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Drainage trench needs to slope downhill but the land goes uphill

Drainage trench needs to slope downhill but the land goes uphill

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  #1  
Old 03-29-09, 05:46 PM
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Drainage trench needs to slope downhill but the land goes uphill

I have a 500 gallon subsurface greywater tank that collects groundwater and downspout runoff---it was dug at the low spot of my property. A sump pump in that tank pushes the water to the corner of the property where it connects to the county trench. Once the water gets to that corner, all is fine. The problem is that I'm dependent on electric pumps to get it there. I'd like to use gravity instead.

I think I could dig an above ground trench (a dry creek) from the greywater tank to the county trench.

The issue is that the trench will have to be dug with a downhill slope but the natural topography of the land is slightly uphill. To get the correct slope, the trench will start at ground level and end up about 3 feet below ground when it connects to the catch basin. The hope is that the catch basin will then fill up and spill over into the pipe going to the county trench.

My concern is that once the water enters the catch basin, it's weight might cause it to back up in the trench as opposed to rising and then spilling into the pipe going to the county.

I suppose this is a simple engineering type calculation but I don't know how to do it.

I've attached a sketch here so you can visualize what I'm thinking of doing.

Drainage Proposal Image.jpg

Thanks for any ideas and help.
 
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  #2  
Old 03-30-09, 02:58 AM
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First thing I would suggest would be to generate some good elevation lines so you really know what your slope will be. With that much up hill to deal with you don't want to guess or it may as stated work in reverse.

Also, I have found things don't always work the same when the ground is frozen or pipes and catch basins freeze solid.

Bud
 
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Old 03-30-09, 07:36 AM
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I already determined the slope using a laser level. A decent downhill slope for the trench, plus the slight uphill slope of the ground over this 100 foot distance would require the trench to be about 3 feet deep at the point it would enter the bottom of a 3 foot deep catch basin.

What I don't know how to determine is if the water will really rise to the top of the catch basin (instead of backing up in the trench) where it would pour into a pipe going to the county trench.

Good point about the freezing Bud.
 
  #4  
Old 03-30-09, 08:08 AM
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I think your concern is correct, but can't quite follow the description, probably my thick head .

Here is what you can do to see it firsthand. Draw a cross section of all drainage involved, land, catch basin, and county drain. Since water will essentially be level, you will see the height at the exit and follow that back the trench where it may be backing up.

Water will only enter the bottom of the basin and fill it to the top if the water outside the basin is as high as the top as well.

Many basements face the same issue of being protected by sump pumps which depend upon electricity. When they are critical, some type of back-up system is needed. Of course gravity would be nice, but it just doesn't sound like you have enough slope to use it.

Bud
 
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