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I believe using damp sand under the liner could be effective in maintaining the slope, as I have previously tried this method with success.
Once you’ve installed the liner, you might consider running some water through the empty grow bed to verify the slope and ensure proper drainage before adding the sand. In my opinion, this is a crucial step for a straightforward and practical drainage test.
If you have any concerns about the slope or drainage, it may be helpful to consider installing a slotted ag pipe down the center.
Installing a slit drain is quite manageable, requiring a skill level similar to that of installing bulkhead fittings into the liner. An alternative could be directing the water from the slit drain into a gutter and then back into a pipe leading to the fish tank.
I believe the holes will be adequate, as bulkhead fittings should help prevent any leaks in the liner. There are plenty of informative videos on building ponds available on YouTube that could provide valuable insights.
A slit drain facilitates faster drainage, which is beneficial for the plants, allowing their roots to spend less time submerged. It also enables the ridges in the grow beds to function as ventilation stacks, drawing fresh air into the sand’s pore spaces.
If you refer to the ‘Vehicle Analogy’ section in the iAVs book, you’ll find that iAVs is designed to perform like a sports engine. Imagine building a sports car but using a lower-quality muffler or having a blocked air intake; while it may still function, it won’t perform to its full potential.
You could certainly implement three drains as you’ve suggested, although it may incur higher costs for parts and require more effort compared to installing a slit drain.