Habitat Fragmentation – Culvert Blockages and Solutions


Culvert that is “perched” due to scour by high velocity flows through the pipe. ©Princeton Hydro.

The Bucks County Chapter of Trout Unlimited (Pennsylvania) and the Cooks Creek Watershed Association were featured in the Summer 2013 edition of Trout magazine, TU’s national publication, for their culvert inventory work in the Cooks Creek watershed.  Princeton Hydro was glad to assist via directly investigating and training of volunteers to inspect and document potential culverts in need of retrofit.  Princeton Hydro also completed design concepts and opinion of costs for two example culverts.  Identified culverts in need of retrofit will help the creek’s wild brown and brook trout.  Princeton Hydro based the training on the Vermont guidelines for rating culverts for pass-ability.  In this small watershed a total of 97 culverts were identified with 32 of them as potential barriers, and 11 identified as “high priority” in need of retrofit.

Why worry about culverts, you say?

One of the most unforeseen danger to the biodiversity in our river networks is habitat fragmentation through un-passable culverts throughout the United States.  While blockages via dams number upward of 100,000 or so, the blockages created by ecologically and biologically inefficient culverts is likely to number in the millions.   The majority of these culverts are located in headwater areas of rivers, which entail greater than 50% of most river miles in a watershed; a large cumulative impact.  As a result, native key headwater species such as brook trout (Salvelinus fontinalis) in the East and cutthroat trout (Oncorhynchus clarkii) in the West have had their historic ranges reduced to a fraction of their former extent.

Historically, culverts were designed by civil engineers to maximize flow capacity and minimize pipe size in order to create the most economical structure for developers, transportation authorities, and municipalities.  The unfortunate by-product of such a design approach is that water velocity through culverts is extremely high, often running in supercritical flow, even during base flow conditions, and the smooth and featureless surfaces in the structure make it extremely difficult to navigate.  To add insult to injury, the high velocity flows also scour and erode the stream channel immediately downstream of the culvert, leaving the pipe too high out of the new channel (“perched pipes”) for organisms to pass.  Downstream water dependent organisms cannot pass upstream to new habitat, and those populations upstream become extirpated due to downstream migration and mortality, and the lack of an ability to return or be replaced.  A study of impacts of fragmentation on brook trout is ongoing by the USGS Conte Anadromous Fish Research Center (USGS CAFRC) and others, and a study recently completed documented the impacts of fragmentation of local populations provides an informative view of the blockage potential of culverted streams.

There is hope in the re-connection of stream habitat through new research and initiatives developed since 1999.  One such approach is through the Stream Simulation design originally developed in its present form at the Washington State Department of Fish & Wildlife and adopted by the US Forest Service, US Fish and Wildlife Service, as well as others, and was also adopted shortly thereafter and refined by the University of Massachusetts, Amherst Extension (Stream Continuity model) for use in Northeastern States (initially in the Massachusetts River and Stream Crossing Standards, and then adopted in similar form by surrounding states).  Through the Stream Simulation/Continuity method, a culvert is not simply measured in terms of hydraulic efficiency, but also in terms of ecological and biological efficiency.

In the most basic terms, Stream Simulation (Continuity) requires a crossing that has a minimum width of the bankfull flow of the natural channel upstream and downstream, plus more width to allow passage of terrestrial organism passage such as reptiles and amphibians (in the UMASS model the increase in width is 20% wider than bankfull, but in the current Washington State model they use 20% plus 2 feet).  The other part of the design requirement is an opening area to length ratio to allow the maximum amount of natural light penetration into the culvert (openess ratio), as many organisms, such as fish, are too intimidated to travel through dark culverts.  Other design requirements include the use of slopes and velocities that allow for fish passage, and roughness (i.e. placement of natural substrate) to also slow down the flow.

The key challenge for the retrofitting of culverts to be more passable is cost.  As with any civil engineering project, the larger it is, the more expensive.  To replace a 36 inch diameter culvert with a 10-14 foot wide structure could increase the cost by 10-fold.  However, there are ways in completing an economic analysis to justify the costs.  For example, most culverts were historically only designed to pass storms up to the 25-year event, but in even more cases, never were sized by engineers.  A larger culvert will increase its capacity and reduce overtopping events that would require road closings and worse, cause the roadway to collapse.  Road closings require emergency management and road crews to set up detours and slowing down commerce, or worse require repetitive reconstruction efforts that, over time, may exceed the cost of installing a Stream Simulation designed culvert.


Same culvert as in photograph above, after the retrofit using Culvert Simulation. ©Princeton Hydro.

Other ways of encouraging installation of these larger and passable culverts is through the permitting process.  In New England, the US Army Corps of Engineers, allows for a by-pass of a formal review for their approval if the Stream Simulation guidelines are followed. This approach can save a significant amount of time to fast-track a retrofit.  To complement the Corps’ permit facilitation process, the states of Connecticut, Massachusetts, New Hampshire, and Vermont, have developed stream crossing guidelines to meet the Corps’ permit by rule compliance.  These states have even instituted state level regulations requiring aquatic organism passage via the Stream Simulation model.

Princeton Hydro was contracted to design a culvert retrofit to replace a 36 inch diameter culvert with a 12 foot wide arch culvert on a tributary of West Brook which is being monitored as part of the USGS CAFRC research project in Massachusetts.  This retrofit will be used to assess the increase in efficiency of headwater stream accessibility by local brook trout populations.

It would appear that the Stream Simulation or Continuity model is catching on, however, there needs to be more outreach and changes to existing rules in other regions of the US.  Further studies, such as that being conducted by USGS and their partners, will determine the true benefits of increasing culvert fish passage efficiency and bolster the economics of protecting fish populations for future generations.
Geoffrey M. Goll, P.E.
Vice President and Founding Partner

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9 thoughts on “Habitat Fragmentation – Culvert Blockages and Solutions

  1. Good article. I have two comments: 1) The agency you likely intended to cite is the Washington State Department of Fish and Wildlife, I do not believe there has been a Washington State Department of Fish and Game since the 1930s, 2)the culvert court case in Washington state is a current case where Native American Tribes have sued Washington State for impacts to tribal treaty rights (an extension of a 1974 case: US vs. WA, aka Boldt Decision). The federal court has ordered the Washington State Department of Transportation , Department of Fish and Wildlife, Department of Natural Resources, and the Department of Parks to replace over 800 culverts in western Washington State by 2030. Estimated cost $1.9 billion. In my opinion, the recognition by a federal court of the importance of fixing culverts to fish populations is significant.

    • Thank you, Gary. I had the link right, but not the name. It is now updated.

      I appreciate the citation of the court case in Washington State. This IS good news and significant that the federal government is recognizing the importance of stream connections for fish passage. I found an article on this recent case here, but if you have the link to the decision that would be appreciated. Unfortunately, it looks as if the case is being appealed by Washington State, so we will keep an eye out for this as it develops.

      It would be an interesting case study to understand the cost implications for the Indian tribes, as well as the commercial marine fisheries as a result of lost breeding habitat for salmon.

  2. Great blog, Geoff! A nice balance of information and cool facts for a blog post.

    One thing that has been particularly encouraging on the West coast has been the consideration, albeit minor, of other aquatic organisms in assessing and designing culverts.

    • Thanks! You bring up a very important point regarding “other aquatic organisms”. While fishery biologists tend to be the representatives of state agencies to weigh in on whether there is a need to install or retrofit a pipe to allow passage, all too often fish are the focus. Especially in higher elevation headwaters within in the Appalachian Mountains for example, fish are absent and the top organisms are stream based salamanders, such as spring salamander (Gyrinophilus porphyriticus), northern dusky salamander (Desmognathus fuscus), and northern two-lined salamander (Euycea bislineata). These important top predators need passable culverts to help avoid exposure on roads from vehicles and predators.

      It needs to be emphasized that it is “aquatic organism passage”, not “fish passage”, to be inclusive to all. Glad you brought this up! Hope all is well.

      • Thanks Geoffrey, this is a great read. It just calls on us that are involve in designing structures like culvert bridges to take into consideration the smaller organisms that ensure that the ecosystem is balanced. Thanks for this in-depth information!

  3. It would be great to work with hydrologists and biologists to further develop solutions that are already proving successful. The aim is to cater to a wider range of species in various situations. Please let me know if you want to collaborate???


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