The Application of the RMEI Scour Assessment for a Heterogeneous Rockmass in an Unlined Spillway Channel

A scour assessment using the Rock Mass Erodibility Index (RMEI) was carried out for the Spillway Return Channel of the Fred Haigh Dam, Queensland. The dam spillway comprises an ogee crest with a 70 m concrete lined chute and a flipbucket, beyond which the unlined Spillway Return Channel discharges into the Kolan River 350 m downstream. Deep scouring (>30 m in places) has occurred in the Return Channel since its construction in 1974, with the most severe events, due to dam spilling, between 2009 and 2017. The complex geology of the site comprises rocks of the Good Night beds and Mount Whacogo Volcanics with several intrusive subvertical dykes and faults. Significant geological mapping, boreholes with televiewers and Sirovision discontinuity mapping was used for geomechanical assessments. This paper discusses the methodology used & the challenges faced to determine the rockmass characterization and parameters selection for a heterogeneous rockmass as inputs into the RMEI assessment, being validated by an alternative eGSI assessment. The rockmass was subdivided in 4 domains with similar geology and geomechanical characteristics. Discontinuities were sorted by domains and statistically analyzed to derive each of the required parameters of the RMEI. Stream Power values were obtained from CFD modelling for a range of flow scenarios. The obtained values were compared and back analyzed against the historic scour behavior of the channel.

Keywords: Spillway, Scour Assessment, RMEI, Heterogeneous Rockmass