Emergent Challenges Adapting Coastal Stormwater and Drainage Systems

The asset value of stormwater and wastewater assets in New Zealand is well over $20 billion, which includes over 17,000 km of stormwater network (White et al., 2017). Historically, infrastructure in coastal areas was designed assuming a static mean sea level (MSL). The so-called local MSL datums still in use are a case in point, where the zero datum was set several decades ago, but MSL is now considerably higher. Consequently, any freeboard (intended or not) built into legacy stormwater and drainage systems has largely disappeared as sea level continues to rise; by around 18-24 cm since 1900.

Building floor levels are generally higher than road networks (and associated stormwater assets) and secondary overland flowpaths. This means that in low-lying coastal areas, flooding of local road networks and decreasing levels of service for stormwater and drainage systems are already emerging as the first signal of significant impacts from sea-level rise (SLR).

A recent risk census for coastal areas for the Parliamentary Commissioner for the Environment (PCE, 2015) shows substantial exposure throughout New Zealand even for modest rises in sea level. It was based on counts of buildings, roads, rail, airports and land-use – but most of these assets are associated or co-located with stormwater networks or drainage schemes, so act as a proxy for their considerable exposure.

Combined indirect effects of SLR and other climate change effects on waves, storms, groundwater, rainfall and river flows will compound the impacts on stormwater and drainage networks. In time, this will alter the type and design of systems that can be used in low-lying coastal areas (e.g., from gravity to vacuum or pressure/pumped systems, or green infrastructure or distributed systems and eventually relocation inland).

The presentation will provide a synthesis of the compounding and cascading impacts from SLR and changes to coastal processes on stormwater and drainage networks, which can inform their consideration in decision making in coastal areas and; how dynamic adaptive pathway planning, with signals and triggers to guide implementation of adaptation options or pathways. Signals (which give early warning) and triggers (defined as the decision point for switching pathways) most relevant to stormwater systems could relate to the changing frequency of coastal flooding (in terms of coping capacity of council services or the community) e.g. number of nuisance floods as the derived indicator of impacts from a rising sea.