Design note 3 - Bioretention

Bioretention assets should be designed in accordance with  the fundamental principles and guidance provided in current version of the  following documents as amended.
  • Adoption Guidelines for Stormwater Biofiltration Systems (CRCWSC, 2015)
  • Draft Design Guide: Bioretention systems in Melbourne Water Development Services Schemes (Melbourne Water, 2019)
  • Melbourne Water’s MUSIC Guidelines, Chapter 12.

The reader is referred to the first two of these for descriptions of what a bioretention asset is and how it works.

The designer should consider whether a bioretention asset is appropriate for the site with reference to Table 9 in Appendix C - When is a Bioretention Asset Appropriate?.

Bioretention assets should be designed and modelled in accordance with the requirements and specifications set out in this guideline unless agreed in writing by the City and this document takes precedence over other guidelines. The City have no obligation to accept any design that does not comply with the following requirements and non-complying designs and models will be considered on a case-by-case basis. The following specific requirements apply:


Design plans and model

  • The asset configuration shown on the design plans must be consistent with the parameters adopted in the MUSIC model.

Flows and velocities

  • Design flows shall be estimated in accordance with methods in the current Australian Rainfall and Runoff.
  • High flows above the 18.13 percent AEP (1 in 5 year ARI) event shall be diverted or the bioretention system protected by ‘feedback control’ to cause high flows to either bypass the asset or enter an overflow in proximity to the inlet after extended detention is filled.
  • Flow velocities should not exceed 1 m/s over the planted filter media area for the 1 percent AEP (1 in 100 year ARI) event.

Design configuration

  • The EDD shall be less than or equal to 300 millimetres.
  • Underdrainage with an invert at a depth of at least 500 millimetres shall be included to ensure the root zone remains aerated.
  • Tree species should be suitable for the wetting and drying conditions expected within a bioretention and deciduous trees should generally be avoided due to significant seasonal leaf drop which may lead to clogging.

Soil moisture and wetting and drying conditions

The following specifications are intended to ensure that soil moisture conditions are suitable to sustain plants:

  • The filter media depth shall be 500-600 millimetres for standard planting and may be 500-800 millimetres for bioretention assets with tree planting. Lesser filter media depths may only be considered for highly constrained sites. Lesser depths will not be accepted where it is possible to achieve a depth of 500 millimetres.
  • A submerged zone is required for all bioretention assets designed within the City and should be included below the outlet invert. The submerged zone must not be less than 300 millimetres and should preferably have a depth of 450-500 millimetres in total (including transition and drainage layer). It is recommended the submerged zone include an appropriate carbon source to support denitrification.
  • The saturated hydraulic conductivity of filter media must be within 100-300 millimetres/hour and it is recomillimetresended it should not exceed 200 millimetres/hour. Hydraulic conductivity should be modelled at 100 millimetres/hour to allow for potential decline in infiltration capacity due to sediment clogging over time.
  • It is essential to ensure bioretention assets retain enough moisture within the filter media, submerged zone and surrounding soils to sustain plants through dry periods. Where any of the above 3 items will be compromised the following is required:
    • Confirm that no other element will be compromised
    • Laboratory testing of the filter media demonstrating the soil moisture retention capacity of the filter media at a root zone depth of 300 millimetres is at least 15 percent.
    • Laboratory testing of the filter media demonstrating that the hydraulic conductivity will be within 100-200 millimetres/hour (if filter media or submerged zone depth reduced)
    • Compensation of loss of soil moisture where practical through:
      • Increased depth of filter media or submerged zone.
      • Choosing plant species with a high level of drought tolerance.
    • A spells analysis indicating the filter media will not experience soil moisture conditions at or below wilting point for more than 20 days within a 10 year period (this can be determined with a spells analysis of soil moisture).
  • The asset must not be subject to continuous or long duration flows.

The design of a bioretention asset should take into consideration the following:

  • Asset dimensions and potential issues with large bioretention assets.
  • Catchment sediment loads and sediment management to minimise clogging risks.
  • Soil moisture conditions.
  • Infiltration and the capacity of underlying soils to absorb and drain away water including soil conditions and any layering, depth to rock, depth to groundwater and slope.
  • Proximity to infrastructure that may be affected by infiltration and provision of adequate offset distances.

Asset dimensions and large bioretention assets

Bioretention assets should be designed within the following dimensional ranges:

  • Maximum recommended cell size 800 square metres.
  • Minimum width 600 millimetres, greater than1 m recommended.
  • Maximum width of 14m where construction access is available from opposite sides.
  • Maximum width of 7m where construction access is available from only one side.
  • Inlets should be designed to ensure inflows will be evenly distributed across the bioretention surface area. This may be achieved using multiple inlets, flush kerbs, weirs, slotted pipe, distribution channels or other methods of distribution where approved. The use of a single large inlet without some form of distribution should be avoided for assets over 100 square metres.

Where a site will have a total surface area of bioretention greater than 800 square metres, serious consideration should be given to preferentially using a wetland and the designer shall be responsible for ensuring the bioretention surface will not be continually or excessively frequently wet by interflow or baseflow leading to excessive algal growth and/or clogging.
For larger catchments, wetlands are generally preferred as they tend to be more resilient, require less maintenance, provide greater amenity and cope better with persistent baseflows that are more likely to occur in larger catchments.


Design to manage sediment loads

Bioretention assets treating larger catchments may receive significant sediment loads that can cause clogging and impact upon treatment performance. These are usually due to:

  • Construction and building activity in the catchment.
  • Sediment inflows to large assets that:
    • Are online to the main flow path.
    • Receive bed load.
    • Receive large event inflows and associated sediment.
  • Fine sediment accumulation in catchments with dispersive clay soils.

Upstream sediment management responses should be designed to minimise sediment inflows to bioretention assets as much as possible. A guide to sediment management responses at different scales is provided in Table 2 and design requirements are provided in Table 3.
Any upstream sediment pond must be designed in accordance with the Draft Melbourne Water Design Guide: Bioretention systems in Melbourne Water Development Services Schemes (Melbourne Water, 2019).
While sediment management tends to target coarse sediment, it is important to understand that it is the fine fraction (2-6 µm) that causes the greatest clogging effect in bioretention assets and this should be considered in the design.

Table 2 : Catchment ranges for coarse sediment forebays and ponds.

Area Coarse sediment management
Roof catchment only less than 2 hectares None
Catchment less than 2 hectares and total filter surface area less than 100 square metres None, where no road runoff received and sediment accumulation at inlet can be regularly assessed and cleared
2 hectares equal to or less than Catchment equal to or less than 5 ha and 100 square metres equal to or less than Total filter surface area equal to or less than 800 square metres Vegetated swale, coarse sediment forebay or coarse sediment pond
Catchment greater than 10 hectares or
Total filter surface area greater than 800 square metres
Coarse sediment pond

Design requirements for coarse sediment forebays and ponds is provided in Table 3.

Table 3 : Design requirements for coarse sediment forebays and ponds.

Area Coarse sediment forebay Coarse sediment pond
Sediment capture Capture 95 percent of coarse particles equal to or greater than 125μm diameter from 4 EY (1 in 3 month ARI) flow
Depth Be equal to or less than300millimetres deep Be equal to or less than 1.6 m deep
Sediment storage Provide adequate sediment storage volume to store 1 year of sediment Provide adequate sediment storage volume to store 5 years of sediment
Velocity - Ensure velocity through sediment pond for 1 percent AEP (1 in 100 year ARI) event is equal to or less than 0.5 m/s
Energy dissipation Provide energy dissipation of incoming flows -
Drainage Be free draining -


Soil moisture conditions

Bioretention assets are expected to have wetting and drying cycles, being wet and even saturated during storm events then allowed to dry out between events. Both aerobic conditions (typically occurring in the filter media root zone) and anaerobic conditions (typically occurring in the submerged zone) are useful for effective nitrogen removal. However, conditions that are too dry or wet for long periods may lead to poor plant health or loss or poor nutrient removal performance and need to be avoided. The design of bioretention assets should aim to minimise the frequency with which complete drying of the filter media occurs to effectively sustain plants and microbes. This includes considering each of the following:

  • Depth and type of filter media and resulting depth of soil water stored.
  • Depth of submerged zone.
  • Catchment to treatment area ratio and frequency of inflows.

The ‘submerged zone’ is intended to provide a reservoir of water that can be accessed by the plants between the events as well as enhance anaerobic denitrification processes that increase overall total nitrogen removal. Either a temporary submerged zone (unlined bioretention in underlying soils with low hydraulic conductivity (less than=36 millimetres/hour) or permanent submerged zone (lined bioretention) in any soils may be used. It is preferable not to line a bioretention asset unless necessary to protect infrastructure or retain moisture.
Specifications to ensure adequate soil moisture conditions are provided above.
As a general guide, conditions that may be considered excessively wet or dry include:

  • Saturated conditions (greater than 80 percent soil moisture) for greater than 5 days at least once in 10 years.
  • Excessively dry conditions (less than11 percent soil moisture) for greater than 35 days at least once in 10 years (this is for Carex Appressa) This may be less for plants that are less drought tolerant and a target of 20 days has been set for Geelong to support a range of planting and minimise loss of biofilms required for treatment. 

Infiltration and proximity to infrastructure

Infiltration to surrounding soils should be encouraged where possible but must be undertaken in combination with an understanding of underlying soil conditions based on desktop data, geotechnical investigations and other information where available. The designer should consider the potential impacts of infiltration and capacity of surrounding soil to absorb infiltrated flows. This includes consideration of:

  • Underlying soil type and capacity to absorb and infiltrate water.
  • Soil conditions where infiltration may be undesirable such as dispersive soils at risk of erosion.
  • Depth to groundwater that may inhibit infiltration.
  • Saline soil or groundwater conditions that may be exacerbated or transmitted through infiltration.
  • Depth to bedrock and capacity of to absorb or transmit flows where relevant.
  • Slope and down-slope infrastructure and conditions.
  • Scale and volume of anticipated infiltration relative to receiving soil environment.

While a partial or full lining may be used where surrounding soils have high infiltration rates (that is: sands), infiltration into soils with low or medium infiltration rates (clays, silts and loams) allow water to be retained within these soils and encourages plant roots to grow into these. This should be seen as complementary and supportive of the submerged zone.

Infiltration can usually be used in combination with a submerged zone without compromising its purpose and function. This is the case where underlying soils have low infiltration rates (for example: clays, silts and loams) and soil moisture storage within the underlying and surrounding soil will be accessible to plants. Where underlying soils are sands or have higher infiltration rates (greater than 3.6 millimetres/hour), a partially or fully lined submerged zone should be adopted to ensure water remains available to plants.

Allowable offset distances for infiltration in proximity to structures should take into consideration the soil type present as outlined in Table 4. For detailed information refer to Australian Runoff Quality (Wong, 2006). The soil types present must be confirmed with a geotechnical report for bioretention assets greater than 100 square metres.

Table 4 : Offset distances

Soil type Soil type hydraulic conductivity (millimetres/h) Offset distance
Deep, confined or unconfined sands (homogenous) equal to or greater than 180 1 metres
Sandy clays (homogenous) 36 to 180 2 metres
Medium clays (homogenous) 3.6 to 36 4 metres
Heavy clays (homogenous) 0.036 to 3.6 5 metres
Constructed clay soils (homogenous) 0.0004 to 0.036 5 metres
Sites with rock or shallow soil over rock 3.6 to 36 2 metres


Construction phase

Bioretention assets must be protected from sediment clogging during construction works within their catchment until at least 90 percent of the catchment is developed. For all greenfield assets, construction should be undertaken in accordance with the Design Guide for Bioretention Systems in Melbourne Water Development Services Schemes (Melbourne Water, 2019).

The proposed construction process, timing and key hold points for construction and inspection must be identified.


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Page last updated: Wednesday, 18 December 2019

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