Browse Forums Owner Builder Forum 1 Mar 12, 2014 9:35 pm Hi everyone, I'm building a house in Melbourne, Victoria, on a subdivided lot, and there is a condition stating that I will need Onsite Stormwater Detention (OSD). I have just applied to Boroondara Council to find out the requirements for OSD. I have been doing some reading about OSD on other council websites, and council's are taking different views. Are there any national/state standards? The Upper Parramatta River has a handbook (Dec 2005), but it contains fairly advanced hydraulic engineering, and was tough for me to follow: http://www.uprct.nsw.gov.au/osd/osd_download_centre.htm An easier read is from Manningham Council. http://www.manningham.vic.gov.au/maccwr ... de.pdf.pdf This took me step by step through calculating the key OSD parameters: Permissible Site Discharge (PSD) Site Storage Requirement (SSR) However, I think this may have been a waste of time, because Boroondara Council will most likely stipulate these critical parameters for me. Reading the Parramatta guidelines, they promote above ground detention, to minimise the costs. It talks about having an area where ponding can occur - the area floods during the heavy rains, and slowly releases the water to the gutters at a controlled rate. The height of the water must not exceed certain depths (say 600mm). Manningham doesn't state their preference for above or below ground. My preference is for above ground ponding, because I will have a gravel area surrounded by retaining walls, which I'd be willing to submerge in heavy rains. I had a chat to the Boroondara drainage team, briefly on the phone, and they recommend underground stormwater detention. This is a little off-putting, because underground stormwater detention is so expensive to build, for the water to simply collect and drain to the gutter. I am in the process of designing OSD for my block with the civil engineer, and would like to optimise the design for effectiveness, cost, and compliance with regulations. I already plan for a 10kL water tank in the front of the house under the drive, but this is planned for retention, rather than detention. This volume is for domestic use. This tank may not go ahead if I run out of funds. It's a desirable, rather than mandatory feature of the development. So, I would prefer that my OSD did not rest on the underground water tank's existence. Furthermore, this tank under the drive is on the high side of the block, because I'm not sure whether there is sufficient space to excavate to 2.5m for the tank at the back of the lot. I'm going off an angle of repose of 45 degrees from the bottom of the footings. No excavation can occur within this region. This will prevent collapse of the footings. The angle of repose also depends on your soil type - clay will not collapse as badly as sand. Anyway, having the tank on the high side of the block might not be the best idea either - though it might not be too bad, since it's not too different to above ground in terms of fall and flow... maybe? So, I was hoping for some advice on OSD design - above ground or below ground? How have other people satisfied council's requirements? Is council open to good alternative custom proposals, if designed well (depends on council, I guess)? Does a retention tank count towards detention? Upper Parramatta has a complicated formula for working this out - they do not allow 100% retention to be counted to detention, because during a storm, the tank may be full. Can I promise to constantly flush the toilet during a heavy storm? Anyway, the parameters that I calculated (using the modified Rational Method) were: Permissible Site Discharge (PSD) = 1.8L/s Site Storage Requirement (SSR) = 3500L Some other parameters: Site Area = 349m2 Impervious Area = 251m2 Thanks David Re: Onsite Stormwater Detention 2Mar 13, 2014 7:04 am I do a lot of work for clients in Warringah council and any OSD that has to be built for a DA I just hand it over to a hydraulic engineer and let them design it. We don't have a choice to provide anything other than something designed by a registered HE. There is no national standard as such that I am aware of - it comes down to the individual councils LEP and DCP requirements. Instead of an OSD tank have you looked at an OSD basin ? Far cheaper but you do need a fairly large area to disperse the water slowly and not all sites can do this unless you have a proposed dwelling with a small footprint on a relatively big site ( or at least in one area ). Seeing as your site only has a small soft landscaped area this may be a moot point. Below ground is a better option. They are mostly not good looking structures although sinking them below the soil does obviously add to the cost . The last one that had to be designed for a DA I submitted three years ago ended up costing the clients about $13,000 to build. Several councils in our local area now give you some concession if the rainwater drains to your rainwater tank first then overflows into the OSD. How much is therefore dependent on the size of both tanks and your roof area but juggling these can result in the OSD being 30% smaller from what I have heard - a saving worth having if you can swing it. Stewie Re: Onsite Stormwater Detention 3Mar 13, 2014 7:55 am Thanks Stewie D. That's good advice. You're right on the mark. I'll try to work closely with my hydraulic engineer, to come up with a good solution. Will definitely look into this OSD basin. If it's just an area that captures the stormwater like a floodplain/pond, and then lets it subside, then that's what I'm interested in. Have to make sure that the distance between the maximum ponding height and the slab has lots of margin, so that in a storm, it doesn't pond in the house! Re: Onsite Stormwater Detention 4Mar 13, 2014 9:10 am There are two not far from us in their front yard so they are easy to see. Only about 250-300mm deep but take up the whole turfed area, probably 6m long by 7m wide but these are for quite large two storey homes so I'd estimate maybe 250 - 300 sqm roofed area on a 600-650 sqm block not allowing for other hard surface areas.
Stewie Re: Onsite Stormwater Detention 5Mar 13, 2014 10:28 am Hi David, I won't be able to spent much if any time on this due to current work pressure and other threads already waiting weeks for replies but I can give some quick basic advice. Stewie's advice is good (as always). Ovals are also detention basins. Councils differ in their requirements. You need to know what those requirements are, particularly the PSD. I have already discussed this in your Roof Plumbing Plan thread. The council requirements will influence your decision as to whether to have above or below ground detention. You can also have both. Above ground is considerably cheaper but if you also have a surface drainage requirement, then above ground won't cut it unless you can manage the surface drainage separately to a rain garden or similar. http://www.melbournewater.com.au/raingardens http://library.melbournewater.com.au/co ... ardens.pdf It pays to do your homework as you are doing and I commend you for that. Have a read of the thread that I have linked below. The relevant posts are close to the start and so you may as well read from the start. It goes on to discuss a person's detention system hydraulic plan and you will go through the mistakes that were made by the hydraulic engineers. You are also able to access the plans. viewtopic.php?f=31&t=66117 Most councils require a hydraulic assessment from a qualified hydraulic engineer and a lot of the cost is drawing the plan as the calculations are simple to do. If you indicate what you want and supply the flow rates, pipe sizes etc, you will save the HE time. Also read this post... viewtopic.php?p=1145148#p1145148 Never assume that the 'experts' are always right. It would pay to read all of the above thread. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Onsite Stormwater Detention 6Mar 13, 2014 8:07 pm SaveH2O, you are a gem. This info is exactly what I'm looking for. OSD is a black art to me and there are probably many ways to skin this cat... hopefully I can figure it out. I really appreciate your time on the forum. You and Stewie D are awesome! THANKS! Re: Onsite Stormwater Detention 7Mar 14, 2014 12:41 am Why do you need OSD? Given that the council prefers underground detention, I assume that there is a connecting stormwater pipe and that you won't be discharging to the gutter. It is also important to confirm the council's ARI requirement. Never assume anything. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Onsite Stormwater Detention 8Mar 15, 2014 6:35 am Hi SaveH2O, the block will be subdivided, into a front and rear lot. The front will be unchanged. The rear will need to provide stormwater pipes through the front lot, to the legal point of discharge (LPOD) on the street at the front. I am confirming with council about OSD, but in the LPOD documents, it states "Should a proposed dwelling be upon a lot subdivided and under existing drainage conditions; onsite detection would be required". I'm assuming the "detection" is meant to read "detention". THANKS! Re: Onsite Stormwater Detention 9Mar 15, 2014 4:41 pm Love it when bureaucrats mandate regulations without checking the grammar or spelling. I thought that your situation as explained would be the case. It makes it easier if the requirement is purely to avoid overloading the stormwater system rather than have a situation where the house is down a slope. You will have noticed on some of the hydraulic plans you have seen where the hydraulic engineer has designed two individual overflow pipes to be fitted to a retention tank that drains to a detention compartment. Having two individual overflow pipes, often fitted to opposite sides of the tank, is amateurish. Just to explain, water tank overflow pipes usually contain up to 75% air when they drain; the pipe has plenty of capacity to accept additional infused flow. By fitting two overflow outlets and connecting those outlets to a single overflow pipe, the installation will be efficient, cheaper and much neater. The two into one overflow plumbing also provides a small detention capacity above the lower overflow outlet as shown in the photo below. The lower pipe will flow a bit faster when submerged but it will still draw a vortex. The overflow capacity still needs calculating and using 100 mm uPVC DWV SN6 overflow pipe on a tank fed by a wet system harvesting multiple downpipes is best. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Onsite Stormwater Detention 10Mar 31, 2014 6:14 am Hi SaveH2O, Thanks for the post, and sorry it's been a while since I've posted. It's taken Council some time to get back to me with the Design Parameters. They quoted 4-6 weeks, just for the design parameters - luckily, it took only about 2 weeks in the end. So, site storage requirement is 2440L and Permissible Site Discharge is 2.9L/s. There was a slight hitch, when I wanted to have an underground concrete water tank for both retention and detention: Council said they do not allow these because the tank would have to take surface drainage from the impervious surfaces, which would contaminate the water retained in the tank for domestic use. So, the surface drainage will not go to the water tank, but will be drained by another series of pits. See plan below: http://i65.photobucket.com/albums/h228/akapacker/StormwaterPlan_zpsb1ab01ac.jpg http://i65.photobucket.com/albums/h228/akapacker/Legend_zps65c0bed0.jpg It looks pretty good to me, but I had a few questions on the details: 1. Why do I need so many grated inlets? Particularly on the West Side of the property. It helps to drain surface water away from the slab, but do I need 3 GIs to drain the Spoon Drain? Wouldn't 1 be sufficient, given that it is only draining 7.2m2? And do I need a GI in the middle of my front lawn? Surely I can grade the lawn toward the pit in the North East corner? 2. Any advice on position of Inspection Points for horizontal pipes? I don't think the design is optimal at the moment. Do I want Inspection points where the pipes go under slabs and difficult to access locations? 3. I will use sewer quality pipes everywhere... http://i65.photobucket.com/albums/h228/akapacker/StormwaterPlanMod_zps4870a14c.jpg The tank design is below, but I don't think much thought has gone into it, beyond being a retention/detention tank (fair enough too). http://i65.photobucket.com/albums/h228/akapacker/Tank_zpsb835e4bb.jpg Since the tank is underground, this is a dry system, since the pipes running to the tank will drain and be empty. Also, the system is sealed. "The system has been designed as a SEALED SYSTEM to the Rain Water Tank. All connections from and including vertical downpipes and horizontal stormwater pipes to be chemically welded and pressure tested." Does it have to be sealed? I would like to have a filter on the downpipes similar to this: http://rainharvesting.com.au/product/le ... an-shield/ The tank will be less than 10kL, since I don't need that much. I had a look at http://tankulator.ata.org.au/, and for my location, I'll get by with 4kL, plus 3kL for detention, so a 7kL tank. It'll mean the tank is shorter, and I won't have to excavate as much near my footings, and hopefully avoid using piers. (You cannot excavate within the footings' angle of repose - I'll double-check with the engineer, but 45degrees). For the First Flush Diverters, I'd like this, plumbed into the pipes just before the water tank, and the slow leak to leak into the surface drainage system. http://rainharvesting.com.au/product/in ... diverters/ FFD volume to be specified for 0.75mm rain on the roof, plus a little extra for the dry horizontal pipes (if necessary?). "The first rain arriving on the roof contains the highest concentrations of these soluble pollutants. Research suggests that up to 80 per cent of pollutants are found in the first 0.5 to 0.75 mm (0.02 to 0.03 inch) of a rain event." rdn.bc.ca/events/attachments/evID6235evattID1344.pdf The inlets to be similar to bashworth's idea: http://www.anewhouse.com.au/2012/11/rai ... rovements/ Just to confirm - you don't want to disturb the anaerobic zone, in a water tank? Or you do, to get air in there, to reduce stagnation? Outlet - the outlet will be towards the top of the tank with a pipe attached to a waterboy, rather than at the bottom. I will have to draw water up to ground level either way, so I cannot see the benefit in having the outlet at the bottom. Water Use: I would like to use the water to flush the toilets, for the gardens, laundry, evaporative cooler, and (possibly) for the solar hot water service (asphalt shingle roof - may have anti-mold chemicals - but I don't drink hot water out of the tap, I boil it. And I don't drink water in the shower.) I've been reading about whether to use a pump alone, or a pump plus pressure tank. http://www.ata.org.au/forums/topic/9703 johnmath says they're not necessary. I'm undecided, but I guess I can decide this later on? So many decisions to make! Pump make: Davey, Grundfos, Chinese no-brand... SaveH2O, I do like this one you recommended: https://www.alkogarden.com.au/products/JET%203500 Thanks for everyone's help! Thanks David Re: Onsite Stormwater Detention 11Mar 31, 2014 1:36 pm Hi David, I won't be able to sit down to this straight away as my work load is unmanageable at the moment and this needs more than a quick reply during a coffee break. I have had a very quick look and there are a few things that I need to mention now. The stated PSD is 2.9 lps (174 lpm). This is very low yet the diagram shows a 100 mm DWV overflow pipe (denoted as sewer grade on the diagram) that will drain at a much higher rate than this!!! Note that the second (higher) 100 mm DWV drain pipe is an emergency measure that is not used in the calculations. You posted that the required detention storage is 2,240 L yet the diagram shows 3,190 L. The 1:20 ARI roof plan area flow rate will be about 565 lpm, nearly 400 lpm faster than the PSD. The PSD is variable, depending on the level of water (head) in the detention compartment. The 3,190 L detention figure is more realistic. When the correct (bottom) size overflow pipe is drawn in, ask for a vortex plate to be added so that you immediately get a higher discharge rate. This will also make the 2,240 L detention compartment more realistic. If you have an external pump, you will need a jet pump as these have good suction. They sacrifice some pumping efficiency to achieve this. The one I previously linked is a jet pump. Underground tanks are commonly installed with submersible pumps but any submersible pump must be raised off the floor to prevent it vacuuming the sediment layer...a common installation fault. It is vital that clean water is delivered to an underground tank. You should have sediment traps to help achieve this. Also, mosquito proof and sealed leaf diverters are necessary because water transferred directly into a tank MUST pass through mesh with apertures less than 1,000 microns in diameter. An underground tank will be expensive and the one in the diagram is huge. Can you possibly have a couple of above ground tanks instead? They don't have to be next to the house or even close together. I am not a fan of underground tanks. Also, I am not keen on below ground first flush diverters. Cleaning issues for a start! If you get one, make your own and use a manual flush valve to drain it. The link below is off the "Need advice on our rainwater collection and storage system" in the Eco Living sub forum. You will need to be on a slope. viewtopic.php?p=942084#p942084 Most debris (not the flush) is washed from the roof and gutters after the first flush fills. You need to ask a qualified drainer about the grates etc but having a GI in the middle of the lawn is a bit weird. I don't have the time to be looking up the regs for IOs on horizontal pipes. Maybe later on. Will have a proper read when I get time. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Onsite Stormwater Detention 12Mar 31, 2014 10:07 pm Hi SaveH2O, Good pick up on the size of the overflow pipe. This pipe overflows into another pit, which has the restricted outlet. I forgot to list the pit specifications: Like ⋅ Add a comment ⋅ Pin to Ideaboard ⋅ Council set the Site Storage Requirement to 2240L The 3190L compensates for the surface drainage that will not be going into the tank. But, I will double check on this figure, because it is 50% higher than required. Council does not permit above ground Onsite Stormwater Detention. Anyway, I prefer the underground concrete water tank, since I will be building it myself - will cost about $2000 to manufacture. And you're right - it is huuuge. Too big. So, I'm going to trim it down in height to be about 7kL. If most debris (not the flush) is washed from the roof and gutters after the first flush fills, what should I do? Get a larger first flush volume? The FFD will be under the decking, so hopefully I will be able to get at them quite easily, but I haven't had a good think about the cleaning yet - thanks for the tip! Thanks David Re: Onsite Stormwater Detention 13Apr 01, 2014 1:45 am The council obviously has a 'template' you need to follow but it is hard to understand the overflow pipes. The tank shows two 100 mm overflow outlets, one at the bottom of the detention compartment and one at the top. Both pipes drain to P2 and this drains through a restricted outlet to P1 that also has a restricted outlet. The very obvious observation is that P2 and P1 are both small pits and P2 will not be able to accommodate the full inflow from the tank during a 1:20 ARI rain intensity because P2's small volume only gives it a few minutes mitigation. I take it that the top of both P1 and P2 are exposed to atmosphere. Melbourne and some surrounding areas had a 1:500 ARI about 3 years ago. How would P2 cope with that? P2 would struggle with a 1:20 ARI if my understanding of the mandated overflows is correct. Have I missed something? The diagram also shows that P1 accepts the discharge from P2 PLUS the surface drainage from two 100 mm pipes servicing the grated inlets (GIs)! How can P1 possibly cope with a 1:20 ARI? Am I reading this correctly? It seems crazy to me. EDIT: Corrected text in above paragraph. You need sediment traps to deliver clean water to the tank. If you Google "sediment trap" or "silt pit" or similar, you should come up with a few commercial units. Rainharvesting use to make one and probably still do. A problem most have is that the bed load is re-suspended when it enters a turbulent small pit and capture depends on gravity. If you can capture and divert the bed load from a non turbulent section of pipe as you can with the DIY one I linked, it is efficient because most of the sediment is no longer in suspension when it reaches the trap. The last thing you want is a regular supply of detritus into an underground tank. The DIY one I linked can also be used as a first flush diverter if the sediment capture chamber is longer. What does get through to the tank will be like a fine talcum powder. Re: "All connections from and including vertical downpipes and horizontal stormwater pipes to be chemically welded and pressure tested." Chemically welding PVC pipe is simply the use of solvent cement. The "pressure tested" bit also stated is BS because stormwater and DWV pipe is not pressure rated plus how would they pressure test them anyway? Re the leaf diverters that you linked, you have to pass water through mesh with apertures less than 1,000 microns in diameter and into a sealed system when it is diverted directly into a tank. Don't get a leaf diverter with a course outer mesh and a fine inner mesh; only use one with a fine outer mesh. Also be prepared to regularly clean them. Having a leaf diverter also gives the downpipe an overflow facility. Bashworth's calming inlet is good. The inflow water is oxygen rich and will also oxygenate the anaerobic zone. A concentrated amount of water falling from above generates a lot of sediment re-suspension through the creation of toroidal vortices impacting on the bottom boundary layer. I think that I have posted info about them before, try doing a search if you are interested. Pressure tanks cut down on a lot of pump start/stops. Cisterns fill slowly. Washing machines fill slowly. This strangles a pump and makes it inefficient. A pressure tank allows the pump to operate efficiently with less start/stops. You need a reasonable size one though. I have a Pressure Wave 35 litre that I paid $190 for a few months ago. I wanted something bigger but it was a good price and still had a 1" connection. The draw down is about 14 litres from memory at 28 psi. The pump needs an adjustable pressure switch if you have a PT. I would not use electronic pressure switches anymore, only manual ones. Your tank will not have much retention capacity. Do some figures on toilet and laundry use and connecting to a couple of garden taps. Don't forget vehicle washing. Also, get an 18 mm garden hose to reduce friction loss. You can connect a small length of 12 mm hose to the end so you can still use the standard 12 mm connections. See link below. viewtopic.php?f=35&t=61588 The last few summers have been very dry (Vic) and the water won't last long. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Onsite Stormwater Detention 14Apr 01, 2014 6:12 am Dang! You're right! "The very obvious observation is that P2 and P1 are both small pits and P2 will not be able to accommodate the full inflow from the tank during a 1:20 ARI rain intensity because P2's small volume only gives it a few minutes mitigation. I take it that the top of both P1 and P2 are exposed to atmosphere. Melbourne and some surrounding areas had a 1:500 ARI about 3 years ago. How would P2 cope with that? P2 would struggle with a 1:20 ARI if my understanding of the mandated overflows is correct. Have I missed something?" How the heck did I miss that? I'll have to chase this up, definitely. Looking at the Pit System, it doesn't make sense really... Why wouldn't I put the restricted overflow on the lower tank outlet (@ IL49.03), and do away with P2 altogether??? Instead, just connect into P1? Regarding P1, I think it only drains the grated inlets, and no downpipes, or am I reading this wrong? All down pipes are plumbed into the tank - or should be! Since the stormwater from the drive and pavement do not go into this tank (to prevent polluting the water for domestic use), the detention volume is increased, but by 30%? The additional impervious area is 14%, so I would have thought the detention volume would increase by this... I'll double check this. Regarding "Your tank will not have much retention capacity." My problem is not having enough storage volume... it's not having enough rain on 220m2 roof to fill the tank. According to tankulator, my usage will surpass my supply, so the tank will hardly ever be full. So, I might as well use a smaller tank. http://i65.photobucket.com/albums/h228/akapacker/Untitled_zps19adee17.jpg Maybe I've overestimated my water usage - is 475L/day for 5 people a lot? I'll double check this. Thanks for your help once again, SaveH2O! Re: Onsite Stormwater Detention 15Apr 01, 2014 10:59 am akapacker Regarding P1, I think it only drains the grated inlets, and no downpipes, or am I reading this wrong? All down pipes are plumbed into the tank - or should be! For whatever reason, I typed grated inlets and downpipes instead of 2 pipes serving grated inlets and missed seeing it when I read through before posting. An obvious and embarrassing mistake as I obviously knew that all downpipes were diverted to the tank. I must admit that I did rush the post as I had to get up early. Big day. I have edited the text. P1 has a restricted outlet yet it will have infeed from P2 plus two 100 mm pipes servicing the GIs! Note that the diagram shows P1 as having a 100 mm PVC outlet!!! So much for the PSD! EDIT: I would like to see their estimated total flow through the two pipes servicing the GIs during a 1:20 ARI. The problem I see you having with officialdom is that any revision could/would impact on you financially. The flow from the GI's pipes during a 1:20 ARI leaves a flow rate margin that is the permissible discharge from the tank. There are figures that give the % flow off different surfaces, I will post them when I get the time. If you have any contact with the council, I would only ask at this stage if they can supply the flow through the pipes during a 1:20 ARI. I will also have to check if ground flow is in fact calculated by a 1:20 ARI, that is something that you could ask them if you do talk to them. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Onsite Stormwater Detention 16Apr 01, 2014 4:11 pm Thanks for spending the time to answer my questions before work! Amazing! I'm not quite following all the flows from the different sources, and how they all add up to give my permissible site discharge of 2.9L/s and whether this is for a 1:5 ARI, 1:10 ARI, or a 1:20 ARI. I might have to talk to my engineer again - I've given him a day to digest all my comments on his plans! Hope he doesn't get upset. I'm hoping that he increased the size of the detention to balance out the extra load to P1 from the surface drainage (mandatory from impervious areas, voluntary from pervious to keep the water away from the footings) so that the flow all adds up to 2.9L/s. "The flow from the GI's pipes during a 1:20 ARI leaves a flow rate margin that is the permissible discharge from the tank." How did you work this out? Using software, or the modified rational approach, or something else? "There are figures that give the % flow off different surfaces, I will post them when I get the time." Didn't know about these, either! Thanks David Re: Onsite Stormwater Detention 17Apr 02, 2014 1:27 am akapacker I'm not quite following all the flows from the different sources, and how they all add up to give my permissible site discharge of 2.9L/s and whether this is for a 1:5 ARI, 1:10 ARI, or a 1:20 ARI. I might have to talk to my engineer again - I've given him a day to digest all my comments on his plans! Hope he doesn't get upset. The roof is 1:20 ARI. For detention, it is plan area, not factored for roof slope for obvious reasons. The maximum permissible pipe flows as per AS2200 is factored for slope but that is separate to the PSD. akapacker I'm hoping that he increased the size of the detention to balance out the extra load to P1 from the surface drainage (mandatory from impervious areas, voluntary from pervious to keep the water away from the footings) so that the flow all adds up to 2.9L/s. PSD is 174 lpm. This is a combined flow of the 2 x 100 mm pipes servicing the GIs and the overflow from the tank (which all goes through P1 anyway). The tank's (bottom) overflow obviously cannot be a 100 mm overflow pipe. akapacker "The flow from the GI's pipes during a 1:20 ARI leaves a flow rate margin that is the permissible discharge from the tank." How did you work this out? Using software, or the modified rational approach, or something else? GI pipe flow + tank overflow = PSD. The ARI applicable to your property for the GI pipes will be known by the engineer. akapacker "There are figures that give the % flow off different surfaces, I will post them when I get the time." Didn't know about these, either! They are referred to as run off coefficients. Roofed area: 1. Unroofed impervious (paved) area: 0.9 Pervious area: This differs between sandy & clay soil, lawn etc and are much lower. I like the nice and simple NZ tables. If you look up gardens and lawns, it is 0.25 etc. The main point of immediate interest is the flow into P1 from the two 100 mm pipes servicing the GIs. It won't be as high as you may be expecting. I assume that you will be fashioning your own anti vortex baffle. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Onsite Stormwater Detention 18Apr 02, 2014 6:11 am Thanks SaveH2O! OK, I found the run-off coefficients. I had seen them before, but forgot about them. They're in AS3500.3. NZ tables are much easier than the complicated Australian Standards! Yeah, the most important thing to confirm is the surface drainage from the GIs. I hope you're right, that it won't be much. If the restricted orifice was put on the tank, do you see any need for P2? I'm scratching my head??? "ask for a vortex plate to be added so that you immediately get a higher discharge rate" I read your previous reference to vortex plate, but thought that I could think about this later. It improves flow, right? I will try to build it myself, if I can. Do you have any advice on vortex plates, please? Thanks David Re: Onsite Stormwater Detention 19Apr 02, 2014 10:52 am I don't know the purpose of having both P1 + P2. Maybe it's to provide additional mitigation...I don't know. Regarding your retention capacity, don't forget that you have used the Tankulator to factor a 7,000 L retention but if the tank is 7,000 L, retention drops to 4,000 L plus not all of the water can be pumped. To increase retention, you could have an additional narrow base (not slimline) above ground tank also connected to the pump. You needn't worry about the tank over-topping or fitting an overflow pipe, diverting downpipes is archaic, you just divert the water and have it automatically stop when the tank fills. A vortex plate simply blocks the vortex when the outlet is submerged, allowing the pipe to fill with water. It will generate a much higher initial overflow rate and aid mitigation. The baffle needs to be fitted immediately above the overflow pipe and it needs to be about 3 times wider than the pipe and extend outwards a similar distance. 3in1 Supadiverta. Rainwater Harvesting Best Practice using syphonic drainage. Cleaner Neater Smarter Cheaper Supa Gutter Pumper. A low cost syphonic eaves gutter overflow solution. Re: Onsite Stormwater Detention 20Apr 02, 2014 1:33 pm Checked with my engineer. He agrees that it makes no sense, but that Council prefers seeing these overflows. He recommended that I submit it, as is. I decided against this, just in case they actually approve this crazy design. I don't want an unnecessary pit if I don't need one. I did do the calcs with a reduced tank size of 4000L on tankulator, and I still couldn't fill it, with my rainfall and usage. I'll take another look though. The vortex plate is installed horizontally, right? It will interrupt the vortex as it traces to the surface... Thanks! David Plumbers 'can be' plumbers, made all the worse by self certification which the building surveyor invariably accepts as proof of compliance! The good thing is that you know know. 3 4859 Grab a hose, insert it at the top of the inlet/down pipe and turn the water on and see where the water is escaping from. Then you'll know. 3 8465 in the stormwater pit or the drain? Those dont look like theyd fit in the drain. Separately, the pits do get stuff in them during the course of the build. For the most… 1 7932 |