I have to clean an in-ground pool that has become green with algae due to us being away. I assume there is debris at the bottom, leaves and twigs. What type of pump can safely get the water and small stuff out without getting clogged?

Some people warn me about the danger of pool rising. Is this real? If a pool is to rise from underground water then that danger exists during construction too. How do they avoid it?

How do you assess the danger? Do I need to find out the height of water table?

  • "dredging" means scraping out the bottom to increase depth, and you need heavy land moving equipment for that. You want to "drain" the pool.
    – dandavis
    Commented Jun 23, 2020 at 15:37
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    If there's an accumulation of tree litter and algae, dredging is a perfectly appropriate term.
    – isherwood
    Commented Jun 23, 2020 at 16:08
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    For those trigger-happy with the shopping close button, this doesn't ask for product recommendations. It's a valid question about pump types.
    – isherwood
    Commented Jun 23, 2020 at 18:17

2 Answers 2


"Trash pumps" 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.

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    The lifting of an empty pool takes place over a period of time. If the pool is cleaned and refilled within a week and the ground is dry, the chances are very low the pool will raise. If the ground is moist from heavy raining or flooding, then there is a possibility of water pressure against the pool. Commented Jun 23, 2020 at 14:32
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    I'm not sure I'd be comfortable taking that risk.
    – isherwood
    Commented Jun 23, 2020 at 14:38
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    I spent 6 years installing electric distribution systems for subdivisions in south Florida. I've seen many pools pop out of the ground, it is a threat. In construction, many would form the drain basin but not install it so rising water would just spill into the pool. Once completed though, all you need is a little tropical storm to pop out of nowhere and an empty pool will be an above ground pool.+
    – JACK
    Commented Jun 23, 2020 at 16:52
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    – Alaska Man
    Commented Jun 23, 2020 at 17:56
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    Some pools have a groundwater suction line built in. In ground liner pools have to have one, it’s the only way to change the liner, you can’t have groundwater under the liner during installation when the pool is empty. It is generally accessed thru one of the skimmers. There will be on open hole leading to the pump, and a second capped fitting. If you have the fitting, you can remove the cap and It will likely be connected to a pipe that runs under the deep end. That will let you dewater the area under the pool with a suction pump, if you are concerned about groundwater lifting it. . Commented Jun 23, 2020 at 18:26

The most cost effective way is to get into the pool and use a rake, etc, to clean debris ,then use an ordinary sump pump . It is what I do when I need to drain a fish pond. And, yes ,you need to find the water table level ; it will easily float a concrete pool. I believe concrete ships or barges have been built ( Norway ?).


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