basement flooding - gravity drain solution

[wbertocci]

This summer our basement flooded with about 12 inches of water. We have two sumps that basically got overwhelmed for several hours. Since we live on a steep hill that had a notch cut out for the house, my intent is to have a contractor dig down and install a gravity drain from the bottom of the sump pit down to the storm drain at the road. Total length of pipe will be about 180 feet. It will need to travel horizontally about 30 feet before it descends following the contour of the front yard. The bottom of the sump pit is about 10-15 feet above the road.

My question is about the flow capacity of different diameter pipes. I'm leaning towards a 6" pipe. The two sumps can pump at about 100 gallons/minute and were overwhelmed. As a result, I'm looking for something that can handle at least 5 times as much so I don't have to worry about the next hurricane or northeaster.

What are the flow capacities of a 4" and a 6" pipe.
Is there a recommended slope for the first 30 feet before it pitches down steeply?

 

[JStephen]

How deep is the sump?

IE, suppose the sump is as full as you want it to get at maximum flow rate. How far from the water surface to the pipe opening?

Is there one of those floor drain fittings over the pipe opening?

That information would be helpful in determining the flow rate.

One issue you may run into is the pipe clogging at the entrance. Any trash that's floating around is going to go right over that pipe.

I'm not a basement expert (maybe you'd want to repost this in the civil engineering section), but it seems you're figuring how to get water out of the basement. I'd be inclined to look for ways to keep it from getting in. Maybe a french drain on the uphill side of the house, something like that.

 

[hacksaw]

a 100 gpm flow in a basement is a river of water and use of a sump pump inside is not the most effective approach.

following up on JStephen's response,what can you outside the basement?

 

[CESSNA1]

WBERTOCCI:

There is a reference book called simply "DESIGN" by SEELEY. It has the information you are seeking.

Regards
Dave

 

[wbertocci]

We have looked at the problem in terms of trying to stop water from getting under the house in the first place. After lots of discussion as well as free and unsolicited informed and uninformed advice , we have come down to improving the flow out of the sump pit as the lowest cost and most likely to succeed.

The gutters already run into an underground piping system to the storm drain so we know that's not the problem.

Rumor has it the the original owners in 2000 once had 6 feet of water in the basement. So with 2 pumps we only had 12 inches so that seems like a good improvement.

The house is on a steep hill primarily shale, which was literally dynamited to flatten out a spot for the house. Most of the other ideas were based on either regrading the back yard, a french drain at the foot of the hill. However, after several experiments I have settled on the better flow strategy as the most likely to succeed. If nothing else, I won't have to worry about power failures. We think that water goes down in a variety of ways, not necessarily right near the house and raises the water table.

The distance from the basement floor to the bottom of the sump pit is about 18 - 24 inches. The pipe will need to come under the footings so the entry will be at the very bottom of the pit. As long as the water level never gets higher than 6 or 7 inches from the basement floor. That could provide about 12 of height in worst case above the entry of the pipe.

 

[katmar]

If you want a safety factor of 5 on your flow of 100 GPM then you will need 250 GPM in each of the two pipes.

To get this in a 4" pipe you will need a slope of 1 in 10, or in a 6" pipe you will need a slope of about 1 in 50 over the initial "horizontal" section.

Theoretically you could rely on the steep portion to syphon the water out, but I would play safe and go for the slopes given above.

All the theory and tables you could possibly want are available at

http://www.cispi.org/handbook/chapter8.pdf

 

[JStephen]

The normal way to approach flow design in a pipe is with Bernouli's equation. By assuming the pressures at either end of the pipe run (IE, atmospheric) and knowing the elevation difference, you can calculate a flow rate. And for better accuracy, figure in pipe friction and minor losses.

The problem is that when you do this, the same calculations will show a partial vacuum along part of the length of the pipe (assuming the pipe is steep enough). If this partial vacuum is achieved in actuality, then the flow rate you calculate should be reasonably close. But if not, you can overestimate the flow rate (and the pipe wouldn't be full of water, but have water and air in it). It's similar to the problem you have when siphoning through a hose, where you can "break the syphon" by letting air back into the line.

Anyway, one approach to the problem is to assume that pressure at the entry point is atmospheric also- like if you have a tub of water with a hole in the side, how fast can water drain out that hole? Making this assumption, the depth to the hole becomes the important factor. And in some past calculations along that line, I've come up with flow rates of 260 GPM for a 4" pipe and 590 GPM for a 6" pipe, based on steel pipe sizes, with 12" of liquid above the pipe opening. This assumes the pipe is an open pipe, not one with the floor-drain type cover on it. If you achieve the partial vacuum condition, actual flow rates might be several times, but they could be as low as those numbers.

A couple of other issues to consider. You give your sump capacity. Is that the actual pumping rate, or is that the "rated capacity" of the pumps? What I mean is, that actual pumping rate would depend on the lift and the piping, and could be a lot less than what the nameplate on the pump indicates. So you might be moving a lot less water than what you imagine.

A second point is that water moving under or around a structure can wash out any kind of soil that is there. Maybe if it's all solid rock and concrete, there won't be a problem. But if you wind up moving a lot of water, don't be surprised to find voids forming whereever there is any loose material.

 

[wbertocci]

Thanks to everyone for the various comments and suggestions and leads into books and tables. It looks like analysis supports the idea of a 6 inch pipe. The incremental cost over a 4 inch pipe I now also have some specific information for the excavator on a minimum slope for the first 30 feet.

Hopefully this will solve the problem. In any case, it will be a substantial improvement and it eliminates the problem of dealing with power failures. It seems silly to have sump pumps when we are so far above the road. Even if we have to do some addition regrading, this will be worthwhile.

 

[katmar]

Only after I made my previous post I thought of the obvious simplification - if you use two 6" pipes for the 30' "horizontal" section you could probably common them into a single 6" line for the steep section down to the road. If the drop is 10' over the 120' down to the road a single 6" line should cope.

 

[dicksewerrat]

If possible you want to add in some kind of trap. You may get ordors coming back up the pipe into the house. the line will act as a vent for the storm drain. Or put a sealed lid on the sump.