Evaluation
The performance of the planters is assessed by monitoring infiltration, flow-metering, water-quality sampling and soil sampling. Monitoring has shown that the planters retain 74 per cent of water during a summer rain storm (defined as six centimetres of rain in less than six hours). They also coped well with added pressures from sewer blockages further up the street. The storm-water planters capture an estimated 681,000 litres of water, which is nearly all of this part of the street’s annual run-off. Water and sediment quality is within safe limits. Ongoing monitoring ensures that maintenance work is adequate to keep the system running effectively.
Sustainability
The City of Portland has a strong tradition of prioritising sustainability in land-use planning and the Green Streets scheme is just one example of this. The impetus behind the Green Streets project was water-management, rather than predicted climate change, and so it is not clear whether the planter system will continue to function effectively as rainfall patterns alter. In future, schemes will need to start with a clear understanding of likely rainfall patterns and use this to guide the design.
The biggest obstacle to creating similar projects is likely to come from the competition for road-side space, which is needed for car parking, pedestrians, bikes, trees and storm-water facilities. One of the biggest challenges that the city has in replicating the Green Streets approach in other areas is the limited space available between the pavement and the curb in many places. In streets where curb extensions are not feasible, alternative designs include lowered planter strips, landscaped swales, porous paving blocks, and pervious asphalt or concrete.
A model for other communities
The Green Streets model may provide a practical alternative to more usual sustainable urban drainage systems for places where space-intensive ditches may not be appropriate.
Although the 12th Avenue green street project has a strong functional component, it is the integration of the landscaped storm-water planters into the urban environment that has gained the interest of the design community, developers, policy makers, and local people. Other local authorities have used Portland’s storm water manual and design details and incorporated them into their own codes.
The 12th Avenue green street project won an award from the American Society of Landscape Architects in 2007. Two further projects also won national design awards in 2008 for NE Siskiyou Green Street project and the Mount Tabor Middle School rain garden.
