The Value of Culvert Replacement

Culverts that hover above the stream are some of the worst for providing fish passage, but many still exist. Image courtesy of klopp-family.com

We all know one. An ugly, impractical beast that you just can’t imagine was purposefully constructed. At the same time, you know Mother Nature would never do one of her streams that dirty.
 
That’s right, today we’re talking culverts. The pipes, concrete boxes, and rebar that tunnel streams under roadways and railroads. The idea behind a culvert is simple: it needs to be strong enough to support the road above ground, and big enough to pass the water below ground. But, turns out, after 150 years in the business, humans are still trying to figure out the right balance between the two.
 
Some of the earliest design criteria for a culvert were published in 1853 in the 6th Edition of A Manual of the Principles and Practices of Roadmaking.  Simply put, they recommended that culvert “size must be proportional to the greatest quantity of water which can ever be required to pass, and should be large enough to admit a boy to enter to clean them out.”
 
Really, we’re using “boy” as the unit of measure? 
 
Today we might scoff at the inadequacy of those basic requirements for culvert design. But, for 1853, that simple recommendation was revolutionary. It also spurred rapid advancement of basic hydrologic theory because, at the time, there was no way to measure the “greatest quantity of water” that would pass through a culvert. We could guess, but flows are tricky- there’s drought and wet years, hurricanes, and heavy snows. So, some brilliant mathematicians worked out the numbers, and by the 1900s they found fairly simple equations that could estimate flood recurrence intervals and peak discharge. Crazy enough, these equations were so good that they still form the foundation of those calculations today.
 
But, something was still missing. We might know how much water passes past a point (otherwise known as stream discharge), but what’s the most efficient structure for facilitating that stream flow?  It wasn’t really until the 1920-1950s where scientists started considering the position of the culvert in the stream. Should the culvert be completely submerged?  Mostly out of water?  What if the inlet is completely submerged, but the outlet not? Vice versa?  Things get complicated fast. And, while we’re now better at designing culverts, we still aren’t 100% sure the answer to some of those questions.
 
Complicating matters is that oftentimes the most efficient way to transport water isn’t the most fish-friendly design. It turns out, fish are really finicky when it comes to culverts. They like a very set amount of flow, substrate sizes, and shade. If the culvert is too long they won’t pass completely through. If the water depth on either side is too deep or shallow, they won’t pass. Some species are more divas than others, but all have a very narrow window of conditions they are willing to tolerate.
 
Do you know how scientists found out that fish weren’t passing through culverts? The hard way. After decades of data collected on millions of culverts and hundreds of studies on fish swimming and jumping abilities, we have refined our understanding of what makes a culvert “passable” or not by fish. Unfortunately, when we started looking at culverts with a critical eye, we started realizing that many need to be replaced in order to achieve adequate fish passage. Replacing a culvert is no easy feat. It’s expensive, requires a lot of work hours, can be a huge hassle with traffic, and could also endanger fish populations in the stream. Making matters worse, a lot of culverts that need replacing aren’t even that old. The really poorly designed culverts- the ones that dangle feet off the stream bed, or are crumpling- may be a few decades old. But, many culverts that score low on the fish passage test are less than 10 years old. Before we start tearing down was is essentially brand new infrastructure, we better be sure that the end result will be restored fish passage, increased population connectivity, and overall increase to stream health.
 
That was part of the motivation behind a study that researchers from West Virginia University recently undertook. Simply put, they sought to determine whether culvert restoration will restore brook trout connectivity. Using genetics, they found that before culvert replacement populations below and above two culverts in West Virginia were structurally dissimilar. Otherwise, very few, if any, brook trout were swimming through the culvert and the populations above the culvert were genetically isolated (to read more on why genetic isolation can spell bad things for brook trout populations, click here). After culvert replacement, they found immediate evidence that fish were swimming upstream and that population connectivity had been restored. Success!
 
But, let’s not go tearing out all the culverts just yet. This was an obvious case where skilled engineers and biologists worked together and installed a culvert that was designed better than the one that was previously in place. But, sometimes it’s not that easy. Sometimes, what should be a great culvert still doesn’t result in great fish passage. And, a culvert that doesn’t seem so great by design is biologically functioning just fine. Biology is oftentimes more than a numbers game, and it’s worth reiterating that there’s no one solution to every problem. That’s what is making the science of culvert design frustrating and at times slow. There’s so many variables, and nature can be so unpredictable.
 
Further, even if we did know the perfect culvert design for every stream, there is also the question of whether populations really should be reconnected. If the isolated population has great genetic diversity, large size, and is seemingly healthy, then maybe it’s okay to place that stream low on the priority list for culvert replacement. Or, if downstream of an impassable culvert there is a thriving population of a nonnative species, maybe we should start considering whether we want to purposefully prevent fish passage.
 
That’s right, I said it. Go against everything I, and science, have ever told you and PURPOSEFULLY keep populations isolated.  But, that’s a story for the next blog…

*Note: Content in this post is my own and may not reflect the opinion of the manuscripts' authors or the agencies they represent.  I encourage you to read the manuscript, found here, so you can contribute to the discussion. 

---