An array of 1:50 scaled groundsill models was installed in a semi-circulating flume to study scour mechanics and effectiveness of riprap protection lengths and riprap sizes. Results of this study show that riprap protection
at the toe of a groundsill is unable to completely eliminate the local scour, but it is capable of reducing scour depth and scour length. A dimensionless relationship was also derived in this study to help select riprap protection lengths and riprap sizes. The maximum percent of scour depth reduction can be reduced from 83% to 76% with respect to the increase of riprap size to channel sediment size ratio when riprap protection covers 1/4 of the groundsill interval. Percent
of reduction remains fairly constant when riprap protection covers 1/2 of the groundsill interval. On the other hand, the maximum percent of scour length reduction increases from 64% to 84% with respect to the increase of riprap size to channel sediment size ratio.
Key Words : Riprap protection, local scour, riprap length, riprap size, groundsills.