Draining Playground Sand 1

[mbph]

This is small potatoes, but I thought, just for fun, I'd throw it out there to the drain brains.

I'm trying to figure out the best way to drain a playground without losing all the sand. Right now, the area is about 25'X 50', the san is about 6" deep and placed directly onto a very clayey impermeable soil. It has three drains--these are 3" PVC pipes with a flat slotted cover (also about 3" DIA) wrapped in a permeable sleeve cloth. These typically get buried under the sand. There's no catch or anything, it's just a vertical pipe elbowed horizontal.

As is, the cloth restricts flow too much (buried or not buried) and letting it runoff is a problem because nearby structures get flodded, plus sand washes out. Without the cloth, sand goes down the drain, and builds-up in the pipe.

 

[dik]

With a proper geofabric (fine non-woven polyester), the sand should not wash through it and clog anything. Can you create a 'trench' with a geofabric liner and fill the trench with clear stone and cover the clear stone with the geofabric? Put the drain tile in the bottom of this trench and let it drain away.

The larger surface area of the trench allows more water to drain out.

 

[gibfrog]

I think that you have a head/capacity problem. I need more details to give you a better answer but here are some generalities.
1) dik is correct. You should consider putting the pipes in gravel trenches to increase the flow ares to the pipe. Right now, you only have a 3" wide flow area, a 2 foot wide gravel trench under the sand with a non woven fabric (mirafi 140n) cover to keep out the sand will probably increase the pipe inflow by a factor of 8 (8x the flow area). Please note, the pipe will not require a fabric sleeve if the gravel if covered with fabric.
2) The 3" pipes should be sloped (I use 1% for smooth 4" pvc, 3" needs steeper slope - use the Manning equation) to flush and flow cleanly. I would use a 2 fps design with terminal clean outs.
3) Calculate your required trench width, head and drawdwon time via the underdrain sand filter drawdown equation. I have it at my office, but it is quite a simple calculation. You will need your sand permeability and required draw down time (this could be based upon your peak rainfall and the time for 2.4" of rain to fall - 40% voids for 6" of sand). The flow length is based upon the geometry. Do an incremental calculation (numeric integration) for each 1.25" or 4 intervals. Someone posted the formula a while back in one of the forums - search for underdrians?. Do not forget a 50% minimum safety factor in your design to account for the sand becoming dirty with silt, clay and organics.
4) As an alternate, use the pipe spacing formula for water table control of fields. Again, I appoligize, I have this at my office. If you cannot find it, I can provide it. This will give you the spacing and depth required without gravel trenches.
Best of Luck Clifford H Laubstein
FL Certified PE #58662

 

[gbam]

Ok,
This is a thought I had after I read your post. Instead of using slotted lids with small pipes, why not remove the sand and grade the clay soil. As gibfrog said cut a trench and place perforated pipe in it. Use small pea gravel to cover the pipe then place the sand over the gravel. The clay soil bottom will direct the flow through the sand to the drain. The gravel will reduce the amount of sand lost and increase the conveyance within the trench. Of course both hydrologic and hydraulic analyses may be required if the objective is to eliminate the flooding. Again just a thought.

 

[mbph]

Thanks. A trench is very doable and the drains are in low spots.

I'm interested in the underdrain sand filter drawdown equation, if it's handy.

I don't think I'll mess with the pipes right now. I'm not positive of the slope is but I think it's OK and I can just flush them.

 

[gibfrog]

Hi MBPH:
Please excuse the delay. The underdrain design is a straight forward numeric integration of the DARCY equation or Q = K*I*A = V/T. I use it to design filter system for stormwater runoff in Florida with low head applications. I use 6" minimum pipe diameter (usually ADS perforated corrugated HDPE @ 1.0% SLOPE), terminal cleanouts, 6" minimum head, lowest flow line as the pipe average centerline and 100% safety factor on th esand permeability. The above design criteria are mostly Southwest Florida Water Management District regulatory requirements.

Q - The flow in cubic feet per second
K - The sand permeability in feet per second, usually around 5-8 feet per hour for coarse sand. Watch your sand carefully. Only use playground sand or a courser sand with less fines due to health requirements.
I - The hydraulic grade or the head divided by the average flow length through the sand.
A - Area of the sand above the gravel filter. If your pipe is in a 2' wide rock bed, you area will be 2' times the pipe length.

Now define your pipe/trench geometry to determine the trench width and average pipe depth. Also define the volume of water ( or height for a fixed sand box area) you want to drain and the maximun time to drain this water. Divide this height in to a minimum of 4 increments or 5 elevations. Calculate the time to drain eace volume segment based upon average values. (T =V/(K*I*A)) Total the time and if it less than the time required, you are done.


Clifford H Laubstein
FL PE 58662

 

[mbph]

Excellent, thanks so much.