Increased Flooding Vulnerability - The Methodology and Flood Models to Assess 120,000 Properties in Christchurch

An overland flow model of the whole of Christchurch was developed by Tonkin & Taylor (T&T) on behalf of the Earthquake Commission (EQC) in TUFLOW 2D software. The model results are applied to the assessment of Increased Flood Vulnerability (IFV) that is defined by EQC as a physical change to residential land as a result of subsidence due to the Canterbury earthquake sequence, which adversely affects the amenity and value that could otherwise be associated with the land by increasing the vulnerability of that land to flooding events. This form of land damage has never been assessed before in the world and the technical (including modelling) response has been unique and unprecedented.

The overland flow model covers the entire urban area of Christchurch City. It can be split into component models covering the individual catchments of the Avon, Heathcote and Styx Rivers if required, but can also be run as a single city-wide model. The city-wide model has benefits in extreme events as cross-catchment flows, which occur in Christchurch due to the flat terrain, can be represented. This model is currently being used to assess and identify (out of all 120,000 properties in Christchurch) approximately 13,500 potential IFV properties that will then have a site specific engineering assessment to confirm their status.

This paper describes the model build of the Christchurch overland model, its sensitivity testing, calibration using the 4 and 5 March 2014 flood event and also its application in assessing IFV. This paper also describes the additional work currently in progress to construct a model to assess IFV for Kaiapoi and adjoining residential areas north of the Waimakariri River.