CULVERT, THE CHOKING POINT OF NETWORK RESILIENCE

B. Kinnear (Woods), T. Wang (Woods)

ABSTRACT

The current population of New Zealand is around 5 million, with approximately 86% inhabiting cities. With this urbanisation, the hydrological cycle is increasingly impacted. Urbanisation impacts the nature of catchments by altering land cover and other hydrology conditions which affects the flow characteristics and conveyance in catchments. Further to this, the climate is also changing. As stated in the Climate Change Projections for New Zealand, moderately extreme rainfall is likely to increase in most areas, and very extreme rainfall is likely to increase in all areas, with increases more pronounced for shorter duration events (NIWA, IPCC 5th Assessment, 2nd edition). In the year 2023 alone, serve events brought with them record rainfalls and slips, which caused extensive damage to the infrastructure and loss of life.

This paper reviews culvert modelling, which is an essential component of drainage systems, allowing flows to be conveyed underneath roads and relieving roadside channels. They are crucial in managing runoff build-up during extreme events and require robust design considerations. Even under normal operational conditions, culverts are occasionally blocked by debris, sediment, and other obstructions, significantly affecting their hydraulic performance.

Traditionally, tools such as nominal charts or software like HEC-RAS 1D and HY8, have been used to design and analyse culverts in one-dimensional (1D) flow scenarios. The limitations of 1D assessments are well recognised in that the approach lacks the ability to capture intricacies of complex flows and is limited to standard culvert shapes. Additionally, it does not provide a comprehensive visual representation of a wide range of flow scenarios. To gain a holistic understanding of culvert performance and flood hazards on individuals, vehicles, and structures, 2D assessments are better suited.

By modelling culverts in 2D, it is possible to gain a better understanding of how flow behaves around them, including backwater effects and energy losses. This understanding becomes particularly crucial when designing culverts to ensure their capability to handle expected flow conditions. 2D hydrodynamic models enable the design of culverts with non-standard shapes, which can be advantageous in situations where the traditional box or circular culverts are not feasible or efficient. These models provide a graphical representation of the flood pattern and, more importantly, assess flood risk. The results obtained from the model can be utilised for detailed hazard assessments, supporting master planning and decision-making. Additionally, they play a crucial role in modelling climate change and resilience in design structures. This information is valuable for comprehending the resilience of stormwater systems and developing appropriate mitigation measures.

This paper further discusses how using a 2D hydrodynamic model could assist engineering design and decision-making and assess flood hazards and risks focused on the Auckland region (using various tools such as Auckland Council Stormwater Flood Modelling Specification and Australian Rainfall Runoff Guidelines). Case studies in Auckland urban areas are used as examples to illustrate these benefits of 2D hydrodynamic modelling and how the model results can be used to facilitate engineering design practices to adapt to the changing climate and build more resilient communities.