Howard Street raingardens

Howard Street slopes down between William Street and Rosslyn Street. We built two new raingardens at the low end of Howard Street. When it rains, the gardens capture water running off the road before it enters the stormwater drains. The water pools in the garden beds and gradually soaks down through the sandy soil.

The raingardens are planted with wetland grasses and look like standard garden beds. Beneath the surface they are made up of several layers of substrate. These are designed to remove litter and pollution from stormwater.

The top layer is called the filter layer. It removes fine particles of pollution from the water and traps litter and debris on top. The middle layer is called the transition layer. It prevents the finer sands from being washed into the drains.

The bottom layer is called the drainage layer. It allows the clean water to drain from the garden bed and back into the stormwater system, eventually flowing into the Yarra River.

Beneath the surface of a raingarden, special layers of sand work to remove pollution from stormwater.

Cost estimate

October 2007

Concept design

November 2008

Detailed design

November 2008

Community consultation

2007/2008

Completion

May 2011

The raingardens in Howard Street were part of broader streetscape redevelopment that cost a total of $1.5 million.

The following tasks were completed during the research and planning phase of the project.

Traffic study

Because the redevelopment of Howard Street involved turning some of the street into parkland, we did a traffic study to understand the impact of such change. We also investigated traffic volumes prior to the design process to support the business case for closing part of the road.

MUSIC modelling

MUSIC modelling was carried out to help determine the size and location of the raingardens.

Concept designs

To aid the community consultation, we developed concept designs. The designs were refined based on community feedback.

This was the first time we used the Biofilta planting system, where mature plants are pre grown at the nursery in large format modules. The modules arrive on site with 100 mm of loamy sand thick with plant roots. We placed these close together in the raingardens, on top of 400 mm of filter layer sand.

The advantages of this type of installation:

• Plants are established from day one and do not need 6 – 12 months on site to mature.
• The system functions to its ability from day one as the mature plants can absorb the nutrients in the stormwater straight away.
• Establishment and maintenance costs are reduced, especially weeding and plant replacements.

We engaged extensively with the community about our plans to turn some of the underused section of Howard Street into parkland. In response to community feedback, we added the raingardens to the original park design.

You can download the concept plan used for consultation at the top of the page.

Each raingarden has four layers; the plant modules, a filtration layer, a transition layer and a drainage layer. Each of these are detailed below.

Filtration layer            

Position: top

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

Depth: 450 mm

Hydraulic conductivity: 180 mm/h (saturated)

Composition: 80% Loamy Sand, 10% vermiculite and 10% perlite, by volume, evenly mixed. A layer of recycled rock aggregate is added to the top to protect the filtration layer.

Transition layer

Position: middle

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

Depth: 100 mm

Composition: course sand.

Drainage layer

Position: bottom

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

Depth: 100 mm

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

Two 90 mm slotted drainage pipes are set within the drainage layer to allow excess water to flow back into the stormwater network. The pipes have a riser that allows for the flush out of any accumulated sediments. The drainage pipes connect in to the overflow pits. This allows for easy inspection of their function during rain events.

The plant modules are 500 mm x 500 mm x 100 mm deep with a well-established root matrix that holds together the sandy soil. These modules are installed like tiles close to one another across the level surface of the filter layer.

Species: Juncus gregiflorus and Juncus procerus

Position: planted throughout the raingardens

Purpose: the biofilm on the plant root system removes nutrients from the water as it soaks through the raingarden

Density: six plants per square metre.

In addition to the grasses, the banks were planted with Lomandra longifolia and a tree was planted in one of the raingardens as a feature to tie it in to the surrounding landscape.

Overflow pits have been installed 100 mm above the surface of the raingarden. In heavy rain, excess water will bypass the raingarden and flow directly to the stormwater system.

Maintenance tasks include:

• Replacing the top layer of substrate annually using a suction hose
• Removing weeds from the biofiltration bed and replacing plants as required
• Monitoring detention time for filter media.