Waikato River Stop Bank Failure Modelling for Rangiriri West – 450-year ARI
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| The purpose of the project is to undertake a flood assessment for a proposed solar farm development in Rangiriri West, Waikato. The existing site proposed for the development of the solar farm is approximately 270 hectares in size, with a most flat and variable land coverings. The existing site predominantly consist of rural farmland, including existing streams and wetlands. The catchment ultimately drains into Waikato River via series of natural and farm drains and exit the site from northern boundary. The site is located within the Franklin Waikato Drainage Scheme within a flood protection zone. The formal flood defence consists of a series of stopbanks, pump stations and spillways, which are designed to provide flood protection up to the 100-year ARI event. Given the required level of resilience for the proposed substation, a modelling simulation was conducted to assess the consequences of a stop bank failure within the flood protection scheme during an extreme design event with a 450-year return period. This analysis informs the design to ensure adequate resilience for the substation. A 2D grid model was built in MIKE21 to an appropriate extent so that the flood protection scheme failure can be appropriately assessed. Boundary conditions were developed based on the model extent. The following boundary conditions were applied to the model: • The Waikato River downstream tailwater level boundary condition. This boundary condition was derived from hydraulic modelling studies completed to date (information source published on the internet) and was applied to the model as a H-Q rating curve. • An upstream inflow boundary condition representing the 1:450 AEP design event Waikato River flow. This boundary condition was derived from modelling assessments completed for the Waikato Regional Council in past, and from publicly available historical observed data. • Other common model boundary conditions (e.g., free flow, land boundary) were established based on the model extent and the topography using an engineering judgment. The following two model scenarios were developed: • Baseline: The baseline scenario represents the stop banks as per current LiDAR (Laser Imaging Detection and Ranging) information (no stop bank failure is assumed). • Stop bank failure: The stop bank failure scenario was developed to assess the impact of a stop bank failure would have on the Rangiriri West Solar Farm site. Three stop bank failure locations were assessed below to understand the potentially most severe flooding on the solar farm site. The three stop bank failure locations were chosen based on the topography from the failure location and geometry of the Waikato river where most of the flow is expected with failure of the stopbank. |