Australia: Knox: Rain garden design

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  • 1. 1 Rain garden: design, construction and maintenance recommendations based on a review of existing systems N. Somes 1, M. Potter2, Joe Crosby1, M Pfitzner3 1 Ecodynamics, nick.somes@ecodynamics.com.au 2 Melbourne Water Corporation, matthew.potter@melbournewater.com.au 3 Kingston City Council,In order to better understand factors that contribute to the successful implementation of street scale WaterSensitive Urban Design (WSUD) assessments were undertaken at 22 sites across Melbourne. The sites use avariety of treatment methods with most using Raingardens. The review was undertaken in response to concernsregarding poor plant growth at a number of sites.The inspections found civil works and maintenance (weed and litter control) were generally undertaken well.Some issues identified with civil works include the use of inappropriate mulches and loss of extended detentionstorage due to overfilling of filters. Infiltration rates of filters were quantified via insitu and laboratory methods.Most sites tested had infiltration rates of less than 80 mm/h. Infiltration rates below design specifications resultsin reduced pollutant removal as systems bypass more frequently. Low hydraulic conductivity of the systemsresulted from use of filter material or mulches containing fines which impeded flow. Surface clogging was notevident. Plant growth was variable with poor growth often linked to water logged soils due to soils with verylow hydraulic conductivity. In response to these findings a revised specification for the design andimplementation of rain garden filters was developed.IntroductionSince the early 1990s a number of activities have been undertaken to reduce the impact ofurban stormwater on the environment in greater Melbourne. The water sensitive urban design(WSUD) measures implemented were at the regional scale and included wetlands andsedimentation basins. These treatments are often referred to as end of pipe treatments andtypically treat large catchments (> 60 ha). By the late 1990s attention was also being focusedon the use of treatment measures at the street scale. This approach avoided the need toconstruct regional scale systems by treating runoff at or near source. To date a large numberof street scale systems have been constructed in new areas of development or retrofitted toexisting urban areas.Common street scale treatment measures include swales, bioretention systems, infiltrationsystems and porous paving. Street scale systems are characterised by treating runoff byfiltration through vegetation and infiltration through soil profiles. The most common form ofstreet scale WSUD are bioretention systems, commonly referred to as raingardens. Streetscale treatments face a number of design pressures not present in regional scale measures.Retrofitted systems have further limitations as they must be accommodated into existingstreetscapes and deal with issues such as existing services and limited space.Figure 1 is a typical section of a raingarden showing the key elements and dominant flowpaths. Raingardens treat runoff by storing catchment flows in the extended detention storageand allowing it to infiltrate through a series of layers. To ensure drainage function ismaintained most systems have an overflow which operates when the storage capacity of theextended detention storage is exceeded.BackgroundIn 2006, Kingston City Council and the Better Bays and Waterways Project embarked on ajoint venture to investigate the condition of 22 street scale WSUD devices across Melbourneand review design, construction, landscaping and maintenance practices. The sites had been
  • 2. 2 Ca Extended tc Ru hme Detention Zone no nt ff Overflow Batter Kerb Overflow Infiltration Infiltration Mulch Filter Transition Drainage Figure 1 Raingarden sectiondesigned and constructed in the past five years and most had been retrofitted into existingstreetscapes. A variety of systems had been built, the majority being raingardens. Infiltrationsystems, swales and wetlands had also been constructed. The review was initiated due toconcerns regarding plant growth in a number of systems. The study also provided anopportunity to inform local government of lessons learnt in implementing raingardens, inparticular for retrofitted systems.The study was undertaken during winter of 2006. Site assessments included a review ofavailable documentation, discussions with Council staff associated with individual projectsand site inspections. The objective of the review was to define the current condition of eachsite compared to the design and the maintenance regime.DesignRaingarden design is an area of ongoing development. The earliest projects reviewed weredesigned in 2002. At this point in time, design tools were in their infancy and limited tooverseas guides and based on results of previous projects (Knox City Council, 2002).Melbourne Water produced the WSUD Engineering Procedures: Stormwater (MelbourneWater, 2005) and represented a significant step forward in the design resources available.The improvement in design resources was reflected in design practice.The sites were designed over a 5 year period and reflected the developments in designpractice that occurred over that period. Many of the changes to design practice related to theaesthetics, maintainability and integration into the streetscape. In early systems, some designsdid not differentiate the raingarden from the streetscape, with edges typically battered andplanting areas adjoining grassed areas. These design decisions have resulted in ongoingmaintenance issues, e.g. weed control is required around the raingardens to prevent grassentering the garden bed, in undertaking these works overspray with herbicide has killed bothgrass and the raingarden plants. To overcome this, recent designs have included provisions offeatures such as concrete edge strips to delineate gardens and assist maintenance. Otherdevelopments are the use of steps and retaining walls rather than batters to reduce the arearequired to take up changes in grade.Further improvements can be made in design practice as new design models are pursued toincorporate raingardens into items such as street furniture, traffic calming devices and streettrees. The design process should also include streetscape design and community engagementto increase the rate of stakeholder buy in. Stakeholder involvement is discussed later in thispaper in relation to its ability to reduce maintenance costs.
  • 3. 3Project ImplementationThe findings of the review of sites are summarised in Figure 2, which outlines the number ofsystems which were rated good, moderate or poor across five functional characteristics(Kingston City council et al, 2007). The selected characters represent the key elements ofstreet scale WSUD elements. It should be noted that the infiltration rate data set was based onresults from 12 sites, while other categories were based on results from all 22 sites. Theresults are discussed in detail in the following sections. 100% 90% Good 80% Moderate 70% Proportion of Sites (%) Poor 60% 50% 40% 30% 20% 10% 0% Civil Works Plant Condition Infiltration Rate Weeds Litter Figure 2 Summary of site investigation findings Civil WorksCivil works are the hard works required to construct raingardens, e.g. concrete paving anddrainage works. The results indicate that civil works were generally well constructed. Thecases of moderate civil construction were related to surface grading of the raingarden notbeing in accordance with the design and reducing the extended detention depth. Anotherissue identified with civil construction was the selection of gravel mulches that containedfines, which formed a compacted and relatively impermeable crust over the surface of theraingarden. Other issues included bypass inlets set at wrong heights and inlet structureswhich restricted flow or were prone to blockage.The good rating for civil works is not surprising, as all of the works were constructed by civilconstruction firms with experience in the construction of hard works. Errors were oftenassociated with departures from standard engineering practice. For example, it is commonpractice to fill around pits and back up kerbs. This had occurred in several raingardens andresulted in the loss of the extended detention storage area, which will significantly reduce theeffectiveness of the systems. From discussions with Council staff it was concluded that thesesorts of errors reflected a lack of understanding on behalf of the contractor and Councilsupervisors of the intended function of the systems.Appropriate supervision and knowledge transfer is essential to ensure construction projectsare built to reflect the design. In most of the projects reviewed, implementation of the workswas supervised by Council Officers, whose background was in the supervision of civil works.Typically officers had not been provided with additional training regarding the design andimplementation of WSUD works. Their experience is reflected in the works, in nearly allcases the civil works are constructed to a high standard. However, the landscape works, inparticular filters, have been implemented poorly, with few of the raingardens installed as perthe design. In most cases the poor implementation has been a result of the specification beingpartially adopted or incorrectly adopted. It is considered that appropriate training is needed to
  • 4. 4provide supervisors with the knowledge to ensure works are constructed to designspecifications.At a minimum the following areas of the works should be checked or confirmed as worksproceed: Connections to drainage system, including sub-surface drains, inlets and overflows; Levels of inlet, overflow pit, and extended detention area base (top of mulch layer); All materials used in the construction comply with the specification, in particular hydraulic conductivities and grading; Placement of the filter to design levels; and That any amelioration to filter material has been undertaken in accordance with the recommendations; Plant Selection and HealthA wide variety of plant species had been used in projects reviewed. Plants used includedtrees, shrubs, tussocks and ground covers. Where filters had been implemented correctly, allspecies of plants grew well. Poor plant growth was typically associated with poor filterfunction or water logged soils. In this regard terrestrial plants, e.g. Dianella species, werefound to offer an advantage as they demonstrated poor growth if the filter had low infiltrationrates (refer to Figure 3). The poor growth prompted managers to investigate reasons for thepoor growth, which subsequently led to filter function being identified as being of concern.Where semi-aquatic plants (e.g. Ficinea nodosa or Juncus species) were used in filters withlow infiltration rates, the plants grew well and the poor filter function was not identifiedthrough plant health. It is considered that the use of some plants which prefer well drainedconditions is preferable, as they will indicate poor filter function.It is considered that a wider variety of plants than has been used to date are suitable forraingardens. It is recommended that designers expand their plant palettes to include plantsthat are not normally associated with raingardens. Good plant cover was achieved best wheregroundcovers were included in the plant palette. Filter Specification and ImplementationTo understand filter function better, samples from filters exhibiting poor plant growth andgood plant growth were analysed at the outset of the study to determine horticultural anddrainage properties and compare these to design values. Soil fertility was found to beadequate at both sites, with plant growth not limited by it. Drainage rates were quantified viahydraulic conductivity tests (AS4419, 2003) and were found to be low compared to designvalues. The designs specified hydraulic conductivities in the range of 80 180 mm/h.Samples from the sites were found top have hydraulic conductivities of less than 5 mm/h.. Atthese sites many of the plants were showing signs of being too wet and soils were waterlogged.To better understand filter function across a wide range of sites, 44 in situ infiltration testswere undertaken at 12 sites. Beds were selected to provide a cross section of plant growth,i.e. poor to good, and in some cases multiple infiltration tests were conducted in the sameraingarden. As a result of the sites not being randomly selected and multiple tests in a numberof beds the results should not be considered definitive. However, they provide a usefulinsight into filter function across the sites and are considered representative of all the sitesinvestigated.
  • 5. 5 Figure 3 Example of good and poor growth at adjacent raingardens planted withDianella tasmanica. The hydraulic conductivity of the right hand site was two orders of magnitude lower than the left hand site.The in-situ test method used a steel collar (Figure 4) that was driven into the filter profile tothe drainage layer (Figure 5). A constant head was maintained within the steel tube andtopped up on a regular basis, with the timing and volumes of additions recorded. The testswere conducted by Land and Water Constructions and the Facility for Advancing WaterBiofiltration (FAWB) with results pooled. Please note, the results are described as infiltrationrates not saturated hydraulic conductivities. This description has been adopted as testconditions could not be controlled as they are in the laboratory tests which comply withAS4419. Figure 4 Infiltration tube Figure 5 Infiltration tube installed in a raingardenThe results of the infiltration tests are summarised in Figure 6 and show that more than halfthe tests indicated infiltration rates below 40 mm/h. These results indicate that manyraingardens are not functioning appropriately and will therefore be bypassing some designflows. An assessment of the impact of reducing infiltration rates was made using MUSIC V3.A simple model of a raingarden was developed and run with a range of infiltration rates, withthe results summarised in Figure 7. Figure 7 shows that optimal treatment is achieved atinfiltration rates between 150 and 250 mm/h. Large reductions in removal efficiency occurwhen the infiltration rate drops below 100 mm/h, due to the increased rate of bypass. Theresults suggest that many of the raingardens reviewed would have the design pollutantremoval effectiveness reduced by 10 to 15% due to their low infiltration rates.
  • 6. 6 25 50 Annual Nitrogen Load Reduction (% 20 45 Infiltration tests (number) 15 40 10 35 5 30 0 25 180 0 100 200 300 400 500 Infiltration rate (mm/h) Infiltration Rate (mm/h) Figure 6 Summary of in situ Figure 7 Effect of altering infiltration rates measured hydraulic conductivity on raingarden performanceFilters were frequently specified in the following manner: A material description e.g. sandy loam; A desired hydraulic conductivity, typically in the range of 80 to 180 mm/h; and In some cases, a grading was provided.In many cases the specifications were in conflict or were contradictory. The description ofsoils, e.g. sandy loam, includes a relatively wide grading range and can contain up to 20% ofclay material (AS 4419-2003 Table I1). This proportion of clay material is considered to betoo high to maintain the required hydraulic conductivity. The term sandy loam is also widelyused within the landscape supply industry to indicate a soil with sandy characteristics. Sandyloams have highly variable gradings and properties reflecting the variety of sources andminimal processing that the soils undergo. Many of the filters reviewed had been constructedwith topsoil and had low infiltration rates. The widespread use of topsoil within filters isthought to be due to ambiguous specification and a lack of experience on behalf ofsupervisory and construction contractors.In light of these findings a new specification was developed for raingarden filters. Thespecification is based on Recommendations for the Establishment of Putting Greens(USGA, 2004) and the experience of the authors in the construction of landscape systems.The objective is the provision of a specification that is robust and can be readily implemented.It is acknowledged that it may appear overly prescriptive, but the experience of the review ofexisting systems has demonstrated that a greater degree of prescription is required to achieveappropriate outcomes. The specification is contained in the project report available on linefrom Melbourne Waters website.(http://wsud.melbournewater.com.au/content/technical_reports/technical_reports.asp)The raingarden specification provides guidance on the following layers: Mulch to suppress weeds and retain moisture within the underlying filter media; Filter soil layer which acts as a pollutant filter and supports plant growth; Transition Layer layer to separate filter layer from the drainage layer to avoid clogging of drainage pipe; and Drainage Layer relatively free draining layer containing perforated drainage pipe.Departures from current specifications (Melbourne Water, 2005, FAWB, 2006, Fletcher et al,2006) include the recommendation of mulch in all systems and the use of washed sand as thepreferred filter material. Stone mulch is recommended to improve plant growth by
  • 7. 7suppressing weed growth and maintaining soil moisture. Timber mulches or jute mat are notrecommended as they are not long lasting and not practical for regularly inundated systems.Suitable stone mulches are sized 4 to 13 mm, screened and applied in a 40 - 75 mm layer.Selection of mulch materials is based largely on aesthetic requirements, with a wide variety ofstone sources available.The filter layer is the critical layer in determining the operation of the system and thereforethe greatest care must be taken in sourcing the appropriate materials. In recognition that thematerials need to be controlled, the approach outlined in the specification recommends the useof a washed sand product. This approach has been adopted to limit the amount of fines thatwill be present. The major determinant of suitability is the hydraulic conductivity of thematerial which should be in the range of 100 300 mm/h.As the filter is a washed product it will not support plant growth and will require ameliorationwith a range of organic and inorganic fertilizers and trace elements. These materials areadded as a one off addition during construction. The total nutrient load of this addition isequivalent to the annual nutrient load (N and P) captured within the filter. Subsequentnutrient additions are not usually required and are provided by stormwater runoff.The transition layer and drainage layer have similar specifications to existing guidelines andare not discussed here in detail.Experience with the supply of materials to the projects reviewed and subsequent projectsindicates that supply of materials is not a simple task. When sourcing materials, a qualitycontrol program must be implemented that test materials prior to and following delivery. Forlarge scale projects this requires prospective materials to be stockpiled at their source andtested prior to delivery. Follow up tests of materials delivered to site should also beundertaken to ensure material properties do not vary throughout the delivery process.MaintenanceMaintenance is considered to include management of the following: Aesthetics management of litter and sediment; Horticultural weed control, replanting and re-mulching; Damage repair of accidental damage and vandalism; and Inspections regular inspections of systems to ensure it is functioning.The review of existing systems found that aesthetic and horticultural maintenance wasgenerally done well. Weeds were only a problem where maintenance responsibility at councilhad not been identified. Litter load generally reflected the surrounding areas, with inner cityareas typically having higher litter loads.As part of the project an attempt to quantify the maintenance cost of raingardens was made.The results varied markedly with many Councils not being albe to identify a maintenancecost. Where costs were identified they ranged between $3.80 and $20 per square metre ofraingarden per annum for planted systems. The costs were considered to reflect themaintenance cost of any landscape at that location, not the specific cost of maintaining araingarden. This finding is reflective of most of the maintenance cost being associated withsite visits for aesthetic purposes. For example a high profile park location will require a visiteach week to ensure litter is managed.Cost effective maintenance is aided by effectively constructed systems with good plant coverand edge delineation. Regular maintenance activity coordinated with maintenance of otherpublic landscapes also reduces costs.
  • 8. 8ConclusionsThe review of 22 street scale WSUD sites found that whilst design practice had improvedsignificantly over the past 5 years and implementation was generally well done, opportunitiesfor improvement still existed.The key areas for improvement are in the specification and implementation of filters. To datemany of the filters constructed have been constructed with topsoil as the filter media. Thesematerials contain too high a proportion of fines therefore impeding infiltration. The reducedhydraulic conductivity significantly reduces the pollutant removal effectiveness. Toovercome these issues it is recommended that a revised specification be adopted and thatgreater attention be given to supervision of the works.Other key findings were: Civil works are generally well constructed; A wide range of vegetation types and species are suitable; and Maintenance costs for raingardens should be considered to be of similar magnitude to adjacent landscapes.AcknowledgementsThe project used as a basis for this paper was conceived and funded by the City of Kingstonand the Better Bays and Waterways - Institutionalising Water Sensitive Urban Design andBest Management Practice in Greater Melbourne Project which is supported with fundingthrough the Australian Government Coastal Catchment Initiative. Fieldwork to quantifyhydraulic conductivity was undertaken in partnership with the Facility for Advancing WaterBiofiltration.ReferencesFAWB (Facility for Advancing Water Biofiltration). 2006. Guideline Specifications for SoilMedia in Bioretention Systems.Fletcher, T., Wong, T. and Breen, P. 2006. Buffer Strips, Vegetated Swales and BioretentionSystems Chapter 10 in Australian Runoff Quality A Guide to Water Sensitive UrbanDesign. Engineers Australia.Kingston City Council and Better Bays and Waterways, 2007, Review of Street Scale WSUDin Melbourne, Technical Report Prepared by Land and Water Constructions, available at(http://wsud.melbournewater.com.au/content/technical_reports/technical_reports.asp)Knox City Council, 2002, Water Sensitive Urban Design (WSUD) ImplementationGuidelines, Technical Guidelines published by Knox City Council, available atwww.knox.vic.gov.au, April 2002.Melbourne Water, 2005. WSUD Engineering Procedures: Stormwater. CSIRO Publishing.Melbourne.Standards Australia, (2003), AS4419 Soils for landscaping and garden uses, StandardsAustralia International, Sydney.USGA, 2004, USGA recommendations for a method of putting green construction, UnitedStates Golf Association Green Section Staff, 11 pages, sourced from www.usga.org