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.
HOUSE/GARAGE ROOF PLANS:
Looking at the north face of house:
Looking at the SW face of house
Looking at garage NE face
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)
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
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.
- 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.
- 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?
- 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?
- See in the photos, the second floor discharges directly onto the lower roof (no 2nd floor guttering) - can you foresee any problems with this?
- Bit confused on how to size the overflow of the tanks.
Hope someone can “dive” in and help a little!