Hi Syd. Welcome to the forum.
Just a few questions first.
Does the pump primarily draw (by using an isolation valve to isolate tank 1) from tank 2 or does it always draw from both tanks? I am assuming that tank 1 is usually isolated except for when you manually transfer water to tank 2. Does the balance/pump line have a valve fitted to it that allows you to isolate tank 1? Are you able to send a photo of this pipe? How long is it? The good thing is that half round gutters are the best for flushing and efficient drainage. The 90mm pvc pipe is measured as an outside diameter, the internal diameter is 86.2 mm which has a volume of 5.8 litres per metre whereas a 100mm pvc DWV pipe has an internal diameter of 104mm and a volume of 8.5 litres per metre but importantly, less friction loss. How long is the current wet system pipe? Is the total roof are 30 x 9 (270 sq m) or (30 x 9) + (7 x 5), i.e. 305 sq m? Where does the gutter overflow? Is it at the downpipe or further along the gutter and is it only the wet system gutter supply that overflows?
thanks for your help:
The 2 tanks are connected to each other and then a single pipe to the pump . There are isolation valves on each tank. We normally draw from tank 2, leaving tank 1 to settle (because its the tank that gets all the input)
The tanks are about 2 metres from the pump and the pipe is underground
The wet system pipe is about 12m long plus the 2 verticals
The total roof is (30 x 9) + (7 x 5), i.e. 305 sq m
Overflows happen at 2 places: primarily at the wet system downpipe but also around the midpoint of the gutter on the front of the house
I've wondered about a Tornada rainhead for the wet system downpipe but thought I'd better look at the whole system first
It is unfortunate that tank 2 only has a 150mm head. It sounds like it was the intended main tank but plans changed when the hydraulic head was found to be inadequate and also the reason why tank 1 (which was almost certainly a later addition) is lower.
Because the
front gutter overflows away from the downpipe, the cause will be either/probably a slope issue or the gutter size is simply too small. The half round gutter would have been fitted to improve on the gutter that was there previously. You first need to check the slope and have it adjusted if necessary.
The
wet system gutter overflowing at one downpipe is most probably caused by the weir flow drainage characteristics at the top of the downpipe
IF the second wet system downpipe doesn't have the same problem and drains a gutter on the same level.
Weir flow causes a vortex to spiral air into the downpipe and this happens even when the gutter is full and overflowing but critical submergence also depends on the velocity of the water going down the downpipe. Are you able to guesstimate the roof area draining to this downpipe?
My understanding is that both downpipes drain to a shared 90mm pvc stormwater charged pipe. Are you able to guesstimate the total roof area that drains to this pipe?
A 90mm pvc stormwater pipe has an internal diameter of 86.2mm and allowing for an effective 300mm head and (wildly and probably optimistically) guessing wet system friction losses of 50 m which includes fittings as equivalent pipe lengths, I arrive at a maximum flow rate of a little more than 250 lpm. Accumulated crud in the wet system and earth movement that may have caused pipe compression will reduce this figure significantly. Your 1:20 Average Recurrence Interval would be about 180 mm/h (can you check your area?) based on an average rainfall intensity of 3 mm/min over a 5 minute duration. The roof slope also needs factoring in calculations to allow for wind driven rain.
The Tornado rainhead is effectively a bell mouth orifice. Bellmouth a vertical pipe at the crest increases the weir flow diameter and when the additional water flows into the smaller pipe, turbulence causes the downpipe to 'flood'. The water 'spiralling' is a natural effect, nothing more. The most efficient drainage method is syphonic drainage which blocks the vortex, allowing the water to completely fill the pipe and fall vertically as laminar flow under self generated head pressure, not around in circles but using syphonics is unrealistic for house downpipes. Despite the puffery, the Tornado is a good product but my feeling is that you have a new 100mm (104mm ID) pvc DWV wet system pipe in your future.
I was considering perhaps also bleeding off some flow to the Tank 1 balance line but filtering and the limited effect has dismissed this thought.
Are you able to guesstimate the roof area draining to this downpipe?
Using google maps and disregarding the pitch of the roof, its about 95m2
I do clean the wet system pipe pretty regularly and Ive flushed it out a couple of times
so in summary I should:
1 check/fix up the slope of the front gutter
2 install Tornado rainhead at the main wet system downpipe
3 Change the wet system over to 100 DWV
thats all sounds pretty straightfoward, thanks for your help
so in summary I should:
1 check/fix up the slope of the front gutter
Yes, investigate the slope and take it from there. How long is the gutter run?
2 install Tornado rainhead at the main wet system downpipe
If possible. Do they make one for half round gutters?
The current downpipe pop is limited to about 220 lpm with a water depth of 50mm.
3 Change the wet system over to 100 DWV
A wise move, 100mm DWV has about 46% more volume and less friction loss than the 90mm (86.2mm ID) pvc stormwater pipe.
Using 100mm DWV also allows you to install a simple but effective sediment trap in a non turbulent section of pipe to capture the bed load. You reduce the flushing pipe size to increase the flushing velocity and to minimise the amount of water that you flush out. We now use an invert taper rather than a pipe reducer.