Flood storage optimisation in the Ōpāwaho / Heathcote Upper Storage Scheme

In a time when flood risk is becoming an imminent reality to many in the community, it is important to hear stories where large infrastructure projects, supported by computational modelling, can result in a significant reduction in the flood risk to an urban area. The Upper Ōpāwaho / Heathcote Storage Scheme is a group of actively managed flood storage basins initiated by Christchurch City Council to optimise flood storage and downstream benefits.

The Upper Heathcote Storage scheme consists of four basins in the west of the catchment. These are the Wigram Basin, Curletts Basin, Cashmere - Worsleys Valley and Dam, and the Eastmans – Sutherlands – Hoon Hay basins. These basins add more than 800,000m3 of flood storage. The basins work by holding back water during a flood event and then releasing the water slowly into the receiving Heathcote River and Cashmere Stream. The basins are actively controlled by gates which are designed to react to the levels in the wider catchment and open and close depending on operational rules. These gates aim to optimise the scheme's operation to minimise flood levels downstream while also minimising the residence time of water in the basins and preventing unnecessary spillway overflows. The modelling works to replicate the full system as closely as possible to what has been built and what will be built. This resulted in many iterations of the modelling over the years to align with the latest design, collating, and interpreting data from various consultants for use in the model and in turn, providing back model results showing the effectiveness of the design and gate operational rules. Because the system is so complex the modelling was a valuable tool in understanding how the operational rules needed to be set up and helped to understand the importance or necessity of specific components.

DHI has been involved in modelling the Ōpāwaho / Heathcote catchment since 2010 and, more specifically, has been a part of modelling the Upper Heathcote Storage scheme since 2015, working to make this a reality and helping ensure that the system of basins will work to best effect.

The scheme was originally modelled and tested using a 1D MIKE 11 model and a 1D/2D MIKE FLOOD model. However, the scheme has now been incorporated into the Christchurch City-Wide model of the Heathcote catchment, including all control structures and operational rules from the latest design (most of which has now been completed). A no basins model has also been developed in parallel to show the impacts of the storage scheme specifically. The results from this show that the scheme has a widespread benefit, reducing peak flood levels in the 24-hour 1 in 50-year ARI design rainfall event, by an average of 210mm along the Ōpāwaho / Heathcote River from Wigram Basin to Opawa Road. This is significant in the context of this catchment and provides a more flexible means of managing storage in the catchment.