Optimising Rain Garden, Tree Pit and Proprietary Filtraion Design for Christchurch

Christchurch City Council is developing a Stormwater Management Plan for the Avon- ?takaro, a highly urbanised catchment of 8,860 hectares. As few sites exist for large treatment devices, retrofitting smaller treatment devices into the existing network is one of the main mitigation options available. Rain gardens, tree pits and proprietary filters have all been identified as potential options suitable for retrofit water quality treatment.

Since design of these devices is highly climate dependent, this study developed a method allowing commonly used design parameters to be assessed against the historic Christchurch 30 minute rainfall record (1963-2013). The aim was to determine the optimum configuration (lowest cost versus treatment capability) by: establishing design parameters based on international and local design best practice; modelling the design parameters against the rainfall record to ensure greater than 80% volume capture and equivalence with other treatment devices; and developing a cost model to arrive at the most efficient design. Significant savings are predicted through optimising the selection and design process.

This paper summarises recent assessments of hydrologic design criteria of these devices undertaken on behalf of Christchurch City Council.