What pump is good for emptying a pool with debris?
&quot;Trash pumps&quot; are designed to handle some degree of debris and sludge, but almost any pump will work if you put a proper filter screen on it. The key is to use a large screen bubble so that the pressure on any part of it is low. This prevents it from vacuuming up junk and clogging. Once you get most of the water out you can start to rake up the debris and move the pump to an area with cleaner water.
The water table may have been low when the pool was constructed, but came up later due to rain or changing grade nearby. You could dig a hole nearby to check. Buoyancy force is calculated as the weight of the displaced water minus the weight of the thing being buoyed (in this case, the pool and its contents). Water is quite heavy, so the force can be large enough to lift a nearly empty concrete pool.