Towards quantifying rate of scour using the Erodibility Index Method: case study

This case study presents relationships between total energy input to plunge pools and scour depth at three BC Hydro dams in British Columbia, Canada. Total energy is defined as the product of stream power and time. The relationship indicates the potential to develop a technique quantifying the rate of scour of rock using the Erodibility Index Method (Annandale 1995; 2006). The scour assessment used the Erodibility Index Method to theoretically quantify scour extent and compare it to observed scour. The Erodibility Index is quantified using in-situ rock parameters including UCS, RQD, joint spacing, aperture, alteration, roughness, and orientation. A graph relating the Erodibility Index and threshold stream power (Annandale 1995) is then used to quantify the ability of the rock to resist the stream power of flowing water. The stream power of the flowing water was quantified using daily discharge records and dam spillway geometries for flip-bucket jets. Numerically generated scour profiles were used to quantify total energy at the surface of the plunge pool and at depth over time. The total energy input was then correlated with both the modeled and surveyed plunge pool depth to develop the relationship. The study showed a statistically significant semi-logarithmic relationship between both modeled and surveyed scour depth and total energy input. In both cases, the rate of plunge pool development decreases over time, and continued energy inputs are required to enact changes to depth. The study revealed that correlations between calculated and observed scour profiles improved with the quality of geologic information and the certainty by which jet stream power and its decay could be quantified. The geologic information at one of the sites was incomplete and resulted in poor comparisons between observed and calculated scour. At the other two sites, where geologic information was more complete comparisons were more favorable