Culvert Crossings

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By Kurt Swearingen

Here’s how to install the right-sized culvert so you can safely travel over streams and ditches.

Small streams often crisscross rural land, and even those that remain dry for most of the year may have steep banks that prevent vehicles from crossing. The simplest way to safely drive across such an obstacle, while protecting the stream, is to install an earth bridge with a culvert.

Culverts are made from corrugated metal or plastic tubes positioned in a crossing to allow water to pass through without damaging the roadway. The culvert’s corrugation provides a greater strength-to-weight ratio than a smooth pipe. This helps support the weight of vehicles, while mimicking the roughness of a natural stream bottom, which slows the speed of flowing water. Follow the steps outlined in this article, and you can install culverts wherever you need them.

Culvert Sizing

To determine the right culvert size for your situation, you need to consider the dynamics of stream flow and how it changes with the seasons. Peak flow usually is during spring runoff, and it tails off from late summer to early fall. These annual fluctuations can seriously erode exposed soil and cause inadequate crossing structures to fail. As a consequence, culverts should be sized — at a minimum — to handle the annual peak flow, and installed in a manner that will protect the culvert’s strength over time.

Many methods exist to determine a culvert’s correct size. The easiest is one in which field measurements are taken of the stream’s cross section when the stream flow is at its lowest, in late summer or early fall. To take the measurements, you will need a tape measure, four stakes, a hammer, string and a notepad.

1. Begin by scouting the intended crossing. Look along the stream banks for the highest water mark. This is where any vegetation meets bare dirt and rock along the bank.rive in stakes at the high water marks on each bank and tie a string between them. Measure the distance between the stakes and record it as the “high water stream width” (HWSW).

2. Next, locate the lowest water mark. Typically, it borders the flattest portion of the stream bottom. Establish the other two stakes at this level and measure as you did for the high water marks; record this length as the “low water stream width” (LWSW). (See the diagram below.)

3. The final measurement is the depth of the stream at high water. This is the distance between the stream bottom and the high-water-line string. If the stream bottom is highly irregular, you will need to take several measurements and average them.

Now use this simple formula to determine the stream’s cross-section area (A):