Typology 7 – Side of road passive irrigation tree trench

Stormwater flow from the kerb is directed into a grated inlet pit where course sediments and debris are trapped and water is piped into the base of a structural soil trench buried beneath the footpath. The structural soil trench contains intermittent water stops which are used to create a retention/detention zone (i.e. wet sump) in the lower part of the structural soil trench.

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

Side of road passive irrigation tree trenches are similar to a system currently installed on the south side of Flinders Street to the east of Swanston Street (however the existing Flinders Street system is configured to be free draining and so provides negligible water quality and passive irrigation benefits).

Vegetation – Tree only.

Growing media depth – There is at least 700 mm of freely draining soil above the retention/detention zone.

Trench area – The planting area and trench dimensions can be adjusted depending on the space available and the size of the contributing catchment. It is recommended that the minimum dimension of the tree planting area is 1 m by 1 m. Ideally the structural soil trench should be 1 – 5% of the contributing impervious catchment area.

Lining – The base and sides of the detention/retention zone have impermeable liner.
Sides of structural soil trench are unlined to allow tree roots to access adjacent soil.

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

Drainage layer – Immediately downstream of each waterstop there is a valve on the solid section of the outlet pipe (refer Detail 18). The valve can be adjusted from the surface to create a water retention zone (valve fully closed) or detention zone (valve partially closed).

Species selection – Refer to Urban Forest Planting Program.

Other

• Passive Irrigation inlet pits should be located immediately upstream of traditional stormwater pits connected to the minor drainage network to maximise flow capture.
• It is important to minimise sediment entering the structural soil trench to minimise risk of blockages. There are two coarse sediment exclusion options (refer Detail 2):
  • An AG pipe filter (recommended when space available) or
  • A permeable paver filter (recommended where space is constrained and AG pipe filter cannot fit).
• The trench contains two AG pipes:
  • An inlet AG pipe that extends downslope from the inlet pit along the structural soil trench and stops before reaching a water stop to introduce water into the trench.
  • An outlet AG pipe that extends upslope from the outlet pit, along the structural soil trench and through the water stop(s) in the trench. The slotted AG pipe transitions to a solid section of pipe where is passes through water stops (this is used to regulate the retention/ detention/ free draining function).

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

City of Melbourne’s WSUD Officer can provide advice if you would like to model in MUSIC.

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.

Coarse sediment filter – Filter present and cleaned

Valve operational – Valve can be freely opened and closed

Tree growth – Average height of trees > 2000 mm

Tree condition – Tree healthy and free from disease

Recommended routine inspection and maintenance activities for side of road passive irrigation tree trench.

Recommended monitoring activities for side of road passive irrigation tree trench