As part of Waitakere, New Zealand's mission to become an eco-city, the city's council along with the Auckland Regional Council provided West Auckland Health Services Foundation a grant to develop Waitakere Hospital, a 121-bed acute inpatient care facility with an extensive rainwater-harvesting system, abundant natural lighting, and a site that incorporates some basic water conservation components.

The site design of Waitakere Hospital combines a landscaped stormwater retention pond, rows of swales, sand filters, and rain gardens to slow and treat polluted runoff from the parking lot and access road before the water reaches the stormwater system and local waterways. The stormwater conservation techniques are part of a number of Environmentally Sustainable Design initiatives on the site, says Chin Young, a director at Rice Daubney Di Carlo Potts (RDDCP) and an assistant project architect and designer during the hospital's design phase (1998-2001). Stormwater that flows directly into storm drains and waterways are more likely to cause erosion, water pollution, and flooding, as well as diminish the amount of groundwater that a location would naturally contain.

The stormwater from the parking lot and access road flows toward the northeast of the site to the pond area, Young says (figure 1). Runoff is also collected and filtered though two sand filters and a series of ground swales that are integrated into the landscape. The swales, dug-out tracts running parallel to the elevation contour line of a hill, slow the runoff, keep soil intact, and let water soak deeper into the ground (see video). About 450 linear meters of swales take in runoff from approximately 3,400 square-meters of paved area (30% of the site’s paved area), according to a case study from New Zealand’s Ministry for the Environment (MfE).

Figure 1

Geoff Lawton, managing director, Permaculture Research Institute of Australia, explains the consequences of swales on contour with a slope.

Figure 2

Rain gardens installed at the end of the swales further treat the runoff before it enters the stormwater system. Wetland-edge vegetation and other plants adapted for varying amounts of water take up a portion of the runoff, while the rest filters through soil layers of root systems and microbial populations.

Two sand filters sit next to the hospital's main parking area to help treat runoff from 300 parking spaces (Figure 2). Although sand filters provided very limited flow-rate control, they are found to be effective in controlling the water’s quality, according to a US EPA Storm Water Technology Fact Sheet. “Sand filters are able to achieve high removal efficiencies for sediment, biochemical oxygen demand (BOD), and fecal coliform bacteria. Total metal removal, however, is moderate, and nutrient removal is often low,” according to the fact sheet.
Remaining runoff from the site collects in a treatment pond that provides irrigation for the landscape and overflows to the local Henderson Creek. The 1,900 square-meter pond receives stormwater from an 18-hectare catchment area and treats it to 75% efficiency, according to the MfE case study. For the landscaping around the pond, designers selected vegetation that would have been found in the area prior to urbanization.

The facility also has a comprehensive roof rainwater harvesting system. "Dual hydraulic systems were installed so if the collected rain water is inadequate, the main water supply will supplement the short fall," says Young. Six 250-cubic-meter tanks collect rainwater run-off from the main hospital roof, with the water used mainly for toilets and cooling tower make-up. The system reduced the facility’s water consumption bills by $15,000 per annum, a significant saving given that water costs in Auckland more than quadrupled in 2005, according to the MfE case study.

The stormwater conservation efforts, which were in operation before the facility was completed, and the plans for the rain-harvesting system earned Waitakere Hospital the high commendation in the MfE 2003 Green Ribbon Awards.