2000 – Risk and Standards Based Approach to Rip Rap Design Alternative for Grahamstown Dam Stage 2 Augmentation

M. B. Barker and D. Holroyde

A detailed study was completed for the Stage 2 works of the Grahamstown Dam augmentation to investigate various alternatives for the slope protection of the Saddle Dam and Subsidiary Dam embankments, including a standards based and a risk management approach. The standards based approach required an evaluation of the slope protection level and least cost option based on the hazard rating of the dam. Due to the sand construction of the embankments, it was possible to apply a wave erosion model SBEACH to develop an economic risk model for optimising  the slope protection alternatives. The erosion model included the effects of the wind direction, reservoir level and wind speed variation during flood events, embankment profile and material parameters. The risk management approach clearly showed that significant cost savings could be achieved by using the risk management approach. Furthermore, the cost curves  indicated  the sections of the embankments for which present capital works would not be economically justified and for which ongoing maintenance works would be economically advantageous.