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?

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.

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.

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