This site has excellent spawning
habitat for a variety of fish species including steelhead, coho salmon
and white suckers.
Evaluating a Problem
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
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
- Consider the bank slopes needed for a stable configuration.
- What is required by county, state and federal highway and
- For keeping installation costs down and for fish passage, how
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.
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).
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:
ability to pass sediment, large wood and debris without scour or
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
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
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
Determining the Bankfull Stage is the most
important measurement for establishing culvert size in handling peak
- Measure and average channel width at 10 locations above the existing
crossing (crossings often alter the normal channel below the
- Measure from the first rooted, and established
vegetation on each side of the channel as shown in this diagram.