Culvert ends are a prime area for erosion. The additional velocities introduced into the stream channel by a culvert can cause maintenance costs to increase and create a burden on the road authority. In this article, I will introduce the main options at your disposal to prevent and protect from erosion damage.
The standard method of erosion control is placing rock, sometimes called riprap, at the inlet and outlet. The greater the velocity, the greater the rock size. Culvert design software such as CulvertPro will calculate culvert inlet and outlet velocities for use in erosion control design. One jurisdiction I have designed culverts for uses the following rock sizes:
- Below 2.0 m/s: 125 – 300 mm
- 2.0 m/s to 3.0 m/s: 200 -450 mm
- 3.0 m/s to 4.0 m/s: 300 – 700 mm
- 4.0 m/s to 4.6 m/s: 500 – 1800 mm
The U.S. equivalent is:
- Below 6.6 ft/s: 6 – 12 inches
- 6.6 ft/s to 9.8 ft/s: 8 – 18 inches
- 9.8 ft/s to 13 ft/s: 12 – 28 inches
- 13 ft/s to 15 ft/s: 20 inches – 72 inches (6 feet)
The rock should uniformly include the full range, for example, if you are dealing with 10 ft/s at a culvert inlet, you should ensure that the rock ranges fairly uniformly from 12 to 28 inches. Also, above the maximum 4.6 m/s (15 ft/s) you can see that some type of engineered erosion control is necessary.
When rock is not practical, several other options exist:
- Prefabricated ends. Most pipe manufacturers make culvert end sections. These are usually very expensive relative to their length but they conform to the slope. One thing to watch out for is that they have no fill above them and many jurisdictions I’ve worked with don’t use them because they don’t stay in place.
- Gabion Baskets can provide a good erosion control solution when the maximum slope of 2:1 for rock is exceeded.
- Biodegradable erosion control matting can be sufficient but I would not count on it unless the culvert is below about 1.5 m (5 feet) in diameter and velocities are minimal.
- Other more exotic erosion control products exist such as rolled blankets but they are not used often and all I can say here is that they can be investigated when needed.
- Concrete can be used as a last resort.