Simplifying the design of bioretention system to minimise complexity, cost and carbon footprint - forebays as a case study

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1 Simplifying the design of bioretention system to minimise complexity, cost and carbon footprint - forebays as a case study Paul Dubowski 1 and Brad Dalrymple 1 1 BMT WBM ABSTRACT Adherence to the fundamental principles of sustainable and liveable cities is crucial to their realisation. Within these principles are a clearly articulated and widely accepted set of water sensitive urban design (WSUD) principles. Stormwater quality management practice is however, often limited to meeting stormwater quality objectives without due consideration of the broader liveable and sustainable cities principles. When these broader objectives are overlooked during design, this can result in either missed opportunities or at worst, outcomes which directly contradict the broader objectives. The structural elements of bioretention systems, such as coarse sediment forebays for example, could potentially be unnecessarily adding to the cost, complexity, carbon and ecological footprints of bioretention systems. In doing so they could be conflicting with objectives such as delivering affordable housing and sustainable solutions. To understand if the benefits of forebays outweigh these costs, their value has been assessed by examining the rational and functional roles of forebays. By integrating forebays, it is generally accepted that the commonly accepted risks from concentrated coarse sediment at the inlet (smothering of plants and limiting infiltration), will be mitigated. In doing so it is further expected that the lifespan of bioretention systems will be prolonged and, by providing easily accessible maintenance access, maintenance reduced. After assessing eight field bioretention systems in South East Queensland, we found limited evidence to support the commonly accepted risks and any benefits would appear to be outweighed by costs. We suggest seven strategies to achieve more simplified and sustainable bioretention systems ranging from changes in the way guidelines are written, to design changes and further research. These recommendations are presented with the view to improve the way bioretention design responds to broader WSUD objectives. INTRODUCTION Water sensitive urban design (WSUD) is an important element of creating and sustaining liveable cities (Roux and Stanley, 2010). While liveability is synonymous with and in many ways dependent upon delivering affordable housing, housing affordability is not given sufficient consideration in the conceptualisation of stormwater quality treatment systems. This applies to many systems that are purportedly designed in accordance with WSUD principles. This disparity between theory and practice is in part associated with the focus of many WSUD policies and guidelines on achieving designs that meet stormwater quality objectives, rather than on 1

2 ensuring that design is responsive to WSUD principles. It can also be attributed however, to the misuse of best practice guidelines by practitioners (including designers and assessment officers), applying textbook, engineered solutions rather than responding to design principles. Out of the 12 Key Principles of WSUD, presented by the National Water Commission (NWC) (NWC, 2012), one of the most important principles in delivering on the affordable housing agenda is to ensure WSUD adds value while minimising development costs. The hard engineering components of bioretention systems recommended by current technical design guidelines e.g. coarse sediment forebays (hereafter referred to simply as forebays ), overflow pits and piped drainage, can add significant cost to these systems. Yet there is a lack of strong documented evidence supporting their need and little discussion on alternative solutions. Current technical design guidelines may therefore potentially be directing practitioners towards designs which may not be providing the expected value. Of more concern however, is the possibility that some of these structural elements may unnecessarily be: adding complexity to design and assessment; adding design, assessment and construction costs; contributing to a greater carbon footprint; and contributing (albeit slightly) to changes in receiving water chemistry. Each of these points is contrary to the principle of adding value while minimising development costs. Each of these points are further discussed below. Complexity and Costs The assessment of stormwater quality devices adds to the cost of the design and development assessment process for Councils and developers. This includes costs associated with: designing treatment systems; assessment of designs; and development holding costs. Where reassessment and redesign is required, further costs are incurred. A recent investigation in South East Queensland (SEQ) (SEQ Council of Mayors, 2012), found that: there was a 75% chance of receiving an information request for a stormwater quality - related item in operational works application; and the average timeframes for WSUD operational works application to be approved was 88 days (including both assessment time and time taken by the developer/consultant to respond to information requests). This would suggest that a high proportion of stormwater quality treatment systems require redesign and reassessment. The reason for this is unclear but overly complex bioretention design and inadequate practitioner capacity may be contributing factors. Given that the design of the structural elements of bioretention systems (e.g. forebays, overflow pits, drainage pipes), is typically more involved than design of other components, the complexity of these elements may be disproportionately adding to design and assessment timeframes and costs. Simplifying the design of structural elements of bioretention systems therefore has the potential to reduce these timeframes and costs. 2

3 By reducing the actual structural components required, a range of other capital costs could also be reduced including costs associated with construction, materials 1, land and holding costs. If the need for structural elements of bioretention systems could be reduced or at least simplified, these costs (which are typically passed onto residents through house/land costs and rates), could also be reduced. Carbon Footprint Moore and Hunt (2012) recently found that for bioretention systems in North Carolina, material transport and construction dominate the greatest proportion of the bioretention carbon footprint 2. Bioretention systems in Australia however are likely to have a higher proportion of embodied carbon compared to those studied by Moore and Hunt 34. This is due to the proportionally larger forebays in some Australian systems and use of concrete as the building material (as opposed to rock used in North Carolina). This is attributed to: the extraction and transport of raw materials for concrete (aggregate, cement materials and water) used on forebay construction; concrete batching required to create concrete used in forebay construction; and transport of concrete used in forebay construction to the construction site. If there was evidence that concrete forebays were not delivering their expected values, forebays could be unnecessarily adding to the carbon footprint of local bioretention systems. The potential for bioretention systems to act as carbon sinks could therefore also be impaired, limiting their sustainability credentials. Urban Stream Syndrome The urban stream syndrome first introduced by Meyer et al (2005), describes the physicochemical and biological degradation of streams associated with the urbanisation of natural catchments. The use of concrete in urban drainage systems has been shown to contribute to urban stream syndrome by multiple authors. Wright et al (2011), for example theorised that the chemistry of urban streams is modified by the leaching of minerals from concrete and tested this hypothesis by examining a range of chemical variables from urban and reference freshwater streams in Sydney. They found changes in water chemistry and attributed these changes to the dissolution of cement products from various concrete materials used in urban drainage. They concluded that: the use of concrete particularly its application for urban drainage is responsible for some of the modifications to urban stream geochemistry. 1 The approximate cost of a forebay with trash rack such as the one shown in 4, 6 and 8 of Figure 1, is $11,500. A forebay sized in accordance with Water by Design 2006 would generally be larger than these systems but without a trash rack. 2 Material transport accounted for 50% (or 2.3 t C) of the bioretention initial carbon footprint. 3 More and Hunt studied designs based on North Carolina Division of Water Quality (2007). 4 Moore and Hunt found that the total carbon footprint of north Carolina bioretention systems was 6.4 kg C/m -2 3

4 A similar study by Davies et al (2010), looked at the influence of both concrete and PVC on urban stormwater chemistry. This study produced comparable results to those of Wright et al with the authors stating that there was a significant influence on water chemistry, from these products. It is acknowledged that forebays constructed from concrete would have only a marginal effect on receiving water chemistry. This is particularly the case when considering the size of contributing catchments and the volume of water which passes over forebays. Further, there would be a significantly greater influence on water chemistry from extensive drainage networks which are dominated by concrete pipes and concrete kerb and channel. Nevertheless, the results from both Wright et al (2011) and Davies et al (2010), highlight the need to pay greater attention to the materials used in the conveyance and treatment of stormwater. Considering the sustainability principles of WSUD again, this is particularly pertinent for stormwater quality systems specifically designed to minimise the deterioration of water chemistry i.e. given the evidence that concrete contributes to urban stream syndrome, alternative materials should be preferred. Forebays case study Given the issues described above, concrete forebays have been considered further to determine whether they perform the functional roles for which they are intended to serve and whether potential benefits would outweigh these costs. This assessment has been undertaken in the context of attempting to more closely align the design of bioretention systems with broader liveability and sustainability objectives for WSUD. METHODOLOGY Phase 1: Desktop assessment To provide a more comprehensive context to this study, a review was undertaken of local, regional, state and national guidelines as well as standard drawings. These documents were interrogated to: 1. identify the origin of forebays including when, where and how the concept of forebays first originated; 2. understand how wide-spread the use of forebays is nationally; 3. clarify the rationale behind forebays use; and 4. check whether there was any documented evidence supporting the use of forebays. A number of people involved in the drafting of the first guideline to present forebays were also interviewed to gain a more in-depth perspective of the issues noted above (pers. comm. T. Weber, 2012 and pers. comm. S. Leinster, 2012). The literature review was also expanded to journal articles and conference proceedings when considering point 4 above. Given that WSUD is a rapidly evolving field in Australia, literature material is often supported by online forums where best practice is regularly debated and clarifications are sought/offered for uncertainty in guidelines. The literature review was therefore also extended to the Water by Design forum (Water by Design, 2012b), to account for broader industry opinion, particularly to clarify the rationale behind forebays use. 4

5 Phase 2: Field Study Based on the Phase 1 investigations, an assessment of eight bioretention systems was undertaken. Each of these systems was located within a fully established catchment and operating for at least three years. Systems 1 and 3 are streetscape retrofit projects while System 2 is a large, end-of-pipe bioretention system broken up into three hydraulically separate cells (results from only one of these cells is discussed in this paper). All the other systems were typical end-of-pipe bioretention systems located in parkland/drainage reserves. The aim of the field investigation was to assess whether bioretention systems without forebays were experiencing the expected operational issues and therefore whether systems without forebays are functioning as designed. Only systems which do not have the type of forebays recommended by local guidelines were therefore assessed. Figure 1 shows the inlet treatment of each system and Table 1 provides a description of each inlet treatment. At each of these systems, the following assessment was undertaken: a measure of the area and volume of sediment accumulated in the inlet zone 5 ; a measure of the depth of sediment (spot checks) accumulated across the bioretention filter surface; a measure of the hydraulic conductivity at the inlet and outlet zones of the bioretention systems. Testing was undertaken in accordance with nationally-accepted guidelines for measuring bioretention hydraulic conductivity (FAWB, 2008); a qualitative description of soil at the inlet compared to spot locations across the filter media surface; an estimate of plant density at spot locations across the filter media surface and at the inlet zone; an estimate of the area of plants affected by accumulated sediment; and a qualitative description of plant health and condition at the inlet compared to spot locations across the filter media surface. photographs taken of the bioretention inlets zones and comparison of inlet zone conditions versus outlet zone conditions. 5 Although the volume of sediment accumulated would be subject to maintenance frequency, sediment was not being removed by asset owners at any of the systems assessed. 5

6 Figure 1 Bioretention Inlet Treatment 6

7 Table 1 provides a summary of the bioretention systems examined. Each of the systems assessed is located within an established low density residential area. Commercial and industrial land uses were not assessed as the higher impervious area would be expected to produce lower sediment loads compared to the residential catchment assessed. Table 1 Summary of Bioretention Systems Assessed (Queensland) System ID Location Age Bioretention filter surface Vegetation Inlet Description (years) area (m²) Melaleuca quinquenervia and Lomandra Short apron extended between wing walls. No 1 Hoyland Street, Bracken Ridge, Brisbane longifolia (originally planted). Allocasuarina sp and Pittosporuum sediment holding capacity. revolutum (colonised). 2 Tilley Road, Wakerley, Brisbane Variety of trees (e.g. eucalypts), and smaller shrubs/ grasses/ groundcover. Short apron extended between wing walls. sediment holding capacity. No 3 Streisand Drive, McDowall, Brisbane 6 20 Diannella spp., Ophiopogon japonicas, Lomandra spp. Appropriately sized forebay although design does not meet guidelines. Limited sediment holding capacity. Carex appressa, Isolepsis nodosa, Extended concrete apron with side ramps and trash Lomandra longifolia, and Juncus usitatus rack. Small wet basin located immediately downstream 4 Promenade Stage 5, Springfield Lakes, QLD of trash rack. Some sediment holding capacity particularly on side ramps and wet basin although high velocities likely to remobilise some sediment onto filter media surface. Isolepsis nodosa, Carex appressa, Juncas Small rock-based inlet pond. Some sediment holding 5 Lakes Entrance Stage 7, Springfield Lakes, QLD usitatus, Cyperus polystachyos, Leptospermum polygalifolium, Melaleuca capacity although high velocities likely to remobilise sediment onto filter media. sp. Isolepsis nodosa, Carex appressa, Juncas Extended concrete apron with trash rack and side 6 Lakes Entrance Stage 12, Springfield Lakes, QLD usitatus, Cyperus polystachyos, Jacksonia scoparia, Leptospermum polygalifolium ramp. Limited sediment holding capacity on side ramps although high velocities likely to remobilise sediment onto filter media. 7 Springfield-Greenbank Arterial 1, Springfield Lakes, QLD Isolepsis nodosa, Gahnia sieberiana, Carex appressa, Juncas usitatus and Cyperus polystachyos Extended concrete apron with trash rack and side ramp. Some sediment holding capacity particularly on side ramp although high velocities likely to remobilise sediment onto filter media. Lomandra longifolia, and Juncus usitatus Extended concrete apron with trash rack and side 8 Creekside Stage 2, Springfield Lakes, QLD and Cyperus sp. ramp. Some sediment holding capacity particularly on side ramp although high velocities likely to remobilise sediment onto filter media. 7

8 RESULTS Phase 1 results The first Australian guidelines which provided comprehensive design guidance for bioretention systems was WSUD Engineering Procedures: Stormwater (Melbourne Water, 2005). This document did not include any bioretention forebay sizing or design advice 6. Soon after the release of the Melbourne guidelines, they were adapted by the Brisbane City Council (BCC, 2005) and Gold Coast Council (GCCC, 2005). Neither of these documents specifically promoted coarse sediment forebays for bioretention systems nor provided design guidance for such systems 7. In June 2006, the BCC guidelines were then further adapted for the South East Queensland region by the Water by Design capacity building program of the Moreton Bay Waterways and Catchments Partnership (now Healthy Waterways) i.e. the WSUD Technical Design Guidelines for South East Queensland (Water by Design, 2006). These guidelines present the first documented requirement for forebays stating that: where stormwater runoff from the catchment is delivered directly to the bioretention basin without any coarse sediment management (through vegetated swale or buffer treatment) a coarse sediment forebay is to be included in the design. The guidelines then go on to discuss design and include forebay sizing calculations to manage coarse sediment, defined by the guidelines as sediment particles 1 mm or greater. As the Water by Design guidelines are the most comprehensive and user-friendly technical design guidelines for stormwater quality treatment systems available (in the author s opinion), it is not surprising that these guidelines are being adopted by multiple authorities across Australia. Figure 2 provides a visual representation showing regions which have either adapted the guidelines to suit local conditions (e.g. Northern Territory and Townsville City Council), referenced the guidelines directly in regulatory documentation (e.g. most SEQ Councils), or promote the guidelines through advisory documents and on their websites (e.g. Sydney Metropolitan Catchment Management Authority/WSUD in Sydney Program). The figure is based on the review of state policy, guidelines published by regional bodies (e.g. catchment management authorities in NSW) and local policy/guidelines 8. 6 It is acknowledged that many other earlier guidelines (e.g. Planning and Management Guidelines for Water Sensitive Urban (Residential) Design, 1994), included design guidance on infiltration systems which are similar to bioretention systems. Given that these systems were not formally recognised as bioretention at the time and were often missing some crucial elements of modern bioretention (e.g. planting), these earlier guidelines are not acknowledged here as the original source of contemporary bioretention design guidance. 7 Many early guidelines such as (GCCC, 2002) and (Melbourne Water 2005) included some discussion on the pretreatment of sediment and some guidelines even mentioned forebays as a pretreatment option. Given that none of these earlier guidelines provided specific sizing or design advice, these were not specifically acknowledged here as the origin of current forebay design guidance. 8 This figure is indicative only. North and mid coast of New South Wales and Sydney areas are approximate and not intended to reflect local government boundaries directly. 8

9 Figure 2 Regions that have Adopted the Water by Design Guidelines From Figure 2 it can be seen that that the use of these guidelines (and therefore implementation of forebays), is widespread across the eastern seaboard and the Northern Territory. Notably, assessment authorities in Melbourne and South Australian typically refer to the Melbourne Water guidelines (2005) while Tasmania has adapted Melbourne s guidelines to suit local condition and integrate new science/best practice. Western Australia relies on its own guidelines (Department of Environment, 2004). The Water by Design guidelines provide guidance for coarse sediment forebays but do not clarify how the concept of forebays first originated. The issue was raised with the guideline authors, one of whom clarified that the inclusion of forebays stemmed from observations from some of the first moderate to large scale bioretention systems in SEQ (pers. comm. S. Leinster, 2012). These systems were observed to be accumulating sediment at the inlet zone. Within the zones where sediment had accumulated, a marked decline in vegetation condition was also observed (i.e. smothering of vegetation and ingress of weed into coarse sediment). Around the same time these observations were made, maintenance crews responsible for these systems had also conveyed to the authors the desire provide easy maintenance of coarse sediment at the basin inlet (pers. comm. Leinster, 2012). In summary, the rationale behind including forebays in the guidelines, was therefore based on observations from sediment accumulating in a series of early moderate to large scale bioretention systems and; 1. an observed decline in the condition of vegetation thought to be associated with the accumulated coarse sediment; 2. the need to design more-maintenance friendly systems; and 3. the need to provide scour protection at piped inflows to bioretention systems. 9

10 Further, the guidelines state that coarse sediment has the potential to reduce infiltration of the filter media, providing a fourth driver; 4. the need to minimise the impact from accumulated coarse sediment on infiltration to the filter media. Experience with the application of coarse sediment forebays since Water by Design (2006), has prompted change in the way forebays are considered for bioretention systems. Water by Design are soon to release the Bioretention Technical Design Guideline (Water by Design, 2012a), which will supersede all previous bioretention references in Water by Design (2006) (pers. comm. J. Mullaly, 2012). The draft of this document adopts the approach to forebay shown in Table 2 (pers. comm. S. Leinster, 2012). Table 2 Recommended coarse sediment removal in the draft Bioretention Technical Design Guideline (Water by Design, 2012a) Roof runoff only Catchment scenario Catchment 0.5 ha (streetscape, civic space, allotment) Catchment > 0.5 ha and 5 ha Catchment > 5 ha Coarse sediment removal methods None None* Vegetated swale, coarse sediment forebay or inlet pond Inlet pond * Sediment accumulation at inflow point should be regularly assessed and accumulated sediment cleared if it is blocking inlet and/or impeding infiltration The functional role of forebays stated in the 2006 guidelines is to reduce the risk of coarse sediment smothering vegetation and limiting infiltration to the filter media. The aim of forebays was later elaborated upon on the Water by Design online forum (Water by Design, 2012b). Forum posts stated that the objective of forebays was to prolong the lifespan of the bioretention system (by protecting vegetation and filter media) and to minimise maintenance costs (by providing an easily accessible place for coarse sediment to be captured and removed). The review of journal articles and conference proceedings failed to identify any further documented evidence supporting the stated functional role of bioretention forebays. Phase 2 results A summary of the results from the field investigation described in the methodology are provided in Table 3 below. The photographs taken of the bioretention inlets are provided in Figure 1 while Figure 3 provides photographs comparing inlet zone conditions against and outlet zone conditions. 10

11 Table 3 Phase 2 Results Summary System ID Estimated volume of accumulated sediment in inlet zone (m 3 ) Estimated depth of sediment across filter media (mm) Hydraulic conductivity measured at inlet zone (mm/h) Hydraulic conductivity measured at outlet zone (mm/h) Accumulated sediment composition at inlet (qualitative description) Coarse sand/gravel which appeared to be road-asphalt. Also, geotextile had lifted and been pushed aside None observed No coarse sediment, just 10-15mm layer of slimy organics then straight into filter media Layer of sandy loam high in organic matter None observed mm layer of fine sediment across entire surface of filter media. No obvious coarse sediment on filter media. Coarse sediment present only in apron with weed growth covering sediment slugs mm (fine Thin broken organic crust over sand. sediment/organ 3mm ic matrix) mm (fine sediment/organ ic matrix) 70 >90% of filter covered with fine sediment/organic matrix but not coarse sediment - 3 mm None observed Layer of deposited leaves then dark sandy loam 30mm thick, then netting then sandy filter media from thereon down. Organic matter had been worked through the top layers and into filter media by bioturbators None observed mm slimy organic layer on surface, 30 mm mix of organic matter/sand, 30 mm sandy filter media then into dark mix of silt/clay/sand at the bottom. Average plant density across filter surface Average plant density at inlet zone Area of plants effected by accumulated sediment (m 2 ) Qualitative assessment of plant health 1-2 m (weed grasses) 7.2 At inlet, original plants scoured away replaced by thick weedy grasses. Beyond that lay a long area of deposited coarse sediment that had deposited as the result of that area being scoured by excessive velocities. Plants in remainder of system in good health Generally 1-4/m 2 Generally 8/m 2 with layer of N/A with layer of dead grass (appears to dead grass (appears to be poisoned) amongst trees be poisoned) 1/m2. amongst trees 1/m 2 5/m 2 No plants over 2.28m 2 area at inlet did not appear due to smothering (presumed stolen). Next to bare areas, plants at 4-5/m 2 appear to have been replanted and much smaller than those in the middle of the system. N/A Scoured area of the inlet missing plants. Large swathe of vegetation appears to be poisoned (herbicide application of nearby weeds). Water ponding in scoured part of inlet after recent rain. Plants in wet areas in better condition compared to elsewhere in this system Plants near inlet missing (presumed stolen). Middle of system plants are healthy. Plants at outlet slightly poorer condition compared to middle 8 to 10/m 2 8 to 10/m 2 N/A Plants very healthy across entire surface of filter media. No obvious difference between plants at inlet and those near the furthest point from inlet 6/m 2 6/m 2 N/A Plants generally in good condition however those plants nearest the inlet are the healthiest 9/m 2 6/m 2 N/A Plants generally in good condition. No significant difference across system 7/m 2 5/m 2 (some scour) ~1m 2 No original plants in scoured area at inlet zone. Elsewhere plants all in good condition apart from a dead zone associated with shape of system where plants not receiving sufficient water Inlet and 2/3 of system >12 /m 2. Last 1/3 of system near outlet ~5 /m 2 >12/m 2 N/A Plants near inlet are much larger and of higher density than those near the outlet. Plants at outlet smaller, less dense and in poorer condition 11

12 System 3 System 1 Scoured area where coarse deposition has occurred (dominated by road asphalt). Large swathe of vegetation killed off (mostly likely by herbicide application). No evidence that this is associated with sediment accumulation. System 4 System 2 Vegetation missing at inlet (presumed stolen). Wet basin downstream of concrete apron. Rocks visible on base of basin with limited sediment accumulation. Plants in good condition immediately downstream. Figure 3 Images Comparing Vegetation at Inlet to Typical Vegetation Across Remainder of Filter Surface 12

13 System 8 System 7 System 6 System 5 Wet forebay downstream of inlet. Rocks visible on base with some sediment accumulation on sides. Plants in good condition immediately downstream. Saturated inlet in this system affected hydraulic conductivity results. Inlet plants scoured allowing sediment deposition. Saturated inlet in this system affected hydraulic conductivity results. Figure 4 cont... Images Comparing Vegetation at Inlet to Typical Vegetation Across Remainder of Filter Surface 13

14 DISCUSSION Coarse sediment forebays were first conceptualised in Water by Design, 2006 and since that time there has been widespread acceptance and implementation of the guidelines particularly in populous regions of the eastern seaboard and Northern Territory (see Figure 2). The rationale behind including forebays in these guidelines was based on some empirical evidence for coarse sediment accumulation at bioretention inlets and requests from a maintenance crew to provide an easy maintenance solution. There was however a lack of any documented evidence to support the use of forebays and experience in bioretention design was still very much in its early stages. The functional roles stated in the guidelines and underlying objectives stated online, are discussed further below in the context of our results. Coarse sediment smothering vegetation The vegetation located at the inlet of bioretention systems 1, 3 and 7 was lower in density and of poorer condition compared to the remainder of vegetation across the filter media surface. There was no obvious difference in vegetation between the inlet and outlet zones of the remainder of the systems. System 2 however, was impacted by what appeared to be an over-zealous application of herbicide as part of weed maintenance activities, but there was no evidence of coarse sediment accumulation. The coarse sediment which had accumulated at the inlet zone of systems 1 and 7 in particular, may have contributed to poor plant densities and condition at the inlet zone. Unfavourable growing conditions associated with high velocities however, appeared to be the more likely cause of the impacts observed. This was particularly likely were it appeared that plants had been first uprooted, filter media scoured and sediment later deposited in the scoured area (systems 1, 2 and 7). Our results may therefore indicate a limited relationship between poor plant condition and coarse sediment accumulation. High velocities however, appear to be a dominant factor in plant condition at the inlet. We believe that the uprooting of plants and scouring resulting from excessive velocities is potentially acting as a catalyst for sediment deposition at the inlet. If this is the case, it is feasible that with suitable velocity control, the concentration of sediment at the inlet would only be slightly greater (due to more regular flows), than accumulation across the remainder of the filter surface. If this is the case, the priority should therefore be on velocity control and sediment distribution rather than on inlet sediment capture. We noted also that plant condition in systems 3, 5 and 8 was actually better at the inlet zone. We also attribute this to more frequent wetting from small, regular rainfall events which do not generate sufficient runoff to wet the entire filter media surface. Coarse sediment reducing infiltration to the filter media For systems 2, 3 and 4 the hydraulic conductivity compared between the inlets and outlets zones appeared to be within natural variation i.e. there was no significant difference in hydraulic conductivity. In systems 1, 2, and 5 the hydraulic conductivity was actually higher at the inlet zone although the reasons for this remain unclear. 14

15 The only systems where inlet hydraulic conductivity was substantially lower (compared to the outlet zone), was in systems 6, 7 and 8. At each of these systems, a layer of organic matter and sand or sandy/loamy material was observed. The particle size of this deposited media was typical of the largest particles found in the underlying filter media and therefore cannot help to explain the lower hydraulic conductivity measured. For system 7 the lower hydraulic conductivity at the inlet would more likely be attributed to a lower plant density however, it should be noted that hydraulic conductivity was still at the design level (~200 mm/hour). Interestingly, across the surface of this system there was a relatively even distribution of coarse sediments (30 mm of accumulated sandy deposits) and a dense layer of decomposing organic matter (mostly eucalypt leaves from directly adjacent bushland). The organic matter of this natural mulch/sediment matrix appeared to be very high and well mixed through the sandy deposits. This formed what appeared to be a well-drained loamy-sand, typical of the type of media desirable for bioretention filter media. The well-mixed nature of this matrix is no doubt attributed to the high density of bioturbators (worms and grubs), observed to be working through these top layers and into filter media. Moisture appeared to be sufficiently high in this layer to maintain a healthy population of bioturbators. These bioturbators, attracted by the mulch/sediment matrix, may have been a contributing factor to the very high hydraulic conductivity observed across this system (almost 700mm/hour). They may also have been assisting in mitigating any impact of sediment deposition on hydraulic conductivity at the inlet zone and potentially improving pollutant removal indirectly (by maintaining hydraulic conductivity). Although there are no known local studies on the relationship between bioturbators and the hydraulic conductivity, biogeochemical processes or pollutant dynamics in stormwater treatment systems, international research has shown direct positive relationships (see for example Nogaro and Mermillod-Blondin, 2009). The issue requires further investigation locally to better inform bioretention design. As for the other systems with lower inlet hydraulic conductivity (systems 6 and 8), these systems were significantly more saturated at the inlet due to recent minor rain events which wetted the inlet zones. This no doubt, contributed in the difference in hydraulic conductivity observed in these systems and may have been the dominant factor affecting results. We believe therefore, that our results provide limited (if any) evidence to support the statement that coarse sediment impacts infiltration to bioretention filter media. Prolonging the lifespan of the bioretention systems The systems we assessed had limited coarse sediment pre-treatment and despite this fact, the vegetation and filter media did not appear to be adversely affected by coarse sediment. Those systems which did have some form of pre-treatment (e.g. systems which 4-8 had extended concrete aprons), were not being actively maintained by asset owners for coarse sediment. Any sediment which had accumulated had therefore either been resuspended and deposited in the filter media or stabilised with weeds. Any additional sediment capture was therefore limited and these concrete aprons were probably not playing an active role in sediment removal. The benefit of these aprons in sediment removal was therefore negligible and it could not be argued that impacts to vegetation of infiltration from coarse sediment deposition, were being effectively mitigated by these concrete aprons. 15

16 The most significant benefit of the aprons to plants and filter media at the inlet zones, was velocity control. High velocities have the potential to scour filter media, uproot plants and create otherwise undesirable conditions for plant survival. By minimising (or altogether preventing), these impacts, velocity control in these systems would be expected to prolong the lifespan of the bioretention systems. The impact of excessive velocities (scouring and plant uprooting/damage), was most pronounced in systems 1, 2 and 7 which also had the most limited form of pre-treatment. The results do not support that there is a risk of coarse sediment smothering plants or that coarse sediment limits infiltration to filter media. Impacts on filter media and plants were however observed from excessive velocity and therefore velocity control may be a more effective method to prolong the lifespan of the bioretention systems. Minimising maintenance cost A concrete sediment forebay does create a zone which can be more readily cleaned out by machinery. If the maintenance protocol for coarse sediment removal is however to use shovels, the need for forebays is questionable from a maintenance perspective. From a cost perspective, the number of maintenance events undertaken manually is far greater than the frequency afforded by machinery. So if the principle of adding value while minimising development costs is to be applied to maintenance activities, preference should be given to maintenance methods which provide the greatest value and the least cost. In this sense, manual removal is that option and therefore we contend that forebays would not be required strictly from a maintenance perspective. From a practical perspective, it is feasible that some maintenance crews may lack the will to undertake manual labour particularly if the option to influence machinery-friendly design is a viable alternative. Pure economics however, should prevail in this decision making process. Creating a designated zone for maintenance crews to clean out sediment is only logical therefore, if: the value of cleaning out sediment is proven; the efficiency of forebays is proven; and the design of the forebay is consistent with the asset owner s maintenance protocols and budget. Given neither the value of cleaning out coarse sediment or the efficiency of forebays are proven, it is difficult to argue the case for forebays on ease of maintenance alone. This is especially the case given the higher costs associated with removal of sediment by machinery. Although the asset owner s maintenance protocols should be considered in design, they should not dictate design at the expense of broader objectives. If creating a concrete forebay for heavy machinery is contrary to other more important objectives therefore, it is the maintenance protocols that should change rather than design. Are coarse sediments a problem in bioretention systems? In systems 5 and 6 where there was no mulch, a 3mm fine sediment and biofilm crust had formed. This layer could feasibly have influenced the hydraulic conductivity of the filter media although the particle size was too small to be effectively captured by forebays designed to capture coarse particles (1mm or greater). Retaining a mulch layer may help to avoid such crusts while dense planting has been shown to assist in improving hydraulic conductivity (Hatt et al, 2009). 16

17 In other systems where mulch was present, coarse sediment which had settled on top of the media appeared to have been mixed in with organic matter and through the top of the filter media. This was most pronounced in system 7 where the well-drained media and high organic mulch had attracted a high population of bioturbators which were observed to be mixing the layers. Coarse sediment spread over the surface of these systems did not appear to negatively impact the functioning of the bioretention systems in any way and may not be a problem in the long term either. A 1 Ha catchment for example with a 100m 2 bioretention basin would experience 6mm/yr of sediment build up 9. Plants can certainly adapt to this loading rate but a 300mm extended detention would be filled after 50 years. What is not considered in such simple calculations however, is the removal of sediment through maintenance and the impact of compaction and washout where the incoming coarse sediment may in fact, be replenishing the filter media (and potentially prolonging its lifespan). The sediment balance of bioretention systems over the long term is however poorly understood. Similarly, the impact of retaining this coarse sediment on the sediment balance of receiving waterways requires further investigation. In any case, two bioretention systems which are identical but one of which has 150mm less extended detention due to coarse sediment accumulation, would have very similar pollutant removal capacity and deliver the same aesthetic, urban cooling and social benefits. So after 25 years of coarse sediment accumulation, the system will still be functioning as intended. After years the system may no longer be performing to design capacity in terms of pollutant removal but would still be providing important ecological and social benefits. After this time period however, the bioretention system would have served its purpose and more advanced ways to manage pollutants (both at local and catchment-wide scale), would no doubt have been developed. Community attitudes may also have changed and the highest and best use for that land may need to be re-evaluated In any case, the management of sediment in bioretention systems should focus more on ensuring sediment is evenly distributed across the filter media rather than being captured at the inlet. This would also assist in minimising the long term maintenance costs by minimising the need to remove sediment removal at the inlet. This should be done in conjunction with better velocity control which can be achieved more effectively by installing inlet scour protection (see for example Section of Water by Design, 2006). Practitioners are also encouraged to reconsider the value of less costly/engineered solutions. This includes pre-treatment solutions (such as vegetated swales and buffers), that are noted in numerous guidelines but which have in recent years been given much less preference compared to forebays. CONCLUSION The demonstrated need for the structural elements of bioretention systems, including coarse sediment forebays, is not well documented and potentially at odds with the WSUD principle of adding value while minimising development costs. By implementing these structural elements, we 9 Assuming even distribution and coarse sediment and a loading rate of 0.6m3/ha/yr (pers. comm. A. O Neill). 17

18 risk unnecessarily adding to the: complexity of design and assessment; costs; carbon footprint and to a lesser extent urban stream syndrome. This then limits our ability to meet broader objectives of sustainable and liveable cities such as delivering affordable housing. When we examined the commonly accepted risks of forebays, we did not find sufficient evidence to support forebay use and therefore conclude that based on our results, the costs of forebays may outweigh any benefits of capturing sediment in forebays. We acknowledge that our study was limited to only eight bioretention systems and further research is required. Nevertheless, the results have prompted us to suggest seven strategies to achieve more simplified and sustainable bioretention systems: 1. The WSUD principles considered at the concept design stage need to be reconsidered at each step of the design process. This includes during detailed design stage where decisions about the type of velocity control for example are often made and decisions about cost and benefits are equally relevant. Technical design guidelines must therefore reiterate these principles and ensure that they are reflected in the design steps presented. 2. In drafting or reviewing technical design guidelines, the functional role of each key component should be clearly articulated. This will assist in promoting a greater understanding of what each component is supposed to achieve, allow assessment of performance, and encourage innovation in design and practice. Encouraging innovation in technical design guidelines should not be limited to a single paragraph in the front of the guideline but rather actively encouraged throughout the guideline. 3. Alternative designs for bioretention systems should be considered which limit accumulation of sediment at the inlet and ensure even distribution of sediments across the filter media. For example, where possible, the design and maintenance of bioretention systems could include a step down (say mm), between the invert of the outlet pipe and the bioretention/inlet swale surface. The aim of this recommendation is to allow the even distribution of flows across the filter media even as sediment builds up. This should in turn help to distribute sediment across the filter media surface and allow a less frequent maintenance regime. Maintenance costs should also therefore be reduced and the lifespan of the filter media prolonged. 4. Maintenance of bioretention systems should be undertaken regularly on an as-needs basis. In systems designed to minimise accumulation of sediment at the inlet, removal (or respreading), of sediment should only be undertaken where there is justification (e.g. where accumulated sediment is likely to impact the even distribution of flow into the system). 5. A greater emphasis should be placed on the importance of inlet scour protection. Guideline advice for energy dissipation techniques should favour solutions which align more strongly with WSUD principles (see Point 1 above). In many cases this may mean returning to less engineered solutions such as buffers or wide vegetated inlet swales. 6. Further research is required on the role of bioturbators in hydraulic conductivity, biogeochemical processes and pollutant dynamics of bioretention systems. Based on this research, we need to consider whether bioturbators can be used to restore hydraulic conductivity in poorly functioning systems and whether the design of bioretention systems is suitable e.g.: if bioturbators are desirable, is mulch preferable over jute mat as a surface treatment?; should we be maximising planting which increases natural mulch that encourages bioturbators; are current filter media specification appropriate in reaching a 18

19 balance between attracting bioturbators and reducing pollutants?; should new biofiltrations be seeded with bioturbators? etc. 7. Further research is required on the sediment budget of bioretention systems and receiving waterways and based on this work we need to reconsider whether capture of coarse sediment in bioretention should be avoided or promoted. ACKNOWLEDGEMENTS The authors would like to acknowledge Tony Weber and Reid Butler for their comments in the first draft of this paper. We are also grateful for the ongoing discussions with Andrew O Neill, Jack Mullaly and Shaun Leinster which have helped shape the contents of this paper particularly our discussion and conclusions. Thanks also to our work experience student, Steven Vernon, for his contribution in the field and thoughts about what was really going on in the bioretention systems we studied. REFERENCES Roux, A. and Stanley, J. (ed.) (2010). ADC Forum Cities Report Enhancing Liveability. Melbourne. Davies, P. J., Wright, I. A., Jonasson, O. J., and Findlay, S. J. (2010). Impact of Concrete and PVC Pipes on Urban Water Chemistry. Urban Water Journal 7, Department of Environment (2004). Stormwater Management Manual for Western Australia. Department of Environment, Perth, Western Australia. ISBN Print: Department of Planning and Urban Development, the Water Authority of Western Australia and the Environmental Protection Authority (1994). Planning and Management Guidelines for Water Sensitive Urban (Residential) Design. Western Australia. Department of Primary Industries, Water and Environment (2012). Engineering Procedures for Stormwater Management in Tasmania. Tasmania. Engineers Australia (2006). Australian Runoff Quality: A Guide to Water Sensitive Urban Design. Wong, T H F (ed), ISBN , Engineers Australia, Canberra, Australia, Facility for Advancing Water Biofiltration (FAWB) (2008). Practice Note 1: In Situ Measurement of Hydraulic Conductivity. Monash University, Melbourne. Hatt, B. E., T. D. Fletcher and A. Deletic (2009). Hydrologic and Pollutant Removal Performance of Stormwater Biofiltration Systems at the Field Scale, Journal of Hydrology 365(3-4): Hunter-Central Rivers Catchment Management Authority (2012). Accessed 25 July Lloyd. S. D. (2001). Water Sensitive Design in the Australian Context. A Synthesis of a Conference held August 2000 Melbourne, Australia. Cooperative Research Centre for Catchment Management Technical Report 01/7. Melbourne Water (2005). WSUD Engineering Procedures Stormwater. CSIRO Publishing. Melbourne. 19

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