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Thread: Force of Water against Plywood @ 6", 1', 1.5", 2', 2.5' etc.

  1. #1
    Associate Engineer
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    Bang Head Force of Water against Plywood @ 6", 1', 1.5", 2', 2.5' etc.

    Hi All, I am not an engineer, however would like to know how much pressure is exerted on the sidewall of a plywood (or other) sided frame when filled with differing depths of water.
    What would the pressure be at different depths. IE; 4",6",12",18",24",30".
    If the area is "Sloped" rather then level, IE; 4" on one side and 18" on the deep end, would it then have a different pressure number than an area with 18" throughout on a level area. Does the force of the water wanting to go down hill have an affect on the amount of pressure exerted on the sidewalls.

    Help.....

  2. #2
    Administrator Kelly Bramble's Avatar
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    The formula that gives the P pressure on an object submerged in a fluid is:


    P = r * g * h

    where

    • r (rho) is the density of the fluid,
    • g is the acceleration of gravity
    • h is the height of the fluid above the object


    r = 1.00 x 103 kg/m3
    Fresh water
    r =
    1.03 X 10 3 kg/m3
    Salt water

    Density of water webpage


    g = 9.8 m/s2

    H = inches x .0254 in meters

  3. #3
    Associate Engineer
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    Kelly - THANK YOU
    Will that also be the pressure against the sidewall then? We're not really submerging an object, but rather filling a shallow "pool" per se' and looking for the outward pressure that will be exerted upon the sidewalls, mainly on the deeper side. Most, will be 4" on the shallow side and then 6-8-10-12" and so on in the deeper side. In doing this for years, we know there's a lot of pressure, as 10-12" will bow out a 3/4" plywood with ease supported every 8'. It will do better supported at 4' intervals. Once it goes to 14"-16"-18". The amount of pressure created isn't relative to the added percentage of water, but rather increases quite quickly, and can bow the boards out and/or push/pull the staked system right out of the ground. Seems that 18" of water carries twice the pressure of 14". Only adding 28.5% more water, however seems to be 100% more pressure exerted against the sidewall.
    Again, I'm not an engineer, and not a math major either. So any real pressure numbers you can provide with the calculations done....all the better



  4. #4
    Administrator Kelly Bramble's Avatar
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    Will that also be the pressure against the sidewall then? We're not really submerging an object
    Doesn't matter - side wall, bottom or what-ever is in the water the equation determines the pressure.

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