U.S. Department

of Agriculture

Forest Service

National Technology &

Development Program

7700—Travel Management

0877 1812—SDTDC

October 2009

FOREST SERVICE

D E P A RTMENT OF AGRICU L T U R E

Culvert Scour 

Assessment
ACKNOWLEDGMENTS This publication is the result of a partnership between the National Technology 

and Development Program of the Forest Service, U.S. Department of Agriculture 

and the U.S. Department of Transportation Federal Highway Administration 

(FHWA) Coordinated Federal Lands Highway Technology Improvement Program. 
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Culvert Scour 

Assessment

Prepared by: 

Inter-Fluve, Inc.,

for

U.S. Forest Service

San Dimas Technology & Development Center

San Dimas, CA

October 2009

 v

1 Introduction................................................................................................................................. 1

2 Methods...................................................................................................................................... 2

 2.1 Study area and site selection............................................................................................. 2

 2.2 Acquisition of background information............................................................................... 4

2.3 Analysis framework............................................................................................................ 4

2.4 Culvert and site conditions................................................................................................. 4

2.5 Topographic survey............................................................................................................ 5

 2.6 Pebble counts and bed material analysis .......................................................................... 6

 2.7 Hydrology........................................................................................................................... 6

 2.8 Hydraulics .......................................................................................................................... 7

 2.8.1 Hydraulic modeling .................................................................................................. 7

2.8.2 Longitudinal profile analysis .................................................................................... 9

2.8.3 Analysis of hydraulic variables .............................................................................. 10

2.8.4 Particle entrainment analysis................................................................................. 10

 2.9 Channel complexity and depth distribution...................................................................... 11

 2.10 Channel units and LWD ................................................................................................... 12

 2.11 Photo documentation....................................................................................................... 13

3 Results ............................................................................................................................... 13

3.1 Culvert and representative channel segment conditions ................................................. 13

3.2 Hydrology......................................................................................................................... 16

3.3 Site evaluations ............................................................................................................... 16

3.4 Overview of scour conditions........................................................................................... 19

3.5 Overview of AOP conditions ............................................................................................ 21

3.6 Overview of substrate conditions..................................................................................... 25

4 Discussion ............................................................................................................................... 29

4.1 Inlet scour and culvert bed adjustment............................................................................ 29

4.2 Effect of culverts on channel-flow geometry and hydraulics............................................ 30

4.3 Culvert substrate conditions ............................................................................................ 32

4.4 Channel complexity and depth distribution...................................................................... 33

4.5 Channel units and LWD ................................................................................................... 34

4.6 Implications to AOP.......................................................................................................... 35

4.7 Comparison of results with WFLHD study ....................................................................... 35

5 Recommendations ................................................................................................................... 37

6 Acknowledgments .................................................................................................................... 40

7 References............................................................................................................................... 41

 Appendix A—Site Evaluations.................................................................... CD inside back cover

 Appendix B—Hydrology Reports................................................................ CD inside back cover

Table of Contents

 1

1 Introduction

The purpose of this study is to quantitatively 

analyze (1) the geomorphic and structure controls 

on channel-bed and footing scour at road-stream 

crossings, and (2) the effectiveness of aquatic 

organism passage (AOP) at these crossings 

by comparing channel characteristics within 

the crossing structure to reference channel 

conditions not influenced by the structure. From 

this analysis, one can determine the design, 

construction, stream, and channel conditions 

that contributed to the success or failure of the 

installation for AOP and scour resistance.

Background information

Embedded or open-bottom culverts that are 

designed to mimic natural channel form and 

function have been used for years at road-stream 

crossings (McKinnon and Hnytka 1985). Such 

designs are intended to provide geomorphic 

continuity through the stream crossing such that 

the passage of water, sediment, debris, and 

aquatic organisms occur in a similar fashion to 

conditions in the adjacent stream channel. This 

technique is often applied on streams where 

traditional hydraulic criteria for fish passage 

cannot be met, such as on mountain streams that 

often exceed the maximum velocities that are 

required for fish passage.

Guidelines for stream-simulation design 

typically include criteria for culvert width, slope, 

and bed material, but may also include other 

considerations (Bates et al. 2003, Oregon 

Department of Fish and Wildlife 2004). 

Guidelines developed by the Forest Service, U.S. 

Department of Agriculture, include detailed site 

assessment and design considerations that relate 

to channel geomorphology, project alignment, 

longitudinal profile, reference reach conditions, 

bed material size and arrangement, structure 

size and elevation, and stability of the streambed 

inside the structure (Forest Service StreamSimulation Working Group 2008).

This study was designed to evaluate past 

culvert installations to determine how they have 

adjusted to local flood history and how their 

modes of response relate to elements of culvert 

design. It was therefore necessary to include 

culverts that had been exposed to high flows; 

otherwise, measured parameters would only 

reflect constructed features/conditions and not 

flood-response conditions. This criteria, however, 

limited the study sites to older installations, most 

of which were not designed according to the 

more contemporary stream-simulation design 

standards described previously. Culvert widths in 

this study ranged from less than half of 1 bankfull 

width to greater than 1.1 bankfull width. Culvert 

slopes ranged from a slope of zero to over 1.7 

times the streamslope. Within these ranges of 

culvert designs, differences can be seen in the 

modes of adjustment in response to flood events. 

These results provide insight into the design 

elements and site considerations that should be 

addressed during culvert-installation projects.

This study includes an evaluation of differences 

between individual culvert sites and channel 

conditions nearby but outside the crossing. 

Conditions are evaluated with respect to bed 

scour, flow geometry, hydraulics (velocity and 

incipient motion), channel complexity, and 

physical habitat. Causative factors are explored 

by relating observed and modeled conditions to 

culvert designs. General discussions are provided 

regarding the implications of study results to AOP. 

Management recommendations are provided that 

we hope will be useful to practitioners involved in 

culvert designs.

Introduction
2

Culvert Scour Assessment

2 Methods

2.1 Study area and site selection

An interdisciplinary team with expertise in 

hydraulic engineering, hydrology, fisheries 

biology, and fluvial geomorphology selected study 

sites in western Oregon. The team selected sites 

according to the following criteria:

n Focus on stream-simulation type culverts 

(may include round, pipe arch, bottomless 

arch, or box).

n Focus primarily on moderate-to-steep 

gradients (2 percent to 6 percent), as these 

are the most common applications that still 

have fish potential. 

n Include a number of low-gradient (less than 

2 percent) culverts, particularly if they have 

experienced significant above-bankfull 

flows, because such a scenario most likely 

results in compromised culvert conditions. 

n Include a range of stream sizes and 

gradients (within the constraints specified 

above) so that results are applicable to the 

range of likely applications. 

n Avoid culverts with internal structures 

(baffles or ladders) because these are not 

typical in stream-simulation design.

n Avoid culverts with other special 

circumstances that are not likely to be 

part of new designs (secondary pipes for 

conveying flood flows).

n Include well functioning as well as poorly 

functioning culverts in order to identify 

the design elements that favor good 

performance.

n Focus on older installations (greater than 

approximately 5 years old) that have 

experienced a range of high flows and 

sufficient channel-forming flow durations 

to make sure culverts have responded 

adequately to local hydrology. More recent 

installations can be utilized on a caseby-case basis if the hydrology has been 

suitable.

n Limit the spatial distribution of the culverts 

to ensure that adequate sample sizes can 

be obtained for each region. 

n Select sites to represent a range of 

geomorphic channel types.

n Select sites that have appropriate 

representative channel conditions 

upstream and/or downstream of the culvert. 

Appropriate representative channels should 

represent similar gradients, planforms, and 

valley conditions that are present at the 

culvert crossing.

The team selected 17 sites that met these criteria. 

The sites range in location from the Coast Range 

to the east slope of the Cascades. Six sites 

are located in the Coast Range, nine sites are 

located in the western Cascades, and two sites 

are located east of the Cascade crest. A map of 

the culvert locations is included in figure 2.1‐1. 

Additional details on culvert locations are included 

in the individual site evaluations in appendix A. 

Appendix A is contained on a CD inside the back 

cover of the publication.

The team selected representative channel 

reaches upstream and/or downstream of each 

culvert crossing. The criterion for selecting 

representative reaches was to select reaches 

close enough to the culvert to have similar 

valley and geomorphic characteristics (slope, 

confinement, bed material), but far enough 

to be outside of the influence of the existing 

or previous culverts. Within each culvert and 

representative reach, the team selected crosssection locations that represented characteristic 

hydraulic, geomorphic, habitat, and substrate 

conditions of the reach. These sections formed 

the basis for the hydraulic and bed-mobility 

analyses. For most sites, the interdisciplinary 

team identified representative reach boundaries 

and representative cross-section locations.
 3

Figure 2.1‐1. Location of the study sites.

Methods
4

Culvert Scour Assessment

2.2 Acquisition of background 

information 

Where possible, the team obtained background 

information for culverts to determine design 

objectives, construction details, flood response 

history, and maintenance activities. These 

factors can help to evaluate culvert performance 

and explain analysis findings. Due to the age 

of many of the sites and attrition of agency 

personnel, most of the sites have very limited, 

if any, available background information. Forest 

Service personnel involved with the installations 

provided construction information for six of the 

culverts. Information was available for three other 

pipes that were analyzed as part of a 1987 study 

by the Western Federal Lands Highway Division 

(WFLHD) (Browning 1990). This study involved 

field assessment by Oregon Department of Fish 

and Wildlife (ODFW) fish biologists and hydraulic 

analysis by WFLHD staff to evaluate fish-passage 

conditions for a range of culvert installation types. 

The WFLHD study also includes an evaluation 

of an older culvert that was replaced by a newer 

culvert that is included in the present study. This 

evaluation gives insight into channel conditions 

that may have existed at the time of culvert 

installation.

2.3 Analysis framework 

The team took measurements to compare 

conditions at the crossing (including the transition 

areas immediately upstream and downstream 

of the culvert) with conditions in the channel 

upstream and/or downstream of the crossing 

that are outside the influence of the culvert. 

They chose metrics that provided insight into the 

condition of the crossing with respect to scour 

and AOP. For each site, the team presents the 

comparison of metrics between the culvert and 

the channel in this context. They include a site 

evaluation for all the components of the site 

analysis for each of the 17 sites.

Comparison metrics included either those 

estimated from hydraulic modeling (referred 

to as predicted conditions) or those measured 

directly in the field (termed observed conditions). 

Hydraulic metrics included cross-sectional flow 

area, top width, wetted perimeter, hydraulic 

radius, maximum depth, flow width-to-depth 

ratio, velocity, and shear stress at a range of 

flows. Field-based measures included vertical 

thalweg sinuosity, depth distribution, crosssection complexity, residual depths, habitat units, 

large woody debris (LWD), and bed material 

distributions. The team conducted a bed mobility 

analysis at selected representative sites.

In order to generate valid comparisons between 

culverts and representative reaches, the entire 

longitudinal profile through the site was broken 

out into separate profile segments, with each 

segment exhibiting a unique gradient. Section 

2.8.2 of this report includes the methods for 

this analysis. For the hydraulic (modeled) 

metrics, the team compared the range of values 

between culvert (and transition) segments 

and representative segments that exhibited 

similar gradient. Field-based vertical sinuosity 

and residual depth measures are presented 

for all delineated profile segments. For fieldbased cross-section measures, the team made 

comparisons between culvert and channel cross 

sections in similar unit types. 

2.4 Culvert and site conditions 

The team recorded culvert types and 

configurations, including corrugation pattern and 

size, mitering, presence of wingwalls, etc. They 

observed culvert structural conditions and: 

n Documented settlement, leaking, or 

structural damage to the pipe, fill, or 

footings. 

n Documented culvert corrosion, torn metal, 

and abrasion. 
 5

n Noted roadfill erosion or piping conditions. 

n Noted, measured, and photographed 

footing exposure compared to total footing 

depth, although at many sites, it was 

difficult to determine total footing depth.

n Documented and photographed other 

relevant site conditions including:

u Debris plugging in culverts and debris, 

stream, or culvert features that were 

creating scour. 

u Stream-channel incision related to 

existing or previous culverts. 

u Areas of significant erosion or 

sediment aggradation. 

u Construction-related changes to 

channel pattern or alignment. 

u Potential backwater effects at the 

crossing from downstream receiving 

streams.

2.5 Topographic survey

The interdisciplinary team used topographic 

surveys to obtain channel-geometry and culvertdimension data that they used in the analysis. 

They conducted topographic surveys at study 

sites using a total-station survey instrument. 

They surveyed thalweg profiles through the 

culvert and upstream and downstream 20- to 

30-channel widths, except for sites that emptied 

into a receiving water body at a shorter distance 

downstream of the outlet. The team surveyed 

thalweg profiles and water-surface profiles at the 

culvert for each representative reach. 

The interdisciplinary team surveyed one to three 

representative cross sections in the culvert and 

in each representative reach, depending on 

site conditions and cross-sections identified for 

analysis. Additionally, the team surveyed two 

additional cross sections at the upstream and 

downstream boundaries of the representative 

segments. They also surveyed additional cross 

sections where necessary for hydraulic modeling 

based on breaks in slope, contraction and 

expansion, or change in flood-plain conditions. 

Cross-section surveys included surveys of 

bankfull boundaries using field indicators of 

bankfull conditions. In some cases, they surveyed 

vegetation-change boundaries to assist with 

identification of flood-plain roughness conditions 

for hydraulic modeling.

The team also surveyed and monumented: 

n Culvert dimensions including footings, 

crowns, inverts, and sidewalls. 

n Road and fillslope locations and 

dimensions. 

n Topographic points, including any 

significant gradebreaks throughout the site. 

Using a Sokkia SDR33 data collector, the team 

collected total station data at each of the 17 

sites. Following the survey, they downloaded 

all survey data from the data collector onto a 

computer using Autodesk Land Desktop as .sdr 

files and converted them to fieldbook (.fbk) files. 

At this point, any survey errors the team noted 

(rod height or descriptor) were corrected in the 

field at the time of the survey. The team imported 

the corrected fieldbook files into computeraided design drawings, converting survey data 

into x, y, z coordinates. Once in a drawing, the 

team evaluated each survey for coherence 

and exported it as a text file of coordinates and 

descriptions. Finally, they imported the text files 

into ArcMap, and each survey was converted into 

a shapefile to facilitate hydraulic modeling.

Methods
6

Culvert Scour Assessment

2.6 Pebble counts and bed-material 

analysis

Pebble-count data provides information on bedmaterial distributions that team members use 

for incipient motion analysis to understand bedmobility thresholds. The team performed pebble 

counts at each representative cross section within 

the culvert and in each representative reach. 

They based the pebble-count methodology on 

the procedures identified by Wolman (1954) and 

sampled particles at random at even-spaced 

paces along the cross-section transect within 

the bankfull channel. They counted at least 100 

particles at each pebble-count location. Then, 

using either a gravelometer or a ruler, they 

measured the intermediate axis of each particle.

Team members assigned particles to standard 

size classes based on the phi-unit scale (Bunte 

and Abt 2001) and plotted frequency distributions 

and cumulative frequency distributions for 

these size classes. They calculated percentiles 

(percent finer than) for the D5

, D16, D50, D84, 

and D95 and used these values to calculate 

sorting and skewness coefficients that are used 

in comparisons between the culvert and the 

representative channel.

The sorting coefficient characterizes the 

dispersion of the particle sizes in the sample. 

A “well-sorted” distribution has a narrow range 

of particle sizes, whereas a “poorly sorted” 

distribution has a wide range of particle sizes. 

The following sorting coefficient, from Folk and 

Ward (1957), was applied (from Bunte and Abt 

2001):

S =

 f84 - f16 

+

 f95 - f5

4 6.6

where S is the Folk and Ward (1957) sorting 

coefficient, fi

 is the phi-unit for the particle size 

at which i

th percent is finer than; f = -log2(Di), 

where Di

 is the particle size in millimeters. 

Sorting coefficient values greater than 4 indicate 

a very poorly sorted bed, where there is a wide 

range of different particle sizes. Values less 

than 0.35 indicate a very well sorted bed, where 

particle sizes tend to be similar throughout the 

distribution. 

The skewness coefficient characterizes the 

skewness of the particle size distribution. 

Streambed particle sizes typically follow 

asymmetrical (nonnormal) distributions, and the 

skewness coefficient represents their degree of 

deviation from normality. The following skewness 

coefficient, from Folk and Ward (1957) modified 

by Warren (1974), was applied (from Bunte and 

Abt 2001):

Sk =

 f84 - f50 

-

 f50 - f5

f84 - f16 f95 - f5

where Sk is the skewness coefficient. This 

equation assumes that the percentile values 

refer to the percent coarser cumulative frequency 

distribution. Because the percent finer values 

were used, the final skewness values were 

multiplied by -1 to obtain the correct sign. 

Skewness values range from -1 to 1. Positive 

skewness values represent skewness towards a 

tail of fine particles, which means large particles 

comprise the bulk of the sample. Negative values 

indicate that fine particles comprise the bulk of 

the sample.

2.7 Hydrology

The team estimated flows for a range of floodrecurrence intervals for each site using U.S. 

Geologic Survey (USGS) regional regression 

equations for western and eastern Oregon, 

as appropriate (Cooper 2005 and 2006). They 

used the Oregon Water Resources Department 

(OWRD) Web-based autodelineation program 
 7

to calculate peak discharges ( http://www.wrd.

state.or.us ). See appendix B (on enclosed CD)

for the equations and parameters used in the 

regressions.

Team members calculated estimated flows for 

the following recurrence intervals: 2-year (Q2

), 

Q5

, Q10, Q50, and Q100. They also calculated 25 

percent of the Q2

 to obtain a lower flow value for 

analysis purposes and used these flow values in 

hydraulic modeling and throughout the analysis.

They made an estimate of bankfull flow using 

a combination of hydraulic modeling and field 

indicators of bankfull flow elevations. More 

information is provided in section 2.8.3.

2.8 Hydraulics

2.8.1 Hydraulic modeling

The team used the U.S. Army Corps of Engineers 

1-dimensional HEC-RAS (version 3.1.3) 

hydraulic modeling program to estimate hydraulic 

conditions at a number of flow levels, and they 

conducted model simulations to predict watersurface elevations, cross-section parameters, 

velocities, and shear stress. 

Geometry data. Incorporation of survey data into 

a hydraulic model requires the transformation of 

three-dimensional data (x, y, and z) data into twodimensional data (x and y) along cross sections 

that are perpendicular to the stream. While data 

was collected along cross sections, deviations 

from a straight line may cause artificial widening 

of the cross section. In order to avoid this, the 

scientists exported three-dimensional (x, y, and 

z) coordinates from the ArcMap shapefiles and 

entered them into a worksheet that corrects for 

any deviations from a straight line, providing a 

two-dimensional cross section. ArcMap measured 

the downstream channel distance between 

each cross section, as well as the left and right 

over-bank downstream distance; the information 

was entered into the model. Cross-section 

interpolation created cross sections at no less 

than 10-foot intervals along the profile. Additional 

cross sections were interpolated where necessary 

to account for bedforms not fully captured in the 

survey.

Using ArcMap shapefiles to extract longitudinal 

thalweg profiles and water-surface profiles, the 

team entered the data into spreadsheets. They 

used the thalweg profile distances and elevations 

to locate cross sections in the model.

The team identified bank stations, or the location 

on either bank that defines the active channel 

width, based on three indicators: (1) an inflection 

point in the geometry of the channel, (2) a 

vegetation line, and (3) survey indicators of top 

of bank. They identified ineffective flow areas 

where water is present but did not add to the 

conveyance within a cross section (backwater 

areas). Additionally, they added levees where 

a low point in the cross section (generally an 

overflow channel in the flood plain) conveys 

flow in the model before water is able to overtop 

the bank and access the overflow channel. The 

levee prevents water from occupying the channel 

until the high point separating the channels is 

overtopped.

Steady flow data and boundary conditions.

The HEC-RAS hydraulic model requires the 

assignment of boundary conditions at the 

downstream end (for subcritical flow conditions) 

and/or upstream end (for supercritical flow 

conditions) of each site. Four different options 

exist for defining these boundary conditions; a 

known water surface, critical depth, normal depth, 

or a rating curve. Lacking detailed discharge 

measurements at the time of the survey or a 

stage-discharge rating curve, normal or critical 

depth boundary conditions were used for all 

Methods
8

Culvert Scour Assessment

hydraulic models. The choice of boundary 

condition used for each site depended on site 

conditions, but was typically calculated using 

normal depth; critical depth was used in a few 

cases where critical-depth conditions were clearly 

present at the reach boundary. All sites used a 

mixed flow regime calculation.

Channel and flood-plain roughness. Team 

members calculated Manning’s n for the channel 

using Jarrett’s (1984) equation and used a first 

approximation (using field-derived values) for 

initial model runs. Hydraulic radius was output 

from the model for flows corresponding to bankfull 

flow (based on field indicators). They obtained 

a local energy grade slope using measured 

water surface measurements bounding the cross 

sections of interest and used the hydraulic radius 

and energy grade slope in Jarrett’s equation 

to obtain Manning’s n for each cross section. 

Jarrett’s equation takes the following form:

n = 0.39S0.38 R-0.16

where n is Manning’s roughness coefficient, S 

is the friction slope (or water surface slope), and 

R is hydraulic radius (ft). Jarrett (1984) specifies 

that the equation is applicable to slopes from 

0.002 – 0.04 and for hydraulic radii from 0.5 feet 

to 7 feet. All sites met these criteria except for the 

upper slope limit, which was exceeded in many 

cases. Hubbard (1997) (as cited in Barnard 2003) 

showed that Jarrett’s equation performed well on 

slopes up to 0.08. Based on this information, they 

applied Jarrett’s equation to slopes up to 0.08. 

For sections with greater slopes, they used the 

average of the most adjacent cross sections with 

slopes less than 0.08. They estimated flood-plain 

roughness values using the approach outlined 

in Arcement and Schneider (1987), except in 

cases where channel roughness (using Jarrett’s 

equation) exceeded the estimate of flood-plain 

roughness, in which case the channel roughness 

was also applied to the flood plain.

Manning’s roughness values predicted by the 

Jarrett (1984) equation were considerably greater 

than those typically used for modeling culverts 

and natural channels. The values predicted by 

the Jarrett (1984) equation ranged from 0.057 

to 0.16. Although the values are large, they are 

within the range of measured values determined 

for high-gradient streams by Jarrett (1984). The 

values are also within the range of other studies 

that have measured roughness, including a study 

conducted in northern California by Lang et al. 

(2004), which recorded values as high as 0.061 

for culverts with native streambed material and as 

high as 0.448 for culverts with baffles.

Culvert modeling. Because of the size of the 

culverts and the presence of natural streambed 

material along the bed, team members modeled 

culverts using an open-channel routine with a lid 

placed over the channel to simulate the culvert 

dimensions. They obtained lid dimensions during 

the topographic survey and applied them to the 

model using the lid function in HEC-RAS. They 

modeled Lowe Creek without a lid because of 

irregular culvert geometry; modeled flows do not 

exceed the culvert height, so little effect on model 

results would be expected. The lid function has 

limitations for modeling culverts as it does not 

address pressure flow, which possibly occurs for 

a couple of sites at the highest flows. Using the 

lid function also means that some values are not 

reported in the model for high flows that fill the 

culvert. Values not reported include top width and 

hydraulic depth. These values were therefore 

not included in the analysis for the affected cross 

sections and flows.
 9

Team members determined Manning’s n values 

for culvert walls based on the guidance provided 

in the HEC-RAS hydraulic reference manual. The 

range of Manning’s n for each site can be found 

in the individual site evaluations in appendix A. 

Vertical variation in n was applied to the culvert 

cross sections by weighting n based on the 

proportion of wetted perimeter comprised of 

culvert bed versus culvert walls. In some cases, 

interpolated sections within the culvert use a 

composite (average) n as opposed to vertical 

variation. Sample runs using average n versus 

vertical variation showed little difference in the 

results. They determined ineffective flow areas at 

the culvert inlet and outlet and culvert contraction 

and expansion coefficients using the guidance 

provided in the HEC-RAS manual.

Model uncertainty. The HEC-RAS model is a 

one-dimensional step-backwater model, which 

uses the energy equation to determine watersurface elevations through a series of cross 

sections. When using the steady-flow program, as 

this study exclusively does, it does so under the 

assumption that flow is (1) steady, (2) gradually 

varied, and (3) one-dimensional, and that the 

slope is “small,” which is cited as less than 

10-percent slope. For natural stream channels, 

the degree to which these assumptions become 

less valid increases with channel-bed slope. 

Large substrate, steep riffles, and steps result 

in greater turbulence, spatially diverse hydraulic 

conditions, and areas of rapidly varied flow. Local 

drops due to steps or steep riffles create short 

sections with bed slopes that far exceed the 

10-percent stated model limit. In these cases the 

model typically is not able to satisfactorily balance 

the energy equation, and it returns critical depth 

as the solution, which indicates there may be 

considerable uncertainty with the hydraulic 

calculations. In general, one can assume that 

the steeper, more complex channels have a 

greater degree of uncertainty than the lower 

gradient, more uniform channels. Although model 

uncertainty was not explicitly quantified in this 

study, uncertainty was addressed by considering 

multiple sources of information at each site, 

including modeled/predicted data, measured 

data, and qualitative site observations.

2.8.2 Longitudinal profile analysis

In order to generate valid comparisons between 

culverts and representative reaches, the entire 

longitudinal profile through the site was broken 

out into separate profile segments, with each 

segment exhibiting a unique gradient. The 

technique generally followed Forest Service 

guidelines (unpublished). The first step was 

the identification of hydraulic controls along 

the channel thalweg profile. The slope of each 

segment between each hydraulic control was 

then calculated and adjacent segments with 

similar slopes (within 20 percent) were combined. 

In some cases, adjacent slope segments with 

similar gradients were not combined in order to 

achieve separate segments for areas of interest 

to the analysis, including within culverts, and in 

the inlet and outlet transition areas. Those areas 

thought to be transition areas were immediately 

upstream and downstream of the culvert within 

the area of expansion or contraction of flow, or in 

areas where other significant culvert effects took 

place (culvert-related incision or aggradation).

The profile segments formed the basis of 

comparisons of hydraulic (modeled) variables 

between the crossing and the representative 

channel. Only segments in the representative 

channel with a similar gradient (within 20 

percent) were identified as comparable to 

the culvert or transition segments. In some 

cases, the 20-percent criterion was relaxed 

if other observations or data suggested that 

the segments were reasonably comparable. 

Segments were not used for comparisons if no 

Methods
10

Culvert Scour Assessment

measured cross sections were present in the 

segment. The rationale for identifying comparable 

segments is discussed in the profile analysis 

segment summary that is included in each site 

writeup.

2.8.3 Analysis of hydraulic variables

Peak discharges determined during hydrologic 

analysis were input into the model. These 

included the 25-percent Q2

, Q2, Q5, Q10, Q50, and 

Q100. An estimate of bankfull flow (Qbf) was also 

determined by varying the discharge in the model 

until the water surface profile best matched the 

elevation of the field-identified bankfull indicators. 

In some cases, it was not possible to select 

a discharge that matched the field-identified 

bankfull elevations at all of the cross sections. 

For these cases, the discharge was matched to 

cross-section bankfull elevations for which the 

investigators had the greatest confidence.

A number of hydraulic variables were exported 

from the HEC-RAS model and used in the 

analysis to compare conditions in the culvert and 

representative segments. These values included 

flow area, top width, wetted perimeter, hydraulic 

radius, maximum depth, channel velocity, and 

channel shear. These metrics are compared for 

the following range of flows: 25-percent Q2

, Qbf, 

Q5

, Q10, Q50, and Q100. For most sites, the Q2 was 

not used because of the similarity with the Qbf; 

however, at Upper and Lower Eightmile Creek, 

the Qbf was closer to the 25-percent Q2

, and so 

the following flows were used at those sites: Qbf, 

Q2

, Q5, Q10, Q50, and Q100.

Because of the important influence of channel 

gradient on channel processes and response, 

hydraulic variables were only compared 

between culvert (and transition) segments and 

representative channel segments with similar 

gradient. The range of values within comparable 

profile segments is presented in box plots in each 

site evaluation.

2.8.4 Particle entrainment analysis

Team members performed analyses of particle 

entrainment (incipient motion) in order to compare 

the bed-mobilization threshold of the channel 

with that of the culvert. Culverts performing well 

as stream-simulation designs would be expected 

to have bed material that is mobilized at similar 

flows that mobilize the bed in the natural channel.

They analyzed bed entrainment by comparing 

the shear stress exerted by the flow to the critical 

shear stress needed to mobilize bed sediments. If 

channel shear stress exceeds critical shear stress 

for a given particle size, then that particle is 

assumed to be mobile. Mobility of the D84 particle 

size was assumed to represent the threshold at 

which the bed is mobilized for these channels, 

which are mostly comprised of step-pool systems. 

This is based on the concept that the larger, 

grade controlling particles that make up steps and 

cascades govern bed mobility and channel form 

in step-pool streams (only once these particles 

become mobile does significant bed reshaping 

occur) (Grant et al. 1990, Chin 1998). The use 

of the D50 particle size may be more appropriate 

for the few pool-riffle streams in the study, but 

the D84 was applied to these streams to maintain 

consistency.

The team used the critical shear stress 

approach for bed-entrainment calculations. It is 

acknowledged that this method may not be the 

most appropriate for the steeper streams in the 

study due to the difficulty in accurately capturing 

the variability in channel conditions and due to 
 11

the moderating effects of form resistance (from 

spill-over steps and LWD) on the shear stress that 

is available to mobilize particles (Bathurst 1987, 

Wilcox et al. 2006, Yager et al. 2007). However, in 

order to maintain a consistent approach at each 

site, and based on early experimentation with 

other methods (Costa 1983 and Bathurst 1987) 

that provided widely variable entrainment flows, 

they selected the critical shear stress approach 

for this analysis.

The selection of appropriate Shields parameters 

can introduce considerable uncertainty in 

determining entrainment thresholds. Shields 

parameters in gravel-bed rivers typically range 

from 0.03 to 0.086 (Buffington and Montgomery 

1997), but in steep mountain rivers may be as 

high as 0.1 (Lenzi et al. 2006). The use of higher 

values may be appropriate in some cases as it 

better represents the increased stability of the 

bed due to particle interlocking and protection 

provided by the accumulations of large particles 

(steps) that form in coarse-grained beds (Church 

et al. 1998). However, in well-graded beds, 

which often characterize step-pool systems, 

larger particles may be entrained at lower flow 

thresholds because of their protrusion above 

smaller neighboring particles, which increases 

their exposure to flow and reduces their pivoting 

angles (Komar and Li 1986).

In order to take into account the potential effect 

of particle exposure, team members determined 

critical shear stress through methods that modify 

the Shields parameter according to the size 

difference between the D50 and the particle size 

of interest (in our case, the D84). Based on the 

methods of Komar (1987), they used following 

equation for critical shear stress:

tci = 102.6 t*D50 Di

0.3 D500.7

where tci is the critical shear stress (lb/ft2) at 

which the Di

 particle size is mobile, t*D50 is the 

Shields parameter for the D50 particle size, Di

 is 

the particle size of interest (feet), and D50 is the 

median particle size (feet). Instead of using the 

typical 0.045 for t*D50, values were obtained from 

Julien (1995), which vary according to the particle 

size of interest. The range of critical shear stress 

values for each site can be found in the individual 

site evaluations in appendix A.

The team members performed incipient motion 

analysis at representative cross-section/pebblecount locations and were limited to those 

cross sections not representing pool units. 

They excluded pools because grain sizes and 

hydraulics are variable and do not necessarily 

represent the principal bed movement dynamics 

of the reach. In order to further limit the impact of 

variability of shear stress between cross sections, 

shear stress was taken as the average of two 

or more cross sections surrounding the pebblecount location, with the criteria that cross sections 

had to be within the same channel unit. In most 

cases, this included the surveyed cross section 

and the two adjacent interpolated sections.

2.9 Channel complexity and depth 

distribution

Channel complexity is important for aquatic 

habitat and passage conditions. Team members 

evaluated complexity at cross sections and 

along longitudinal profiles. Profile complexity was 

assessed by looking at thalweg vertical sinuosity 

and residual depths. They analyzed crosssection complexity by using the sum of squared 

height difference method and by conducting a 

depth-distribution analysis. These methods are 

discussed below.

Methods
12

Culvert Scour Assessment

Vertical sinuosity and residual depth. They 

evaluated profile complexity using vertical 

sinuosity, also known as the chain-and-tape 

method (Bartley and Rutherford 2005), which is 

the length of the topographic bed distance divided 

by the reach distance. This metric reveals how 

sinuous the bed is in the vertical plane. Uniform, 

plane-bed reaches will have very low vertical 

sinuosity, whereas complex step-pool channels 

will have high vertical sinuosity. Team members 

calculated vertical sinuosity for each of the 

longitudinal profile segments.

The team also assessed profile complexity 

through a comparison of residual depths between 

longitudinal profile segments. Residual depth is 

defined as the difference in elevation between 

the maximum pool depth and the elevation of the 

downstream hydraulic control.

Cross-section complexity – sum of squared 

height difference. The team evaluated crosssection complexity by calculating the sum of 

squared height differences (∑dh2

) along cross 

sections (Bartley and Rutherford 2005). This 

metric reflects the degree of variation of the 

channel bed. Height differences were calculated 

between each successive surveyed point along 

the section, squared, and the results summed. In 

order to remove the potentially spurious effect of 

channel banks, this analysis was performed only 

for the channel bed (left toe of bank to right toe of 

bank).

Depth distribution. In recognition of the value of 

shallow water habitats, Barnard (2003) developed 

a method to quantify the amount of shallow-edge 

habitat that is available in culverts compared 

to corresponding natural channels. The team 

performed this “depth-distribution” analysis for 

this study, generally following the procedures 

described by Barnard (2003). Depth distribution 

is calculated as the amount of the cross section 

that has flow depths less than 0.3 feet. Depth 

distribution was calculated by interpolating points 

along the cross section at 0.5-foot increments 

and calculating depth for each point. The number 

of points with depths less than 0.3 feet was 

summed.

The team conducted this analysis for the 

25-percent Q2

 flow. For most streams, the 

25-percent Q2

 is likely to represent a flow that is 

meaningful for fish passage. In their study of fish 

passage at culvert sites in northern California, 

Lang et al. (2004) found that juvenile salmonids 

(3 inches to 8 inches fork length) typically tried to 

pass upstream through culverts between the 8 

percent and 26 percent annual exceedance flows 

(an 8-percent annual exceedance flow is the flow 

that is exceeded only 8 percent of the time, on 

average, over the course of a year). For adult 

anadromous salmon, nearly 60 percent of fish 

attempted to pass at flows greater than or equal 

to the 5-percent annual exceedance flow. As a 

reference, the 25-percent Q2

 flow for the Middle 

Fork Willamette River near Oakridge, Oregon, 

(USGS #14144800, nearest long-term gauging 

station near the upper Willamette River sites) is 

approximately equivalent to the 5-percent annual 

exceedance flow.

2.10 Channel units and LWD

Team members performed a channel-unit (or 

habitat-unit) survey for the culvert and for the 

natural channel outside the crossing. They 

identified unit boundaries during the survey of 

the longitudinal thalweg profile and they were 

amended in some cases after review of profile 

characteristics; for instance, after review of 

residual-depth measurements. Unit types were 

limited to pools, riffles, steps, and glides. They 

identified: 

n Pools if there was significant residual 

depth and backwatered conditions from a 

hydraulic control. 
 13

n Riffles for rapid and steeper flow with 

surface turbulence. 

n Steps if there was an abrupt drop or 

sequence of abrupt drops ending in 

hydraulic jumps. 

n Glides for areas of rapid flow with moderate 

slopes, little surface turbulence, and no 

residual depth. 

They calculated percent composition by channel 

unit in order to compare conditions between the 

culvert and the natural channel.

They counted LWD in each of the study 

segments. LWD counts followed the Washington 

timber, fish, and wildlife protocols (SchuettHames et al. 1999). Minimum size to qualify as 

LWD is 10-centimeter diameter over 2 meters of 

length. They compared wood counts between 

the culvert and the natural channel and used this 

data to help explain analysis results.

2.11 Photo documentation

Photos taken at the crossing and throughout the 

channel outside the crossing documented site 

conditions and were used by team members 

as a reference for the assessment. The photos 

also will be useful for any future assessments 

of change at the sites. They took photos 

looking upstream and downstream from the 

upstream and downstream boundaries of each 

representative channel segment and the culvert 

segment. They photographed each representative 

cross-section/pebble-count location from one 

or more angles and took other shots to show 

representative conditions in the natural channel 

and the culvert. 

They took photos of the culvert inlet and outlet 

as well as from atop the culvert inlet (looking 

upstream) and atop the culvert outlet (looking 

downstream). They took close-up photos of 

footing conditions if scour conditions were 

present. They photographed other notable 

conditions throughout the site, as appropriate, 

to document site conditions that may have 

relevance to the assessment.

3 Results

3.1 Culvert and representative channel 

segment conditions

A summary of basic site conditions is included in 

table 3.1‐1. Sites consist of 4 pool-riffle channels 

and 13 step-pool systems. Stream bankfull widths 

range from 7 feet to 31 feet. There are 14 openbottom arch culverts, two embedded pipe-arch 

culverts, and one embedded round culvert. All 

culverts are corrugated metal pipe except for one 

site, Little Zigzag, which is a Con/Span® precast 

modular concrete culvert. Six of the installations 

date back to the 1980s or earlier. Four sites were 

constructed in the 1990s, and the remaining 

seven sites are year 2000 to 2003 installations.

Each of the 17 sites had 1 or 2 representative 

channel reaches that were surveyed upstream or 

downstream of the crossing. A summary of reach 

conditions, including the number of representative 

cross-section/pebble-count locations in each 

reach, is included in table 3.1‐2. One site, Pine 

Creek, had two representative channel reaches 

upstream of the crossing. In some cases, they 

chose representative reaches a considerable 

distance away from the crossing in order to get 

beyond culvert-related channel incision or to 

avoid areas where significant human alteration 

(instream structures) was present. For a couple of 

sites, hydraulic analysis performed following the 

survey showed that the upstream representative 

reach was not entirely free from culvert backwater 

impacts for some of the modeled flood stages. 

These instances are discussed in the individual 

site evaluations. One site in particular, Eames 

Creek, had backwater effects that impacted the 

Results
14Stream County Region Location Culvert Year Culvert Stream Span Rise Culvert Bankful Channel

Type Installed Gradient1 gradient2 (max (max) Length width3 Type

width) (ft) (ft)

Velvet Creek Lane Coast range Siuslaw NF Rd 32 Open-bottom 1981 1.6% 1.3% 16 8 92.5 24.0 pool-riffle

arch

Deadwood Trib North Lane Coast range Deadwood Rd Open-bottom 2000 6.2% 5.9% 8 7.3 56 15.8 step-pool

arch

Deadwood Trib South Lane Coast range Deadwood Rd Round 2000 12.6% 9.3% 7.5 7.1 63 6.6 step-poolBuck Creek Lane Coast range Deadwood Rd Open-bottom 2003 0.1% 1.4% 17 12.7 60 19.9 pool-riffle

arch

Falls Creek Lane Cascades, south Fall Cr Rd Open-bottom ~1999 3.0% 3.6% 19 9.8 80 21.2 step-pool

Willamette Valley west of Eugene arch

Hehe Creek Lane Cascades, south Willamette NF Open-bottom ~1986 4.5% 4.7% 17 11.3 60 19.5 step-pool

Willamette Valley Rd 1831 arch

Eames Creek Lane Coast range Wolf Creek Trib Open-bottom pre-1987 0.03% 0.83% 13 8.2 55 31.0 pool-riffle

(~0.6 hr SW arch

of Eugene)

Haight Creek Lane Coast range Upper Siuslaw Open-bottom pre-1987 0.5% 0.8% 19 10 62 18.9 pool-riffle

 RiverTrib arch

(SW of Eugene)

Pine Creek Lane Cascades, south Willamette NF Pipe arch ~1999 5.0% 6.2% 13.5 7 60 13.1 step-pool

Willamette Valley Rd 21 (MF Will)

Youngs Creek Lane Cascades, south Willamette NF Pipe arch ~2002 6.7% 6.4% 14 9 62 13.7 step-pool

Willamette Valley Rd 21 (MF Will)

Simpson Creek Lane Cascades, south Willamette NF Open-bottom 1995 8.3% 5.2% 22 13 114 24.8 step-pool

Willamette Valley Rd 21 MP 21.1 arch

(MF Will)

Tire Creek Lane Cacades, south Willamette NF Open-bottom 1996 9.4% 5.2% 17 11.8 62 22.0 step-pool

Willamette Valley Rd 5826 MP 2.63 arch

(MF Will)

Cool Creek Clackamas Cascades, north 2.5 Mi off US 26 Open-bottom 1984 2.2% 5.2% 13 6.7 48 19.6 step-pool

Willamette Valley on Still Ck Rd arch

(FR 2612)

Little Zigzag Creek Clackamas Cascades, north Mt. Hood NF Open-bottom 2000 4.5% 4.2% 15.5 8.7 133 14.2 step-pool

Willamette Valley Rd 2639 arch

MP 1.4 (concrete)

Lowe Creek Clackamas Cascades, north Mt. Hood NF Open-bottom pre-1987 5.0% 4.5% 19 15.5 70 23.6 step-pool

Willamette Valley Rd 4671 arch

Upper Eightmile Wasco Cascades, east Mt. Hood NF Open-bottom 2000 4.7% 4.2% 12 6.2 42 20.5 step-pool

Creek of Mt. Hood Rd 4430 arch

150 spur

Lower Eightmile Wasco Cascades, east Mt. Hood NF Open-bottom 2000 2.4% 3.7% 12 7.4 56 16.5 step-pool

Creek of Mt. Hood Rd 4440 arch

1 Slope of pipe structure.2 Longitudinal thalweg profile extending up to 20- to 30-channel widths up and downstream of crossing.3 Bankful width of channel determined through field indicators and cross-section geometry.

Table 3.1-1. Summary of study sites and culvert characteristics.
 15

1

Obtained from water surface profile from survey.

2

Both representative segments in Pine Creek are upstream of the culvert.

3

Also corresponds to number of representative cross sections used for critical shear stress, cross-section complexity, and depth distribution 

analysis.

Table 3.1‐2. Summary of representative reaches and numbers of representative pebble counts taken in each reach.

 Upstream Representative Reach Downstream Representative Reach

Stream Length Slope1 No. of Length Slope No. of

(ft) Pebble (ft) (ft/ft)1 Pebble

Counts3 Counts3

Velvet Creek 200 1.5% 2 

Deadwood Trib North 140 4.8% 3 

Deadwood Trib South 145 10.6% 3 

Buck Creek 394 1.0% 3 208 1.0% 2

Falls Creek 158 3.9% 2 

Hehe Creek 123 4.8% 2 122 4.3% 2

Eames Creek 180 0.7% 2 

Haight Creek 165 1.1% 2 

Pine Creek2 98 3.1% 2 67 8.0% 2

Youngs Creek 135 6.2% 2 80 4.4% 1

Simpson Creek 126 6.5% 2 150 4.5% 2

Tire Creek 115 4.8% 2

Cool Creek 128 4.0% 2 

Little Zigzag Creek 118 3.6% 2 

Lowe Creek 110 5.8% 2 95 4.0% 2

Upper Eightmile Creek 125 2.5% 2 

Lower Eightmile Creek 150 3.7% 2 105 1.8% 2

Results
16

Culvert Scour Assessment

entire upstream representative reach for nearly all 

modeled flow levels.

The relationship of culvert width and slope to the 

channel width and slope is an important factor 

in stream-simulation culverts, as these are often 

included in design criteria. Table 3.1‐3 displays 

these relationships for each of the 17 sites. 

Five culverts have widths that equal or exceed 

the bankfull width of the natural channel. Two 

culverts, Deadwood Tributary North and Eames 

Creek, have widths that are half that of the 

bankfull channel or less.

3.2 Hydrology

Discharges for the 0.25 Q2

, Q2, Q5, Q10, Q25, Q50, 

and Q100 for each site are included in table 3.2‐1. 

Discharges were calculated differently depending 

on the location and elevation of the site. Detailed 

hydrologic analysis results are included in the 

OWRD autodelineation reports in appendix B. 

Estimates of bankfull flow were calculated by 

matching HEC-RAS water surface profiles with 

field-identified bankfull indicators. The bankfullflow estimates are included in the site evaluations 

in appendix A.

3.3 Site evaluations

The focus of this study was at the individual-site 

scale. Site evaluations in appendix A include sitescale results for the 17 sample sites. 

The site evaluations in appendix A contain the 

following information for each site: (1) overview 

of culvert and site information, (2) site map, (3) 

history of the site (if known), (4) site description, 

(5) survey summary, (6) profile analysis 

segment summary, (7) scour conditions, (8) 

AOP conditions, and (9) design considerations. 

Analysis results appear as graphs, tables, and 

other figures and include longitudinal-profile plots, 

cross-section plots, HEC-RAS output data, box 

plots of hydraulics results, tables of complexity 

and habitat measures, bed-material distributions, 

and site photos.

The scour and AOP sections summarize and 

interpret the analysis results with respect to the 

performance of the structure for scour resistance 

and AOP. These sections contain the following 

subsections for presentation of results.

Observed conditions (measured)

Footing scour

Culvert bed adjustment

Profile characteristics

Residual depths

Substrate

Predicted conditions (modeled)

Cross-section characteristics

Shear stress

Excess shear stress

Velocity

Cross-section complexity

Profile complexity

Depth distribution

Habitat units

Residual depths

Substrate/bed material

Large woody debris

The following sections contain a brief summary of 

conditions among all sites. Scour Conditions AOP Conditions
 17

Table 3.1‐3. Culvert width and slope ratios.

Ratio Ratio Ratio

Width (culvert (culvert bed (culvert 

Bankfull Ratio Culvert Long slope/long slope/long slope/

Width Culvert (culvert/ Bed Profile Culvert profile profile culvert bed 

Site (ft)1 Span BFW) Slope Slope2 Slope3 slope) slope) slope)

Velvet Creek 24.0 16 0.7 1.1% 1.3% 1.6% 1.2 0.8 1.5

Deadwood Trib North 15.8 8 0.5 6.3% 5.9% 6.2% 1.0 1.1 1.0

Deadwood Trib South 6.6 7.5 1.1 6.6% 9.3% 12.6% 1.4 0.7 1.9

Buck Creek 19.9 17 0.9 1.5% 1.4% 0.1% 0.1 1.1 0.1

Fall Creek 21.2 19 0.9 3.7% 3.6% 3.0% 0.8 1.0 0.8

Hehe Creek 19.5 17 0.9 4.1% 4.7% 4.5% 0.9 0.9 1.1

Eames Creek 31.0 13 0.4 0.8% 0.8% 0.0% 0.0 0.9 0.0

Haight Creek 18.9 19 1.0 0.1% 0.8% 0.5% 0.6 0.1 5.0

Pine Creek 13.1 13.5 1.0 3.8% 6.2% 5.0% 0.8 0.6 1.3

Youngs Creek 13.7 14 1.0 2.8% 6.4% 6.7% 1.1 0.4 2.4

Simpson Creek 24.8 22 0.9 4.4% 5.2% 8.3% 1.6 0.9 1.9

Tire Creek 22.0 17 0.8 3.3% 5.2% 9.4% 1.8 0.6 2.8

Cool Creek 19.6 13 0.7 1.5% 5.2% 2.2% 0.4 0.3 1.5

Little Zigzag Creek 14.2 15.5 1.1 3.9% 4.2% 4.5% 1.1 0.9 1.2Lowe Creek 23.6 19 0.8 3.5% 4.5% 5.0% 1.1 0.8 1.4

Upper Eightmile Creek 20.5 12 0.6 3.1% 4.2% 4.7% 1.1 0.7 1.5

Lower Eightmile Creek 16.5 12 0.7 2.6% 3.7% 2.4% 0.6 0.7 0.9

1BFW of channel determined through field indicators, cross-section geometry, and hydraulic analysis.2Longitudinal thalweg profile extending up to 20 channel widths up and downstream of crossing.3Slope of pipe.

Results
18

Culvert Scour Assessment

Table 3.2‐1. Hydrology for each site obtained from USGS regional regression equations using OWRD 

autodelineation process. Hydrology reports for each site are included in appendix B.

 Basin Q (cfs) for indicated return interval (year)

Area 25% of

Site (sq mi) 2-year 2 5 10 25 50 100

Velvet Creek 1.81 35 140 208 256 318 366 414

Eames Creek 5.53 78 311 486 613 785 919 1060

Haight Creek 3.76 41 165 242 295 365 418 471

Deadwood Trib North 0.43 10 40 58 70 86 99 111

Deadwood Trib South 0.21 5 18 26 32 40 46 52

Buck Creek 1.54 30 119 171 208 255 290 326

Hehe Creek 1.86 45 178 258 310 375 424 471

Pine Creek 1.91 15 58 91 113 142 165 188

Youngs Creek 2.92 21 85 135 170 215 250 286

Simpson Creek 11.30 65 258 407 511 646 751 858

Tire Creek 3.90 48 193 308 388 489 566 644

Cool Creek 1.70 21 83 134 172 222 262 302

Little Zigzag Creek 4.08 30 118 168 204 253 292 333

Lowe Creek 5.99 35 141 227 292 380 448 520

Fall Creek 3.55 67 269 401 491 604 689 774

Upper Eightmile Creek 4.09 43 171 237 285 353 406 460

Lower Eightmile Creek 4.78 43 171 236 287 360 418 478
 19

3.4 Overview of scour conditions

Site evaluations in appendix A include site analysis information. The authors present a brief overview of 

scour conditions among sites here.

Except for a few sites, the culverts in the survey are in relatively good shape with respect to scour. 

There are two sites with severe scour; these include Lowe Creek and Deadwood Tributary South. The 

Lowe Creek culvert has footing scour along the upstream portion of the culvert on the left bank that is 

severely undermining the left-base footing and is creating a potential risk for structural failure if scour 

continues. The Deadwood Tributary South culvert is a closed-bottom culvert (pipe arch) that is scoured 

to the culvert base for the upstream half of the pipe. Structural integrity does not appear to be at risk, 

but fish-passage conditions are poor. There are a handful of other sites with localized scour, typically at 

the inlet area or along the footings, but none of these sites are compromised with respect to structure 

integrity. There are also a few sites where the main flow is along the footing and may scour footings 

over time. A brief summary of observed scour conditions is included in the table 3.4.1.

Table 3.4‐1. Brief summary of observed scour conditions at culvert sites. See site evaluations (appendix A) for 

additional information.

Velvet Creek No significant scour. Some bedrock is exposed in the inlet area, which is 

atypical of conditions in the natural channel outside the influence of the 

crossing.

Eames Creek Culvert bed composed of sandstone bedrock. Downstream transition segment 

composed of bedrock. Little to no bedrock present in upstream channel reach. 

Unknown if streambed material was placed in culvert during construction. 

Base footings slightly undermined by scour in places.

Haight Creek Culvert bed composed of sandstone bedrock. Some bedrock present in 

upstream channel but not dominant. Unknown if streambed material was 

placed in culvert during construction. Scour of bedrock along left bank footing 

in inlet area and along right bank at downstream end.

Deadwood Trib North No significant scour. Riprap placed on right bank upstream of inlet failed into 

inlet area. Flow was wall-to-wall at the time of the survey.

Deadwood Trib South Pipe scoured to base in upper half. Material aggraded in downstream half.

Buck Creek No significant scour. Footings are exposed up to 3 feet but pipe was 

constructed that way.

Hehe Creek No significant scour. Some bedrock present just upstream of inlet may be 

related to culvert inlet scour but bedrock sections are also found in the natural 

channel outside the crossing.

Pine Creek There is currently scour to the culvert base at the inlet of the culvert. Originally 

placed material scoured out of the pipe after construction but refilled.

Youngs Creek There is currently scour to the culvert base at the inlet of the culvert. Originally 

placed material scoured out of the pipe after construction but refilled.

Results
20

Culvert Scour Assessment

Table 3.4‐1. Brief summary of observed scour conditions at culvert sites. See site evaluations (appendix A) for 

additional information (continued).

Simpson Creek No significant scour. There has been incision upstream of the culvert and 

aggradation of material in the downstream portion of the culvert.

Tire Creek There is scour in the inlet region and associated channel incision upstream of 

the inlet. Pipe footings are not compromised.

Cool Creek There is minor inlet scour along the left bank inside the pipe. Material has 

aggraded along the right bank and in the downstream portion of the pipe, 

potentially reducing hydraulic capacity.

Little Zigzag Creek No significant scour. Flow was wall-to-wall at the time of the survey.

Lowe Creek Severe scour on left bank at inlet and extending downstream 30 feet. Left 

bank base footing severely undermined.

Fall Creek No significant scour. Flow was wall-to-wall at the time of the survey.

Upper Eightmile Creek No significant scour. Flow was wall-to-wall at the time of the survey.

Lower Eightmile Creek No significant scour. Flow was wall-to-wall at the time of the survey.

A common condition found at many of the sites is the adjustment of the channel bed post installation. 

Survey measures indicate that many sites have culvert beds that have a flatter slope than the culvert 

structure itself. Assuming that culvert beds were originally constructed at the same slope as the 

culvert structure, these sites have adjusted since installation through inlet scour, aggradation in the 

downstream portion of the pipe, or a combination of the two. At many sites, bed degradation at the 

upstream portion of the pipe and aggradation of material in the downstream portion were clearly visible. 

Figure 3.4‐1 shows the incidence of culvert-bed flattening among all sites.

Figure 3.4‐1. Ratio of culvert slope to culvert-bed slope. Culvert slope is the slope of the actual culvert structure. 

Culvert-bed slope is the slope of the thalweg profile through the culvert.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

Culvert Slope / Culvert Bed Slope

Culvert slope/culvert bed slope
 21

The occurrence of culvert-bed flattening was 

plotted against the width ratio in order to evaluate 

the effect of culvert widths on culvert-bed 

slope (figure 3.4‐2). This comparison shows no 

relationship between culvert-bed adjustment 

and culvert width as it related to bankfull width. 

There is no relationship present, suggesting that 

different culvert widths have little effect on culvert 

bed slope adjustment.

Figure 3.4‐2. Relationship of culvert slope/culvert-bed 

slope ratio to the width ratio.

3.5 Overview of AOP conditions

Site analysis information for AOP is included 

in the site evaluations in appendix A. A brief 

overview of AOP conditions among sites is 

presented here.

Most sites appeared suitable for fish passage at 

the flow encountered during the survey. The one 

exception was Deadwood Tributary South, where 

the upstream half of the culvert was scoured to 

the base and rapid shallow flow in this section 

would inhibit fish passage. This site, as well as 

Deadwood Tributary North, also has potential 

passage limitations created by large rock steps 

that were constructed downstream of the outlet in 

order to maintain grade through the culvert.

Several other sites had conditions that might 

limit AOP during some flow conditions. At the 

time of the survey, several sites had shallow 

wall-to-wall flow without flow concentration that 

may be necessary to provide fish passage at low 

flows. These include Velvet Creek, Fall Creek, 

Deadwood Tributary North, Deadwood Tributary 

South, Little Zigzag Creek, Eames Creek, Haight 

Creek, Lower Eightmile Creek, and Upper 

Eightmile Creek. These sites generally showed a 

lack of streambed complexity, shallower residual 

depths, and low depth-distribution values when 

compared to the natural channel. These sites had 

no defined streambanks to facilitate terrestrial 

organism passage and had little or no stable bed 

elements (steps) to provide complexity. 

Vertical sinuosity and residual depth. Vertical 

sinuosity in the natural channel was consistently 

greater than that in the culvert except for 

Simpson Creek and Tire Creek, where culvertbed sinuosity was greater (figure 3.5‐1). This 

indicates that, in nearly all cases, the culvert beds 

reviewed have less complex bed topography 

than their corresponding natural channels. Values 

generated in the profile complexity analysis are 

included in appendix A.

Maximum residual depth is greater in the natural 

channel than in the culvert for all but two sites; 

Pine Creek and Cool Creek (figure 3.5‐2). On 

average, maximum residual depth in the culverts 

reviewed was about 0.6 that of the natural 

channel. This is consistent with the results of 

the vertical-sinuosity assessment. Greater pool 

depths would generally be associated with 

greater vertical sinuosity. Values generated in the 

residual-depth analysis are included in appendix 

A. Factors contributing to similarities and 

differences in residual depths at individual sites 

are discussed in the site evaluations in appendix 

A.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Width Ratio (Culvert / BFW)

Culvert Slope / Culvert Bed Slope Culvert slope/culvert bed slope

Results
22

Culvert Scour Assessment

Figure 3.5‐1. Vertical sinuosity in the culvert and natural channel at each study site. For sites with more than one 

reference segment, the vertical sinuosities were averaged. Reference channel values are significantly greater 

than the culvert values using a one-tailed paired t-test (a = 0.05). 

Figure 3.5‐2. Maximum residual depth in the culvert and natural channel. For sites with more than one 

representative natural channel segment, the residual depths were averaged. Values for culverts and reference 

channels are significantly different using a paired t-test at a = 0.05.

Cross-section complexity – sum of squared height difference. The sum of squared height 

difference did not show that the measured culvert cross sections were less complex than those 

measured in natural channel segments. Five sites had higher values in the culvert and seven sites 

were within the range of the values in the natural channel. At only two sites did the culvert have lower 

values than the channel, and one of these was Hehe Creek, which site observations suggest has a 

complex bed. 

1.00

1.01

1.02

1.03

1.04

Velvet Creek

Deadwood Trib North Deadwood Trib South

Buck CreekFall CreekHehe CreekEames Creek Haight Creek Pine Creek

Youngs Creek Simpson Creek

Tire CreekCool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile Creek Lower Eightmile Creek

Vertical Sinuosity

Culvert Representative Channel

Vertical sinuoxity

0

0.5

1

1.5

2

Velvet Creek

Deadwood Trib North Deadwood Trib South

Buck Creek Fall Creek

Hehe Creek

Eames Creek Haight Creek

Pine Creek

Youngs Creek Simpson Creek

Tire Creek Cool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile Creek Lower Eightmile Creek

Maximum Residual Depth

Culvert Representative Channel

Maximum residual depth

Velvet Creek

Deadwood Trib north

Deadwood Trib south

Buck Creek

Fall Creek

Hehe Creek

Eames Creek

Haight Creek

Pine Creek

Youngs Creek

Simpson Creek

Tire Creek

Cool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile CreekLower Eightmile Creek

Velvet Creek

Deadwood Trib north

Deadwood Trib south

Buck Creek

Fall Creek

Hehe Creek

Eames Creek

Haight Creek

Pine Creek

Youngs Creek

Simpson Creek

Tire Creek

Cool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile Creek

Lower Eightmile Creek
 23

Depth distribution. At the 25-percent Q2, the depth distribution analysis showed that at least one cross 

section in the culvert had less available shallow water habitat at 13 of 17 sites. The following sites had 

at least one cross section with depth-distribution values within or greater than the range found in the 

natural channel: Tire Creek, Buck Creek, Little Zigzag Creek, Deadwood Tributary South, Fall Creek, 

Lowe Creek, Lower Eightmile Creek, Pine Creek, and Youngs Creek.

Four sites had both culvert values within or greater than the range of the channel (Tire, Deadwood 

Tributary South, Fall, and Youngs). In order to look at broad trends in depth distribution between 

the culvert and the natural channel, a comparison was made between averaged depth-distribution 

measures of culvert cross sections and channel cross sections. These comparisons indicate a strong 

influence of culverts on the availability of shallow flow areas (figure 3.5‐3). At 13 of the 17 sites, the 

natural channel has a greater abundance of shallow channel margin habitat at the 25-percent Q2

. 

Values generated in the depth-distribution analysis are included in appendix A. Individual depthdistribution measures are presented and discussed further for each site in the site evaluations in 

appendix A.

Figure 3.5‐3. Depth distribution results for all sites. Bars depict the number of cross-section increments with 

depths less than 0.3 feet.

Channel units and LWD. The distribution of channel-unit types differs between culverts and natural 

channels. In general, pools and steps are more common in natural channels, riffles are about equally 

as common in natural channels and culverts, and glides are more common in culverts. Percent pool 

values for all sites are displayed in figure 3.5‐4. Percentage of pools is greater in natural channels for 

13 of 17 sites. Total habitat-unit composition results for all sites are included in appendix A.

These results are consistent with the results of the vertical-sinuosity analysis, which shows that vertical 

sinuosity is greater in natural-channel reaches. Culverts tend to be dominated by riffle and glide habitat, 

which is more uniform and shows less vertical variation and complexity.

0

2

4

6

8

10

12

14

Velvet Creek

Deadwood Trib North Deadwood Trib South

Buck CreekFall Creek

Hehe Creek

Eames CreekHaight Creek

Pine Creek

Youngs Creek Simpson Creek

Tire Creek Cool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile Creek Lower Eightmile Creek

Depth Distribution (25%Q2)

Culvert Repres entative Channel

31

Depth distribution (25% Q2

)

Velvet Creek

Deadwood Trib north

Deadwood Trib south

Buck Creek

Fall Creek

Hehe Creek

Eames Creek

Haight Creek

Pine Creek

Youngs Creek

Simpson Creek

Tire Creek

Cool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile CreekLower Eightmile Creek

Results
24

Culvert Scour Assessment

Figure 3.5‐4. Percent pool (as percent of total stream-surface area) of natural channels and culverts for all sites. 

For sites with two representative natural channel segments, percent pool values were averaged. Natural channel 

values are significantly greater than culvert values using a paired t-test (one-tailed) at a=0.10.

LWD loading is considerably different between the culverts and natural channels. In all cases, LWD 

per channel width is greater in representative channel segments (figure 3.5‐5). The data from the LWD 

analysis is provided in appendix A. An LWD loading of two pieces per channel width is considered 

“good” according to the Washington State standards (WFPB 1997). Natural-channel segments in only 

six sites exceed this target, although several other sites have between one and two pieces per channel 

width. Wood is absent in most culverts, with only one or two pieces counted at seven of the sites.

Figure 3.5‐5. Counts of LWD per channel width for culverts and natural channels. For sites with two 

representative natural-channel segments, LWD and channel-width values were averaged.

0%

20%

40%

60%

80%

Velvet Creek

Deadwood Trib North Deadwood Trib South

Buck CreekFall CreekHehe CreekEames Creek Haight Creek Pine Creek

Youngs Creek Simpson Creek

Tire CreekCool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile Creek Lower Eightmile Creek

Percent Pool

Culvert Representative Channel

Percent pool 0

2

4

6

8

Velvet Creek

Deadwood Trib NorthDeadwood Trib SouthBuck CreekFall Creek Hehe CreekEames CreekHaight CreekPine Creek

Youngs Creek

Simpson Creek

Tire Creek Cool CreekLittle Zigzag

Creek

Lowe Creek

Upper Eightmile CreekLower Eightmile Creek

Pieces of LWD per Channel Width

Culvert Representative Channel

Pieces of LWD per channel width

Velvet Creek

Deadwood Trib north

Deadwood Trib south

Buck Creek

Fall Creek

Hehe Creek

Eames Creek

Haight Creek

Pine Creek

Youngs Creek

Simpson Creek

Tire Creek

Cool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile CreekLower Eightmile Creek

Velvet Creek

Deadwood Trib north

Deadwood Trib south

Buck Creek

Fall Creek

Hehe Creek

Eames Creek

Haight Creek

Pine Creek

Youngs Creek

Simpson Creek

Tire Creek

Cool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile CreekLower Eightmile Creek
 25

Low wood counts in culverts are no surprise considering the absence of channel banks, riparian 

forests, and road maintenance activities. In some culverts, fluvially transported wood had accumulated 

within the pipe. LWD that is retained in culverts may not be viewed favorably, because wood can create 

debris jams that plug culverts or create footing scour. Nevertheless, a lack of wood demonstrates that 

channels at culvert crossings are not functioning like their natural counterparts. The prevalence of 

riffle-and-glide habitat in culverts may be partially due to a lack of wood that is available to form steps, 

create scour pools, and generally increase channel complexity.

3.6 Overview of substrate conditions

Grain-size distributions. Pebble-count data for each site are included in the site evaluations (appendix 

A). When analyzed together, pebble-count data showed some interesting trends. In order to look at 

broad trends among sites, size-fraction ratios were developed by dividing the size for each size class in 

the culvert by the size for the same size class in the representative channel. A summary of these ratios 

for all sites is included in figure 3.6‐1. The larger values in the chart indicate that the material in the 

culvert is larger than the natural channel for the given size fraction. On average, material in culverts is 

bigger than material in the channels. This is especially true for the smaller size classes. For most sites, 

the culvert has larger material for the smaller size fractions. This was further analyzed by averaging the 

ratios for each size class (figure 3.6‐2). There are large differences in sizes for the smaller size classes 

(D5

 and D16) but little differences for the large size classes (D84 and D95). The D50 shows a moderate 

difference, with culverts having, on average, about 50 percent larger D50s. D5

 and D16s, however, 

average more than three times larger in the culvert, indicating that culverts typically have fewer particles 

in the smaller size classes, which is indicative of armored bed conditions. Results of a paired onesample t-test show significantly greater values for the culvert for the D5

 and D16 size classes (a = 0.05) 

but not for the D50, D84, or D95. The complete substrate data are included in appendix A.

Figure 3.6‐1. Size-class ratios for each site (culvert/representative natural channel). Data from pools are 

excluded.

0

2

4

6

8

10

Velvet Creek

Deadwood Trib NorthDeadwood Trib South

BuckCreek

Fall Creek

HeheCreek Haight Creek PineCreek Youngs Creek

Simpson Creek

Tire Creek

CoolCreek LittleZigzag LoweCreek Upper

Eightmile

Lower

Eightmile

Site and Size Class

Size class ratio (Culvert / Representative 

Channel)

D5 D16 D50 D84 D95

26.4

Velvet Creek

Deadwood Trib north

Deadwood Trib south

Buck Creek

Fall Creek

Hehe Creek

Haight Creek

Pine Creek

Youngs Creek

Simpson Creek

Tire Creek

Cool Creek

Little Zigzag Creek

Lowe Creek

Upper Eightmile CreekLower Eightmile Creek

Size class ratio (culvert/

representative channel)

Results
26

Culvert Scour Assessment

Figure 3.6‐2. Ratios (culvert/representative natural channel) for a range of size classes. 

Substrate size ratios were compared to stream width ratios to see if culvert width had an impact on the 

size of material in the culvert. The width ratio did not appear to impact the size of the D50 or D84 (figure 

3.6‐3).

Figure 3.6‐3. Effect of width ratio (culvert/natural channel bankfull width) on the D50 (left) and D84 (right) ratios. 

There are no significant correlations. Upper Eightmile Creek was removed as an outlier from the D50 analysis 

because of the effect of a large sand deposit on the D50 for the natural channel. Pebble-count data in pools are 

not included in these data.

Sorting and skewness. In order to look at broad trends in bed-material sorting and skewness among 

all sites, the values for each location (natural channel or culvert) at each site were averaged and ratios 

were developed for each site. The sorting and skewness coefficients are displayed in table 3.6‐1. 

Guidelines for evaluating these coefficients are included in table 3.6‐2 and table 3.6‐3. Box plots are 

also included that show the range of data (figure 3.6‐4). Culverts tend to have lower sorting coefficients 

than the natural channels. This corresponds to a greater degree of sorting; that is, culvert beds tend to 

be more “well-sorted” than the natural channel. This implies a narrower size range of particle sizes in 

1

2

3

4

D5 D1 6 D5 0 D84 D95

S iz e C las s

Average Size Ratio (Culvert / 

Representative Channel)

Average size ratio (culvert/

representative channel)

0.0

1.0

2.0

3.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Width Ratio (Culvert / Channel BFW)

D50 Ratio

0.0

1.0

2.0

3.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Width Ratio (Culvert / Channel BFW)

D84 Ratio D50 ratio

D84 ratio
 27

culverts and broader, more heterogeneous ranges of material in natural channels. Differences between 

the culvert and representative natural channels are significant using a paired t-test at a = 0.05.

Bed-material distributions appear to be slightly more negatively skewed in culverts than in 

representative channels, which would indicate that culverts have a greater abundance of particles 

on the fine side of the distribution. These results, however, are not significant using a paired t-test at 

a = 0.05. 

Table 3.6‐1. Substrate sorting and skewness coefficients.

 Substrate Sorting Coefficient Substrate Skewness Coefficient

Site* Culvert Natural Sorting Culvert Natural Skewness

Channel ratio Channel Difference 

(culv/ref) (culv/ref)

Velvet Creek 1.5 2.3 0.64 0.18 0.30 -0.12

Deadwood Trib North 2.0 2.1 0.97 0.14 0.35 -0.21

Deadwood Trib South 1.6 2.0 0.81 0.29 0.35 -0.05

Buck Creek 1.9 2.1 0.90 0.19 0.46 -0.27

Fall Creek 1.6 1.5 1.03 0.15 0.04 0.11

Hehe Creek 1.9 2.4 0.78 0.24 0.37 -0.13

Haight Creek 2.7 2.4 1.14 -0.03 0.48 -0.51

Pine Creek 1.6 2.0 0.80 0.33 -0.09 0.42

Youngs Creek 1.7 2.0 0.84 0.42 0.40 0.02

Simpson Creek 1.5 1.5 0.99 0.03 0.19 -0.16

Tire Creek 1.3 1.6 0.80 0.00 0.32 -0.33

Cool Creek 1.4 1.3 1.08 0.23 0.16 0.07

Little Zigzag Creek 1.7 3.1 0.55 0.35 0.47 -0.12

Lowe Creek 1.5 1.8 0.82 0.26 0.25 0.00

Upper Eightmile Creek 1.3 2.8 0.45 0.23 -0.15 0.38

Lower Eightmile Creek 1.2 2.4 0.51 -0.15 0.36 -0.51

* Eames Creek was removed from this analysis because of the large effect of the culvert on upstream bed material distributions.

Results
28

Culvert Scour Assessment

Table 3.6‐2. Interpretation of sorting coefficients. From Folk and Ward 1957

Sorting Coefficient Characterization

> 4 Extremely poor

2 – 4 Very poor

1 – 2 Poor

0.71 – 1 Moderate

0.5 – 0.71 Moderately well

0.35 – 0.5 Well

< 0.35 Very well

Table 3.6‐3. Interpretation of skewness coefficients.

Skewness Value Characterization Particle Sizes

-0.3 to -1 Strongly negative skewed More fine particles

-0.1 to 0.3 Negative skewed

-0.1 to 0.1 Nearly symmetrical

0.1 to 3 Positive skewed

0.3 to 1 Strongly positive skewed More coarse particles

1

1.5

2

2.5

3

3.5

Culvert Natural Channel

Sorting Coefficient

-0.2

0

0.2

0.4

0.6

Culvert Natural Channel

Skewness Coefficient

Figure 3.6‐4. Box plots of sorting coefficients (left) and skewness coefficients (right). Sorting coefficient Skewness coefficient
 29

4 Discussion

Culvert sites included in this study represent 

a range of culvert types that generally fit the 

description of stream-simulation design. All of 

the pipes are either open-bottomed arches or 

embedded-pipe arches with culvert beds made 

up of natural streambed materials. Some of 

the culverts were installed in the 1980s, long 

before the more contemporary standards for 

stream simulation were in common use. For 

most sites, the original design objectives and 

project constraints are unknown. This evaluation 

is therefore not intended to be a critique of how 

well these culverts meet stream-simulation 

design objectives, but instead is focused on 

learning from these sites in order to advance the 

practical application of stream-simulation design 

techniques.

The following sections discuss patterns of culvert 

behavior in response to bed-mobilizing flood 

events that have been observed among all the 

sites. The effect of culvert characteristics (width) 

and designs (slope and bed material) are related 

to modeled and observed culvert bed scour 

conditions. There are, of course, many variables 

that affect culvert bed conditions at each site, and 

these influences may obscure underlying patterns 

of culvert response. Thus, an attempt is made at 

identifying trends, while also highlighting specific 

site conditions that may be influencing the results. 

A discussion on the implication of designs to AOP 

is also included. And finally, the results are used 

to develop recommendations for future culvert 

design and construction.

4.1 Inlet scour and culvert bed 

adjustment

Assuming culverts were installed to a similar 

grade as the channel (except for Buck and 

Eames, which were placed at zero slope), 

most culverts have adjusted their profiles since 

installation. In most cases, the culvert bed has 

adjusted to a flatter slope (see figure 3.4‐1). 

Thirteen out of 17 sites (76 percent) have culvert 

bed slopes that are less than the pipe slope, 

indicating adjustment. This adjustment has 

occurred through inlet scour and/or aggradation 

at the downstream end of the culvert. A high 

incidence of inlet scour has been reported in 

other regional studies (Lang et al. 2004) and was 

observed at many of the sites included in the 

present study. Inlet scour was observed primarily 

along culvert sides adjacent to footings/stem 

walls, but was occasionally observed to occupy 

much of the inlet area.

At least two sites (Pine and Youngs Creek) have 

lost most if not all of their originally placed bed 

material; most of the material has subsequently 

been naturally replaced by new material from 

upstream. This may be an acceptable or even 

a desired scenario for a stream-simulation type 

design; except that in closed-bottom pipes like 

Pine and Youngs it presents a risk of scour to the 

culvert base that may not always be replaced by 

upstream sources. Use of open-bottom arches, or 

installation of a foundation of stable bed material 

over which mobile material can move, would 

ensure that base scour of the culvert does not 

persist.

Results show that the incidence of culvert bed 

flattening does not vary with the culvert width 

ratio (figure 3.4‐2), suggesting that other factors 

may be influencing this condition. Presumably, 

the degree of bed adjustment would depend on 

the number of large flow events experienced by 

the culvert, and so some sites may have adjusted 

more than others depending on their flood 

history. The following discussions highlight other 

potential causes for bed adjustment, which are 

primarily related to site evaluation, design, and 

construction.

Discussion
30

Culvert Scour Assessment

Culvert elevation. New culvert installations 

are typically larger than the culverts they are 

replacing. An increase in the elevation of the 

culvert crown may require raising the height of 

the roadfill to accommodate the larger structure, 

which may add considerable expense to a 

culvert replacement project. If resources are 

unavailable to raise the road prism, then culverts 

may be either undersized or placed at too low 

an elevation. In cases where culverts are placed 

too low, the slope may remain similar to the 

natural stream slope, but the pipe is lower than 

the channel profile. This condition can create 

upstream channel incision. Material eroded 

from the channel and banks upstream is then 

made available to aggrade within the pipe. This 

occurs more readily in the downstream portion 

of the pipe that is free from the contraction scour 

at the inlet. Based on study results and site 

observations, this dynamic may be contributing 

to bed adjustment at Pine Creek, Youngs Creek, 

Tire Creek, and Hehe Creek.

Backwater-induced sediment aggradation.

At some sites, accumulations of coarse material 

were observed upstream of the culvert inlet (Pine 

Creek, Youngs Creek). The material was likely 

deposited during mobile-bed flood events, when 

the upstream bed was mobilized and contributing 

material that was then deposited in the backwater 

created by the culvert (potentially plugged with 

debris or exceeding capacity). This material 

appears to have caused localized channel 

widening in Pine and Youngs Creeks, and also 

creates an oversteepened section just upstream 

of the inlet, which may be exacerbating inlet 

scour.

Maintenance and restoration. Slope adjustment 

may also be the result of maintenance activities 

during or following storms, when debris plugs at 

culvert inlets are removed by heavy machinery 

to improve conveyance. Debris removal efforts 

may have a tendency to overexcavate the inlet 

area to ensure that culverts remain free of 

debris that can plug culverts and cause roadway 

overtopping. Other potential managementrelated causes of culvert bed flattening are grade 

control structures placed downstream of the 

culvert outlet that create backwater areas where 

sediment aggrades, thus reducing slope through 

the crossing. Such structures are present at Cool 

Creek and Upper Eightmile Creek.

4.2 Effect of culverts on channel-flow 

geometry and hydraulics

Culverts have a significant effect on channel-flow 

geometry and hydraulic variables including widthto-depth ratio, cross-sectional area, velocity, and 

shear stress. There are many factors that affect 

hydraulic conditions at each site, and it is difficult 

to separate out the many variables that are 

influencing observed conditions. Nevertheless, 

some trends can be seen from the results of this 

study, and these are discussed below under their 

respective headings.

Cross-section geometry. The effect of culverts 

on cross-section flow geometry indicates 

considerable flow constriction caused by culverts 

at every site. Modeling shows that six sites (Tire, 

Cool, Eames, Lowe, Lower Eightmile, and Upper 

Eightmile) have some degree of backwater 

created by the culvert, typically with outlet 

control characteristics where the water surface 

is elevated through a portion of the length of the 

pipe. This condition typically only affects flows 

above the Q2

 and, for some sites, only affects the 

Q50 and Q100. Backwater conditions have obvious 

impacts to flow geometry, including increasing the 

depth and decreasing the width-to-depth ratio. 

Culverts that were at least 90 percent of bankfull 

width or greater showed no backwater impacts; 

however, some culverts as small as 50 percent 

bankfull width (Deadwood Tributary North) also 

showed no backwater effects.
 31

All sites have some degree of impact on crosssection flow geometry, at least at the higher flows 

(Q10 and above). Top width and wetted perimeter 

tend to show the greatest differences from the 

natural channel.

Velocity. All sites show an effect on velocity at 

least at the highest modeled flows (Q50 and Q100). 

Even at low-to-moderate flows (25-percent Q2

), 

most of the sites have average velocities that 

exceed 2.0 feet per second, which is considered 

near the upper limit for juvenile salmonid 

swimming ability (Bell 1990). Even though 

average velocities are high, there are typically 

lower velocity areas near culvert margins or within 

boundary layers adjacent to substrate where 

fish may be able to pass (see literature review in 

Kahler and Quinn 1998). However, as average 

velocities exceed 5 feet per second, which occurs 

at many sites, low-velocity areas may be limited 

in the culvert.

Velocity differences between the channel and 

the culvert are related to flow constriction in the 

culvert, but the degree of effect on velocity does 

not necessarily track well with culvert bankfull 

width ratios. Many other factors, including culvert 

roughness, culvert slope, culvert cross-sectional 

area, and the degree of backwater created by 

the culvert all affect velocity. More investigation 

is needed to explore the relationships among 

all sites with respect to velocity. For discussion 

of velocity results at the site scale, see the site 

evaluations in appendix A.

Shear Stress. For many sites, shear stress 

is lower in culverts than in the representative 

channels, a condition that is attributable to lower 

culvert bed slopes and backwatered conditions 

in culverts in some cases. Many of these sites 

have experienced culvert bed adjustments in the 

form of inlet scour or locally reduced slopes due 

to aggradation, resulting in reduced shear stress. 

Other sites, such as Little Zigzag, are dominated 

by uniform beds without the step sequences 

that elevate the shear in the natural channel. 

Steeper culvert beds tend to show greater shear 

stress than flatter beds. Judging from the high 

incidence of culvert bed adjustment (flattening), 

shear stresses were likely greater in culverts 

immediately following construction but prior to 

any bed-mobilizing flows. These initially higher 

shear stresses would have resulted in scour 

and bed adjustments that have reduced culvert 

bed slopes and have therefore moderated shear 

stresses within culverts. More investigation is 

needed to explore the relationships among all 

sites with respect to shear stress. For discussions 

of shear stress results at the site scale, see the 

site evaluations in appendix A.

Critical shear stress. Results of the excess 

shear analysis suggest that the D84 particles are 

mobilized at lower flows than would typically 

be expected to mobilize the bed in step-pool 

channels (Grant et al. 1990, Chin 1998). 

Many of the sites have calculated shear stress 

that exceeds critical shear stress below the 

25-percent Q2

 (appendix A). This runs counter 

to other regional studies that have estimated 

that bed mobilizing events in step-pool channels 

typically occur close to the 50-year event (Grant 

et al. 1990). The low entrainment thresholds 

calculated in the analysis may be due to the 

difficulty in accurately modeling incipient motion 

of particles in step-pool channels using traditional 

critical shear stress approaches (Bathurst 1987). 

Furthermore, applying the Komar (1987) method 

to estimate critical shear stress assumes that an 

increase in particle protrusion above the bed (a 

condition that often occurs in step-pool channels) 

will increase the potential for particle entrainment. 

However, the equation does not take into account 

the increased stability that is provided by the 

interlocking of particles at stable step structures, 

nor does it account for high degrees of form 

Discussion
32

Culvert Scour Assessment

resistance (spill resistance and LWD resistance) 

that may limit the amount of shear stress that is 

available to initiate particle motion (Wilcox et al. 

2006).

In consideration of the factors discussed above, 

the critical shear stress approach may not be 

the most appropriate technique to apply to 

the steep boulder-bed channels in this study. 

However, applying different methods for the steppool systems versus the lower gradient poolriffle systems may have introduced significant 

variability. Moreover, initial trials using the Costa 

(1983) and Bathurst (1987) methods, which 

were developed for step-pool channels, provided 

widely variable results that made interpretations 

and comparisons difficult. In summary, even 

though entrainment thresholds calculated for 

the steeper sites in this study are potentially 

lower than the actual entrainment thresholds 

experienced in these streams, it is assumed that 

they provide reasonable relative comparisons 

of bed mobility between culverts and natural 

channels.

Excess shear stress is typically smaller in 

culverts than in the representative channels at 

low flows. This is attributable to lower average 

shear in the culvert for many sites as discussed 

previously, but is further influenced by coarser 

D84s in culverts. Coarser bed material in culverts 

is a result of a combination of bed coarsening 

and artificially larger material placed during 

construction. A few of the sites exhibit an increase 

in culvert excess shear with respect to the natural 

channel as flows increase. These include Velvet 

Creek, Haight Creek, and Cool Creek. This is 

generally due to more rapid increases in depth 

of flow as flows rise in the culvert with respect 

to the natural channel. A few sites have lower 

excess shear in culverts when compared to 

natural channels due to a reduction in the energy 

slope in the culvert as a result of outlet control 

conditions in the pipe. These include Tire Creek, 

Lowe Creek, Upper Eightmile Creek, and Lower 

Eightmile Creek.

4.3 Culvert substrate conditions

Coarser bed material in culverts compared 

to natural channels is a ubiquitous condition 

measured at nearly all the study sites (figure 

3.6‐1). Differences in bed material size are most 

pronounced for the smaller grain sizes, with 

culverts typically having much less of the smaller 

material that is found in the natural channels. In 

some cases, the differences are severe, such as 

in Velvet Creek, Deadwood Tributary South, Buck 

Creek, Little Zigzag Creek, Upper Eightmile, and 

Lower Eightmile, where the size of the smaller 

size classes far exceed the natural channel 

conditions. Only Haight Creek and Cool Creek 

have culvert material that is significantly smaller 

than the representative channels. In Haight Creek 

this is likely the result of backwater influence from 

the mainstem Siuslaw River, which is a short 

distance from the culvert outlet.

Culverts have material that is more “well-sorted” 

(poorly graded or only belonging to a few size 

classes) than representative channels (table 

3.6‐1). Culverts therefore do not represent the 

wide range of size classes that are found in the 

natural channels. This is likely related to the lack 

of smaller material in culverts. Even though a lack 

of smaller material may not have a direct impact 

on aquatic habitat or passage conditions, it does 

suggest that there is reduced hydraulic and 

physical habitat complexity available for sorting 

and storing smaller material.

The size and gradation of material originally 

placed in culverts during construction obviously 

would have a large influence on the material 

measured at the sites. Larger material than that 

found in the natural channel is sometimes placed 
 33

in the culvert in order to ensure that culvert beds 

do not experience excessive scour. This likely 

occurred at a number of the sites in this study, 

based on site observations and discussions 

with practitioners that were involved in design/

implementation. In particular, Deadwood Tributary 

North and Buck Creek have D84 and D95 material 

that is considerably larger than the representative 

channels, likely due to the size of material placed 

during construction. These sites show very little 

incidence of culvert bed flattening, possibly a 

result of the construction-related bed armoring 

that is able to resist adjustment.

The pronounced difference in size of the smaller 

size fractions suggests that bed coarsening has 

occurred through transport of the smaller material 

and that flows competent to move the larger 

material have occurred less frequently. Transport 

of the smaller material from culverts may be 

exacerbated by a lack of material sorting or 

bedform construction during installation. Several 

culverts have uniform plane-bed type channels 

where bedform structures (steps) do not appear 

to have been constructed. Plane-bed channels 

decrease the presence of low energy areas 

where smaller material is able to accumulate and 

remain.

4.4 Channel complexity and depth 

distribution

Channel complexity was lower in culverts than 

in representative natural channels. Measures of 

complexity were evaluated by looking at thalweg 

(vertical) sinuosity, residual depth, and depth 

distribution (important for passage). Crosssection complexity, using sum of squared height 

difference, was also analyzed but the technique 

does not appear to provide a reliable assessment 

of complexity in culverts. The various complexity 

metrics applied to the study sites are discussed 

below.

Vertical sinuosity and residual depth.

Vertical sinuosity is less in culverts than in the 

representative natural channels at most sites 

(63 percent). Only two sites have higher vertical 

sinuosity in the culvert; these include Simpson 

Creek and Tire Creek. For Deadwood Tributary 

South and Pine Creek, the vertical sinuosity in 

the culvert is considerably less than the natural 

channel (over 7 times less in the case of Pine 

Creek). Less vertical variation in the culvert is 

related to more plane-bed type channels that lack 

the step-pool sequences and residual depths of 

natural channels. Maximum residual depth in the 

culverts is, on average, about 60 percent that of 

the natural channel outside the crossing. Sites 

with low values for residual depth also show big 

differences in substrate when compared with their 

natural channels, particularly with respect to the 

presence of small material. This may reflect a 

lack of complexity that is needed to retain smaller 

material. The clear differences in residual depths 

and vertical sinuosity demonstrate differences 

in complexity between culverts and natural 

channels. This has implications for fish habitat 

and passage conditions.

Cross-section complexity – sum of squared 

height difference. Sum of squared height 

difference did not appear to be a good metric for 

evaluating complexity. In many cases, results did 

not match visual estimates of complexity. This 

may be an artifact of where the boundaries were 

chosen for this metric. Use of this metric may 

require more detailed cross-section measures 

to make sure the detailed bed topography is 

captured and included in the calculation.

Depth distribution. In general, the culverts in 

this study tended to have less available shallow 

channel margin habitat than natural channels at 

the 25-percent Q2

. These results have important 

implications for AOP because of the effects of 

shallow flows on velocity. Velocity profiles in 

Discussion
34

Culvert Scour Assessment

culverts show lower velocity areas closer to the 

bed (Ead et al. 2000; Lang et al. 2004; House 

et al. 2005). In addition, areas of shallower flow 

(areas closer to the culvert sides) have lower 

velocities at a given depth than areas of deeper 

flow. This follows the classic Prandtl Von Karmen 

universal velocity distribution law (Chow 1959). 

Velocity in shallow flow areas may be further 

reduced through increased boundary roughness 

as one approaches obstructions along flow 

margins, or from secondary currents near the 

surface (Ead et al. 2000).

Because juvenile fish have been shown to seek 

out low velocity areas in culverts for passage 

(Belford and Gould 1989), passage conditions 

may be degraded if shallow, low energy areas are 

less available, such as occurs due to a reduction 

in width-to-depth ratios in culverts. Not only is 

this important for fish that rely on the low velocity 

habitat, but it may also be important for passage 

of other organisms, including amphibians 

and mammals that may use shallow areas for 

migration through culverts.

The two culverts with the most available shallow 

channel margin habitat with respect to their 

natural counterparts are Youngs Creek and Tire 

Creek. These culverts both have well-developed 

banks within the pipe that allow for the presence 

of shallow channel margin flow. Well-developed 

banks constructed within a culvert will increase 

the availability of shallow channel-margin habitat 

and will provide more boundary roughness and 

complexity that can provide velocity refuge for 

passage.

4.5 Channel units and LWD

Channel-unit and LWD analysis indicates that 

aquatic habitat conditions are less favorable 

in culverts compared to natural channels. The 

channel-unit and LWD results and implications 

are discussed below.

Channel units. Percentage of pool habitat is 

often used as an indicator of habitat quality and 

complexity. In 13 of 17 sites, the natural channel 

had greater percentage of habitat in pools than 

the culvert; however, 4 of these have percent 

pool within 10 percentage points of the natural 

channel, which indicates fairly good simulation of 

natural pool abundance. Youngs Creek, Simpson 

Creek, Tire Creek, and Cool Creek have greater 

percent pools in the culvert than the natural 

channel; all with pool abundance that is at least 

19 percentage points greater than the natural 

channel. These sites are among the best of all of 

the sites with respect to bedforms or banks that 

simulate natural channel conditions. Simpson 

Creek, which is a long culvert with high pool 

abundance, has many constructed step-pool 

sequences that contribute to the high number of 

pools.

Most culverts have less pool abundance due to 

plane-bed channels, lack of constructed step-pool 

sequences, and a prevalence of glide-like habitat. 

For short culverts, a lack of pool habitat may not 

significantly impact passage conditions, and it 

may actually serve to reduce the potential for 

footing scour. However, for long culverts, a lack 

of pool habitat may reduce resting areas that are 

important for migrating fish.

Large woody debris. LWD frequency is 

much less in the culverts than in the channel. 

Stream-simulation designs should be capable 

of transporting some wood; however, large logs, 

especially those with attached root wads, may be 

too large to be transported through the pipe. This 

is because at high flows the stream hydraulics 

may be able to transport wood of a length 

that exceeds the width of the culvert, even if 
 35

designed under contemporary design standards. 

Wood transport is therefore an issue that is not 

adequately addressed by stream-simulation 

design criteria. During large flood events, there 

will continue to be the potential for plugging of 

the culvert with wood. Even small wood can plug 

the pipe or reduce capacity. In natural channels, 

the stream can adjust laterally or vertically to 

accommodate in-channel wood. In a culvert, 

this is not possible. For these reasons, wood 

collecting within culverts or at culvert inlets will 

continue to be a maintenance issue for all but the 

largest of pipes. Capacity will be maintained at 

many sites by removing wood from culvert barrels 

and inlet areas during or following large floods. 

In order to compensate for the lack of wood in 

culverts, the structural and habitat benefits of 

wood will need to be met in other ways, such 

as building rock steps and bank features that 

simulate the geomorphic function and habitat 

complexity provided by wood.

4.6 Implications to AOP

A number of studies have demonstrated the 

importance of low velocity areas in culverts 

for fish passage (Kahler and Quinn 1998). It 

is important that these low-velocity areas are 

available at relatively large flows since fish are 

motivated to migrate through culverts during the 

rising and falling limbs of flood hydrographs. Lang 

et al. (2004) demonstrated that most juvenile fish 

migration through culverts occurred between the 

11-percent and 15-percent annual exceedance 

flows (flows exceeded only 11 percent to 15 

percent of the time) and that adults tended 

to pass at even greater flows, with the peak 

between the 1-percent and 2.5-percent annual 

exceedance flows. As a reference, the 25-percent 

Q2

 flow for the Middle Fork Willamette River 

near Oakridge, Oregon, (USGS #14144800, 

nearest long-term gauging station near the 

upper Willamette River sites) is approximately 

equivalent to the 5-percent annual exceedance 

flow. Maintaining low-velocity areas at these 

higher flows is difficult because of the increased 

constriction of flow at higher discharges. Modeled 

velocities in culverts for the study sites in this 

study exceeded 2.0 feet per second for all but 

one site at the 25-percent Q2

. All sites had 

velocities that exceeded 2.0 feet per second at 

the 25-percent Q2

 in the natural channel. Juvenile 

fish swimming capabilities are typically 2.0 feet 

per second or less (Bell 1990). Although velocities 

at the study sites exceed this threshold, lower 

velocity areas near culvert/channel boundaries or 

in shallow flow areas may be within the range of 

fish swimming abilities. It was beyond the scope 

of this study to evaluate the spatial distribution of 

velocities in channels and culverts.

Channel complexity and habitat quality were 

lower in culverts than in natural channels. Lack 

of habitat complexity is partially a result of 

culvert design and construction methods but is 

also related to the inherent constraints imposed 

on streams by culverts. The lack of channel 

sinuosity, channel migration zones, streambanks, 

and adjacent riparian areas limits the ability of 

culverts to provide functional habitat and passage 

conditions. The individual site evaluations 

(appendix A) discuss the factors affecting AOP 

conditions at each site.

4.7 Comparison of results with WFLHD 

study

Several sites analyzed as part of this study 

were also analyzed as part of a culvert study 

conducted by the Western Federal Lands 

Highway Division (WFLHD) in the late 1980s 

(Browning 1990). The two studies have five sites 

in common. However, for one of the sites (Pine 

Creek), the structure was replaced since the early 

study, and for another site (Eames Creek), two 

very similar culverts in close proximity made it 

impossible to tell which culvert was included in 

Discussion
36

Culvert Scour Assessment

the original study. The three remaining culverts that were included in both studies include Haight Creek, 

Cool Creek, and Lowe Creek. The WFLHD study only looked at hydraulic variables for two flow events; 

the 2-year recurrence interval and the 50-year recurrence interval event. A comparison of the results of 

the two studies is provided in table 4.7‐1.

Table 4.7‐1. Comparison of results of present study with results of the WFLHD study (Browning 1990) for the 

three sites assessed in both studies.

Flow Depth (ft) Velocity (ft/s) Shear Stress (lb/ft2)

Site Flow WFLHD Present WFLHD Present WFLHD Present

Event Study Study Study Study Study Study

(range) (range) (range)

Haight 2-year 2.1 2.68 – 3.08 3.06 2.99 – 3.43 0.66 0.13 – 0.24

50-year 3.1 4.56 – 5.11 3.99 4.99 – 5.6 0.97 0.29 – 0.57

Cool 2-year 2.5 1.64 – 2.67 4.31 2.78 – 4.36 1.56 0.61 – 2.18

50-year 3.8 3.9 – 6.06 5.51 5.39 – 7.28 2.37 1.61 – 4.01

Lowe 2-year 2.5 1.21 – 3.05 9.65 2.42 – 5.6 7.8 0.7 – 7.51

50-year 3.9 1.03 – 9.75 12.12 1.43 – 22.15 12.17 0.25 –123.26

There is fairly good correlation of values for some parameters at some flows, but many parameters 

diverge considerably. There are several reasons related to the different methods of the two studies that 

would explain much of the difference in the values:

1. The use of different regional regression equations. The older study used equations from an 

earlier USGS publication (USGS 1979). The estimated flows in the WFLHD study are larger 

than those using the newer USGS equations. In Lowe Creek, the estimated flows are over twice 

as large as in the current study.

2. Different calculations for watershed area. Older calculations for Lowe and Haight Creek were 

larger than the more recent calculations, resulting in larger flow estimates.

3. Different assumptions for roughness. The WFLHD study assumed Manning’s roughness values 

of 0.040 or 0.045, whereas the present study used Jarrett’s equation to determine roughness 

(see section 2.8.1).

4. Different methods for modeling. WFLHD study used a normal depth approach (Manning’s 

equation) and assumed trapezoidal channel dimensions to represent the culvert bed.

It is also probable that the culvert beds have adjusted since the late 1980s, thus changing the hydraulic 

conditions. However, short of recreating the original methods used in the WFLHD study, it is impossible 

to separate out the influence of the methods versus the influence of channel adjustments.
 37

5 Recommendations

The following recommendations and 

considerations for culvert design were developed 

from study results and site observations.

Design for a bed mobility threshold in the 

culvert that equals or exceeds that of the 

natural channel. If bed mobilization occurs more 

readily in the culvert than in the natural channel, 

then scour of culvert beds or footings will occur. 

This appears to have occurred in Deadwood 

Tributary South and may be responsible for 

scoured portions of other culverts including Pine, 

Youngs, Eames, Haight, and Lowe. In the majority 

of culverts in this study, even those that exceed 

bankfull channel width, flow constriction occurs 

even at relatively frequent flood recurrence 

intervals. It therefore must be acknowledged that 

there will often be more energy available to cause 

bed scour in culverts than in natural channels 

during high flows.

An important step in culvert design is the 

determination of the different thresholds for 

bed mobilization in the culvert and the natural 

channel. This information can be used to select 

appropriate culvert sizes and bed-material 

gradations. The objective should be designing for 

bed mobility in the culvert that occurs at similar 

flows as in the natural channel. This will simulate 

natural channel adjustment and will reduce bed 

degradation or aggradation in the vicinity of the 

crossing.

Compensating for higher energy in culverts 

(shear and velocity) by increasing rock sizes 

comes with some risk at higher flows. If and 

when the placed material is transported out of the 

culvert (during large flows), new material recruited 

from upstream may be too small to be retained 

in the culvert; resulting in persistent instability 

and scour potential from future floods. This is 

especially a concern in step-pool channels, where 

significant bed mobilization occurs only when the 

key step-forming particles are entrained (Grant 

et al. 1990) and bed mobility ceases once they 

become stable again. The lack of continued 

recruitment of material into the culvert at the 

descending limb of the flood hydrograph, or 

during subsequent lower magnitude flow events, 

may perpetuate culvert bed scour conditions. This 

may be a concern for the Buck Creek site, where 

culvert shear stress and velocity are higher in the 

culvert at high flows. So far, the bed has been 

stabilized because there is coarser material in the 

culvert than in the natural channel. However, if 

this material is transported out of the pipe during 

a large flow event, there will be no material of this 

size available to replace it, leading to potential 

culvert bed scour and channel incision.

Create bedforms in culverts that simulate 

natural channel bedforms. A number of sites 

in this study had uniform culvert beds that did 

not resemble the natural channel outside the 

crossing. These include Velvet Creek, Fall Creek, 

Deadwood Tributary North, Deadwood Tributary 

South, Little Zigzag Creek, Eames Creek, Haight 

Creek, Lower Eightmile Creek, and Upper 

Eightmile Creek. These sites were characterized 

by plane-bed or bedrock channels with shallow 

wall-to-wall flow at the time of the survey. A lack 

of bedforms or banks that mimic those in the 

natural channel reduces complexity and depth 

distribution and likely results in adverse impacts 

to AOP. Sites with bedforms that better mimic 

those in the natural channel include Tire Creek, 

Simpson Creek, and Hehe Creek.

Creating bedforms akin to those in the natural 

channel not only benefits AOP, but also helps to 

achieve bed stability that is similar to the natural 

channel. This is especially the case for step-pool 

channels, where stability is provided by rock 

steps that consist of interlocked particles. The 

Recommendations
38

Culvert Scour Assessment

interlocking of the particles increases particle 

stability above what would be expected if the 

rocks were arranged in an unsorted pattern on 

the streambed. Bedform construction in steppool systems may include creating embedded 

rock steps that mimic natural steps or using large 

rocks to mimic the function of wood in a woodforced system. The Forest Service guidelines 

(Forest Service Stream-Simulation Working 

Group 2008) outline several considerations 

and techniques for designing culvert beds that 

satisfy these objectives. Step height and spacing 

should also be similar to the natural channel. 

Step dimensions can be measured using an 

appropriate reference reach or can be determined 

through the use of empirical equations and 

relationships measured in other streams (Grant 

et al. 1990, Papanicolaou and Maxwell 2000). 

Bedforms should ideally be constructed before 

the culvert is placed on the footings. In closedbottom pipes, constructing bedforms within the 

pipe will be necessary except in cases where the 

top portion of the culvert can be removed or left 

off for access (on structural plate culverts).

Create stable channel banks in wide culverts.

Most sites showed a lack of defined streambanks 

within the culvert. Constructed streambanks can 

protect footings, concentrate flow for low-flow 

fish passage, create greater habitat complexity 

and velocity refuge, create shallow flow on 

channel margins at a range of flows, and allow 

for passage of terrestrial organisms. Pine Creek, 

Tire Creek, Buck Creek, Cool Creek, Lowe Creek, 

Hehe Creek, and Youngs Creek all had some 

degree of exposed streambanks at the survey 

flow, but most of these only had a bank on one 

side, with flow along the culvert wall on the other 

side.

Banks should consist of stable configurations of 

rocks that are placed along culvert margins in a 

manner that simulates conditions found in the 

natural channel. Construction of banks will only 

be feasible in wide culverts. It may be counterproductive to provide banks in narrower culverts 

where preserving flow capacity may outweigh the 

benefits of banks.

In several culverts in this study, flow was 

observed to concentrate along the concrete 

footings on one or both sides of the culvert. This 

is a result of the lower resistance of the smooth 

wall of the footing; but it is undesirable as it can 

contribute to footing scour and abrasion. Placed 

bank features can protect footings and culvert 

walls from scour and other damage.

Control for inlet scour. Many of the sites in 

this study have experienced at least some 

degree of inlet scour. These include Lowe Creek, 

Deadwood Tributary South, Eames Creek, Haight 

Creek, Tire Creek, Pine Creek, Youngs Creek, 

and Cool Creek. Significant outlet scour was 

not observed. Because of the predisposition 

for inlet scour, special precautions should be 

taken to address it. These include placing larger 

than normal bed material or stable steps in the 

upstream portion of the culvert, or making sure 

that at least the upstream portion of the pipe has 

a solid foundation of stable material. For closedbottom pipes, bed retention sills near the inlet 

may be necessary. Modified culvert inlets such 

as wingwalls may reduce the incidence of inlet 

scour.

Design for bed adjustments. Most of the sites 

in this study had evidence of bed adjustment 

post-installation. Ten of the 17 sites experienced 

a flattening of the culvert bed through inlet scour 

and/or aggradation in the downstream portion 

of the pipe (see section 3.4). Inlet scour has 

been identified as a common occurrence in 

other studies (Lang et al. 2004). The width of 

the culvert with respect to the natural channel 

does not appear to be a good predictor of bed 
 39

adjustment. There are many other contributing 

factors, and bed adjustment may be difficult 

to predict. Designs should therefore anticipate 

potential adjustments by installing culverts that 

are large enough to accommodate aggradation 

and deep enough to accommodate scour. 

This may include the use safety factors for 

culvert size and embedment/footing depth. 

The Forest Service guidelines (Forest Service 

Stream-Simulation Working Group 2008) outline 

considerations and techniques for sizing and 

placing culverts that satisfy these objectives.

Alignment considerations. Scour in several 

of the pipes in this study was at least partially 

related to the culvert alignment where tight bends 

at the inlet were contributing to scour of culvert 

beds or footings. This includes Pine Creek, 

Youngs Creek, Eames Creek, and Haight Creek. 

Deadwood Tributary North experienced failure of 

bank material into the inlet due to a sharp bend at 

the inlet. At Pine and Youngs Creeks, aggradation 

of bed material upstream of the inlet (assumed 

to be a result of backwatered conditions during 

floods) induced lateral channel migration that 

resulted in a skewed alignment. Designers should 

recognize the potential for this dynamic and 

select culvert sizes accordingly.

Culvert replacements may mistakenly assume 

that the observed alignment of the stream at the 

time of replacement is the correct alignment, 

when in reality the original culvert installation 

may have changed the alignment. Alignments 

are often changed to allow for culverts that 

are perpendicular to the stream and therefore 

have the least possible length and road fill 

requirements. These situations not only change 

stream alignments, but they also increase the 

slope of the channel at the crossing. Efforts 

should be made to determine the natural stream 

alignment and to place structures as close to this 

alignment as possible. 

Channel-unit considerations. Although pool 

habitat is generally viewed as a benefit for fish 

rearing and spawning, pool habitat in culverts 

may not be considered a positive attribute 

because of potential scour issues. During large 

flows, scour in step-pool channels occurs in 

pools due to local high slopes and shear stresses 

created by the steep channel units located just 

upstream (Grant et al. 1990). If these conditions 

occur in culverts, then the incidence of footing or 

culvert base scour may be increased. For long 

culverts, it would be unreasonable to design 

culvert beds without pool sequencing similar 

to that of the natural channel, since designing 

otherwise might create passage issues (long, 

uniform plane-bed channels). However, for short 

culverts, placement of the pipe can take channel 

units into consideration to ensure that scour 

potential is minimized. For example, if location 

options are available, culverts can be placed in 

sections of long riffles to avoid potential issues 

with scour in pool units.

Recommendations for future assessments.

This assessment was focused primarily on 

conditions at the individual-site scale. A further 

examination of trends among all sites would help 

to further the understanding of the response of 

culvert sites to elements of design. Predictor 

variables such as culvert size, culvert shape, 

culvert slope, and the size of placed bed material 

can be related to observed scour, predicted 

scour, and AOP conditions in order to evaluate 

how components of culvert design affect scour 

and AOP among all sites.

Future assessments should look at how culvert 

designs affect species-specific fish passage 

requirements. This would include detailed 

measures of the spatial extent of velocity and 

depth throughout the culvert and in adjacent 

natural channels outside the crossing at fishpassage flows.

Recommendations
40

Culvert Scour Assessment

One of the objectives of this study was to 

examine sites that had adjusted to local flood 

history (all sites were at least 5 years old); 

therefore newer sites constructed according to 

more contemporary stream-simulation design 

criteria were not included. Future studies should 

attempt to include newer pipes constructed 

with the more recent criteria. Although these 

culverts may not have fully responded to local 

hydrology, modeling can be used to predict 

channel response, which can later be compared 

to observed adjustments. 

Modes of culvert response will vary depending on 

channel type, sediment conditions, and regional 

differences in hydrology and geomorphic setting. 

Future studies should attempt to represent the 

range of conditions in which culvert installation 

projects are likely to occur, so that appropriate 

prescriptions can be developed for particular site 

conditions.

6 Acknowledgments

This investigation would not have been possible 

without the assistance of several Forest Service 

personnel who contributed many suggestions and 

perspectives on culvert design and installation. 

These include Ed Gililland, Kim Johansen, Dan 

Cenderelli, Mark Weinhold, and Sandra Wilson 

Musser. These individuals provided technical 

guidance and thoughtful review for this effort, and 

we thank them for their collaboration.

SDTDC’s national publications are available on 

the Internet at: http://www.fs.fed.us/eng/pubs/

Forest Service and U.S. Department of the 

Interior, Bureau of Land Management employees 

also can view videos, CDs, and SDTDC’s 

individual project pages on their internal computer 

network at: http://fsweb.sdtdc.wo.fs.fed.us/

For additional information on culvert scour, 

contact Greg Napper at SDTDC. Phone: 909–

599–1267 ext. 290. Email: gnapper@fs.fed.us
 41

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