Drainage System Tips
DEAR TIM: I live on a slight grade and my yard is very wet when we get days of rain or after the snow melt in early spring. Do you think there’s a spring in my yard? Is there an affordable way to dry out my yard so it becomes something other than a mud pit? Surely I’m not the only person with this dilemma. Can you explain why some yards are well drained and others are not? Susan E., Akron, OH
DEAR SUSAN: You’ve come to the right place for this advice. My college degree was in geology, and two of my favorite subjects were geomorphology and engineering geology. Suffice it to say, I know a little bit about land forms and what you can do to intercept and divert the flow of underground water. It’s absolutely possible you have a seasonal spring in your yard.
Let’s first explore what’s happening in the ground. Think of soil as a giant sponge, and I mean giant. If you have a dry or slightly damp sponge sitting on a dry countertop and drip water into it, the water drop disappears into the sponge. If the sponge is really dry, the water would never make it to the countertop. It’s absorbed into the dry sponge matrix. The same thing happens in dry soil when it rains.
But when soil starts to get too much water, the weight of the water and gravity act to pull water through soil. Because this is happening below the surface, it’s invisible. Believe me, water is on the move through soil and it can happen for days or even months after a rain. This is what causes real springs to produce water 24 hours a day.
Some soils have a clay component or a dense hard clay deep in the soil sometimes called hardpan. Clay and hardpan block water from moving down deeper into soil. This is one reason it’s used to line pond bottoms and it’s a great raw material for pottery that holds liquids.
Add to this the fact that most soils have a vertical profile that changes the deeper you go into the soil. The very spongy top of the soil that you often call topsoil, is the part that really absorbs water. But below it, the soil becomes more dense and water can have a very hard time traveling through it if there’s lots of clay. In these cases, and those of hardpan, the water moving through the soil starts to travel sideways along the border between the topsoil and the denser zone. If the ground is sloped then the water moves sideways and downslope.
This is undoubtedly what’s happening in your yard. I happen to know the soil profile where you live as it was victimized by one or more continental glaciers that left layers of clay behind many feet thick.
To solve your problem, you need to use the same engineering technology that we’ve used for years to collect rainwater from roofs. You need a gutter and downspout solution, but inside your soil. You’re going to install a drainage trench in your yard that collects the subsurface water and transports it to the lowest part of your land where it would end up by default if you didn’t live there.
Think of it on a big scale. The slope of your land and that above your property is like a roof. Many thousands of gallons of water could be flowing downslope in the soil towards your property and you need to capture it just like a gutter collects rainwater. You do this with a simple trench that can be anywhere from 2 feet deep up to 8 feet or more.
The depth of the trench is a function of many things. The most important one is the elevation of the lowest spot on your property. Since you’ll rely on gravity to move the water, you need to make sure the water can flow by itself through the trench to the lowest part of your land.
This trench will have a perforated drain pipe in it and will be filled with clean washed stones or gravel that are the size of golf balls or slightly smaller, perhaps the size of a large glass marble.
The trench is dug so that it protects your yard, your basement, your crawlspace, etc. like a moat protects a castle. The trench may only need to be L or U-shaped so that one or both ends eventually daylight. This happens if you keep the bottom of the trench level or just with a slight slope and the ground falls away. Eventually, just like a cave or mine entrance on a hillside, the trench ends.
If you want to really make your yard dry, you should fill the trench up to nearly the top with the stone. This makes for a highly effective gutter in the ground.
The drainage system works because water finds it infinitely easier to flow down through the gravel than push its way through the soil. Water always wants to travel the path of least resistance.
The trench only needs to be a foot wide, and can often be just 6 inches wide to be wildly successful. If you need to use a digging machine, you may end up with a trench 16 or 24 inches wide. There is nothing wrong with this, it’s just that you’ll end up buying more gravel.