Author Topic: What is HYDROSTATIC PRESSURE?  (Read 40 times)

Michael Caswell

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« on: August 29, 2021, 07:38:48 am »
What is Hydrostatic Pressure?

The deeper you go under water, the greater the pressure of the weight of the water pushing on you will be. For every 33 feet (10.06 meters) you go down, the pressure increases by 14.5 psi (1 bar).

Imagine a pile of bricks placed on your chest. Just one is say only a pound, put another on and its now 2 pounds, and the more you pile on top, the heavier it gets.

The Erie Canal was dug or built to a depth of twelve feet. so for every foot of water the pressure is 33/14.5 which equals .44 lbs per foot. At 12 feet deep, the pressure is therefore 12 x .44 = 5.28 lbs. It doesn't sound a lot, but that pressure is there twenty four hours a day. There is nothing stopping it except dirt.

Eventually, the water permeates through the soil and escapes. We all know water won't travel uphill, so where does it go? It seeps through the dam, until gravity pulls it toward the bottom of the dam and it drains away into the level ground. This leaves a line of wet soil below and dry soil above. It's called THE PHREATIC LINE.

But, things can go wrong, and the phreatic line can move. If it is raised, say due to a steep slope, (as in most of our dams here) then there is a chance the water will exit above ground level and this can become catastrophic.

Those few pounds of constant hydrostatic pressure don't seem like much of a problem, but a small dribble can remove a few grains of soil, which allows a bigger volume of water to escape, taking more soil, so the path becomes bigger and bigger, until the dam collapses with just 5.28 lbs of pressure.

Control of the pressure and The Phreatic Line are crucial in keeping our residents safe.

Frequent inspections are crucial, and at this time, this is impossible (see Rizzo Engineering Reports) because of the trees and vegetation.

« Last Edit: August 29, 2021, 12:37:42 pm by Doug K »

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Doug K

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« Reply #1 on: August 29, 2021, 01:03:57 pm »
In practical terms the MAIN goals of the Erie Canal Embankment Integrity work is to IDENTIFY where the Phreatic Line is leaking, by allowing for proper, regular embankments inspection of a cleared, turf covered, earthen dam surface and to MINIMIZE the opportunity for the Phreatic Line to leak by removing the invasive vegetation and root systems that cause the "piping" that disrupts a normal Phreatic Line water flow.

That minimization the opportunity part is done by removing large trees and root balls, and then replacing those tree roots with turf grass roots.

Here's a section of cleared canal embankment at Canal Mile Marker 278, note that this is in the Brockport section, where barrier screen plantings have been added.

So did you see the three leaks on this Earthen Embankment? Look again, there's one right in the bottom foreground.

Hmm, you say you can't see it? Probably because you are not a qualified Dam Safety Inspector.

It's a good thing the Canal Corporation hires those professionals who DID see these leaks and marked them for monitoring right after they cleared the vegetation, and remediated the outboard embankment to the proper slope angles. When water started running out they saw the phreatic line integrity issue here and marked these leaks.

Go look again, I'll give a hint to what Mike is talking about...phreatic line leaking.

Still unsure it's a leak? Let's get a real CLOSE-UP look at it.

I can assure you the canal embankment is leaking, one of 4 NEW leaks that were found along our 400' of Canal Embankment Dam frontage here in Brockport.

Thank God our Town Supervisors & Village Mayors on the Westside decided to take care of the safety of their Village & Town residents instead of sticking their heads in sandy embankments and ignoring public safety, in the name of "pleasing voters". This story is the same all along the western end of the Erie, where in 27 miles of clearing embankments the Canal Corporation reportedly found over 3-4 new canal leaks per mile of cleared earthen structure.

And another fact about Phreatic Line Leaks... after watching the Toe Ditches that collect the water from these leaks, these Phreatic Leaks get stronger near the end of the Canal Navigation season, normally start leaking visibly after 4-5 months of having the canal filled each year. Which makes perfect sense as the saturation level of the embankment dam increases over time, with that 5-6 lbs of pressure at the BOTTOM of the canal, pushing water outward through old dead roots, and poorly maintained dams.

On the bright side, because there always is one.. we never have to pay a bill to water our lawn, it's the rule of thumb when you live below an earthen canal embankment, green grass, even in the worst drought.