Looking after all our water needs Looking after all our water needs Water sensitive urban design Biofilters

Summary Main benefits Design factors Target pollutants • They will work in a variety of • Ensure they are integrated into landscape design. • coarse sediment Biofilters (also known as biofiltration climate, soil and groundwater • Consider location as part of planning and design of roads and • suspended solids systems, bioretention systems and conditions. lots. • phosphorus rain gardens) are excavated basins or • Their flexible design (linear, basins, trenches that are filled with porous • Design to infiltrate or connect to the larger stormwater system • nitrogen tree pits, planter boxes) allows them where appropriate. filter media and planted with vegetation to fit into many different locations. • heavy metals to remove pollutants from stormwater • Make the surface area at least 2% of the constructed, directly • Soil and sand filters provide good connected impervious catchment for water quality treatment. runoff. They use natural and physical removal of sediment and heavy • Size the storage volume to suit system hydraulics – ideally a processes to treat stormwater. metals. 1-year critical average recurrence interval event. • Specially selected soil filter media This brochure is part of a series that • Use vegetation appropriate to the climate and desired and appropriate vegetation selection explain various aspects of water pollutant removal. improves nutrient removal rates. sensitive urban design. Please see • Incorporate a submerged zone and carbon source to • They require less space than other Water sensitive urban design in Western promote vegetation health and resilience during dry periods infiltration systems due to their Australia for background information on and aid nitrogen removal. higher infiltration rate. water sensitive urban design. • Consider the impact of potential acid sulfate soils on filter media and/or structures where relevant. Where they can be used in the water sensitive urban design process Treatment train

Safe Land use Source control Runoff control conveyance and planning and discharge

Capture, use Retention, and in ltrate detention and Flood rainfall conveyance management Total water cycle outcomes Appropriate Prevent and Reduce through reduce discharge and location and transmission of reuse pollutants at pollutants good design their source Protect people and buildings Minimise runoff Minimise erosion from ooding

Up to the 1 in 1 Up to the 1 in 5 Up to the 1 in 100 year ARI event year ARI event year ARI event

Design scale

Precinct Public open space biofilter, Roadside biofilter retrofit, Busselton CBD On-lot biofilter, Evermore Heights, Baldivis District Street Lot (subdivision) Meadow Springs, Mandurah Looking after all our water needs Looking after all our water needs Water sensitive urban design Biofilters

Biofilter elements Required reading Design function Design guidelines Adoption guidelines for stormwater biofiltration systems,2009 , Note: Transition and drainage layers may not be Extended detention depth required if direct infiltration of treated Facility for Advancing Water Biofiltration, Monash University, stormwater to surrounding soils is possible. See available at At least 50% of the plants to be effective at nutrient removal. FAWB guidelines for diagrams demonstrating Remainder to be local, native, ephemeral plants e.g. Ficinia nodosa for the most recent advice. submerged zones, outlets, etc. (formerly Isolepis nodosa) Australian runoff quality: a guide to water sensitive urban 2

200–300mm 8–12 plants per m , depending on species design, 2006, Engineers Australia, available at . Sedimentation of primary sediments and metals Construction and establishment guidelines: swales, Stone 100% 4–13mm in size. No fines. Note that wood mulch will bioretention systems and wetlands, 2009, Water by Design, Suppress weeds and retain moisture in underlying filter STONE float under flooded conditions South East Queensland Healthy Waterways Partnership, media MULCH 50mm Brisbane, available at . Sandy loam Guideline specifications for soil media in bioretention systems, Saturated hydraulic conductivity in range 100–300mm/hr Soil filters fine sediments and colloidal particles 2008, Monash University Facility for Advancing Water Clay and silt <3% (<0.05mm) Soil layer for plants to grow in Very fine sand 5–30% (0.05–0.15mm) Biofiltration. Sorption of metals and nutrients by filter particles Fine sand 10–30% (0.15–0.25mm) In situ monitoring of hydraulic conductivity, 2009, Monash Side liner required if in clay sites to prevent contamination Medium to coarse sand 40–60% (0.25–1.0mm) FILTER University Facility for Advancing Water Biofiltration. of filter, and in coarse sandy sites if the native soil has a Coarse sand 7–10% (1.0–2.0mm) Fine gravel <3% (2.0–3.4mm) higher hydraulic conductivity than the filter media Stormwater management manual for Western Australia, 2004– pH 5.5–7.5 Include a carbon source to improve chemical processes in 07, Department of Water, available at .

300–800mm Electrical conductivity <1.2dS/m soil and promote vegetation health At least 3% low nutrient-content organic matter See Section 4.2 of Chapter 9 – Structural controls. Suitable Nutrient sorption and pollutant decomposition by soil and <80mg/kg orthophosphate plant species for use in biofilters in Western Australia are being bacteria <1000mg/kg total nitrogen content researched. Section 4.2 will be updated to reflect the findings Amend soil with 5% mulch and 5% hardwood chips (6mm) by from this research and the Monash University FAWB research. volume

Separates filter layer from drainage layer to avoid clogging Clean, well-graded sand <2% fines and stop transition of the filter media into the drainage Clean, fine gravel or crushed rock 2–5mm and carbon source, if TRANSITION Department of Water layer submerged 168 St Georges Terrace at least 100mm Perforated PVC subsoil pipe (if required) Perth Western Australia Free draining layer containing pipe (if required) DRAINAGE Drainage layer (if there are no subsoil pipes) - should be at least PO Box K822 Perth Subsoil pipe can aid in watertable control and is also re- as deep as the extended detention depth, however this should be

200mm Western Australia 6842 quired in impervious systems to collect and convey water based on individual site characteristics Drainage layer (if subsoil pipes are used) - completely surround the Ph: 08 6364 7600 • Fax: 08 6364 7601 Infiltration from base if applicable subsoil pipe and at least 50mm over pipe www.water.wa.gov.au (Adapted from: Thompson McRobert Edgeloe Group 2008) The department acknowledges the past project contributions of the Leschenault Catchment Council. June 2011 21592_100_06/11