Draining Pool -- Risk of floating shell?

[peterl1365]

Here in Southern California, we are in the midst of a series of storms that are expected to drop about 6-12 inches of rain over the course of 4-5 days. I'm using this as an opportunity to do a partial drain of my pool, which has very high calcium hardness values (over 1000 ppm). I figure: free water (with very low mineral hardness), and cloudy, humid air make for ideal conditions to protect the plaster.

Now I know that in general, it is not a good idea to completely drain a pool during the winter because of the risk of floating the pool shell. However, we live in a pseudo-desert (Temecula valley) area, and we've had less than 3" of rain in the last 8 months. The pool has a hydrostatic valve in the main drain which is supposed to prevent pressure from building up under the shell.

Given that, approximately how much water could I safely drain? The pool depth is 3.5'-5.5'-4.5' (sport pool). FWIW, when the pool was dug about 3 years ago (also in January), the ground was bone dry, so it appears that the normal water table is well below the bottom of the pool shell.

I've already drained about 10" of water, and I'd like to drain about 24-30", which I estimate is about half the volume of the pool.

Thanks in advance for any advice.

 

[Guest]

The fact that you have a hydro-static valve in the pool is a red flag! Obviously you are in a high water table area or the builder would not have installed the hydro valve.

While I do not advocate pool draining, you do need to address your hardness levels. But, this "free water" is not the way to do it! Your water rate is somewhere around $4.00 per thousand gallons (I live up the road from you, and I'm pretty familiar with water rates and the rain we are supposed to be getting also!) at the high end. 10,000 gallons of water would run you about $40.00! Worth the risk of popping the pool or taking advantage of the free water? Not in my book!

 

[peterl1365]

I agree that the cost of the water is not that great. I was mostly hoping to take advantage of the low CH in the rainwater.

I was under the impression that hydrostatic valves were required by code no matter where the pools were built in CA.

 

[waste]

Bruce, are you speaking from experience with that area when you say

The fact that you have a hydro-static valve in the pool is a red flag! Obviously you are in a high water table area or the builder would not have installed the hydro valve.

?

As I captured this quote, I see that Peter has responded with a similar question, i.e. hydrostats are usually installed on IG MDs.

While I also doubt the ability of a hydrostatic valve to keep up with significant water, I think it'd be safe for Peter to drain 2-3' of water and let Ma Nature help refill the pool.

 

[peterl1365]

I guess I should add that about 2 years ago, I did a partial drain of about 15" of water. Considering that the last two years have been pretty dry, wouldn't it be fairly safe to assume that the water table is lower than it was 2 years ago?

That wasn't meant to be a rhetorical question. I know very little about the mechanics of groundwater.

 

[polyvue]

I guess I should add that about 2 years ago, I did a partial drain of about 15" of water. Considering that the last two years have been pretty dry, wouldn't it be fairly safe to assume that the water table is lower than it was 2 years ago?

That wasn't meant to be a rhetorical question. I know very little about the mechanics of groundwater.

I can't advise you on whether your water table is high, but I would think the amount of draining you're planning (30" total or 2½ feet) from a pool that is 5½ ft deep should be OK.... unless you plan on waiting for the rest of the season for it to fill au naturale. How much calcium is in your fill water? I'm thinking some sort of compromise might be your best bet. Assuming fill is CH ≤ 200 ppm, gaining a few inches of rain while refilling slowly with fill water should result in a fair depletion of the calcium.

I was under the impression that hydrostatic valves were required by code no matter where the pools were built in CA.

That may be a bit broad in scope. My pool's main drain has no such valve (built mid-2007) and I wasn't aware of any state-wide code that governed this -- besides the strictures in California code that require anti-suction measures (dual main drains, etc.), however I could be wrong.

 

[Richard320]

I'd like to do what you propose, too. CH is 800, CYA somewhere above 200...

But I'm afraid to drain that much water right now. When I was out in the yard yesterday afternoon starting the submersible pump, I noted the lawn area was swampy. If it were taller grass, it could pass for a rice paddy. I'm not so much afraid of floating the pool as having the walls cave in or crack.

It's not as efficient to drain 4', let the rain refill it, drain another 4", and so on, but I'd rather live with chemistry out of whack than walls out of whack. If the pool absorbed 4" of rain, so did the ground around it.

 

[JasonLion]

If you see flooding/swampy areas anywhere near the pool I would be extremely cautious about draining any water out of the pool. If the ground is dry, there has been very little rain, and you don't live in an especially swampy area, it is nearly always alright to drain 18" to 24". The only way to be completely sure is to determine the depth of the water table. At the same time, draining 18" when winterizing is extremely common and problems are quite rare. There is far more risk when completely draining the pool to the bottom.

 

[Guest]

In this area (I am about 20 miles north of Peter) hydro-static valves are not required. I have built many pools on the beach and in high water table areas, so I have installed my share of hydro-static valves (my first pool I ever had built, before I was in the industry, has a hydro-static valve). They are not a big deal, nor are they expensive, but they are not common here. In pools with hydro-stats, I always send a diver down to undo the main drain(s) and hook a pump to the valve to "pull" the water out. I do not count on the valve itself to release enough ground water to protect the pool.

I agree that Peter is not running any risk with the low amount of water that he is contemplating draining, but I question how much he will really offset his high calcium levels. The rain water will help, but I don't think it will make much of a dent! He is in a very dry area, and rain is not plentiful. Just the fact that the builder installed the valve tells me that he is sitting on precarious ground in the dry season, and with the amount of rain that we are supposed to get I would be concerned as to what may be happening underground! I do not want to see a post here for an out of the ground pool!

 

[carolina pool]

Bruce is correct that you should be very cautious. Even if the water table is lower than the pool, under certain conditions its possible to float the pool with the water pumped out of the pool.

 

[peterl1365]

Thanks for the advice everyone.

Bruce, I respect your opinion (better safe than sorry, in this regard). I stopped draining my pool last night with the water about 12-15" below the normal waterline. I may drain some more tonight, but I won't go any more than 16" or so.

As for CH levels, my current readings are 1000+. I believe the fill water is in the range of 150-250ppm. I was hoping that rainwater would be less than 50ppm, especially once the air is cleaned from the first storm.

I am definitely going to be refilling the pool to the top immediately. I just wanted to take advantage of the 6" or so of rain that we're getting this week.

renovxpt, what type of things could cause the pool to float, assuming a low water table?

JasonLion, this area is very close to being a desert. We do have patches of grass that are very soggy after a heavy rain, but that is generally because the water doesn't penetrate past the first 4-6 inches of soil. lawns in this area generally have to have some type of drainage plan (either a buried drain system, or slopes). A flat lawn is just asking for trouble.

Bruce and PolyVue: I must have been misinformed about the hydrostatic valves being generally required. Is it really that much safer to drain the pool in the spring or summer? I don't think the water table around here is significantly impacted by the weather.

I am shooting to get the CH into the 500-600 range. Doing partial drains of 4-6 inches would probably result in no more than a 5-10 percent reduction in CH levels. To get a 50% drop in CH means that I'd have to perform the draining at least 6-7 times.

 

[Guest]

Peter-

Draining is best in cool months, as plaster wants to remain wet (even though you have the quartz, which is much more durable than regular plaster). In this case, and with the hydro-stat, there must be a reason it was installed (did you have the pool built or did you buy the house with the pool? Any chance you can contact the builder (do you know who it is?), and this bout of weather could be a bad thing for you if you tried to drain it now.

Your calcium hardness wants to be in the 200-400 ppm range. I doubt that your water is as "sweet" as you say it is coming out of the tap, as we tend to see water closer to the 500 ppm range in our area. That is better than what you have, but still out of ideal range. Only so much you can do when they give us that hard of water to start with!

There really are only a couple of reasons that you have the hydro-static valve; either the builder hit ground water during the dig and put it in, he always puts them in (maybe a selling point??), or he is aware that the table is volatile in the area and was protecting the build just in case. You may be worried for nothing, or it may be a valid concern. I think I would try and get some info on the build/builder to find out why it is in there. And I also think I would wait for the rain to pass!

PM me if you want and I will give you my cell. I'd be happy to talk and see if this is something I can point you in the right direction on.

 

[carolina pool]

If the pool is pumped out faster than the surrounding soils can release the water, the pool can float.

For example, I have seen this happen when the plumbing trench and pool backfill was sand or gravel (pervious) in an otherwise impervious site. The water followed the course of least resistance from the equipment to the pool and created a perched water table effect that lifted the pool. Another example would be if the water was pumped out above the grade of the pool on a perched water table site.

By slowly removing the water you have nominalized this potential. However draining it before a big rain adds a lot back to the risk factor.

 

[peterl1365]

Peter-

Draining is best in cool months, as plaster wants to remain wet (even though you have the quartz, which is much more durable than regular plaster). In this case, and with the hydro-stat, there must be a reason it was installed (did you have the pool built or did you buy the house with the pool? Any chance you can contact the builder (do you know who it is?), and this bout of weather could be a bad thing for you if you tried to drain it now.

The PB told us at the time that they put it in every build. Could be the area, or just being conservative.

Your calcium hardness wants to be in the 200-400 ppm range. I doubt that your water is as "sweet" as you say it is coming out of the tap, as we tend to see water closer to the 500 ppm range in our area. That is better than what you have, but still out of ideal range. Only so much you can do when they give us that hard of water to start with!

I got my estimates from the Western Municipal Water District water quality report
http://www.wmwd.com/pdfs/WMWD_CCR_2009_WEBFINAL.pdf

FWIW, when the pool was first filled, the hardness readings were around 200-250 (Leslies Pool testing, so could be somewhat off).

There really are only a couple of reasons that you have the hydro-static valve; either the builder hit ground water during the dig and put it in, he always puts them in (maybe a selling point??), or he is aware that the table is volatile in the area and was protecting the build just in case. You may be worried for nothing, or it may be a valid concern. I think I would try and get some info on the build/builder to find out why it is in there. And I also think I would wait for the rain to pass!

PM me if you want and I will give you my cell. I'd be happy to talk and see if this is something I can point you in the right direction on.

Thanks for the offer. For the time being, I am doing just a slow, partial drain, no more than 15" from the normal water level.

 

[Guest]

Hope I was some help! Batten the hatches for the next storm!

 

[PaulR]

The most efficient way to reduce ppm via water replacement is to drain as much as you possibly can first time around. This of course has to be tempered with considerations such as groundwater levels (already addressed in depth here), and for vinyl pools, shifting the liner (not a consideration in this case).

I think the original idea was sound: rainwater should have CH 0, so it is the optimal fill water for lowering CH. Drain at least as much as you expect the rain to fill back in; if you do more draining after it rains, then you get less benefit (because you're partly draining the good rainwater back out).

Regarding a series of partial drains to get the same effect, you end up using more water that way (i.e. it's less efficient). For example, if you repeatedly drain and refill 5% of the pool, it will take you about 13 rounds (roughly 2/3 of the pool volume, over time) to replace half of the original water.

Finally, I don't know what the rules are in SoCal, but in the SF Bay area, you have to drain into the sewer, not the ground or storm drain. If you're on a city sewer that's probably a good idea anyway, it's that much less water soaking into the ground and trying to float your pool.
--paulr