How to Build a Grassy Drainage Swale

By |Last updated on |Yard|16 Comments

A grassy drainage swale can solve standing water, flooding and yard drainage problems. The advantages of a grassy swale are:

  • Nice appearance
  • Inexpensive to install
  • Easy maintenance
  • Can run the lawn mower across the swale
  • Effectively drains the water
Yard Drainage Swale

Yard Drainage Swale

Drainage Swale Construction Planning

Before installing a drainage swale, consider the following factors:

  1. Where is the water coming from? Standing rain water? Or is the water flowing onto the property from elsewhere?
  2. Where should the water go? Water always flows downhill. Is something preventing the water from draining away? Is a storm drain clogged with debris? Is a drainage ditch overgrown with weeds or full of mud? Is some man-made barrier blocking the waters natural path?
  3. How much water to be handled? Is a wide drainage channel needed or will a shallow channel suffice?
  4. How steep is the slope? A gentle slope will have a slow current, while a steep slope will need breaks to slow the water and prevent erosion.

The yard at this house is very flat and flooded during heavy rains:

Flooded Yard Caused by Poor Drainage

Flooded Yard Caused by Poor Drainage

Why the Yard Flooded

The flooding and standing water problem was caused by an overgrown and clogged drainage ditch that couldn’t handle the rainwater runoff. The drainage ditch is hidden behind the wood fence along the back of the property line in the above photo. Annual maintenance of the ditch had been neglected for so long that it was choked with trees, weeds and mud.

Fixing the Flooding Problem

The flooded yard problem was solved by:

  1. Called the County Stormwater Management Dept. (this function may be within the County Dept. of Transportation in your area) to clear and dredge the drainage ditch. The Stormwater Dept. promptly performed the clean-up and repair work because the drainage ditch is connected to an upstream ditch along a road that is maintained by the County Government. I said to the County representative “You’re putting the water into the ditch, it’s your responsibility to make sure it handled properly.”
  2. Built a grassy swale and buried a 6″ PVC drain pipe that empties into the large ditch that is now maintained by the County.

The results were very satisfactory.

Grassy Drainage Swale

Grassy Drainage Swale

How to Build a Grassy Drainage Swale

The County Stormwater Dept. cleared the overgrowth and dredged the drainage canal as shown in the photo below. The county used a 40-ton trackhoe to cut down the trees and weeds. A Gradall excavator dredged out the mud and contoured the sides of the ditch. The rotted wood fence (see the first photo above) was removed and replaced with a post and wire farm fence set well back from the ditch to provide access for the county’s heavy equipment to maintain the ditch.

Cleared and Dredged Drainage Ditch

Cleared and Dredged Drainage Ditch

A tractor service was hired for $600 to dig two swales roughly 200 feet in length on the left and right sides of the property line. The main trench was made by a single-furrow v-blade plow. The depth of the swale began at ground level and increased to about 1 foot at the far end. The sides of the swale were then graded and smoothed using a flat scraper blade tractor attachment.The slope of the swale is very gentle and no lining material for the bottom was necessary, other than grass seed and grass to hold the soil in place.

Making a Yard Drainage Swale

Making a Yard Drainage Swale

A 6-inch PVC drain pipe was buried on a 1/2 bubble slope (set using a carpenter’s level) that emptied into the drainage ditch. The mouth of the drain pipe is lined with cement to prevent erosion and keep away the grass.

Yard Drainage Swale - PVC Drain Pipe

Yard Drainage Swale – PVC Drain Pipe

Grassy Drainage Swale in Heavy Rain

The swale worked great during a heavy rain and thunderstorms. Notice the swale full of water from left-to-right in the photo below. The water would pond temporarily only during the heaviest downpours and the yard is high and dry soon after the rain had stopped.

Grassy Drainage Swale in Heavy Rain

Grassy Drainage Swale in Heavy Rain

Hope this helps,

Bob Jackson

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16 Comments

  1. NORMAN CAMPBELL May 29, 2009 at 12:03 am - Reply

    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.

  2. Bob Jackson May 29, 2009 at 6:55 am - Reply

    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

  3. guest June 28, 2011 at 12:41 pm - Reply

    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.

    • Bob Jackson June 28, 2011 at 9:05 pm - Reply

      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.

  4. Jerry Perron December 24, 2015 at 9:41 am - Reply

    I live on a small farm on which my barn/shop is often under water after heavy rain. My neighbor says I should pull a yard box behind the tractor to make a swale. Can’t find a yard box online. Help?

    Jerry

  5. Jackson April 2, 2016 at 10:19 am - Reply

    @Jerry Perron

    This is what a “yard box” is – the term “yard box” is misapplied but some people use it but its not common or correct usage which is why you can not find one on line – more correctly (generically) its called a “box scraper” or “box blade” – they range in sizes, for example (some examples – search on line for ‘box scraper” or “box blade”):

    for pulling behind a lawn/garden tractor > http://www.homedepot.com/p/Brinly-Hardy-38-in-Sleeve-Hitch-Tow-Behind-Box-Scraper-BS-38BH/100609659

    For a Category 1 subcompact and compact tractors up to around 35 horsepower > http://www.everythingattachments.com/Box-Scraper-For-Compact-Tractors-p/eta-xd-cbb.htm

    and here for other sizes and tractors > http://www.everythingattachments.com/Tractor-Box-Blades-s/85.htm

    You may be able to rent one also, consult your local equipment rental place (construction type equipment maybe)

    Your neighbor is correct. I used one for my drainage swale’s and worked like a charm. Good Luck.

  6. Jake K May 10, 2017 at 3:33 pm - Reply

    Thanks for this. I have a swale that the builder had put in during the build process of my new home that isn’t as far back as I’d like it to be.

    1) If I keep the same pitch away from the house, is there any problem with filling in their swale and making my own swale 8′ further away? Would give me more usable yard for kids to play before the swale.

    2) If I want to make the pitch of the yard SLIGHTLY less severe than it is now (though still flowing away from the house), does that change the answer to #1, or does it change the size of the swale (IE- do I need a bigger/smaller swale if I’m pushing water at a slower run rate to the swale)?

    3) To accomplish all of this I have 8 ft that I’d need cleared and distributed over the existing swale/leveled over the current pitch of the backyard. Does a tractor with v-plow accomplish that or should I be looking to hire a different type of beast of burden to get my project completed?

    Thanks in advance.

    • Bob Jackson May 11, 2017 at 8:00 pm - Reply

      1) Shouldn’t be if there are no issues with property lines, Homeowner Association drainage rules (if applicable) and I would contact the Call-Before-You-Dig agency. In your area I believe it’s http://www.oups.org/.

      2) Any amount of downward slope will cause rainwater to move away from the house. A 1/4 inch per foot will drain the water quickly.

      3) It depends on the type of soil and how many inches of topsoil need to be cut into. The loose sandy soil at my former home in Florida is easy to scrape and plow with a tractor equipped with a box scraper and V-plow. If a tractor service says it’s too big a job contact a Bobcat or backhoe service. I hired both in the past and they’re surprising affordable. Lookup Excavation & Earthmoving Contractors.

      • Jake K May 17, 2017 at 9:11 am - Reply

        Bob, thanks for the great reply! Appreciate your help. I’m in Ohio with an HOA, so I’ll look any drainage rules and if none exist I’ll get a backhoe/bobcat service to help with the reconfiguration of the swale/pitch.

      • Jake K July 19, 2017 at 12:42 pm - Reply

        Hi Bob,
        Updating….I’ve gotten multiple quotes back in the ~$3,000 range (includes reseeding the torn up sod). Is that in your range of ‘surprisingly affordable’? Just curious b/c I was envisioning less. Using skid steer, rock hound and harley rake…seems overkill to me, but I’m green when it comes to projects like this.

        • Bob Jackson July 19, 2017 at 7:57 pm - Reply

          My yard in Florida was sandy and easy to work. I paid the tractor guy a couple of hundred dollars. Solicit other estimates to determine what’s a reasonable rate for your job.

  7. Patricia January 27, 2018 at 7:48 am - Reply

    Hi Bob, our property is the second house on our street in an older subdivision (1987) and sits lower than all of the other houses above us. We are at the bottom of the hill when you first turn on our street. The city has a large stormwater headwall at the top of our property about and its drains on our property! The former owner who had the house built (1987) had stuck a 15″ black pipe in the headwall and it runs about 120 ft out to the wooded area of the property. We think they did this around 1995. The property is just 1 acre total, and over the last 2 years, the ground has started flooding much more when it rains and water pours off the hill and thru the stormwater drain! We have now dug the area up around the drain now to see whats going on. Pipe appears to be clogged alot from over the years and the headwall pipe diameter is 21″!! The city says its not their problem since its on our property! But their huge stormwater drain is turned and exiting in our yard. We get all the rainwater from the whole street! Both sides run into this drain. Help! Need any suggestions. We cant move, we just bought the house 5 yrs ago!

    • Bob Jackson January 27, 2018 at 9:42 am - Reply

      Your situation is different from mine because the storm water drainage course is wholly within your property and wasn’t originally installed by the city/county. Maintenance of the storm water drainage system outside the public right-of-way is the responsibility of the property owner.

      Try meeting with the city storm water official again at your home to review the storm water headwall and walk the property to discuss what can be done. If nothing else, the city official may be provide advice on the best solution. Also, does the headwall have a grate to keep out large debris?

      An obvious issue is the 15 inch drain pipe installed by the former homeowner is too small for the 21 inch headwall outlet. The advantages of a rocky drainage swale are it can handle a much greater water flow and it’s easy to inspect and maintain.

  8. Rocky July 17, 2018 at 9:59 am - Reply

    Hello Bob,

    I have been in a new residential home for 6 months. We are in a resort style community with several homes everywhere. Our property is at the bottom of a hill with a retaining wall and trees in the back. There’s also two homes on the hill that drains down onto our property. There is a storm drain at the corner of our property as well, that has an swale moving the water from the top of the hill and the property on the other side of me. The water is supposed to drain into the storm drain. We are experiencing standing water in the swale. The county guys came through and said to contact the builder to fix the problem. When I contacted the construction manager he said he needed the water to dry out a little before he can attempt to fix it. They just put new sod on the property on the hill and has been watering the sod, day and night for the last 5 days. The standing water was there before the sod was put down. It has been about a month of having standing,smelly, green water in the back yard. My question is does it have to dry out before the work can be done? How to I assure that the work is done correctly the second time, so it does not continue to be a semi or annual issue?

    • Bob Jackson July 18, 2018 at 8:10 am - Reply

      There’s no reason to wait and it wasn’t an issue for the County when they excavated the drainage ditch behind my home.

      > How to I assure that the work is done correctly
      > the second time, so it does not continue to be a semi
      > or annual issue?
      The swale must be graded such that it has positive drainage with no low spots to hold water. An unlined grassy swale is good for low slopes and slow moving water. If the swale is subject to fast moving water during heavy rains that might cause erosion, consider a rock drainage swale. If appearances are a concern, line the swale with decorative rounded river stones instead of granite rip-rap.

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