Science Centres: Freshwater

Grab samples of road runoff showing the so-called first-flush where accumulated sediments and contaminants are washed off urban surfaces by the first rainwater.

An ancient Greek roof-top spout (Delphi)

An ancient Greek drain (Athens).

Newmarket stream has become urbanised and has steep sides which have been reinforced.

The urban stormwater management triangle puts equal emphasis on stormwater regulation, treatment and wider community function

Dry weather flow at the pond, water is regulated by the slot weir on the side of the standpipe.

29 March 2007, the standpipe is overtopped following a heavy downpour.

Observed inflow and outflow with modelled outflow for the pond standpipe.

Observed and modelled loads of total suspended solids at the outlet.

Stormwater refers to runoff from urban surfaces generated by rainfall or snowmelt events. 

Urban stormwater is a major concern for New Zealand.  Urbanisation leads to increased surface water flows as vegetation is removed and soils are variously compacted or covered by impervious surfaces such as roofing, asphalt and concrete.  This stormwater is conveyed quickly and efficiently via underground reticulated pipe networks to local receiving environments. 

Moreover, human activities, particularly industry and traffic, lead to the build-up and eventual wash-off of sediments and contaminant which are conveyed to receiving environments with stormwater.  Urban stormwater quality is closely linked to the quantity and quality of urban sediments.  These sediments accumulate in receiving environments and can become a hazard for local biota. 

The impacts of urbanisation include:

• increased stream bank erosion and sediment transport;
• increased deposition of sediments (and associated contaminants) in estuaries and harbours;
• increased local flood risk for urban flood plains;
• increased overflow frequency and volumes from sanitary sewers, particularly in areas with combined stormwater and wastewater sewers;
• contamination following storm events (acute) or due to accumulation over time (chronic) or both.

A Brief History of Stormwater

Stormwater and stormwater management have been around as long as there have been towns.  The pictures right shows an ancient Greek roof-top spout (Delphi) and drain (Athens).

Reticulated stormwater pipe networks common to most cities in Europe and North America were first constructed in the mid 19th century following a period of the rapid urbanisation following the industrial revolution.  Cities in Australia and New Zealand followed suit.  The first sewers were urban streams that were covered and later buried and lined with pipes or brickwork.  Many of these "combined sewers" are still with us, particularly in city centres.  Eventually, waste and stormwater carried in combined sewers was led to waste water treatment plants before realease into receiving enviroment. 

From the 1950s, separate pipe networks for waste- and stormwater have become standard.  Even so, streams have continued to be replaced with hydraulically efficient buried pipes or chanalised.  The example below is from Newmarket near the NIWA Auckland office. 

A paradigm shift

Over the last decade or so, the realisation that urban receiving environments are at risk both due to flooding and poor water quality has lead to changes in stormwater management towards systems that reduce and attenuate flows and treat contaminated stormwater.  These systems are variously called low impact design (LID), water sensitive design (WSD), sustainable urban drainage systems (SUDS) and best management practices (BMPs). 

Common examples of stormwater control and treatment devices for LID include:

• detention ponds and wetlands for sediment settling
• infiltration surfaces
• permeable paving
• swales (vegetated ditches)
• media filters
• bio-retention
• green-roofs

Many are visually appealing and can be cunningly incorporated into landscaping to provide a blue-green space in the otherwise concrete and asphalt urban environment.  Stormwater has truly become a liquid asset.

At present, LID is mainly found on greenfield developments, but there are moves to design systems for retrofitting existing stormwater systems. There are also moves to restore urban waterways to their natural to both improve water quality and the urban landscape. NIWA scientists from Christchurch and Hamilton have been involved in urban stream restoration.

NIWA and Stormwater

Much of the work undertaken bythe Urban Aquatics Group is to monitor flow, take and analyse water samples for sediments and contaminants and model the hydrological processes in operation in urban areas. We also work with other NIWA groups to look at the impacts of urbanisation on urban environments including stream erosion, flooding, and ecosystem degradation.  We have recently started a program for innovation in stormwater treatment, especially for fully developed sites where devices must be retrofitted into existing systems.

The example of our work below shows the effects of the heavy rainfall on 29 March 2007 on a highway stormwater treatment pond near Silverdale north of Auckland.  The same event was responsible for devastating floods in Northland (including the Kerikeri river discussed on the Urban Aquatic homepage new bulletin).  The Silverdale pond is being monitored as part of two projects.  The first is to track the fate of particulate metals from roads through the drainage network to receiving environments.  The second is to provide data that can be used to develop a catchment contaminant annual load model that includes treatment devices.