| Summary: | Hydraulic models, using either cohesive or non-cohesive beds, have traditionally been
employed to gauge scour below a spillway. Empirical equations, based on world-wide prototype
and model experience, have been developed for estimating scour depth depth. In this study both
the hydraulic modelling and analytical approach were used to assess the maximum potential
scour in the rock lined channel below Ruskin Dam.
Ruskin Dam, built in 1928, has an overflow spillway with seven bays discharging into a
rock-lined channel. The spillway has a complex arrangement of concrete benches lining the
sides and a large concrete bridge pier in directly downstream of the spillway. A non-cohesive
bed, hydraulic model was employed to understand how the degree of scour is affected by the
presence of the bridge pier and redirection of the spillway discharge by the concrete benches, and
to locate areas of maximum scour intensity. Five scour equations were used to determine
theoretical scour depths in both the prototype and model. Data on scour depths gathered from
the model study was compared to the theoretical values calculated from the equations.
From the model study it was shown that if the bridge pier was removed from the spillway
channel the point of maximum scour moves downstream and the intensity is reduced. The
location of the point of maximum scour was also influenced by gate openings. High scour
intensities were recorded for gate openings where flow was deflected to one side of the channel
and localized by the pier armature.
The results of the scour depth comparisons using theoretical and measured model depths
showed that 2 of the 5 equations reasonably represented the model scour. This result was
generalized to the scour calculations using prototype data and an estimate of scour depth was
made for the rock lined spillway channel below Ruskin Dam.
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