Designing a Sump Overflow for Walk Out Basement?

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PapaDisco

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I have 4 foot deep sump with dual pumps and battery backup (Basement Watchdog system) installed in a walk out basement. I've posted about it here: https://terrylove.com/forums/index.php?threads/plastic-sump-repair-glue-goo-to-use.110581/

Since this basement is a walk out, I'm thinking about installing a curb around the basin and overflow gravity drain with a new 1 1/2" pipe running to the outside. Here are the unknowns:

1. Local hydrostatic pressure of the groundwater during rainy season: i.e., without pumps running, how high will the water get in the basin? Since this site is on a slope (walk out basement), it's possible that the 4 foot deep basin never overflows because the water can always find an easier path down the hill underneath the slab, but how to be certain?

2. Height of the curb to be installed: This is more of a traditional plumbing problem. I have 30' of run, so the pipe needs 1/8-1/4"/foot of fall to drain, so a curb of 4-8" high should do. The basin is in the up hill corner of the basement, so a curb won't be in the way and the pipe can run along the outer wall under shelving.

So if the power fails, I'll have about 24 hours at high flow rates of battery power, and then the overflow drain takes over. This means that the underside of my slab is sitting in water of course (it's built as: slab, 6mil plastic sheet, sand and 6" of 3/4" drain rock) which is not ideal, but until the power comes back on at least this solution should keep the basement from turning into a swimming pool.

Any flaws in this idea? I could build the curb like a shower pan, or maybe bolt a 12" extension to the top of the basin (it's an Everbilt and these are available) with a proper gasket?
 
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