Description
The 12th Avenue Green Street project converted a previously underused landscape area between the pavement and curb into a series of four attractive landscaped planters that were specially designed to hold and manage water run-off. The planters divert the water run-off away from the storm sewer that drains directly into the Willamette River.
When it rains, water flows downhill along by the curb as usual until it reaches the first of planters. It is then channelled into the first planter through a 30 centimetre-wide trench drain. The water can collect inside the planter until it reaches a depth of 15 centimetres. Within the planter a landscape system allows the water to percolate down into the ground at a rate of 10 centimetres an hour. From here it gradually finds its way back into the natural water table, bypassing the traditional drainage system.
If the rainfall is particularly intense, the planter could fill. When this happens, the excess water comes out of the other end of the planter, runs along the curb and eventually enters the next planter where the process repeats itself. Depending on the intensity of the storm, run-off continues downhill from planter to planter until all four are full. It is only if all four planters reach their capacity that the excess water will water run into the storm sewer. As a whole, the planter system decreases the quantity of water entering the traditional drains, and slows it down, minimising the likelihood of flash floods.
Trees and plants were chosen for the planters based on their function as well as their aesthetics. A native grooved rush (juncus patens) was chosen because it slows water flow and captures pollutants, while its deep roots are effective at absorbing water. The tupelo tree (nyssa sylvatica) was chosen because it is tolerant to both wet and dry conditions and has an attractive colour in the autumn.
Before the Green Streets project was implemented, the width of the landscape area between the curb and the pavement along 12th Avenue was 2.5 metres. This was divided to provide a one metre wide area along the curb for people to get in and out of parked vehicles and 1.5 metres between this and the pavement for the storm water planters. The planters are 5.5 metres long, 1.2 metres wide and have a depth of 33 centimetres. In total, the four planters provide a landscaped area of 6.7 square metres. At the end of each one there is a 60 centimetre-wide space so that people can get round them easily.
After they were installed, it was clear that there was a problem getting the water that was running down the road along the curb to make a sharp turn into the planters. This was solved by installing small asphalt ledges that channel the water inside.
