NZSOLD ANCOLD Conference 2025 Christchurch
Urban dams for flood management, water quality treatment, amenity, ecology, and cultural value represent a large proportion of New Zealand’s dam inventory. Many, perhaps most, of these dams require spillway upgrades due to changes in flood standards over time. The recent commencement of the Building (Dam Safety) Regulations 2022, which covers Classifiable dams, will give greater impetus to such projects in the coming years.
A remarkable number of urban dam embankments also function as road or rail embankments, which introduces special considerations for spillway upgrade. By way of example, of the 43 Classifiable dams managed by Auckland Council’s stormwater team, 18 convey road or rail along their embankment crests.
This paper presents considerations for spillway upgrade of dams that are also road or rail embankments, many of which were designed first as road or rail embankments with little consideration of dam design. These dams often have a culvert as a primary spillway, but no auxiliary spillway. Dam safety assessments, or observed overtopping and damage, often highlight that the spillway falls short of modern dam design practice and poses an unacceptable risk. This paper presents examples of how the spillway system can be improved to safely convey the Inflow Design Flood using three case studies of recent upgrade designs.
The preferred approach of an open channel spillway in natural ground is often not practicable for these dams. For Kennedy Road dam, a 3.2 m diameter culvert was installed using trenchless methods. For Paremuka Lakes rail embankment dam, 1.5 m diameter culverts are to be installed inside twin existing, damaged 1.8 m by 1.8 m box culverts. For Rosedale Road dam, a 1.5-1.8 m diameter culvert is to be installed by open trench with staged traffic management. The paper examines the selection of different approaches to suit specific sites. The paper also covers special design and construction features for retrofitting a culvert through a dam that is a road or rail embankment, which relate to mitigating internal erosion risk, maximising hydraulic capacity, unusual aspects of structural analysis, staging to reduce rail and road disruption, and minimising confined space entry.
This paper is intended to be of value to dam owners and practitioners facing similar challenges upgrading spillways for dams that are road or rail embankments. The paper also highlights these challenges as a valuable area for wider industry attention and for advances in practice to mitigate dam safety risk in New Zealand.
