Introduction: Reduce Yard and Basement Flooding! Install a Downspout/Sump Drywell.
Spring has sprung! And with it comes all the rain that causes issues in basements and back yards. Those heavy thunderstorms can drop tons of rain on your lot, causing ponding in your back yard, or worse: flooding in your basement. Mitigate these issues by installing a drywell (or multiple drywells) in your back yard!
This is a simple project that any homeowner can tackle - however, it is fairly strenuous given all of the digging required. Also, prior to any digging, contact your local Diggers Hotline or equivalent so that they may come and map out any buried utilities in your yard. The last thing you want is to hit the power lines going to your house. That's a bad day.
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Step 1: Assemble Drywell
Start by removing the drywell pieces from the packaging and assembling everything. This will give you an idea of the size of the unit. The side pieces have interlocking finger joints that slip together, and the lid is simply friction fit on the top.
Note that this unit does not come with a bottom, nor do we want it to. The idea is that water enters the drywell from a drainage pipe and then slowly seeps out the bottom over time.
Also note that these are a bit squirrely when it comes to staying together. I had to reconnect the pieces more times than I can count. Some reviews I read online recommended using a waterproof sealant as a glue on the joints. I didn't find that really necessary, as once it's installed in the ground and backfilled with gravel, it shouldn't come apart.
Step 2: Establish Hole Size, Remove Grass
Hole size (red circle in the first image) is a critical parameter for any drywell installation. NDS, the maker of this particular drywell, has a handy calculator for determining the hole size, depth, amount of gravel backfill, and number of drywells to chain together, if needed. In hindsight, I should have dug a wider and deeper hole for my drywell, so learn from my mistake in this regard.
In any case, establish the perimeter of your hole with a shovel, being sure to penetrate below the grass roots with the blade. When you've circled the drywell, use a flat spade to slice a line through the middle of the circular plug of grass to make it easier to handle. Work the spade under the grass to separate it from the soil beneath, and remove the grass for later replacement.
Depending on the size of your hole, you might want to cut the grass plug into fourths. Up to you.
Step 3: Dig....and Dig....and Dig..........
Now for the "fun" part, digging the hole. Channel your inner six year old, who always wanted to dig that hole to China, and get to digging the hole to the diameter and depth you determined previously.
I recommend putting a tarp next to your hold to hold all of the dirt you dig up. This keeps your yard clean of the dirt and makes disposing of the excess dirt much easier. Alternately, you could back up a trailer and toss the dirt on it for easy disposal.
In the last photo in this set, you'll see the old French drain pipe. This drywell is replacing a silted-up French drain system that the previous owners installed. You can see about an inch of silt on the bottom of the pipe, which I removed as best I could before proceeding.
Step 4: Line Hole With Soil Separator Fabric
With the hole dug, now is time to line it with soil separator fabric. This fabric is a mesh that's sized to keep the fine soil particles from migrating from the surrounding soil to the drywell pit, which would clog the drywell over time and reduce its effectiveness in draining. As such, it's non-optional.
This fabric is fairly thin, so I would recommend doubling it up. For my hole, it was sufficient to put the fabric in a cross-shaped pattern to cover the entire hole. Use a shovel, your feet, and hands to spread the fabric around the hole for complete coverage.
Step 5: Line Base With Gravel, Prep Flo-Well
Once satisfied with the spread of the filter fabric, line the bottom of the hole with an appropriate amount of gravel (determined in Step 2). Tamp it down with your feet or a tamper.
Figure out where your drain line enters the drywell, and use a hammer to knock out the drain plug matching this location. It might be helpful to set the drywell on a couple bricks to brace it while you do this, and expect the pieces to pop apart when you try.
Also take the time now to pop out all of the 1" emitter holes on the drywell. I used a screwdriver and a hammer to pop these out. Some might be more stubborn and require cutting out.
Step 6: Add Flex Drain to French Drain Outlet, Install Flo-Well
This step could be a little different depending on your install. Since I had an existing drain pipe, I installed a flex drain pipe to connect it to the drywell. This was as easy as slipping one end into the drain pipe and pulling some of the flex pipe to extend it.
If you're doing an entirely new drain pipe system, then just run the new drain directly into the drywell. No flex hose needed.
Be sure to re-cover around the drain line with the soil separator fabric as well as you can. You want this to be as tight against soil intrusion as possible.
Next, take the drywell body and install it in the hole. Line up the drain plug hole and feed the flex hose through the hole.
Step 7: Backfill Sides With Gravel
Now is time to backfill around the sides of the drywell with gravel. First, use some rocks, clumps of soil, or whatever else to hold the fabric in place on the surface. This just makes it easier to pour the gravel in the hole since the fabric isn't trying to slide in the entire time.
Proceed to fill the gravel around the drywell, stopping every 6" or so to tamp the gravel down. Try to keep the drywell as round as possible when you backfill since this makes putting the lid on easier.
Step 8: Add Lid to Flo-Well, Fill Remaining Gravel
Use a hammer or some other implement to knock the emitter hole out of the top of the drywell. If you have an installation with multiple drywells, you may not need to do this on every drywell. Put the lid on the drywell and fit it into place. This might require some persuasion since it's likely that the drywell got out of round when backfilling.
Fill in the remaining gravel up to the rim of the drywell, and tamp everything down.
Step 9: Seal Filter Fabric
Take the filter fabric and fold it neatly over the top of the drywell, trying to keep it as flat as possible.
Using a utility knife, cut a slot for the emitter hole, and slip the filter fabric around the hole. Put a piece of 4" PVC pipe into this hole to block it during the next steps.
Step 10: Fill Topsoil, Replace Grass
Ok, home stretch.
Start backfilling with topsoil above the drywell. Smooth things out when the topsoil is at an appropriate level, and replace the soil plugs you cut out earlier. Use a shovel to cut out a space for the emitter pipe. Lightly tamp the grass plug down onto the topsoil.
Step 11: Add Outlet
Remove the long section of PVC pipe from the emitter hole. Using a short section of drain pipe and the drain grate, replace the longer pipe in the emitter hole. This hole will allow excess water to leak out the top of the drywell when its capacity is exceeded.
They also make pop-up emitters, which are a little better against allowing grass clippings and debris to enter the drywell. In my climate, these have a tendency to freeze up in the winter, so I didn't opt for using one.
Step 12: Finished!
Congratulations, your drywell installation is complete!
If done correctly, you shouldn't even know it's there. All that will be visible from the surface is the emitter, which should blend into the grass pretty well.
I've included a couple photos of the inside of the drywell during its first rain storm. You can just make out the water draining into the drywell from the drain pipe. We had a very rainy year the first year I had this installed, and it certainly got a workout. In hindsight, I should have made my hole larger and deeper, and maybe even chained an additional drywell into the system. Luckily, these systems are fairly simple to expand if needed.
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