Drainage Pipe Slope Leaving Storm Water Catch Basin -- three proposals 4

[FromHollywood]

I have a question with regards to design of a gravity overflow of a storm water catch basin.

I'm about to install a gravity overflow from a 500 gallon catch basin which runs down a 140' drainage pipe into another Tank (already existing on a neighboring property) which then gravity feeds eventually to a county trench. I'm constrained by the existing specs of the neighbor's tank so need to design my side of this system so it will work with his. He already put his tank in for me to use so I cannot easily change it. I can do anything I want on my side since I'm still in the design phase.

It's difficult to determine the needed capacity given the various sources that end up in my Catch Basin (all rain runoff and ground water). We're in a rainy Pacific Northwest rural farmland area so the land is relatively flat. My gut tells me 6" pipe will work.

I sketched out three proposed ideas using 6", smooth walled (not corrugated) drainage pipe at various slopes. See the attached jpg's (and one that has everything). There are pros and cons to each of the three. Which of the three do you think is best? Are there any glaring issues I'm not thinking of?

Thanks for your time.

Here's everything in one pdf:

http://files.engineering.com/getfil...1-e1cf687d0d63&file=DrainageProposal6inch.pdf

Here is each page as a jpg:

http://files.engineering.com/getfil...cedd07&file=DrainageProposal6inch_Current.jpg

http://files.engineering.com/getfil...a53e99&file=DrainageProposal6inch_Option1.jpg

http://files.engineering.com/getfil...be04fa&file=DrainageProposal6inch_Option2.jpg

http://files.engineering.com/getfil...e0edfd&file=DrainageProposal6inch_Option3.jpg

 

[dicksewerrat]

First of all you have to get the agreement in writing and it should be attached to both deeds. Second you should find out what his outfall capacity is. Third run the numbers for the two scenarios that have your overflow pipe above the outfall pipe.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[GoldDredger]

I do see one potential problem. If the outfall capacity of the 40 gallon tank get overwhelmed, then the water level "could" raise to the top of the 40 gallon tank.

Since the 40 gallon tank is 3' higher than the top of your tank, your new gravity connection would serve as a conduit for storm flows to backflow if you will back to your property.

This 40 gallon tank, how is storm water fed to it. Is the rim of this tank flush with the ground, so storm water is fed into it overland from the top? In other words where is natural grade with respect to the rim? (This makes a difference on the potential for water to fill the 40 tank to the top)

I would recommend you look into a 'Tideflex' type check valve at the end of your gravity line. It is like a rubber flapper valve, where a higher water level downstream closes it, should the situation arise.

Otherwise I think option 2 or 3 will probably work fine, if this is just an emergency overflow deal for you in the event of pump outage.

 

[TerryScan]

In ALL of your scenarios, the controlling factors are the 40 gallon tank outfall (and water surface elevation). The 40gal tank outfall elevation sets the static water level. This renders most of the storage in the 500gal basin useless.

It seems to me that you need to either bypass the neighbor's tank or lower it as well as the pipe to the county trench for a gravity system to work properly.

Another option would be to put a backup generator on the existing pump as well as upgrade the pump if the existing downstream system will take additional flow.

You may want to hire an engineering experienced in drainage systems to design a solution.

 

[cvg]

during a heavy rainfall, both tanks will be filled. I see no way that you would be able to gravity flow from your full tank to the neighbors full tank. As suggested earlier, you will then have water backup from the neighbors tank to yours, making your flooding problem a lot worse.

I would suggest better options to be:

1) install a second pump and emergency power supply so you can always rely on pumping
2) install a larger or additional catch basin / pond to catch the entire storm. Calculate the size you need. It is not that difficult to do. However, it is probably larger than 500 gallons.
3) install dry well to help drain your catch basin

 

[FromHollywood]

Thanks for the very useful suggestions.

I did think the outfall capacity of his 40 gallon tank might be an issue. An new additional factor I did not realize at the time of my posting is that there is another neighbor feeding into his 40 gallon tank with a 4" pipe. That tank would get fed by this neighbor's 4" gravity pipe and my intended 6" pipe. But, his outflow from that tank is a 7" pipe. That means there would be a total of 10" coming in and 7" going out. Even without factoring in head calcs, I assume 10" going in versus 7" going out doesn't sound good. None of the slopes of these lines is very steep.

I think I better go back to the drawing board and think about your other suggestions like a pond, dry well, standby generator or lowering his 40 gallon tank and installing a bigger outflow from it.

Thanks very much everyone. I really appreciate your helpful suggestions.

 

[jgailla]

FromHollywood,
This is probably superfluous to the discussion, but the flow rate is correlated to the area, not the diameter.
Of course there are many other factors, but generally the gravity flow capacity of a 7" line is greater than the combined capacity of a 4" and 6" lines provided they have the same hydraulic grade line slope.

 

[polariman]

This system looks like an adaquate system for low rainfall amounts. Any high rainfall amounts all bets are off.

I would run a flatter grade 8" instead of a 6" and have the outlet in the 40 gallon at the same invert elevation or slightly above the the outlet elevation of the 6" in the 40 gallon. This will keep the pipe open in freezing situations.

Labor costs would be the same and the pipe costs for 8" is no much more than 6"/

I would agree with a check valve flapper.

Good luck.

 

[brandoncdg]

The basic idea is have the inflow lower than the outflow of the tank. Not sure what the purpose of keeping the tank is though at the end if you are gravity flowing out. The tank is more so you aren't shooting the pump water.

I would suggest doing a drainage area study to find out how much water is going into the beginning of the system for a 25 year storm. That Q=Flow is what will set your minimum pipe size and minimum slope.

For your example say the 500 Gallons per hour = 0.02 cfs. A 6" PVC pipe can handle 0.81 cfs at a 1% slope.

Also the 500 gallon catch basin will just have standing water in it up to the level of the outflow pipe. That is gonna get nasty very fast.

CDG, Civil Engineering specializing in Hillside Grading in the Los Angeles area

http://www.CivilDevelopmentGroup.com