Please use this guide only as a quick reference to the material presented.
Refer to the documentation presented throughout this web site for more detailed information.

1.  We recommend a road crossing design process called the Stream Simulation Design Method.

2.  Generally, when flow velocities through a culvert exceed .6 meters (or 2.0 feet) per second, some sizes and species of fish will be blocked. 

3.  Measure bankfull width of the stream (average of 10 measurements).

4.  Establishing the elevations of the culvert inlet and outlet are critical elements in the installation using the Stream Simulation Method. 

Determine the stream slope by surveying the vertical drop in the stream bed from 100 feet above the culvert (Point A) to 100 feet below (Point B).  See images below.  Set culvert inlet and outlet depths (embed) to the appropriate amount below the slope line.  Round Culverts may be embedded from 20% to 40 %, ellipse or squashed arch culverts from 10% to 25%.

5.  Determining the Bankfull Stage is the most important measurement for establishing culvert size in handling peak stream flows (refer to image). 

6.  If the Bankfull Width is more than 20 feet or if stream slope is more than 6%, use a span bridge.

7. If the Bankfull Width is less than 20 feet and  stream slope is less than 6%, a span bridge may be used or one of the four culvert types may be selected.

8. If slope is 3% to 6%, a span bridge is preferred but a bottomless arch culvert may also be used.

9. If slope is from 0% to 3%, bottomless arch, round, box or ellipse culverts may be used.  If using a round culvert, the best design will include natural stream materials in the bottom of the culvert that help to minimize and alter water flows, thus allowing for easy fish passage. 

10. Choice of the structure type is an important consideration, but the placement and installation are just as critical for the success of the project.

11.  For all culverts, the width (D) of the opening will be the Bankfull Width of the stream, plus 1 foot. 

12.  Length of the culvert will be the total road width plus 2 times (fill height X slope).

13.  Survey and establish the desired height of the culvert inlet and outlet. 

14.  Culvert should be embedded to correct for elevations at inlet and outlet.

15.  Stream bank erosion and stream sedimentation at culvert crossing can be a major problem for fish habitat if the appropriate control measures are not adopted.

16.  Soil erosion problems at fish crossings can be minimized if not completely eliminated, with advance planning.  The 1st step is to protect the work area long before the heavy construction begins.

17. Another very important consideration is the potential for geomorphic effects, in particular, channel incision. Channel incision or down-cutting of the streambed tends to propagate upstream by means of migration of a headcut. Often, this headcut migration is arrested when it encounters a culvert. If the culvert is replaced, the headcut may then proceed onward upstream, spreading channel incision, and inducing a period of channel destabilization and habitat degradation. 

Click here to review the document (pdf)  Geomorphologic Impacts of Culvert Replacement and Removal: Avoiding Channel Incision.

Return to Home Page or    Click here to proceed to the "FINAL" section