If heavy rain is falling and the ground around your swimming pool is saturated, think carefully before significantly lowering or emptying the pool.
The water inside an in-ground pool helps resist groundwater pressure acting on the outside of the pool shell. If you remove too much of that water while groundwater levels are high, the upward pressure beneath the pool can become significant.
In extreme cases, an empty or nearly empty pool can lift, distort or suffer structural damage.
Why Heavy Rain Changes the Risk
During prolonged or heavy rain, groundwater levels can rise and poorly drained sites can direct additional water toward the pool area.
At the same time, pumping water out of the pool reduces the downward weight resisting that groundwater pressure.
That is why draining a pool during wet weather can be very different from draining the same pool during a long dry period.
What Is the Hydrostatic Valve For?
Many in-ground pools have a hydrostatic relief valve or similar system near the deepest part of the pool.
Its purpose is to relieve groundwater pressure beneath the pool rather than allowing a large pressure difference to develop between the outside and inside of the shell.
But you should not assume that simply having a hydrostatic valve makes it safe to empty a pool in any weather. The condition of the valve, surrounding drainage, pool construction and groundwater conditions all matter.
Do Not Empty a Pool Just Because It Is Raining
Sometimes owners want to lower the pool because rain has filled it close to the top.
Removing a modest amount of excess water is different from substantially lowering or emptying the pool.
If the ground is saturated or groundwater may be high, avoid significantly reducing the water level without first considering the pool manufacturer’s instructions and obtaining advice from an experienced pool professional where necessary.
The Risk Is the Difference in Water Levels
The important issue is the difference between the water pressure outside the pool and the water pressure inside it.
A full pool contains a large mass of water helping hold the shell down. An empty pool does not.
If groundwater rises beneath an empty pool, the situation can become similar to pushing an empty container down into water: the surrounding water wants to lift it.
Poor Site Drainage Can Increase the Problem
Pools on sites where surface water runs toward the pool area deserve particular care.
Blocked drains, poor falls, saturated fill and concentrated stormwater can all increase the amount of water around the pool structure.
This is one reason good site drainage matters around swimming pools as well as around houses, retaining walls and other structures.
For more information about drainage and structural movement, see our drainage and stormwater inspection advice.
If You Need to Drain the Pool
If an in-ground pool needs to be substantially lowered or emptied for repairs, resurfacing or maintenance, the safest approach is to plan the work rather than simply start pumping.
Consider the recent weather, groundwater conditions, site drainage, pool construction and the operation of the hydrostatic relief system.
Where there is any uncertainty, obtain advice from an experienced pool contractor familiar with the particular pool before draining it.
Has the Pool Already Moved?
If a pool has lifted, cracked or moved following heavy rain or a period when it was empty, the cause may need investigation before repairs are undertaken.
Useful information can include photographs, rainfall history, site drainage conditions, how long the pool was empty and what happened immediately before the damage appeared.
If the matter involves significant structural damage or an insurance claim, Cornell Engineers can review the circumstances and advise whether a structural engineering investigation is appropriate.