Is 2 x 150mm overflow sufficient for 1 x 225mm inlet in a rainwater harvesting system?

dan_walls · Nov 28, 2024 8:22 am

Hello,

I have been given a design with a 225mm pipe coming from a house down to a tank on a lower level (3-4m lower), about 30-40 meters away. Due to aesthetic considerations, it doesn't work to have the system completely dry so the pipe comes out near the bottom of the tank and has a vertical riser, making the last portion of the system 'wet'. I am aiming to put a first flush device here that will easily allow me to drain/dry the system.

The rain water tank has a 400mm standard opening on top. It has been specified with 2 x 150mm overflow outlets. Ideally I would like to have an internal elbow and bell mouth on both of these outlets in order to raise the height of the overflow and maximise water storage

1. Will this be enough overflow given the 225mm pipe coming in?
2. I would like to have a calm water inlet. Is there a product that will work for 225mm pipe?

Thanks!

SaveH2O · Nov 28, 2024 6:15 pm

Before I give a full answer, I need to know the harvested roof area and your 1:20 ARI/AEP.

dan_walls · Nov 28, 2024 7:15 pm

Sorry - I meant to include those. 1:20 ARI is 220mm/h and total roof area is 900m2. There are actually two x 225mm each one going to a separate tank. I was just trying to keep the example simple.

SaveH2O · Nov 28, 2024 9:10 pm

So, two 225mm pipes each diverting water from 450 sq m of roof to separate tanks.

A 150mm DWV pipe has an internal diameter of 146mm. This equates to a volume of 16.74 L/metre.

I assume that you will have a mosquito proof meshed flap valve at the end of the overflow pipes rather than having flow restricting mesh at each tanks overflow outlet.

A standing pipe has a greater discharge capacity than has a horizontal outlet with the same water level above its invert.

Your 450 (sq m) x 3.67 lpm (the 1:20 ARI average rain intensity over a 5 minute duration) is 1,651 lpm.

The following overflow rate figures are for a 150mm ID vertical pipe, NOT a 146mm ID vertical pipe. The first figure is the water height above the crest, the second figure is the overflow rate in lpm.

100mm 950
150mm 1,090
200mm 1,260

Bell mouthing a standing pipe's weir (increasing the weir wall) substantially increases a standing pipe's discharge capacity. The figures below are for a modest 200mm bell mouth.

100mm 1,584
150mm 1,940
200mm 2,240

You also have to consider the overflow pipe's velocity and this is easily calculated by remembering that a 146mm ID pipe has a flow rate of 1,004 lpm at a velocity of one metre/second.

It is recommended not to exceed 2 m/s but 1.5 m/s is considered ok.

Also calculate the tank's volume for every 25mm rise above the vertical pipe's crest and remember that the overflow rate increases as the water level rises. With just a 50mm depth above the crest, a 200mm bell mouth will discharge at 840 lpm.

You need to factor the tank's mitigation capacity above the invert and crest, assuming the two pipes start overflowing at the same water depth. A tank's mitigation capacity is the safety margin plus the higher the water level is above the invert or crest, the greater the discharge capacity will be.

A vertical pipe's discharge capacity can be substantially increased by drilling a horizontal line of weirs (holes) near the top of the pipe. Fifteen equidistant 20mm holes drilled in a 200mm pipe and submerged 100mm above their centre point will flow at a combined flow rate of 396 lpm while twelve equidistant 25mm holes drilled in a 200mm pipe and submerged 100mm above their centre point will flow at a combined flow rate of 497 lpm.

If you chose to drill a horizontal line of either eleven 20mm or nine 25mm equidistant holes into the 146mm ID pipe, at 100mm submergence at their centre points, the respective combined flow rates would be 290 lpm and 372 lpm.

The flow rates are calculated by using Torricelli's theorem and the 100 mm critical submergence in this case is sufficient to prevent air entrainment or vortex formation.

If you raise a 200mm diameter vertical pipe 100mm higher than a 150mm horizontal outlet's invert and the water levels are thus 100mm and 200mm above the respective crest and invert, the vertical pipe will discharge at 1,584 lpm whereas a horizontal 150mm outlet with 200mm of water above its invert will flow at a more modest 996 lpm.

SaveH2O · Nov 28, 2024 10:18 pm

dan_walls

Due to aesthetic considerations, it doesn't work to have the system completely dry so the pipe comes out near the bottom of the tank and has a vertical riser, making the last portion of the system 'wet'.

Is there an embankment close to the tank that is higher than the top of the vertical riser?

If not, then the wet system will be full to the same level as the elbow invert at the top of the riser.

dan_walls

I am aiming to put a first flush device here that will easily allow me to drain/dry the system.

I assume that you are referring to a subsurface first flush. This will NOT serve as a first flush diverter unless the wet system pipe is drained after each rain event.

You are better off fitting a proper sediment trap with a manual drain facility.

dan_walls

I would like to have a calm water inlet. Is there a product that will work for 225mm pipe?

Just join three 45-degree elbows together but you need to have water diverting into a tank to first pass through mosquito proof mesh. This would be satisfied by fitting good leaf diverters to every downpipe.

What size are the downpipes? If you have suitable leaf diverters fitted to the downpipes, you can then also fit a supplementary low restriction inlet that will equalise the water retained in the wet system to the same level of the water in the tank.

Also, will you have 225mm pipes all the way, assuming that you are diverting multiple downpipes to the 225mm subsurface carrier pipes?

Why not have two 225mm pipes diverting to one tank also fitted with two bell-mouthed vertical overflows. This tank would then serve as a settling tank to transfer decanted water to the second tank which would not need an overflow. The pump would be fitted to the second tank and you could put the icing on the cake by having the pump drawing water through a floating intake filter.

dan_walls · Nov 30, 2024 7:06 pm

Thanks SaveH20 for such a detailed reply. I appreciate it.

There's a bit to digest in that and I have some follow-up questions to help my understanding if thats ok?

First the extra information you requested:

- The downpipes are all 100mm.
- I am aiming to use leaf diverters (as opposed to gutter guard).
- The downpipes fall into carrier pipes of increasing size (eg. 150mm, 200mm, 225mm) as more and more downpipes are added to the flow. Once all of the downpipes are added then it is a 30-40m run of 2 x 225mm pipes. There is an approximately 1.5-2M drop in height from the last downpipe to the top of the tanks.
- The roof areas diverted to each 225mm pipe are roughly equal. So 450m2 per pipe is a good assumption.
- Embankment? Yes the tanks will be nestled into the side of the hill down from the house and the system will be 'wet' with a vertical riser to avoid the aesthetic of having the pipes coming horizontally out of the ground and into top of the tank. I think it will only be wet in that vertical riser though - probably level with the top of the bell mouth overflow inside the tanks.

Terminology

I'm uncertain what you mean by a standing pipe and weir vs a horizontal outlet and where the crest and invert would be referring to, but using this post of yours (viewtopic.php?f=35&t=85013) I believe that the horizontal outlet is just the standard overflow hole they put in the side of most tanks and the standing pipe is the internal elbow solution with the bell mouth as pictured below:

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I'm going to concentrate on the internal elbow/bellmouth solution as that seems a sensible way of maximising overflow rates.

Given that I will have roughly 1,651L per minute coming in on a single tank, then the two 150mm outlets with internal elbows and 200 mm bell mouths and 100mm clearance above the crest of the bell mouth, should provide 2 x 1,584L/m (ie. 3,168L/m) if the tank somehow got to full capacity, which seems like plenty of capacity.

The storage capacity would then be up to 100mm from the top of the tank which seem pretty good. The key takeaway will be asking the tank manufacturer to put the 150mm overflows a bit lower than normal to allow room for the elbow and bell mouth internally.

Are these bell mouth and elbows etc able to be purchased as a kit somewhere?

Calm Inlet

Just using a few 45deg elbows - great idea! Nice and simple. I was wondering whether the final piece could be 90 deg so that it points straight up? Also - my main issue here is getting that 225mm pipe through the 400mm inlet opening on top of the tank and sealing around it. Is there a product that allows this?

First flush device

I was going to use one of these on each 225mm pipe (note there is a 225mm version and they can be installed horizontally).
https://rainharvesting.com.au/products/ ... ommercial/

I believe they can automatically drain off a timer and I would also be able to manually open them up to flush out as well.

Pipes into single tank?

I'm achieving the decanting in a slightly different manner. For further context, I'm planning a high volume, low pressure transfer pump on the second tank to move water to a further two other tanks (tanks 3 & 4) up behind the house (I need a fair bit of storage). This is actually where the filtration and main house pump will be. Tanks 3 & 4 will only be fed in this manner and I was going to use the floating intake valve on tank 2 as you suggested.

I probably should draw a picture!

SaveH2O · Dec 01, 2024 7:37 pm

dan_walls

- I am aiming to use leaf diverters (as opposed to gutter guard).

Good move. The best leaf diverter currently available is the ICON Leaf and Debris Controller. Bunnings use to sell it but you can still order it at the special orders desk because Bunnings still sells other ICON products.

The ICON unit has a steep sloped straight outer filtering screen and a large internal mosquito proof mesh that is easily cleaned by pulling out a drawer at the front.

The best clean gutter system by a huge margin is Leaf Safe Guttering.

dan_walls

- The downpipes fall into carrier pipes of increasing size (eg. 150mm, 200mm, 225mm) as more and more downpipes are added to the flow.

You are fortunate in that the head of the wet system will be dry gravity drainage which will give good upstream flushing.

A (DN) 100mm DWV pipe will flow full at 510 lpm with a one metre per second velocity whereas a downpipe, unless blocked, will never have less than 70% air. For flushing efficiency and economics, try not to upsize the subsurface carrier pipes too soon.

dan_walls

Yes the tanks will be nestled into the side of the hill down from the house and the system will be 'wet' with a vertical riser to avoid the aesthetic of having the pipes coming horizontally out of the ground and into top of the tank. I think it will only be wet in that vertical riser though - probably level with the top of the bell mouth overflow inside the tanks.

It will be level with the water level in the tank if you fit a low restriction inlet.

dan_walls

I'm uncertain what you mean by a standing pipe and weir vs a horizontal outlet and where the crest and invert would be referring to, but using this post of yours (viewtopic.php?f=35&t=85013) I believe that the horizontal outlet is just the standard overflow hole they put in the side of most tanks and the standing pipe is the internal elbow solution with the bell mouth

Standing (vertical) drainpipe. Often called a standpipe, they can also be pressurised and have a variety of uses. I like the term "standing pipe" because it conjures an upright vision.

The open top of an open standing drainpipe is called the crest and the open circumference is the weir. Gravity drainage is referred to as weir flow. If you drill holes in the standing pipe just below the crest, you are adding additional weirs and this greatly increases the flow capacity when the initial water height is not far above the crest. This helps retard a rising continuance during the onset of heavy rain.

dan_walls

Given that I will have roughly 1,651L per minute coming in on a single tank, then the two 150mm outlets with internal elbows and 200 mm bell mouths and 100mm clearance above the crest of the bell mouth, should provide 2 x 1,584L/m (ie. 3,168L/m) if the tank somehow got to full capacity, which seems like plenty of capacity.

The ARI figures can be misleading. The 1:20 ARI figure is the qualifying average rain intensity. The 1:50 ARI is obviously a higher rain intensity than a 1:20 and the % increase actually varies between regions but a 30% increase in intensity is typical. This means that you could be experiencing a 1:20 ARI (it needs to only achieve the 5-minute average intensity duration) but be experiencing a rain intensity that falls just short of being a 1:50 ARI.

dan_walls

Are these bell mouth and elbows etc able to be purchased as a kit somewhere?

The elbows are standard pipe fittings that should fit inside the overflow outlet. The tank manufacturer should know.

The bellmouths I have seen all looked commercial.

dan_walls

Just using a few 45deg elbows - great idea! Nice and simple. I was wondering whether the final piece could be 90 deg so that it points straight up?

Also - my main issue here is getting that 225mm pipe through the 400mm inlet opening on top of the tank and sealing around it. Is there a product that allows this?

I would drill large hole in the roof of each elbow.

Never seen it. Maybe the tank manufacturer will know.

dan_walls

I was going to use one of these on each 225mm pipe (note there is a 225mm version and they can be installed horizontally).
https://rainharvesting.com.au/products/ ... ommercial/

I believe they can automatically drain off a timer and I would also be able to manually open them up to flush out as well.

It is very disappointing to see a large company promoting an endemic, extreme bad practice and I refer to their diagrams showing first flush diverters fitted to the top of vertical risers. Simply unforgivable!

The timer is an aid well worth having. Drippers on first flush diverters discharge a lot of water and the cleaning regime is outdated.

dan_walls

I'm achieving the decanting in a slightly different manner. For further context, I'm planning a high volume, low pressure transfer pump on the second tank to move water to a further two other tanks (tanks 3 & 4) up behind the house (I need a fair bit of storage). This is actually where the filtration and main house pump will be. Tanks 3 & 4 will only be fed in this manner and I was going to use the floating intake valve on tank 2 as you suggested.

Excellent.

You are better off having a floating intake made for you by an irrigation store. The commercial ones are overpriced, the 'filter' often has large apertures (1200-1400 micron are not uncommon) and you should also have a collar/disc fitted to keep the filter above the sediment layer should the water level ever get low. The H1 member who started the thread linked below had a disc 3D Printed and we had some initial discussion about the design criteria. The discussion started around page 3.

Rainwater Harvesting Help • Home Renovation & Building Forum

Are you doing this installation for yourself? If so, how many have you previously installed? You have very good knowledge and understanding about RWH best practice.

dan_walls · Dec 01, 2024 9:16 pm

In answer to your question, I would like to try the installation myself (at least around the tanks - not the downpipes etc.) or perhaps in close concert with a plumber friend. I've never installed a RWH system before but I enjoy understanding how things work and doing the research. It's been quite interesting to discover the RWH community/industry! I'm particularly focused because this will be our main source of water (before we've been on town water).

FFD
In relation to your comments on the FFD - were you saying that the solution I was looking at is not a great idea or product? I understand that fitting any FFD at the top of the vertical riser in a wet system will do nothing, but having the FFD I linked at the lowest point in the pipe (ie. just before the vertical riser up into the tank) seems to be good design with the following main benefits as far as I can tell.

- captures the first flush off the roof
- will allow the 'wet' system to discharge slowly and avoid water becoming stale and anaerobic (thereby making it more of a 'dry' system).

I'm more than happy to consider other solutions as well but I liked that it was a ready made 225mm FFD and came with the timer.

Weir Holes

Drilling those extra holes that you mention in the internal vertical elbow would give greater overflow mitigation, however if I am understanding you correctly, it will also reduce the amount of storage in the tank which was one of the advantages of the elbow and bell mouth.

It would seem to be a tradeoff between total storage capacity vs having the weir bell mouth lower (to leave more room above allowing greater discharge rates) or leaving the bell mouth higher and drilling the holes.

SaveH2O · Dec 02, 2024 7:19 pm

In post 6, you wrote "There is an approximately 1.5-2M drop in height from the last downpipe to the top of the tanks." Is this drop from the top of the downpipe or is it an elevation drop from the buried pipe?

I have been assuming an elevation drop and that the upstream section of subsurface pipe will be dry after it stops raining. Have I made a mistake thinking this?

dan_walls · Dec 02, 2024 8:17 pm

It’s an elevation drop from the buried down pipe. Sorry for the confusion. It would be another 2.5 meters to top of down pipe.

SaveH2O · Dec 02, 2024 10:07 pm

Ok., thanks. I had to make sure.

Given that a substantial part of the pipe run will be dry even when the tank is full and also considering the cost of the commercial unit, I would make my own and fit it to a dry section. Your set up is really the ideal scenario.

I would.....
Plumb to the 225mm pipe a 225x150mm junction with the 150mm branch pointing down.
Connect a 150mm 90-degree elbow to the junction's branch.
Connect a 150x100mm invert taper to the elbow.
Connect 100mm pipe to the taper.

You can determine how much flush you want to capture and using the 100mm pipe will save heaps. 100mm DWV pipe holds 8.5 L/m. There are also several options re what to fit to the end of the pipe for flushing.

The most important time to capture dirty roof wash is after a dry period.

I wouldn't install this type of first flush and sediment trap in a flooded section because of the yield loss when flushing the pipe.

Wet systems are only bad when they retain crud that bacteria feed on. Unfortunately, this is most of them! When designed to best practice, the water retained in the pipe will be the last and cleanest water diverted from the roof with nothing to contaminate it.

A four tank RWH system with sediment traps was designed on Homeone a few years ago. I have linked its 12-month summary below.

Need advice on our rainwater collection and storage system. • Home Renovation & Building Forum

Bell mouth

I numbered the holes by having spacings one hole diameter apart. The top of the holes would be 10mm below the weir, a total of 35mm below the weir for a 25mm hole. It is an option that you can play with but if you are having two overflows, then the combined overflows will need no assistance.

The bellmouth wall is angled and I don't know how this 'angling' would affect the flow rates through each hole. I will try to find out.