We have had the same issue with two of our water features in a work pond. The only time it is anissue is when we clean out in the spring, but when you put your liner inand fill with water, the water in your pond keeps the liner down and the under water pressure doesn't affect it. The only time you will have any issues is if you have a leak or evaporation happens and the under water pushes up the liner, re-filling takes care of the problem.

If you raise the pond with a berm around the surrounding soil, any water above the water table will act as a weight on the liner. The pvc drain is a good idea only if the exit is below the final water level of the pond, and a high enough berm could achieve this. Rocks would help, but may not completely solve the problem on their own.

Keep in mind that you have had some extreme weather lately, and this won't always happen.

...berm...........water level...........berm
../====\~~~~~~~~~~~~~~~/====\.........vent pipe
=/=====\...........pond.........../======\==/===orig grade
................\------------------/................../
................_______________________/vent pipe (perf in gravel)

So the height of the berm creates the water weight that holds down the liner. That causes the pressure to make build up under the liner to be forced out the vent.
Hope this helps.

-TT

is the berm like building the pond above ground a little? & how much higher should it be built, could it just be a few inches above ground?

Hi, I am also building a new pond (15x25) with bog/filter on both ends(one at head of stream and the other at far end of pond/pump return). I also struck water but only about 3 inches max. While the pond is built into a slope it ends near a natural bog. The water table is at about the 3' level of the pond or about 1' below natural bog. Plan is to put about 3-4" of river rock/pea gravel on bottom and am hoping this will along with weight of water hold it down.

Raflipp, please, please reconsider the rock/pea gravel on the bottom. You will have a disaster with anaerobic bacteria and a pond that smells like sewage that you won't be able to clean. You would have to remove all that rock in order to clean it every year and sometimes more than that. Hippos are really a rare occurrence and are easy to eliminate or prevent. While bottom rocks or no bottom rocks is a volatile and frequently fought subject on this forum I doubt any of the forum contributers would go along with this. There is simply no way to flush enough water through the gravel to remove fish poop, uneaten food and rotting plant debris. Why don't you start a new thread where it will get the attention it deserves. Sandy

At my only possible pond location, I had groundwater within 1" of the surface during the high water season. I didn't want to go above ground and I couldn't accept on faith that I could control the pressure of all that water.

I was fortunate enough to have an existing drain to the municipal stormwater system and chose to drain the pond location with a buried pipe system. This limited the depth of my pond (2') because the existing drain was fairly shallow and I still had to trench 75' to get the drain pipe and existing pipe to the same levels for flow. While I wish I could have gone deeper, but never planned on fish anyway, I just drained my pond for fall cleaning (drained through an overflow to the same system) and left it empty for a few days, without a worry.

Surprisingly, for all the groundwater moving around on my property, I rarely see any trickle going to the municipal catch basin.

Despite all the advice I got about a drainage system being unnecessary, I am a firm believer. A flat lot such as yours requires a different approach but it is doable with some effort and money. Dry well, cistern, small filter pond come to mind.

That said, it seems your groundwater is relatively deep and the solution of raising and allowing water pressure in the pond to counteract it should prevent serious problems down the road. But, water never stops doing what it does over the short or long term and I recommend you think about a simple system to remove a bit of it.

Then again, I tend to over-think and over-design about everything at my age as once was more than enough to do my pond. Just my opinion and circumstances. This topic comes up time to time, so search the forum archives for more opinions. Best of luck.

Mike

Hi, I have now finished digging my hole and found I have about a Foot of water from water table. I had to change my original plans to fit the situation. I placed my underlayment, then carpet then liner. I pumped out the water and then used 12 inch boulders for the bottom of pond(the first ones I put in floated, but as I added it gave a nice solid base. This was enough to keep liner down without water in pond (should make it easier to drain and clean pond).