Erosion Control Treatment Trials on Loess Soils

During earthworks on Canterbury’s Port Hills, highly erodible loess sub-soil is exposed to potential erosion. Stormwater discharged from such sites can contain large quantities of fine sediments that stay in suspension and are challenging for treatment systems to remove. The ecological impact of these fine suspended sediments on downstream receiving environments, including the Cashmere Stream, Heathcote River and Avon Heathcote Estuary, is significant. Utilising effective methods to minimise erosion of exposed soils is key to reducing the amount of loess reaching such sensitive receiving environments.

An experimental field study to test the effectiveness of erosion control treatments was commissioned by the Cashmere Working Group of the Christchurch–West Melton Zone Committee and undertaken by EOS Ecology. Five erosion control treatments applied over a loess sub-soil were tested against an exposed loess sub-soil control during multiple controlled one hour rainfall simulations. The study showed that such erosion control treatments were effective in reducing soil loss, but that proper application of the products was critical to their effectiveness. Even at the higher rates of erosion control, suspended sediment in runoff still exceeded most local consent-based limits, reiterating the importance of construction sites needing to use a treatment train solution of erosion control and sediment control measures.

The findings of the study helped inform an update of the Environment Canterbury Erosion and Sediment Control Guidelines and will be useful in developing innovative construction-phase discharge consent conditions, which are flexible enough to be applied during site development while achieving objectives, policies and water quality outcomes set in the Canterbury Land and Water Regional Plan. A collaborative approach to investigating the effectiveness of erosion control measures through field studies in a specific highly-erosive area has been extremely helpful in demonstrating what is required to limit sediment loss from challenging hillside development areas.