Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

Hydraulic Considerations 

for Pipelines Crossing 

Stream Channels

Technical Note 423

April 2007
Suggested citations:

Fogg, J. and H. Hadley. 2007. Hydraulic considerations for pipelines crossing stream channels. Technical Note 423. BLM/

ST/ST-07/007+2880. U.S. Department of the Interior, Bureau of Land Management, National Science and Technology 

Center, Denver, CO. 20 pp. http://www.blm.gov/nstc/library/techno2.htm.

U.S. Department of the Interior. 2007. Hydraulic considerations for pipelines crossing stream channels. Technical Note 

423. BLM/ST/ST-07/007+2880. Bureau of Land Management, National Science and Technology Center, Denver, CO. 

20 pp. http://www.blm.gov/nstc/ library/techno2.htm.

Production services provided by:

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P.O. Box 25047

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BLM/ST/ST-07/007+2880
Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

Hydraulic Considerations for Pipelines Crossing Stream Channels

Technical Note 423

By:

Jim Fogg

Surface Water Hydrologist

Bureau of Land Management

Denver, CO

and

Heidi Hadley

Salinity Coordinator

Bureau of Land Management

Salt Lake City, UT

2007

Hydraulic Considerations 

for Pipelines Crossing 

Stream Channels

Technical Note 423

April 2007

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

iii

Contents

Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

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

Surface Crossings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Reconnaissance Method . . . . . . . . . . . . . . . . . . . . . . . . 4

Physiographic Method . . . . . . . . . . . . . . . . . . . . . . . . . 6

Analytical Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Detailed Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Subsurface (Buried) Crossings . . . . . . . . . . . . . . . . . . . . 11

Channel Degradation . . . . . . . . . . . . . . . . . . . . . . . . . 11

Channel Scour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423



Abstract

High flow events have the potential to damage pipelines 

that cross stream channels, possibly contaminating runoff. 

A hydrologic analysis conducted during the design of the 

pipeline can help determine proper placement. Flood 

frequency and magnitude evaluations are required for 

pipelines that cross at the surface. There are several methods 

that can be used, including reconnaissance, physiographic, 

analytical, and detailed methods. The method used must be 

appropriate for the site’s characteristics and the objectives 

of the analysis. Channel degradation and scour evaluations 

are required for pipelines crossing below the surface. Proper 

analysis and design can prevent future pipeline damage and 

reduce repair and replacement costs.



Introduction

In 2002, the U.S. Fish and Wildlife Service raised 

concerns about the potential for flash floods in ephemeral 

stream channels to rupture natural-gas pipelines and 

carry toxic condensates to the Green River, which would 

have deleterious effects on numerous special-status fish 

species (Figure 1). In November of the same year, BLM 

hydrologists visited the Uinta Basin in Utah to survey 

stream channels and compute flood magnitudes and depths 

to better understand possible flooding scenarios. From 

this they developed construction guidance for pipelines 

crossing streams in Utah. This guidance was later modified 

so that it was generally applicable to the arid and semiarid 

lands of the intermountain west. It may also have general 

applicability in other areas of the western United States. 

The purpose of this document is to present the modified 

guidance for placement of pipelines crossing above or below 

the surface of stream channels to prevent inundation or 

exposure of the pipe to the hydraulic forces of flood events.

Figure 1. Pipeline breaks during flooding can release condensate toxic to sensitive fish species.

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

Surface Crossings

Pipelines that cross stream channels on the surface should 

be located above all possible floodflows that may occur at 

the site. At a minimum, pipelines must be located above 

the 100-year flood elevation and preferably above the 500-

year flood elevation. Two sets of relationships are available 

for estimating flood frequencies at ungaged sites in Utah. 

Thomas and Lindskov (1983) use drainage basin area 

and mean basin elevation for flood estimates for six Utah 

regions stratified by location and basin elevation (Table 1). 

Thomas et al. (1997) also use drainage area and mean basin 

elevation to estimate magnitude and frequency of floods 

throughout the southwestern U.S., including seven regions 

that cover the entire State of Utah. Results from both sets 

of equations should be examined to estimate the 100- and 

500-year floods, since either of the relations may provide 

questionable results if the pipeline crosses a stream near the 

boundary of a flood region or if the drainage area or mean 

basin elevation for the crossing exceed the limits of the data 

set used to develop the equations.

Regression equations for peak discharges for Uinta Basin

(from Thomas and Lindskov 1983)

Discharge Q in cubic feet per second, Area in square miles, Elevation in thousands of feet

Recurrence interval 

(yrs)

Equation Number of stations 

used in analysis

Average standard 

error of estimate (%)

2 Q = 1,500 A0.403 E -1.90 25 82

5 Q = 143,000 A0.374 E-3.66 25 66

10 Q = 1.28 x 106 A0.362 E-4.50 25 64

25 Q = 1.16 x 107 A0.352 E-5.32 25 66

50 Q = 4.47 x 107 A0.347 E-5.85 25 70

100 Q = 1.45 x 108 A0.343 E-6.29 25 74

Table 1. Examples of flood frequency equations for ungaged sites in Utah.

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423
Procedures for estimating 100-year and 500-year flood 

magnitudes for other States are described in the U.S. 

Geological Survey’s National Flood Frequency Program 

(Ries and Crouse 2002) (Figure 2). Full documentation 

of the equations and information necessary to solve 

them is provided in individual reports for each State. The 

National Flood Frequency (NFF) Website (http://water.

usgs.gov/software/nff.html) provides State summaries of the 

equations in NFF, links to online reports for many States, 

and factsheets summarizing reports for States with new or 

corrected equations. Background information in each State’s 

flood frequency reports should be checked to ensure that 

application of the equations is not attempted for sites with 

independent variables outside the range used to develop the 

predictive equations.

Figure 2. View of the output from NFF.

Once the flood frequency for a site has been estimated, 

determining the depth of flow associated with an extreme 

flood (i.e., the elevation of the pipeline at the crossing) 

may be approached in a number of ways. Procedures 

for estimating depth of flow for extreme floods in Utah 

are presented in Thomas and Lindskov (1983). Similar 

procedures presented in Burkham (1977, 1988) are 

generally applicable for locations throughout the Great 

Basin and elsewhere. The reconnaissance, physiographic, 

analytical, and detailed methods described in those reports 

will be summarized briefly in this paper. Burkham (1988) 

describes an additional method (historical method) not 

presented here, since the data for its use (high-water marks 

for an extreme historical flood with known discharge and 

recurrence interval) are rarely available in public land 

situations for which this guidance is intended.

Reconnaissance Method

The reconnaissance method (as the name implies) is a fairly 

rough and imprecise method for delineating flood-prone 

areas (Burkham 1988; Thomas and Lindskov 1983). It is 

most applicable to stable or degrading alluvial channels 

with multiple terrace surfaces, although such terraces 

may be difficult to detect on severely degrading streams. 

In this procedure, the channel of interest is examined 

to approximate the area that would be inundated by a 

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423


large flood. A geomorphic reconnaissance of the site is 

conducted, and it may be supplemented with aerial photos, 

maps, and historical information available for the reach 

of interest. In addition to the morphology of the channel, 

floodplain, and terraces, information on vegetation (e.g., 

species, flood tolerance, drought tolerance) and soils (e.g., 

development, stratification, and drainage) can be helpful 

for identifying flood-prone areas (Burkham 1988). For best 

results, the geomorphic analysis should include reaches 

upstream and downstream of the site and should attempt 

to determine the general state of the stream channel as 

aggrading, degrading, or stable. (Additional guidance on 

detection of stream degradation is presented in the section 

on subsurface crossings).

In the reconnaissance method, identification of bankfull 

elevation and the active floodplain (i.e., floodplain 

formed by the present flow regime) provides inadequate

conveyance information for extreme flood events (Figure 3). 

Past floodplains or present terraces also must be identified, 

since these surfaces may be inundated by extreme floods 

in the present flow regime, especially in arid and semiarid 

environments. Pipelines should be constructed so that they 

cross at or above the elevation of the highest and outermost 

terrace (Figure 4). The highest terrace is unlikely to be 

accessed in the modern flow regime by any but the most 

extreme floods. 

Figure 3. Although this pipeline crossed above the bankfull channel indicators, it was not high enough to escape more 

extreme floods.

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423


Practitioners of the reconnaissance method need 

considerable experience in geomorphology, sedimentation, 

hydraulics, soil science, and botany. Also, since this method 

is based on a geomorphic reconnaissance of the site, no 

flood frequency analysis is required and no recurrence 

interval can be assigned to the design elevation. An 

additional drawback to the method is that the accuracy 

of the results is unknown. However, the reconnaissance 

method may be the most rational one for delineating floodprone areas on some alluvial fans and valley floors where 

channels become discontinuous (Burkham 1988). While 

this is the quickest approach to designing a stream crossing, 

it likely will result in the most conservative estimate 

(i.e., highest elevation and greatest construction cost) for 

suspension of the pipeline.

Physiographic Method

A slightly more intensive approach to designing a stream 

crossing is based on the physiographic method for 

estimating flood depths at ungaged sites described by 

Thomas and Lindskov (1983) and Burkham (1988). The 

procedure uses regional regression equations (similar to 

the flood frequency equations described above) to estimate 

maximum depth of flow associated with a specified 

recurrence-interval flood (Table 2). Flood depth is then 

added to a longitudinal survey of the channel thalweg in 

the vicinity of the crossing (10 to 20 channel widths in 

length), resulting in a longitudinal profile of the specified 

flood. Elevation of the flood profile at the point of pipeline 

crossing is the elevation above which the pipeline must be 

Figure 4. This New Mexico pipeline crosses the channel near the elevation of the highest terrace, which places it 

above even the most extreme flood events.

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423


suspended. The method is generally applicable where 

1) the project site is physiographically similar to the 

drainage basins used to develop the regression equations 

and 2) soil characteristics are the same at the project 

site as in the basins where the regression equations were 

developed. While this procedure requires a field survey and 

calculation of flood depths at points along the channel, it 

may result in a lower crossing elevation (and possibly lower 

costs) for the pipeline. Also, since the regional regression 

equations estimate flood depths for specific recurrenceinterval floods, it is possible to place a recurrence interval 

on the crossing design for risk calculations. However, 

regional regression equations linking depth of flood to 

recurrence interval have not been developed for many areas. 

In States where they have been developed (e.g., Alabama, 

Colorado, Illinois, Kansas, and Oklahoma), standard errors 

of the estimates have ranged from 17 to 28 percent, with an 

average standard error of 23 percent (Burkham 1988).

Table 2. Examples of depth frequency equations for ungaged sites in Utah.

Regression equations for flood depths for Uinta Basin

(from Thomas and Lindskov 1983)

Flood depth D in feet, Area in square miles, Elevation in thousands of feet

Recurrence interval 

(yrs)

Equation Number of stations 

used in analysis

Average standard 

error of estimate (%)

2 D = 1.03 A0.159 16 30

5 D = 13.3 A0.148E-1.03 16 28

10 D = 68.6 A0.131 E-1.69 16 26

25 D = 556 A0.128 E-2.59 16 24

50 D = 1330 A0.123 E-2.95 15 24

100 D = 1210 A0.130 E-2.86 14 22

Analytical Method

The analytical method described by Burkham (1988) uses 

uniform flow equations to estimate depth of flow associated 

with a particular magnitude and frequency of discharge. 

Typically, a trial-and-error procedure is used to solve the 

Manning uniform flow equation for depth of flow, given 

a design discharge (i.e., a flood of specified recurrence 

interval), a field-surveyed cross section and channel slope, 

and an estimate of the Manning roughness coefficient (n). 

Numerous software packages are available to facilitate the 

trial-and-error solution procedure (e.g., WinXSPRO). Since 

the Manning formula is linear with respect to the roughness 

coefficient, estimating this coefficient can be a significant 

source of error and is likely the most significant weakness in 

this approach. Estimating roughness coefficients (n values) 

for ungaged sites is a matter of engineering judgment, but 

n values typically are a function of slope, depth of flow, 

bed-material particle size, and bedforms present during the 

passage of the flood wave. Guidance is available in many 

hydraulic references (e.g., Chow 1959). Selecting n values 

for flows above the bankfull stage is particularly difficult, 

since vegetation plays a major role in determining resistance 

to flow. Barnes (1967) presents photographic examples 

of field-verified n values, and Arcement and Schneider 

(1989) present comprehensive guidance for calculating n 

values for both channels and vegetated overbank areas (i.e., 

floodplains). Depth of flow determined with uniform flow 

equations, such as the Manning equation, represents mean

depth of flow to be added to the cross section at the site of 

the pipeline crossing.

Burkham (1977, 1988) also presented a simplified 

technique for estimating depth of flow, making use of the 

general equation for the depth-discharge relation: 

d = C Q f

Values of f (the slope of the relationship when plotted 

on logarithmic graph paper) can be determined from 

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423


“at-station” hydraulic geometry relationships at gaging 

stations in the region. Only the upper portion of the 

gaging-station ratings should be used to derive the slope (f

value) for application to extreme floods, since a substantial 

portion of the flow may be conveyed in the overbank 

area. Alternatively, Burkham (1977, 1988) presents a 

simplified procedure for estimating f that requires only a 

factor for channel shape. Leopold and Langbein (1962) 

computed a theoretical value of 0.42 for natural channels, 

while Burkham (1988) computed a theoretical value of 

0.46 for parabolic cross sections. Burkham (1977) earlier 

reported an average f value of 0.42 from 539 gaging stations 

scattered along the eastern seaboard and upper Midwest, 

while Leopold and Maddock (1953) reported an average 

f value of 0.40 for 20 river cross sections in the Great 

Plains and the Southwest. Park (1977) summarized f values 

from 139 sites around the world and found most values 

occurred in the range of 0.3 to 0.4. Additional assumptions 

in Burkham (1977, 1988) enable an estimate of the 

coefficient C in the depth-discharge relationship with only 

a single field measurement of width and maximum depth 

at some reference level in the channel (e.g., bankfull stage) 

(Burkham 1977, 1988). Depth of flow determined from 

Burkham’s simplified technique represents maximum depth 

of flow to be added to the thalweg at the cross section.

The analytical methods described by Burkham (1977, 1988) 

generally will be more accurate than the physiographic and 

reconnaissance methods described previously; thus, they 

may result in lower pipeline elevations and construction 

costs than the previous methods. However, analysis of flood 

elevations for the most sensitive situations should probably 

be conducted with the detailed method described below.

Detailed Method

Additional savings in construction costs for pipelines 

crossing channels may be realized by applying a detailed 

water-surface-profile model of flow through the crossing 

site. The water-surface-profile model requires a detailed 

survey of both the longitudinal channel profile (at least 

20 channel widths in length) and several cross sections 

along the stream (Figure 5). Design flows (e.g., 100-year 

Figure 5. Application of a water-surface-profile model requires both a longitudinal channel profile and several 

surveyed cross sections (Federal Interagency Stream Restoration Working Group 1998).

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423
Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

and 500-year floods) are calculated for the channel at the 

crossing with the regional regression equations described 

above and routed through the surveyed channel reach using 

a step-backwater analysis. The step-backwater analysis uses 

the principles of conservation of mass and conservation 

of energy to calculate water-surface elevations at each 

surveyed cross section. Computed water-surface elevations 

at successive cross sections are linked to provide a watersurface profile for the flood of interest through the reach 

of interest. The computations are routinely accomplished 

in standard software, such as the U.S. Army Corps of 

Engineers’ HEC-RAS model. Whereas the analytical 

methods described previously assume steady, uniform flow 

conditions through the reach, a detailed water-surfaceprofile model is capable of handling both gradually and 

(to some extent) rapidly varied flow conditions. Since the 

computation uses a detailed channel survey, it is the most 

accurate method to use; however, it is likely the most 

expensive method for the same reason. Burkham (1988) 

indicates that the error in flood depths predicted from 

step-backwater analysis can be expected to be less than 

20 percent. The step-backwater computations require an 

estimate of the Manning roughness coefficient (n) as an 

indicator of resistance to flow and assume fairly stable 

channel boundaries. Estimation of the roughness coefficient 

(n) includes the same considerations discussed previously 

for the analytical methods. The assumption of fairly stable 

channel boundaries is not always met with sand-bed 

channels and is an issue of considerable importance for 

designing subsurface pipeline crossings as well.

Of the methods presented for determining elevation of 

floods for pipelines crossing channels, the detailed method 

is the most accurate and should be used for situations 

with high resource values, infrastructure investment, 

construction costs, or liabilities in downstream areas. 

In undeveloped areas, the physiographic and analytical 

methods may be used to provide quick estimates of flood 

elevations for sites with fewer downstream concerns. The 

reconnaissance method provides the roughest estimates but 

may be all that is warranted in very unstable areas, such as 

alluvial fans or low relief valley floors (e.g., near playas). The 

detailed, analytical, and physiographic methods all assume 

relatively stable channel boundaries but may be used on 

sand channels with an accompanying loss of accuracy. 

In very sandy channels, the accuracy of results from the 

detailed method may not be significantly better than the 

results from one of the intermediate methods unless a 

mobile-boundary model is used (Burkham 1988).

11

Subsurface (Buried) Crossings

Since many of the pipelines are small and most of the 

channels are ephemeral, it is commonplace to bury the 

pipelines rather than suspending them above the streams. 

The practice of burying pipelines at channel crossings likely 

is both cheaper and easier than suspending them above all 

floodflows; however, an analysis of channel degradation and 

scour should be completed to ensure the pipelines are not 

exposed and broken during extreme runoff events (Figure 6). 

Without such an analysis, channels should be excavated to 

bedrock and pipelines placed beneath all alluvial material.

Figure 6. Channel degradation or scour during flash-flood events may expose buried pipelines, resulting 

in costly breaks.

Buried pipelines may be exposed by streambed lowering 

resulting from channel degradation, channel scour, or a 

combination of the two. Channel degradation occurs over 

a long stream reach or even the entire drainage network 

and is generally associated with the overall lowering of the 

landscape. Degradation also may be associated with changes 

in upstream watershed or channel conditions that alter the 

water and sediment yield of the basin. Channel scour is a 

local phenomenon associated with passage of one or more 

flood events or site-specific hydraulic conditions that may 

be natural or human-caused in origin. Either process can 

expose buried pipelines to excessive forces associated with 

extreme flow events, and an analysis of each is required to 

ensure integrity of the crossing.

Channel Degradation

Detection of long-term channel degradation must be 

attempted, even if there is no indication of local scour. 

Conceptual models of channel evolution (e.g., Simon 

1989) have been proposed to describe a more-or-less 

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423
Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

12

predictable sequence of channel changes that a stream 

undergoes in response to disturbance in the channel or the 

watershed. Many of these models are based on a “space for 

time” substitution, whereby downstream conditions are 

interpreted as preceding (in time) the immediate location 

of interest, and upstream conditions are interpreted as 

following (in time) the immediate location of interest. 

Thus, a reach in the middle of the watershed that 

previously looked like the channel upstream will evolve 

to look like the channel downstream (Federal Interagency 

Stream Restoration Working Group 1998). Since channel 

evolution models can help predict current trends where 

a pipeline crosses a channel, they may indicate areas to 

be avoided when relocation of the crossing is an option. 

Most conceptual models of channel evolution have been 

developed for landscapes dominated by streams with 

cohesive banks; however, the same processes occur in 

streams with noncohesive banks, with somewhat less 

well-defined stages. 

Geomorphic indicators of recent channel incision 

(e.g., obligate and facultative riparian species on present-day 

stream terraces elevated above the water table) also may 

be helpful for diagnosing channel conditions. However, 

long-term trends in channel evolution are often reversed 

during major flood events, especially for intermittent and 

ephemeral channels in arid and semiarid environments. 

Thus, a stream that is degrading during annual and 

intermediate flood events may be filled with sediment 

(i.e., it may aggrade) from tributary inputs during a major 

flood, and channels that are associated with sediment 

storage (i.e., aggrading) during the majority of runoff 

events may be “blown out” with major degradation during 

unusual and extreme large floods.

In some situations, a quantitative analysis of channel 

degradation may be warranted. Plots of streambed elevation 

against time permit evaluation of bed-level adjustment 

and indicate whether a major phase of channel incision 

has passed or is ongoing. However, comparative channel 

survey data are rarely available for the proposed location for 

a pipeline to cross a channel. In instances where a gaging 

station is operated at or near the crossing, it is usually 

possible to determine long-term aggradation or degradation 

by plotting the change in stage through time for one or 

more selected discharges. The procedure is called a specificgage analysis (Figure 7) and is described in detail in Stream 

Corridor Restoration: Principles, Processes, and Practices

(Federal Interagency Stream Restoration Working Group 

1998). When there is no gaging station near the proposed 

channel crossing, nearby locations on the same stream or 

in the same river basin may provide a regional perspective 

on long-term channel adjustments. However, specific-gage 

records indicate only the conditions in the vicinity of the 

particular gaging station and do not necessarily reflect river 

response farther upstream or downstream of the gage. Therefore, 

it is advisable to investigate other data in order to make 

predictions about potential channel degradation at a site.

Figure 7. Specific-gage plots of the gage heights associated with index flows through time may indicate general channel lowering in the 

drainage basin (adapted from Federal Interagency Stream Restoration Working Group 1998; Biedenharn et al. 1997).
13

Other sources of information include the biannual 

bridge inspection reports required in all States for bridge 

maintenance. In most States, these reports include channel 

cross sections or bed elevations under the bridge, and a 

procedure similar to specific gage analysis may be attempted 

(Figure 8). Simon (1989, 1992) presents mathematical 

functions for describing bed-level adjustments through 

time, fitting elevation data at a site to either a power 

function or an exponential function of time. Successive 

cross sections from a series of bridges in a basin also may 

be used to construct a longitudinal profile of the channel 

network; sequential profiles so constructed may be used 

to document channel adjustments through time (Figure 

9). Again, bridge inspection reports so used indicate only 

the conditions in the vicinity of those particular bridges 

(where local scour may be present) and must be interpreted 

judiciously for sites upstream, downstream, or between the 

bridges used in the analysis.

In the absence of channel surveys, gaging stations, and 

bridge inspection reports (or other records of structural 

repairs along a channel), it may be necessary to investigate 

channel aggradation and degradation using quantitative 

techniques described in Richardson et al. (2001) and 

Lagasse et al. (2001). Techniques for assessing vertical 

stability of the channel include incipient motion analysis, 

analysis of armoring potential, equilibrium slope analysis, 

and sediment continuity analysis. Incipient motion analysis 

and analysis of armoring potential are equally applicable to 

Figure 8. Plots of bed elevation versus time may be developed 

from biannual bridge inspection reports to document systemwide 

degradation or aggradation (Federal Interagency Stream Restoration 

Working Group 1998).

Figure 9. Sequential 

longitudinal profiles 

also may be used to 

document channel 

lowering through time 

(Federal Interagency 

Stream Restoration 

Working Group 1988; 

Biedenharn et al. 

1997).

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423
Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

14

both long-term degradation and short-term scour and fill 

processes, while equilibrium-slope and sediment-continuity 

analyses are more closely tied to long-term channel 

processes (i.e., degradation and aggradation). 

Channel Scour

In addition to long-term channel degradation at subsurface 

crossings, general channel scour must be addressed to 

ensure safety of the pipeline. General scour is different from 

long-term degradation in that general scour may be cyclic 

or related to the passing of a flood (Richardson and Davis 

2001). Channel scour and fill processes occur naturally 

along a given channel, and both reflect the redistribution 

of sediment and short-term adjustments that enable the 

channel to maintain a quasi-equilibrium form. In other 

words, channels in dynamic equilibrium experience various 

depths of scour during the rising stages of a flood that 

frequently correspond to equal amounts of fill during the 

falling stages, resulting in minimal changes in channel-bed 

elevation. Where pipelines cross channels, it is important 

to determine the potential maximum depth of scour so that 

the pipeline is buried to a sufficient depth and does not 

become exposed when bed scour occurs during a flood.

General scour occurs when sediment transport through 

a stream reach is greater than the sediment load being 

supplied from upstream and is usually associated with 

changes in the channel cross section. General scour can 

occur in natural channels wherever a pipeline crosses a 

constriction in the channel cross section (contraction 

scour). Equations for calculating contraction scour generally 

fall into two categories, depending on the inflow of bedmaterial sediment from upstream. In situations where 

there is little to no bed-material transport from upstream 

(generally coarse-bed streams with gravel and larger bed 

materials), contraction scour should be estimated using 

clear-water scour equations. In situations where there is 

considerable bed-material transport into the constricted 

section (i.e., for most sand-bed streams), contraction scour 

should be estimated using live-bed scour equations. Livebed and clear-water scour equations can be found in many 

hydraulic references (e.g., Richardson and Davis 2001). 

In either case, estimates of general scour in the vicinity of 

the pipeline crossing must be added to the assessment of 

channel degradation for estimating the depth of burial 

for the crossing.

Other components of general scour can result from 

placement of subsurface crossings relative to the alignment 

of the stream channel. Pipelines crossing at bends in the 

channel are particularly troublesome, since bends are 

naturally unstable and tend to collect both ice and debris 

(which can cause additional constrictions in the flow). 

Channel-bottom elevations are usually lower on the 

outside of meander bends and may be more than twice 

as deep as the average depth in straighter portions of the 

channel. Crossings in the vicinity of stream confluences 

also create difficulties, since flood stages and hydraulic 

forces may be strongly influenced by backwater conditions 

at the downstream confluence. For example, sediment 

deposits from tributary inputs may induce contraction 

scour opposite or downstream of the deposit. Additional 

complications are introduced where pipelines are located 

near other obstructions in the channel. Channel-spanning 

obstructions (e.g., beaver dams or large wood) may induce 

plunge-pool scour downstream of the structure, and 

individual obstructions in the channel induce local 

scour akin to pier scour characteristic of bridge piers at 

highway crossings.

Even in the absence of contraction scour, general scour will 

still occur in most sand-bed channels during the passage of 

major floods. Since sand is easily eroded and transported, 

interaction between the flow of water and the sand bed 

results in different configurations of the stream bed with 

varying conditions of flow. The average height of dune 

bedforms is roughly one-third to one-half the mean flow 

depth, and the maximum height of dunes may nearly equal 

the mean flow depth. Thus, if the mean depth of flow in 

a channel was 5 feet, maximum dune height could also 

approach 5 feet, half of which would be below the mean 

elevation of the stream bed (Lagasse et al. 2001). Similarly, 

Simons, Li, and Associates (1982) present equations 

for antidune height as a function of mean velocity, but 

limit maximum antidune height to mean flow depth. 

Consequently, formation of antidunes during high flows 

not only increases mean water-surface elevation by one-half 

the wave height, it also reduces the mean bed elevation by 

one-half the wave height. Richardson and Davis (2001) 
15

reported maximum general scour of one to two times the 

average flow depth where two channels come together in a 

braided stream. 

Pipeline crossings that are buried rather than suspended 

above all major flow events should address all of the 

components of degradation, scour, and channel-lowering 

due to bedforms described above. In addition, once a 

determination is made on how deep to bury the pipeline 

at the stream crossing, the elevation of the pipe should be 

held constant across the floodplain. If the line is placed at 

shallower depths beneath the floodplain, channel migration 

may expose the line where it is not designed to pass beneath 

the channel (Figure 10).

Figure 10. Lateral migration of this stream channel during high water excavated a section of pipeline under the 

floodplain that was several feet shallower than at the original stream crossing.

In complex situations or where consequences of pipeline 

failure are significant, consideration should be given to 

modeling the mobile-bed hydraulics with a numerical 

model such as HEC-6 (U.S. Army Corps of Engineers 

1993) or BRI-STARS (Molinas 1990). The Federal 

Interagency Stream Restoration Working Group (1998) 

summarizes the capabilities of these and other models and 

provides references for model operation and user guides 

where available.

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

17

Conclusion

Pipelines that cross perennial, intermittent, and ephemeral 

stream channels should be constructed to withstand floods 

of extreme magnitude to prevent rupture and accidental 

contamination of runoff during high flow events. Pipelines 

crossing at the surface must be constructed high enough 

to remain above the highest possible floodflows at each 

crossing, and pipelines crossing below the surface must 

be buried deep enough to remain undisturbed by scour 

and fill processes typically associated with passage of peak 

flows. A hydraulic analysis should be completed during 

the pipeline design phase to avoid repeated maintenance 

of such crossings and eliminate costly repairs and potential 

environmental degradation associated with pipeline breaks 

at stream crossings.

Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

19

Literature Cited

Arcement, G.J., Jr. and V.R. Schneider. 1989. Guide for 

selecting Manning’s roughness coefficients for natural 

channels and flood plains. U.S. Geological Survey 

Water-Supply Paper 2339. 38 pp.

Barnes, H.H., Jr. 1967. Roughness characteristics of natural 

channels. U.S. Geological Survey Water-Supply Paper 

1849. 213 pp.

Biedenharn, D.S., C.M. Elliott, and C.C. Watson. 1997. 

The WES stream investigation and streambank 

stabilization handbook. Prepared for the U.S. 

Environmental Protection Agency by the U.S. Army 

Corps of Engineers Waterways Experiment Station. 

Vicksburg, MS.

Burkham, D.E. 1977. A technique for determining depths 

for T-year discharges in rigid boundary channels. 

U.S. Geological Survey Water-Resources Investigations 

77-83. 38 pp.

Burkham, D.E. 1988. Methods for delineating flood-prone 

areas in the Great Basin of Nevada and adjacent states. 

U.S. Geological Survey Water-Supply Paper 2316. 20 pp.

Chow, V.T. 1959. Open-channel hydraulics. McGraw Hill, 

New York. 680 pp.

Federal Interagency Stream Restoration Working Group. 

1998. Stream corridor restoration: Principles, processes, 

and practices. National Technical Information Service, 

Order No. PB98-158348INQ, Washington, DC.

Lagasse, P.F., J.D. Schall, and E.V. Richardson. 2001. 

Stream stability at highway structures. Hydraulic 

Engineering Circular No. 20, Third Edition, FHWA 

NHI 01-002. Federal Highway Administration, 

Washington, DC.

Leopold, L.B. and W.B. Langbein. 1962. The concept of 

entropy in landscape evolution. U.S. Geological Survey 

Professional Paper 500-A. 20 pp.

Leopold, L.B. and T. Maddock, Jr. 1953. The hydraulic 

geometry of stream channels and some physiographic 

implications. U.S. Geological Survey Professional Paper 

252. 57 pp.

Molinas, A. 1990. Bridge stream tube model for alluvial 

river simulation (BRI-STARS), user’s manual. National 

Cooperative Highway Research Program, Project 

No. HR 15-11. Transportation Research Board, 

Washington, DC.

Park, C.C. 1977. World-wide variations in hydraulic 

geometry exponents of stream channels: An analysis and 

some observations. Journal of Hydrology 33:133-146.

Richardson, E.V. and S.R. Davis. 2001. Evaluating scour 

at bridges. Hydraulic Engineering Circular No. 18, 

Fourth Edition, FHWA NHI 01-001. Federal Highway 

Administration, Washington, DC.

Richardson, E.V., D.B. Simons, and P.F. Lagasse. 2001. 

Highways in the river environment. Report FHWA 

NHI 01-004, Hydraulic Design Series No. 6. Federal 

Highway Administration, Washington, DC.

Ries, K.G., III and M.Y. Crouse. 2002. The National Flood 

Frequency Program, version 3: A computer program 

for estimating magnitude and frequency of floods for 

ungaged sites. U.S. Geological Survey Water-Resources 

Investigations Report 02-4168. 42 pp.

Simon, A. 1989. A model of channel response in 

distributed alluvial channels. Earth Surface Processes 

and Landforms 14(1): 11-26.

Simon, A. 1992. Energy, time and channel evolution in 

catastrophically disturbed fluvial systems. In Phillips, 

J.D. and W.H. Renwick (eds.). Geomorphic systems: 

geomorphology. Vol. 5. pp. 345-372. 

Simons, Li, and Associates. 1982. Engineering analysis of 

fluvial systems. Fort Collins, CO.
Hydraulic Considerations for Pipelines Crossing Stream Channels—TECHNICAL NOTE 423

20

Thomas, B.E. and K.L. Lindskov. 1983. Methods for 

estimating peak discharge and flood boundaries 

of stream in Utah. U.S. Geological Survey WaterResources Investigations Report 83-4129. 77 pp.

Thomas, B.E., H.W. Hjalmarson, and S.D. Waltemeyer. 

1997. Methods for estimating magnitude and 

frequency of floods in the southwestern United States. 

U.S. Geological Survey Water-Supply Paper 2433. 195 pp.

U.S. Army Corps of Engineers. 1993. HEC-6 scour and 

deposition in rivers and reservoirs: Users manual. 

Hydrologic Engineering Center, Davis, CA.
The mention of company names, trade names, or commercial products does not constitute endorsement or recommendation for use 

by the Federal Government.