Integrated stormwater management in the Avon River catchment

Abstract

This thesis sets out to establish whether low impact urban design structures can be retrofitted into existing urban catchments and what cleaning efficiencies multiple systems could achieve. Christchurch City has an intimate relationship with its waterways and wetlands, with two major lowland rivers, the Avon and Heathcote dissecting the city centre and surrounding suburbs. The large volume of stormwater runoff that enters these urban river networks, increase flooding, endanger private and public infrastructure, decrease water quality and causes erosion to riverbanks, destroying wildlife habitats. The Canterbury Regional Council is in the process of setting new stormwater discharge requirements for water that is discharged into existing river networks. Improving the quality of stormwater runoff is a challenge in existing urban areas, where land available for the development is scarce. Treatment systems such as swales, detention basins and constructed wetlands are widely acknowledged to be able to reduce pollution levels in contaminated stormwater. Many of these systems require large land areas, making them difficult to retrofit into existing developed sites. To be able to retrofit the Avon River catchment with low impact urban design (LIUD) structures, a fully integrated approach to stormwater management is needed, treating stormwater produced by different land use types as close to the source as possible. Existing research into the treatment abilities of LIUD structures is based around a single system, not what the pollution removal efficiencies would be if multiple smaller systems were combined. To establish if it is physically possible to retrofit the Avon River catchment with LIUD structures and whether these structures can treat all of the stormwater produced, a typical sub-catchment has been chosen. This sub-catchment will represent the various land use types found throughout the Avon River Catchment. The best LIUD structures for each land use have been explored through a series of cases studies based on each land use type; what their cleaning abilities are; and the benefits integrated stormwater management can provide to the local community and environment. These case studies established that it is physically possible to retrofit existing urban environments with LIUD structures, treating all of the stormwater produced by small storm events. Larger events, such as the volumes produced by a one in fifty year flood, however, are not able to be addressed through retrofitting strategies. The case studies also demonstrated that using an integrated approach would reduce stormwater contaminate levels by an average of fifty percent for small storm events and forty percent for larger storm events. Nonetheless, this significant reduction in pollution levels it is not enough to meet the Regional Councils proposed stormwater discharge requirements for urban rivers. To increase pollution removal rates the volumes of stormwater being treated would need to be significantly reduced. Further research into how this can be achieved is needed.