Negative Perimeter Edge - calculating flow rates

[AlanH]

I need some help with a calculation (and "informed guesses" based on experience also welcome), that I believe is essential for specing a pump (or pumps) for my pool. So far everything I've been able to find is very anecdotal and I would appreciate a better understanding of the "background math", if possible. This is a new area for me so please let me know if I am even approaching this the correct way and asking the right questions?

Here's the situation. I am building a perimeter negative edge pool (see http://www.troublefreepool.com/diy-pool-build-latin-latitudes-t28572.html for initial posting although planned shape and size changed radically due to recently passed by-laws on "water containment vessels" and associated permit fees and property tax assessments). The Hydraulics 101 thread and a few others have been helpful in calculating flow rates (GPM) for pool turnover/filtration and in calculating volumes/dimensions for the catchment basin (pending other information from this post). The flow rate for pool turnover however is based on simply the volume of the pool with zero flow over the perimeter edge.

A few observations from other posts in TFP and some other web sites and then some details about my pool. The posted discussions usually deal with one wall or a small segment that is a negative edge. Out of curiosity, and perhaps essential in my case, are flow rates for spillage over a negative edge dependent only on the number of linear feet of negative edge or is there some "factor" included as part of the calculation for water having to spill over the entire perimeter as opposed to (for example) only one of four walls? One TFP post mentions "projects with over 100` of edge powered by a half HP pump" (but gives no further details). An article in Watershapes (ezine, August 2005) mentions "wetting" over 160 ft of perimeter negative edge using only a 3/4 HP pump with flows of 65-75 GPM (original spec was a 3HP pump). It is this latter example that interests me the most and if there is a calculation that can be done for the depth of flow over the edge (from "wetting" or, more precisely, "X" inches), the GPM entering the pool required for a depth of flow of "X" inches, and the appropriate pump to deliver it.

Pool details:
In-ground Pool: 24x13x4 (on average) = 9400 gal.
Surface area: 325 sq ft (exact)
Catchment Basin: 16x2x2.5=600 gal.
Estimated GPM for 8 hour pool/basin turnover = 21
Max GPM for Pentair CC50 filter = 50 (as an example only)
No heater
Suction vacuum cleaner
***AND***
Water feature:
- perimeter negative edge, 75 linear feet (approx.)
- water spills into perimeter gutter system about 4-6" below edge.
- "sheeting/cascading" water NOT part of the design plan.
- assume a dead level edge tolerance (not beyond my capabilities or others on the build).

The flow of water into an open topped container would equal the flow of water over its' edges given no other outlet. Even 1 GPM would overflow the edges. As would 50 and 100 GPM; but at what depth? Here's where I'm stuck and need some help. How do you calculate the flow rate entering the pool required to rise the level of water so that it flows evenly over the entire perimeter edge at a given depth? I suspect (but don't really know at this point) that 1/4" flowing over the edge would be a good starting point. What flow into the pool would support this? I'd also like to know the calculation/requirement for a flow of 1/8" and 1/2".

Look forward to hearing from you or if you can point me in a direction to find out more. Thanks for stopping by and reading the post.

 

[JasonLion]

It doesn't make any difference how many sides of the pool are negative edge, only the total number of linear feet of negative edge and how level you can make the edge. The longer the edge the more important it is that it be level, because even small amounts of out of level require huge flow rates to compensate for.

Approximate GPM required per linear foot for various water depths:
1/8” - 5 gpm
1/4” - 10 gpm
1/2” - 17 gpm
3/4” - 28 gpm
1” - 40 gpm
1.5” - 70 gpm
2” - 105 gpm

Even at 1/8" you are talking about 5*75=375 GPM, which is going to require a massive plumbing system.

 

[AlanH]

Jason, thanks for the numbers. Indeed, a lot of water. And perhaps I'm overstating the depth of flow I'd like, or even need, over the edge. The visualization I have is the pool surface just kissing the top of the edge and "wetting" it as opposed to any "great" quantity flowing over it - although, "on occasion" that would be nice. The numbers you provided certainly help to give me a top end in thinking about the plumbing but I'm also intrigued by the following quote from Lew Akins who seems to have garnered a reputation (good I hope) for negative edge pools - “You don’t need much,” says Akins. “If you are level at the edge, you can keep it full and flowing with hardly any water. You can literally create flow across a negative edge with a garden hose.” Quite a range to work with.

 

[JamesW]

Another issue that you need to consider is how much water has to be moving when the pool is being used.

When people are generating waves (jumping, splashing, swimming etc), there will be a lot of water that goes over the edge, and if the amount that goes over the edge is not moved back into the pool, then the overflow will flood.

 

[solarboy]

Another issue that you need to consider is how much water has to be moving when the pool is being used.

When people are generating waves (jumping, splashing, swimming etc), there will be a lot of water that goes over the edge, and if the amount that goes over the edge is not moved back into the pool, then the overflow will flood.

I have found that about 700 gallons of extra capacity in the overflow allows 10 people to use the pool without problems. I originally found the calculation somewhere and it's not failed me yet, although you could probably use less.

 

[AlanH]

Thanks for the comments. I'm sure it was somewhere in TFP and a few other sites that I found the following: for bather surge allow 2 inches extra over the surface area of your pool. Or 1/6 of a cubic foot for every square foot of pool area. In my case, with a 325 sqft pool, that works out to 55 cuft (x 7.5 gal/cuft=410 gal - approx).

Curious how such a number (2") came about. According to various google/wiki searches, the human body has an average density equal to water. One Kg of water equals one liter in volume. An 80 Kg (176 lb) person would occupy/displace 80 liters (21 gallons) of water - plus splash and drip when exiting. My pool is fairly small and I can't imagine more than 6 people in it at any one time. (Closest neighbour is 3 Km away and other friends/neighbours probably 12-50 Km - like herding cats trying to get folks together). Six times 21 equals 126 gallons displaced. If the conventional wisdom (rule of thumb) dictates 410 gallons, can (asking somewhat rhetorically) 6 people slash out 284 gallons? Better to err on the safe side and the cost, in relation to the whole project, of building a bigger catchment basin/surge tank is minimal. Also factored in the following: an amount to prevent drain vortexes and accommodate rain (to an extent); controlled overflow; basin free-board to control splash-out.

The attached Sketchup model shows a conceptual drawing (and I emphasize conceptual at this point) of just the gutter system. The scale is expanded more for clarity than accuracy. The gap leading to the below-deck gutter is 1 inch. The deck, from about 3 feet out from the gutter gap, slopes down to the gutter so that any splash (or most of it) will return to the pool's circulation system. Comments, suggestions welcome.

 

[solarboy]

You need to make sure you can access the channel for cleaning etc, hence the grill normally.

 

[AlanH]

Good point, particularly when you live in coastal tropical forest and have 24/7/365 bugs, critters, leaves, flower petals, dust, pollen, and whatever else mother nature can throw at your pool.

The idea for this type of gutter comes from dozens of photos I've looked at on various pool builders' web sites, and from a few concept and engineering reference drawings that I've stumbled across. Aesthetically, the idea was to avoid the plastic/fiberglass grill look (if that's not your taste), although I understand that cut stone grills are also available at a much higher cost. What I didn't show in the sketch was that at each corner there is a "sump", accessible from the deck, that acts as a primary filter and also connects to the catch basin/surge tank. This would allow access to run what's called a "pig", a ball slightly smaller than the diameter of the gutter tube, that is attached to a rope and can be pulled through the gutter from one sump to the other (the rope being led through the slot/gap or snaked through). Could also use a wadded-up towel. This would only be needed if something really clogged the gutter or perhaps once every month just as general routine. Others with this type of gutter say that flushing with a garden hose (or a pressure washer if available) is sufficient.

 

[solarboy]

Sounds like a great solution Alan. I've seen that kind of drain constructed by grinding a slot in concrete piping at the side of the road.