Pretty much everyone is familiar in some way with culverts. If you walk or drive on the road, you’ve likely seen them many times. In my experience, even children seem to notice culvert openings, and they’re quick to make judgments about them (my nephew is convinced that monsters and tigers could be living inside).
As a watershed forester, I also often notice culverts. Culverts are popular choices for roads, trails, and driveways in the woods. If you ever foresee installing a culvert in your woodlot, keep the following three tips in mind.
Culvert Tip #1: Get the right size.
Culvert prices increase rapidly when you look at larger and larger diameters. As a result, thrifty shoppers often buy the smallest size they can find. Unfortunately, those savings are wasted, because these smaller culverts often simply don’t work. Culverts that are too small won’t drain water quickly enough. During heavy rains, the culvert fills, causes flooding above it, and ultimately gives way, damaging both the crossing and the area below the culvert. Instead of saving money, that too-small culvert leads to more cost when you need to replace it plus fix the damage caused. If you want to fix problems once, be sure to ask a qualified professional or consult the New York State BMP Field Guide to make sure you buy a large enough culvert and don't waste money.
Culvert Tip #2: Install it correctly.
On a well-installed culvert, only the culvert openings should be visible. The culvert needs enough fill over it so it doesn't get crushed. The engineering rule to follow for this is to always place at least 12 inches of fill or 1/2 the diameter of the culvert, whichever is greater.
That might sound like a lot of fill, but it’s needed. It is the fill on top of the culvert that bears the weight of vehicle traffic and keeps the culvert intact.
Most culverts on private property are not installed with enough fill. The picture below is of a 30 inch diameter culvert that has about 4 inches of fill.
Without enough fill, the culvert will bear the weight of vehicles going over it, and it isn’t designed to handle those loads. That can lead to the culvert becoming crushed and no longer working.
Culvert Tip #3: Maintain your culvert.
If you install your culvert at a slight angle, it will usually self-clean under normal rain and snowmelt conditions. That means soil or stone that deposits inside the culvert will wash away along the culverts rounded bottom.
Unfortunately, culverts don’t always self-clean. I've seen 12 inch and smaller culverts rendered useless by just a handful of oak leaves that block the entrance. Check your culverts a few times each year to make sure they’re functioning correctly and are not becoming clogged or damaged. During fall and following heavy rainstorms are good times to do these checks. If you can't commit to this maintenance, I don’t recommend using culverts on your woodlot.