# Forum Home Renovation Plumbing  Charged stormwater question?

## JohnnyD

I recently dug up my sewer line and in the process discovered that the stormwater pipe didn't actually go anywhere, it just ended underground halfway down the side of the house (and was full of roots). As my block slopes away from the street a plumber friend suggested using a charged system. The ground level at the downpipe at the front of the house is approx 800mm below the street level and the one at the back of the house is about 1.4 m below the street level. 
 Is this my best option? 
I asked about an ag pit, but he said besides being a hassle to dig, they often collapse and cause other problems. 
Any advice would be greatly appreciated.
Thanks
John

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## Danny

> The ground level at the downpipe at the front of the house is approx 800mm below the street level 
> the one at the back of the house is about 1.4 m below the street level.

  The available head is the height difference between the top of the  downpipe and the outlet at street level. Obviously l do not know the  available head, the downpipe size or the roof area serviced to  be able to calculate the maximum flow rate a pipe would need to handle but even  with a 1 metre head, a 100 mm DWV pipe should be ok if other influences are not abnormal. 
To give you an idea, l design small syphonic systems and the  50 mm pipe we commonly use will flow at about 200 lpm  with a 1 metre head over a distance of about 32 metres. Note that this  distance is conversion factored with fitting losses. A 100 mm DWV pipe  has roughly 4 times more volume and less friction loss. 
You will need to plumb a drain valve at the lowest point as the slope will be conducive to the pipe accumulating sediment.  
Note that stormwater is suppose to have a minimum slope of 1:100. Wonderplumb will be able to advise as to the legality of running uphill to the street but l hear that it is commonly done.

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## wonderplumb

I don't like to do it, but in a lot of cases involving the fitting of rainwater tanks to existing dwellings this is often the only option. Out to the street, I have done this a couple of times with a charged system but you need to run a drain off point somewhere with a screwed cap so you can drain all the crap out of it periodically.

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## JohnnyD

Thanks for the advice so far, especially about the drain off point. I assume this would be underground where it bends to go to the street. Speaking of the bend, is it ok to use a 90 degree bend or is it better to use two 45 degree bends.
Also, can I connect the front and back to the one outlet that runs to the street, or will this cause problems with the flow (especially with the heavy falls we can get in Sydney).
Do you also recommend using 100 mm instead of the 90 mm?

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## Danny

> the drain off point. I assume this would be underground where it bends to go to the street.  
> is it ok to use a 90 degree bend or is it better to use two 45 degree bends. 
> can I connect the front and back to the one outlet that runs to the street, or will this cause problems with the flow 
> Do you also recommend using 100 mm instead of the 90 mm?

  The "drain off point" is the end of the pipe.  
At low flow rates, friction loss through two 45 degree elbows is the same as the loss through a single 90 degree elbow. It is a bit different at high flow rates as other factors come into play, shear causing low vapour pressure that can boil water etc. It is a very interesting subject but not applicable to this situation. I would however use two 45 degree elbows as debris transfer would be less restricted. If your 100 mm (102 mm I.D.) pipe was flowing at just 1 metre per second, it's flow rate would be 492 litres per minute. If l knew the static head, l could tell you the design flow rate. The roof area drained would then have to be factored for a major storm event. One thing that you have to be aware of is that if you don't have the static head to generate the required flow rate, the downpipes will fill and flow back into the gutter during a torrential downpour unless you have a leaf diverter that also provided an air gap. This would however reduce the available head. The upper 300 mm should also not be used to calculate head. The downpipe's structural integrity also needs determination. The charged pipe should be drained after rain events to not only flush sediment and protect the downpipes but to also prevent mosquitoes breeding.  
No problems connecting the front and back. The outlet point has negative pressure to the head, this means that while the water is pushed by positive head, it is also being sucked towards the outlet by the lower pressure in this region. All said and done, it is only a syphon.  
They do not make 90 mm DWV pipe as far as l know. The 90 mm PVCu downpipe is not pressure rated, very thin for its size and should never be buried. The main enemy is ground movement.

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## wonderplumb

I always use 100mm sewer pipe for stormwater. I only use 90mm to make downpipes.

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## Danny

> Thanks for the advice so far, especially about the drain off point. I assume this would be underground where it bends to go to the street.

  G'day John, 
I just realised that l misunderstood the above when l replied to it earlier as l thought you were referring to the head's lowest point being used to calculate the static head. Sorry about that; not the first time l have wrongly answered a query when having something else in mind.  :Blush7:  
From your description, the lowest point of the pipe would be at the back where it is 1.4 metres below street level. You need an I.O. fitted here but the problem is that you will have 1.4 metres of head when you undo it! Design carefully with this in mind. Having it at the end of the pipe is a lot better that at a bend where it rises to the street as the former option will take advantage of the head to flush the entire length.

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## JohnnyD

Thanks Danny.
This is what I had in mind. 
Would this work ok?  Attachment 88791

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## Danny

Looks fine. How high is the gutter from the ground at the back? 
Just in case you don't know, l should point out here that l am not a plumber. At the moment l purely have a design and regulatory involvement in roof drainage and a couple of other select areas within the plumbing industry and my advice here has focused on the hydraulic feasibility of using a charged system in your situation.

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## Farmer Geoff

A couple of thoughts:
1. When there is heavy rain on your roof, there will be a lot of water and possibly debris in the stormwater system you are connecting to at the street. It may effectively be charged as well - at least uphill to the nearest opening (eg street drain) so check that you have some space between that point and your gutter level. Certainly a good idea to have a leaf diverter or similar open escape between gutter and downpipe to make sure that any malfunction doesn't result in overflowing gutter. 
2. Check that there are no potential air traps in the system. From any point in the pipe, air needs to be able to bleed uphill to either a downpipe or to the street. There will be air enter the pipe with the water and any pockets in the pipe will restrict flow even if they don't totally stop the flow.
3. If slope and access allows, the drainage point might be better placed well downhill from the rear gutter entry (eg a 6m length away).  That way you can have the drainage end fairly shallow, put an elbow and have the cap on top of vertical riser at ground level. There will be a fair bit of head - if it is hard to remove cap, try using an expanding pipe plug inside under the cap then with a hooked rod you can release it without getting wet.
4. It seems odd that your stormwater pipe was found to just end in a tangle of roots underground. Has the ground level been raised over the years to cover a surface run-off point? Might the stormwater have once been connected to sewer line? Might there be a damaged section in an old stormwater connection that may be retrievable?

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## JohnnyD

Thanks Geoff and Danny
Here are some answers to the questions posed.
Surface area, front half of the house is 110 sq.m, but it also has another downpipe on the other side (which I don't know where it goes, probably nowhere too!) so to will catch half this area. The back half of the house is 50 sq.m.
Height from ground to the eaves, front 3.0m, back 3.3m.
I intend to put in leaf diverters, inticated by the triangles in the diagram. The approx height difference between the leaf diverter and the estimated charged height is 1.7m. 
Thanks for the help so far. 
PS @Geoff, when I mentioned to a plumber about it just finishing nowhere, he just laughed and said it's not uncommon in old houses!

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## JohnnyD

Here's an updated diagram.
Do I need to add other inspections like in the sewer? I understand that they would be under pressure, and spout water as soon as they were opened.  Attachment 88814

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## Danny

> Surface area, front half of the house is 110 sq.m, but it also has another downpipe on the other side so will catch half this area. The back half of the house is 50 sq.m. 
> I intend to put in leaf diverters, inticated by the triangles in the diagram. The approx height difference between the leaf diverter and the estimated charged height is 1.7m.

  You have a huge hydraulic reserve. No problems there. 
EDIT: Re head pressure when draining the pipe, you can reduce the pipe at the end by using a Tee, reduce the tail pipe size, bring the smaller pipe to the surface and then fit an above ground inline tap to relieve the pressure. Just point the tap at 45 degrees towards the lawn (provided the fence isn't too close). The I.O. can be fitted upstream of the Tee.

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## JohnnyD

Thanks for the advice.
Time to start digging!

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