Slope stability analysis of potential bank failure as a result of toe erosion on weir-impounded lakes: an example from the Nepean River, New South Wales, Australia

Abstract

The consequences of weirs present on the Upper Nepean River on the long-term slope stability of both vegetated and devegetated riverbanks were investigated using models that account for the reinforcement of bank sediments by tree roots. The effects of the weirs in concert with channel widening and deepening caused by dredging in 1970s and 1980s, as well as natural processes, have turned the Upper Nepean from a small upland river into a series of quiet, narrow lakes, measuring 3–5 m deep, 30–70 m wide and several kilometres long. The surface of these lakes is located currently within the steep mid-bank zone. Wind-generated waves have eroded 1–3-m high scarps in the mid-bank region. These scarps are receding laterally at an average rate of 10 cm per year and this process is gradually undermining and destabilising the upper banks. In contrast, the mass of water impounded by the weirs currently acts to provide lateral support to the banks and improves their stability. Therefore, the existence of the weirs and their impounded lakes has currently both positive and negative effects on bank stability. The retention of the weirs will promote continued erosion at the waterline of the weir lakes that will eventually lead to the destabilisation and collapse of both vegetated and devegetated banks during future large floods. Demolition of the weirs would also lead to a renewed phase of bank failure during future floods as the stabilising effects of the weir lakes on the banks would be removed. The size of eventual failures will be larger and the distribution of such failures probably more widespread if the weirs are retained.