Comments on: How to Build a Grassy Drainage Swale

By: Bob Jackson

Bob Jackson — Wed, 29 Jun 2011 02:05:38 +0000

A 4″ corrugated slotted pipe with a polyester sock (sock pipe) is suitable for low volume drainage when covered with gravel and maybe soil for a grassy surface finish. The purpose of the sock is to keep out dirt and rock so the pipe doesn’t clog.

The sock pipe can be used in two ways:
1) Pipe the water into the mouth (high end) of the sock pipe so the water can soak into the soil over the length of the pipe. The pipe in this application has no surface outlet, similar to a septic tank drainage field. This requires an elevated drain field so the water will flow into the surrounding area.
2) Lay the pipe in a ditch, cover with gravel (and optionally soil) to take in the water along the length of the pipe to the open discharge end located at a more distant lower elevation.

The problem with a 4″ diameter sock pipe is the area of the pipe is only 0.09 square feet versus 12 square feet for a rectangular (6 ft wide x 2 ft deep) ditch. Let’s assume the ditch is roughly V shaped instead of a rectangle, then the V ditch has maybe 6 square feet of cross sectional drainage area. The open ditch with 6 square feet of cross sectional area has about 66 times greater drainage capacity (=6 sq ft divided by 0.09 sq ft) compared to the 4″ diameter sock pipe. In other words, you’d need at least 66 of the 4″ inch sock pipes stacked together to equal the drainage capacity of the open ditch. In reality you’d need more than 66 sock pipes due to friction losses of the many pipes.

I buried a 6″ PVC pipe at the end of the swale to empty into the large ditch at the back yard because a 6″ pipe was the largest size sold at Home Depot. I would’ve bought a 10″ PVC pipe if they sold it.

The 6″ PVC outlet pipe emptied my swale in an hour or so after a heavy rainfall. Overall it did great job of preventing the ponding problem and water would continue to drain in smaller amounts after a day or two of wet weather.

Remember that I started from a flat yard with no swale and was guessing at how big a drain pipe would be needed to handle the water. If the 6″ outlet pipe didn’t work, then I would have replaced it with a larger pipe. It’s obvious your swale that fills with 2 feet of water can’t be replaced with a 4″ pipe that’s at least 66 times too small.

By: guest

guest — Tue, 28 Jun 2011 17:41:37 +0000

Can you get the same results with a 4″ sock pipe? My neighbor has filled in the drainage swale in his yard and replaced it with a sock pipe system. He wants us to do the same. I’m not sure. The swale is ~6ft wide and about 2 feet deep when it fills up. I don’t see how that little pipe can diplace all that water.

By: Bob Jackson

Bob Jackson — Fri, 29 May 2009 11:55:52 +0000

Use the lawnmower you already have, that’s the idea behind a shallow grassy swale. If the swale is too wet or has water in it, such as Florida’s summer season, use the weedeater to cut the grass to just above the water level.

A good swale should drain completely within a few hours to a day after the rain stops. You might want to check the slope for proper runoff.

Take Care,
Bob

By: NORMAN CAMPBELL

NORMAN CAMPBELL — Fri, 29 May 2009 05:03:20 +0000

PLEASE ADVISE ME WHAT TYPE OF LAWNMOWER OR MOWING EQUIPMENT THAT YOU WOULD RECOMMEND THAT I USE TO MOW MY SWALES. BEAR IN MIND THAT THEY ARE TYPICALLY WET ESPECIALLY AFTER A SHOWER OF RAIN. ASIDE FROM A WEEDEATER WHAT MOWER OR EQUIPMENT WOULD MAKE THE JOB LESS LABOR INTENSIVE AND EFFECTIVE?
tHANKS FOR YOUR REPLY TO MY EMAIL ADDRESS.