Monitoring of stream response to integrated water resource management in a low density residential watershed in New Zealand

Abstract

In New Zealand Integrated Water Resource Management (IWRM) as part of urban development is most common on the city-edge. This integration results from the stimulus to innovation by developers who have consents to subdivide land close to but beyond the reach of reticulated water, sewage and stormwater networks. In some cases, as at Regis Park described below, elements of the alternative integrated infrastructure may be regarded by the city council as an interim measure until such time as city networks for separated water types are extended. Whether or not this reversion to conventional systems is desirable should be debated. This paper reports on advances in IWRM within Regis Park. This is a low-density residential development extending over four small watersheds in Manukau, New Zealand. The development is situated in the headwaters of the Otara Stream, which discharges to the Tamaki Estuary. High concentrations of urban contaminants in the sediments and shellfish of this estuary have also been a stimulus to innovation in water resource management within the watershed. The District Plan for the site allowed for 5000 square metre lots. The designer and developer respectively of Regis Park understood the importance of vegetation in the maintenance of the hydrological cycle within this urbanising site. This understanding led to a site design where the 66 houses were clustered instead of dispersed evenly across the watersheds. This clustering enabled sixty percent of the land area in valleys to be revegetated in forest or wetland plants according to location. These revegetated valleys and riparian zones have subsequently become infiltration zones. Stormwater from street surfaces is collected and dispersed evenly across valley plantings. Stormwater from each house roof and impervious surfaces around each house is conveyed to a backyard raingarden in each lot. These raingardens overflow to the revegetated valleys. Sewage is treated in a neighbourhood processing plant and the effluent is dispersed to vegetated areas of one of the four watersheds. One method of assessing the efficacy of IWRM is to assess the aquatic ecosystem response to urban discharges. Subdivision of the Regis watersheds occurred in 2004 followed immediately by revegetation. The author has been monitoring Macroinvertebrate Community Indices for the Regis Park Streams between 2005 and 2014. During this period there has been a gradual maturation of plants and an increase in the number of completed houses. Over the monitoring period therefore there has also been a steady rise in population serviced by the sewage treatment plant and a rise in house-site impervious areas. This paper demonstrates trends in Macroinvertebrate Community Indices as a measure of receiving water impacts during this transition. Three of the four streams indicate a steady improvement in aquatic ecosystem condition during the period of increasing house density and population growth. The watershed with the sewage effluent irrigation has not shown the improvement in aquatic ecosystem condition shown in the other watersheds. This trend to improvement in three watersheds is contrary to expected receiving water degradation during urbanisation using conventional approaches.