Tanjannerup Dam is an 11 metre high zoned earthfill embankment dam located near the township of Nannup in the south-west of Western Australia. Constructed in 1961, the dam impounds a storage of 156 ML and is used for public water supply.
Originally, the spillway was constructed with a “peaked” crest and tapered chute on a straight line alignment on the left abutment. A simple stilling basin structure consisting of baffle walls and boulders embedded in concrete was also constructed at the downstream end. This failed on the first spillway flow and was reconstructed.
In 1994, the Full Supply Level was raised by 1 metre by construction of a labyrinth weir within the crest area. A portion of the chute was also replaced to a contraction joint located 5.1 metres downstream of the dam centreline. At the same time, the embankment crest was raised by 0.36 metres by the construction of a precast concrete wave-wall and accompanying backfill across the entire length of the embankment.
Dam safety inspections over a period of many years highlighted problems with the spillway chute and identified deterioration and displacement along many of the joints between adjacent concrete slabs. Inspections of the chute slabs found that they were not well constructed, consisting of 75 mm thick slabs with mesh reinforcement, which, as often as not, was found to be on the ground under the concrete. In addition, the discharge capacity was insufficient for the “significant” hazard rating for the dam.
Accordingly, during the period February to May, 2016, the Water Corporation replaced the older (1961) section of spillway with a new structure capable of passing a 10-4 AEP outflow flood. The chute tapers from 9.2 metres near the crest to a 5 metre wide straight section with a USBR Type III stilling basin situated at the downstream end.
Author: John Waters, Water Corporation