Capacity of gravity-fed subslab water discharge pipe

[toodman]

I have what I would call a "challenging" subslab water problem in my home. About twice or so per year, the ground becomes saturated enough that a substantial amount of water has to be pumped out of my sump basin. When I say substantial, I mean it's not unusual to pump 20,000 gallons of water in a 24-hour period. I estimate this based on my observations, and using the published data on my two sump pumps. My sump basin is perforated all around, and the water flows in from the sides as the subslab water level rises.

The highest water inflow I ever saw was in October 2005, when my area received the highest rainfall ever recorded in a 30-day, 7-day, and 1-day (7.5 inches)period. At the peak, my two pumps were running continuously, not cycling, and the water level was holding steady. The pumps were keeping up with this enormous inflow, keeping the water from rising any further, but were not able to lower the water level for about 8 hours. At 120GPM (the published rating for my pumps @10 feet TDH), that's in the neighborhood of 50,000 gallons in 8 hours.

Most of the time, say 10 months out of the year, my sump basin is bone dry. But dealing with the occasional huge inflow of water is a challenge. I have a finished basement and I'd like to keep it above water.

I recently took some measurements on my property and determined that it would be possible to run a pipe from the top of my sump basin, just below my basement slab, out the foundation wall, to a discharge point about 150 feet away. The total drop would be about 2 feet. The idea is to have a passive discharge pipe that would be able to deal with the water during a pump failure or power failure during one of these periods of intense inflow.

I have JPEG showing a rough drawing that I will try to attach here. I would go ahead with the project if I had some confidence that this passive discharge would be able to keep up with an inflow of 120GPM into the sump basin, and prevent the water from reaching the top of my slab.

I'd appreciate some opinions from those with more practical experience than me. I have a physics background, but I'm not a professional engineer. I started with the Manning formula and used Manning N=0.010 (for 6" PVC sewer pipe). The result I got was about 0.8 cubic feet per second, or about 360GPM. That's based on a 1.5-foot drop and 150-foot run (1%). I believe this also assumes a full pipe, and I don't know how realistic an assumption that is. I did not know how to take into account the several (gentle) turns that would probably be necessary in this run. Probably there would be about four 22.5 degree turns. If necessary I could probably fit three of these pipes side by side to increase the capacity.

 

[civilperson]

50000 gallons in eight hours= 6684 cubic feet in 28800 seconds=0.232 cubic feet per second. (This is 104 gpm). Q=AV. A=0.982 sq ft and R=0.25 feet, (assuming half full) V=5.4 ft/sec and Q=0.52 cubic ft/sec. This more than double what you require so go ahead.

 

[civilperson]

Whoops! A=0.098 sq ft

 

[cvg]

you need to include headloss components for the pipe entrance and for the bends. you will not get the maximum flow through your pipe until the water ponds up to the level of the floor. If you build it as shown in your sketch right below the slab, you will have at most 10 - 12 inches of head (to the invert of the pipe) at the sump to work with. It would be better to lower the pipe to allow more. You also need to have a free outfall at the end of your pipe. If you don't this may reduce the capacity of your pipe. If you plan a bubble up structure, or anything with a screen, add additional head loss for that.

 

[civilperson]

A gravity fed pipe at a constant slope flowing less than full needs no excess head above the supposed level in the pipe to achieve the flow in Manning's formula. A restriction at the outlet will change the regime to pressure flow as stated by cvg.