innovations maintenance development monash inlet for
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Innovations in Maintenance – Development of the Monash Kerb Inlet for Passive Irrigation of Street Trees in residential suburbs
Ari Triskelidis1, Kanch Withana1, Stephen Livesley2 , Chris Szota2, Robyn Mitchell1, Kyesha Milenkovic3 1City of Monash, Engineering Department – Design Mgt, 293 Springvale Rd, Glen Waverley, 3150, Victoria, Australia (E‐mail: [email protected]; [email protected]) 2The University of Melbourne, School of Land and Environment, 500 Yarra Blvd, Burnley, Victoria, 3121, Australia (E‐mail: [email protected]; [email protected]) VIC, Australia 3Melbourne Water, Regional Water Planning, 990 La Trobe Street, Docklands, Victoria, Australia Abstract
This project involves the passive irrigation of street trees and the development of a new kerb inlet that deals with minimal maintenance and costs when retrofitting and modifying existing kerb and channel systems. This project was a great opportunity to innovate and push the boundaries of conventional thinking. We knew there were great challenges with regards to blockage and debris control and ultimately called upon fluid dynamics and airfoil technology to solve this problem. We realized that we need to keep the debris away from the face of kerb and the operational principles were simply to collect the heavy debris in a central dip and use airfoil flow dynamics in applying that technology to the kerb inlet. The design also relies on a maintenance program of street sweeping and cleaning of secondary filter baskets that collect the fine debris.
This kerb inlet diverts catchment runoff into the nature strips while dealing with the debris at the grated interface. The anticipated benefits being: (i) reduced maintenance costs (ii) the volume of polluted runoff entering waterways will be reduced, and (iii) street trees will benefit from additional water and nutrients, increasing resilience to drought or heat stress.
Catchment runoff is directed into a gravel trench installed just outside tree canopy drip line. Changes in soil moisture profile are continuously monitored within the nature strip and road pavement, to monitor any movement and potential defects. Tree health is measured through pre‐dawn leaf water potentials and tree growth through stem diameter change.
This study will enable the costs of maintenance and installation to be considered against runoff reduction and street tree health benefits. If these systems prove cost‐effective, residential suburbs can be WSUD retrofitted which will have huge impacts upon catchment hydrology, stream ecology as well as street tree ecosystem services.
Keywords Passive irrigation; Street Trees; Kerb Inlet; Debris; Maintenance Costs; Stormwater Introduction So why was this project necessary and why are trees important? We all have realized the benefits of trees and the interaction between the soil and atmosphere and the real benefits that stormwater can offer. We know about the benefits of greening the suburbs and the connection it has between water. A real goal of this project is the use of wsud to definitely improve a very simple element in all suburbs – street trees. One of the over arching goals of this project, was to look for something that is going to be simple, cost effective and eventually something any Council could consider, with minimal maintenance in mind.
Like many other Councils, Monash street trees suffered quite badly during the drought so this project is about getting more water to these trees, especially during these difficult times. So we are essentially focusing on tree growth, shade from canopy and to reduce some floodwater runoff issues. We have very simple aims and they are to improve tree drought tolerance and improve tree growth and health. Background As design managers, the brief was to design a kerb inlet that would redirect stormwater runoff into a gravel trench. The kerb inlet had to be low cost with minimal installation and maintenance costs. The typical layout below shows the redirection of stormwater into a 600mm deep gravel trench. The runoff flows in and then the water will slowly seep out, depending on the soil type within that nature strip. From the beginning, we could have settled on a simple vertical grate in the face of the kerb, but we decided that this was an opportunity to innovate, to research and to push the boundaries of conventional thinking. Some of the issues we identified as important were being very clear about the brief and its intent; having a degree of confidence and self‐belief; needing to take some calculated risk and thinking forward; we also knew we needed to learn from others, to collaborate and do a lot of research and testing. The result is what we call the Monash Kerb Inlet (MKI).
During the early stages we arranged site visits with other Councils and in particular City of Port Philip and City of Melbourne. We were advised of their challenges and they shared their issues so we could address them in our project. We observed that most existing systems where not functioning very well and that sediment and debris blockage was a common problem. Shown below are some existing systems performing with varying levels of success.
The MKI isn’t designed to catch a lot of water, it is designed to divert floating debris and heavy
sediments and catch water that is less likely to contain suspended solids than other kerb inlets, and so keep maintenance of the system to a minimum. We sought advice from Prof Saman Fernando (Swinburne University), who is an expert in fluid dynamics and together we began to solve this problem. The real innovation was to use aerofoil flow dynamics and apply that technology to the kerb inlet. Development of the MKI The aerofoil design The aerofoil design is an innovation in kerb edge treatments. It remodels the kerb face into an aerofoil‐type form with an inset grate. The MKl is designed with two shapes. The vertical aerofoil and the horizontal dip, which together cause acceleration of flow through the kerb. Flows and the debris will have a higher tangential velocity and will quickly pass the grated opening. Water passes through the grate when the flow depth increases in a similar manner to a weir. The grated opening has 2 main roles: 1. To prevent the majority of floating debris (such as leaves) and suspended solids from entering
the trench. The opening size dictates the size of particles getting through, which is about 5mm. 2. Reduce velocity drop across openings
• Orientation of the openings is important. • If the openings are horizontal most of the debris will get into the trench and also will not
give an opportunity for the flows to travel through the air foil to develop speed.
The spacing of the vertical openings was carefully considered. We had to strike a balance between capturing water and having enough of a gap between the openings, to minimise the loss of velocity across the openings. We tested a few combinations with varying opening sizes and gaps and ultimately decided upon 8mm openings with 10mm gaps. In the acceleration zone, (location of the dip) the flows closer to the kerb will speed up, compared to flows further away from the face. The difference in this speed creates a swirl or vortex. The grate is then best placed in this acceleration zone. Likewise in the deceleration zone, the speed is slower near the face, thus a swirl turning the other way is formed at the aerofoil nose. The vortex helps some of the smaller, heavier particles to settle to the bottom of the dip and that is the sole purpose of the dip. Given removal of the debris from the grate is crucial for proper function, then the dip is a very important part of the design. When the flows increase the first swirl will get pushed towards the second swirl and the debris will gets washed downstream. This unique pre‐cleaning means that the water that does enter the system through the grate has been filtered of exactly the sort of particles that are most likely to cause blockage of the grate
What happens to the debris? Debris that remains in the dip or on the downstream end of the profile is later removed either by regular street sweeping or during a downpour of rain, by the first flush of stormwater travelling along the kerb.
The bottom of the grate opening is at the 'invert level' of the kerb and the bottom of the dip is a further 3 cm below, which means water pools in the depression before it starts to enter the grate. At the start of a light rain fall, low flows of water begin to run along the kerb, slowly fill the depression, sediments will drop out and collect at the bottom of the depression because the flows are too slow to carry them, and eventually, when the depression fills, the water will begin to spill through the grate. In heavier rain falls, more substantial flows of the first flush push the larger debris downstream past the grate.
Sometimes debris that can float will have collected in the depression before a light rain. In that case, the depression will slowly fill up and the debris that are floating on its surface will have a chance to spill through the grate. This could be avoided by having the base of the grate higher than the invert level. In such a case, the floating debris would be carried away in the 'first flush' of the kerb inlet system. However, water would only enter the grate in heavier rains when flows were greater. This payoff between water capture and water cleanliness could be further investigated. The lintel The aerofoil curved face of the MKI does not extend up to the top of the kerb. Instead, it is covered in a strong lintel. The effect of this design feature is twofold: firstly, it means from the pavement side the MKI blends in with the pre‐existing kerb, and secondly, by setting back the grate and aerofoil face, it protects them from tyre damage. Development of the 3D surface profile Professor Saman Fernando in his report to us detailed the shape that would give us the outcomes we were after. He determined some equations for the horizontal and vertical shapes and also spoke about the test apparatus manufacturing process. The horizontal shape is given by a National Advisory Committee for Aeronautics (NACA) airfoil equation and dip profile was also given as an equation. The white rectangle (acceleration zone) is the location of the vertical grate. Then X,Y values were derived using these equations and then transferred to Autodesk Civil 3D civil to create a 3d surface profile. (NACA ‐ National Advisory Committee for Aeronautics)
We took our inital 3d surface model and then began thinking about how we could construct this within a kerb and channel. We further developed the surface profile into a complete 3d model with the entire cast kerb profile. Prototype Development with SVC Concrete Products (SVC) 3d mould designs During the onsite sessions with SVC we decided to go with a complete 1200mm wide cast concrete section. We had the ability to use existing moulds thereby reducing some costs. The 3d shapes were then developed in cad which were used to cast aluminium moulds for the airfoil and channel dip sections. At $9000 for mould construction we could not afford to get this wrong at this stage. SVC development of 3d cad models
Mould Development with SVC Concrete Products Kerb and channel mould with the aluminium inserts attached to create the Monash Kerb profile.
(First precast unit without the kerb inlet openings and grate) Detailed construction design While we developed the moulds for the kerb inlets, we finalised our construction drawings for final installation in the streets. As can be seen in the following drawing section we have included a secondary filter behind the back of kerb. During our trials we had realized that some maintenance will be required as small particles still passed through the 8mm gap in the grate. We observed particles were about 4‐5mm wide. Our secondary filter has 1mm openings thereby minimizing small debris entering the system. The filter needed to be simple, low cost, easy to clean and with low maintenance. The filter basket is accessed by an I/O opening at surface level and has a stainless steel basket with handle for easy removal. Final passive irrigation installation with Monash kerb inlet Monash kerb inlet installation section view
Development of standard drawings
SVC MKI prototype Here you can see the additional openings completed by SVC to cater for the connection at the rear of the unit. This in itself was quite difficult to manufacture due to the size constraints. You can see the stainless steel grate being fitted which also required special grout mix to do some hand sealing of the gap. Further development of the precast unit to include connections & grates
These are some images of our first completed concrete kerb inlet. First MKI concrete precast product (Prototype) Final working model during rainfall
Summary & Discussion Summary of MKI elements System element
Description Notes on selection Future implications
Chord length 800 mm Chosen by council engineers after testing showed good performance.
Optimal performance will depend on site conditions (e.g. slope, catchment size). Further research useful.
Total unit length
1200 mm 1200 mm is a standard mould length. Additional length makes unit heavier. A 1200 mm unit weighs approximately 300 kg.
Further research useful.
Grate dimensions
8mm opening, with 10 mm closed spaces between
Two options tested during prototyping, 10‐8 and 10‐10. 10‐8 considered by council engineers to block more litter while still maintaining good inflow rates.
Further research useful.
Bowl depth 30 mm Bowl depth supplied as optimal depth by hydrological consultant.
Further research useful.
Bowl slope and form
Symmetrical dip
Vortex action highly responsive to variation in slope and curve and further refinement possible.
Further research useful.
Perceived barriers to passive irrigation uptake “It’ll damage the road” A common perception, especially from road engineers, is water from the infiltration trench will wet the soil beneath the adjacent road, i.e. the road subgrade, and cause damage, in particular road slumping. Damage to roads is expensive to repair and potentially dangerous. The City of Monash passive irrigation trials are measuring soil moisture content near and beneath the road at various distances from the kerb. A number of engineering solutions may help minimise wetting, for instance by including waterproof barriers at the road edge at the time of passive irrigation system installation; by including a connection to direct excess water back into the stormwater drainage system, and by grading the base of the infiltration trench away from the road.
The perception that damage to the road will occur needs to be clearly considered side by side with the following: 1) How does the cost of road damage compare with the benefits provided by street trees and
flowing from good water management? 2) How serious is the damage to the road? For instance, is the damage confined to the low‐traffic
area near the kerb? 3) Road damage can be caused by many factors, including poor construction, heavy vehicle use
etc. 4) Wetting of the road subgrade can occur from groundwater in general, not just water
specifically from a passive irrigation system. The presence of groundwater is in fact inevitable. 5) If excessive wetting is a problem, what is its cause? Is it, for instance, a failure of the road
engineering to successfully convey water away from the subgrade, for instance through aggi pipe along its edge? Roads really should be able to tolerate some level of subgrade wetting. If they can’t then perhaps they are poorly constructed.
6) Healthy street trees protect road surfaces from solar radiation, and the consequent swelling, shrinking and deterioration of the road surface.
7) Passive irrigation systems offset the negative impacts of roads. They minimise atmospheric pollution, filter runoff, help abate noise, cool streets made hot by heat‐trapping expanses of bitumen, encourage walking and strengthen local biodiversity.
8) Roads damage trees at least as much as trees damage roads. The creation of impervious surfaces, the production of particulate pollution, the increasing of the UHI, severing of roots during construction, and pruning required for passage of vehicles, all adversely affect the health of trees.
“Costs too much” / “Not worth it” The cost of installing a passive irrigation system is by no means negligible. However, it is reasonable to consider whole of life cycle approach in assessing the worth of a project. The financial benefits of street trees are now beginning to be recognised, and LGAs can make future savings by investing in green infrastructure. Upfront costs will decrease with increased LGA capacity. “Needs too much maintenance” / “Going to make too much work” The passive irrigation systems being trialled are intended to be low maintenance, in recognition of both the costs of maintenance and the limited availability of maintenance resources. Maintenance is increasingly seen as a vital component of urban systems. In the future, systems may be in place that facilitate more regular maintenance. Capacity building is important. “Won’t do anything” The benefits of healthy street trees are well documented. Less well documented is the response, and degree of response, of street trees in particular to additional water availability. The results of these trails notwithstanding, it is generally well‐known that trees grow better with greater water availability. The City of Monash passive irrigation project has IWM benefits beyond street tree health. By filtering pollutants, it minimises downstream pollution. “Not important” Street trees are important because they are valuable assets that contribute multiple benefits at a range of levels. Put simply, trees save lives.
“Not my problem” The multiple stakeholders involved or affected by a passive irrigation project all have different priorities. This can contribute to lack of project uptake. Different priorities can be reflected in different guiding policies and legislation. For instance the City of Monash maintenance department needs to comply with the Road Management Act and to Monash’s Customer Service Charter. Maintenance works directly relating to these will be prioritised above passive irrigation system maintenance. “Too risky” It is important to be risk aware, rather than risk adverse. In an environment threatened with rapid climate change, for instance, it is useful to presume in favour of mitigation strategies. The risks of passive irrigation systems can be foreseen, and future iterations can respond to minimise future risk. Without initial implementation, further implementation is impossible. Moreover, new approaches to urban planning and design require time for a change of attitudes and mechanisms to occur within the implementing organisation. Often risk is more perceived than real, and cross‐disciplinary, team‐oriented, communication will reduce that perceived risk. Learnings from installation Location of services A service‐location company was contracted to mark the location of underground services. This was essential. While infiltration trenches were dug to 800 mm to avoid pipes nominally at 900 mm, in practice many services occurred above 800 mm. It became evident during installation that it was important to use a skilled excavation crew. Most likely problems could occur with cable telephone services and underground electricity cables running from pole to box. Also problematic can be gas and water services running at angles out from the residential properties. Installed drains Many roads have ag pipe drains running along under kerb & channels. Often these are blocked, due to their age. However, if they do transport water, they can drain the infiltration trench if the trench intersects or is in close proximity to them. Where possible, the infiltration trench should be set back from the kerb as much as possible. Drainage from other service installation If excavation lines for other services cross the infiltration trench, then the infiltration trench may drain along the line of those services. If conditions are right, drainage may be to beneath residential housing, leading to issues of liability. Contractor skills Excavation needs to be done by a skilled crew to avoid damage to services. Maintenance will be best performed by crew experienced in IWM systems. Root occurrence In only one of the 24 excavations was a root greater than 40 mm diameter encountered. Order of excavation Trenches should be excavated before kerb units installed to avoid kerbs directing water into excavations in progress.
Where there any other challenges? As discussed we realised during our testing the need for a secondary filter but we may still have issues with any soluables that will pass through. So the challenge here was to design a system that was not only simple, low cost but easy to maintain and clean. Early testing is suggesting maintenance of the secondary filter to be 3‐4 times per year, with about 20cm of debris in a four month period. This is still well below the capacity of the filter. Field testing has shown that debris diversion in the kerb is at least 95% when stormwater is flowing, but build up is likely to occur during very minor rainfall. There are some limitations of use on steep grades and areas of high leaf litter. We are also undertaking soil moisture testing of the nature strips and road pavement to measure any potential changes beneath the pavement, with some concern regarding movement of kerbs and road failure. And there are always challenges in educating the community on issues relating to dumping of grass clippings and leaf litter in the kerb and channel.
Conclusions Where to from now? No real results just yet as installations have just been completed.
• Construction was completed August 2014 and testing has begun but no real results just yet! • Testing and monitoring will continue for a least 2 years and this will be undertaken by
Melbourne University. • Testing will include the measurement of debris within the kerb inlet & filter basket with an
assessment of how the kerb inlets are coping with sediments in the tray and at the vertical inlets. Testing within the trenches will also indicate how much rainfall is entering the trenches and what volume is being used by the trees.
• We realise that the first prototype will require further refinement and testing. Installation will be monitored for their effectiveness.
Costs/Benefits By diverting the debris and the heavier sediments away from the trench the Monash kerb Inlet increases the gap between renewals. This in turn reduces the assets whole of life costs by reducing the number of renewals. The benefits of greener environments are many, successful passive irrigation improves the drought resilience of the street trees and reduces the potable use of water during drought conditions. There is benefit to the community with reduced heat island effect, cooling from tree canopy and the visual amenity of healthy trees within a street. Although not designed as a stormwater management device, City of Monash kerb inlet assists in reducing the flows. The cost of installation is currently at $4k, this is likely to reduce with a wider roll‐out of the product. The typical cost of loss of a mature tree is $40k and with a typical life span of 40 years, then there is considerable benefit in successfully passively irrigating our street trees. What can go wrong Detailed survey has been undertaken prior to works starting and a further survey will be undertaken at the completion of the trials , up to 2 years, so we can check for any movement in road pavement and kerbs. Maintenance may be higher than what we anticipate and we realize that maintenance is the number one issue that we should be focusing on as it can determine the success of a project.
Acknowledgement This work was undertaken as part of funding received from Melbourne Water and the Office of Living Victoria. We wish also to acknowledge the technical support provided by Prof. Saman Fernando, Safer Engineering Solutions, Mr. Ralf Pfleiderer, City of Melbourne, Mr. Sam Innes, City of Port Philip, Prof. Tim Fletcher, Stephen John Livesley, Dr Chris Szota and Harry Virahsawmy, University of Melbourne. REFERENCES FERNANDO, S. 2013. Passive Tree Irrigation – Kerb Inlet Design. Sa‐Fer Engineering Solutions,
Report No: 2013/10A1, 4 October 2013.