Hi!
I love this forum, and I love this topic. The original post is brilliant, so I read all pages of it. However, I'll have to make some point that realism will really only be reachable in a very limited way, so live with it:
Sand should have crudely 15% of porosity, meaning that approx. the amount of 1/7 water is enough to wet 7/7 sand, and 7 + 1 = 7. In fact, the water goes through the sand, so for a proper treatment, nested volumes would be necessary: 7/7 sand can take 1/7 water which can be filled itself by 1 - 7.
While the difference in how the 2 materials move is the slope the fluid is going for eventually (sand 2/7 per tile, water 0/7 per tile) water flowing through sand actually gets a slope.
And even worse does it get when less water goes through the sand bottleneck it has to do this SLOWER instead of FASTER to be physically accurate...
Another thing on using up the sand: How much glass can you make from 0.5 x 1 x 1 m? We don't know the dimensions, maybe that's 1/7 or 7/7, doesn't matter, in reality it is ~ 1 metric ton of sand, meaning 20 sand bags that are heavy enough not to want to haul them around anymore. So never mind using up any kind of deposit.
Exhaustion of big deposits only occures since mankind industrially exploits them at rates that are meant to exhaust them within 30-40 years as that's most optimal regarding interest rates and such. We do that now at at least 20 times the rate we did that before 1850.