2000 – Grahamstown Dam Stage 2 Augmentation Selection and Design of a Labyrinth Spillway and Baffle Chute

M. B. Barker, R.M. Holroyde, J Williams and T. Qiu

Grahamstown Dam is a major water supply source for the Newcastle area and it is proposed to raise the full supply level by 2.4m from RL 10.4m to RL 12.8m. The present spillway is inadequate to pass the PMF without overtopping of the existing embankments at the new FSL and part of the raising comprises construction of a new embankment of about 10m high with a right bank spillway upstream of the existing spillway capable of passing the PMF. The Pacific Highway is located some 600m downstream of the new spillway and a 60m wide culvert below the Pacific Highway is being constructed with capacity sufficient to pass the PMF. Significant changes were made to the feasibility design for the spillway and the Pacific Highway culvert using a labyrinth spillway and a baffle chute energy dissipator respectively. Both of these designs are uncommon and the process of finalising the designs as well as some of the problems in the use of a labyrinth spillway and the cost savings realised in the use of these designs are presented.