Digging a drainage trench/swale with an uphill topography

[FromHollywood]

I have a 500 gallon subsurface greywater tank that collects groundwater and downspout runoff---it was dug at the low spot of the property. A sump pump in that tank pushes the water (through buried 1.5" PVC) 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 the dependency on electric pumps to get it there. During a big rainstorm and power failure, there is a guaranteed big flood in the basement since the house is near the greywater tank. I'm trying to design a gravity fed overflow to this greywater tank pump system.

I've attached a sketch here so you can visualize what I'm thinking of doing. Start at the bottom of the drawing and work your way up.

http://img208.imageshack.us/img208/8964/drainageproposal.jpg

I'm thinking a ground level trench/swale (a dry creek) from the greywater tank to the county trench would solve the problem. Then if the pumps failed, the water would rise to the top of the tank and spill out into this trench.

The issue is that the trench will have to be dug over about 100' of a slight uphill topography to reach the catch basin. 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 base of a three foot deep catch basin, the water will backup into the trench instead of rising to the top of the catch basin.

My engineering background is not civil so I'm probably missing a basic point here like head pressure or weight of water or something.

Is this going to work? Are there mechanical siphons that could help? Here's a couple of mechanical siphons:

http://www.watermanusa.com/PDF/Siphons PDF.pdf

http://chestofbooks.com/crafts/scientific-american/sup4/Automatic-Siphon-For-Irrigation.html

Thanks for any ideas and help.

 

[MiketheEngineer]

They make battery backup sump pumps....I have one... Might be a cheaper answer...

 

[FromHollywood]

Hi Mike, I have a battery backup sump pump but the water flow, during big storms, is too much for it to keep up with. Can't use those venturi backup solutions either since I don't have city water.

 

[WL123]

A small gas generator for the sump pump may be cheaper.

Your design is good, if the catch basin rim is lower than the house foundation.

 

[WL123]

Also, the gravity pipe and county ditch would have to be lower than the foundation for the trench solution to work.

I thing a gas generator would be better if you don't have gravity working for you.

 

[Hoagie]

Pump as a last resort......I would explore the gravity-type system a little further. Can you discharge into Dry Creek below the house? If not, are there any existing drainage facilities below you? If so, you may be able to tie into it if the neighbor permits--he will need to agree to a drainage easement.

What's the soil like around the house? Can you infiltrate with dry well/s?

My answer to your question (on the sketch) is that there will be an inline check-valve to keep the water from backing up or entering into the line from the catchbasin.

 

[coloeng]

You mentioned that you especially have a problem during a large rainstorm. If during a rain storm the water level in the county tench is higher than the outflow from the catch basin, the water will backup to what ever the water level in the county trench is. You may only be creating a reservoir from the catch basin back to your grey water tank. If you can determine and share the relative elevations of the the different components, i.e. bottom of county trench, high water level in county trench, top of grey water tank, etc. it would probably help to determine if your proposed solution would work.

 

[cvg]

further to coloeng post, an exagerated profile is attached. I came to the same conclusion...

 

[PELS]

I agree with Hoagie that you can place a backflow preventer valve after the pump.

Also if you have the room, you could build an earthen berm between the house and the outlet to the ditch with the top of berm higher than the release point into the county ditch so that if the outfll ditch begins to fill, it will relaase towards the county ditch before backing up towards the house.

 

[cvg]

a check valve should always be installed in sump pumps, however is that the source of the basement flooding? or is it coming through the walls or floor or windows?

berming and flood proofing around the perimeter of the house are options that should be strongly considered if secondary pumps or emergency generators etc. are not considered feasible.

 

[FromHollywood]

Thanks for the ideas!

The house was built in the 1920's and they unfortunately situated it on the lowest spot of the property. The greywater tank is about 20 feet from the house. The ground in the basement and carport are at the same level as the top of the greywater tank. So, when the pumps fail, the water rises to the top of the greywater tank and then starts backing up into the floor drains in the basement and carport. This is the only situation that causes this basement to flood.

I understand that the water will level itself between any two points. But, won't the velocity of the water going down the new dry creek trench prevent some of the water from backing up into the trench? In other words, doesn't the force created by a volume of water flowing downhill prevent some of it from backing up onto itself?

If designed correctly, I would think the force of the head of the water going into the trench might allow the water to then raise 3 feet into the catch basin to the level it needs to go to reach the county trench. Maybe a buried drainage pipe would be a better idea than an open dry creek trench? Would a buried drainage pipe create even more head pressure than an above-ground open dry creek, thereby allowing even more rise in the catch basin?

The soil around the house is spongy at best during half the year. I don't think dry wells will work during downpours as the percolation rate won't keep up with the amount of rain at times (I'm in the Pacific Northwest). There is too much clay. I'm surrounded by a 100 acre farm on all sides but I'm on just 2 acres. French drains would work if they went out 100's of feet in all directions but then they would be on my neighbor's land.

I'm not sure I understand the check valve suggestion since I'm proposing a dry creek as opposed to another buried pipe. There is a check valve on the buried 1.5" PVC pipe that the pump is now using as its outlet from the tank. But, how do you install a check valve in a dry creek? Or, are you suggesting another buried drainage pipe (instead of an above-ground dry creek) and install another check valve in that pipe?

Thanks for all the suggestions so far. I really appreciate your help.

 

[cvg]

you have a certain amount of potential energy which is related to the elevation difference between your overflow and the catch basin. Energy is lost due to friction as it flows down the pipe or ditch. Potential energy is simply converted to kinetic energy in the form of velocity / momentum. if you are going to design the ditch or pipe option, you need to plot the profile and run hydraulic calculations to determine the headloss and velocity to prove it works. it appears to me that it doesn't. however, post a profile on here - drawn to scale or at least showing the elevations and perhaps I am missing something...