A large amount of rain or runoff water entering your pool may have an effect on the cleanliness of the pool, the pool's water chemistry and the landscape around the pool. Rain and runoff, as well as the wind that sometimes accompanies heavy rain, often carry with them dirt and debris that can settle into the pool and contaminate the water, and you'll have to clean up the mess after the rain stops.
Rain may also have an effect on pool water chemistry. Rain tends to be acidic, and large amounts of rain can lower the pH of pool water. The concentration of free chlorine in the water and the water's total alkalinity level may also drop after heavy rains as a result of dilution of the pool water. Contaminants brought into the water by the rain may also lower the free chlorine level as the chlorine reacts with the foreign matter.
Pool overflows may also cause damage to decks and structures around the pool if the area has not been landscaped so that runoff water drains away quickly.
Draining Excess Water
If heavy rains have raised the water level of your pool to the point of overflowing, you may need to drain some the excess water. If your filter has a backwash or waste setting, you can run the pump with the filter set at these settings to remove water from the pool, directing the draining water to a safe area via a hose connected to the pump. If your filter does not have these settings, you may be able to use a submersible pump and hose to remove water. In either case, remove enough water so that the water level is 1 to 2 inches below the coping at the edge of the pool.
Cleanup and Water Chemistry
After heavy rains, you'll likely have to clean debris out of the pool. Use a skimmer or rake to remove any leaves, branches or other large debris, and check your pump strainer and skimmer basket to be sure that they're free from debris. Brush the sides and floor of the pool and then vacuum up any remaining dirt and debris.
After the water is clean and at the proper level, use a test kit to check pH, chlorine and total alkalinity levels, and adjust the levels if necessary using the proper chemicals. In most cases, it isn't necessary to shock, or superchlorinate, the pool after a rain, but if heavy rains or storms have introduced a lot of debris into the pool, shocking can help to restore the water's chemical balance, which can be thrown off by large amounts of organic material.