Role of existing industrial land in providing supplemental flood mitigation for low-lying coastal cities in a context of climate change : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Flooding in low-lying coastal cities is projected to worsen with climate change. Planners are concerned about the inadequacy of their current flood protection capacity and the risks associated with under- or over-investment in expensive grey infrastructures under climate change uncertainty. Implementing green stormwater infrastructure (GSI) on strategically located developed land, particularly industrial land, maybe a lower-risk alternative; however, little is known about its potential for providing substantial catchment flood mitigation over time. In addition, there is no effective methodology for determining its potential in the context of climate change. In this research, a novel Hydrology-based Land Capability Assessment and Classification (HLCA+C) methodology is proposed and used to evaluate the flood mitigation capability (FMC) of strategic land use, having large properties over the long term (80 to 100-year period) with climate change. The methodology was then applied to a case study (the City of Christchurch) to determine the potential of industrial land for providing substantial flood mitigation. The results indicated industrial land has substantial flood mitigation capability in four of its six catchments, capturing both on-site and off-site runoff volumes. While their increased runoff volumes associated with climate change in two catchments can be reduced to a manageable level within the mid-term period (2031-2050), industrial land in the other two catchments had this capability up to the long-term period (2081-2100) and under much larger storm events, though not for all climate change scenarios. The methodology was also applied to one Christchurch catchment to evaluate the FMC of individual industrial properties further. The highly capable properties within the catchment were prioritised to be included in adaptative flood mitigation pathways. Considering the most appropriate implementation approach associated with properties’ FMC and timing for implementation (whether through a retrofit only, retrofit and transfer, redesign or relocation), their flood mitigation capabilities can be maximised in response to increased climate change impacts through time. This would result in flood mitigation just shy of that associated with the high emission scenario up to the end of this century. The findings of this research indicated that the size of drainage area controlled by GSI networks is dominantly responsible for enhancing FMC in the long term if the high groundwater level of industrial land was deeper than 2 m below the surface. Therefore, policies encouraging off-site runoff collection in large private properties, where capable, are deemed necessary to utilise the maximum GSI capability. The research demonstrates the effectiveness of this novel methodology over existing methods for helping planners develop adaptive flood mitigation plans through time with climate change. These outputs can facilitate planners in developing flood mitigation policies and strategies for the long-term protection of their communities. The findings from the application of this methodology demonstrate that implementing such GSI networks on capable industrial land can provide effective low-risk supplemental flood mitigation to ensure communities in low-lying cities are protected from climate change–induced flooding in the long term. It will also allow costly investments in flood mitigation structures, such as barriers and levees, to be safely delayed until their cost-effectiveness has been confirmed under increased climate certainty. To maximise the FMC of existing industrial land, the research recommends that planners designate strategic stormwater management zones in city plans. These can be used in support of implementing policies to encourage on-site and off-site runoff collection and the establishment of new governing bodies to regulate the uses of land for implementing GSI networks and ensure long-term flood mitigation.