Rainwater Woes

mrsxtro · #1 12/03/06, 03:06 AM

Hi all.
We live in Queensland, Australia and are lucky enough to be having a few showers today. However, it is not making me smile!
We live on 8 acres, with no town water and have two 23,000L water tanks. The house is brand new and sits on the side of a slope (cut and filled to provide a building pad), with the tanks lower than the house. The eleven 90mm down pipes run in to 100mm pipes underground which run downhill either side of the house, to the bottom of the tanks, up the side of the tanks and then over the top and into the tanks via a screened opening on top. I'll attach some links to photos so you get a better idea of what I am talking about.
Anyway.... we are having troubles collecting water. A steady drizzle is fine. We collect all that falls on our large roof area. However, when it gets any harder than a drizzle the pipes back up and all our precious water just overflows from the rainwater heads out onto the ground. The rainwater heads are clean so that is not obstructing the flow. We have been told all sorts of reasons and solutions from lots of 'experts' but are still confused as to why and how to fix it.
Our best guess is that during a decent downpour there is too much water running through the pipes and trying to run up the sides of the tanks. The water can't be lifted quickly enough up the pipes so backs up and overflows. We have 2 ideas which might fix the problem.
1. Place a 'Y' piece at the bottom of the tanks and have two pipes running up the sides of each tank.
2. Have pipes running straight from the gutters at the end of the house closest to the tanks, straight to the top of the tanks. This would reduce the amount of water trying to run through the underground pipes. Ugly but could be disguised by building a pergola on that side of the house for the pipes and passionfruit vines to run over.

So I am after advice. Does anyone here have any experience with this sort of problem? Are our assumptions right about what is causing the problem? If so, would either of our proposed solutions work?
Please help as it makes me want to cry seeing our water spill all over the ground!!!
Thanks.
-mrsxtro.

http://www.flickr.com/photos/tetlaws...7594112082608/

http://www.flickr.com/photos/tetlaws...7594112082608/

http://www.flickr.com/photos/tetlaws...7594112082608/

boonieman · #2 12/03/06, 05:12 AM

Are the tanks you're going into vented to the atmosphere so they don't build up pressure as the water comes in?

Are both roofs all going to a single underground line? May be too much volume of water going thru one pipe.

How much difference in elevation from the gutters to the top of the tanks? It doesn't look like much. The higher the difference in elevation, the greater the flow. Since you realistically can't change the elevation, you may have to maybe run each half of the roofs individually to the tanks. Just my thoughts.

uncle Will in In. · #3 12/03/06, 08:44 AM

The vvertical pipes going up the side of the tank require enough pressure to push the water up to the top. A hard rain doesn't create any more pressure than a drizzle. It would eliminate the bottleneck to run the water directly from the roof to the top of the tank without the down and up again route.

GREEN_ALIEN · #4 12/03/06, 08:52 AM

If indeed the tanks have a screened opening (visible) on the top then that would be plenty of vent for the tank. Three problems I see is 1- that 11, 90mm pipes (approx 3.5" for us westerners) feed into what appears to be 1 or possibly 2 100mm pipes (approx 4.0" for us westerners). If all of those 3.5's feed full into a couple of 4's there is too much volume for the 4's to handle. 2 - There needs to be an open vent somewhere before the climb back up the tanks and 3 - the tanks could stand to be lower or the drop and rise taken out and coming out the slope direct to the tank top.

Ted
Crabapple Ranch

Cyngbaeld · #5 12/03/06, 09:37 AM

I'd go with choice #2. And I know exactly what you mean about losing your precious rainwater. Have a cistern myself that needs tweaking. My 'handiman' who set it up thought he knew better than I did and I have a similar bottleneck. Now I have to get him back to do it the way I wanted it done in the first place.

agmantoo · #6 12/03/06, 11:19 AM

Hi, I have spent a few minutes to review your problem and here are the rough conclusions that I have reached. PS....I looked at a lot of your pics, nice place! Whomever designed the 11 each 90mm drains for the roof did a decent task. That should handle the majority of the rains. These 90mm pipes should handle more than 680 liters/minute each allowing for some line losses. With a rapid rainfall your roof should yield somewhere in the range of 7384 liters per minute. Given you have 11 of the 90mm lines that is capable of carrying away 7480 liters. The 2 each 100mm pipes are capable of optimumly handling 2206 liters only however. Therefore you have a major restriction with these 100mm lines. The 100mm lines should have been 2 each 150mm lines IMO. These are all rough calculations and a lot was assumed. However, this does ID the cause of your problem.

goatlady · #7 12/03/06, 12:32 PM

Seems to me that the rainwater pipes goping into the BOTTOM of the storage tanks is the bottleneck. There is not enough pressure from the slope from the house to the tanks to raise the weight of the water collecting in those tanks. Most probably if the pipes were going into those storage tanks at the tope of the tanks you would be able to catch most every drop.

Hammer4 · #8 12/03/06, 02:03 PM

I'm with agmantoo, you have 11 90 mm lines feeding into 2 100 mm lines? I'm afraid you have some digging to do, you need bigger lines going to the tank or more of them. You will have to dig back up to where the 90 mm lines feed into the 100 mm lines and either put in the bigger lines or split some of those 90mm lines onto new 100 mm lines to the tank.

Area = piersquare

So the area of each of those 90 mm lines are 6358 square mm so 635 square cm per pipe.

7850 square mm so 785 square cm per 100 mm pipe.

So you have 6985 square cm of drain pipe ( 11 * 635 ) feeding into 1570 square cm of pipe ( 785 * 2 ).....

I don't know how to do the fluid flow numbers, but agmantoo covered those....

(Edit...I'm not sure I got my metric conversions right, but I am confident the ratio itself is right, imo you can see that you have about 1/4 the amount of pipe you need going from the downspouts to the tanks. )