I like the shelf to be 2-6" below the normal water line. I also like some areas to vary so when putting plants along the edge you have places to put pots as well as bare rooted. Berm needs to be 2-6" above the very highest mark you have ever seen collect during a hard rain in the yard. The actual need of a berm is to help guide water away from the pond and to prevent water from the yard getting in the pond and from getting behind the liner.
Thanks Larkin! As you might recall from a fairly recent PM, I ended up making my "collar" a little higher than originally anticipated, so in this case, my berm is, well, actually going to be the top of the pond's wall, with the collar being the shelf which is at or close to ground level. Oh, well..... Or it might have been an unconscious attempt to have the pond slightly ABOVE grade.... I have no idea what I was thinking at the time....
With our strange soil & drainage here, there is rarely any run-off or water collecting in the yard. Rain water might sit there for a few hours, but then just soaks into the soil and is sponged into the clay. In the 2 years so far, even with our worst rains, have never seen any indication of berm erosion or water getting between the liner via surface water. But because of the apparent high water table and all of this "sponged" ground water, yes, there is [and has been evidence of] problems with liner-float [but only when the pond's water level was low due to working on it]. Guessing that's WHY my subconscious made me do the collar too high, LOL!
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Back to the original questions...... In addition to it's being a protective barrier,
this berm is also going to be forced into service as the top part of that upper wall -- planning to make it extra wide to accomodate any extra height, so hoping it will be structurally sound
*. So, having the shelf 2"-6" below the water level sounds reasonable. Since I like a little distance from the water to the top of the pond [our rains DO fill the pond and cause flooding -- just not the other way around, thank goodness!], that would make having my berm be in the 8"-10" tall range.
What about the size of the shelf, which will primarily hold rocks to hide liner [as well as some bare-root plants among the rocks]? Would 6"-8" there be a good fit?
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*In an attempt to stabilize the soil in the berm, am trying an experiment.
It might not work, but it's not going to make anything worse. And all I'll be out is the cost of supplies and my extra labor, LOL! I'm tilling Portland and sand into ALL of my clay soil before working it back into the berm. I've done some research, and while there really isn't a recipe for this [except that with clay soil, you need a higher PC to soil ratio -- and the opposite goes if you have sandy soild], there is evidence that this "soil-crete" will stabilize the soil. My hope is that it's going to work with the clay and not let it shift and ooze as much as clay usually does. It's NOT going to harden into lumps or anything, which is what you'd think when you hear "Portland." I'm tilling the PC & sand into the clay until it turns it all into a nice dirt-like consistency -- which means tilling it almost TO DEATH to break up the clay clumps. That gives the PC more particles to attach to. Then with good tamping.....
I have hopes, anyway. And you know, it is so strange so see this dirt in my yard [the tilled stuff] that actually LOOKS like real dirt! It still has a slight gooey-ness to it, but it's so fine and NOT clumpy! I like!
