Floods are among the most frequent and damaging natural hazards. The ramifications of flooding extend beyond the immediate physical danger to include damage to homes, infrastructure, agriculture, ecosystems, and public health that can take communities years to recover from. Flood frequency and magnitude will likely worsen in the future due to climate change and urbanisation. Recognising flood risks is increasingly crucial as populations exacerbate the risk by moving to concentrate in vulnerable areas. Understanding and addressing this flood risk is vital for developing effective mitigation strategies, resilient infrastructure, and safeguarding communities from adverse consequences.

Flood protection systems play a pivotal role in reducing flood risk by shielding communities from the impacts of floods. These systems employ a range of strategies to manage and control water flow. Among these are defensive mechanisms, which include dams and levees. Levees increase the conveyance capacity of rivers to prevent inundation, ostensibly protecting 10% of the global population (ICOLD, 2022). Dams, in contrast, increase the storage capacity, allowing controlled releases and regulated outflows to mitigate the downstream flood risk. Both dams and levees are integral components of flood defence infrastructure, protecting urban and rural communities alike. However, these systems face increasing risk due to ageing infrastructure, urbanisation, the increasing complexity of management plans and stakeholder relations, and climate change. Understanding these systems and their inherent risk becomes critical for catchment management and selecting adaptation options.

This thesis explores the effects of levee breaching and dam operation on flooding and the potential consequences of their interaction. This is done by simulating floods in computational models of four New Zealand dam-levee catchments. The research proposes novel methods for simulating levee breaching and representing dam management in models so impacts can be captured in future risk assessments. Combining these approaches, the research explores cascading consequences. The thesis advances flood risk assessments by acknowledging the interconnectedness of levees and dams. In addressing this gap, these systems may be improved to aid in addressing wider flood risks.

If not omitted entirely, flood maps in practice that include levee breaches are often deterministic (van Kalken et al., 2007; Vorogushyn et al., 2010; Wild, 2017). The research developed a method for simulating levee breaches to determine the probability of inundation, given that a breach occurs. Unlike the current probabilistic methods, the method does not rely on computationally intensive Monte Carlo simulations and detailed geotechnical information. Empirical equations were used to set breach development parameters and final dimensions. The method ran one breach per simulation, initiated where the depth exceeded 50% of the levee height. Results from a historical flood in one catchment found that including breaching increased the average inundation by 48%. Including breaching in another catchment reduced the inundation by up to 12% as flows were contained between a levee and floodway. This highlights the possible use of floodways and fuse plug levees to cause preferential flooding. Given that the breaching predominantly flooded cropland and pastures, it may be prudent to develop these before the land is developed and becomes more expensive to purchase and repurpose.

Managing dams is critical in not exacerbating flood risk. However, quantitative research on the importance of management in dam operation during floods is still developing. This thesis developed a method for representing stakeholder communication and understanding roles & responsibilities in a computational flood model by varying parameters such as the initial level, release timing, and maximum gate opening. These were simulated at various levels in a range of rainfall patterns and return periods, enabling a more nuanced perspective than previous studies. The hours of gate pre-release were more correlated with outflows in dams designed to provide flood control, as these had larger mechanisms to release water, allowing the flood volume to be evenly distributed throughout the event. However, these mechanisms can cause outflows greater than dams’ inflow without appropriate management. A different catchment showed that legislative regulations can remove sources of misoperation, enabling more consistent outflows, though these need to be created collaboratively to set realistic expectations. The volume five-days before the flood peak was more correlated with outflows in dams with large storages, as these relied on storing water to reduce outflows. However, the outflow reductions in these dams decrease in large events as they act as run-of-river schemes once full. Flow control mechanisms may make these dams more effective in large events and provide a potential climate change adaptation option.

Cascading consequences are the chain reaction of events where the consequences of each event amplify or propagate the consequences. Understanding these cascading effects is crucial for comprehensive risk assessment. This thesis investigates the cascading consequences of dam operation and levee breaching. This was achieved by pairing the maximum, median, and minimum dam operation flows with the levee breaching method to create cascading consequence flood maps. Using the maps with land cover and building data, the consequences were calculated in terms of monetary building and land losses. In all four case study locations, at least 20% of the levee length reached the breaching threshold under median dam flows, and two locations reached at least 55%. Dam operations significantly impacted this potential breach length, with the average difference between the minimum and maximum dam flow cases being 25% of the total levee length. Including breaching tripled the potentially exposed area in half the case study catchments. The affected land was predominantly cropland and paddocks. However, this can be converted to higher-value orchards, vineyards, and built environments. Because of this, the potential losses will likely increase under climate change without additional measures to manage flooding.

When the full range of scenarios is considered, there is at least a 950% increase in losses between minimum flow dam operation without breaching and maximum flow dam operation with breaching (i.e. best and worst-case scenarios). This range of cascading consequences is often unconsidered in modelling practices. By emphasising the risks, this research contributes to addressing the levee paradox, advocating against building developments in areas presumed to be flood-safe due to levees’ perceived absolute protection. This approach promotes greater resilience in flood-prone areas.

As global flood losses increase, understanding our flood protection systems is vital for making informed adaptation decisions. As such, acknowledging risks like levee breaching, dam operation, and their cascading consequences is crucial. This thesis explores these topics, finding they significantly impact inundation and the resultant losses. Implementing this research in practice could transform the understanding of flood risk as by improving our knowledge of these systems, aiding in addressing broader flood risks and improving our communities’ resilience.