane, Gareth Jack, Paul Weber
Keywords:   erosion, armouring, mining, sediment yield, acid rock drainage

The loss of valuable topsoil due to erosion and the environmental impacts of acid rock drainage are two major concerns in the process of restoring landscapes in open cast coal mines in New Zealand’s rainy West Coast region. To minimize acid drainage, waste rock material is covered with an engineered subsoil capping layer which reduces infiltration and is designed to minimize oxygen exposure of the underlying potentially acid forming waste rock. Topsoil, a valuable commodity at West Coast coal mining sites, is then used as a protective cover for the capping layer and as a medium for vegetation growth. The performance of a crushed granite material, a mixture of fly ash and granite, and silt pond sediments as capping materials to reduce infiltration and oxygen ingress were evaluated in the laboratory. The mixture of fly ash and granite were found to perform the best. Furthermore, a series of laboratory rainfall simulation experiments were conducted to quantify the sediment yield from rainfall events with intensities ranging from 22 to 80 mm/h at slopes of 18 degrees. The combination of high concentrations of gravel and rocks in the topsoil and the high intensity rainfall resulted in a rapid soil armouring process. Fine soil particles were quickly eroded leaving a coarse surface layer of rocks and gravel which offered enhanced erosion protection. Results showed over 75% reduction in erosion between highly armoured soils and freshly applied soil on steep hillslopes. Future modeling of armouring and soil loss at different slopes angles and lengths will help improve the design and management of restored landforms.

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