The Urban Impacts Toolbox: An example of modelling the effect of climate change and sea level rise on future flooding

This paper presents good practice methods for flood risk assessment under climate change in urban areas of New Zealand, following techniques selected from the "Impacts of Climate Change on Urban Infrastructure and the Built Environment Toolbox" (see the first paper in this special issue and NIWA, MWH, GNS and BRANZ, 2012). A case study evaluating flood risk in Westport is used to demonstrate the methods. Hence this paper has a dual purpose to explain the modelling methods and to provide a flood risk assessment for Westport under selected climate change scenarios. We show how physically-based climate, hydrological and hydrodynamic models can be used together to simulate changes in meteorological and hydrological processes under future climates, and evaluate the effect of those changes on projections of flood inundation and risks to people and assets. Using a historic 1-in-50 year event as a baseline, we predicted how the severity of that event would change under each climate scenario. Statistically downscaled projections from Global Climate Models were used to define appropriate adjustments to the historical rainfall and temperature measurements. Using the hydrological model TopNet, this data was used to simulate flood hydrographs at the Te Kuha gauging station upstream of Westport. The resulting hydrographs predicted for the future time period 2080-2099 correspond to events in the current climate with recurrence intervals of 78, 98 and 113 years for the B1, A1B and A2 IPCC SRES scenarios respectively. The flood hydrographs provided upstream boundary conditions for a 2D hydrodynamic model simulating inundation of the Buller floodplain. Predictions for inundated area increase from 50% of Westport town in the current climate to 67%, 70%, and 72% for the B1, A1B and A2 scenarios for the 2080-2099 time period. Resulting maps of inundation depths and velocities allow detailed planning for mitigation of flood events. We used the hazard assessment tool RiskScape to calculate the impact of the flood on people and assets (buildings, contents and vehicles) within the inundated area. The predictions showed that under the A1B 2080-2099 scenario, present day Westport could expect risk to life classified as 'Medium' or greater to 560 people, building damage of $72M and contents damage of $68M.