Raingarden tree pit program

Each raingarden tree pit works like a small raingarden. When it rains, stormwater runs off road surfaces and into the gutters.

Raingarden tree pits are set into the kerb and intercept and clean the water before it goes into the drain.

The pit contains layers of substrate that work with the tree’s root system to filter pollution from the stormwater. This includes nitrogen, phosphorus and oils.

Tree pits also include a rock mulch layer to retain moisture in the soil and prevent erosion during rain.

A grate sits around the base of the tree, flush with the pavement. This protects the tree pit from damage and ensures that pedestrians don’t trip over it.

A pipe is located at the bottom of the well. It allows cleaned stormwater to drain back into the drainage system.

We first began installing raingarden tree pits in 2006. The successful program is ongoing.

Competing objectives and uses in city streets

Inner Melbourne is a densely developed area and public spaces must accommodate many different needs. Some considerations in a typical central city street are:

• pedestrian flow
• car parking
• canopy cover for cooling and shade
• amenity
• Melbourne’s character

• public transport

• urban ecology and biodiversity.

Raingarden tree pits work well in the city because they are smaller than standard raingardens. We install them in footpaths to prevent loss of road or car parking spaces.

A single tree is planted in the raingarden, rather than grasses. This is consistent with the iconic style of Melbourne’s streets. Installing trees also helps us meet our target of increasing canopy cover to aid urban cooling.

Redesigning raingardens for city streets

Typically, raingardens are large garden beds planted with wetland grasses that remove pollution from stormwater.

This type of design is not suitable for inner city streets because there is limited space to install raingardens. In busy streets, normal raingardens present a tripping hazard for pedestrians and can be damaged by vehicles.

Our challenge was to maintain the stormwater cleaning capability of a raingarden, while modifying the design for inner city streets.

The tree pit designs have evolved since 2006. The first pit lids had a large, 400 mm diameter hole for the tree trunk. This meant that litter collected in the pit. It was also a trip hazard while the tree was young.

In later designs, we have included tree growth rings. These reduce the hole around the trunk to 100 mm and allow for the rings to be gradually cut away as the trunk grows.

In the municipality of Melbourne we have two preferred standards for tree pit design. Our design for the Docklands area includes wavy tree pit lids and a kerb inlet. For central city areas we prefer bluestone pit lids with a grated pit inlet. The differences in design cater to the different urban design styles and local conditions in these areas.

Raingarden tree pits contain three layers of drainage substrate: a filtration layer, a transition layer and a drainage layer.

Filtration layer:          

Purpose:  This is where all the work happens. This layer filters sediments and larger litter. Phosphorous is absorbed into the filter layer with the sediments. Nitrogen is removed via the plant roots and soil bacteria that exist in the filter media.

Position: This is the top layer

Depth:  Minimum 450 mm. Depth will vary to suit the depth of the stormwater drain. Hydraulic conductivity: 100 mm per hour

Composition: 80% loamy Sand, 10% vermiculite and 10% perlite, by volume, evenly mixed. A small amount of compost is added to the top 10 cm.

Transition layer:

Purpose:  To prevent fine filter sand entering the drainage layer while facilitating drainage and root growth

Position: This is the middle layer

Depth: 100 mm

Composition:  Course sand

Drainage layer:

Purpose: The drainage layer surrounds the drainage pipe. It is free draining and prevents the blocking of the holes in the drainage pipe

Position:  This is the bottom layer

Depth: 100 mm to 150 mm

Composition: Fine, clean washed gravel (2.5 mm particles).

The top of the substrate is positioned a minimum of 50 mm below the invert point in the kerb. This air void allows stormwater to gather on top of the substrate before filtering down through the layers. It provides additional treatment within a confined space.

Any tree that grows well in a streetscape setting can be grown in a tree pit, but specific consideration is given to:

• root structure
• local climate conditions
• interaction with surrounding infrastructure – buildings, power lines, footpaths.

A range of tree species have been planted in raingarden tree pits across the central city, including waterhousia, pin oaks, london planes and turkish hazel.

We have carefully chosen tree species that work towards our species diversity and resilience targets. These targets are set out in our Urban Forest Strategy.

A drainage pipe with slots in it sits in the drainage layer at the bottom of the tree pit. It is connected to the stormwater drain. The pipe allows filtered stormwater to flow to waterways and prevents water logging at the trees roots.

The high flow bypass makes sure that heavy rain flows are safely conveyed to the conventional drainage network. The bypass is provided by side entry pits located downstream of the raingarden tree pit inlets.

Excessive sediment load will cause the filter media to clog. Best practice site management during construction is required to minimise the sediment load.

During the construction phase, the raingarden tree pit can be protected with a geotextile layer. This is placed on top of the filter media and then covered by turf.

Once construction is completed the turf can be removed, along with any sediment build up. The tree can then be planted.

Each raingarden tree pit costs between $4000 and $8000. The price varies depending on the type of design, which is influenced by how busy the street is and the existing infrastructure.  Much of the cost is associated with the tree pit lids, bollards and tree guards.

Comparatively, a planting a normal tree in a street usually costs less than $1000. However, in the long run raingarden tree pits provide better value for money because they require significantly less mains water for irrigation. We have found that in the first two years after planting, a tree in a raingarden pit uses 70% less water than a standard street tree.

Raingarden tree pits are designed to need minimal maintenance. However to ensure stormwater can flow from the gutter to the tree pit, street sweepers do need access to clean litter from the inlet.

Covering the tree pits with a grate around the base of the tree helps to prevent litter accumulating on the surface of substrate.

Periodic cleaning is required on all tree pits. The amount of cleaning needed depends on the design of the cover and the location of the tree pit.

We clean the pits by hand or use a suction hose to remove any litter and sediment build-up from the top layer of the substrate.

The typical annual maintenance cost of a raingarden tree pit is five to seven per cent of the total construction cost. This equates to between $200 and $560 for each tree pit in the City of Melbourne.