Rainwater catchment in tropics (New build)

Hello

Please excuse the length of this post!

Hoping to get some advice/checking of my calculations for my house build here in the Philippines. I know these are Australian forums but I’m originally from the West Australian wheatbelt and hoping to have a functioning rainwater collection system for gardens and hopefully future drinking water (we have mains supply)

We have a 2.5Ha block with two buildings; house and garage. Construction is about two months away from completion. Gutters are installed, downpipe holes cut but no downpipes (90mm) installed yet. Builder’s scope is to just run a ring of 150mm around the house to collect the rainwater but I have advised him I will run this to the future tank location. The builder has already installed commonly used guttering and put downpipes approximately every 6m. The downpipe holes cut into the guttering are 77mm.

I’ve found some rainfall intensities which I’ve used in my calculations; they’re horribly high rainfalls and I fear I am going to have overflowing gutters when the next typhoon blows through. From the calculations it also appears I will be needing a lot of pipe in the ground to convey the water to our future tanks.


SITE PLAN:

https://drive.google.com/open?id=0B52grw6hCinQSGYtd08tVy1kM0k




HOUSE/GARAGE ROOF PLANS:

http://drive.google.com/uc?export=view&id=0B52grw6hCinQcmJfQ2RZVWo1c28


Looking at the north face of house:


Looking at the SW face of house


Looking at garage NE face



RAINFALL INTENSITY:

I’ve done a lot of digging and this is what I could come up with: From http://unesdoc.unesco.org/images/0018/001802/180223e.pdf, closest measurement site mentioned was Ambulong, 30km to the NW, however two mountain ranges between us!


From this, the 5 min intensity: 1 in 20 year event is 29.84mm/5 minute or 358mm/h.

From https://en.climate-data.org/location/20576/, Precipitation here averages 1904 mm (Tropical Monsoon Climate)


GUTTERS:

https://drive.google.com/open?id=0B52grw6hCinQaG1EZ0hPczZGWFE



I’ve measured the gutters, the builder said they did install with slight slope to each of the gutters (can’t confirm yet). As mentioned, the downpipes are going to be 90mm (though the holes in the bottom of the gutter are about 77mm so I may ask them to expand these a bit?).

I am going to try to get the builder to install some DIY leaf catchers, it may just be PVC reducer added to the top of the 90mm pipes with some insect screening on top. Most of the gutter rubbish will be dust, as we leave in a typhoon country I have no plans of having tall trees adjacent to the buildings.


CALCULATIONS (DOWNLOADABLE EXCEL SPREADSHEET) SHOWING CAPTURED ROOF AREAS

https://drive.google.com/open?id=0B52grw6hCinQVFNuQnNhOVpuRXM



BUILDING ELEVATIONS:

https://drive.google.com/open?id=0B52grw6hCinQU0luWlZQMDdBUUE


As you can see above I’ve copied extensively from the system Jnk40 and SaveH2O developed (plus the sediment trap also printed in the reNew magazine issue 127).

I am looking at having between 50kL and 100kL of rainwater tank capacity and see the benefit of having the two tank system. Due to the slope of my property, I am wondering whether it would be best to have the settling tank (largest tank) near to the buildings (and low point of the property) and then pump water up to another tank located at the high point where it would be more useful for future drip lines and watering of the orchard/gardens. One disadvantage of this is if the pump can’t keep up with a high rainfall event (or loss of power) and the lower settlement tank overflows while the capacity of the upper tank is under-utilised.

The tanks will most probably be the “cube” type (galvanised steel). Most tanks here are Stainless steel 304 and on stands and rust to bits in a few years.

BIGGEST QUESTIONS

1. In the calculations, you will see I got stuck once it came to gutter capture and the downpipe sizing, I’m a little confused how to get further.
2. In order to cope with a 1:20 storm I believe I may need to run several 150mm horizontal pipes between house and tank in order to cope? Could someone help me a little on how to go forward?
3. Sediment trap: the horizontal runs will be at least 25m long, what would be the best place to put the sediment trap along this run (I know at least 3m past the last Tee or Elbow). Because of the long runs I am thinking the sediment trap is placed as close to the lowest point in the pipe (i.e. close to the tank)? Should I increase the length of the sediment trap (ie after the 45deg elbow add a length of 100mm DWV before connecting to the reducer)? Does smaller diameter pipe need to be lower than the sediment trap to drain properly?
4. See in the photos, the second floor discharges directly onto the lower roof (no 2nd floor guttering) - can you foresee any problems with this?
5. Bit confused on how to size the overflow of the tanks.

Hope someone can “dive” in and help a little!

Cheers
Leon

Hi Leon.

I should have the time to answer this tonight, I couldn't log in for 2 days but all is fine now.

Thanks for all the vital information, it certainly makes it easier to answer your queries.

I see that you are near Batangas, did the earth move for you too?

Hi SaveH20! Thanks for chiming in, thought I wasn't going to get any replies for a while!

Yep the ground moved for us, though at the moment we are up in Manila and even there the tremors could be felt.

Hopefully it's not a precursor to something bigger... I've built our house with steel frame rather than concrete hollow block (the typical materials here) so hopefully will withstand most of what gets thrown at it in the future (fingers crossed).

Look forward to your response!

Cheers
Leon

Your posted information shows that you have a very good understanding of best practice. Best practice isn't involved nor should it be expensive, in fact it is frequently cheaper.

Your 1:20 ARI is severe but the Phils get typhoons every year and their size, intensity and frequency is increasing. The waters around the Phils are warmer now but not spectacularly so like many other regions are but with the atmosphere now holding about 7% more moisture and twin and triple typhoons no longer rare in your zone, the Phils will face many challenges in the coming years.

A few notes.

A 1:20 storm's figures are based on an average intensity over 5 minutes, that intensity is not transposed to an expected hourly figure.

The roof multiplier factors wind driven rain but the total roof area does not collect more water than a flat roof would. Depending on wind direction during severe winds, a lot of the water draining from the upper roof will probably never reach the lower roof and a lot of water falling on the lower roof would also be blown off. The real test will come with heavy rain when the wind has dropped.

I can see in the house photo that you have a large gutter. If your roof drainage installation is like here in Australia, the 77mm hole in the gutter is for the gutter 'pop' which is also referred to by various other names. Your 90mm downpipes will attach to the pop, usually by pop rivets.

In Australia, 90mm round downpipes use to be rated at 3.5 litres per second (lps) but manufacturers now usually quote 4.2 lps although I have also seen claims of 4.5 lps. Given that a pop drains the gutter and not the downpipe, we should look at AS3500.1 Section 8 which quotes a vertical 75mm pipe with a 50mm head as 2.63 lps and 3.22 lps with a 75mm head when draining a storage tank. That same pipe/pop will however drain more water from a sloped gutter because of the flow turbulence within a restricted space. Having a good gutter slope is important because strong winds can impact on the gutter's flow plus the gutter's high points determine the areas of roof that drain to each downpipe.

Abutting walls also need to be factored for additional wind driven rain.

A (nominal) 150mm DWV pipe holds about 18 litres per metre, this equates to a flow rate of 1,080 litres per minute at a velocity of 1 metre per second. I would be looking at either using a calming inlet or preferably diverting into a settling tank via an inlet fitted a couple of hundred mm above the bottom of the tank at the inlet's lowest point. This would also give you more head but if either alternative is used, then you really need mosquito proof leaf diverters fitted to the downpipes but finding a typhoon proof leaf diverter could be a real challenge.

Large tanks have some mitigation above the overflow but you would still need a massive overflow capacity and the best way to achieve this would be to install a vertical bell mouth on top of a 150mm diameter elbow extension coming off the overflow pipe. A bell mouth can generate a syphon but that won't happen in your situation but a degree of bubble flow which is the third stage of syphonic purge would happen during extreme inflows. Having a larger bell mouth and bubble flow substantially increases the overflow pipe's flow capacity but bubble flow will also ensure that full syphonage with its associated negative atmospheric pressures within the pipe are not realised.







Re pumping water to a second tank, I would have either a fossil fuelled or a solar powered pump on hand rather than rely on grid electrical power. If massive rain was forecast, I would fill the second tank prior to the event and also mitigate the settling tank. I would not consider a 50k settling tank and would have 100k as a minimum. You also need to ensure that the high velocity overflow did not cause problems at the discharge point.

The two calculators below should be of benefit to you. PVC pipe has a roughness coefficient of 150 and you must also allow for friction losses through fittings (elbows etc) by adding an equivalent pipe length for each fitting.

http://www.calctool.org/CALC/eng/civil/hazen-williams_g

http://www.nationalpump.com.au/calculators/friction-loss-calculator/

A sediment trap is of benefit if you have a lot of dust blown onto the roof and it will take 3-4 metres for the majority of suspended sediment to settle as bed load. You would have to reduce the fitting to 100mm and then to 40mm or 50mm. If you have a sediment trap and also a low restriction inlet, you need to close the inlet valve and use the head provided by the downpipes. With your large roof areas and massive annual rainfall, minimising the volume of discharge from the sediment trap would obviously not be an issue.

Bed load is mostly stationary and it clumps in small 'colonies', only breaking off at the front once the 'colony' becomes over populated, the water becomes turbulent or the flow rate increases. For fine detritus, the critical flow rate is about 0.6 metres per second and higher for heavier materials. Because a pipe's laminar flow is almost stationary at the boundary layers, fine particles actually have more resistance to flow than do chunkier materials that protrude into the faster flowing streamlines closer to the core.

The video below is worth watching, note how the turbulence created by the elbow allows the larger heavier particles to travel up the vertical pipe but it is disappointing that the video didn't also show fine particle clumping.

https://www.youtube.com/watch?v=c1xX90ZfBj4

Having a settling tank's pump's draw outlet a minimum of 300mm above the bottom of the tank at the outlet's lowest point will supply very good quality water to the pump. A floating suction outlet hose that takes water from a few hundred mm below the water surface allows you to fit the valve a lot lower and will also provide the best quality water.

http://www.crystalclearwater.com.au/waterboy/

Thank you SaveH2O for your valued response.

I’ve learnt pretty much everything from your replies here and on the ATA website. The Texas AgriLife Extension also has some good info but your ideas about the sediment systems/first flush make more sense.

Might take me a little time to digest and apply some of what you’ve said and I might come back with more questions…

Firstly, can you be a bit more explicit about your comment regarding the 1:20 figure: "A 1:20 storm's figures are based on an average intensity over 5 minutes, that intensity is not transposed to an expected hourly figure.”

Have I done my initial flow rate calculations based on the wrong figures? Or are you just implying not to expect that rate over an hour (for instance, I found this quote regarding the rainfall for one of the larger typhoons in recent years: "Ondoy’s onslaught dumped 455 mm of rain over a 24-hour period. But 341 mm fell in just six hours.”)

For example, using 5 minute intensity 1:20 of 29.84mm
For my flow rate calculations should I be using 29.84/5 to get my flow rates (Lpm)?

Your advice about the size of the 100k settlement tank makes sense and I’ll add it to our plans.

Cheers
Leon

The ARIs are a measure of rain intensity and you are correct with your figures apart from the wrong application of the multiplier when determining the total amount of water harvested. The 29.84mm is the total over a 5 minute duration.

If you lived in Adelaide where the 1:20 ARI is 120mm/hr (an average of 2mm per minute over a 5 minute duration), you could experience a 1:20 ARI yet only have maybe 11mm of rain for the hour. It does create confusion when the ARI is expressed as an hourly figure which is why I try not to forget to explain that the figure is based on an average over a 5 minute duration.

I recommend that people do all hydraulic calculations as per the 1:20 ARI and then add a safety margin but that is my judgement and recommendation, nothing official.

Some peripheral information.....

The 1:20 ARI is used here in Australia for eaves gutter and downpipe roof drainage compliance, there are no similar regulations for harvesting rainwater to rainwater tanks but the authorities often erroneously and unknowledgeably state what must be done. As an example, I am in Victoria and the Victorian Building Authority (VBA) has a technical solution sheet that states that the tank's overflow pipe must be the same size as the inflow pipe. The diagram was inherited from the former disgraced Victorian Plumbing Industry Commission.

http://www.vba.vic.gov.au/__data/assets ... -Tanks.pdf

A charged wet system pipe could be connected to several downpipes and the inflow capacity during heavy rain could easily exceed a tank's overflow capacity...which often happens. During the Millennium Drought, the Brisbane City Council (BCC) mandated that rainwater tanks must harvest 50% of the roof area or 100 sq m, whichever was the greater. Brisbane's 1:20 ARI is 240/250mm per hour yet there was no information given about tank overflow capacities and when the drought broke, over-topping tanks became a big problem.

Water discharge flow rates through a tank's horizontal overflow pipe is subject to Torricelli's theorem and the calculations should have immediately raised concerns about inadequate overflow capacities yet most tank overflow outlets also have mesh that has not much more than 50% open area that restricts the flow. The BCC gave no advice about the hydraulic benefits of removing the mesh and installing an external flap valve or any other recommendations...nothing!

If a water tank is connected back to the stormwater via the overflow, the water tank is effectively part of the stormwater system but this often escapes the attention of many plumbers and other industry 'professionals'.

Thanks again SaveH2O.

I’ve been working through my calculations and this is what I’ve come up with. Can you let me know if I’m on the right track?

If I use the roof slope multiplier (I understand your comment about the wind driven rain not collecting more total rain, but will still use the multiplier instead of adding a safety margin.)


Total catchment area of HOUSE = 615m2
Captured rainfall = 615 x (ARI/5min) = 3673 l/min
Number of downspouts = 16Downspouts with 77mm “pop” drain at 3.22 l/s w/75mm head (AS/NZ3500.1 Fig 8.3) = 193.2 l/min
Total drainage of all downspouts = 193.2 x 16 = 3091 l/min
i.e. the roof will Array drain in a 1:20 event.

However, my gutter profile is approximately 170mm deep, could I assume worst conditions that the downspouts with 77mm “pop” will drain at 4.55 l/s w/150mm head (AS/NZ3500.1 Fig 8.3) = 273 l/min?
Total drainage of all downspouts = 273 x 16 = 4368 l/min
i.e. the roof will drain in a 1:20 event.

Working the other way, could I say that worst case; a downspout with 77mm “pop” will drain a maximum roof area of 273 l/min / (ARI/5 min) = 45m2.
However, with my 2nd floor roof draining directly onto the lower roof this gives a maximum area of about 75m to two of the downspouts, so I should probably consider having guttering put on the upper roof to drain onto the west side (smaller) roof?

Regarding the 150 DWV pipe to the tank, you quoted 1080 l/min (wasn’t sure where you got this from, when I used the calc tool I got 1419 l/min but I assume yours is a more accurate figure).
Obviously a single 150mm DWV won’t drain my total captured rainfall of 3673 l/min. I tried dividing up my roof into the areas covered by each downspout and determined that I would need 3 separate 150mm DWV conveyance pipes from the house to the settling tank! This is a lot of pipe to run and I’m leaning towards risking having only two.

For the garage a single 150mm DWV will suffice.

Regarding the tank overflow, do I basically sum the total captured rainfall l/min in an 1:20 ARI and size the overflow pipe accordingly? If my total from both house and garage is 3235 l/min have the overflow sized at 250mm?

Tanks of 100kL size are very expensive here. First quote I got was about AUD $35k.

My builder has suggested (he'd like his guys to remain in work so eager to do some more jobs for me) that they build it using concrete hollow block and add a non-toxic coating to prevent leaking. It'd be a square shape of about 7m x 7m x 2m. I'm happy for them to do this but wondering if anyone can point me to some good info about building tanks with concrete, especially from a civil perspective. I want to be sure that what they build will hold that amount of water with no problems...

My main worry is the tank won't be very earthquake proof, but considering the amount that a proper tank is going to cost me, I'm willing to take the risk. I'll situate it at the lowest point on the property so if it does happen to burst I think the house and garage will be safe.

Your 1:20 5 minute intensity is mind blowing. We had a 1:500 a few years ago which flooded parts of Melbourne, it was quite an extraordinary event.

Your gutters will drain a bit faster than the figures shown in AS 3500.1 because those figures are for a closed tank and the discharge is a constant weir flow whereas water in a gutter flows and is turbulent.Water inefficiently swirls down a round downpipe because of the annular vortex but I think that your gutters and downpipes will cope.

The 1080 lpm came from 18 litres per metre volume at 1 metre per second, it was just an example but having a required velocity of 2 metres per second is on the high side. Just be careful when you do your head calculations because a lot of plumbers here take the figure from the gutter's sole and then scratch their heads wondering why the system doesn't cope during heavy rain. Water that isn't a solid mass does not add to the head.

The tank should have some mitigation, that is, it should have space between the top of the overflow pipe and the level where the tank would over-top. That will allow you to have a 150mm overflow pipe. A bell mouth is very efficient but you need strong DWV pipe.

Round tanks are inherently stronger than square tanks. You may need to talk to an engineer if you are considering that option.

They build the bigger round tanks on site here and cost nowhere near the amount you have been quoted. You tube should have examples.

22,500 litre tanks are popular here and are made en masse. Do they have a rainwater harvesting industry and manufacturers in the Phils?

Hi SaveH2O

Thanks again for your replies, you're giving me much more confidence about this system.

Noted about being careful about the head levels; the measurements I take should be from the bottom of the leaf catcher/rain head.

There is a huge tank industry, they're everywhere here. The main's supply can be unreliable (has gotten a lot better where we live in the city, probably only lose water twice a year). The most popular brand is Bestank and they make SS304 tanks (which rust easily here in the tropics). Usually installed on elevated stands to gravity feed main's water to their houses. Mostly sized just for a few days' worth of water.

I'm not sure I've ever seen a rainwater catchment tank here, people seem to believe it's dirty water and just a breeding place for mosquitoes and diseases (of course it would be if not managed properly...). We are designing our property with permaculture principles and so saving rain water was a no-brainer for me. A friend who is doing the permaculture design course at the moment and is using my property as his project, wants to use dams, but I'd like a tank for access to cleaner water. During the summer we can have very long dry spells and I can see us using up the water easily just on gardens...

To get a tank of 100 kl size it's either a bolted steel round tank with a liner (expensive - the price I mentioned earlier was actually from Steelfab in Perth who must have a distributor here) or modular tanks, which can be SS304, FRP, or galvanised.
Some prices for modular tanks (foundation not included) have been :

• 50 kl SS304 - AUD $14,400
• 50kl FRP - AUD $10,200
• 90 kl FRP - AUD $17,000

I'm trying to get a quote for galvanised as that will probably be the best option for me (at least that's what I remember all our Australian rainwater tanks were built of).

What is FRP?

FRP is Fibre Reinforced Plastic I believe, about halfway down this page they list "Composition:

"Fundamental composition of FRP modeling sheet: mixtures such as unsaturated polyester resin, thickener (MgO), initiator (curing agent), cross linking agent, low shrinkage additive, packing (calcium powder), internal releasing agent ( Zinc stearate ), colorant etc.
These mixtures dip the glass fiber . And then adding protective films to the two surfaces (polyethylene or polypropylene film) to form sheet compression molding material.
Removing the thin film, clipping according to the required size, and then compression molding."


Galvanised modular tank 98 kl - AUD $21,600

Sigh, I think concrete hollow block will be the way to go....

Instead of having the inlets flow through calming pipes (inverted U-tubes at the bottom of the tank) maybe I can section off part of the tank as a settling area.

Cheers
Leon

I've seen mentioned a few times that the tank inlet should be at least 300mm below the bottom of the leaf eater/ rainhead. If I calculate friction losses for about 70m of pipe I get 600mm. Do I then add that to the aforementioned 300mm? Or is the aforementioned 300mm just a rough figure people throw around to take into account friction losses on a simple setup?

I am glad that you have asked about this and your question also reinforces that you have a good understanding of proper design.

It is an irresponsible and unknowledgeable figure that appears on numerous websites and government documents. It really annoys me to see that figure quoted and when I hear industry 'professional' also quote it, I then know that they don't know what they are talking about.

The required minimum head depends on the pipe's internal diameter, the pipe's length plus an allowance expressed as additional pipe length for the number and type of fittings along the pipe as well as the system's required maximum flow rate. Higher flow rates require greater head and/or often a larger diameter pipe.

There are also other considerations such as allowing for the usual mass of bubbles at the top of the pipe lowering the atmospheric pressure in that section of pipe which negates that sections head pressure plus also the need to take the head measure from a point above the bottom of the vertical riser's horizontal pipe at the highest point above the tank and not from the pipe's invert (the bottom of that horizontal pipe)...for very obvious reasons because the flow of water through that horizontal section will not be paper thin.

A further vital consideration is that larger air bubbles (2mm or greater) rise at about 0.25 metres per second but smaller bubbles rise slower and so they become entrained in the liquid and carried downwards in a downpipe flowing at velocities that are less than 0.25 metres per second. A larger air bubble would not usually be carried downwards in a 90mm round pvc downpipe because the pipe's internal diameter of 86.2mm gives a volume of 5.83 litres per metre and the drain velocity is usually less than 0.25 metres per second BUT a quick calculation shows that 1/4 of 5.83 is 1.46 litres per second or 87.6 litres per minute. If the downpipe was draining faster than 0.25 metres per second, larger air bubbles will also be carried downwards. This not only affects the pipe's liquid volume, it also affects flow capacity due to the bubbles displacing water which lowers the pipe's internal atmospheric pressure (reduced head), for example, an *additional 10% air entrainment will lower the flow rate by about 16-17%.

*All water has dissolved air (micro bubbles) and sometimes a small degree of micro air entrainment that is commonly seen as 'cloudy' water in a basin or similar, particularly when running hot water.

*Actually, not all water has dissolved air but I have tried to keep the explanation simple. You can use 'airless' water to do some pretty nifty tricks that appear to defy the laws of physics but the water will absorb air once exposed to atmosphere.

A 100mm SN4 DWV pipe has an internal diameter of 104mm and its internal volume is about 46% greater than 90mm pvc stormwater pipe. In a typhoon prone area, I would use 100mm DWV downpipes for both strength and to minimise air entrainment during extreme rain events.

Thanks SaveH2O

Unfortunately the builder already has the 90mm DWV downpipes on site, but I might see if I can get him to change just a few to 100mm (the ones that will get the highest flow rate) as well as increasing the size of the "pop".

"reinforces that you have a good understanding of proper design" - most of this comes from reading your posts here and on the ATA!! I feel like I'm back at school ! It took me a few weeks of procrastinating before I fully dived into the rainwater design, was one of my last big worries with the house build. Now that I've sunk my teeth into it I feel a lot more confident about doing it.

Thank you heaps for your assistance - it's great to get so much professional advice from someone on a forum, it is very much appreciated.

Sorry with all these gradual questions but trying to finish this design with two young kids underfoot and not much sleep...

So I've been looking at my tank overflow and now I've understand the reason for your bellmouth and the overflow being vertical. This is all based on the "Rate of Overflow" from AS/NZS 3500.1 Section 8**. The rate of overflow from vertical outlet (Figure 8.3) is much higher than for the horizontal outlet (Figure 8.2).

So my combined inlet flow from the garage and house will be 3235 l/min (54 l/sec). Even with a overflow pipe of 200mm (8" DWV) and height of 200mm above crest of pipe (hence I now understand why tanks need this mitigation space between overflow and the roof of tank!) it only gives me 37 l/sec !

Hence also your recommendation for the bellmouth. Now I've looked for a bellmouth here in the Philippines but have failed to locate something appropriate so this will probably be a homemade job for the bellmouth. Two questions:
1. Should I maybe go up to a 10" DWV (250mm) overflow with bellmouth (can't find a formula/results to back this up this diameter)?
2. Is there a recommended way to run the overflow piping to increase the flow: i.e. I imagine that having a maximum length of vertical pipe inside the tank exiting right at the very bottom of the tank increases the flow due to the gravity fall?

I'm starting to realise the expense of all this piping, especially the larger stuff!! Hopefully the savings from having a concrete hollow block will more than cover it!!

**For a free copy of these standards see https://ia801901.us.archive.org/20/items/as-nzs.3500.1.2003/as-nzs.3500.1.2003.pdf

The standards were updated in 2015 but the relevant information is the same.

A tank's mitigation capacity is critical when there is a statistical probability of the inflow exceeding the overflow at times when the tank is full but the mitigation compartment of course increases more slowly than the inflow.

The other thing to remember is what I mentioned in an earlier post about preparing for a severe rain event by filling the second tank and emptying the larger settling tank. If there was a drain valve on the bottom that ran to a suitable discharge point, then this could be left open and the inflow would also give the tank a good clean. Once the severe rain intensity eased off, the drain could be closed and the recharge would be pristine water. The drain outlet may need to be only 40-50mm maximum.

Another consideration is the mosquito proof mesh that should be on the overflow outlet and I assume that you will also have this. BTW, mosquitoes do go up pipes.

Mosquito proof metal mesh has a very low % of open area...barely 50% and this restricts the flow. The actual flow restriction is complicated because round wire effectively creates micro bell mouths but the restriction (flow rate reduction) is significant regardless.

Using an external flap valve in place of mosquito proof mesh is very highly recommended for extreme overflow situations. I haven't seen the (nominal) 150mm flap valve that I have linked below but our stormwater and DWV pipes both have outside diameters of 160mm, the difference being that stormwater pipe contains a higher % of regrind in the core and the two wall thicknesses are 3.2mm and 4.2mm respectively.

https://plumbingsales.com.au/pvc-flap-valve-150mm.html

There is a trend now here in Australia for some tanks to be manufactured with very high overflow outlets that provide negligible mitigation to increase the tank's capacity as a sales advantage but if the public was aware of the shortcomings of doing this, would they buy them? Unfortunately, the public is treated like mushrooms...kept in the dark and fed bs. Who would buy a tank with a side 90mm meshed overflow outlet if they wanted to harvest more than one downpipe and were told that the overflow's discharge capacity was <120 litres per minute?

A note here about vertical drain pipes. When you pull the plug out of a basin or bath etc, you will see an annular vortex. This is not air escaping from the pipe, it is air being drawn into a pipe, the reason why downpipes never contain less than 3/4 air unless blocked or flow restricted such as a charged (head restricted) system. The vortex is also the reason why water swirls down a round downpipe.

Increasing the orifice diameter (adding a bell mouth) increases the pipe's capacity because it floods (adds a higher % of water to) the smaller pipe. If you had a 150mm pipe with a 250mm bell mouth, you will have a massive increase in overflow capacity. Combined with a reasonable mitigation capacity that you would most probably (and unintentionally) have anyway and not forgetting that mitigation also increases the head, you will not have any worries. A 150mm to 250mm diameter increase is a much lesser % increase than the bell mouth in the photo I posted and there is no reason you couldn't go bigger than 250mm.

You might be able to modify the end of the pipe to a bell mouth shape by applying a heat gun to the end of the pipe and shaping the softened uPVC into shape. I have never done that to this size pipe but it should work, the trick is to keep the heat source moving, don't keep it in the one spot. There are some You Tube videos showing it being done to smaller uPVC pipes.

Re question 2, you really don't want to create even a short duration syphonic effect down a vertical pipe exiting at the bottom of the tank. A standing pipe would also be hard to brace. Just keep it simple and remember that it is insurance.

Thanks for the fast response - I'm a bit lost on your last line about not wanting to create even a short duration syphonic effect. I thought you'd want to encourage a syphonic effect to get faster flow??

Is this because it'll alternate between syphonic drainage and then normal gravity drainage and this is inefficient (or does it knock around the pipe, or is the pipe diameter too large to sustain it?). I tried to google an answer but all I find are the products designed to do syphonic drainage like yours with the baffle.

Cheers
Leon

Both.

The bell mouth could potentially provide enough water and turbulence in your system to generate an occasional syphonic pulse down a longer vertical pipe. If so, the sudden acceleration and the resultant vacuum break would be undesirable due to 'kick' transferring via the elbow at the bottom of the pipe to the tank’s outlet unless the pipe was somehow braced. I have never conducted tests of this scenario but I wouldn’t chance it.