Bénéfices des techniques alternatives d’assainissement des eaux pluviales pour les nouveaux développements urbains du bassin versant Mangakootukutuku : retour d’expérience de la première année de suivi de la qualité des cours d’eau

In a context of increasing housing demand, many greenfield areas at the outskirts of major cities are gradually being turned into urban areas. Green infrastructure systems and/or Water Sensitive Urban Design (WSUD) are alternatives to conventional drainage systems that aim to capture, manage and treat urban runoff to maintain a good ecological health of receiving water bodies. This long-term experimental study, i.e. 10-15 years, aims to track through time, the changes occurring as one catchment (Tiireke) changes from rural to urban land use, integrating WSUD, compared to a conventional (i.e. “Business as usual” (BAU), no WSUD) catchment (Rukuhia). Initial results suggest that early stages of development and earthworks have resulted in slightly higher suspended solids (SS) exports compared to upstream rural undeveloped areas. SS export was however much higher from the nearby conventional urban catchment. Although it is expected that the BAU urban subcatchment contributes to copper export, this was concealed by in-stream attenuation of the high input loads from its upstream rural/peatland subcatchment. An alternative BAU catchment, with no upstream contaminant export influence, is being investigated for the next phase of monitoring. Initial data analysis of the broad list of monitored pollutants and planned activities in the Tiireke catchment will help refine the monitoring strategy for future years to better capture the impact of development activities and the benefits provided by the widespread use of WSUD once developed, when compared to a conventional urban catchment.

Auteurs
Karine Borne, Yalden Sharleen, Gadd Jennifer, Mckergow Lucy, Moores Jonathan, Hudson Neale, Van Assema Gareth, Bellingham Margaret, Colliar Jackie, Phillips Andrea, Young Nick

Résumé court
Du fait de l'augmentation de la demande en logements, de nombreuses zones rurales en périphérie des grandes villes se transforment progressivement en zones urbaines. Les techniques alternatives (TA) d’assainissement des eaux pluviales visent à capter, gérer et traiter les ruissellements urbains pour maintenir une bonne qualité écologique des cours d’eau. La présente étude expérimentale vise à suivre, à long terme (10-15 ans), la qualité des cours d’eau lors de la transformation d’un bassin versant rural en zone urbaine intégrant des TA (Tiireke), par rapport à un bassin versant urbain conventionnel (BUC), i.e. utilisant un réseau d’assainissement enterré (Rukuhia). Les résultats suggèrent que les premiers stades de développement et de terrassement ont entraîné des exports de sédiments légèrement plus élevés que les zones rurales non développées en amont. L'export de sédiments du bassin versant urbain conventionnel voisin était cependant nettement plus important. Bien que l'on s'attende à ce que le BUC contribue à un export de cuivre, ceci a été masqué par les exports élevés du bassin versant rural amont comprenant des tourbières (possible source de cuivre). Un autre bassin versant conventionnel, ne subissant pas l’influence de potentiels exports de contaminants en amont, est actuellement à l'étude pour inclusion dans le programme de suivi. L'analyse préliminaire de l’ensemble des polluants suivis et des activités prévues dans le bassin versant Tiireke aidera à affiner la stratégie de suivi pour les années à venir. Ceci permettra de mieux capturer l'impact des activités de développement et les bénéfices liés à l’utilisation généralisée des TA une fois la zone développée, par rapport à une gestion conventionnelle des eaux pluviales.

Summary
In a context of increasing housing demand, many greenfield areas at the outskirts of major cities are gradually being turned into urban areas. Green infrastructure systems and/or Water Sensitive Urban Design (WSUD) are alternatives to conventional drainage systems that aim to capture, manage and treat urban runoff to maintain a good ecological health of receiving water bodies. This long-term experimental study, i.e. 10-15 years, aims to track through time, the changes occurring as one catchment (Tiireke) changes from rural to urban land use, integrating WSUD, compared to a conventional (i.e. “Business as usual” (BAU), no WSUD) catchment (Rukuhia). Initial results suggest that early stages of development and earthworks have already resulted slightly higher suspended solids (SS) and copper exports compared to upstream rural undeveloped areas. SS export was however much higher from the nearby conventional urban catchment. Although it is expected that the Rukuhia BAU subcatchment contributes to copper export, this was concealed by in-stream attenuation of the high input loads from its upstream rural/peatland subcatchment. An alternative BAU catchment, with no upstream contaminant export influence, is being investigated for the next phase of monitoring. Initial data analysis of the broad list of monitored pollutants and planned activities in the Tiireke catchment will help refine the monitoring strategy for future years to better capture the impact of development activities and the benefits provided by the widespread use of WSUD once developed, when compared to a conventional urban catchment.