Effectiveness of Engineered Log Jams in Reducing Streambank Erosion to the Great Barrier Reef: The O'Connell River, Queensland, Australia

摘要

The delivery of fine-grained sediment and nutrients to the Great Barrier Reef (GBR) is posing a threat to the sustainability of the reef's ecosystem. Streambank erosion can be an important source of sediment in disturbed channel systems. Reducing sediment loads from banks can be accomplished using a variety of mitigation measures that vary in cost and effectiveness. The Bank-Stability and Toe Erosion Model (BSTEM), a simple spreadsheet tool to simulate the hydraulic and geotechnical processes operating on streambanks in a completely mechanistic framework was used. The O'Connell River has been identified as producing the greatest amount of sediment per unit drainage area to the GBR. Streambank loadings from an actively eroding 4 m-high meander bend were simulated over three successive bankfull events. Simulations were conducted for: (1) existing bank conditions, (2) existing bank geometry between installed engineered log jams (ELJs), and (3) existing bank geometry at installed (ELJs). The development of root networks and associated root reinforcement with time was accounted for based on established relations. Results show that that for existing conditions the 270 m-long meander bend produced about 4220 t of sediment, or 15.6 t/m, for an average of 5.21 t/m/y from two large mass failures. Simulations over the same period between and at the ELJs showed a single upper-bank failure, order of magnitude reductions in hydraulic erosion and load reductions of 56% and 74% respectively. Using an estimate that 5% of the 600 km of streambanks along the O'Connell River have recently failed, bank erosion estimated by the SedNet model may be underestimated by an order of magnitude for both existing and "natural" conditions.