2021 – Ewen Maddock Dam Upgrade – Draining down the risk (with siphons)

Anna Hams, Lindsay Millard, Elizabeth Jackson, Zara Bostock, Helena Sutherland

The Queensland dam regulator requires that dam safety risk during construction must not increase from its existing profile. The Stage 2A upgrade of Ewen Maddock Dam required excavation of its homogeneous embankment to retrofit chimney and filter blankets, and also the construction of a concrete parapet wall. Due to the constraints of the embankment profile and a constricted site, it was necessary to excavate the downstream face of the embankment. This excavation increased the risk of embankment failure due to overtopping, piping and instability. This paper discusses the measures taken to manage those dam safety risks, and includes:

● use of a temporary system consisting of six large siphons to regulate the lake level to a Restricted Full Supply Level (Restricted FSL). This encompassed the optimisation of lake level and capacity of siphons required to balance competing risks; dam safety, environmental, community and water security. This optimisation was based on a probabilistic assessment of hydrological inflows and lake levels, the development of a flow management plan;

● implementation of a Dam Safety Management Plan which outlined the roles and responsibilities for
managing dam safety during construction at each pre-determined lake level trigger levels. This includes how the contractor was involved to ensure quick response from the “eyes and ears on the ground”; and,

● development of recommended construction methodologies including a “rolling front” and placing
filters vertically to increase production, maintain quality and limit the extent of embankment excavation underway.