When calculating a lot's ISR (Impervious Surface Ratio), things like concrete sidewalks, paved driveways, rooftops & hard-surfaced patios obviously count as impervious surface.
But when it comes to swimming pools and hot tubs, does your jurisdiction count the water surface area of a pool as pervious surface, or impervious surface? Why?
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I'm not a Zone Master, but it would seem that a full pool would be considered impervious. The issue here is determining how much storm water runoff the lot will experience and if adequate steps have been made to infiltrate X percent on-site and allow X percent to flow into the storm sewers. A full pool will not hold any additional water (well, maybe a small bit, but lets not quibble about those details right now) and so that rain will have to go somewhere.
In addition to not being a Zone Master, I also don't own a pool. But I would imagine during rain and off-season, there is a cover on the pool and that also would make it equivalent to something like a deck, patio, or other hard, non-permeable surface.
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Clearly and unambiguously impervious. No infiltration is going on under the concrete structure.Originally posted by WhenIGrowUp
But neither is any run-off.Hot tub with the cover on: impervious. Uncovered pool or hot tub: pervious (the water is soaking into the other water in the pool or tub. Case closed.
As much as I hate to say it, Otis is correct. If the pool is at its proper level, it will likely accept at least a 25-year storm event. My pool has never overflowed (except for the time I inadvertently left the hose running in it overnight).
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Pervious is much more than just accepting a 25-year storm event. It includes soil and groundwater recharge. The precipitation falling on the pool is not going to the soil. It is true that there is somewhat less sheet flow from the surface of the pool, but if that is your only criterion then your reference frame is much too narrow, especially from a 'sustainability' perspective.As much as I hate to say it, Otis is correct. If the pool is at its proper level, it will likely accept at least a 25-year storm event. My pool has never overflowed (except for the time I inadvertently left the hose running in it overnight).
If a swimming or other pool was 'pervious', its contents would fairly quickly percolate into the ground. Since, for a pool, that is not a very good idea and many steps are continually taken to prevent that, they are, IMHO, fully impervious.
Mike
Sir: quite clearly so.
Many codes here in Florida consider it impervious surface. There is no right or wrong answer... It really depends on the context if your community and how much real green space your community wants.
IMO, it really depends on what your calculations are going to be used for. If you're trying to minimize stormwater runoff, swimming pools will not contribute excess runoff under reasonable conditions. Groundwater recharge and percolation/infiltration may be completely separate considerations. Depending on your local geography/geology, water management, development patterns, etc they may or may not be significant considerations for your locality.
From a stormwater management perspective, there is no benefit in overdesigning your SWM system, especially since engineers tend to overdesign them by a ridiculous factor anyway. In these cases, including the water surface area of swimming pools will only serve to fudge your SWM needs calculations and lead to overdesigned SWM infrastructure.
Edit: I just noticed you're in Atlanta. Based on what I know about water in your area and some of the issues you all are facing, I think it would be wise to consider swimming pools an impervious surface.
Last edited by kalimotxo; 09 Apr 2011 at 3:19 PM. Reason: clarification
Process and dismissal. Shelter and location. Everybody wants somewhere.
Sure there is. If it doesn't allow infiltration, it is impervious. If your code doesn't state such then it is faulty. Green space has zero relevance to the issue.
Splitting hairs, it is impervious. However, I would exempt pools and ponds from any impervious lot coverage area ratios in a zoning code.
Now, if I provide a dry well on my lot, have I increased the pervious area by the third dimension of depth?
By basic definition a concrete structure is impervious. And a dry well likely increases the rate of percolation, but not the surface area able to percolate.
As others said before, it comes down to why you are making the distinction. If it is about stormwater run-off and the load on the storm drainage systems, I think it makes no sense to consider a pool as impervious, since there will be no run-off from the pool. If it is about groundwater recharge, then it clearly is impervious. I was assuming the former.
The problem may lie in the term itself, and perhaps different, more precise terminology is needed for the specific context in which the term is being used.
Exactly, the pool essentially acts as a detention pond. Holding what would be runoff until it evaporates. So more importantly, the reason for the distinction is what determines the definition.
I'll give it a rest after this, as I see the overarching problem.
Nonetheless, non-concrete detention ponds, ponds, swales, depressions, creeks, lakes, etc allow water percolation and infiltration to the soil. Concrete structures do not - thus by definition they are impervious.
I will now go bang my head on an impervious surface.
That's the way I thought as well. I understand the distinction others are making, but if I'm going one way or the other I'm going with impervious. It's concrete, and if it's not full of water there's probably a cover on the pool.
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It might depend on the purpose of your impervious cover standard. If it is about aesthetics and stormwater runoff, they might get some credit (see RJ's argument about the 25-year storm event). If that is the case, you probably need to look at adding an impervious cover credit schedule to your technical manual or actual code.
In my part of the world, impervious cover standards are all about groundwater aquifer recharge and water quality. As a result, a swimming pool would be considered impervious here.
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Sadly, my swimming pool is completely impervious in EVERY debatable definition of the word for 10 months out of the year.
Impervious surface calculations
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