This site has excellent spawning habitat for a variety of fish species including steelhead, coho salmon and white suckers.

Evaluating a Problem Road Crossing
for Remedial Action:

1.  Is there important fish habitat in the stream on the work site?  What is the composition of the material on the stream bottom?  Gravel or cobble size rock are spawning habitat for many fish species.  Groundwater or springs are too. These are important habitats and must be protected from disturbance and sedimentation during and after construction.

These culverts are severely perched, damage habitat and block fish passage.

Too short a culvert, soft, sandy soil and excessively steep banks spelled disaster for this installation. Replacing it correctly will approximately double the cost of the initial project.

2.  Evaluate and record soil types at work site.  Soil and available fill composition are important factors in determining how slopes may be contoured to stable configurations.  Cohesive soils such as clay are less prone to erosion or slumping than sand.  Slope or bank ratios include a number of considerations:

• Consider the bank slopes needed for a stable configuration.
   
• What is required by county, state and federal highway and safety authorities?
   
• For keeping installation costs down and for fish passage, how can the slope and total culvert lengths be kept to a minimum, while maintaining a safe and lasting structure?

In this example, a roadside ditch is contributing suspended red clay to this trout stream. 

3.  Note and record potential erosion or sediment sources entering the work site.  Are there ditches or banks contributing significant amounts of water flow or sediment to the site?  

Plan an approach to sediment control !

Slope determines the type of culvert or bridge that may be used.

Critical Stream Measurements:

1.  What is the slope of the stream bottom?  Slope determines the type of culvert or bridge that may be used.  Measure stream slope from a point 100 feet above the existing culvert to a point 100 feet downstream.  Measure elevations from bottom of the stream bed.

Culvert designers need to measure profile (i.e. elevations and longitudinal distances of a series of points on the streambed), not just slope through the pipe.  This requires extending the survey far enough upstream & downstream to get a representative profile of the channel away from the area affected by the culvert itself.  For some projects, this may be more than 100 ft (some authorities recommend a distance of 10 bankfull channel widths in each direction).

Compute stream slope using this formula:

Slope = Length (200 + length of Culvert) / elevation change 

For example: If the existing culvert is 80’ long and we measure from 100’ above to 100’ below, the total length equals 280'.  If the elevation change is 9’, then 9’/280’= .032 or 3.2% slope.

Bankfull stage is the vital measurement for properly sizing culverts.

---

Advantages to having a culvert somewhat wider than bankfull include:

• Better ability to pass sediment, large wood and debris without scour or blockage.
   

• Less constriction of the flow during high water, and thus less increase in average velocity during peak flow development of a natural channel edge within the culvert, which promotes a more extensive low-velocity zone (technically, the hydraulic “boundary layer”).  Small fish typically hug this zone to stay out of high-velocity water when moving through a culvert.

2.  Determine the width of the bankfull stage of the channel.  This measurement is a good, on-the-ground  indicator of flow levels in the stream over a long period of time.  It may be used to determine culvert sizing that will be capable of handling peak flow levels, but, it must be measured accurately.

The important element in determining the bankfull stage is the presence of the depositional (floodplain) surface.  This surface may be correlated to a transition in vegetation type (e.g. presence of certain species, transition from moss to rooted plants or annual to perennial woody plants), or transition in substrate texture (e.g. gravel to sand or silt), but these secondary indicators need to be locally calibrated by correlating them to the “point of incipient flooding”.

  Determining the Bankfull Stage is the most important measurement for establishing culvert size in handling peak stream flows. 

• Measure and average channel width at 10 locations above the existing crossing (crossings often alter the normal channel below the installation).

• Measure from the first rooted, and established vegetation on each side of the channel as shown in this diagram.