For waterproofing in Florida sand, I used a rather expensive Firestone EPDM rubber pond liner. It's about inner tube thickness. The height and girth of the berm got larger than in this early photo. When finished, this front yard pond needed a 40' by 40' liner that weighed about 600 pounds. Earlier attempts (in a smaller crater) that used a vinyl swimming pool liner only lasted a few years before cracking from acquired brittleness.
I applied used carpet over the EPDM liner to add a layer of protection from claws and other sharp hazards. After tucking into the berm, you wouldn't know it was there. Plants have colonized the carpet and tilapia suck algae from its texture.
I kept the wood chunks away from the "shell" of the pond cavity. That is, there is a vertical wall of sand / soil nearest the water all the way around. I was conscious of a slumping berm over time. There may still be unexpected settling, but I hope I can back fill and adjust if necessary. With a flexible liner, you can peel it back and re-stuff problem edges. For example, I moved the spill way fifteen feet a month ago, by re-stuffing the old lowest point along the rim thereby forcing the sheet flow to find the new lowest point more to my liking.
But I did use thin, uniform layers of fine mulch, and biochar (from wood chips) as I layered the sand on the expanding crater even into the vertical layer of sand / soil that is free of hugel chunks. Uniform compression (micro slumping) throughout, is not seen as a threat.
Seems like an awesome plan for your climate and soil. Do the beds get saturated well above the water level ? How will you prevent it from breeding mosquitos ? Will you add only the amount of water that the beds can absorb or will the system be allowed to fill and overflow ?
The pond was the initial feature in this part of this residential forest garden. It catches and stores rain water high in my flat landscape. Is seemed natural to combine these raised pond berms with hugel techniques--to increase fertility and bolster moisture holding capacity. The raised beds (berms, actually) are planted with perennials--olives along the crest, papayas, white sweet potatoes, moringas, peppers and even bananas at the bottom edge where it blends to original grade. Except for bananas, these crops cannot tolerate soggy soil, even for short periods.
The pond is six feet deep and has tilapia and gambusia (mosquito fish) at present. More species may be added later. Any overflow is absorbed by the berm. Wicking occurs along the protective carpet and (I suppose) adds some moisture to the top of the berm during dry periods. I don't think the berms have ever been saturated because the sand back fill is porous and so are the wood chunks and biochar and mulch.
Roof gutters drain into 4 inch pvc thin wall pipes, which flow into the pond(s). I don't control the water level unless I siphon or pump some out.
It seemed like a happy marriage between ponds, which I love, and hugel magic, which I just encountered.
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