J.H. Cockerell Consulting Engineers

Pool Water Circulation

Water In the Pool

It is important for water in a pool to be regularly removed for filtration and chemical balancing. Poor water circulation in a pool creates bodies of stagnant water which are a potential health risk to bathers. A pool’s turn-over period is defined as the time taken for a volume of water equivalent to that of the pool’s to pass through the pool’s water treatment plant. Recently there has been a move to significantly reduce the turn-over period of pools, in an effort to improve pool water quality. Reduction in a pool’s turn-over period adds significantly to a pool’s cost of construction, and will be largely wasted if there is not good even circulation of water in the pool.

Pool Surface Water

Pool shells were first designed to have a top water level approximately 100mm below the top of the pool wall, and were said to have 100mm of free-board. This allowed the pool water level to rise as bathers entered the pool and then lower when they left. It also meant that if the pool shell rose or dropped on one side (e.g due to movement of the supporting ground) pool water quality was largely unaffected. Initially, free-board pools had wall and/or floor mounted inlet and outlet pipes to return water to the pool after water treatment and to take water from the pool for water treatment.

When it was discovered that the majority of pool water contaminants were in the pool’s surface water, an effort was made to remove pool surface water for treatment.

Scum gutters, constructed towards the top of the pool wall, were an early attempt at removing pool surface water. Pools with scum gutters were still designed with approximately 100mm of free-board. This meant that if the pool’s shell experienced differential vertical movement, although there was reduced removal of surface water, an even flow of water out of the pool could still be maintained, by slightly raising the pool’s top water level.

Skimmer boxes were developed to more reliably remove surface water, even after there had been differential movement in a pool’s concrete shell. They are now widely used in domestic and resort pools. Their use in municipal and school pools is only occasional because they do not provide uniform surface water removal for the entire length of a pool.

So-called “wetdeck edge” pools, where pool water is designed to flow uniformly over the side of a pool into a channel, provide the best opportunity for removal of a pool’s surface water. They also significantly reduce wave action in a pool’s side lanes and are now widely used in the construction of municipal and school swimming pools. “Wetdeck edge” pools that experience vertical differential movement in excess of ±2mm will not have a uniform flow of pool water for the full length of the “wetdeck edge”. As a result bodies of water in the pool will become stagnant creating a potential health risk for pool users. Our condition reports on pools with wetdeck edges regularly include this problem.