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Comprehensive review of plunge pool performance at four of BC Hydro dam sites and assessment of scour extent Monfette, Maryse

Abstract

The plunge pool performance at four of BC Hydro dam sites is reviewed. The sites of Peace Canyon Dam, Seven Mile Dam, Portage Mountain Project, and Revelstoke Dam are described in terms spillway design, plunge pool geology, and historical spillway outflows. The main factors susceptible to affect plunge pool scour are examined and the importance of each is established through a comparative analysis between sites. The spillway layout affects the plunge pool scour development mainly through the design unit discharge and total head drop for a given outflow and reservoir level. The plunge pool scour configuration is affected by the prevailing geological conditions. The magnitude of spillway flows appears predominant over the frequency and duration of spills. Observations support the concept of equilibrium conditions. A total of 14 sets of scour data were assembled from the review of plunge pool performance at the four sites of study. This information was used to evaluate the conventional methods of scour assessment such as hydraulic model studies and empirical equations in comparison with a new approach called the Erodibility Index Method. Ten well-known empirical formulas were tested. The Erodibility Index Method was applied methodically to the four sites of study and a sensitivity analysis was performed. The results of downstream scour tests from small-scale model studies were comparable to prototype observations in one of the four study cases. The empirical equations were seen to provide on average conservative values for design, but the great variability in results for a single data set and the inconsistency of a given formula from one site to another are problematic. The expression by Damle (1966) was seen to give the best combination of precision and accuracy with a standard error of estimate of 16 ft. The performance of the Erodibility Index Method in the assessment of maximum scour depth was disappointing and did not outclass the conventional methods. The Erodibility Index Method was characterized by a tendency for underestimation and a standard error of estimate of 53 ft. The main weakness of the approach is that the vertical distribution of power available for scour in the plunge pool is essentially related to the submerged jet velocity profile which do not reflect the changing magnitude of spillway discharges.

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