In the previous sections we measured the bankfull width and slope of the stream bed at the planned crossing. 

  The Bankfull Width measurement determines the width of the new culvert (all types) using
the following formula:

Culvert Width = Bankfull Width + 1 foot

We have also determined whether important fish spawning habitat occurred on the site.  Based on these factors, we decided which of the four culvert types to install.  The final dimension of the culvert, length, is determined by measurements made on the site.  The measurements needed are the desired, final width of the road bed and the slope of the embankments.  The design width of the road bed (at the culvert) should be at least the width of the existing road.  However, in some cases it should be set at the width of any planned or expected improvements that might occur during the life of the structure.

The new culvert should be aligned with the existing stream channel.  Sharp turns in the channel above or below the structure will direct currents into banks, eventually causing undercutting and collapse.  However, if stream bends adjacent to culverts can't be avoided, it is preferred to locate culverts so they are aligned with the downstream channel.  Even if there is a sharp bend upstream.

The length of the culvert for any installation is based on the total width of the road plus the distance added by the slopes of the embankments on each side of the road to the bottom of the culvert (see diagram below). 

The road width is measured from the point on each side of the road where slope meets level road shoulder.   The height of the fill is the vertical distance from the bottom of the culvert at the middle of the road to the finished level of the center of the road.

The actual slope of the embankments should provide a stable configuration and may be based on one-size-fits-all recommendations or requirements, or be determined by knowledge and experience with local soil and fill characteristics.  General recommendations tend to be conservative and based on unstable fill materials such as sand.  This can result in longer slopes and culverts than are really needed in applications where fill materials are more cohesive and stable (such as clay). 

While the general rule approach may provide an extra measure of stability to slopes in some cases, it also results in longer culverts, higher costs and more habitat and fish passage problems.  We recommend consultation with agencies with local expertise such as U.S. Natural Resources Conservation Service (NRCS), State natural resources agencies and experienced road building contractors. This point in the project development is an excellent time for consultation with regulatory agencies for both advice and information regarding regulations and permits.

In the example above, the fill height needed to maintain a level road at the crossing is 16 feet.  Assume that we will use local fill materials that are fairly cohesive and capable of supporting strong plant growth for further stabilization.  Based on the observations made in this example, we have assumed that a 2:1 bank slope is adequate for this site.  We now have the information needed to determine culvert length by using the following formula:

Culvert Length = Road Width + 2(Fill Height X Slope)

Culvert Length = 32’ + 2(16’X2)

Culvert Length =96’

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 image above.  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%.

Note:  The method described above is the simplest method for finding stream slope. However, a more complete profile of the stream slope may be needed in some cases. In so, several elevations are measured on the stream bed at points above and below the structure to create an actual bed profile (images right).

When using either method you will often note that the stream bed is elevated directly above the existing culvert.  This typically occurs when the initial culvert placement is too high.  Natural stream processes deposit sediment, rock and debris above the culvert to a level above the culvert inlet.  When a new structure is placed at the correct depth, the sediment deposit may be considerably higher than the new culvert inlet. In most cases, the stream flow will quickly cut through and redistribute these materials downstream to normalize the channel. 

The process is called “head cutting” and will change the upstream channel shape and configuration considerably.  If the materials lodged above the old installation are clean sand, wood or rock, downstream deposition of these materials may not be a severe problem.  However, if large amounts of fine silt/sediment are involved, fish habitat downstream may be damaged.  In some cases, these materials can be removed mechanically before the flow is released through the new structure.

At this site, the original culvert placement was too high above the natural stream slope.  Rocks and sand have accumulated at the upper end of the culvert blocking more than 50% of the opening and creating a fish passage barrier.

Only a few hours after installation of a new culvert (embedded 20% below the slope line), head cutting is beginning to normalize the channel.

Culverts may be purchased with square or bevel cut ends that match the bank slope.  Bevel cut ends (see diagram) are thought to be less prone to blockage by debris washing downstream.  Placing rocks in the bottom of culverts will also help to simulate the natural stream bed. 

With an outline diagram of the planned project, we are now ready to plan the final steps before permits are requested. Requirements for construction permits are highly variable among jurisdictions. In most cases however, the project diagram will need to be supplemented with an erosion control plan and a description of sediment control during construction. This image shows one example of an erosion control plan for a small stream.

A plan to control sedimentation of the stream during construction is often required.  This plan should describe how stream flow will be handled during the construction period.  In very low flow streams, simply damming the stream with clean fill above the work site may provide time for the installation.  On streams where flows are too large for this technique, water may be pumped around the work site, or a temporary diversion channel may be required.

Construction scheduling for stream work can be very difficult.  Work must be done during low stream flow periods to minimize sediment deposition and transport, but must also consider the life stages of fish present in the stream.  Adult fish are usually not highly vulnerable to the type of sediment depositions created by culvert projects. However, fish eggs and fry developing in spawning beds are extremely vulnerable to being smothered by sediment.

Culvert installations should be scheduled when no fish eggs or developing fry are in substrates downstream of the project site.  Most fish species spawn either in spring or fall. These diagrams outline life cycles of some common game fish.  They also help to illustrate, that when spring and fall spawning species are present in a stream, the construction window may be limited to the months of July and August.  In this situation, some jurisdictions will only issue construction permits during that time frame.   Be sure to check with your local offices of the U.S. Fish and Wildlife Service and state Natural Resource Departments to determine what fish are present in the stream and permitting requirements. 

Permits are usually issued for a certain time period, such as 60 or 90 days.  Within that time it will be the builder’s responsibility to schedule the work when streams are low and work can be completed under good conditions.  Often this means keeping some flexibility in work schedules to provide for contingencies such as equipment problems or severe weather events. 

Culvert Installation, At last!

 Be Sure to Review Your Final Checklist:

·                    Contractor and machine operator are completely informed on the final installation plan, elevation requirements and sediment controls are in pace.

·                    The proper permits have been issued and the responsible natural resource agencies have been notified.

·                    Electrical, gas and water utilities lines have been located or cleared at the site.

·                    Project supervisor will be on-site throughout the construction period.

·                    All construction materials are present and ready.

·                    No bad weather forecast for today/tomorrow.

Excavate culvert and bypass channel.  Elevations of inlet and outlet should be checked often to ensure they are positioned correctly.

Appropriate use of a flow bypass channel during placement of a large, multi-section box culvert.  Note the clear water in the diversion channel and the alignment of the new culvert with the natural stream channel.

Water flow bypass during
culvert placement.

The fill should be clean mineral soil and free of organic material, large rocks, tree roots or ice that will break down and leave holes.  Bedding and filling around culverts is an important step in insuring that a culvert installation will not erode, settle or deform.  It also prevents “piping” or the leaking of water around the culvert.   

Compact the soil carefully using the process outlined below: 

Bedding and compacting fill
around the culvert.

When the culvert installation is complete, trim and stabilize the work site as soon as possible.

• Trim slopes to a stable configuration        

• Apply grass seed or plantings                     

• Apply mulch or erosion control blankets