2018 – Localised Repairs of Spillway Concrete Scour at the Base of a 42 m High Arch Dam

Oliver Giudici

A common concern for large spillways is erosion/abrasion of the receiving plunge pool and potential impacts on the stability of the dam. An example of this was presented at the 2017 ANCOLD Conference in a paper that discussed the detection and repair of spillway scour erosion at the base of Devils Gate Dam, an 84 m high, double curvature arch concrete dam. The focus of this paper is the partial repair of scour and abrasion within another concrete lined plunge pool, at the base of Repulse Dam in Southern Tasmania.

Repulse Dam consists of a 42 m high double curvature concrete arch with post-tensioned abutments and an adjoining earth embankment with a reinforced concrete upstream face. The stepped dam crest acts as a free-overflow spillway which discharges onto a concrete apron designed to protect the valley sides and floor immediately downstream of the dam. The permanent tailwater rises part-way up the dam during high flows which lessens the impact on the apron.
Previous underwater inspections had not identified a pressing need for maintenance. However, an upcoming twelve month Repulse Power Station outage would generate constant spill and therefore a more thorough assessment of the spillway apron was undertaken. Inspection was limited to underwater methods due to the inability to lower the tailwater; the downstream lake forming the tailwater is solely regulated by a hydro-power station and this station was being refurbished at the time. Sonar scanning enabled the spillway apron condition to be mapped and revealed areas of exposed reinforcing steel and deposits of river rock and gravel. The information provided by the scan justified temporary disruption to the lakes and power stations which form the Lower Derwent Power Development in order to dewater the area and work safely below the spillway. This was necessary to expose the apron for detailed inspection in dry conditions and thereby make a full assessment of the need for concrete repairs prior to the station refurbishment.

This paper presents a case study of the actual performance of a spillway apron below an arch dam and the inherent challenges in accessing and maintaining these types of structures when a permanent tailwater is present.