Confronting the Residual Effects of Development on Stormwater

Urban stormwater management is undergoing a paradigm shift. In a generation, New Zealand cities have moved from seeing stormwater as a waste to dispose of quickly and unobtrusively, towards being a vital part of a healthy urban environment. The tools we have to hand to manage stormwater today are potent and evolving. We can remove the majority of contaminant loads from waters before they hit our streams, lakes, rivers and oceans. We can reduce flash flows which cause erosion, instability and flooding. We understand more than ever the vital role that greenspaces play in our environmental, cultural and social systems.

However, there are limitations to our current stormwater toolbox. Stormwater from new developments, even when treated using well managed and maintained devices, contains residual contaminant load. Runoff volumes from developing land also increase, even when peak flows are attenuated. Increased stormwater volumes lead to increased scour and erosion. This leads to loss of land, vegetation cover, and sediment being discharged into streams.

These residual effects are sometimes written off as less than minor on an individual basis. Cumulatively these effects can be enormous. Hamilton City Council has begun to address some cumulative residual impacts of development through our Integrated Catchment Management Planning (ICMP) programme, and associated works. Managing residual effects is shaping up to be the next front in stormwater management, and one to be decisively acknowledged and addressed.

Two case studies are presented where residual effects have been addressed in Hamilton City.

The first case study is the developing Rotokauri Catchment, whose receiving environment is Lake Rotokauri. The lake is sensitive to nutrient load, particularly phosphorus. The recently completed Rotokauri ICMP has an objective of ensuring no further degradation of the lake. A key indicator for this is keeping chlorophyll a concentrations, which are dependent on phosphorus load, to predevelopment levels, even while allowing for development of approximately 550 Ha from pastoral to urban land use (Cooke, 2015). Under a standard TP10 approach, development on this scale would result in a substantial increase in contaminant discharge to the lake, including phosphorus, resulting in the lake becoming increasingly degraded. The ICMP has identified an innovative three stage treatment train approach to remove phosphorus and prevent chlorophyll a concentrations in the lake from increasing at all. This represents complete mitigation of residual effects of development on stormwater for a single critical contaminant.