Typology 6 – Bluestone channel raingarden tree pit

An existing bluestone strip at the side of the road is reformed into a channel (refer Detail 14) to convey stormwater runoff to trees. Trees are planted in line with the channel invert and strip of raised bluestone immediately downstream of each tree allows water to pond temporarily around the tree and soak into the filter media. The raised bluestone strips are at the level of the previous bluestone strip to ensure there is no reduction in peak flow conveyance capacity.

Capital and operational costs are influenced by site specific factors including proximity to underground services and ease of access for construction and maintenance. The indicative cost estimate below is for a straightforward site (i.e. assuming service relocation not required and access is unconstrained) and the infrastructure dimensions shown in the standard drawings. The indicative cost estimates will be updated as we gain more experience with this typology.

Capital cost – $4,000
Operational cost – $200 per year

Vegetation – Tree only

Filter media depth – At least 600 mm (ideally 700 mm)

Filter media surface area – The standard drawings show the Bluestone channel raingarden tree pit being 1.0 m long and three to four bluestone blocks wide (i.e. 0.75 to 1.0 m). The raingarden dimensions can be increased depending on the space available and the size of the contributing catchment.

The filter media surface area should be 11-30% of the contributing impervious catchment area to ensure adequate soil moisture. If allotment runoff discharges into the bluestone channel via the back of kerb, the allotment should be included in the catchment area.

Lining – If a detention or retention outlet is used, the lower 300 mm of the drainage/transition layer has an impervious liner to detain/retain water between rainfall events.

Sides of filter media unlined to allow tree roots to access soil adjacent to raingarden

Filter media soil specification – City of Melbourne Tree Planter Soil Specification (contact Urban Forest team)

Drainage layer – Outlet configured to:

• retain water, if agreed to by Urban Forest (Detail 5 Option 3) OR
• freely drain (Detail 5 Option 2)

Species section – Refer to Urban Forest Planting Program

Other design elements

• The trees are planted in filter media and the surface of the filter media is covered with permeable pebble mix paving to prevent scouring of filter media during peak flows yet allow water to infiltrate. A gap in the pebble mix is provided around the tree trunk for growth.
• The surface of the permeable pebble mix paving is set 50 to 100 mm below the raised bluestone strip on the downstream side of the tree.
• The bluestone channel is constructed with sand between the pavers so that it is permeable, allowing water to soak into the underlying structural soil.
• Hold points must be noted on construction drawings so construction supervisors can easily ensure that key staff are invited to hold point inspections.

The performance of Bluestone channel raingarden tree pits can be assessed using MUSIC. The recommended modelling parameters are shown in Table 1 . The typical performance of a covered back of kerb raingarden tree pit is shown in Table 2.

Table 1 Recommended MUSIC parameters for a Bluestone channel raingarden tree pit

Table 2 Typical performance of a Bluestone channel raingarden tree pit

The long-term performance of assets can be strongly influenced by practices and procedures adopted during the construction and establishment phase. It is recommended that hold-points are used to allow City of Melbourne, or the system designer, to check that the design intent is met during critical stages of the construction and establishment period.

Hold points must be noted on construction drawings so construction supervisors can easily ensure that key staff are invited to hold point inspections.

It is the contractor’s responsibility to provide a minimum of 24 hours weekday’s notice (unless agreed to prior) to the superintendent to arrange for an inspection for the hold points.

Recommended construction and establishment phase hold points:

It is important to ensure that assets are functioning properly before responsibility for them is transferred to the asset owner (generally at the end of the establishment phase). The following aspects should be checked prior to handover.

Documentation – Construction documentation AND as constructed survey received.

Extended detention depth (i.e. height between top of filter media and street gutter) – At least 75% of depth shown on drawings measure using a tape measure.

Filter media condition – No significant mounds or depressions. Visual judgement.

Drainage layer water level (measured via inspection opening) – At least 600mm below filter media

Filter media hydraulic conductivity – Between 50 and 100mm/hr for tree

Tree growth – Average height of trees > 2000mm

Tree condition – Tree healthy and free from disease

Recommended routine inspection and maintenance activities for Covered back of kerb raingarden tree pits.

Recommended monitoring activities for Bluestone channel raingarden tree pits.