Culvert Inspection and Repair

In the autumn water levels are usually low, making it a great time to inspect culverts and perform routine maintenance. This article will cover inspections to determine needed cleaning, repair or replacement, and methods to minimize erosion.

Inspections

Thorough inspection is essential to effective maintenance. Knowledge of culvert material can predict the types of problems a culvert may have. Each material has specific weaknesses.

• Steel culverts are subject to corrosion and abrasion, and have a shorter life span than other materials.
• Aluminum culverts can sustain abrasion and have less strength.
• Plastic culverts bend easily and are subject to ultraviolet degradation. They are subject to impact damage at low temperatures.
• Concrete is the most durable material, but concrete and reinforcing steel still deteriorate.

Inspections frequently show that culverts require cleaning. During cleaning, certain maintenance tasks should be performed:

• Remove obstructions and clean inside the pipe.
• Examine the culvert for visible cracks, changes in shape, corrosion or abrasion.
• Check upstream for trees, dead branches, and other debris that may obstruct the pipe.
• Cut vegetation that may hinder flow near inlet and outlet.
• Look for evidence of past overflow to indicate whether the culvert is the correct size.
• Remove debris and sediment, or add material to make the ditch bottom level with the culvert invert elevation.

Bent or broken culvert ends should be repaired for smooth water flow. Repair headwall cracks, loose mortar, and displaced stones. The chart on the facing page will help to diagnose other problems and choose a repair.

The most important reason to replace a culvert is to minimize the possibility of structural failure. Weak culverts beneath a road are especially dangerous. Replace a culvert if it cannot handle the expected water flow. Water and debris lines above the culvert indicate a larger culvert is needed. Area residents and town records can indicate water levels of past storms and the maximum level to expect.

Replacing a culvert with one only slightly larger significantly increases the capacity of flow. For example, an 18" culvert has about 50% more capacity than a 15" culvert. Before installing a larger culvert, check culverts downstream to ensure they can handle the increased flow.

Multiple culverts might be necessary if cover is insufficient for a larger culvert. The table below shows the number of smaller diameter culverts equal in water carrying capacity to that of one larger sized culvert. It is based on culverts laid on the same slope. For example, one 24" diameter culvert is equivalent in water carrying capacity to five 12" culverts or two 18" culverts.

Water exiting the culvert can erode the land at the outlet. The greater the velocity of flow, the greater the erosion. Erosion at the outlet of an upstream culvert will loosen sediment and debris, which can build up in ditches and inside a culvert downstream. Buildup slows the flow. Ultimately sediment is carried into streams, ponds, or lakes. A solution for low velocities is to plant vegetation. For higher velocities, crews should construct a stone splash pad or plunge pool at the outlet. Typically, for culverts with a diameter of 30 inches or less, the depth of the plunge pool should equal the diameter of the culvert.

Inspection determines whether culvert cleaning, repairs, or replacement are needed. Where necessary, obtain a wetlands permit before performing maintenance. If replacement is necessary, it is important to choose a culvert the right size for the location and water flow. Regular maintenance, such as removing sediment and debris, is essential to keep the culverts working properly.

Common Culvert Problems and Solutions