Avon River Modelling Bears Fruit

Christchurch City Council (‘council’) has been improving its stormwater models across the city to inform development of the Land Drainage Recovery Programme (LDRP) and the Long Term Plan (LTP), assess impacts of the Canterbury Earthquake Sequence (‘the earthquakes’) and a wide range of other activities including, informing floor level setting across the city.

The Citywide Modelling project began in 2015 with the aim of delivering updated river catchment models for ‘flat land’ Christchurch with much greater detail and a single combined hydraulic model of the City’s pipe networks (greater than 300 mm), waterways and the estuary and to an open sea level as the model boundary condition (Preston, Parsons, 2016).

Modelling of the Avon River catchment has been progressed in advance of the other large river catchments. The model now provides detailed results across the catchment in ways that previous models could not, along with the new capability to predict flood inundation for a wide range of design storm event durations and magnitude.

The surface model includes an area of 12,000 ha and contains 1.8 million computational elements, the smallest of which is 12 m2. The one-dimensional models include 160 km of waterways, 360 km of piped networks, 11,000 manholes and representation of all 14,000 sump inlets.

A rain on grid hydrological approach has been adopted across the catchment with multiple design storms that enable an assessment of critical duration for small catchments and an evaluation of the risk down to the street level. This level of detail is useful as the aim of the modelling is to be able to inform council of flood risk across a broad spectrum of event magnitudes; from 10% AEP up to 0.5% AEP and for a range of durations from 0.5 hour up to 36 hour.