Pressure Relief Valves in Liner

[Birdman]

I know they make pressure relief valves you can put in your liner to keep it from floating if you get to much ground water under the liner. If the ground water pressure builds to much the relief valve will open and bleed off the pressure into the pond. Has anyone ever used these? Or are there any other ideas for a high water table, no hillside, so the water could not be drained off from under the liner.

[Regenmeneer]

I have a big diameter pipe next to my system, which runs down to the level of the bottom of the pond and in which I can place a pump. This pump will remove the groundwater, placing less stress on the liner. I will only run this pump when I know I will empty the pond.

[MikeM]

Birdman: It is not uncommon in low lying areas of Florida for swimming pools to be built with a pump/drain system to de-water when the water table rises too much risking stress on the pool structure... even if the pool is not being emptied. I am sure there are readily available designs/equipment to deal with this. I'm not personally familiar with the details. I do not like the idea of having a valve that would allow groundwater into the pond. Shallow groundwater often has heavy concentrations of pollutants from lawn fertilizers, pesticides, etc., as well as hydrogen sulfide and the like in some areas.

[gregbickal]

I ran into ground water when constructing a pond for a customer. I sealed up the spot with some bentonite clay, then used field tile to make a loop and run it out to my drain chamber. Once the pond was full of water, I had no bubbling of the liner.

[Birdman]

The biggest problem this guy has is level ground. No place to drain a curtain drain to. I suggested he run a drain from under the liner to the same sump he will be using for backflushing.

[Kohaku]

I have heard of those being used in large ponds. Not sure how effective.

[bekko]

I do not think you would be happy with the results. The water from behind the liner will be muddy and nasty. The relief valve will probably leak some in the other direction during dry periods. When there is no pressure holding the liner against the side of the hole (e.g. after the pressure relief valve has allowed water to enter) the liner will tend to float up anyway. Why not just top off the pond if there is danger of the water level behind the liner becoming higher than the pond water level? If the pond is level-full, the water behind the liner cannot get higher - unless you are as flood prone as we are.

We have had almost constant rain for two weeks - probably 3 to 4 feet of rain since February 14th. We got a foot of rain yesterday before dark and it sounded like another six inches or so in the middle of the night. It was raining faster than the water could run off so some ponds were already overflowing late yesterday. I am afraid to go look this morning for fear that the pumps are under water and the fish are swimming from one pond to another.

-stevehopk ins

[KoiCop]

know what you mean about that torrential Hawaiian rain. When I lived there in the 60's I didn't much worry about the flooding. My text books sank but my surfboards floated -- primo, eh?

But it's not a pretty picture to think about your ponds flooding and your koi swimming free. Good luck.

[bekko]

Don,

The spot where we are was historically used for growing taro and, then later, rice - both semi-aquatics. Much of it is low-lying and the average rainfall is about 160 inches per year. Luckily, the house is up the hill some. I guess floods are to be expected and we have to take the good with the bad. Luckily, our pumps and air blowers were not under water this time. The koi could swim to another pond if they set their mind to it, but they seem content to stay put and the water should have receided some by tomorrow morning. Mike T is just across the mountains and only about 10 miles away as the crow flies - but has very different weather. He probably averages 50 inches of rain per year and had some sunshine today.

-steve

[MikeM]

From what I've read (no personal experience on this), liner below the water table can shift without regard to whether the pond is full. Stress points can occur at bottom drains and the like. If I recall correctly, this is because the weight/pressure of the pond water above the ground water level is reduced as the ground water level rises. At a point well before groundwater rises to the surface, the groundwater pressure becomes too nearly equal to the pressure of the pond water, allowing the liner on the bottom to shift, even if only negligibly. This creates undesirable stresses.

I do not understand the hydrology sufficiently to explain it well. But the impression I have is that a brief seasonal rise in the water table to a point about one-fourth up the liner wall is likely not a cause for much concern, but anything more is; and a permanent water table issue is always a concern.

Somebody with a real understanding of hydrologic pressures should jump in here and point out how these "rules of thumb" I've tucked in the back of the brain are too simplistic, etc.