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U.S. Department of the Interior

U.S. Bureau of Land Management Papers

University of Nebraska - Lincoln Year

Hydraulic Considerations for Pipelines

Crossing Stream Channels

Jim Fogg Heidi HadleyBureau of Land Management Bureau of Land Management

This paper is p osted at DigitalCommons@University of Nebraska - Lincoln.

http://digitalcommons.unl.edu/usblmpub/14



Hydraulic Considerations

or Pipelines Crossing

Stream Channels

echnical Note 423

April 2007



Suggested citations:

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

S/S-07/007+2880. U.S. Department o the Interior, Bureau o Land Management, National Science and echnology

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

U.S. Department o the Interior. 2007. Hydraulic considerations or pipelines crossing stream channels. echnical Note

423. BLM/S/S-07/007+2880. Bureau o Land Management, National Science and echnology Center, Denver, CO.20 pp. http://www.blm.gov/nstc/ library/techno2.htm.

Production services provided by:

Bureau o Land Management

National Science and echnology Center

Branch o Publishing Services

P.O. Box 25047

Denver, CO 80225

Copies available online at:

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tp://tp.blm.gov/pub/nstc/TechNotes

BLM/S/S-07/007+2880



Hydraulic Considerations for Pipelines Crossing Stream Channels

Technical Note 423

By:

Jim Fogg

Surace Water Hydrologist


Denver, CO

and

Heidi Hadley

Salinity Coordinator


Salt Lake City, U

2007

Hydraulic Considerations

or Pipelines Crossing

Stream Channels

echnical Note 423 April 2007



Contents

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

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

S u r a c e C r o s s i n g s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

R e c o n n a i s s a n c e M e t h o d . . . . . . . . . . . . . . . . . . . . . . . . . 4

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

A n a l y t i c a l M e t h o d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

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

Subsurace (Buried) Crossings . . . . . . . . . . . . . . . . . . . . . 11

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

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

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

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



Abstract

High ow events have the potential to damage pipelines

that cross stream channels, possibly contaminating runo.

A hydrologic analysis conducted during the design o the

pipeline can help determine proper placement. Flood

requency and magnitude evaluations are required or

pipelines that cross at the surace. Tere are several methods

that can be used, including reconnaissance, physiographic,

analytical, and detailed methods. Te method used must be

appropriate or the site’s characteristics and the objectives

o the analysis. Channel degradation and scour evaluations

are required or pipelines crossing below the surace. Proper

analysis and design can prevent uture pipeline damage and

reduce repair and replacement costs.



Introduction

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

concerns about the potential or ash oods in ephemeral

stream channels to rupture natural-gas pipelines and

carry toxic condensates to the Green River, which would

have deleterious eects on numerous special-status sh

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

hydrologists visited the Uinta Basin in Utah to survey

stream channels and compute ood magnitudes and depths

to better understand possible ooding scenarios. From

this they developed construction guidance or pipelines

crossing streams in Utah. Tis guidance was later modied

so that it was generally applicable to the arid and semiarid

lands o the intermountain west. It may also have general

applicability in other areas o the western United States.

Te purpose o this document is to present the modied

guidance or placement o pipelines crossing above or below

the surace o stream channels to prevent inundation or

exposure o the pipe to the hydraulic orces o ood events.

Figure . Pipeline breaks during ooding can release condensate toxic to sensitie fsh species.



Surace Crossings

Pipelines that cross stream channels on the surace should

be located above all possible oodows that may occur at

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

the 100-year ood elevation and preerably above the 500-

year ood elevation. wo sets o relationships are available

or estimating ood requencies at ungaged sites in Utah.

Tomas and Lindskov (1983) use drainage basin area

and mean basin elevation or ood estimates or six Utah

regions stratied by location and basin elevation (able 1).

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

elevation to estimate magnitude and requency o oods

throughout the southwestern U.S., including seven regions

that cover the entire State o Utah. Results rom both sets

o equations should be examined to estimate the 100- and

500-year oods, since either o the relations may provide

questionable results i the pipeline crosses a stream near the

boundary o a ood region or i the drainage area or mean

basin elevation or the crossing exceed the limits o the data

set used to develop the equations.

Regression equations or peak discharges or Uinta Basin

(rom 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 stationsused in analysis

Average standarderror 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 . Examples o ood requency equations or ungaged sites in Utah.



Procedures or estimating 100-year and 500-year ood

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

Geological Survey’s National Flood Frequency Program

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

o the equations and inormation necessary to solve

them is provided in individual reports or each State. Te

National Flood Frequency (NFF) Website (http://water.usgs.gov/sotware/nf.html ) provides State summaries o the

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

and actsheets summarizing reports or States with new or

corrected equations. Background inormation in each State’s

ood requency reports should be checked to ensure that

application o the equations is not attempted or sites with

independent variables outside the range used to develop the

predictive equations.

Figure 2. View o the output rom NFF.

Once the ood requency or a site has been estimated,

determining the depth o ow associated with an extreme

ood (i.e., the elevation o the pipeline at the crossing)

may be approached in a number o ways. Procedures

or estimating depth o ow or extreme oods in Utah

are presented in Tomas and Lindskov (1983). Similarprocedures presented in Burkham (1977, 1988) are

generally applicable or locations throughout the Great

Basin and elsewhere. Te reconnaissance, physiographic,

analytical, and detailed methods described in those reports

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

describes an additional method (historical method) not

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

or an extreme historical ood with known discharge and

recurrence interval) are rarely available in public land

situations or which this guidance is intended.

Reconnaissance Method

Te reconnaissance method (as the name implies) is a airly

rough and imprecise method or delineating ood-prone

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

most applicable to stable or degrading alluvial channels

with multiple terrace suraces, although such terraces

may be difcult to detect on severely degrading streams.

In this procedure, the channel o interest is examined

to approximate the area that would be inundated by a

H

y d r a u l i c C o n s i d e r a t i o n s f o r P i p e l i n e s C r o s s i n g S t r e a m C h a n n e l s

— T E C H N I C A L N O T E 4 2 3



large ood. A geomorphic reconnaissance o the site is

conducted, and it may be supplemented with aerial photos,

maps, and historical inormation available or the reach

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

oodplain, and terraces, inormation on vegetation (e.g.,

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

development, stratication, and drainage) can be helpulor identiying ood-prone areas (Burkham 1988). For best

results, the geomorphic analysis should include reaches

upstream and downstream o the site and should attempt

to determine the general state o the stream channel as

aggrading, degrading, or stable. (Additional guidance on

detection o stream degradation is presented in the section

on subsurace crossings).

In the reconnaissance method, identication o bankull

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

ormed by the present ow regime) provides inadequate

conveyance inormation or extreme ood events (Figure 3).

Past oodplains or present terraces also must be identied,

since these suraces may be inundated by extreme oods

in the present ow regime, especially in arid and semiaridenvironments. Pipelines should be constructed so that they

cross at or above the elevation o the highest and outermost

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

accessed in the modern ow regime by any but the most

extreme oods.

Figure 3. Although this pipeline crossed aboe the bankull channel indicators, it was not high enough to escape moreextreme oods.



Practitioners o the reconnaissance method need

considerable experience in geomorphology, sedimentation,hydraulics, soil science, and botany. Also, since this method

is based on a geomorphic reconnaissance o the site, no

ood requency analysis is required and no recurrence

interval can be assigned to the design elevation. An

additional drawback to the method is that the accuracy

o the results is unknown. However, the reconnaissance

method may be the most rational one or delineating ood-

prone areas on some alluvial ans and valley oors 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) or

suspension o the pipeline.

Physiographic Method

A slightly more intensive approach to designing a stream

crossing is based on the physiographic method or

estimating ood depths at ungaged sites described by

Tomas and Lindskov (1983) and Burkham (1988). Te

procedure uses regional regression equations (similar to

the ood requency equations described above) to estimate

maximum depth o ow associated with a specied

recurrence-interval ood (able 2). Flood depth is then

added to a longitudinal survey o the channel thalweg in

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

length), resulting in a longitudinal prole o the specied

ood. Elevation o the ood prole at the point o pipeline

crossing is the elevation above which the pipeline must be

Figure 4. This New Mexico pipeline crosses the channel near the eleation o the highest terrace, which places itaboe een the most extreme ood eents.

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— T E C H N I C A L N O T E 4 2 3



suspended. Te 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 eld survey and

calculation o ood depths at points along the channel, itmay result in a lower crossing elevation (and possibly lower

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

equations estimate ood depths or specic recurrence-

interval oods, it is possible to place a recurrence interval

on the crossing design or risk calculations. However,

regional regression equations linking depth o ood to

recurrence interval have not been developed or many areas.

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

Colorado, Illinois, Kansas, and Oklahoma), standard errorso the estimates have ranged rom 17 to 28 percent, with an

average standard error o 23 percent (Burkham 1988).

Table 2. Examples o depth requency equations or ungaged sites in Utah.

Regression equations or food depths or Uinta Basin(rom Thomas and Lindskov 1983)

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

Recurrence interval(yrs)

Equation Number of stationsused in analysis

Average standarderror 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

Te analytical method described by Burkham (1988) uses

uniorm ow equations to estimate depth o ow associated

with a particular magnitude and requency o discharge.

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

Manning uniorm ow equation or depth o ow, given

a design discharge (i.e., a ood o specied recurrence

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

and an estimate o the Manning roughness coefcient (n).

Numerous sotware packages are available to acilitate the

trial-and-error solution procedure (e.g., WinXSPRO). Sincethe Manning ormula is linear with respect to the roughness

coefcient, estimating this coefcient can be a signicant

source o error and is likely the most signicant weakness in

this approach. Estimating roughness coefcients (n values)

or ungaged sites is a matter o engineering judgment, but

n values typically are a unction o slope, depth o ow,

bed-material particle size, and bedorms present during the

passage o the ood wave. Guidance is available in many

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

or ows above the bankull stage is particularly difcult,

since vegetation plays a major role in determining resistance

to ow. Barnes (1967) presents photographic examples

o eld-veried n values, and Arcement and Schneider

(1989) present comprehensive guidance or calculating n

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

oodplains). Depth o ow determined with uniorm ow

equations, such as the Manning equation, represents mean

depth o ow to be added to the cross section at the site o

the pipeline crossing.

Burkham (1977, 1988) also presented a simplied

technique or estimating depth o ow, making use o the

general equation or the depth-discharge relation:

d = C Q f

Values o f (the slope o the relationship when plotted

on logarithmic graph paper) can be determined rom



“at-station” hydraulic geometry relationships at gaging

stations in the region. Only the upper portion o the

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

value) or application to extreme oods, since a substantial

portion o the ow may be conveyed in the overbank

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

simplied procedure or estimating f that requires only aactor or channel shape. Leopold and Langbein (1962)

computed a theoretical value o 0.42 or natural channels,

while Burkham (1988) computed a theoretical value o

0.46 or parabolic cross sections. Burkham (1977) earlier

reported an average f value o 0.42 rom 539 gaging stations

scattered along the eastern seaboard and upper Midwest,

while Leopold and Maddock (1953) reported an average

f value o 0.40 or 20 river cross sections in the Great

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

rom 139 sites around the world and ound most values

occurred in the range o 0.3 to 0.4. Additional assumptions

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

coefcient C in the depth-discharge relationship with only

a single eld measurement o width and maximum depth

at some reerence level in the channel (e.g., bankull stage)

(Burkham 1977, 1988). Depth o ow determined rom

Burkham’s simplied technique represents maximum depth

o ow to be added to the thalweg at the cross section.

Te 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 o ood

elevations or the most sensitive situations should probably

be conducted with the detailed method described below.

Detailed Method

Additional savings in construction costs or pipelines

crossing channels may be realized by applying a detailed

water-surace-prole model o ow through the crossing

site. Te water-surace-prole model requires a detailed

survey o both the longitudinal channel prole (at least

20 channel widths in length) and several cross sections

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

Figure . Application o a water-surace-profle model requires both a longitudinal channel profle and seeralsureyed cross sections (Federal Interagency Stream Restoration Working Group 99).

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— T E C H N I C A L N O T E 4 2 3



and 500-year oods) are calculated or the channel at the

crossing with the regional regression equations described

above and routed through the surveyed channel reach using

a step-backwater analysis. Te step-backwater analysis uses

the principles o conservation o mass and conservation

o energy to calculate water-surace elevations at each

surveyed cross section. Computed water-surace elevationsat successive cross sections are linked to provide a water-

surace prole or the ood o interest through the reach

o interest. Te computations are routinely accomplished

in standard sotware, such as the U.S. Army Corps o

Engineers’ HEC-RAS model. Whereas the analytical

methods described previously assume steady, uniorm ow

conditions through the reach, a detailed water-surace-

prole model is capable o handling both gradually and

(to some extent) rapidly varied ow 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 or the same reason. Burkham (1988)

indicates that the error in ood depths predicted rom

step-backwater analysis can be expected to be less than

20 percent. Te step-backwater computations require an

estimate o the Manning roughness coefcient (n) as an

indicator o resistance to ow and assume airly stable

channel boundaries. Estimation o the roughness coefcient

(n) includes the same considerations discussed previously

or the analytical methods. Te assumption o airly stable

channel boundaries is not always met with sand-bed

channels and is an issue o considerable importance or

designing subsurace pipeline crossings as well.

O the methods presented or determining elevation o oods or pipelines crossing channels, the detailed method

is the most accurate and should be used or situations

with high resource values, inrastructure investment,

construction costs, or liabilities in downstream areas.

In undeveloped areas, the physiographic and analytical

methods may be used to provide quick estimates o ood

elevations or sites with ewer downstream concerns. Te

reconnaissance method provides the roughest estimates but

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

alluvial ans or low relie valley oors (e.g., near playas). Te

detailed, analytical, and physiographic methods all assume

relatively stable channel boundaries but may be used on

sand channels with an accompanying loss o accuracy.

In very sandy channels, the accuracy o results rom the

detailed method may not be signicantly better than the

results rom one o the intermediate methods unless a

mobile-boundary model is used (Burkham 1988).



Subsurace (Buried) Crossings

Since many o the pipelines are small and most o the

channels are ephemeral, it is commonplace to bury the

pipelines rather than suspending them above the streams.

Te practice o burying pipelines at channel crossings likely

is both cheaper and easier than suspending them above all

oodows; however, an analysis o channel degradation and

scour should be completed to ensure the pipelines are not

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

Without such an analysis, channels should be excavated to

bedrock and pipelines placed beneath all alluvial material.

Figure . Channel degradation or scour during ash-ood eents may expose buried pipelines, resultingin costly breaks.

Buried pipelines may be exposed by streambed lowering

resulting rom channel degradation, channel scour, or a

combination o the two. Channel degradation occurs overa long stream reach or even the entire drainage network

and is generally associated with the overall lowering o the

landscape. Degradation also may be associated with changes

in upstream watershed or channel conditions that alter the

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

local phenomenon associated with passage o one or more

ood events or site-specic hydraulic conditions that may

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

expose buried pipelines to excessive orces associated with

extreme ow events, and an analysis o each is required to

ensure integrity o the crossing.

Channel Degradation

Detection o long-term channel degradation must be

attempted, even i there is no indication o local scour.

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

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



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— T E C H N I C A L N O T E 4 2 3

2

predictable sequence o channel changes that a stream

undergoes in response to disturbance in the channel or the

watershed. Many o these models are based on a “space or

time” substitution, whereby downstream conditions are

interpreted as preceding (in time) the immediate location

o interest, and upstream conditions are interpreted as

ollowing (in time) the immediate location o interest.Tus, a reach in the middle o 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 o the crossing is an option.

Most conceptual models o channel evolution have been

developed or landscapes dominated by streams with

cohesive banks; however, the same processes occur in

streams with noncohesive banks, with somewhat less

well-dened stages.

Geomorphic indicators o recent channel incision

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

stream terraces elevated above the water table) also may

be helpul or diagnosing channel conditions. However,

long-term trends in channel evolution are oten reversed

during major ood events, especially or intermittent and

ephemeral channels in arid and semiarid environments.

Tus, a stream that is degrading during annual and

intermediate ood events may be lled with sediment

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

ood, and channels that are associated with sediment

storage (i.e., aggrading) during the majority o runo

events may be “blown out” with major degradation during

unusual and extreme large oods.

In some situations, a quantitative analysis o channeldegradation may be warranted. Plots o streambed elevation

against time permit evaluation o bed-level adjustment

and indicate whether a major phase o channel incision

has passed or is ongoing. However, comparative channel

survey data are rarely available or the proposed location or

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 or one or

more selected discharges. Te procedure is called a specic-

gage 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, specic-gage

records indicate only the conditions in the vicinity o the

particular gaging station and do not necessarily reect river

response farther upstream or downstream of the gage. Terefore,

it is advisable to investigate other data in order to make

predictions about potential channel degradation at a site.

Figure . Specifc-gage plots o the gage heights associated with index ows through time may indicate general channel lowering in thedrainage basin (adapted rom Federal Interagency Stream Restoration Working Group 99; Biedenharn et al. 99).



Other sources o inormation include the biannual

bridge inspection reports required in all States or bridge

maintenance. In most States, these reports include channel

cross sections or bed elevations under the bridge, and a

procedure similar to specic gage analysis may be attempted

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

unctions or describing bed-level adjustments throughtime, tting elevation data at a site to either a power

unction or an exponential unction o time. Successive

cross sections rom a series o bridges in a basin also may

be used to construct a longitudinal prole o the channel

network; sequential proles 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 o those particular bridges

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

judiciously or sites upstream, downstream, or between the

bridges used in the analysis.

In the absence o channel surveys, gaging stations, and

bridge inspection reports (or other records o 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). echniques or assessing vertical

stability o the channel include incipient motion analysis,

analysis o armoring potential, equilibrium slope analysis,

and sediment continuity analysis. Incipient motion analysis

and analysis o armoring potential are equally applicable to

Figure . Plots o bed eleation ersus time may be deelopedrom biannual bridge inspection reports to document systemwide

degradation or aggradation (Federal Interagency Stream RestorationWorking Group 99).

Figure 9. Sequentiallongitudinal proflesalso may be used todocument channellowering through time(Federal Interagency

Stream RestorationWorking Group 9;Biedenharn et al.99).



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— T E C H N I C A L N O T E 4 2 3

4

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

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 subsurace

crossings, general channel scour must be addressed to

ensure saety o the pipeline. General scour is dierent rom

long-term degradation in that general scour may be cyclic

or related to the passing o a ood (Richardson and Davis

2001). Channel scour and ll processes occur naturally

along a given channel, and both reect the redistribution

o sediment and short-term adjustments that enable the

channel to maintain a quasi-equilibrium orm. In other words, channels in dynamic equilibrium experience various

depths o scour during the rising stages o a ood that

requently correspond to equal amounts o ll during the

alling stages, resulting in minimal changes in channel-bed

elevation. Where pipelines cross channels, it is important

to determine the potential maximum depth o scour so that

the pipeline is buried to a sufcient depth and does not

become exposed when bed scour occurs during a ood.

General scour occurs when sediment transport througha stream reach is greater than the sediment load being

supplied rom 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 or calculating contraction scour generally

all into two categories, depending on the inow o bed-

material sediment rom upstream. In situations where

there is little to no bed-material transport rom upstream

(generally coarse-bed streams with gravel and larger bedmaterials), 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., or most sand-bed streams), contraction scour

should be estimated using live-bed scour equations. Live-

bed and clear-water scour equations can be ound in many

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

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

the pipeline crossing must be added to the assessment o

channel degradation or estimating the depth o burial

or the crossing.

Other components o general scour can result rom

placement o subsurace crossings relative to the alignment

o the stream channel. Pipelines crossing at bends in thechannel are particularly troublesome, since bends are

naturally unstable and tend to collect both ice and debris

(which can cause additional constrictions in the ow).

Channel-bottom elevations are usually lower on the

outside o meander bends and may be more than twice

as deep as the average depth in straighter portions o the

channel. Crossings in the vicinity o stream conuences

also create difculties, since ood stages and hydraulic

orces may be strongly inuenced by backwater conditions

at the downstream conuence. For example, sediment

deposits rom tributary inputs may induce contraction

scour opposite or downstream o 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 o the structure, and

individual obstructions in the channel induce local

scour akin to pier scour characteristic o bridge piers at

highway crossings.

Even in the absence o contraction scour, general scour will

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

major oods. Since sand is easily eroded and transported,

interaction between the ow o water and the sand bed

results in dierent congurations o the stream bed with

varying conditions o ow. Te average height o dune

bedorms is roughly one-third to one-hal the mean ow

depth, and the maximum height o dunes may nearly equal

the mean ow depth. Tus, i the mean depth o ow in

a channel was 5 eet, maximum dune height could also

approach 5 eet, hal o which would be below the meanelevation o the stream bed (Lagasse et al. 2001). Similarly,

Simons, Li, and Associates (1982) present equations

or antidune height as a unction o mean velocity, but

limit maximum antidune height to mean ow depth.

Consequently, ormation o antidunes during high ows

not only increases mean water-surace elevation by one-hal

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

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



reported maximum general scour o one to two times the

average ow depth where two channels come together in a

braided stream.

Pipeline crossings that are buried rather than suspended

above all major ow events should address all o the

components o degradation, scour, and channel-lowering

due to bedorms described above. In addition, once a

determination is made on how deep to bury the pipeline

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

held constant across the oodplain. I the line is placed at

shallower depths beneath the oodplain, channel migration

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

the channel (Figure 10).

Figure 0. Lateral migration o this stream channel during high water excaated a section o pipeline under theoodplain that was seeral eet shallower than at the original stream crossing.

In complex situations or where consequences o pipeline

ailure are signicant, consideration should be given to

modeling the mobile-bed hydraulics with a numerical

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

1993) or BRI-SARS (Molinas 1990). Te Federal

Interagency Stream Restoration Working Group (1998)

summarizes the capabilities o these and other models and

provides reerences or model operation and user guides

where available.



Conclusion

Pipelines that cross perennial, intermittent, and ephemeral

stream channels should be constructed to withstand oods

o extreme magnitude to prevent rupture and accidental

contamination o runo during high ow events. Pipelines

crossing at the surace must be constructed high enough

to remain above the highest possible oodows at each

crossing, and pipelines crossing below the surace must

be buried deep enough to remain undisturbed by scour

and ll processes typically associated with passage o peak

ows. A hydraulic analysis should be completed during

the pipeline design phase to avoid repeated maintenance

o such crossings and eliminate costly repairs and potential

environmental degradation associated with pipeline breaks

at stream crossings.



Literature Cited

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selecting Manning’s roughness coecients or natural

channels and food plains. U.S. Geological Survey

Water-Supply Paper 2339. 38 pp.

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

1849. 213 pp.

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20

Tomas, B.E. and K.L. Lindskov. 1983. Methods or

estimating peak discharge and ood boundaries

o stream in Utah. U.S. Geological Survey Water-

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