Cover: Oblique aerial photograph of Glenn Highway bridge crossings of Eagle River, Alaska. 

Photograph by Jeffrey S. Conaway
Summary and Comparison of Multiphase 

Streambed Scour Analysis at Selected 

Bridge Sites in Alaska

By Jeffrey S. Conaway

Prepared in cooperation with the

Alaska Department of Transportation and Public Facilities

Scientific Investigations Report 2004-5066

U.S. Department of the Interior

U.S. Geological Survey
U.S. Geological Survey, Reston, Virginia: 2004

For sale by U.S. Geological Survey, Information Services

Box 25286, Denver Federal Center

Denver, CO 80225-0286

U.S. Department of the Interior

Gale A. Norton, Secretary

U.S. Geological Survey

Charles G. Groat, Director

For more information about the USGS and its products:

Telephone: 1-888-ASK-USGS

World Wide Web: http://www.usgs.gov/

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Conaway, J.S., 2004, Summary and comparison of multiphase streambed scour analysis at selected bridge sites in 

Alaska: U.S. Geological Survey Scientific Investigations Report 2004–5066, 34 p.
iii

Contents

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

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

Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Site Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Phase 1.5 Data Collection and Model Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Phase 2 Data Collection and Model Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Boundary Conditions and Computation of Water-Surface Profiles . . . . . . . . . . . . . . . . . . . . 6

Estimation Methods for Contraction Scour, Pier Scour, Complex Pier Scour, and 

Pressure Scour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Contraction Scour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Pier Scour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Complex Pier Scour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Pressure Scour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Hydraulic Modeling Results and Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Phase 1.5 and Phase 2 Analyses and Evaluation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Comparison of Phase 1.5 and 2 Analyses with Phase 1 Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Comparison of Hydraulic Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Contraction Scour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Pier Scour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
iv

Figures

Figure 1. Map showing location of bridges in Alaska selected for Phase 1.5 or 

Phase 2 analysis of susceptibility to streambed scour . . . . . . . . . . . . . . . . . . . . . . . . 2

Figure 2. Bar graphs showing change in streambed elevation at bridge 

between the lowest elevation streambed from the as-built bridge 

survey and the elevation measured for the Phase 1.5 or Phase 2 for 

selected bridge sites in Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 3. Bar graphs showing percentage of change between Phase 1 values 

and Phase 1.5 and Phase 2 values for channel slope and roughness 

at bridge for selected bridge sites in Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Figure 4. Graphs showing comparison of modeled Froude numbers at the bridge, 

determined by Phase 1 and Phase 1.5 analyses for the 100-year and 500-year 

recurrence-interval discharge for selected bridge sites in Alaska . . . . . . . . . . . . . . 18

Figure 5. Graphs showing comparison of modeled Froude numbers at the bridge, 

determined by Phase 1 and Phase 2 analyses for the 100-year and 500-year 

recurrence-interval discharge for selected bridge sites in Alaska . . . . . . . . . . . . . . 19

Figure 6. Bar graphs showing percentage of change in approach-channel width 

between the Phase 1 analysis and the Phase 1.5 and Phase 2 analyses

for selected bridge sites in Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Figure 7. Graphs showing comparison of contraction-scour depths computed 

by Phase 1 and Phase 1.5 analyses for the 100-year and 500-year 

recurrence-interval discharge and factor responsible for the difference 

in scour depths for selected bridge sites in Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

Figure 8. Graphs showing comparison of contraction-scour depths computed 

by Phase 1 and Phase 2 analyses for the 100-year and 500-year 

recurrence-interval discharges and factor responsible for the 

difference in scour depths for selected bridge sites in Alaska . . . . . . . . . . . . . . . . . 22

Figure 9. Graphs showing comparison of pier-scour depths computed by 

Phase 1 and Phase 1.5 analyses for the 100-year and 500-year 

recurrence-interval discharges and factor responsible for the 

difference in scour depths for selected bridge sites in Alaska . . . . . . . . . . . . . . . . . 23

Figure 10. Graphs showing comparison of pier-scour computed by Phase 1 

and Phase 2 analyses for the 100-year and 500-year recurrence-interval 

discharges and factor responsible for the difference in scour depths 

for selected bridge sites in Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
v

Tables

Table 1. Bridges in Alaska from the Phase 1 streambed scour analysis selected 

for further analysis, and the 100- and 500-year recurrence-interval flood 

discharges and depths of scour estimated in the Phase 1 analysis . . . . . . . . . . . . . 3

Table 2. Sensitivity analysis of channel-roughness values for six bridge sites 

in Alaska in the Phase 1.5 scour analysis where the calibration discharge

is less than 20 percent of the 100-year recurrence-interval discharge . . . . . . . . . . 11

Table 3. Summary of computed scour depths for 100- and 500-year 

recurrence-interval discharges for the Phase 1.5 and Phase 2 

streambed scour analyses at selected bridge sites in Alaska . . . . . . . . . . . . . . . . . . 13

Table 4. Selected hydraulic variables used to construct hydraulic models for 

analyses of streambed scour at selected bridge sites in Alaska. . . . . . . . . . . . . . . . 27

Table 5. Computed contraction-scour depths, and hydraulic variables used in 

computation for the 100-year and 500-year recurrence-interval flood 

discharges at selected bridge sites in Alaska. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Table 6. Computed pier-scour depths and hydraulic variables used in computation 

for the 100-year and 500-year recurrence-interval flood discharges at 

selected bridge sites in Alaska . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Table 7. Computed pressure-scour depths and hydraulic variables used in 

computation for the 100-year and 500-year recurrence-interval flood 

discharges at selected bridge sites in Alaska. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Table 8. Computed pier-stem scour depths, and hydraulic variables used in 

computation for the 100-year and 500-year recurrence-interval flood 

discharges at selected bridge sites in Alaska. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
vi

CONVERSION FACTORS AND DATUM

CONVERSION FACTORS

Multiply By To obtain

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per second per square mile [(ft3/s/mi2] 0.01093

cubic meter per second per square

kilometer [(m3/s/km2]

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

square mile (mi2) 259.0 hectare

2.590 square kilometer (km2)

DATUM

Datum: Local datums used in this report were arbitrary set at the time of the fieldwork or by the 

Alaska Department of Transportation and Public Facilities and are not necessarily referenced to 

any National Geodetic Vertical Datum. 
Summary and Comparison of Multiphase Streambed 

Scour Analysis at Selected Bridge Sites in Alaska

By Jeffrey S. Conaway

Abstract

The U.S. Geological Survey and the Alaska Department 

of Transportation and Public Facilities undertook a cooperative 

multiphase study of streambed scour at selected bridges in 

Alaska beginning in 1994. Of the 325 bridges analyzed for 

susceptibility to scour in the preliminary phase, 54 bridges 

were selected for a more intensive analysis that included site 

investigations. Cross-section geometry and hydraulic 

properties for each site in this study were determined from field 

surveys and bridge plans. Water-surface profiles were 

calculated for the 100- and 500-year floods using the 

Hydrologic Engineering Center’s River Analysis System and 

scour depths were calculated using methods recommended by 

the Federal Highway Administration. 

Computed contraction-scour depths for the 100- and 

500-year recurrence-interval discharges exceeded 5 feet at six 

bridges, and pier-scour depths exceeded 10 feet at 24 bridges. 

Complex pier-scour computations were made at 10 locations 

where the computed contraction-scour depths would expose 

pier footings. Pressure scour was evaluated at three bridges 

where the modeled flood water-surface elevations intersected 

the bridge structure.

Site investigation at the 54 scour-critical bridges was used 

to evaluate the effectiveness of the preliminary scour analysis. 

Values for channel-flow angle of attack and approach-channel 

width were estimated from bridge survey plans for the 

preliminary study and were measured during a site 

investigation for this study. These two variables account for 

changes in scour depths between the preliminary analysis and 

subsequent reanalysis for most sites. Site investigation is 

needed for best estimates of scour at bridges with survey plans 

that indicate a channel-flow angle of attack and for locations 

where survey plans did not include sufficient channel geometry 

upstream of the bridge. 

Introduction

The U.S. Geological Survey (USGS), in cooperation with 

the Alaska Department of Transportation and Public Facilities 

(ADOT&PF), began studying the susceptibility of Alaskan 

bridges to streambed scour in 1994. A multiphase approach 

was applied to bridges selected by ADOT&PF as potentially 

susceptible to scour. Heinrichs and others (2001) documented 

results from the initial phase of this project, referred to 

hereafter as Phase 1 analysis. Phase 1.5 and Phase 2 analyses 

were completed in 2002.

Phase 1 evaluated streambed scour for the 100- and 

500-year recurrence-interval floods at 325 of Alaska’s 

approximately 800 bridges (Heinrichs and others, 2001). In this 

initial analysis, the step-backwater water-surface profile 

(WSPRO) model (Shearman, 1990) was used to construct 

hydraulic models of 100- and 500-year discharges using 

channel geometry determined from existing bridge plans and 

either assumed or, when available, measured hydraulic 

properties. Hydraulic variables computed by these models were 

used to calculate contraction and pier scour using federally 

recommended techniques and equations. From these initial 

results, ADOT&PF selected 54 sites (fig. 1, table 1) for further 

evaluation consisting of detailed field surveys and new 

hydraulic models and scour analyses. The addition of field data 

allowed for an updated evaluation of scour and documented 

changes in channel geometry since the bridge plans were 

surveyed.

Sites from the Phase 1 analysis were selected for either 

Phase 1.5- or Phase 2-level analysis. These two levels of 

investigation are distinguished by the quantity of field data 

collected; the Phase 1.5 analysis was the less intensive. 

Division into these groups was based on the magnitude of 

existing scour, scour depths calculated for the Phase 1 analysis, 

and data that were available for the Phase 1 analysis. No sites 

were evaluated at all three levels of analysis.
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2 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Figure 1. Location of bridges in Alaska selected for Phase 1.5 or Phase 2 analysis of susceptibility to streambed scour.
Introduction 3

Table 1. Bridges in Alaska from the Phase 1 streambed scour analysis selected for further analysis, and the 100- and 500-year recurrence-interval flood 

discharges and depths of scour estimated in the Phase 1 analysis

[Data from Heinrichs and others (2001). ADOT & PF No.: Alaska Department of Transportation and Public Facilities. ft3/s, cubic feet per second; ft, feet. 

–, no data]

ADOT & 

PF No. Bridge name Route name

Road 

mile 

point

Year 

built

Discharge

(ft3/s)

Depth (ft)

Contraction 

scour

Pier scour

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

Phase 1.5

205 Copper River near Chitina McCarthy Road 34.6 1971 126,000 151,000 1.0 1.1 13.7 14.0

240 Little Susitna River Parks Highway 57.1 1964 3,800 4,570 .0 .0 10.3 10.8

277 Taylor Creek Taylor Highway 50.3 1977 1,350 2,360 4.4 .9 no pier no pier

308 Skagway River Klondike Highway 1.7 1974 18,500 27,500 – – – –

396 Deception Creek Fishhook/Willow Road 48.4 1972 1,600 2,060 .3 .3 3.6 3.8

401 Moose Creek Petersville Road 7.1 1974 4,360 7,010 2.0 9.6 no pier no pier

505 Tanana River Near Tok Alaska Highway 1,303.3 1944 51,900 59,700 .0 .0 28.1 28.7

509 Robertson River Alaska Highway 1,345.3 1944 9,520 11,700 .0 .0 21.6 22.4

518 Johnson River Alaska Highway 1,380.4 1944 7,520 9,250 .0 .0 26.6 28.1

520 Gerstle River Alaska Highway 1,392.7 1944 5,100 6,340 .1 .2 18.6 19.2

530 Little Salcha River Richardson Highway 328.4 1967 3,130 3,920 1.5 2.2 12.2 13.5

543 Granite Creek Glenn Highway 62.4 1958 1,920 2,390 .4 .5 17.0 18.1

544 Kings River Glenn Highway 66.4 1961 7,600 9,310 2.0 2.4 21.6 23.1

547 Hicks Creek Glenn Highway 96.5 1956 1,370 1,700 .0 .0 12.0 12.5

548 Caribou Creek Glenn Highway 106.9 1953 9,550 11,800 4.5 5.0 34.5 36.6

556 Valdez Glacier Stream Richardson Highway 0.9 1965 24,400 30,600 5.8 7.3 9.7 11.7

572 Klutina River Old Richardson 0.4 1957 10,700 11,800 .6 .5 16.1 16.3

573 Tazlina River Richardson Highway 110.7 1973 79,400 109,000 2.9 2.8 34.6 37.2

603 Snow River West Channel Seward Highway 17.1 1965 2,100 2,600 .6 .7 9.3 9.8

658 Little Tok River Old Tok Highway 7.5 1951 9,430 11,200 .0 .0 4.5 4.7

678 Little Goldstream Creek Parks Highway 314.8 1958 2,260 2,870 2.5 1.8 3.5 3.7

686 Clearwater Creek Denali Highway 55.9 1957 4,420 5,400 1.1 1.6 12.4 13.4

687 Susitna River Denali Highway 79.2 1956 37,600 48,200 .3 .5 18.7 19.5

690 Seattle Creek Denali Highway 110.9 1954 3,090 5,730 5.9 .0 no pier no pier

742 Chilkat River Haines Highway 23.8 1958 32,600 42,900 .3 .3 4.5 4.8

832 Boulder Creek Steese Highway 125.3 1971 2,130 3,840 .7 .3 no pier no pier

833 Albert Creek Steese Highway 131.2 1978 3,890 4,860 2.2 2.8 no pier no pier

999 Glacier Creek Alyeska Road 2.3 1967 7,020 10,200 3.3 4.4 12.5 14.3

1220 Cowee Creek Glacier Highway 1.9 1971 9,870 12,400 3.2 .7 4.9 5.0

1261 Middle Fork Koyukuk River No. 1 Dalton Highway 188.5 1972 25,100 28,500 .9 1.0 8.2 8.4

1282 Middle Fork Koyukuk River No. 2 Dalton Highway 190.8 1972 20,300 23,400 .3 .4 13.1 13.7

1283 Middle Fork Koyukuk River No. 3 Dalton Highway 204.3 1972 10,300 12,000 1.5 3.1 3.4 3.4

1284 Middle Fork Koyukuk River No. 4 Dalton Highway 204.5 1972 7,880 9,180 1.5 1.6 4.6 4.8

1329 Little Chena River Nordale Road 3 1975 7,910 11,800 .0 .0 5.1 5.8

1389 No Name creek near Seward Exit Glacier Road 4.9 1994 980 1,230 .1 .1 3.6 3.7
4 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Background

Streambed scour is the leading cause of bridge failure in 

the United States (Murillo, 1987). The costs associated with 

restoring damaged structures are substantial, but are less than 

five times the indirect costs associated with the disruption of 

traffic (Rhodes and Trent, 1999). These costs and the societal 

repercussions are even greater in Alaska, where alternate 

ground transportation routes between many cities do not exist. 

Bridge and culvert damage associated with the effects of flow 

hydraulics occurs several times in Alaska every year. 

Quantifying the susceptibility of these structures to streambed 

scour helps to prioritize mitigation, monitoring, and redesign 

efforts.

Streambed scour at bridges results from the complex 

hydraulic conditions created either by the contraction of flow 

through a bridge or by the interaction of flow with bridge piers 

and abutments that results in the hydraulic erosion of the 

channel bed or banks. Streambed scour at bridges is separated 

into general scour, contraction scour, and local scour. General 

scour is the natural channel degradation and lateral erosion that 

occur regardless of the bridge structure. Contraction scour 

results from the decrease in channel width through a bridge 

reach and the attendant increase in flow velocity and sediment 

transport capability. Local scour at piers results from 

horseshoe vortices that form at the upstream, downstream, and 

sides of piers. Literature on streambed scour is copious; 

Richardson and Lagasse (1999) have edited a compendium on 

the subject that provides a thorough overview of historical and 

current research.

Purpose and Scope

The USGS developed a multiphase approach for the 

analysis of streambed scour at Alaska’s bridges. ADOT&PF 

used the data from Phase 1 to identify scour-susceptible 

structures for further analysis. This report presents Phase 1.5 

and Phase 2 estimations of contraction and pier scour derived 

from hydraulic modeling of 100- and 500-year floods at 

bridges identified as scour critical. The hydraulic models were 

constructed from existing data and field data collected for this 

study. The methodologies for data collection, model 

generation, and scour estimates are described and evaluated. 

Phase 2

233 Chena River at 40 mile Chena Hot Springs Road 39.5 1967 22,100 27,400 8.3 15.9 8.0 8.8

242 North Fork Chena River Chena Hot Springs Road 55.3 1967 4,220 5,220 .8 .8 4.5 4.7

254 Susitna River at Sunshine Parks Highway 104.2 1965 186,000 206,000 6.2 6.5 16.7 17.3

255 Chulitna River Parks Highway 97.5 1970 83,200 103,000 .4 .4 23.8 25.0

355 Birch Creek Steese Highway 147.1 1957 36,000 42,000 .6 .8 9.1 9.5

527 Salcha River Richardson Highway 323.3 1967 50,600 64,900 3.3 3.9 20.7 21.7

535 Eagle River Glenn Highway 140.1 1981 6,920 8,710 2.1 3.0 9.9 10.6

539 Knik River Old Glenn Old Glenn Highway 8.6 1975 79,400 104,000 11.7 13.4 10.8 11.7

557 Lowe River Richardson Highway 14.8 1974 22,100 28,500 1.1 1.5 7.2 7.7

558 Lowe River Richardson Highway 16.5 1985 19,400 25,100 .3 .3 17.2 18.3

574 Gulkana River Richardson Highway 126.9 1974 18,400 22,200 5.3 6.3 6.8 6.9

608 Ptarmigan Creek Seward Highway 23.1 1952 1,610 2,070 .1 .1 6.7 7.0

609 Falls Creek Seward Highway 25 1951 1,220 1,750 .5 .6 3.4 3.7

694 Nenana River at Park Bend Creek Parks Highway 231.2 1973 25,400 30,300 .2 .2 12.5 12.9

1025 Resurrection Creek Hope Road 16.9 1969 4,590 6,520 1.8 2.4 7.5 8.1

1147 Nenana River at Park Station Parks Highway 237.9 1970 43,200 53,600 .8 1.7 9.9 10.3

1255 Fish Creek Dalton Highway 114 1972 3,800 4,710 1.4 1.9 9.5 10.1

1341 Eagle River Glenn Highway 139.9 1974 6,920 8,710 1.8 1.7 12.9 13.6

1513 Ft. Hamlin Hills Dalton Highway 72.6 1982 1,830 2,330 1.8 2.6 no pier no pier

Table 1. Bridges in Alaska from the Phase 1 streambed scour analysis selected for further analysis, and the 100- and 500-year recurrence-interval flood 

discharges and depths of scour estimated in the Phase 1 analysis–Continued

[Data from Heinrichs and others (2001). ADOT & PF No.: Alaska Department of Transportation and Public Facilities. ft3/s, cubic feet per second; ft, feet. 

–, no data]

ADOT & 

PF No. Bridge name Route name

Road 

mile 

point

Year 

built

Discharge

(ft3

/s)

Depth (ft)

Contraction 

scour

Pier scour

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year
Methodology 5

Methodology and results from this study are compared with 

those of the preliminary Phase 1 analysis of Heinrichs and 

others (2001), which did not include the collection of field data 

and used less intensive hydraulic modeling software.

Methodology

The Phase 1.5 and Phase 2 analyses built upon existing 

information by providing current field data. These data were 

used to construct and calibrate a one-dimensional surfacewater model capable of computing water-surface elevations 

and hydraulic variables necessary to estimate scour at bridges. 

The models were used to estimate scour for 100- and 500-year 

floods using techniques outlined by the Federal Highway 

Administration’s Hydraulic Engineering Circular No. 18, 

hereafter referred to as HEC-18 (Richardson and Davis, 2001). 

Hydraulic variables needed to calculate scour were computed 

with the Hydrologic Engineering Center River Analysis 

System (HEC-RAS; Brunner, 2001), which is a onedimensional surface-water modeling program.

Existing data for the sites in this study consisted of the 

Phase 1 analysis and the bridge as-built plans that are provided 

by ADOT&PF. The as-built plans generally include a detailed 

topographic map of the channel near the bridge at the time of 

construction. Other possible sources of existing data include 

discharge measurements, streamflow-gaging station records, 

indirect computations of discharge, or ADOT&PF bridge 

inspections. The discharges for the 100- and 500-year floods 

for each river crossing in this study (table 1) were estimated by 

Heinrichs and others (2001) using standard flood-frequency 

analyses (Interagency Advisory Committee on Water Data, 

1982) and regional regression equations as outlined by Jones 

and Fahl (1994). 

Site Selection

The results from the Phase 1 analysis were used to select 

sites for more intensive study. A site was selected for further 

study if it met any of the following criteria.

• Phase 1 estimates of contraction scour were greater 

than 5 ft.

• Phase 1 estimates of local scour at piers were greater 

than 10 ft. 

• The water-surface profile calculated during Phase 1 

intersected the bridge at the 100- and(or) 500-year 

discharge (pressure flow).

• Bridge as-built plans or discharge measurements 

indicate high channel-flow angles of attack at the 

bridge piers.

• Piers are set on shallow or exposed footings.

• Bridge inspections reported scour.

• Channel data available from bridge as-built plans were 

insufficient for accurate Phase 1 scour analysis.

Sites were selected for study at the Phase 1.5 level if it met 

any of the following criteria.

• They lacked detailed bridge plans,

• Slope could not be accurately estimated, 

• Channel-flow angle of attack determined from bridge 

plans was significant and needed verification. 

• Phase 1 hydraulic modeling indicated pressure-flow 

conditions. 

Thirty-five bridges were selected for Phase 1.5 analysis 

(table 1). Bridges with large calculated scour depths from the 

Phase 1 study or those with known potential for significant 

scour during extreme floods were selected for Phase 2 analysis. 

Nineteen bridges were selected for Phase 2 analysis (table 1). 

Phase 1.5 Data Collection and Model 

Parameters 

Standard field data collected for a Phase 1.5 analysis were 

a discharge measurement, bridge sounding, water-surface 

slope, site sketch, and photographs. Data collection was 

abbreviated at sites that had recent bridge as-built plans, bridge 

inspections, or discharge measurements. Discharge was not 

measured at some locations because the discharge at the time 

of the site visit was insufficient to calibrate a hydraulic model. 

At a minimum, the channel-flow angle of attack was field 

verified and the channel slope was measured. 

Hydraulic models were constructed using a combination 

of field data and the bridge as-built plans. Bridge soundings or 

discharge-measurement data, in combination with geometry 

from the bridge as-built plans, were used to construct cross 

sections immediately upstream and downstream of the bridge. 

Channel-geometry data from the cross section at the bridge also 

were integrated with overbank data from the bridge as-built 

plans or the field survey to construct a full-valley cross section. 

This section was copied two times upstream and two times 

downstream of the bridge using a surveyed water-surface slope 

to adjust cross-section elevations used to construct the model. 

Cross sections typically were separated by a distance of one 

bridge opening. This spacing usually was sufficient to span the 

difference over which contraction and expansion of flow 

occurred in response to the bridge structure. At locations where 

the channel constricts through the bridge, the sounding or 

discharge-measurement data were expanded laterally to 

integrate with the approach and exit overbank data.

The Phase 1 study assumed Manning’s roughness values 

of 0.035 and 0.10 for the channel and overbanks, respectively, 

unless other data sources were available. These default values 

are a good approximation for Alaskan streams and were 

retained for Phase 1.5 locations unless a discharge 

measurement and surface-water slope were available. Slope 

and discharge-measurement data and the Manning equation 
6 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

can be used to calculate a roughness coefficient for the 

channel. When photographs were available, overbank 

roughness values were estimated using techniques outlined by 

Hicks and Mason (1991). 

Phase 2 Data Collection and Model Parameters

The comprehensive field survey for Phase 2 sites included 

surveying channel cross sections at the bridge, in the approach 

to the bridge, and the exit from the bridge. Channel data 

obtained by boat or wading were integrated with bank and 

overbank data to complete each cross section. The number of 

surveyed cross sections varied, but a minimum of three was 

obtained at each bridge. Ideally, cross sections were separated 

by a distance equivalent to the bridge opening width, but this 

distance can vary depending on field conditions. When 

surveyed cross sections were insufficient to construct the 

hydraulic model, the field data were copied using the surveyed 

water-surface slope in the approach and exit sections as 

needed. All surveys were tied to the vertical datum from the 

bridge as-built plans.

In addition to surveying cross sections, data collection for 

Phase 2 included measuring discharge and surveying the 

corresponding water-surface slope, observing site 

geomorphology, channel-flow angle of attack at the piers, 

current scour conditions, and initial estimation of roughness 

coefficients for the channel and overbanks, and describing 

streambed material.

Roughness coefficients for the Phase 2 sites were 

calculated using the same procedures that were used for the 

Phase 1.5 sites. Roughness values were further calibrated with 

HEC-RAS. 

Boundary Conditions and Computation of 

Water-Surface Profiles 

HEC-RAS was used to calculate water-surface profiles 

for the Phase 1.5 and Phase 2 analyses. HEC-RAS is based on 

the solution of the one-dimensional energy equation and is 

capable of modeling flow regimes that are subcritical, 

supercritical, and a combination of the two (mixed flow 

regime). Energy losses are evaluated with roughness 

coefficients and contraction and expansion coefficients. The 

momentum equation is used in place of the energy equation 

where the flow varies greatly, including hydraulic jumps, 

bridge constrictions, pressure flow, and locations where flow 

transitions between subcritical and supercritical. Hydraulic 

variables used to construct the models are summarized in

table 4 (at back of report).

Water-surface profiles were calculated for the 100- and 

500-year floods and for calibration discharges. The calibration 

discharge was either a discharge measurement made during the 

field visit for the Phase 1.5 or Phase 2 analysis or an existing 

high-flow direct or indirect discharge measurement. The stepbackwater models for the Phase 1.5 and Phase 2 analyses were 

identical with the exception of the calibration process. 

The Phase 1.5 and Phase 2 models were constructed and 

calibrated using the following procedure.

1. Entered cross-section geometry and defined overbanks on 

the basis of field observations or slope breaks. Model 

geometry also includes deck, abutment, and pier 

elevations and dimensions that were obtained from 

existing bridge as-built plans. 

2. Assigned computed or estimated roughness coefficients 

to each cross section as a composite value or separately 

for the channel and overbanks. 

3. Defined boundary conditions. Boundary conditions were 

set at the downstream cross section for subcritical flow 

regime and at the downstream and upstream cross 

sections for the mixed flow regime. Water-surface slope 

was used for the normal-depth boundary condition for 

the Phase 1.5 models. If the slope was not available, 

either a friction slope was calculated from a discharge 

measurement or a slope from a topographic map was 

used. For Phase 2 models, the surveyed water-surface 

elevation at the downstream cross section was used as a 

boundary condition for the calibration discharge. 

4. Calculated initial water-surface profiles and calibrated 

the model to measured values. For Phase 1.5 models, the 

average velocity, flow area, and flow width of the 

calibration discharge were compared with the measured 

values. Channel-roughness coefficients were adjusted 

until agreement was reached between modeled and 

measured values. Phase 2 models were calibrated by 

adjusting roughness coefficients at each cross section 

until the modeled water-surface elevation for the 

calibration discharge matched the surveyed watersurface elevation at each cross section. After roughness 

coefficients for the channel were calibrated and the 

model was rerun, the energy-gradient slope at the 

downstream cross section was calculated and used as 

the normal-depth boundary condition for models of the 

100- and 500-year floods. 

5. Calculated water-surface profiles for the 100- and 

500-year floods with the calibrated model. If Froude 

numbers exceeded 1.0 for any cross section, an 

upstream boundary condition (the slope of the energy 

gradient at this section, determined from the calibration 

discharge) was established and the model was rerun 

using the mixed flow regime. Interpolated cross sections 

were inserted between existing sections if the water 

surface varied greatly, the channel expands or contracts 

rapidly, or the gradient changed abruptly. 

6. Computed final water-surface profiles and hydraulic 

variables necessary for the scour computations.
Estimation Methods for Contraction Scour, Pier Scour, Complex Pier Scour, and Pressure Scour 7

Estimation Methods for Contraction 

Scour, Pier Scour, Complex Pier 

Scour, and Pressure Scour 

Estimates of scour were computed using the equations and 

methodology outlined in HEC-18 (Richardson and Davis, 

2001). Contraction scour was computed at all sites, pier scour 

was computed at crossings supported by piers, complex pier 

scour was computed if the foundation of the pier was wider 

than the pier and was exposed to the flow, and pressure scour 

was computed at locations where a modeled water-surface 

elevation intersected the bridge structure. Abutment scour was 

not evaluated because of the large computational uncertainties 

and because most abutments on Alaskan bridges are armored 

with riprap to inhibit scour (Heinrichs and others, 2001). Flow 

widths, depths, and velocities were calculated with HEC-RAS 

and were used to compute the scour estimates for the 100- and 

500-year floods. The reference surface for all calculations was 

the streambed elevation at the bridge determined from the asbuilt survey plans. 

Contraction Scour

Contraction scour at the bridges in this study was 

evaluated with the live-bed contraction-scour equation. Under 

live-bed conditions, contraction scour in the bridge section 

reaches a maximum when sediment transport into the 

contracted section equals sediment transport out or when the 

mean velocity equals the critical velocity of the mean-diameter 

bed material. This equilibrium is reached when the transport 

capacity in the contracted section decreases because of 

increasing channel area and the attendant decrease in flow 

velocity. At constrictions that result from bridge crossings, 

cross-section width generally cannot increase, so channel area 

is increased by degradation of the channel. The equation 

recommended in HEC-18 (Richardson and Davis, 2001) for 

estimating live-bed contraction scour is a modified version of 

an equation developed by Laursen (1960) that simplifies 

sediment transport functions to balance sediment transport 

through to contracted section with sediment transport through 

the approach section. 

ys y1

Q2

Q1

------ ⎝ ⎠ ⎛ ⎞

6

7

-- W1

W2

------- ⎝ ⎠ ⎛ ⎞ k1

y0 = – , (1)

where

ys is the contraction scour depth, in feet;

y1 is the average depth in the upstream main 

channel, in feet;

y0 is the existing depth in contracted section before 

scour, in feet;

Q1 is the discharge in the main channel of the 

approach section that is transporting sediment, in 

cubic feet per second;

Q2 is the discharge in the contracted section, in cubic 

feet per second;

W1 is the width of the main channel of the approach 

section that is transporting sediment, in feet;

W2 is the width of the main channel in the contracted 

section that is transporting sediment, in feet; and

k1 is a coefficient that accounts for the method of 

sediment transport. For this study, transport at all 

sites is assumed to be mostly in contact with bed 

material and the coefficient for this condition is 

0.59.

The estimated contraction scour depth in equation 1 (ys) is 

the difference between the maximum flow depth in the 

contracted section after scour has occurred and the flow depth 

that existed prior to any scour (y0). Estimation of y0 is difficult 

because the channel geometry in the contracted section at the 

time of the study likely had already been modified by 

contraction scour. The channel depth in the approach section 

(y1) can be used as an approximation of y0, based on the 

assumption that channel depth in the uncontracted approach 

section is a good representation of channel depth in the bridge 

section before any contraction scour occurred. The equation 

used to estimate contraction scour for this study and for the 

Phase 1 analysis is

ys y1

Q2

Q1

------ ⎝ ⎠ ⎛ ⎞

6

7

-- W1

W2

------- ⎝ ⎠ ⎛ ⎞ k1

y1 = – , (2)

where all terms have been previously defined.

The approach section used in the contraction-scour 

equation was located at least one bridge-opening width 

upstream of the bridge. If the channel contraction begins 

upstream of this cross section, the next upstream section that is 

above the constriction was used in the computation. 

Pier Scour

Estimates of pier scour depend on the flow characteristics 

directly upstream of the bridge pier, the characteristics of the 

bed material, and the geometry of the pier and its footing. The 

equation recommended in HEC-18 for pier scour includes 

correction factors for pier shape, channel-flow angle of attack, 

bed configuration, and the armoring effect of large-size bed 

material. The pier-scour equation is described in detail in 

HEC-18 (Richardson and Davis, 2001) and is summarized 

here. The equation for local scour at a pier is

ys 2.0K1K2K3K4a

0.65

y1

0.35Fr10.43 = , (3)
8 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

where

ys is the pier scour depth, in feet;

K1 is the correction factor for pier nose shape;

K2 is the correction factor for channel-flow angle of 

attack;

K3 is the correction factor for channel bed form;

K4 is the correction factor for armoring of bed 

material;

a is the pier width, in feet;

y1 is the flow depth directly upstream of the pier in 

feet; and

Fr1 is the Froude number just upstream from the pier.

For round-nosed piers that are aligned with the flow, 

scour is limited to 2.4 times the pier width for Froude numbers 

less than 0.80 and 3.0 times the pier width for Froude numbers 

equal to or in excess of 0.80 (Richardson and Davis, 2001). 

The correction factor for channel-flow angle of attack, K2, can 

be computed with

K2 Cosθ La–1 ( ) + Sinθ

0.65 = , (4)

where

L is the length of the pier along the direction of 

flow, in feet;

q is the channel-flow angle of attack, in degrees; 

and 

all other terms are previously defined.

The correction factors for channel bedform and armoring 

of the bed material were constant for the sites used in this 

study. The correction factor for channel bedform, K3, is 1.1 for 

bedforms with a magnitude less than 10 ft. This condition was 

assumed to exist at all locations in this study. The correction 

factor for armoring of the bed, K4, is applied when the median 

diameter of the bed material is greater than 0.006 ft. This 

correction decreases the estimation of pier scour to account for 

armoring of the scour hole by large bed material. This 

condition could occur at some locations in this study, but 

grain-size estimations for this study were not accurate enough 

to apply this correction factor. Not applying this factor to 

locations where bed material armors scour holes will result in 

a more conservative estimate (overestimate) of scour. For this 

study, equation 4 was reduced to 

ys 2.2K1K2a

0.65

y1

0.35Fr10.43 = , (5)

where all terms have been previously defined. 

Complex Pier Scour

Local scour at a pier is affected by the pier’s foundation if 

the foundation is wider than the pier and is exposed to the flow. 

For situations where the footing is located at or below the 

streambed, the footing can limit local scour at the pier by 

disrupting the flow vortices induced by the pier (Parola and 

others, 1996; Melville and Coleman, 2000). When the footing 

is exposed to the flow, it can induce vortices that will cause 

scour in front of and along side of the foundation (Parola and 

others, 1996; Melville and Coleman, 2000). 

HEC-18 recommends independent calculation of scour 

for the pier stem, the pier foundation, and any piles supporting 

the foundation. These scour components are then summed to 

determine complex pier scour. Estimates of complex pier scour 

were made at sites with observed exposed footings and for sites 

where the streambed elevation was lower than the top of the 

footing after estimated contraction scour for the 100- and 

500-year floods was subtracted from the lowest point in the asbuilt cross section. For this study, scour was estimated for the 

pier and foundation components, but not for the pile-group 

component. Computing the pile-group component was 

unnecessary for the sites in this study because either the pile 

group was not exposed by the estimated contraction scour or 

the foundation is surrounded by sheet piling. The equation 

recommended by HEC-18 for the pier-stem scour component 

of complex scour is similar to equation 5 and is expressed as 

follows.

ys pier kh pier2.2K1K2a

0.65

y1

0.35Fr10.43 = , (6)

where

ys pier is the scour component of the pier stem, in 

feet; 

Kh pier is a coefficient to account for the height of 

the pier base above the bed and the shielding 

affect of the footing; and

all other terms are previously defined.

The equation to determine Kh pier is

Kh pier 0.4075 0.0669 f a–1 = ( ) –

0.1615 0.0455 f a–1 + ( ) – h1 a–1 ( )2

 0.4271 0.0778 f a–1 ( ) – h1 a–1 –

 0.269 0.012 f a–1 ( ) – h1 a–1 ( )3 – , (7)
Estimation Methods for Contraction Scour, Pier Scour, Complex Pier Scour, and Pressure Scour 9

where

f is the distance between front edge of the pier 

foundation and pier stem, in feet; 

h1 is the height of the pier base above the bed, in 

feet; and 

all other terms are previously defined.

The scour estimation for the pier foundation treats the 

foundation like a short, wide pier and uses equation 5, but the 

exposed footing height is used for flow depth and the approach 

velocity is the average velocity in the segment of flow 

intersecting the foundation. The average velocity in the water 

column intersecting the foundation is estimated by

Vf V2

ln 10.93 yf

Ks

----- + 1 ⎝ ⎠ ⎛ ⎞

ln 10.93 y2

Ks

----- + 1 ⎝ ⎠ ⎛ ⎞

= --------------------------------------- , (8)

where

Vf is the average velocity in the segment of flow 

intersecting the foundation, in feet per second;

V2 is V1(y1y2

-1), the average adjusted velocity 

upstream of the pier, in feet per second; V1 is the 

approach velocity, in feet per second, and y1 is 

the approach flow depth, in feet;

yf is h1 + 0.5 (ys pier), the distance from the bed to 

the top of the footing after contraction scour and 

pier scour, in feet;

Ks is the D84 (the particle size for which 84 percent 

are finer by weight) of the bed material, in feet

(default of 0.16 ft for this study); and

y2 is y1 + 0.5 (ys pier), the adjusted flow depth 

upstream of the pier including contraction scour 

and one-half of the pier stem scour, in feet;

The equation to estimate scour for the foundation is

ys found 2.2K1K2a

0.65

yf

0.35 Vf

gyf

-----------

⎝ ⎠ ⎜ ⎟ ⎛ ⎞ 0.43

= , (9)

where

ys found is the scour component for the pier 

foundation exposed to the flow, in feet; 

g is the constant for acceleration due to 

gravity, in feet per second squared; and 

all other terms are previously defined.

Complex pier scour is then the sum of the pier-stem component 

and the pier-foundation component

ys found ys pier ys found = + , (10)

where

ycps is the total scour depth of the pier stem and its 

foundation, in feet; and

all other terms have been previously defined.

Pressure Scour

Free-surface flow changes to pressure-flow conditions 

when rising floodwaters intersect or submerge the bridge deck, 

resulting in an additional component of scour. This term, 

pressure scour, is evaluated using an equation developed by 

Arneson and Abt (1999) for live-bed conditions. The equation 

is 

yps y1 – 5.08 + 1.27 y1

Hb

------ ⎝ ⎠ ⎛ ⎞ =

+4.44

Hb

y1

------ 0.19

Va

Vc

+ -----

, (11)

where

yps is the depth of scour resulting from the pressure 

flow condition, in feet;

Hb is the distance from the bridge low steel to the 

average streambed elevation before scour, in 

feet;

Va is the average velocity inside the bridge before 

scour, in feet per second; and

Vc is the incipient motion velocity of the D50 of the 

bed material, in feet per second.

Vc is defined by

Vc 11.17y1

0.17D500.33 = , (12)

where 

D50 is the particle size for which 50 percent are finer 

by weight; and 

all other terms are previously defined.

Under pressure-flow conditions, the magnitude of pier 

scour is approximately the same as it is under free-surface flow 

conditions (Jones and others, 1999). Pressure-flow scour is 

computed after lateral contraction scour is estimated (Arneson 

and Abt, 1999). For this study, contraction scour was evaluated 

independent of the pressure flow conditions by modeling the 

flood flows without a bridge deck for the flows to intersect. The 

contraction scour that occurred under this condition was then 

subtracted from the cross sections at the bridge. The bridge 

deck then was included in another hydraulic simulation with 

the modified cross sections at the bridge to evaluate pressure 

scour after the contraction scour had occurred. Total scour at 

pressure-flow sites is then the sum of the local pier scour, the 

contraction scour, and the pressure scour calculated using 

equation 11 (Arneson and Abt, 1999).
10 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Hydraulic Modeling Results and Evaluation

The hydraulic models were constructed and calibrated 

with existing information and with data collected at the time of 

the field visit. The hydraulic parameters used to generate the 

surface-water models and the hydraulic data needed for the 

computation of scour are presented in tables 4-8 (at back of 

report). The calibrated models then were extrapolated to 

accommodate the 100- and 500-year flood flows. Simulations 

of large flood flows with models calibrated to smaller 

discharges can result in hydraulic inaccuracies. 

The magnitude of the 100- and 500-year discharges and 

the geometry of the bridge openings can create scenarios that 

are difficult to model with HEC-RAS or other programs. 

Supercritical flow, velocities in excess of 10 ft/s, hydraulic 

jumps, and pressure flow are conditions found at many sites in 

this study that can vary three-dimensionally in both time and 

length scales. For highly varied flow, the one-dimensional 

modeling solution averages the momentum equation over the 

channel instead of computing the velocity at each point. The 

velocity at a pier could be greater or less than this averaged 

velocity. One-dimensional models capture a small portion of 

the active processes in the channel, but are efficient at making 

predictions over long length and time scales (Nelson and 

others, 2003). The hydraulic variables generated by HEC-RAS 

for adverse flow conditions are considered the best available 

for this level of study, and multi-dimensional modeling should 

be considered for additional studies at locations with large 

estimates of scour and complex flow regimes.

Insufficient channel-geometry data limits the accuracy of 

hydraulic data. Most of the Phase 1.5 sites lack channelgeometry data upstream and downstream of the bridge and rely 

on templating, or copying, and expanding the surveyed bridge 

section on the basis of field observations and channel slope or 

information from the ADOT&PF bridge as-built plans. Data 

available from the bridge as-built plans vary from site to site 

and were limited in upstream and downstream extent from the 

bridge. Copying cross sections from the bridge data is 

particularly inadequate for sites where the channel is 

constricted significantly by the roadway approaches to the 

bridge. The depth of the channel at the bridge section will be 

greater than that in the approach and exit sections, and copying 

this section through the reach results in an over-estimation of 

channel area and overall lower modeled water surfaces and 

flow velocities. For many locations, cross-section end points 

were extended vertically because the as-built plans did not 

include overbank topography. This procedure accommodated 

the recurrence-interval discharges within the channel, but 

resulted in a greater amount of effective flow in the channel.

For sites with significant channel contraction or overbank flow, 

the Phase 2 analysis is more appropriate than the Phase 1.5 

analysis because it includes approach, exit, and overbank 

channel geometries. 

The extent of geometric data available was a limiting 

factor for models of pressure flow. The bridge section 

surveyed for the Phase 1.5 sites rarely extended laterally 

beyond the bridge itself. Without these data, flow over the 

roadway approaches to the bridge could not be modeled and all 

discharge was routed through the bridge section. If the 

roadway approaches were lower than the bridge deck, enough 

discharge could pass over this area to limit or eliminate 

pressure flow. Phase 1.5 pressure-flow sites would be better 

evaluated with the addition of detailed hydraulic cross sections 

and roadway geometry. 

Hydraulic models are sensitive to the value selected for 

channel roughness. An overestimate of roughness will produce 

models with lower flow velocities, higher water-surface 

elevations, and greater flow widths than may occur naturally. 

Roughness values used in the hydraulic models for this study 

were calibrated to measured data when possible, rather than 

relying on estimation procedures. Roughness values were 

calibrated for the discharge at the time of the survey and used 

for the simulations of the 100- and 500-year flood flows. 

Calibration discharge at 25 of the Phase 1.5 sites and 15 of the 

Phase 2 sites was less than 25 percent of the 100-year 

discharge. Calibration of channel roughness to these low 

discharges for the flood models was likely an overestimation 

because channel roughness normally decreases with increasing 

depth. In many cases, flow in the overbanks, where the 

roughness is generally higher, compensates for the decreased 

roughness in the channel that results from the greater flow 

depths. For braided channels, the flow tends to expand rather 

than deepen and roughness values do not decrease, except 

through constricting bridge sections. To determine the 

sensitivity of pier and contraction scour computations to the 

value computed for channel roughness, six sites were selected 

for additional hydraulic modeling. These sites had Manning’s 

roughness values ranging from 0.045 to 0.080 and were 

calibrated to discharges less than 20 percent of the 100-year 

discharge. The hydraulic models for these six sites were rerun 

with a Manning’s roughness of 0.035, which was the default 

value used in the Phase 1 analysis (Heinrichs and others, 

2001). The hydraulic variables from these simulations were 

then used to recalculate pier and contraction scour. The 

hydraulic variables and resulting computed values for pier and 

contraction scour for the original simulations using the 

calibrated roughness values and those from the second 

simulations using the default roughness of 0.035 are presented 

in table 2. 
Table 2. Sensitivity analysis of channel-roughness values for six bridge sites in Alaska in the Phase 1.5 scour analysis where the calibration discharge is less 

than 20 percent of the 100-year recurrence-interval discharge 

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. ft3/s, cubic feet per second; ft, feet; ft/s, feet per second]

ADOT & 

PF No.

 Manning's 

roughness 

coefficient 

n

 Average 

approach 

depth (ft)

Approach 

velocity 

(ft/s)

 Approach 

channel 

width (ft)

 Bridge 

average 

depth (ft)

 Bridge 

section 

width 

(ft)

 Depth of 

contraction 

scour (ft)

 Depth 

upstream of 

pier (ft)

 Velocity 

upstream of 

pier (ft/s)

 Froude 

number 

at bridge

 Depth 

of pier 

scour (ft)

Phase 1.5 calibrated roughness

543 0.055 1.9 7.7 144 1.8 136 0.2 4.7 8.6 0.70 9.1

556 .064 12.0 6.9 292 10.7 215 3.7 17.3 13.5 .57 3.7

557 .049 7.3 9.1 333 7.5 334 .0 14.1 11.7 .55 7.1

694 .050 11.4 8.0 281 11.6 253 .5 15.1 9.0 .41 11.0

742 .080 13.8 5.5 1415 12.7 463 .6 16.6 6.3 .27 4.3

999 .045 5.4 5.5 236 5.2 157 1.6 8.7 10.5 .63 11.9

Phase 1 default roughness values from Phase 1

543 0.035 1.6 9.0 130 1.7 109 0.1 3.7 14.4 1.33 10.9

556 .035 10.1 8.5 284 8.9 204 3.4 15.2 17.3 .78 4.0

557 .035 5.8 12.0 319 5.9 319 .0 12.3 15.4 .77 7.7

694 .035 9.5 10.2 262 9.4 238 .7 12.4 11.6 .58 12.0

742 .035 8.2 9.6 415 7.5 441 .5 11.0 11.4 .61 5.3

999 .035 4.8 6.3 232 4.5 151 1.7 7.9 12.3 .77 12.6

1Approach channel width determined from as-built survey plans.

Estimation Methods for Contraction Scour, Pier Scour, Complex Pier Scour, and Pressure Scour 11

Decreasing the channel roughness from the calibrated 

value to the default estimate had an equal influence on all the 

hydraulic variables used in the computation of live-bed 

contraction scour. For example, a decrease in flow depth was 

accompanied by a decrease in approach channel width. 

Sensitivity analysis of equation 2 shows that all variables have 

a moderate to significant influence on the contraction scour 

depth (Glenn, 1994). Therefore, the accuracy of the channel 

roughness was not critical to the computation of contraction 

scour at most sites. However, the geometry of the approach 

channel at some locations may have limited changes in the 

approach width. Under these conditions, the approach depth, 

rather than the approach width was more responsive to variance 

in channel roughness, and the computation of contraction scour 

was affected. 

The computation of pier scour was more responsive than 

the computation of contraction scour to variances in channel 

roughness. Flow depth upstream of the pier and the Froude 

number at the bridge were sensitive to changes in roughness, 

whereas most of the other variables in equation 5 remained 

constant. The Froude number was used in equation 5 to 

represent the approach flow velocity, which increased with 

decreased channel roughness. Sensitivity analysis of equation 

5 indicated that the approach velocity’s influence on pier scour 

was second only to pier width, which was a constant for most 

scenarios (Glenn, 1994). Decreasing channel roughness 

increased pier scour by as much as 20 and 23 percent for 

bridges 543 and 742, respectively, and by less than 10 percent 

for the other four sites (table 2).
12 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Phase 1.5 and Phase 2 Analyses and 

Evaluation

Scour depth was computed for contraction scour, pier 

scour, pressure scour, and complex pier scour for 100- and 

500-year floods at 35 Phase 1.5 and 19 Phase 2 bridge sites. 

Computed scour depths are summarized in table 3 and 

complete calculations are included in tables 5-8 (at back of 

report). Computed contraction and pier-scour depths greater 

than 5 and 10 ft, respectively, were used as criteria to identify 

Phase 1 bridges for further analysis. Computed contraction 

scour depths for Phase 1.5 and Phase 2 sites were greater than 

5.0 ft at six sites for the 100-year discharge. Computed pierscour depths were greater than 10.0 ft for the 100-year 

discharge at 23 bridges. Complex pier-scour depth was 

computed for six sites where the pier-footing elevation was 

above the lowest streambed elevation after the contractionscour calculations. Pressure scour was evaluated at 10 bridges, 

but pressure-flow conditions persisted at only three sites after 

contraction-scour depths were accounted for. Total maximum 

scour at a bridge in this study was the summation of the 

contraction and pier-scour depths. For pressure-flow sites, the 

pressure-scour depth was added to the contraction and pierscour depths. When the pier footing is exposed to the flow, the 

complex pier-scour depth should be used in place of the pierscour depth.

Potential scour at bridges is estimated using equations 

derived mainly from simplified studies in laboratory flumes. 

The applicability of these equations to actual conditions is 

limited. The scour estimates tend to be conservative when 

compared with the limited field measurements of scour that 

have been made. Mueller (1996) compared predictions from 

the pier-scour equations recommended in HEC-18 with field 

data and concluded that the scour estimates rarely underpredicted measured scour depths. In a study of measured 

versus predicted pier scour in New Hampshire, Boehmler and 

Olimpio (2000) also found that the HEC-18 equations 

consistently over-predicted scour. In a study of streambed 

scour at bridges in Alaska, Norman (1975) found that equation 

2 is a good estimate of contraction scour at streams with gravel 

or cobble beds. Results from a comparison of measured and 

calculated scour depths for two bridges on the Copper River in 

Alaska by Brabets (1994) varied. Equation 3 provided the 

closest value to measured pier scour at one bridge and underpredicted scour by 25 percent at the other bridge (Brabets, 

1994). Results from a comparison of measured and predicted 

contraction scour for the Copper River study also varied. 

Equation 2 over-predicted scour at the first bridge by 3.4 ft and 

at the second bridge by only 0.5 ft (Brabets, 1994).

The effectiveness of the contraction-scour predictions for 

the Phase 1.5 analysis was limited by the lack of channel and 

overbank data at the approach to the bridges. Channel widths 

typically were estimated in the field or from the bridge as-built 

plans rather than being surveyed. Estimating approach-channel 

widths from bridge as-built plans can be insufficient to predict 

contraction scour accurately. Even with a recent and accurate 

bridge plan it is difficult to accurately delimit the portion of the 

channel that is actively transporting sediment. Limited or no 

overbank data in the approach to the bridge resulted in 

modeled discharges and depths larger than what would be 

expected for actual conditions where some portion of the flow 

would extend beyond the main channel. Channel and overbank 

data in the approach to the bridge that are required for an 

accurate estimate of contraction scour using the methodology 

recommended by HEC-18 were collected for the Phase 2 

analysis. 

Uncertainty in roughness values and channel-flow angle 

of attack on piers at high flows influenced the computation of 

local scour at piers or the Phase 1.5 and Phase 2 analyses. 

Based on the sensitivity analysis of pier scour to changes in 

channel roughness (table 2) and the comparisons of measured 

to calculated values discussed previously, uncertainty in 

channel-roughness estimates for higher discharges will 

produce a result that is within the resolution of equation 5. 

Channel-flow angle of attack was a significant factor in the 

calculation of pier scour and varied with changes in stage and 

flow conditions. Changes in channel-flow angle of attack 

could not be determined without field verification at higher 

stages or multi-dimensional flow modeling. Estimates of 

channel-flow angle of attack for each site were based on the 

measured value and the channel morphology upstream of the 

bridge that would control the angle at higher discharges. 

Estimates of complex pier scour were made for sites 

where the footing elevation was above the streambed after the 

computed contraction scour for the 100- and 500-year floods 

was subtracted from the lowest point on the as-built cross 

section at the bridge. Identifying complex scour sites by this 

method was a conservative approach for locations where the 

thalweg was not located near a pier and channel migration was 

not of concern. The complex pier-scour estimate, rather than 

the normal pier-scour estimate, should be used at the sites in 

table 3 unless the thalweg is not in proximity to bridge piers 

and long-term observation indicates channel stability. 
Phase 1.5 and Phase 2 Analyses and Evaluation 13

Table 3. Summary of computed scour depths for 100- and 500-year recurrence-interval discharges for the Phase 1.5 and Phase 2 streambed scour analyses at 

selected bridge sites in Alaska

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. ft, feet. –, no data]

ADOT & 

PF No.

Depth (ft)

Contraction scour Pier scour Pressure scour Complex pier scour Total scour

100-year 500-year 100-year 500-year 100-year 500-year 100-year 500-year 100-year 500-year

Phase 1.5

205 0.7 0.9 15.1 15.6 – – 26.9 28.1 27.6 29.0

240 .0 .0 12.1 12.7 – – – – 12.1 12.7

277 6.9 13.3 – – – – – – 6.9 13.3

308 .7 .9 16.2 17.4 – – – – 16.9 18.3

396 .9 1.3 12.2 13.1 – – – – 13.1 14.4

401 1.9 6.4 – – – 8.1 – – 1.9 14.5

505 6.0 6.5 8.9 9.1 – – 14.3 14.6 20.3 21.1

509 .0 .0 24.9 26.9 – – – – 24.9 26.9

518 .0 .0 20.8 21.5 – – – – 20.8 21.5

520 .0 .0 9.1 9.6 – – – – 9.1 9.6

530 .0 .0 10.2 10.7 – – – – 10.2 10.7

543 .0 .0 9.1 9.4 – – – – 9.1 9.4

544 .0 .0 7.2 7.2 – – 13.9 14.4 13.9 14.4

547 .0 .0 12.8 13.4 – – – – 12.8 13.4

548 3.6 4.3 32.6 33.9 – – – – 36.1 38.2

556 2.3 2.5 4.0 4.2 – – – – 6.2 6.7

572 .0 .0 14.1 14.5 – – 14.8 15.0 14.8 15.0

573 8.3 10.6 12.2 12.8 – – – – 20.4 23.4

603 .6 .6 10.1 10.6 – – – – 10.7 11.2

658 1.7 2.3 – – 0 .2 – – 1.7 2.5

678 2.2 3.0 3.5 3.8 2.1 .2 – – 7.8 7.0

686 .8 1.0 3.3 3.4 – – – – 4.1 4.4

687 1.4 1.6 13.0 13.8 – – – – 14.4 15.4

690 12.0 11.6 – – – – – – 12.0 11.6

742 .0 .0 4.3 4.6 – – – – 4.3 4.6

832 .6 .8 – – – – – – .6 .8

833 1.9 1.9 – – – – – – 1.9 1.9

999 1.5 1.8 11.9 12.7 – – – – 13.4 14.5

1220 2.4 2.3 3.1 3.1 – – – – 5.5 5.4

1261 1.1 1.2 9.4 9.7 – – – – 10.6 11.0

1282 .0 .0 19.9 20.5 – – – – 19.9 20.5

1283 1.2 1.9 3.3 3.6 – – – – 4.5 5.5

1284 2.2 2.4 3.4 3.5 – – – – 5.6 5.9

1329 .4 .6 10.3 11.0 – – – – 10.6 11.6

1389 .0 .0 – – – – – – .0 .0
14 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Limited research has been done on the vertical 

contraction scour that results from full or partial submergence 

of a bridge. Arneson and Abt (1999) have conducted the most 

extensive study to date of pressure-flow scour under live-bed 

conditions, and their equation is recommended in the current 

edition of HEC-18. Contraction scour and pressure scour were 

evaluated independently because submergence of the bridge 

structure impedes the flow and can create additional backwater 

at the bridge constriction. This backwater can reduce the 

approach velocities below the critical velocity of the bed 

material required for contraction scour. Contraction scour will 

occur up to a point, until the flow changes to pressure flow and 

backwater conditions develop. Evaluating contraction scour 

independently, and then modifying the channel to reflect the 

scour, eliminated modeled pressure-flow conditions at several 

bridges. Calculations of pressure scour are summarized in 

table 7 for the three sites where pressure-flow conditions 

persisted after the channel area was increased to reflect the 

contraction scour. 

Comparison of Phase 1.5 and 2 

Analyses with Phase 1 Analysis

The Phase 1 analysis was a rapid, cost-effective 

assessment of scour susceptibility using existing data. The 

quality of this assessment was based on the quantity and 

accuracy of data from the bridge as-built plans and other 

sources. Most bridges in this study were built across dynamic 

channels that could be aggrading, degrading, and migrating 

laterally. These changes in the channel can be a response to the 

change in hydraulics caused by the bridge structure or to more 

long-term adjustments that could be related to tectonics or 

changes in flow regime or sediment supply. The lowest 

elevation from the as-built bridge survey for the streambed at 

the bridge was compared with the elevation measured during 

the site investigation for the Phase 1.5 and Phase 2 analyses, 

and the change in elevation is shown in figure 2. 

Phase 2

233 6.5 6.4 8.1 9.2 – – – – 14.6 15.6

242 .8 .9 4.4 4.5 – – – – 5.1 5.4

254 5.0 5.3 17.6 18.1 – – – – 22.6 23.4

255 .0 .0 20.1 20.9 – – – – 20.1 20.9

355 1.6 1.9 8.5 8.9 – – – – 10.1 10.8

527 2.7 3.1 18.0 19.0 – – – – 20.7 22.1

535 2.6 3.2 15.0 15.0 – – – – 17.6 18.2

539 12.0 14.4 11.4 12.7 – – 20.3 31.2 32.3 45.6

557 .0 .0 7.1 7.3 – – – – 7.1 7.3

558 .0 .0 11.7 12.2 – – – – 11.7 12.2

574 2.7 3.0 6.8 7.1 – – – – 9.5 10.1

608 .0 .0 6.5 6.8 – – – – 6.5 6.8

609 .4 .5 3.7 3.9 – – – – 4.1 4.4

694 .9 1.1 11.0 11.5 – – 19.8 20.5 20.7 21.6

1025 .0 .0 6.6 6.9 – – – – 6.6 6.9

1147 1.4 1.8 7.2 7.2 – – – – 8.6 9.0

1255 .5 .5 9.4 9.6 – – – – 9.9 10.1

1341 1.5 1.7 8.8 9.3 – – – – 10.3 11.0

1513 2.2 3.3 – – – – – – 2.2 3.3

Table 3. Summary of computed scour depths for 100- and 500-year recurrence-interval discharges for the Phase 1.5 and Phase 2 streambed scour analyses at 

selected bridge sites in Alaska–Continued

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. ft, feet. –, no data]

ADOT & 

PF No.

Depth (ft)

Contraction scour Pier scour Pressure scour Complex pier scour Total scour

100-year 500-year 100-year 500-year 100-year 500-year 100-year 500-year 100-year 500-year
"2)$'%.5-"%2





















$)3#(!2'%!44)-%/&3526%9!30%2#%.4!'%/&
9%!22%#522%.#%
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#(!.'%).342%!-"%$%,%6!4)/.).&%%4

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0(!3%!.!,93)3

342%!-"%$

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Comparison of Phase 1.5 and 2 Analyses with Phase 1 Analysis 15

Figure 2. Change in streambed elevation at bridge between the lowest elevation streambed from the as-built bridge survey and the 

elevation measured for the Phase 1.5 or Phase 2 for selected bridge sites in Alaska. 

Discharge at the time of measurement is shown as percentage of the 100-year recurrence-interval discharge. Five locations that were not 

resurveyed for the Phase 1.5 analysis are not shown.
16 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

The discharge measured during the Phase 1.5 or Phase 2 

analysis is included on figure 2 as a percentage of the 100-year 

recurrence-interval flood discharge to illustrate that the 

measured elevation used in this study may not have necessarily 

been obtained during a high flow event. A general degradation 

in channel depth can be seen for most sites. Determining the 

process behind the degradation was beyond the scope of this 

study, but these data confirm the dynamic nature of these 

channels. Scour values presented in this study were in 

reference to the streambed elevation from the as-built plans and 

did not take into consideration the channel changes measured 

during the Phase 1.5 or Phase 2 analyses. 

Comparison of Hydraulic Models

Hydraulic variables used in the scour computations for 

the Phase 1 analysis were calculated using the step-backwater 

water-surface profile (WSPRO) model (Shearman, 1990). 

WSPRO is a water-surface profile computational model for 

one-dimensional, gradually varied, steady flow in open 

channels. WSPRO and HEC-RAS are both sufficient for 

determining hydraulic variables for subcritical flow and flow 

that does not intersect the bridge structure. In situations where 

the flow is supercritical, changes to supercritical through the 

reach, or intersects the bridge structure, HEC-RAS provides a 

more rigorous hydraulic analysis with multiple methods for 

determining water-surface profiles. 

Comparison of HEC-RAS and WSPRO outputs for a site 

is difficult because the data used to construct the models are 

not at the same level of detail. The hydraulic parameters used 

to construct the models can be compared. Comparison of the 

estimated channel-roughness values for the Phase 1.5 analysis 

and calibrated channel-roughness values for the Phase 2 

analysis with values from Phase 1 (either an assumed channel 

roughness of 0.035 or a value estimated from existing 

information) indicated an average increase from the Phase 1 

values (fig. 3). The surveyed channel slopes used for the Phase 

1.5 and Phase 2 analyses were compared with the slope 

estimated from 1:63,000 topographic maps and used to copy 

cross sections for Phase 1, and the changes varied greatly 

(fig. 3). The variation in surveyed slope values when compared 

to those measured from topographic maps illustrated the 

importance of this field measurement for accurate templating 

of channel geometry and determination of roughness 

coefficients using the Manning’s equation. 

Model output and detail of data used to generate it can be 

compared generally by evaluating the modeled Froude number 

at the bridge for the different levels of analysis. The magnitude 

of the Froude number was affected by channel slope, 

roughness values, and method used to compute the watersurface profile. The Froude values for Phase 1 and Phase 1.5 

analyses were in good agreement for values less than 0.5, but 

the variance increased for higher Froude values (fig. 4). 

(Modeled Froude values for the Phase 1.5 and Phase 2 analyses 

did not exceed 1.0.) Several of the outliers on figure 4 were 

locations with modeled pressure flow that were evaluated 

under a different procedure for the Phase 1.5 analysis. The 

Froude values for Phase 2 showed less agreement with those 

from the Phase 1 models, but generally tended to be lower 

numbers that were attributed to the increased channel 

roughness values selected for the Phase 2 analyses (fig. 5).

Contraction Scour

The validity of the contraction-scour depths determined 

in the Phase 1 analysis was limited by the extent, availability, 

and age of channel and overbank data for the approach section 

of the river. This computation was improved for the Phase 1.5 

analysis by including field estimation of approach-channel 

widths and for the Phase 2 analysis by including surveys of the 

approach channel. Comparison of the approach widths for the 

Phase 1 analysis and the Phase 1.5 and 2 analyses showed a 

general increase in the widths in the later analyses (fig. 6). 

At several sites, contraction-scour depths for the 100- and 

500-year flood discharges computed for the Phase 1 and Phase 

1.5 analyses differed greatly (fig. 7). The following three 

factors, determined through a sensitivity analysis of input 

variables, accounted for the difference in computed scour 

between the analyses. 

1. Channel geometry in the approach: Increases or decreases 

in the Phase 1.5 modeled flow width in the approach 

channel or bridge section explained the differences in 

computed scour for most of the sites (fig. 7). 

2. Channel geometry at the bridge: Changes in the modeled 

bridge-section width reflect measured channel 

aggradation or degradation in the bridge section since 

the time of the survey for the bridge as-built plans. 

3. Pressure flow: Procedures by which the sites with 

pressure-flow conditions were evaluated explain 

differences in computed scour at some sites. Contraction 

scour was not evaluated independent of the pressureflow conditions for the Phase 1 analysis, and the values 

were based on the backwater conditions that develop 

when the flow intersects the bridge. 

The difference in scour values at one site was attributed to 

a three-fold increase in the measured slope of the channel and 

resultant change in hydraulic variables. Bridges that have 

estimated scour-depth values in excess of 5 ft for the Phase 1 

or 1.5 level analyses would be better evaluated with the 

inclusion of detailed hydraulic cross sections. Detailed 

hydraulic cross sections would also improve simulations of 

pressure flow for Phase 1.5 sites. 
0(!3%!.!,93)3

0(!3%!.!,93)3































0%2#%.4!'%/&#(!.'%&2/-0(!3%6!,5%

"2)$'%.5-"%2

















#(!..%,2/5'(.%33

#(!..%,3,/0%















Comparison of Phase 1.5 and 2 Analyses with Phase 1 Analysis 17

The field measurement of the approach-channel geometry 

in the Phase 2 models resulted in the most significant 

differences in contraction-scour depth relative to Phase 1 

models. Phase 1 contraction scour was overestimated when the 

Phase 2 data indicated the approach channel was narrower than 

the geometry determined from the as-built bridge plans. Phase 

1 contraction scour was overestimated at one site when the 

Phase 2 data indicated the channel roughness was lower than 

the Phase 1 value (fig. 8). Results from the Phase 2 contraction 

scour computations generally were lower at sites where the 

Phase 1 estimate of scour was greater than 4.0 ft. 

Pier Scour

The estimation of pier scour by equation 5 was computed 

using hydraulic variables from HEC-RAS and pier dimensions, 

pier shape, and the channel-flow angle of attack. The pier 

dimensions and shape are consistent for a site for the different 

levels of analysis, the exception being sites where further 

investigation indicated a need to model a group of piers as one 

solid pier or vice versa. The hydraulic variables flow depth and 

Froude number can be used to show a difference between the 

levels of analysis based upon the hydraulic parameters used to 

generate the model. For the Phase 1 analysis, the channel-flow 

angle of attack that was estimated from the as-built plans was 

measured in the field for the Phase 1.5 and 2 analyses. 

Figure 3. Percentage of change between Phase 1 values and Phase 1.5 and Phase 2 values for channel slope and roughness at bridge for selected bridge 

sites in Alaska. 

Note that slope for the Phase 1.5 Bridge 1329 increases 600 percent.














      

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02%3352%&,/7

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18 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Figure 4. Comparison of modeled Froude numbers at the bridge, determined by 

Phase 1 and Phase 1.5 analyses for the 100-year and 500-year recurrence-interval 

discharge for selected bridge sites in Alaska.














      

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0(!3%&2/5$%.5-"%2

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YEARDISCHARGE

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Comparison of Phase 1.5 and 2 Analyses with Phase 1 Analysis 19

Figure 5. Comparison of modeled Froude numbers at the bridge, determined by 

Phase 1 and Phase 2 analyses for the 100-year and 500-year recurrence-interval 

discharge for selected bridge sites in Alaska. 






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#(!..%,7)$4(&2/-0(!3%6!,5%

































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20 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Figure 6. Percentage of change in approach-channel width between the Phase 1 analysis and the Phase 1.5 and Phase 2 analyses for 

selected bridge sites in Alaska.
































       

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Comparison of Phase 1.5 and 2 Analyses with Phase 1 Analysis 21

Figure 7. Comparison of contraction-scour depths computed by Phase 1 and Phase 1.5 analyses for 

the 100-year and 500-year recurrence-interval discharge and factor responsible for the difference in 

scour depths for selected bridge sites in Alaska. 




































        

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YEARDISCHARGE

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0(!3%3#/52$%04().&%%4

22 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Figure 8. Comparison of contraction-scour depths computed by Phase 1 and Phase 2 analyses for the 100-year 

and 500-year recurrence-interval discharges and factor responsible for the difference in scour depths for selected 

bridge sites in Alaska. 
Comparison of Phase 1.5 and 2 Analyses with Phase 1 Analysis 23

Comparisons of pier-scour depths for the 100- and 

500-year discharges for the Phase 1 and Phase 1.5 analyses are 

shown in figure 9 and for the Phase 1 and Phase 2 analyses in 

figure 10. There was a positive association between the levels 

of analysis, with a few outliers. Sensitivity analysis on these 

outliers determined that angle of attack, Froude number, and 

pier shape were responsible for the variance in the scour-depth 

values between the levels of analysis. 

The channel-flow angle of attack at a bridge accounted for 

most of the difference between scour values for the different 

levels of analysis in this study (figs. 9 and 10). Increases or 

decreases from the angle used for the Phase 1 study can be 

attributed to a better measure of the value from field 

observations and also to migration of the channel since the asbuilt survey of the bridge. Field verification, preferably at high 

flow, was critical for an accurate estimate of angle of attack. 

The Phase 1 analysis used dated survey plans that do not reflect 

the current flow conditions at sites with dynamic channels.





































        

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YEARDISCHARGE

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YEARDISCHARGE

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&2/5$%.5-"%2

!.',%/&!44!#+

%15!,6!,5%,).%

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Figure 9. Comparison of pier-scour depths computed by Phase 1 and Phase 1.5 analyses for the 100-year and 500-year 

recurrence-interval discharges and factor responsible for the difference in scour depths for selected bridge sites in Alaska. 




























      

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24 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Figure 10. Comparison of pier-scour computed by Phase 1 and Phase 2 analyses for the 100-year and 500-year 

recurrence-interval discharges and factor responsible for the difference in scour depths for selected bridge sites in 

Alaska. 
Summary and Conclusions 25

Summary and Conclusions

The U.S. Geological Survey began studying the scour 

susceptibility of bridges in Alaska in 1994. The initial phase of 

this project used existing data to perform a preliminary Phase 1 

scour assessment at 325 bridges. Based on these assessments, 

the Alaska Department of Transportation and Public Facilities 

(ADOT&PF) selected 54 bridges for further analysis. These 

analyses included on-site inspection and collection of detailed 

hydraulic cross sections. The bridges were evaluated at either 

the Phase 1.5 or Phase 2 level analysis. Division into these two 

groups was based on the magnitude of measured scour and the 

calculated scour depths from Phase 1. The Hydrologic 

Engineering Center’s River Analysis System (HEC-RAS) was 

used to calculate the hydraulic variables needed to compute 

estimates of scour for 100- and 500-year recurrence-interval 

flood discharges. Contraction scour, pier scour, complex pier 

scour, and pressure scour were calculated using federally 

recommended techniques and equations outlined in the Federal 

Highway Administration’s Hydraulic Engineering Circular 

No. 18 (HEC-18).

The less-intensive Phase 1.5 analysis was completed at 35 

of the 54 bridges. Data collection included a discharge 

measurement, water-surface slope survey, verification of the 

channel-flow angle of attack, and estimation of the approachchannel width. Computed contraction scour depths for the 

100-year flood discharge were greater than 5 feet for four 

bridges and pier-scour depths were greater than 10 feet for 16 

bridges. 

The Phase 2 analysis was performed at 19 bridges. Data 

collection for the Phase 2 analysis was analogous to the Phase 

1.5, with the addition of detailed hydraulic cross sections 

surveyed upstream and downstream of the bridge. Computed 

contraction-scour depths from the 100-year flood discharge 

were greater than 5 feet at two bridges and pier-scour depths 

were greater than 10 feet at seven bridges.

Comparing the analysis methodologies and results of the 

Phase 1.5 and Phase 2 analyses highlighted the effectiveness 

and weakness of each phase of scour analysis. Estimation, 

rather than field surveys, of the approach channel width limited 

the validity of the Phase 1.5 contraction-scour computation. 

Phase 1.5 analyses at bridges with modeled pressure flow lack 

sufficient hydraulic cross-section data for an accurate 

representation of flow hydraulics. The Phase 2 analysis would 

be more appropriate at locations with contracted openings and 

modeled pressure flow. Most channel-roughness values for 

both levels of analysis were calibrated to discharges less than 

the 100- and 500-year floods. Contraction and pier scour were 

evaluated for sensitivity to this variable. At selected bridges, 

there was little resultant change in the contraction scour depths 

and the change in the pier-scour depths was within the 

resolution of the pier-scour equation. Locations where an 

increase or decrease to the channel-flow angle of attack will 

result in significant change in pier scour should be visited at 

high flow or considered for multi-dimensional flow analysis. 

Supercritical flow, velocities in excess of 10 feet per second, 

hydraulic jumps, and pressure flow are conditions found at 

many sites in this study and are subject to multi-dimensional 

variation in both time and length scales. A more accurate 

simulation of these adverse flow conditions would be provided 

by a multi-dimensional flow model. 

Because field measurements were made at the sites for the 

Phase 1.5 and 2 analyses, the results of these analyses were 

useful in evaluating the effectiveness and limitations of the 

Phase 1 analysis. Estimated channel-roughness values for the 

Phase 1.5 analyses and calibrated channel-roughness values for 

the Phase 2 analyses were on the average larger than those used 

for the Phase 1 study. The surveyed channel slopes both 

increased and decreased when compared with the slopes 

measured from topographic maps for the Phase 1 analysis. The 

differences in slope and roughness values were reflected in the 

general lack of agreement between modeled Froude numbers. 

Sensitivity analysis of the contraction-scour equation 

determined that changes in the approach-channel geometry 

were responsible for most major differences in scour depths 

between the levels of analysis. Contraction scour for locations 

that had modeled pressure flow was computed for Phase 1 

using a procedure different from the subsequent analyses, and 

the results cannot be compared. The Phase 2 contraction-scour 

depths, which include more data than the Phase 1.5, generally 

were less than the Phase 1 depths. Pier-scour depths for Phase 

1.5 were either larger or smaller than those for the Phase 1 

analysis. Most of the inconsistencies were explained by an 

increase or decrease in the channel-flow angle of attack used in 

the computation. Most of the sites in the Phase 1.5 analysis 

were selected because the existing estimates of channel-flow 

angle of attack were either high or were unsupported. Pierscour depths at locations selected for Phase 2 analysis generally 

were less than the values computed for the Phase 1 analysis. 

The Phase 1 analysis was a cost-effective preliminary 

assessment of both pier and contraction scour that was used to 

select bridges for site investigation. The Phase 1.5 analysis was 

sufficient for most estimates of pier scour, but should not have 

been considered adequate at sites with significant channel 

contractions through the bridge. The Phase 2 analysis provided 

the best estimate of one-dimensional scour analysis using 

HEC-18 equations. Scour depths summarized in this report are 

being used by ADOT&PF to focus monitoring efforts and 

emplacement of countermeasures at river crossings in Alaska 

that are susceptible to scour.
26 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

References Cited

Arneson, L.A., and Abt, S.R., 1999, Vertical contraction scour 

at bridges with water flowing under pressure conditions, in

Richardson, E.V., and Lagasse, P.F., eds., Stream stability 

and scour at highway bridges: Reston, Va., American 

Society of Civil Engineers, p. 189-204.

Boehmler, E.M., and Olimpio, J.R., 2000, Evaluation of pierscour measurement methods and pier–scour predictions 

with observed scour measurements at select bridge sites in 

New Hampshire, 1995-98: U.S. Geological Survey WaterResources Investigations Report 00-4183, 48 p.

Brabets, T.P., 1994, Scour assessment at bridges from Flag 

Point to Million Dollar bridge, Copper River Highway, 

Alaska: U.S. Geological Survey Water-Resources 

Investigations Report 94-4073, 57 p.

Brunner, G.W., 2001, HEC-RAS, River Analysis System 

User's Manual, Version 3.0, January 2001: Davis, Calif., 

U.S. Army Corps of Engineers Hydrologic Engineering 

Center (HEC), CDP-68, variously paged.

Glenn, J.S., 1994, Sensitivity analysis of bridge scour 

equations, in Proceedings of the 1994 National Conference 

on Hydraulic Engineering, Buffalo, New York, American 

Society of Civil Engineers, p. 11-15.

Heinrichs, T.A., Kennedy, B.W., Langley, D.E., and Burrows, 

R.L., 2001, Methodology and estimates of scour at selected 

bridge sites in Alaska: U.S. Geological Survey WaterResources Investigations Report 00-4151, 44 p.

Hicks, D.M., and Mason, P.D., 1991, Roughness 

characteristics of New Zealand Rivers: Wellington, New 

Zealand, Water Resources Survey, 329 p.

Interagency Advisory Committee on Water Data, 1982, 

Guidelines for determining flood-flow frequency: 

Hydrology Subcommittee, Bulletin 17B, 28 p. [available 

from the National Technical Information Service as report 

PB-86-157278/AS].

Jones, S.H., and Fahl, C.B., 1994, Magnitude and frequency of 

floods in Alaska and conterminous basins of Canada: U.S. 

Geological Survey Water-Resources Investigations Report 

93-4179, 122 p.

Jones, J.S., Bertoldi, D.A., and Umbrell, E.R., 1999, Interim 

procedures for pressure flow scour, in Richardson, E.V., and 

Lagasse, P.F., eds., Stream stability and scour at highway 

bridges: Reston, Va., American Society of Civil Engineers, 

p. 288-297.

Laursen, E.M., 1960, Scour at bridge crossings: American 

Society of Civil Engineers, Journal of the Hydraulics 

Division, v. 86, no.2, p. 39-54.

Melville, B.W., and Coleman, S.E., 2000, Bridge Scour: 

Highlands Ranch, Colo., Water Resources Publications, 

550 p.

Mueller, D.S., 1996, Local scour at bridge piers in nonuniform 

sediment under dynamic conditions: Fort Collins, Colorado 

State University, Civil Engineering Department, Doctoral 

Dissertation, 212 p.

Murillo, J.A., 1987, The scourge of scour: Civil Engineering, 

American Society of Civil Engineers, v. 57, no. 7, 

p. 66-69. 

Nelson, J.M., Bennett, J.P., and Wiele, S.M., 2003 Modeling 

flow, sediment transport, and bed evolution in channels, in

Kondolf, M, and Piégay, H., eds., Tools in fluvial 

geomorphology, New York, Wiley, p. 696.

Norman, V.W., 1975, Scour at selected bridge sites in Alaska: 

U.S. Geological Survey Water-Resources Investigations 

Report 32-75, 160 p.

Parola, A.C., Mahavadi, S.K., Brown, B.M., and El Khoury, 

A., 1996, Effects of rectangular foundation geometry on 

local pier scour: Journal of Hydraulic Engineering, v. 122, 

no. 1, p. 35-40.

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

bridges (4th ed.): Federal Highway Administration 

Hydraulic Engineering Circular 18, publication no. FHA 

NHI 01-001 HEC-18, 378 p.

Richardson, E.V., and Lagasse, P.F., eds., 1999, Stream 

stability and scour at highway bridges: Reston, Va., 

American Society of Civil Engineers, 1040 p.

Rhodes, J., and Trent, R.E., 1999, Economics of floods, scour, 

and bridge failures, in Richardson, E.V., and Lagasse, P.F., 

eds., Stream stability and scour at highway bridges: Reston, 

Va., American Society of Civil Engineers, p. 1013-1018.

Shearman, J.O., 1990, Users manual for WSPRO—A 

computer model for water-surface profile computations: 

Federal Highway Administration Publication FHWA-IP89-027, 177 p.
Table 4 27

Table 4. Selected hydraulic variables used to construct hydraulic models for analyses of streambed scour at selected bridge sites in Alaska

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities number. Discharge: 100- and 500-year discharges were estimated using 

methodology of Jones and Fahl (1994). Cross-section source: a, discharge measurement; b, survey; c, bridge sounding; d, as-built survey. ft3/s, cubic feet per 

second; ft, feet; ft/ft, feet per foot. –, no data]

ADOT & 

PF No.

Discharge (ft3/s) Watersurface 

slope 

(ft/ft)

Manning’s roughness 

coefficient

Minimum bed elevation

(ft) Crosssection 

source

Date 

surveyed

100-year 500-year Measured Channel Overbank Measured As-built

Phase 1.5 analysis

205 126,000 151,000 48,300 0.0017 0.030 0.05 448.0 448.0 a 08/02/99

240 3,800 4,570 3,320 .0002 .030 .050 375.0 375.0 a 10/12/86

277 1,350 2,360 426 .0027 .083 – 1,884.0 1,886.0 c 08/03/97

308 18,500 27,500 1,160 .0074 .040 – 54.1 65.2 a 08/31/01

396 1,600 2,060 36.3 .0060 .033 .070 164.2 1,64.0 c 07/16/99

401 4,360 7,010 5,790 .0060 .038 .050–.100 86.1 88.2 c 10/11/86

505 51,900 59,700 21,700 .0001 .032 – 1,513.5 – a 08/23/01

509 9,520 11,700 – .0080 .040–.045 – 1,445.2 1,445.0 b 05/30/91

518 7,520 9,250 – .0035 .035 – 60.4 – c 09/24/99

520 5,100 6,340 133 .0063 .041 – 1,326.8 1,331.0 b 09/15/98

530 3,130 3,920 1,900 .0050 .035 .090 609.1 612.5 c 07/31/99

543 1,920 2,390 184 .0335 .055 – 478.8 480.0 c 07/15/99

544 7,600 9,310 – .0100 .055 – 79.8 83.9 b 07/19/99

547 1,370 1,700 35.9 .0200 .055 – 85.0 89.8 c 07/19/99

548 9,550 11,800 5,210 .0178 .059 – 1,766.7 1,770.0 a 06/16/73

556 24,400 30,600 3,640 .0073 .040 – 20.1 23.0 a 08/05/98

572 10,700 11,800 9,320 .0069 .041 .100 1,006.0 1,001.4 c 09/02/98

573 79,400 109,000 5,230 .0030 .026 .034–.092 1,105.7 1,106.9 c 07/22/97

603 2,100 2,600 2,400 .0010 .030 .100 440.0 440.0 a 12/02/85

658 9,430 11,200 240 .0019 .023 .065 93.1 93.1 c 08/01/99

678 2,260 2,870 – .0025 .040 .045 373.5 373.6 c 08/20/67

686 4,420 5,400 – .0100 .044 – 2,880.9 2,881.0 c 09/12/96

687 37,600 48,200 18,800 .0010 .032 – 2,417.7 2,422.0 a 10/17/67

690 3,090 5,730 358 .0160 .046 .100 2,305.9 2,304.8 c 08/03/99

742 32,600 42,900 6,200 .0025 .080 .100 113.7 112.9 a 08/30/01

832 2,130 3,840 – .0080 .035 .100 1,053.7 1,053.7 d 05/14/70

833 3,890 4,860 – .0023 .035 .100 844.2 846.0 c 08/09/99

999 7,020 10,200 440 .0090 .045 – 99.8 104.0 a 07/18/99

1220 9,870 12,400 – .0041 .035 .100 39.8 39.8 d 04/01/70

1261 25,100 28,500 5,950 .0010 .026 .030–.070 1,187.5 1,188.0 a 06/11/98

1282 20,300 23,400 – .0030 .026 – 1,210.7 1,216.3 c 07/26/99

1283 10,300 12,000 611 .0013 .030 .075 1,376.3 1,377.0 c 07/26/99

1284 7,880 9,180 467 .0020 .040 – 1,370.5 1,379.8 c 07/26/99

1329 7,910 11,800 – .0070 .035 .100 445.1 446.0 c 08/06/99

1389 980 1,230 31.0 .0450 .070 – 207.5 208.0 c 07/17/99
28 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Phase 2 analysis

233 22,100 27,400 733 0.0013 0.032 0.058–0.145 726.8 734.0 b 06/06/98

242 4,220 5,220 – .0066 .045 .06 1,092.3 1,098.0 b 09/22/94

254 186,000 206,000 59,900 .0010 .02–.028 .06–.07 242.6 248.0 b 08/26/98

255 83,200 103,000 24,800 .00145 .028–.04 .05 480.7 482.0 b 08/25/98

355 36,000 42,000 8,520 .0005 .03 .08 631.0 634.0 a 08/09/99

527 50,600 64,900 36,400 .0025 .036 .09 629.0 634.0 a 09/06/95

535 6,920 8,710 1,600 .0080 .04 .12–.15 258.9 263.7 b 09/01/98

539 79,400 104,000 23,000 .0007 .027–.037 .08 19.5 36.0 b 07/13/99

557 22,100 28,500 3,270 .0070 .049 .049 365.4 366.0 a 08/05/98

558 19,400 25,100 3,940 .0050 .042 .13 429.7 440.0 a 08/04/98

574 18,400 22,200 1,580 .0035 .03–.04 .06–.1 1,362.8 1,365.0 a 08/31/98

608 1,610 2,070 – .0090 .035 .075 451.8 453.5 b 09/01/95

609 1,220 1,750 – .0200 .04 .1 473.5 472.0 b 08/31/95

694 25,400 30,300 4,850 .0030 .05 .1 1,784.2 1,786.0 a 08/12/98

1025 4,590 6,520 339 .0055 .03–.035 .08 18.9 20.0 b 07/18/99

1147 43,200 53,600 1,030 .0050 .04 .1 1,504.8 1,505.0 b 08/13/98

1255 3,800 4,710 181 .0050 .03–.037 .05 836.2 834.9 b 06/27/99

1341 6,920 8,710 1,600 .0080 .04 .12–.16 198.8 202.5 b 09/01/98

1513 1,830 2,330 – .0057 .05 .01 80.0 – b 06/24/99

Table 4. Selected hydraulic variables used to construct hydraulic models for analyses of streambed scour at selected bridge sites in Alaska—Continued

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities number. Discharge: 100- and 500-year discharges were estimated using 

methodology of Jones and Fahl (1994). Cross-section source: a, discharge measurement; b, survey; c, bridge sounding; d, as-built survey. ft3/s, cubic feet per 

second; ft, feet; ft/ft, feet per foot. –, no data]

ADOT & 

PF No.

Discharge (ft3/s) Watersurface 

slope 

(ft/ft)

Manning’s roughness 

coefficient

Minimum bed elevation

(ft) Crosssection 

source

Date 

surveyed

100-year 500-year Measured Channel Overbank Measured As-built
Table 5 29

Table 5. Computed contraction-scour depths, and hydraulic variables used in computation for the 100-year and 500-year recurrence-interval flood discharges 

at selected bridge sites in Alaska

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. Discharge at bridge: 100- and 500-year discharges were calculated using 

methodology of Jones and Fahl (1994). ft3/s, cubic feet per second; ft, feet]

ADOT & 

PF No.

Discharge at bridge 

(ft3/s)

Width of 

approach channel 

(ft)

Discharge at 

approach (ft3/s)

Flow depth in 

approach (ft)

Width of channel 

at bridge (ft)

Depth of flow

at bridge (ft)

Depth of 

contraction 

scour (ft)

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100- 

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

Phase 1.5 analysis

205 126,000 151,000 843 844 123,415 146,419 12.4 13.9 791.1 793.8 12.3 13.6 0.7 0.9

240 3,800 4,570 181 186 3,800 4,570 7.8 8.6 179.4 186.7 7.7 8.4 .0 .0

277 1,260 2,210 30.0 30.0 859 1,224 9.0 12.1 20.1 20.1 9.4 10.9 6.9 13.3

308 18,500 27,500 491 500 18,500 27,500 5.3 6.9 402.5 408.9 4.9 6.3 .7 .9

396 1,600 2,060 90 99 1,600 2,059 3.4 3.8 59.5 61.1 5.3 5.9 .9 1.3

401 4,360 7,010 79 79 3,575 4,276 7.1 10.1 70.7 71.0 9.1 10.8 1.9 6.4

505 51,900 59,700 1,482 1,485 51,900 59,700 18.2 19.8 914.4 915.8 18.1 19.7 6.0 6.5

509 9,520 11,700 257 321 9,520 11,700 4.3 4.3 248.6 304.5 4.2 4.2 .0 .0

518 7,520 9,250 394 471 7,520 9,250 3.9 3.4 379.5 453.2 3.3 3.4 .0 .0

520 5,100 6,340 223 223 5,100 6,340 3.6 4.1 216.4 216.6 3.4 3.9 .0 .0

530 3,130 3,920 32 32 2,587 2,931 9.8 11.3 46.3 49.5 6.6 7.2 .0 .0

543 1,920 2,390 144 147 1,920 2,390 1.8 2.1 136.4 139.0 1.8 2.1 .0 .0

544 7,600 9,310 176 178 7,600 9,310 4.8 5.4 168.8 171.0 4.4 4.9 .0 .0

547 1,370 1,700 84 85 1,370 1,700 2.5 2.9 80.7 83.4 2.5 2.8 .0 .0

548 9,550 11,800 183 202 9,550 11,800 6.4 7.1 86.4 90.2 7.2 8.1 3.6 4.3

556 24,400 30,600 286 292 24,400 30,600 10.4 12.0 204.6 211.5 9.0 10.1 2.3 2.5

572 10,700 11,800 160 162 10,700 11,800 6.9 7.3 155.8 157.9 6.8 7.2 .0 .0

573 79,400 109,000 921 921 78,154 105,630 13.6 17.3 422.7 428.8 12.4 15.0 8.3 10.6

603 2,100 2,600 146 147 2,095 2,578 3.5 4.0 113.6 116.0 3.6 4.0 .6 .6

658 9,430 11,200 93 93 9,081 10,423 9.9 11.3 75.0 74.9 9.0 9.8 1.7 2.3

678 2,260 2,870 75 76 1,947 2,357 7.9 8.9 62.0 62.0 7.6 8.6 2.2 3.0

686 4,420 5,400 219 221 4,420 5,400 3.5 4.0 151.5 152.1 3.6 4.0 .8 1.0

687 37,600 48,200 1,299 1,301 37,600 48,200 6.3 7.4 928.6 930.1 7.1 8.1 1.4 1.6

690 3,090 4,360 106 109 3,090 5,620 9.4 13.6 26.4 26.4 8.8 14.4 12.0 11.6

742 32,600 42,900 415 415 31,656 41,313 13.8 16.3 463.1 466.5 12.7 15.0 .0 .0

832 2,130 3,840 34 34 1,651 2,746 6.3 9.3 42.2 48.6 4.9 6.6 .6 .8

833 3,890 4,860 76 76 3,571 4,383 8.6 9.8 61.5 65.9 5.9 6.5 1.9 1.9

999 7,020 10,200 236 241 7,015 10,164 5.4 6.9 157.4 162.8 5.2 6.4 1.5 1.8

1220 9,870 12,400 174 174 9,363 11,492 7.5 9.0 118.1 131.3 6.0 6.9 2.4 2.3

1261 25,100 28,500 368 370 25,100 28,500 8.0 8.6 295.1 295.7 7.8 8.3 1.1 1.2

1282 20,300 23,400 223 226 20,300 23,400 8.4 9.1 217.5 219.1 8.3 9.0 .0 .0

1283 10,300 12,000 165 165 9,799 10,580 6.6 10.1 134.6 148.3 6.9 8.1 1.2 1.9

1284 7,880 9,180 125 129 7,880 9,180 9.3 10.0 86.6 89.8 8.0 8.6 2.2 2.4

1329 7,910 11,800 83 98 7,910 11,800 7.7 8.9 76.1 87.1 6.9 8.3 .4 .6

1389 980 1,230 99 104 980 1,230 1.6 1.8 98.2 103.1 1.6 1.8 .0 .0
30 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Phase 2 analysis

233 22,100 27,100 255 255 21,261 26,405 13.9 13.7 141.0 138.6 14.6 12.8 6.5 6.4

242 4,220 5,220 108 112 4,220 5,220 5.1 5.8 84.9 87.7 5.8 6.4 .8 .9

254 186,000 206,000 1,555 1,560 186,000 206,000 14.4 15.3 936.7 938.8 14.4 15.0 5.0 5.3

255 83,200 103,000 379 379 82,257 101,340 19.2 21.9 455.9 469.6 17.4 19.6 .0 .0

355 36,000 42,000 368 368 34,914 40,012 14.2 15.4 319.5 323.2 15.0 15.9 1.6 1.9

527 50,600 64,900 673 673 50,502 64,695 9.7 11.5 446.7 451.1 13.1 14.6 2.7 3.1

535 6,920 8,710 149 160 6,920 8,706 5.3 6.5 77.0 80.6 6.3 7.1 2.6 3.2

539 79,400 104,000 1,567 1,636 79,400 104,000 9.9 11.5 410.2 412.6 17.3 17.8 12.0 14.4

557 22,100 28,500 333 348 22,100 28,500 7.5 8.6 333.8 348.8 7.5 8.5 .0 .0

558 19,400 25,100 319 321 19,400 25,090 6.8 8.0 319.3 325.4 6.2 7.2 .0 .0

574 18,400 22,200 430 435 18,400 22,200 8.2 9.0 264.6 266.2 8.2 8.8 2.7 3.0

608 1,610 2,070 77 80 1,610 2,070 3.1 3.4 96.1 97.5 2.1 2.4 .0 .0

609 1,220 1,750 96 102 1,220 1,750 1.6 1.9 66.8 69.1 2.2 2.7 .4 .5

694 25,400 30,300 290 310 25,400 30,300 11.2 11.9 252.9 267.8 11.6 12.3 .9 1.1

1025 4,590 6,520 52 52 4,443 6,207 7.0 8.7 130.0 141.5 5.9 7.3 .0 .0

1147 43,200 53,600 334 345 43,200 53,600 12.5 14.5 277.9 284.0 12.3 14.6 1.4 1.8

1255 3,800 4,710 87 91 3,415 4,189 6.9 7.6 90.5 97.5 4.7 5.4 .5 .5

1341 6,920 8,710 341 349 6,920 8,710 5.6 6.4 227.3 231.2 6.4 7.2 1.5 1.7

1513 1,830 2,330 51 53 1,792 2,160 5.5 6.8 30.0 30.0 7.3 8.4 2.2 3.3

Table 5. Computed contraction-scour depths, and hydraulic variables used in computation for the 100-year and 500-year recurrence-interval flood discharges 

at selected bridge sites in Alaska—Continued

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. Discharge at bridge: 100- and 500-year discharges were calculated using 

methodology of Jones and Fahl (1994). ft3/s, cubic feet per second; ft, feet]

ADOT & 

PF No.

Discharge at bridge 

(ft3/s)

Width of 

approach channel 

(ft)

Discharge at 

approach (ft3/s)

Flow depth in 

approach (ft)

Width of channel 

at bridge (ft)

Depth of flow

at bridge (ft)

Depth of 

contraction 

scour (ft)

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100- 

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year
Table 6 31

Table 6. Computed pier-scour depths and hydraulic variables used in computation for the 100-year and 500-year recurrence-interval flood discharges at 

selected bridge sites in Alaska

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. K2: Correction factor for channel-flow angle of attack. ft, feet. –, no data]

ADOT & 

PF No.

Froude no.

at bridge

Depth of flow 

at bridge

(ft)

Pier

nose

shape

Angle of 

attack 

(degrees)

K2

Pier

width

(ft)

Pier

length

(ft)

Depth of 

flow at pier 

(ft)

Depth of 

pier scour 

(ft)

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

Phase 1.5 analysis

205 0.59 0.61 12.3 13.6 sharp 0 1.00 6.0 28.5 22.7 24.1 15.1 15.6

240 .17 .18 7.7 8.4 round 45 4.23 1.3 30.0 11.4 12.4 12.1 12.7

277 .39 .52 9.4 10.9 no pier – – – – – – – –

308 .71 .72 4.9 6.3 sharp 45 4.23 1.0 28.0 7.5 8.9 16.2 17.4

396 .36 .40 5.3 5.9 square 45 4.23 1.0 34.0 7.6 8.3 12.2 13.1

401 .40 .49 9.1 10.8 no pier – – – – – – – –

505 .13 .13 18.1 19.7 sharp 0 1.00 6.0 50.0 32.6 34.2 8.9 9.1

509 .93 .67 4.2 4.2 sharp 30 1.91 9.0 33.0 3.0 5.6 24.9 26.9

518 .49 .49 3.3 3.4 sharp 20 2.04 6.0 36.0 6.9 7.5 20.8 21.5

520 .61 .63 3.4 3.9 sharp 0 1.00 6.0 35.0 5.2 5.7 9.1 9.6

530 .61 .63 6.6 7.2 square 15 2.49 1.0 43.0 10.7 11.8 10.2 10.7

543 .70 .72 1.8 2.1 sharp 9 1.98 1.7 29.0 4.7 5.1 9.1 9.4

544 .68 .70 4.4 4.9 round 0 1.00 3.0 35.0 9.5 10.2 7.2 7.2

547 .67 .70 2.5 2.8 sharp 25 3.20 1.2 29.0 6.5 6.9 12.8 13.4

548 .75 .74 7.2 8.1 round 35 3.65 3.0 34.0 10.1 11.5 32.6 33.9

556 .76 .76 9.0 10.1 sharp 0 1.00 .8 35.2 15.3 17.8 4.0 4.2

572 .53 .55 6.8 7.2 sharp 0 1.00 8.0 31.0 12.5 12.9 14.1 14.5

573 .73 .73 12.4 15.0 sharp 0 1.00 4.0 20.0 20.1 23.2 12.2 12.8

603 .44 .46 3.6 4.0 round 20 1.72 3.0 12.0 5.9 6.4 10.1 10.6

658 .32 .41 9.0 9.7 no pier – – – – – – – –

678 .28 .31 7.6 8.6 square 0 1.00 1.0 1.0 13.8 15.6 3.5 3.8

686 .70 .72 3.6 4.0 square 0 1.00 .8 .8 5.5 6.0 3.3 3.4

687 .37 .39 7.1 8.1 sharp 24 3.13 1.5 20.0 9.9 10.9 13.0 13.8

690 .46 .49 8.8 14.4 no pier – – – – – – – –

742 .27 .28 12.7 15.0 sharp 0 1.00 1.8 25.0 16.6 19.0 4.3 4.6

832 .82 .38 4.9 6.3 no pier – – – – – – – –

833 .73 .73 5.9 6.5 no pier – – – – – – – –

999 .63 .65 5.2 6.4 sharp 30 3.50 .8 57.0 8.7 10.1 11.9 12.7

1220 .78 .78 6.0 6.9 round 0 1.00 1.3 35.0 8.0 9.1 3.1 3.1

1261 .69 .71 7.8 8.3 sharp 15 2.49 .8 25.0 10.6 11.1 9.4 9.7

1282 .69 .70 8.3 9.0 sharp/round 20 2.86 2.0 25.0 11.7 12.5 19.9 20.5

1283 .72 .84 6.9 6.3 sharp 0 1.00 .8 24.0 9.4 9.4 3.3 3.6

1284 .66 .66 8.0 8.6 sharp 0 1.00 .8 24.0 10.9 11.8 3.4 3.5

1329 .77 .76 6.9 8.3 sharp 5 1.59 2.0 40.0 11.1 13.8 10.3 11.0

1389 .85 .87 1.6 1.8 no pier – – – – – – – –
32 Summary and Comparison of Multiphase Streambed Scour Analysis at Selected Bridge Sites in Alaska

Phase 2 analysis

233 0.43 0.61 14.6 12.8 sharp 10 2.07 0.8 28.0 20.5 19.3 8.1 9.2

242 .59 .60 5.8 6.4 sharp 0 1.00 1.5 34.0 8.7 9.5 4.4 4.5

254 .61 .64 14.4 15.0 sharp 0 1.00 7.0 28.0 25.3 25.9 17.6 18.1

255 .44 .44 17.4 19.6 sharp 7 1.61 4.5 40.0 24.0 26.8 20.1 20.9

355 .34 .36 15.0 15.9 sharp 0 1.00 4.0 40.0 18.3 19.4 8.5 8.9

527 .45 .48 13.1 14.6 round 10 1.77 3.7 30.0 19.0 20.5 18.0 19.0

535 .80 .77 6.3 7.1 round 0 1.00 5.0 32.5 10.4 11.8 15.0 15.0

539 .46 .57 17.3 17.8 sharp 0 1.00 4.3 26.0 26.0 26.7 11.4 12.7

557 .57 .56 7.5 8.5 sharp 0 1.00 2.5 68.0 14.1 15.5 7.1 7.3

558 .58 .59 6.2 7.2 circular 0 1.00 5.0 5.0 11.7 12.9 11.7 12.2

574 .47 .49 8.2 8.8 sharp 0 1.00 3.0 38.0 11.1 11.9 6.8 7.1

608 .63 .63 2.1 2.4 square 0 1.00 3.4 24.0 3.1 3.5 6.5 6.8

609 .71 .70 2.2 2.7 group cyl. 0 1.00 1.5 24.0 3.1 3.9 3.7 3.9

694 .41 .42 11.6 12.3 round 0 1.00 5.0 19.5 15.1 16.5 11.0 11.5

1025 .42 .40 5.9 7.3 circular 0 1.00 3.0 3.0 8.6 10.5 6.6 6.9

1147 .56 .54 12.3 14.6 round 0 1.00 3.0 22.0 18.9 21.4 7.2 7.2

1255 .65 .62 4.7 5.4 sharp 20 2.86 .8 24.0 7.5 8.5 9.4 9.6

1341 .30 .31 6.4 7.2 circular cyl. 0 1.00 6.0 6 8.35 9.26 8.8 9.3

1513 .16 .17 7.3 8.4 No pier – – – – – – – –

Table 6. Computed pier-scour depths and hydraulic variables used in computation for the 100-year and 500-year recurrence-interval flood discharges at 

selected bridge sites in Alaska—Continued

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. K2: Correction factor for channel-flow angle of attack. ft, feet. –, no data]

ADOT & 

PF No.

Froude no.

at bridge

Depth of flow 

at bridge

(ft)

Pier

nose

shape

Angle of 

attack 

(degrees)

K2

Pier

width

(ft)

Pier

length

(ft)

Depth of 

flow at pier 

(ft)

Depth of 

pier scour 

(ft)

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year
Table 7. Computed pressure-scour depths and hydraulic variables used in computation for the 100-year and 500-year recurrence-interval flood discharges at 

selected bridge sites in Alaska

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. –, no pressure flow for 100-year discharge. ft, feet; ft3/s, cubic feet per second; 

ft2, square feet; ft/s, feet per second]

ADOT 

& PF 

No.

Depth at

approach

(ft)

Discharge in 

approach

(ft3/s)

Area of 

approach 

(ft2

)

Approach 

velocity

(ft/s)

Depth of 

overflow

(ft)

Average 

depth at 

bridge 

(ft)

Median 

particle 

diameter 

(ft)

Incipient motion 

velocity (ft/s)

Pressure 

scour 

(ft)

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

401 – 8.9 – 4,781 – 701 – 6.8 – 0 9.0 0.03281 – 5.3 – 8.1

658 11.1 11.2 8,824 10,449 1,024 1,035 8.6 10.1 3.5 4.0 6.0 .03281 5.5 5.5 0.0 .2

678 7.8 8.9 1,953 2,355 589 677 3.3 3.5 0 .6 6.5 .03281 5.2 5.3 2.1 .2

Table 7 33
Table 8. Computed pier-stem scour depths, and hydraulic variables used in computation for the 100-year and 500-year recurrence-interval flood discharges 

at selected bridge sites in Alaska

[ADOT & PF No.: Alaska Department of Transportation and Public Facilities. Khpier: Coefficient to account for height of pier base above streambed and 

shielding effect of footing. K1: Correction factor for pier-nose shape. K2: Correction factor for channel-flow angle of attack. D84: particle size for which 84 

percent are finer by weight. ft, feet; ft/s, feet per second]

ADOT & 

PF No.

Average depth 

of approach (ft) Khpier

Distance 

between 

front edge 

of pile cap 

and pier 

(ft)

Width 

of pier 

(ft)

Height of pier 

stem above bed 

(ft) Maximum 

approach 

velocity (ft/s)

K1 K2

Depth of pier 

stem scour

(ft)

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

205 22.7 24.1 0.0 0.0 1.3 6.0 4.8 5.3 16.0 17.0 0.9 1.0 0.3 0.4

505 32.6 34.2 .1 .1 6.0 6.0 13.0 13.0 4.1 4.3 .9 1.0 .4 .4

544 9.5 10.2 .0 .0 1.7 3.0 2.3 2.4 11.9 12.7 1.0 1.0 .3 .3

572 6.5 7.3 .1 .1 5.3 3.8 11.0 11.1 10.7 11.1 .9 1.0 .7 .7

539 26.0 26.7 .1 .1 5.0 4.3 1.1 6.4 13.3 16.7 .9 1.0 .9 1.7

694 15.1 16.5 .3 .3 8.5 5.0 3.3 3.5 9.0 9.6 1.0 1.0 2.8 2.9

ADOT & 

PF No.

Velocity at 

footing 

(ft/s)

Adjusted flow 

velocity

(ft/s)

Width of 

pile cap 

(ft)

Distance from 

bed to top of 

footing after 

scour (ft) 

D84 of bed 

material 

(ft)

Adjusted depth 

of flow 

(ft)

Depth of 

footing scour 

(ft)

Depth of 

complex 

pier scour

(ft)

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

100-

year

500-

year

205 12.6 13.5 15.9 16.9 23.0 5.0 5.5 0.2 22.9 24.3 26.6 27.7 26.9 28.1

505 3.6 3.7 4.0 4.3 16.0 13.2 13.2 .2 32.8 34.4 13.9 14.2 14.3 14.6

544 9.2 9.9 11.7 12.6 10.0 2.4 2.5 .2 9.7 10.4 13.7 14.1 13.9 14.4

572 11.0 11.3 10.2 10.6 8.0 11.3 11.4 .2 6.9 7.6 14.1 14.3 14.8 15.0

539 8.5 13.5 13.1 16.2 24.0 1.6 7.2 .2 26.4 27.5 19.4 29.5 20.3 31.2

694 6.8 7.2 8.3 8.8 15.0 4.7 5.0 .2 16.5 18.0 17.0 17.6 19.8 20.5

Table 8 34
Manuscript approved for publication, April 28, 2004

Prepared by the Publishing Group, U.S. Geological Survey, 

Washington Water Science Center, Tacoma, Washington

USGS Publishing staff

Judith A. Wayenberg

Bill Gibbs 

Bob Crist

Linda Rogers

Chief, Water Resources Office

U.S. Geological Survey

Alaska Science Center

4230 University Drive, Suite 201

Anchorage, AK 99508-4664

http://alaska.usgs.gov