Culvert-Design Tool

Culvert-Design Tool

SC2 members conducted an analysis of projected changes in streamflow, due to climate change, which may cause a particular culvert to fail to provide fish passage over the course of its design lifetime. The 2018 analysis, conducted by the University of Washington Climate Impacts Group (CIG), was initiated as a complement to a similar analysis conducted in 2016 by the Washington Department of Fish and Wildlife. The interactive tool, developed by the UW Climate Impacts Group, allows users to assess the probability that a particular culvert size will fail to meet regulatory standards over its design lifetime using available streamflow data.

Launch the tool

The work completed by CIG resulted in both a report and tool:

The work completed by WDFW resulted in a report:

How to Use and Interpret the CIG Tool

Use the +/- buttons and the hand tool to select the area on the map that contains the culvert location you wish to explore. Click on the boxed area.


Change the auto-prompted setting with your information on the measured bankfull width at the culvert site, the design lifetime for your culvert, and the proposed culvert size.
The yellow box at the bottom will provide an estimate of the likelihood that your proposed culvert will fail to meet stream sim standards during its design lifetime. This means that a score of 100% means that it is very certain this culvert design will not allow fish passage during its lifetime. A score of 0% means it is highly likely that fish passage will continue to be provided throughout the culvert’s design lifetime.
The two graphs show the outputs of the 6 models used in the analysis.


Bankfull Width: Provide your measured or estimated bankfull width at the culvert site you want to explore. The stream sim estimated culvert size prompted below the bankfull width number can be used to explore a starting place for proposed culvert size. The user must select the Update Graphs button for the stream sim culvert size to reflect any new input entered in the three prompts.
Design Lifetime: The design lifetime is the length of time the user expects the culvert to be used prior to replacement.
Culvert Size: The culvert size explores the size of the culvert needed to achieve a given probability that a culvert will or will not fail within its design lifetime.
Update Graphs: It is necessary to click on the Update Graph button for any change to the numbers provided for bankfull width, design lifetime, or culvert size to be reflected in the stream sim culvert size estimate, the two graphs, or the probability of exceeding design over the design lifetime.
Probability of Exceeding Design Over Design Lifetime: Likelihood of failure, displayed in the yellow box in the lower left of the screen, is based on the number of years during which one or more models project that flows will exceed the proposed culvert size.

The number of models that project a failure, within the design lifetime, is also listed below the probability number. This is another measure of the likelihood of failure: if only one model shows that the proposed culvert will be undersized, users should put less confidence in the result than if all six show a failure within the design lifetime.

Some users may wish to know approximately when the proposed culvert will be undersized. As a result, the box includes one final piece of information: the average year (average among all six model projections) in which the proposed culvert size is first exceeded.

Top Graph: The top graph, on the right-hand side of the tool, shows the range among model projected changes. Specifically, the plot shows the average, and range, among the 5th percentile estimates of culvert size for each year in each of the six model projections. The low-end 5th percentile estimates were chosen so that any time flows are projected to exceed the proposed culvert size, this would correspond to a 95% confidence of failure. These low-end estimates were derived using a monte carlo approach, as described in the project report.
Bottom Graph: The bottom graph shows the fraction of models that projects a culvert failure, in each year of the simulations. Since there are six models, these are in units of 1/6th (or about 17%).

Why Small Changes in the Size of a Culvert Quickly Change the Failure Rate

Users may observe that small changes in the proposed culvert size can result in large changes in the likelihood of failure as seen in the images below. There are two primary reasons for this. The first is that culvert failure is a threshold response: either flows are within the capacity of the culvert, or they are not. As a result, small differences in culvert size will make a big difference in its likelihood of meeting regulatory standards. Second, we define failure as the likelihood that flows exceed the regulatory standard at any time within the design lifetime. This means that the likelihood of failure increases rapidly with the number of years that show a likelihood of exceedance. For example, even if there is only one out of six models that shows a culvert failing in any given year (i.e.,: a 1 in 6 chance of failure for that year), the overall probability of exceedance is 60% if there are just five such years within the design lifetime.

What is Stream Sim?

Stream sim (or stream simulation) is a simple equation that defines an ecological approach to determining the necessary culvert size to provide fish and aquatic organism passage. The equation is based on the bankfull width measurement. Stream width, including bankfull width, adjusts to match changes in streamflow. Since climate change is projected to lead to changes in streamflow, this means that bankfull width will change as well. This is why the stream sim design, based on today’s bankfull width, is often shown to fail with this tool: as flows change in the future, those flows may sometimes exceed a design that is based on the current bankfull width. Stream sim is currently a regulatory requirement for Washington Department of Transportation as a result of the US v. Washington “culvert case.” WDFW is currently using the stream sim approach in their climate change and culvert work as well.

The report and tool were completed through the Skagit Climate Science Consortium (SC2) with funding provided by the United States Environmental Protection Agency under assistance agreement PA-00J99101 to the Swinomish Indian Tribal Community. The contents of this website do not necessarily reflect the views and policies of the Environmental Protection Agency, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. Website development was supported with funding provided by the Skagit Climate Science Consortium and Seattle City Light.