CASHMERE VALLEY, APPLYING DAM PRINCIPLES TO STORMWATER

S. Woods, M. Burrows & P. O’Bery (Stantec), M. Jackson (Christchurch City Council)

ABSTRACT

Following the major earthquakes in Canterbury in 2010 and 2011, flooding of properties along the Ōpawaho-Heathcote River significantly worsened due to a combination of effects such as settlement of buildings and deposition of silt in the river. In order to respond to this change in the environment and build resilience in the stormwater system against future climate change, Christchurch City Council (CCC), amongst other flood mitigation projects, has constructed a network of significant flood detention basins and naturalised wetlands.

The construction of larger flood detention basins in urban areas is likely to be part of the flood mitigation strategies in many parts of New Zealand in the future. New Zealand is in the process of adopting new building regulations that relate to dam safety, and will come into force in May 2024. New regulations fundamentally change what is classified as a dam, which will require dam engineering principles to be adopted for many more flood detention basins in the future.

Many embankments constructed to retain or detain stormwater have historically been designed using empirical methods and engineering judgement. This paper highlights CCC’s newest flood storage basin, the Cashmere Valley Dam, and presents key considerations required to apply dam engineering principles to flood detention dams, in particular:

• Understanding the potential consequences of a dam failure so that a Potential Impact Classification of low, medium or high can be assigned.
• Flood modelling required to complete the consequence assessment, particularly in a highly populated urban environment.
• Using different soils through the embankment cross section to achieve seepage control and stability.
• Detailing around pipe penetrations through earth embankments to control seepage.
• Spillways designed and constructed to operate safely under significant flows.
• Construction monitoring required of engineers – where to look and what to focus on.
• The size of floods that dams are designed to safely pass, which are generally much larger than the floods considered for stormwater design.
• The obligations of the Cashmere Valley Dam owner, planning for an emergency and what documentation is required.

Construction of dams requires engineering monitoring that is focused on dam engineering principles and is significantly more rigorous than what may be considered typical for earth and civil structures. The Cashmere Valley Dam monitoring and surveillance began from the ground up including foundations, fills, earthworks construction methods, and testing. Monitoring of the construction of the civil structures (reinforced concrete inlet, outlet, and pipes) required specific attention to quality and durability of concrete for flood water conveyance, and consideration of seepage paths through structures and adjacent fills.

The Cashmere Valley Dam stakeholders are diverse and considerable effort was placed on the multiple use aspect of the dam and reservoir. This paper highlights how ‘softer’ engineering solutions can be applied to dams in urban setting so that they integrate sympathetically with the surrounding environment and the ability to use the reservoir formed behind flood detention dams for recreation and environmental enhancement purposes.