Structures, water, computers, languages and people (not necessarily in this order)

Fauna-friendly culverts

Frogs, turtles, snakes, trouts and salmons among others are blocked by the culverts that lay beneath our roadways.

The importance of a good drainage is known since the work done by the Scottish engineer Mac Adam in the dawn of 19th century. Surface drainage would have to be completed by ditches and culverts as a mean of driving the drained water away from the road and into the streams that existed before.

Traditionally, culverts have been installed with consideration for their hydraulic capacity. The cross-sectional flow area is smaller than that of the stream approaching it and the velocity of the water entering the culvert is greater than that of the natural stream. However, culverts that are correctly designed and installed (see figures) do not hinder animal passage.

Upper pic: George Washington National Forest.
Lower pic: British Columbia Min. of Transp.


  1. Looking at the lower pic, I was wondering whether the "proper installation" is less stable. I mean, it seems to me that the whole thing could slide down, with the road included. (Sorry if this comment makes no sense, I'm just a renegade engineer).

    1. It is almost 6 year after so maybe the coment doesnt make sense now. The proper instalation is necesary from the hydraulic point of view. Without the gradient water can not flow from one side to the other.

    2. Thanks for the informative comment, Edlira. Hydraulics first, structures then.

  2. Another comment. Doesn't the topic of this post drift a little from the beginning to the end? Proper and improper culverts are equally unfriendly to salmons and trouts, aren't they?

  3. I suppose that the bottom of the culvert is buried to be stable during high flow events but I must admit that I do not have a clear idea about this. Some piping companies tend to use finite element codes to check stability, stress, fatigue and deformation with a range of fills.

    If I get time, I will rewrite this post. I aimed to underline the fact that the culverts had been first used under the roads by MacAdam and others. In those times, the calculations were driven by hydraulic criteria. Nowadays the main idea when designing culverts should be to reduce water velocities in order to make it easy for the fauna and to reduce the erosion (as far as I know subcritical flow is not allowed in most of the standards).

    Finally, I agree with you, an animal-friendly culvert is not as friendly as a bridge, but it is less expensive.