Stu Farrant and Reuben Ferguson (Morphum Environmental Ltd).
The implementation of stormwater management strategies has become commonplace across New Zealand. As the objectives and drivers of these strategies become increasingly complex and multi-faceted, a clear understanding of the changes in hydrology and water quality resulting from urban development is recognised as being critical to support decision making and align with regulations.
The planning of stormwater management assets in New Zealand has historically relied on conceptualising elements largely in isolation from other assets. The design and sizing of devices such as wetlands and bioretention systems, and the treatment performance inferred from these, has typically depended on calculating target water quality volumes and attenuation requirements. In reality, however, the temporal variability of rainfall and runoff are fundamental to the operation of these stormwater management devices, and the inter-relationships of multiple devices in series needs to be reflected to accurately quantify the performance of management strategies at a sub-catchment or precinct scale.
Continuous simulation modelling provides a means of evaluating the effectiveness of stormwater management strategies by simulating the relationships between rainfall, runoff and stormwater devices in urbanised catchments. This reflects the highly variable characteristics of stormwater during rainfall events which in turn affects the hydraulic function of systems and the corresponding water treatment performance. The Model for Urban Stormwater Improvement Conceptualisation (MUSIC) is one such tool developed to support the design of complex stormwater management strategies at a range of scales. MUSIC has become the industry standard across Australia and has more recently been used in other parts of the world. In New Zealand, MUSIC is being used on an increasing number of projects in both greenfield and retrofit situations.
Morphum Environmental Limited (Morphum) have used MUSIC as an effective means of assessing the hydrological and water quality impacts downstream of urban developments, and for evaluating the benefits of fully integrated solutions. These include stormwater treatment devices combined with rainwater and stormwater harvesting and reuse, to emulate pre-development hydrology. The model allows for iterative refinement of designs and clear communication of benefits resulting from comprehensive stormwater management strategies to improve the protection of receiving environments from urbanisation.