About Stormwater Management

Increases in impervious surfaces lead to an increase in the volume of stormwater runoff discharged, as well as increased peak flow rates. In addition, impervious surfaces collect litter, dust, metals, oils and sediments which become entrained in stormwater, and are discharged to our streams, estuaries and beaches. Given the high level of development in New Zealand, and particularly in Auckland, the cumulative increase in impervious surfaces, and the resulting increase in stormwater runoff, is causing the single biggest impact on our aquatic receiving systems. The three main effects of stormwater runoff are:

1. Increased flooding and flood levels.
2. Decline in water quality.
3. Accelerated channel erosion on the physical structure of streams.

In order to manage these effects practioners have traditionally used constructed stormwater practices to either treat or attenuate stormwater runoff. For example, ponds, wetlands, rain gardens, swales, infiltration practices, rain tanks and sand filters are all common mitigation methods. However, in order to satisfy the Resource Management Act (1991) principles, and to attempt to first avoid or remedy the effects of stormwater discharges, we need to go beyond the use of constructed stormwater practices. Stormwater effects are closely linked to land use, given that it is the increase in impervious surfaces which increases stormwater runoff. The way we consume land for housing, roads and infrastructure, and commercial and industrial land, has a profound effect on stormwater discharges. In addition, many earthwork practices do not seem to take account of the natural resources within sites; nor, do they attempt to minimise the amount of disturbance, and therefore the resulting discharge of sediment.

Water Sensitive Design (WSD) or low impact design (LID) is a design philosophy for site development which assists us in meeting the RMA goal of preventing and minimising effects. This is achieved by taking land use decisions and earthwork practices into account when designing subdivisions. The key principles of low impact design include:

1. reducing impervious areas;
2. minimising disturbances;
3. maximising site resources;
4. source control of both contaminants and runoff volume;
5. considering earthwork and stormwater mitigation (constructive practices) at the beginning of the design phase;
6. bringing nature back into the urban environment and using practices such as green infrastructure; and
7. incorporating stormwater management into sustainable urban design.

In many instances WSD leads to lower development costs and higher profits for developers due to reduced construction costs and the potential increase in lot yield. In addition, there can also be lower long term stormwater management costs for local network operators which result from reduced mitigation requirements.  Furthermore, green infrastructure practices elicit a number of non-water benefits such as carbon sequestration, linking green corridors, reducing the urban heat island effect and increasing biodiversity.  This means that they are likely to achieve greater value and more benefits across a range of project outcomes than grey treatment infrastructure.

At Koru Environmental Consultants, we use a WSD philosophy with regard to stormwater management. By incorporating stormwater design, together with sediment and erosion control management, into the conceptual site design phase, we can blend these facets of the development phase with sustainable urban design and planning.

Stormwater management should be seen as an opportunity, not a problem!

Our aim is to assist developers, as well as local, regional and national operators to create cost effective solutions, whilst ensuring that the outcome results in valued urban environments for communities. And, most importantly, where possible, to work towards the protection and enhancement of the quality of New Zealand’s valuable aquatic resources.

Please contact us if we can help you.