It is increasingly becoming apparent that unmitigated stormwater runoff from urban areas is linked to degradation of the stormwater receiving waterways. Specifically, the hard surfaces, such as roads, that are impervious to rainfall prevent infiltration of rainfall and instead very efficiently transport the rainfall via stormwater drainage infrastructure to our urban streams.
This has changed the hydrology of our urban areas so that much more stormwater is being transported more quickly to the streams which in turn causes erosion of the banks and bed.
Base flows of ground water into urban streams are critical to the ecology of the steams. However, the rapid transport of rainfall by the stormwater infrastructure to the urban stream removes the opportunity for the rainfall to infiltrate the ground where it can contribute to the base flow.
Another critical change to urban environments is the development of a “heat island” effect where the temperatures in highly developed areas of cities comprising vast quantities of concrete, steel and asphalt are elevated compared to surrounds that are more vegetated.
The heat island effect has been amplified by climate change where it is demonstrated that better management of vegetation in city areas can have a significant positive impact on the quality of life as well as significant energy reduction.
The combination of reduced base flow and the heat island effect has resulted in significantly drier conditions in urban environments.
At the same time, however, communities are recognizing the value of trees in our urban areas that improve the health of the environment as well as improve aesthetics, general quality of life and reduce energy consumption. However these trees are typically planted in hard surfaces (footpaths and roads) that are formed so that rainwater is directed into the stormwater infrastructure. The trees typically struggle to obtain adequate water supply and therefore local government invest in watering schemes.
Over the summer months trees are watered by hand once or twice a week from a water truck with up to 60 L per application in an attempt to keep them alive. The water is typically dumped into an agricultural drain that protrudes from the ground and or simply dumping water on the surface at the base of the tree. It takes some time for the water to infiltrate into the soil where the roots can access the water. Often the dry soil results in the water simply sitting on the surface or even running off into the stormwater infrastructure. Many trees, therefore, can be left with inadequate watering. In recent drought conditions, many of the trees died due to lack of water.
Recent efforts have been made to promote water sensitive urban design (WSUD) tree pits that are primarily designed to improve stormwater quality, but which have a secondary benefit of passively watering trees.
Installation of the WSUD tree pits involves re-configuring pavement surfaces to divert stormwater, from typically road surfaces, into pit. The pit is prepared to include engineered materials placed in specific layers in which a tree is to be planted. An appropriate tree species to suit the moisture conditions must be selected and drainage is installed under the pit and connected into the stormwater drainage system. Stormwater entering the pit is filtered as it passes downwardly through the engineered materials so that when it reaches the drainage, the stormwater has a substantially reduced contamination.
The WSUD tree pits are often criticized due to cost of engineered materials, drainage connection and pavement re-configuration. They are only applicable to new plantings and rely on consistent periodic maintenance of sediment removal to ensure water quality improvement performance.
There is a need to provide a more cost-effective alternative to the WSUD tree pits that provides irrigation for trees and plants surrounded by pavements and roads.
Beneficially, the alternative stormwater provides passive irrigation to street trees and, optionally, reduces stormwater flows to urban streams.