Sizing sump pit for external french drain

Discussion in 'Pumps and Tanks Well Forum & Blog' started by Teehee99, May 13, 2010.

  1. Teehee99

    Teehee99 New Member

    Messages:
    2
    Location:
    Los Angeles
    Hi all,
    I live at the bottom of a large hill so when the rain falls during the winter (only the winter as I'm in Los Angeles) a ton of subterranean water comes down the hill and under my house and ends up in my crawl space . I'm putting an external French Drain 10ft back/uphill from the house to catch that water and send it to a 1/2 hp cast iron sump pump ...but I have no idea on how big to size the pit .

    The French drain contains 2 x 3" perf pipes side-by-side at bottom of 4ft deep trench ..actually 4" from the bottom resting on gravel ..so 3' 8" deep from the surface .

    Does my pit need to be 18" diameter or greater ?
    At what depth of the pit would the perf pipes meet the pit wall?
    How much deeper than the perf pipes is the pit ?

    Regards
    Daniel
  2. drick

    drick In the Trades

    Messages:
    392
    Its difficult to tell you how big to make the pit because we have no idea how many gallons per minute we're dealing with. Essentially you want to size the pit so that the pump will remain off at least 2 minutes before restarting to drain the pit. If your not sure error on the side of caution and make it bigger.

    The area you can count as being part of your sump pit is the area below your perforated drain pipe, maybe even below the lowest point of the gravel. Otherwise your perf pipe will fill up with water before the pump would kick on.

    -rick
  3. SewerRatz

    SewerRatz Illinois Licensed Plumber

    Messages:
    1,705
    Location:
    Chicago, IL
    Making a sump pit deeper does not help with the turn on and off levels of the pump that has a fixed switch. The pit should be at least 18" deeper than the drain tiles to give the pump adequate turn on room. You want the pump to turn on below the seepage piping. Now as to the sizing for the GPM coming into the pit you need to make the pit wider. I have a formula in one of my books. I will post it up later. As Drick said you want to have it sized where during the hardest rain it will take at least 2 minutes for the pit to fill enough to turn on the pump.
  4. Teehee99

    Teehee99 New Member

    Messages:
    2
    Location:
    Los Angeles
    Thanks all for the info and I understand without GPM it's just guesswork . For now I'm going to go with 2ft diameter hole with 2 ft below the drain tiles for the pit based on your recommendations .
    I've already dug down 6ft for the pit and it looks like 4ft down is the level of the water table (at this point in the year , x weeks after rains) ..so my sump will be both pumping french drain water and rising water (unless the french drain can catch enough water to keep the water level down low enough at that point ) .
    So I should leave the bottom of the sump pit open to catch that rising water right ?
  5. drick

    drick In the Trades

    Messages:
    392
    It sounds like your drainage tile is deep enough to keep your crawl from flooding. Given that I'd cement the bottom of the pit. Otherwise if you have dug down below the water table and you do not seal the pit you will be pumping more than just rain water from the hill, you will be attempting to lower the water table which could turn out to be a pointless exercise since as soon as you pump it out it will be replaced by more water until the water table falls on its own. I think you will just be running up your electric bill with no real gain if you leave the bottom open in this application.

    -rick
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