Prepared in cooperation with the Alaska Department of Transportation and Public Facilities

Evaluation of Streambed Scour at Bridges over Tidal 

Waterways in Alaska

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5245
Cover: Hartney Bay, Alaska, during an incoming tide at bridge number (BN) 1085. 

(Photograph taken by J.S. Conaway, U.S. Geological Survey, September 5, 2009.
Evaluation of Streambed Scour at 

Bridges over Tidal Waterways in Alaska

By Jeffrey S. Conaway and Paul V. Schauer

Prepared in cooperation with the Alaska Department of Transportation and 

Public Facilities

Scientific Investigations Report 2012–5245

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Conaway, J.S., and Schauer, P.V., 2012, Evaluation of streambed scour at bridges over tidal waterways in Alaska: U.S. 

Geological Survey Scientific Investigations Report 2012-5245, 38 p. 
iii

Contents

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

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

Study Approach..............................................................................................................................................5

Tidal Analysis..................................................................................................................................................5

Storm Surges.........................................................................................................................................8

Velocity Measurements during Tidal Exchanges ..........................................................................13

Riverine Analysis..........................................................................................................................................15

Data Collection and Model Parameters..........................................................................................16

Estimation of Contraction and Pier Scour .............................................................................16

Contraction Scour.............................................................................................................18

Pier Scour ..........................................................................................................................18

Evaluation of Streambed Scour Computations.....................................................................19

Repeated Cross-Section Surveys ....................................................................................................21

Overall Risk to Bridges over Tidal Waterways........................................................................................22

High-Risk Bridges and Potential Mitigating Factors.....................................................................24

BN 347 Bonanza Creek .............................................................................................................24

BN 1127 Safety Sound...............................................................................................................24

BN 385 Salt Creek ......................................................................................................................24

BN 444 Salmon River.................................................................................................................24

Bridges Crossing Rivers Draining into Turnagain Arm........................................................25

BN 627 Placer River Overflow and BN 629 Placer River Main Crossing.................25

BN 630 and BN 631 Bridges Crossing Portage Creek.................................................25

BN 634 Twentymile River.................................................................................................28

BN 638 Virgin Creek..........................................................................................................28

BN 639 Glacier Creek .......................................................................................................28

BN 1017 Seldovia Slough..........................................................................................................28

BN 1085 Hartney Bay.................................................................................................................28

BN 1121 Knik River and BN 1124 Matanuska River..............................................................29

BN 1149 Kenai River..................................................................................................................30

BN 1188 Salmon Creek..............................................................................................................30

BN 1197 and BN 1863 Lemon Creek........................................................................................30

BN 1274 Monashka Creek ........................................................................................................31

BN 1385 Tununak River .............................................................................................................31

BN 1783 Spruce Creek ..............................................................................................................31

BN 2150 Ship Creek ...................................................................................................................31

Summary........................................................................................................................................................31

References Cited .........................................................................................................................................33

Glossary.........................................................................................................................................................35

Appendix A. High and Low Tide Elevations and Water-Surface Elevations at the 

1-Percent Annual Exceedance Probability (AEP) Discharge at Cross Sections at 

33 Bridges over Tidal Waterways in Alaska ..............................................................................37

Appendix B. Multiple Cross-Section Surveys for 42 Bridges over Tidal Waterways in 

Alaska...............................................................................................................................................37
iv

Figures

Figure 1. Map showing location of bridges over tidal waterways in Alaska selected for

analysis of susceptibility to streambed scour .......................................... 3

Figure 2. Diagrams showing classification of tidally controlled, affected, and influenced

bridges ....................................................................................... 5

Figure 3. Graph showing water-surface elevations from the Anchorage tidal station and

at the tidally-controlled bridge number 2150 over Ship Creek, Alaska ............... 6

Figure 4. Graph showing water-surface elevations from the Sunriset tidal station and at

bridge number 634 over the tidally affected Twentymile River, Alaska ............... 7

Figure 5. Diagrams showing examples of the three categories used to define the degree

to which a bridge can constrict tidal exchanges and backwater riverine flow...... 8

Figure 6. Map showing location of bridges subject to coastal flooding and detailed

location of bridge numbers 347 and 1127 along the Nome-Council Road, Alaska ... 10

Figure 7. Diagrams showing surveyed channel cross sections at bridges BN 347 and 

BN 1127 along the Nome-Council Road showing the 1-percent annual 

exceedance probability water-surface elevation (AEPWL) for coastal flooding 

at Nome, Alaska ........................................................................... 11

Figure 8. Graph showings the predicted and measured tides for Nome, Alaska, and the

measured water-surface elevation at bridge number (BN) 1127 from 

October 27–31, 2010, and October 6–November 17, 2010 .............................. 12

Figure 9. Graph showing water-surface elevations and velocity measurements made

during a tidal exchange on September 23, 2010, at BN 1085, Hartney Bay, Alaska 13

Figure 10. Map showing bridges over tidal waterways along Turnagain Arm, Alaska ......... 26

Figure 11. Graph showing surveyed cross sections at bridge number (BN) 627 and BN 629, 

along Seward Highway, Alaska............................................................ 27

Figure 12. Oblique aerial photograph looking downstream at an Alaska Railroad bridge

(upstream) and Seward Highway bridge number 634, Twentymile River, Alaska ... 29

Tables

Table 1. Bridges over tidal waterways in Alaska selected for streambed-scour analysis

and characteristics of riverine and tidal conditions at the bridges .................. 4

Table 2. Hydrograph analysis and ranking of the upstream storage potential at selected

bridges over tidal waterways in Alaska................................................... 9

Table 3. Bridges at risk from storm surge flooding and the 1-percent annual

exceedance probability water level in feet, local datum of the bridges near

Nome and Toksook Bay, Alaska............................................................ 9

Table 4. Hydraulic measurements made during tidal exchanges at selected bridges

over tidal waterways in Alaska ............................................................ 14

Table 5. Selected hydraulic variables used to construct hydraulic models for the

analyses of streambed scour at selected bridges over tidal waterways in

Alaska ....................................................................................... 17

Table 6. Computed contraction-scour depths, and hydraulic variables used in

computation for the 1-percent annual exceedance probability (AEP) discharge

at selected bridges over tidal waterways in Alaska .................................... 19
v

Table 7. Computed pier-scour depths and hydraulic variables used in computation for

the 1-percent annual exceedance probability (AEP) discharge at selected

bridges over tidal waterways in Alaska................................................... 20

Table 8. Sites where channel instability was identified from repeated cross-sectional

surveys at the bridge........................................................................ 21

Table 9. Range of values used to assign levels of risk to bridges over tidal waterways in

Alaska ....................................................................................... 22

Table 10. Categories and the sum of the assigned levels of risk to bridges over tidal

waterways in Alaska ........................................................................ 23

Tables—Continued

Conversion Factors, Datums, and Abbreviations and 

Acronyms

Conversion Factors

Multiply By To obtain

Length

foot (ft) 0.3048 meter (m)

foot (ft) 304.8 millimeter (m)

mile (mi) 1.609 kilometer (km)

Volume

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

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

Pressure

pound per square foot (lb/ft2) 0.04788 kilopascal (kPa) 

Density

slug per cubic foot (lb/ft3) 515.378 kilogram per cubic meter (kg/m3)

Datums

Vertical coordinate information is referenced to the local bridge datum used by the Alaska 

Department of Transportation and Public Facilities. Some tidal elevations were not translated to 

the local bridge datum and are referenced to Mean Sea Level (MSL). 

Horizontal coordinate information is referenced to the World Geodetic Standard of 1984 

(WGS 84).

Abbreviations and Acronyms

ADCP acoustic Doppler current profiler

ADOT&PF Alaska Department of Transportation and Public Facilities

AEPWL Annual exceedance probability water level

BN bridge number

HEC Hydraulic Engineering Circular

HEC-RAS Hydraulic Engineering Center River Analysis System

USGS U.S. Geological Survey
vi

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Evaluation of Streambed Scour at Bridges over Tidal 

Waterways in Alaska

By Jeffrey S. Conaway and Paul V. Schauer

Abstract

The potential for streambed scour was evaluated at 41 

bridges that cross tidal waterways in Alaska. These bridges are 

subject to several coastal and riverine processes that have the 

potential, individually or in combination, to induce streambed 

scour or to damage the structure or adjacent channel. The 

proximity of a bridge to the ocean and water-surface elevation 

and velocity data collected over a tidal cycle were criteria 

used to identify the flow regime at each bridge, whether tidal, 

riverine, or mixed, that had the greatest potential to induce 

streambed scour.

Water-surface elevations measured through at least one 

tide cycle at 32 bridges were correlated to water levels at the 

nearest tide station. Asymmetry of the tidal portion of the 

hydrograph during the outgoing tide at 12 bridges indicated 

that riverine flows were stored upstream of the bridge during 

the tidal exchange. This scenario results in greater discharges 

and velocities during the outgoing tide compared to those 

on the incoming tide. Velocity data were collected during 

outgoing tides at 10 bridges that experienced complete flow 

reversals, and measured velocities during the outgoing tide 

exceeded the critical velocity required to initiate sediment 

transport at three sites. 

The primary risk for streambed scour at most of the sites 

considered in this study is from riverine flows rather than 

tidal fluctuations. A scour evaluation for riverine flow was 

completed at 35 bridges. Scour from riverine flow was not the 

primary risk for six tidally-controlled bridges and therefore 

not evaluated at those sites. Field data including channel 

cross sections, a discharge measurement, and a water-surface 

slope were collected at the 35 bridges. Channel instability 

was identified at 14 bridges where measurable scour and or 

fill were noted in repeated surveys of channel cross sections 

at the bridge. Water-surface profiles for the 1-percent annual 

exceedance probability discharge were calculated by using 

the Hydrologic Engineering Center’s River Analysis System 

model, and scour depths were calculated using methods 

recommended by the Federal Highway Administration. 

Computed contraction-scour depths were greater than 2.0 feet 

at five bridges and computed pier-scour depths were 4.0 feet 

or greater at 15 bridges. 

The potential for streambed scour by both coastal and 

riverine processes at the bridges considered in this study were 

evaluated, ranked, and summed to determine a cumulative 

risk factor for each bridge. Possible factors that could mitigate 

the scour risks were investigated at 22 bridges that had high 

individual or cumulative rankings. Mitigating factors such 

as piers founded in bedrock, deep pier foundations relative 

to scour depths, and lack of observed scour during field 

measurements were documented for 13 sites, but additional 

study and monitoring is needed to better quantify the 

streambed scour potential for nine sites. Three bridges prone 

to being affected by storm surges will require more data 

collection and possibly complex hydrodynamic modeling to 

accurately quantify the streambed scour potential. Continuous 

monitoring of water-surface and streambed elevation at one or 

more piers is needed for two bridges to better understand the 

tidal and riverine influences on streambed scour. 

Introduction

Alaska, with its nearly 34,000 mi of coast, has more 

than twice the shoreline of the continental United States and 

tidal ranges that are the second largest in North America. The 

state’s limited coastal infrastructure is subject to the effects 

of tidal fluctuations, storm surges, littoral drift, wave action, 

and ice. In addition to these processes, bridges over tidal 

waterways can be subject to riverine processes that can induce 

streambed scour, which is the major cause of bridge failure in 

the United States (Murillo, 1987).

After several tragic bridge failures in the late 1980s, the 

Federal Highway Administration recommended that every 

bridge over a waterway be evaluated for susceptibility to 

streambed scour (U.S. Department of Transportation, 1988). 

In response to that recommendation, 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 multi-phase approach was applied to bridges 

selected by ADOT&PF as potentially subject to scour. 

Heinrichs and others (2001) documented procedures and 

results from the initial phase of this project at 325 bridges. 
2 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

Conaway (2004) selected 54 bridges from the initial study and 

performed a more intensive analysis of scour susceptibility. 

Both studies followed the guidance outlined in the Hydraulic 

Engineering Circular (HEC)-18 (Richardson and others, 1993) 

and included estimates for contraction and pier scour at each 

bridge computed from one-dimensional hydraulic models of 

the 1- and 0.2-percent annual exceedance probability (AEP) 

flows corresponding to the 100- and 500-year recurrenceinterval peak flows. The complexity of tidal hydraulics and 

limited federal guidance on analysis procedures exempted 

bridges over tidal waterways from these initial scour analyses. 

The fourth edition of HEC-18 (Richardson and Davis, 2001) 

provided the first guidance for evaluating bridges over tidal 

waterways and HEC-25 (Zevenbergen and others, 2004; 

Douglass and Krolak, 2008) expanded upon this work. 

Bridges over tidal waterways are subject to several 

coastal processes that have the potential to induce streambed 

scour or damage the structure or roadway approaches 

to the structure. These processes include scour during 

tidal exchanges or storm surges, scour and erosion due to 

hydrodynamic loading from waves, and erosion or deposition 

from long-shore drift. Most bridges in this study cross rivers 

and creeks rather than estuaries or tidal inlets; therefore, the 

local topography limits the effects of these coastal processes. 

Bridges over tidal waterways also can be subject to 

riverine processes that induce streambed scour at a bridge. 

Streambed scour at bridges results from the complex hydraulic 

conditions created either by the contraction of flow through 

the bridge or by the interaction of flow with bridge piers 

or abutments that results in the hydraulic erosion of the 

streambed or stream banks. Streambed scour is commonly 

separated into three processes: long-term degradation of 

the channel, contraction scour, and local scour. Long-term 

degradation of the channel accounts for the natural channel 

degradation that would occur regardless of the bridge. 

Contraction scour results from the decrease in channel width 

caused by the presence of the bridge and the attendant increase 

in flow velocity and the potential for sediment transport in this 

area. Local scour at piers results from horseshoe and wake 

vortices that form at the upstream, downstream, and sides of 

piers and from flow acceleration at the pier obstruction. There 

is a vast amount of literature on the subject of streambed 

scour at bridges; Richardson and Lagasse (1999) have edited a 

compendium on the subject that provides a thorough overview 

of historical and recent research. 

Tides and storm surges can intensify scour conditions 

at a bridge by temporarily increasing the volume of water 

upstream of the bridge from both backwater effects and tidal 

flooding. The bridge can potentially act as a constriction to 

the outgoing flow, thereby creating a head difference between 

the area upstream of the bridge and the ocean. The potential 

for storage upstream of the bridge and the degree to which the 

bridge opening limits the return of stored volume back to the 

ocean are important factors in the assessment of scour. 

In 2009, the USGS, in cooperation with ADOT&PF, 

began an investigation to evaluate streambed scour potential 

at 41 bridges over tidal waterways in Alaska (fig. 1, table 1). 

The objectives of this investigation were to (1) determine the 

degree of tidal influence at each bridge; (2) identify the flow 

regime (tidal or riverine) that presents the greatest risk to 

the bridge and determine the appropriate method to evaluate 

this risk; (3) measure riverine discharge, velocity, and watersurface elevations during a tidal cycle, and channel cross 

sections at selected sites; (4) investigate long-term degradation 

from repeated channel surveys; and (5) estimate the scour 

potential for the 1-percent AEP riverine flow using the current 

scour evaluation techniques in HEC-18 (Richardson and 

Davis, 2001) and HEC-25 (Douglas and Krolak, 2008).

There are varying degrees of complexity associated with 

the hydrologic and hydraulic processes at bridges over tidalaffected waterways. Relatively simple one-dimensional steady 

flow models are sufficient for evaluating hydraulic variables 

at some sites, but at other sites complex data intensive, 

multi-dimensional hydrodynamic models may be required to 

determine these same variables. The approach taken in this 

study was to determine the level of complexity required for an 

individual site and then utilize the simplest evaluation method 

available to accurately evaluate the potential for streambed 

scour. Several sites would require multi-dimensional 

hydrodynamic models to fully evaluate the hydrologic and 

hydraulic processes, but if these processes do not have the 

potential to scour the streambed at the bridge, then a complex 

model is unwarranted. Sites that cannot be evaluated fully 

with the procedures outlined in this study are recommended 

for more intensive study.

This report describes (1) the techniques developed to 

evaluate streambed scour at 41 bridges over tidal waterways 

in Alaska; (2) the results of these analyses; and (3) potential 

mitigating factors at high risk sites and recommendations for 

further analysis. Contraction and pier scour were evaluated at 

each bridge using variables computed from one-dimensional 

steady state step-backwater models. Abutment scour was 

not evaluated because all of the bridges in this study have 

abutments that are either founded in bedrock or armored by 

riprap or sheet piling. Hydraulic models were constructed 

from existing data and field data collected for this study. The 

degree of tidal influence at a bridge was quantified using 

water-stage data collected over one or more tidal cycles. 

Velocity data were collected during the falling tide at bridges 

that experienced flow reversals during tidal exchanges. 
Introduction 3

Figure 1. Location of bridges over tidal waterways in Alaska selected for analysis of susceptibility to streambed scour.

aktac12-5196_fig 01

BN 444

RUSSIA

Chukchi Sea

ARCTIC OCEAN

Beaufort Sea

CANADA

ALASKA

Bering Sea

PACIFIC OCEAN

60°N

50°N

Gulf of Alaska

170°E 180° 170°W 160°W 150°W 140°W 130°W 120°W

BN 347

BN 1127

BN 1385

BN 1764

BN 399

BN 400

BN 402

BN 1017

BN 1783

BN 1085

BN 1124

BN 1121

BN 2150

BN 214

BN 1149

Cook Inlet

BN 387

BN 385

BN 989

BN 990

BN 992

BN 1274

BN 429

BN 2078

BN 301

BN 724

BN 418

BN 732

BN 787

BN 1188

BN 1197

BN 1863

BN 620

BN 627

BN 629

BN 630

BN 631

BN 634

BN 636

BN 638

BN 639

0

0 100 200 300 400 500 KILOMETERS

100 200 300 400 500 MILES

EXPLANATION

Bridge and bridge No. BN 724
4 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska Table 1. Bridges over tidal waterways in Alaska selected for streambed-scour analysis and characteristics of riverine and tidal conditions at the bridges. [Tide elevations are the maximum and minimum since 1901. Bold values indicate tides referenced to mean sea level. Abbreviations: WGS, World Geodetic Standard of 1984; ft3/s, cubic foot per second; ft, foot; mi, mile] Bridge No. Bridge name Latitude (WGS 84) Longitude (WGS 84) Magnitude of 1-percent annual exceedance probability discharge (ft3/s) Tide elevation (ft) Tidal classification Nearest tidal station Station type Distance to tidal station (mi) Minimum Maximum 214 Swanson River 60°47'57.68"N 151° 0'47.47"W 3,040 -14.8 17 Affected East Foreland Subordinate 14.7 301 Klawock River 55°32'54.95"N 133° 5'49.12"W Only tidal -3.4 12.9 Controlled Craig Subordinate 4.9 347 Bonanza Creek 64°32'13.59"N 164°29'25.72"W Only tidal -0.5 1.7 Controlled Nome Reference 27.1 385 Salt Creek 57°39'0.62"N 152°31'34.86"W 578 77.9 92.2 Controlled Women’s Bay Reference 2.6 387 Chilkoot River 59°19'27.81"N 135°33'27.53"W 12,200 -10.0 16.4 Affected Haines Inlet Subordinate 7.7 399 King Salmon Creek 58°41'45.80"N 156°41'51.20"W 3,120 -0.6 9 Affected Omakstalia Point Subordinate 2.0 400 Leader Creek 58°44'49.18"N 156°56'37.82"W 100 -1.8 21.9 Affected Morakas Point Subordinate 1.0 402 Pauls Creek 58°43'28.81"N 156°46'50.21"W 1,700 -0.6 9 Affected Omakstalia Point Subordinate 2.0 418 Sheep Creek 58°15'37.45"N 134°19'31.91"W 1,140 -7.4 18.2 Influenced JUNEAU Reference 4.3 429 Blind River 56°36'54.92"N 132°49'12.64"W 2,710 -4.5 20.6 Influenced Anchor Point Subordinate 4.5 444 Salmon River 58°24'45.47"N 135°44'17.31"W 10,100 -4.9 19.1 Affected Bartlett Cove Subordinate 5.8 620 Ingram Creek 60°50'47.90"N 149° 3'36.56"W 5,300 -23.9 21 Influenced Sunrise Subordinate 13.1 627 Placer River Overflow 60°49'11.49"N 149° 0'9.01"W 10,200 -23.3 21.6 Affected Sunrise Subordinate 16.0 629 Placer River Main Crossing 60°49'2.15"N 148°59'20.10"W 10,200 -23.8 21.1 Affected Sunrise Subordinate 16.0 630 Portage Creek 1 60°49'15.48"N 148°58'38.39"W 7,160 -22.8 22.1 Influenced Sunrise Subordinate 16.0 631 Portage Creek 2 60°49'36.62"N 148°58'45.81"W 7,160 -22.7 22.2 Influenced Sunrise Subordinate 16.0 634 Twentymile River 60°50'41.46"N 148°59'22.63"W 18,420 -21.7 23.2 Affected Sunrise Subordinate 15.4 636 Peterson Creek 60°53'4.56"N 149° 2'55.07"W 547 -22.4 22.5 Influenced Sunrise Subordinate 12.9 638 Virgin Creek 60°55'58.58"N 149° 9'23.69"W 563 -26.2 18.7 Affected Sunrise Subordinate 9.5 639 Glacier Creek 60°56'19.12"N 149°10'7.26"W 4,920 -25.3 19.5 Influenced Sunrise Subordinate 9.3 724 Ketchikan Creek 55°20'37.85"N 131°38'26.75"W 3,180 -3.5 20.9 Controlled Ketchikan Reference 0.8 732 Gold Creek 58°17'56.67"N 134°25'11.50"W 3,250 -8.0 17.9 Controlled JUNEAU Reference 0.1

787 Salmon Creek Twin Lakes Drive 58°19'52.17"N 134°28'19.26"W 3,290 -4.9 20.7 Influenced JUNEAU Reference 2.9

989 Sargent Creek 57°42'33.09"N 152°34'6.28"W 4,230 -2.8 11.5 Influenced Women’s Bay Reference 2.7

990 Russian River 57°42'22.77"N 152°34'16.76"W 4,730 -2.9 11.4 Influenced Women’s Bay Reference 2.7

992 Salonie Creek 57°41'46.76"N 152°33'37.91"W 5,090 -2.8 11.5 Influenced Women’s Bay Reference 2.7

1017 Seldovia Slough 59°26'13.45"N 151°42'31.00"W 200 -8.8 20.5 Controlled Seldovia Reference 0.9

1085 Hartney Bay 60°30'7.18"N 145°51'48.04"W 2,810 -9.2 10.9 Controlled Gravel Point Subordinate 4.3

1121 Knik River NB 61°28'55.87"N 149°15'11.65"W 120,000 7.1 21.4 Influenced Anchorage Reference 26.9

1124 Matanuska River NB 61°30'15.42"N 149°15'2.07"W 47,000 7.1 21.4 Influenced Anchorage Reference 27.9

1127 Safety Sound 64°28'19.73"N 164°44'47.07"W Predominantly tidal -0.5 1.7 Controlled Nome Reference 19.51149 Kenai River at Kenai Bridge 60°31'36.06"N 151°12'32.34"W 42,300 -10.0 21.3 Affected Kenai City Pier Subordinate 1.81188 Salmon Creek at Egan Drive 58°19'49.39"N 134°28'26.33"W 3,290 -4.9 20.7 Influenced JUNEAU Reference 2.91197 Lemon Creek NB 58°21'2.73"N 134°30'28.64"W 6,940 -8.5 16.9 Affected Fritz Cove Subordinate 4.1

1274 Monashka Creek 57°50'28.60"N 152°26'45.11"W 2,210 -7.5 6.77 Influenced Kodiak Subordinate 4.1

1385 Tununak River 60°34'40.70"N 165°16'7.66"W 1,220 -1.6 5.3 Controlled Tachikuga Bay Subordinate 74.9

1764 Indian Creek 56°17'54.16"N 158°24'39.88"W 1,390 -2.5 11.7 Influenced Anchorage Bay Subordinate 0.4

1783 Spruce Creek 60° 4'26.04"N 149°26'43.61"W 3,930 -3.5 13.8 Influenced Seward Reference 3.0

1863 Lemon Creek SB 58°21'2.12"N 134°30'29.82"W 6,940 -8.5 16.9 Affected Fritz Cove Subordinate 4.1

2078 Deer Creek 55°54'8.24"N 132°30'6.72"W 810 -4.6 20.2 Affected Karta Bay Subordinate 8.6

2150 Ship Creek 61°13'25.41"N 149°53'33.78"W 1,900 -22.7 17.8 Controlled Anchorage Reference 0.7
Tidal Analysis 5

Study Approach

Variable coastal and riverine conditions at the study 

sites required that a flexible approach be taken to evaluate the 

scour potential for bridges in the intertidal zone. This study 

followed the general guidelines of HEC-18 (Richardson and 

Davis, 2001) and applied one or more of the following levels 

of approach to: 

1. Determine the magnitude of tides and storm surges, the 

stability of the crossing, and determine if the hydraulic 

analysis should include tidal flows, riverine flows, or both.

2. Determine hydraulic variables needed to evaluate 

contraction and pier scour, typically computed from 

a one-dimensional steady or unsteady step-backwater 

model. 

3. Determine hydraulic variables using complex multidimensional hydrodynamic modeling. 

All of the bridges in this study were analyzed through 

the second level, which corresponds to a HEC-18 Level 

2 analysis. For sites where the third level of approach 

corresponding to a HEC-18 Level 3 analysis seemed 

appropriate, velocity data were collected during one tidal cycle 

to determine if the velocities during the outgoing tide were 

high enough to induce scour at the bridge sites. 

Tidal Analysis

The degree to which astronomical tides affect the 

hydraulic conditions at a bridge is related to the magnitude of 

the tides, proximity of the bridge to the coast, bridge opening 

geometry and the area upstream where incoming tidal flow can 

be detained or stored (upstream storage area), and streambed 

slope. In Alaska, astronomical tides are semidiurnal, with two 

high tides and two low tides for every lunar day (24.8 hours). 

The two daily high tides are of similar magnitude at most sites 

in Alaska, but some areas experience mixed tides, whereby 

one high tide is significantly higher than the other. Several 

bridges in this study are in or near upper Cook Inlet, which has 

the second largest tidal range in North America. 

The extent of tidal influence on flow through each 

bridge was determined from a variety of sources including 

tidal-station data, water-surface elevations measured through 

tide cycles, and aerial photography. These data were then 

used to group all of the sites into three categories (1) tidally 

controlled, (2) tidally affected, and (3) tidally influenced 

(fig. 2). Tidally-controlled bridges have a full flow reversal at 

every tide cycle. Tidally-affected bridges have reverse flow 

during some tide cycles, but tidal action is not the dominant 

flow condition. Tidally-influenced bridges are dominated by 

riverine flow and experience backwater at the bridge, but no 

flow reversal. 

The highest and lowest predicted tides since 1901 

were determined for each bridge from data at the nearest 

tidal station (table 1). Tidal stations are either reference 

stations operated by the National Oceanic and Atmospheric 

Administration or subordinate stations at which tide and 

current predictions are calculated based on observations at 

the reference stations. The magnitude of tidal fluctuations 

at the tidally-controlled bridges are usually similar to 

those at the tidal stations, and the hydrograph at the bridge 

matches that at the tide stations for the incoming and the 

outgoing tides (fig. 3). The magnitude is less at the tidallyaffected and tidally-influenced bridges, and the relationship 

between the water-surface elevations at bridges and tidal 

stations are dependent on several factors. Tidally-affected 

and tidally-influenced bridges can have a slightly different 

hydrograph than the tidal station on the outgoing tide, due to 

a combination of the riverine inflow to the bridge being stored 

and the outgoing tide being restricted by the bridge opening 

(fig. 4). During the incoming tide, however, those influences 

are minimized and the shape of the hydrograph at the bridge is 

virtually the same as that at the tide station. 

Pressure transducers were installed at 33 bridges for 

at least one tidal cycle to develop a relation between watersurface elevations at the bridge and at the nearest tide station 

during an incoming tide. Transducers were not installed 

Figure 2. Classification of tidally controlled, affected, and 

influenced bridges.

aktac12-5196_fig 02

Tidaly-Affected

Extent of mean high

high water from tides 

(Flow reversals at the bridge

occur during some tide cycles)

Tidaly-Influenced

Extent of mean high

high water from tides

(No flow reversals

at bridge)

A.

B.

OCEAN

Tidal Flow

Bridge

Riverine Flow

Riverine Flow

Tidal Flow

BAY

OCEAN

Bridge

Storage area of tidal flow

Tidaly-Controlled

Flow reversal during

each tide cycle

Extent of mean high

high water from tides
6 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

at three bridges because of logistical constraints or at an 

additional five bridges because the affect of tides on the 

riverine flow was negligible. The water-surface elevations at 

tidally-affected and tidally-influenced bridges may not display 

the full signature of the incoming tide due to the elevation 

of the bridge above mean sea level. A minimal or no tidal 

signature would be expected in the water-surface elevation at 

these bridges for the low-tide part of the tidal cycle. The first 

step in the correlation between the nearest tide station and the 

bridge was to determine the time difference between the hightide elevations at the tide station and at the bridge. Because 

the tide stations typically are not near the bridge, and the river 

is affected by backwater caused by the incoming tide, there is 

a lag between the sites. The time lag was considered to be the 

difference between the time of the maximum water surface 

at the bridge and the time of the high tide at the tide station. 

Tide elevations from the tide station were converted into the 

local bridge elevation datum (bridge datum). Elevations of 

the maximum high and minimum low tides (since 1901), 

mean higher high water (MHHW), and mean lower low 

water (MLLW) were plotted on cross sections at each of the 

33 bridges to determine the tidal elevations relative to the 

bridge structure (appendix A). The MHHW is the average of 

the higher high water height of each tidal day observed over 

a period specified by the National Ocean Service. In Alaska, 

tidal datums are computed on a 5-year epoch. The MLLW is 

the average of the lower low water height of each tidal day for 

the same time period. Plots of tidal elevations at the bridges do 

not reflect the cumulative water-surface elevations that would 

result by adding the tidal-elevation component to the riverineelevation component shown individually in the plots (tide plus 

riverine). These components are not strictly additive because 

of varying channel geometries, valley slopes, discharges, and 

backwater effects. At several locations, the mean low-low 

water elevation is below the surveyed streambed elevations at 

the bridge. This indicates that the bridge is located above this 

elevation and would be classified as either tidally influenced 

or tidally affected. The MLLW elevation is above the surveyed 

cross section at the bridge sites classified as tidally controlled. 

Figure 3. Water-surface elevations from the Anchorage tidal station and at the tidally-controlled bridge number 2150 over Ship 

Creek, Alaska. Tidal station elevations have been adjusted to the bridge datum. 

aktac12-5196_fig03

08-06-2009 (1445) 08-06-2009 (1645) 08-06-2009 (1845) 08-06-2009 (2045) 08-06-2009 (2245) 08-07-2009 (0045) 08-07-2009 (0245) 

-20

-15

-10

-5

0

5

10

15

Elevation, in feet, local bridge datum

Tidal Influence

Riverine

Base Flow

Riverine

Base Flow

EXPLANATION

Water-surface elevation at bridge

Tide level from tide station

Time when tide enters and exits the bridge

Date and time
Tidal Analysis 7

Figure 4. Water-surface elevations from the Sunriset tidal station and at bridge number 634 over the tidally affected Twentymile 

River, Alaska. Sunrise tidal station elevations have been adjusted to the bridge datum.

aktac12-5196_fig04

Tidal Influence Riverine

Base Flow

Riverine

Base Flow

EXPLANATION

Water-surface elevation at bridge

Tide level from tide station

Time when tide enters and exits the bridge

-20

-15

-10

-5

0

5

10

15

20

06-08-2009 (1715) 06-08-2009 (1915) 06-08-2009 (2115) 06-08-2009 (2315) 06-09-2009 (0115) 06-09-2009 (0315) 06-09-2009 (0515) 

Effect of Channel Contraction

and Riverine Backwater

Date and time

Elevation, in feet, local bridge datum

Tidal hydrographs are typically symmetrical through 

a tidal cycle, because the tide rises as fast as it falls. 

Asymmetrical hydrographs can occur at bridges when tidal 

flow encounters riverine flow and riverine flow is temporarily 

affected by backwater caused by the opposing tide. The 

duration of the outgoing tide is then greater than that of the 

incoming tide because the outgoing tide includes the release 

of the backwater-affected riverine component induced by the 

bridge constriction. The discharge and velocities at the bridge 

are also greater during the outgoing tide. This situation can 

lead to scour because the potential for sediment transport 

through the reach on the outgoing tide is increased due to 

higher velocities and sustained flow durations that are greater 

than that of the incoming tide. 

The shape of the hydrograph for the tide gage nearest 

the bridge was compared to the hydrograph measured at the 

bridge. Tidal analysis from the 33 bridges identified 14 sites 

where the hydrographs were asymmetric for the outgoing 

portion of the tide (table 2). The tidally-affected portion of the 

hydrographs at the other 18 bridges is not always symmetrical, 

but asymmetry was not evident in the data collected for this 

study. An attempt was made to collect data at each bridge 

during the highest annual tide, but the limited field season 

and duration of this study precluded data collection during the 

highest tides at some sites. 

The potential for a bridge to act as a constriction during 

the exchange of tides was also evaluated with aerial imagery. 

The ratio of the average width of the upstream storage area 

to the width of the bridge opening was used as a measure of 

the degree to which a bridge constricts the exchange of tidal 

and riverine flow (fig. 5). Sites with high ratios of approach 

channel width to bridge width are susceptible to contraction 

scour from the increase in flow velocities through the bridge 

constriction. Bridges in this study were divided into three 

categories, wherein the width of the approach channel 

including any overbank area that might be overtopped by tidal 

or riverine flows is (1) less than or equal to the bridge width, 

(2) greater than the width of the bridge but less than twice 

the bridge width, and (3) greater than twice the bridge width 

(table 2).
8 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

aktac12-5196_fig 05

X

EXPLANATION

Riverine flow 

 direction

Extent of 

 high tide

Bridge

Width

OCEAN

River

BAY > 2X

3.

OCEAN

≤

2X

OCEAN

River

River

2.

 X

1.

 X

X

X

Figure 5. Examples of the three categories used to define the degree to which a bridge can constrict tidal exchanges and 

backwater riverine flow. Numbers 1–3 indicate the value assigned to the risk posed by upstream storage during a tidal exchange.

Storm Surges

Storm surges occur when coastal waters are forced 

above the expected high-tide elevation by a combination of 

low atmospheric pressure and strong winds blowing onshore 

or along shore where the coast is to the right of the wind 

direction due to the Coriolis Effect. Areas of Alaska that are 

particularly susceptible to storm surges are the Bering Sea 

coast from Bristol Bay to the Bering Strait, Kotzebue Sound, 

the Chukchi Sea coast, and the Beaufort Sea coast (Wise and 

others, 1981). Blier and others (1997) analyzed storm-surge 

induced coastal flooding in Nome, and numerical modeling 

experiments were conducted for three historical storms. Of the 

14 coastal flooding events that occurred from 1900 to 2000 in 

Nome, all except 2 occurred in the autumn (Blier and others, 

1997). Coastal flooding typically ceases from mid-November 

through late spring because most of the Bering Sea is covered 

by sea ice. Although many communities are affected by storm 

surges along the western coast of Alaska, infrastructure is 

limited and only three bridges in this study are along those 

shores: ADOT&PF bridge numbers (BN) 1127, 347, and 1385. 
Tidal Analysis 9

The U.S. Army Engineer Research and Development 

Center, Coastal and Hydraulics Laboratory developed 

frequency-of-occurrence relationships of storm-generated 

water levels for 17 communities along Kotzebue and Norton 

Sounds, the Bering Sea, and Bristol Bay (Chapman and others, 

2009). Advanced-Circulation model (ADCIRC) simulations 

were performed for 52 historical storms to develop the return 

period of storm-surge water levels at each community. The 

1-percent AEP water level (AEPWL) prediction for each 

bridge at risk from storm-surge flooding was based on the 

AEPWL from the community nearest the bridge (table 3).

Two of the bridges susceptible to storm surges, BN 347 

and BN 1127, are east of Nome along the road from Nome to 

Council (fig. 6). The two bridges cross tidally-controlled inlets 

to Safety Sound from Norton Sound. Storm surges into Safety 

Sound are of particular concern because of the large storage 

area upstream of the bridges and flow restrictions created by 

the bridge openings. This stretch of coastline is exposed to 

open waters that extend into the Bering Sea. The two largest 

storm surges recorded in Nome produced a 10.0 ft rise in sea 

level on November 10–12, 1974 and 10.5 ft on October 18–20, 

2004 (National Climate Data Center, 2011). The 1-percent 

AEPWL for storm-surge flooding in Nome is 9.7 ft. The 

October 2004 storm surge damaged the Nome-Council Road 

a half mile to the east of bridge BN 1127, but no damage was 

reported at the bridge. 

The 1-percent AEPWL for storm-surge flooding at Nome 

was plotted on cross sections for both bridges (fig. 7) and 

is only 3.8 ft below the low-beam elevation of the bridge at 

BN 347. The maximum elevation of woody debris deposited 

at BN 347 by an undocumented storm that occurred between 

field visits on September 18, 2008, and September 28, 2010, 

was surveyed at an elevation only 2.5 ft below the low-beam 

elevation of the bridge. This debris likely was deposited 

by wave action rather than a large storm surge because no 

major storms occurred during that period. Wave action that 

impinges on a bridge, or wave loading, has the potential to 

displace a bridge from its foundation. The low-beam elevation 

of BN 1127 is 8 ft higher than the low-beam elevation of 

BN 347, so wave loading of the structure is not likely for the 

1-percent AEPWL at BN 1127.

Table 2. Hydrograph analysis and ranking of the upstream storage 

potential at selected bridges over tidal waterways in Alaska.

[N/A, not available, these data were not collected at the bridge]

Bridge 

No. Bridge name

Asymmetry 

in tidal 

part of the 

hydrograph

Rank of upstream

storage area 

determined from 

aerial photographs

(1 = low, 3 = high)

214 Swanson River Yes 2

347 Bonanza Creek N/A 3

385 Salt Creek No 3

387 Chilkoot River Yes 1

399 King Salmon Creek No 1

400 Leader Creek No 1

402 Pauls Creek No 1

429 Blind River No 2

444 Salmon River N/A 2

620 Ingram Creek Yes 1

627 Placer River Overflow Yes 1

629 Placer River Main Crossing Yes 1

630 Portage Creek 1 Yes 1

631 Portage Creek 2 Yes 1

634 Twentymile River Yes 2

636 Peterson Creek No 1

638 Virgin Creek Yes 3

639 Glacier Creek No 1

724 Ketchikan Creek No 1

989 Sargent Creek N/A 1

990 Russian River Yes 1

992 Salonie Creek N/A 3

1017 Seldovia Slough Yes 3

1085 Hartney Bay No 3

1121 Knik River NB Yes 1

1124 Matanuska River NB N/A 1

1127 Safety Sound Yes 3

1149 Kenai River at Kenai Bridge Yes 3

1197 Lemon Creek NB No 3

1274 Monashka Creek No 3

1385 Tununak River N/A 2

1863 Lemon Creek SB No 3

2150 Ship Creek No 2

Table 3. Bridges at risk from storm surge flooding and the 1-percent annual exceedance probability 

water level in feet, local datum of the bridges near Nome and Toksook Bay, Alaska. 

[The return period of storm-surge water levels at each location was determined from Advanced-Circulation (ADCIRC) 

model simulations of 52 historical storms (Chapman and others, 2009)]

Bridge No. Bridge name Nearest

community

1-percent 

annual exceedance

probability surge level 

(feet, local bridge datum)

Standard

deviation

Low elevation

of bridge

(feet)

1127 Safety Sound Nome 9.7 1.2 20.5

347 Bonanza Crossing Nome 9.7 1.2 13.5

1385 Tununak River Toksook Bay 11.7 1.2 14.5
10 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

Figure 6. Location of bridges subject to coastal flooding and detailed location of bridge numbers 347 and 1127 along the Nome-Council 

Road, Alaska. Background image from NASA’s Earth Observatory using data from Moderate Resolution Imaging Spectroradiometer (MODIS). 

aktac12-5196_fig06

Safety Sound

Norton Sound

To Nome

To Council

BN 1127

BN 347

BN 347

EXPLANATION

Nome-Council RoadBridge and bridge number

164°50'W 164°40'W 164°30'W

64°30'N

64°25'N

0 250 500

CANADA

Beaufort Sea

Chukchi Sea

RUSSIA

Bering Sea

Area of detail

ALASKA

Gulf of Alaska

BN 1385

1,000 MILES

0 250 500 1,000 KILOMETERS
Tidal Analysis 11

Figure 7. Surveyed channel cross sections at bridges (A) BN 347 and (B) BN 1127 along the Nome-Council Road 

showing the 1-percent annual exceedance probability water-surface elevation (AEPWL) for coastal flooding at 

Nome, Alaska. 

aktac12-5196_fig07

-30

-20

-10

0

10

20

30

-30 20 70 120 170 220 

Distance from left bank, in feet 

to Council to Nome

-30

-20

-10

0

10

20

30

-30 70 170 270 370 470 570 670 770 

Elevation, in feet, local bridge datum 

Distance from left bank, in feet 

to Council to Nome

EXPLANATION

EXPLANATION

Bridge

09-28-2010

06-02-2011

1-percent AEPWL

Debris 

Bridge

Asbuilt (August 1990)

09-29-2010

06-02-2011

1-percent AEPWL 

BN 347

BN 1127

A.

B.
12 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

The relation between storm-surge elevations at the Nome 

tide gage and tidal elevations at Safety Sound was studied 

in autumn 2010. A non-contact stage sensor was installed on 

BN 1127 to measure water-surface elevations at the bridge at 

15-minute intervals from October 2010 until mid-November 

2010, when the water began to freeze (fig. 8). The measured 

water-surface elevations are similar in magnitude and timing 

to the measured tides at Nome. A storm in late October 

concurrently increased the water-surface elevations in Nome 

and at BN 1127 during two tidal cycles, but the water level 

decreased at a slower rate at the bridge during the second tide 

cycle. This could result from the temporary storage of river 

inflows that were affected by backwater caused the high tide, 

even though rivers at this latitude are usually frozen by late 

October. Another possibility is that onshore winds backed the 

tide up at the bridge, but not at the tide gage in Nome. 

-4

-3

-2

-1

0

1

2

3

4

5

6

10-27-2010 10-28-2010 10-29-2010 10-30-2010 10-31-2010

Elevation, in feet, local bridge datum

Date

Predicted nome tides

Measured Nome tides

Water elevation at BN 1127 

EXPLANATION

-4

-3

-2

-1

0

1

2

3

4

5

6

10-06-2010 10-11-2010 10-16-2010 10-21-2010 10-26-2010 10-31-2010 11-05-2010 11-10-2010 11-15-2010 

Date

A.

B.

Figure 8. The predicted and measured tides for Nome, Alaska, and the measured water-surface elevation at 

bridge number (BN) 1127 from (A) October 27–31, 2010, and (B) October 6–November 17, 2010.
Tidal Analysis 13

The Tununak River bridge (BN 1385) is approximately 

7 mi northwest of the village of Toksook Bay. The bridge 

is located on a stretch of coastline that faces north into the 

Bering Sea and is at risk to storm surges. The 1-percent 

AEPWL for storm-surge flooding in Toksook Bay is 11.7 ft, 

and this elevation would be 2.8 ft below the lowest elevation 

of the bridge. The bridge is protected from wave action by a 

spit, but there is potential for upstream storage during a storm 

surge. No field measurements of tidal elevations at BN 1385 

were made as part of this study. Measurements of watersurface elevations during tidal cycles and storm surges are 

needed to quantify the impact of storm surges at this bridge.

Velocity Measurements during Tidal Exchanges

Maximum tidal discharge and highest tidal velocities at 

tidally-controlled and tidally-affected bridges occur during 

the outgoing tide, which includes any riverine flow that was 

affected by backwater during the incoming tide. Velocity 

data were collected during outgoing tides at several tidallycontrolled and tidally-affected bridges to (1) determine the 

scour potential of these flows and (2) compare the magnitudes 

of tidal and riverine velocities for selected flows at a site. 

Measurements were made using an acoustic Doppler current 

profiler (ADCP) tethered either from the downstream 

side of the bridge or from a manned boat. Tethered boat 

measurements were stationary and made in the section of 

the channel where earlier ADCP cross-sectional transects 

identified the highest velocities. Velocity data were collected 

from the manned boat along cross sections at the bridge. The 

highest velocities during a tidal exchange occur at or later than 

the midpoint between the high and low tides depending on 

the amount of upstream storage and the degree of contraction 

caused by the bridge opening (fig. 9). Velocity data were 

collected during this time frame (the outgoing tide) at 10 

bridges, and the peak instantaneous and average velocities 

were determined (table 4). Most measurements were made 

after a high tide that was near or exceeded the MHHW. 

Velocity data also were collected at some of these sites during 

low tide to compare the magnitude of the riverine velocity 

to the velocity measured during the tidal exchange. These 

data measured at low tide were collected only when the flow 

regime was purely riverine and not affected by the tide. 

At sites where both tidal and riverine velocities were 

measured, the riverine values were always higher, except 

at BN 1149. The highest velocity during the outgoing tide 

at BN 1149 was 6.7 ft/s, and the highest riverine velocity 

measured was 5.1 ft/s. The discharge of the river during the 

low-tide measurement was 13,100 ft3/s, much lower than the 

1-percent AEP discharge of 42,300 ft3/s. Flow velocities at the 

1-percent AEP would be higher and are discussed later in the 

riverine analysis. 

Figure 9. Water-surface elevations and velocity measurements made during a tidal exchange on September 23, 2010, 

at BN 1085, Hartney Bay, Alaska. 

-6

-4

-2

0

2

4

6

8

10

0

0.5

1

1.5

2

2.5

1:26 PM 2:24 PM 3:21 PM 4:19 PM 5:16 PM 6:14 PM 7:12 PM 8:09 PM 9:07 PM 10:04 PM 11:02 PM 

Velocity, in feet per second

Time

Water-surface elevation, in feet, local bridge datum

Water-surface elevation at bridge 

Tide level at tide station 

1-minute average of water velocity

Instantaneous water velocity 

EXPLANATION
14 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska Table 4. Hydraulic measurements made during tidal exchanges at selected bridges over tidal waterways in Alaska.

[Abbreviations: ft, foot; ft/s, foot per second; lb/ft2, pound per square foot; ft3/s, cubic foot per second; –, no corresponding riverine data were collected]

Bridge No. Bridge name

Average

depth of

flow

(ft) 

Velocity at tidal exchange 

(ft/s) Mannings 

roughness

Drag

coefficient

Estimated

boundary

shear stress

(lb/ft2

)

Critical 

boundary

shear stress

(lb/ft2

)

High-tide

elevation at

measurement

(ft)

Mean high

high water

elevation

(ft)

Measured

riverine

velocity

(ft/s)

Riverine

discharge

at low tide

(ft3/s) Maximum Average

385 Salt Creek 3.6 2.6 1.1 0.037 0.0125 0.03 0.16 9.6 8.8 – –

634 Twentymile 4.4 7.0 3.8 0.025 0.0053 0.15 0.16 35.9 33.3 10.1 14,400

638 Virgin Creek 8.5 1.3 0.5 0.032 0.0070 0.00 0.16 35.9 33.3 1.6 17

1017 Seldovia Slough 9.5 5.5 3.8 0.032 0.0068 0.19 0.16 19.7 18.0 5.5 761085 Hartney Bay 13.8 2.1 1.1 0.032 0.0060 0.01 0.16 12.0 10.8 2.9 10.9

1127 Safety Sound 19.5 2.8 1.5 0.025 0.0033 0.01 0.16 1.3 2.3 – –

1149 Kenai at Kenai 19.6 6.7 4.7 0.036 0.0067 0.29 0.16 23.9 20.7 5.1 13,100

1197 Lemon Creek NB 4.3 1.2 0.7 0.030 0.0078 0.01 0.16 19.3 16.3 – –

1863 Lemon Creek SB 4.3 1.2 0.7 0.030 0.0078 0.01 0.16 19.3 16.3 – –

2150 Ship Creek 7.0 2.2 0.6 0.032 0.0075 0.01 0.16 28.4 29.2 4.5 67
Riverine Analysis 15

The ability for tidal or riverine flow to transport sediment 

is related to the shearing forces of the flow, known as shear 

stress, that are exerted on the streambed. Sediment transport of 

noncohesive grains is initiated when the shear stress exceeds 

the critical shear stress for that grain size. Shear stress cannot 

be measured directly, but is estimated from measurements of 

velocity or flow geometry and their relation to bed shear stress 

(Wilcock, 1996). The bed shear stress was estimated with the 

quadratic stress law method:

( )

( )

2

2

3

,

where

is boundary shear stress, in pounds per 

square foot lb/ft ,

is fluid density, in slugs per cubic foot 

slugs/ft ,

 is the non-dimensional drag coefficient, and

 is vertically a ra e ve g d

b d

b

d

C u

C

u

τ =ρ

τ

ρ

 flow velocity, in feet 

per second.

(1)

The non-dimensional drag coefficient, Cd, can be related 

mathematically to the Manning’s roughness coefficient with 

the following equation:

2

1/3

is the dimensionless drag coefficient,

is the dimensionless Manning’s roughness 

coefficient,

is the acceleration of gravity, in feet per 

second, and

is the mean depth of

1.515 ,

where

m

d

d

m

n g

C

H

C

n

g

H

      =

 flow, in feet.

(2)

The estimated shear stress from equation 1 was compared 

to critical shear stress values determined by Julien (1998) for 

the estimated sediment size at the bridges. Grain-size data 

are not available for most sites in this study, but Heinrichs 

and others (2001) determined that 0.033 ft (10 mm) was a 

reasonable estimate of the median diameter of the bed material 

for evaluating scour at bridges in Alaska. The critical bed 

shear stress for a 10 mm particle is 0.16 lb/ft2 (Julien, 1998). 

If the estimated shear stress calculated from the velocity 

measurements exceeds 0.16 lb/ft2, then sediment movement of 

grain sizes up to 10 mm can be expected. If sediment transport 

during the outgoing tide is possible, then the tidal exchange is 

considered a potential scour process at the bridge.

Estimated shear stress values computed from the 

measured velocities on the outgoing tide exceeded 0.16 lb/ ft2

at BN 1017 (Seldovia Slough) and BN 1149 (Kenai River 

at Kenai) (table 4). The sensitivity of this computation to 

grain size was evaluated by reducing the estimated grain 

size diameter to 0.016 ft (5 mm). This reduced the critical 

boundary shear stress to 0.07 lb/ft2 and then BN 634 would 

be included in the sites where the estimated shear-stress value 

exceeded the critical shear-stress value. The risk of scour at 

BN 1017 is minimal because both bridge piers are founded on 

bedrock. Scour during tidal exchanges is primarily a concern 

at sites where sediment transport occurs only during the 

outgoing tide and there is no sediment replenishment from 

riverine inflow. Bedload sediment transport was noted in the 

ADCP data collected during riverine discharges at BN 634 and 

BN 1149, indicating that any scour during an outgoing tide 

would be limited by infilling during subsequent riverine input. 

Riverine Analysis

The majority of the bridges studied were characterized 

as either tidally affected or tidally influenced (table 1), and 

the primary risk for streambed scour at these sites is from 

riverine flows rather than tidal fluctuations. Streambed scour 

was evaluated for these bridges using guidelines from HEC-18 

(Richardson and Davis, 2001) and previous scour studies in 

Alaska by Heinrichs and others (2001) and Conaway (2004). 

Hydraulic variables were computed for the 1-percent AEP 

riverine discharge with the Hydrologic Engineering Center 

River Analysis System (HEC-RAS; Brunner, 2010).

One-dimensional steady-state models of the 1-percent 

AEP discharge were constructed using a combination of 

existing data and data gathered as part of this study. Existing 

data consisted primarily of the bridge as-built plans provided 

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

topographic map of the channel near the bridge at the time 

of construction and limited hydrologic information. Other 

sources of existing data include discharge measurements, 

streamflow-gaging station records, or ADOT&PF bridge 

inspection reports. The 1-percent AEP discharge was either 

obtained from the as-built plans or it was estimated using 

regional regression equations as outlined by Curran and others 

(2003).

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 and pier 

scour was computed at crossings supported by piers. Abutment 

scour was not evaluated because of the large computational 

uncertainties and because most abutments on bridges in Alaska 

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 1-percent AEP discharge. 
16 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

Data Collection and Model Parameters

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 at the upstream and downstream face of the bridge. 

Channel-geometry data from the cross section at the bridge 

were used with overbank data from bridge as-built plans or 

a field survey to construct full-valley cross sections of the 

approach and exit sections associated with the bridge. Field 

data collected at a bridge for the riverine analysis typically 

included a discharge measurement, channel soundings from 

the bridge deck, water-surface slope through the reach, flow 

angle of attack on the piers, geomorphic observations, and 

photographs. Full valley cross sections were surveyed at 

sites at which topographic data included in the as-built plans 

were limited. Stream discharge was not measured at some 

sites because the discharge at the time of the site visit was 

insufficient to calibrate a hydraulic model of the 1-percent 

AEP discharge. 

Channel roughness coefficients for each site were 

computed by the Manning equation using slope and dischargemeasurement data obtained for this study (Hicks and Mason, 

1991). If these data were not available, roughness values 

of 0.035 for the channel and 0.045 for the overbanks were 

assumed for the model. These default values were considered 

to be a good approximation for streams in Alaska (Heinrichs 

and others, 2001). When photographs were available, the 

overbank roughness coefficients were estimated from 

inspection of the photographs in combination with interpretive 

techniques outlined by Hicks and Mason (1991).

Water-surface profiles were computed with the HEC‐RAS 

model for discharges that were measured and for the computed 

1-percent AEP discharges. Models were calibrated for sites 

where discharge and corresponding water-surface elevation 

data were collected. In those instances, channel-roughness 

values were adjusted until agreement was reached between 

measured and modeled water-surface elevation and velocity. 

Data used in the determination of the downstream boundary 

condition for modeling hydraulic conditions varied and either 

was a known water-surface elevation, a surveyed watersurface slope, or a slope obtained from a topographic map. 

The downstream boundary condition for modeling hydraulic 

conditions associated with the 1-percent AEP discharge 

was either the energy-gradient slope computed from the 

calibration discharge or the water-surface elevation for a 

low tide. The low-tide elevation was used at sites where the 

downstream-most cross section was located in the intertidal 

zone. A high-flow low-tide scenario represents the hydraulic 

conditions with the steepest water-surface slope and highest 

velocities and is therefore the most conservative estimate of 

scour at a tidally-affected and tidally-influenced river crossing. 

If Froude numbers exceeded 1.0 for any cross section, an 

upstream boundary condition (the slope of the energy gradient, 

determined from the calibration discharge) was included and 

the model was rerun as a mixed-flow regime. Interpolated 

cross sections were inserted between surveyed cross sections 

if the water-surface profile varied greatly over a short reach, 

the channel expanded or contracted rapidly, or the channel 

gradient changed abruptly. Hydraulic variables used to 

construct the models are summarized in table 5.

The use of hydraulic models poses several limitations 

and introduces several sources of inaccuracies. Hydraulic 

models in this study were constructed and calibrated on the 

basis of existing information and data collected at the time of 

the field visits. The calibrated models then were extrapolated 

to accommodate the 1-percent AEP flood flow. Simulations of 

flood flows with models calibrated to smaller discharges can 

result in hydraulic inaccuracies. Channel roughness values can 

vary with stage as the influence of channel features changes 

with depth. Calibrating channel roughness to low discharges 

and then extrapolating to higher discharges can be an 

overestimation because channel roughness normally decreases 

with increasing depth. In many cases, flow in the overbank 

areas, where the roughness is generally higher, compensates 

for the decreased roughness in the channel that results from 

the greater flow depths (Conaway, 2004). One-dimensional 

models capture a small part of the active processes in the 

channel, but are efficient at making predictions over long 

length and time scales (Nelson and others, 2003). For many 

of the sites in this study, a multi-dimensional flow model 

would better capture the complex hydraulics associated with 

flood flows at bridges. Data requirements for these types of 

models, however, were prohibitive for this level of study. 

The hydraulic variables generated by HEC-RAS for complex 

flow conditions are considered the best available for this level 

of study, and multi-dimensional models are recommended 

for sites with complex flow regimes. Several models were 

calibrated with hydraulic variables collected at discharges 

that were much lower than the expected magnitude for the 

1 percent AEP flow. This can result in a poor calibration 

and affect the computed scour values for the 1-percent AEP 

discharge. Additional hydraulic data collected at higher 

discharges is recommended for the locations with high scour 

estimates and calibration data collected at low flows. 

Estimation of Contraction and Pier Scour

Estimates of contraction and pier scour for the 1-percent 

AEP discharge were computed from the HEC-RAS modeling 

results using the methods and equations outlined in HEC-18 

(Richardson and Davis, 2001). The HEC-18 equations 

were chosen so that the methodology would be consistent 

with previous evaluations of streambed scour in Alaska. 

Contraction scour was computed for all sites and pier scour 

was computed for bridges supported by piers. 
Riverine Analysis 17

Table 5. Selected hydraulic variables used to construct hydraulic models for the analyses of streambed scour at selected bridges 

over tidal waterways in Alaska.

[Abbreviations: AEP, annual exceedance probability; ft3/s, cubic foot per second]

Bridge No. Bridge name

Manning’s roughness coefficient Watersurface

slope

Source of 

water-surface

slope

Discharge for 

calibration

(ft3

/s)

1-percent

AEP discharge

(ft3/s) Channel Overbank

214 Swanson River 0.035 0.045 0.0022 Survey 45.2 3,040

385 Salt Creek 0.037 0.045 0.0025 Survey 392 578

387 Chilkoot River 0.032 0.050 0.0010 Survey 185 12,200

399 King Salmon Creek 0.032 0.045 0.0016 Survey 590 3,120

400 Leader Creek 0.032 0.045 0.0139 Survey 5.99 100

402 Pauls Creek 0.032 0.045 0.0018 Survey 350 1,700

418 Sheep Creek 0.040 0.045 0.0050 Survey 62 1,140

429 Blind River 0.030 0.045 0.0017 Survey 67 2,710

444 Salmon River 0.032 0.045 0.0003 As-built 119 10,100

620 Ingram Creek 0.028 0.045 0.0020 Survey 373 5,300

627 Placer River Overflow 0.018 0.045 0.0008 Survey 4,710 10,200

629 Placer River Main 0.026 0.045 0.0004 Survey 4,480 10,200

630 Portage #1 0.020 0.045 0.0004 Survey 1,000 7,160

631 Portage #2 0.025 0.045 0.0004 Survey 5,990 7,160

634 Twentymile 0.025 0.045 0.0004 Survey 8,677 18,420

636 Peterson Creek 0.040 0.040 0.0058 Survey 20.3 547

638 Virgin Creek 0.032 0.045 0.0090 Survey 17 563

639 Glacier Creek 0.032 0.045 0.0026 Survey 309 4,920

724 Ketchikan Creek 0.040 0.045 0.0046 Survey 821 3,180

732 Gold Creek 0.055 0.060 0.0300 As-built 23 3,250

787 Salmon Creek at Twin Lakes 0.041 0.045 0.0084 Survey 38 3,290

989 Sargent Creek 0.022 0.045 0.0022 Survey 70 4,230

990 Russian River 0.034 0.045 0.0003 Survey 706 4,730

992 Salonie Creek 0.040 0.044 0.0005 Survey 151 5,090

1017 Seldovia Slough 0.032 0.045 0.0360 Survey 76 200

1085 Hartney Bay 0.032 0.045 0.0035 Survey 10 2,810

1121 Knik River NB 0.023 0.045 0.0009 Survey 73,600 120,000

1124 Matanuska NB 0.028 0.045 0.0001 Survey 37.1 47,000

1149 Kenai at Kenai 0.036 0.045 0.0005 Survey 13,100 42,300

1188 Salmon Creek at Egan Drive 0.041 0.045 0.0084 Survey 38 3,290

1197 Lemon Creek NB 0.030 0.045 0.0020 Survey 128 6,940

1274 Monashka Creek 0.035 0.045 0.0030 Survey 59 2,210

1783 Spruce Creek 0.035 0.045 0.0090 Survey 323 3,930

1863 Lemon Creek SB 0.030 0.045 0.0020 Survey 128 6,940

2150 Ship Creek 0.032 0.045 0.0100 Survey 170 1,900
18 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

Contraction Scour

Contraction scour was estimated using Laursen’s (1963) 

live-bed contraction-scour equation (eq. 3). The live-bed 

equation was selected because active bedload transport is 

occurring at all of the sites being considered in this study. 

The equation is based on the understanding that scour reaches 

a maximum when sediment transport into the contracted 

section (bridge cross section) equals sediment transport out 

of the contracted section or when the mean flow velocity 

equals the critical velocity of the mean-diameter bed material. 

For all sites studied, the bed material was assumed to be 

mobile because bed-material size was generally classified 

as fine gravel or smaller and, therefore, would be expected 

to be mobile during periods of high flow. The equation 

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

estimating live-bed contraction scour is

1

6

7 2 1

1 0

1 2

1

0

,

where

is the contraction scour depth, in feet;

is the average depth in the upstream main 

channel, in feet;

is the average depth in the contracted section 

k

s

s

Q W

y y y Q W

y

y

y

 

     =    −         

1

2

before scour, in feet;

is the discharge in the main channel of the 

approach section that is transporting 

sediment, in cubic feet per second;

is the discharge in the contracted section,

in cubic feet 

Q

Q

1

2

per second;

is the width of the main channel of the 

approach section that is transporting 

sediment, in feet;

is the width of the main channel in the 

contracted section that is transporting 

sediment,

W

W

1

 in feet; and

is a coefficient that accounts for the 

predominant sediment transport 

condition. For this study, transport at 

all sites was assumed to be mostly 

through bed-material discharge; the 

coeff

k

icient for this condition is 0.59.

 (3)

Determining the depth in the contracted section before 

scour (y0) is difficult because the current channel geometry 

in the bridge section has likely been modified by contraction 

scour. For this study and the previous USGS studies on 

streambed scour in Alaska (Heinrichs and others, 2001, 

Conaway, 2004), the depth in the approach channel (y1) 

was substituted for y0 in equation 2. If y1 is measured far 

enough away from the bridge so that the channel geometry 

is not affected by the downstream contraction, it is a good 

approximation of the average depth in the contracted section 

before scour (y0). Estimated contraction-scour depths and 

hydraulic variables used in the computation for the 1-percent 

AEP discharge are presented in table 6.

Pier Scour

Local scour at bridge piers was evaluated with the 

techniques and equation outlined in HEC-18. This empirically 

derived equation accounts for the shape and dimensions of the 

pier, flow depth and velocity, the flow angle of attack on the 

pier, and the size of the bed material. The HEC-18 equation 

for estimating pier scour is

0.65

0.43

01234 1

0

1

2

,

where

is pier-scour depth, in feet;

is the correction factor for pier-nose shape, 

dimensionless;

is the correction factor for flow angle of 

attack on the 

2.0

pier, 

s

s

a y y K K K K Fr

y

y

K

K

  =    

3

4

0

dimensionless;

is the correction factor for channel bedform, 

dimensionless;

is the correction factor for armoring of the 

streambed, dimensionless;

is the pier width, in feet;

is the flow depth dire

K

K

a

y

1

ctly upstream of the pier 

in feet; and

is the Froude number just upstream from the 

pier, dimensionless.

Fr

(4)

The correction factor for channel bedform, K3, is 1.1 

for bedforms with a magnitude less than 10 ft, which can be 

assumed for all sites in this study. The correction factor for 

armoring of the bed material, K4, was not used in this analysis 

or in previous USGS studies of streambed scour at bridges in 

Alaska because of paucity of data on bed-material size and 

gradation. Estimated pier-scour depths and hydraulic variables 

used in the computation for the 1-percent AEP discharge are 

presented in table 7.
Riverine Analysis 19

Table 6. Computed contraction-scour depths, and hydraulic variables used in computation for the 1-percent annual exceedance 

probability (AEP) discharge at selected bridges over tidal waterways in Alaska.

[Abbreviations: ft3/s, cubic foot per second; ft, foot]

Bridge No. Bridge name

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)

214 Swanson River 3,040 120 3,040 6.2 106.8 5.7 0.4

385 Salt Creek 578 138 578 2.0 95.7 2.8 0.5

387 Chilkoot River 12,200 171 12,200 7.9 163.3 9.9 0.2

399 King Salmon Creek 3,120 116 3,120 5.7 93.0 6.9 1.2

400 Leader Creek 100 11 100 1.6 10.9 0.8 0.0

402 Pauls Creek 1,700 117 1,700 4.7 73.5 5.0 1.2

418 Sheep Creek 1,140 96 1,140 1.2 43.3 2.8 0.7

429 Blind River 2,710 204 2,710 7.4 117.6 7.2 2.9

444 Salmon River 10,100 118 8,963 12.7 149.0 10.9 1.4

620 Ingram Creek 5,300 73 4,930 7.5 141.0 5.2 0.5

627 Placer River Overflow 10,200 290 10,159 5.1 234.6 6.1 0.7

629 Placer River Main 10,200 452 10,200 5.2 413.5 5.1 0.3

630 Portage number 1 7,160 132 7,072 7.0 155.4 5.8 0.1

631 Portage number 2 7,160 151 7,105 6.6 212.6 9.0 0.0

634 Twentymile 18,420 481 18,420 11.3 483.4 7.6 0.0

636 Peterson Creek 547 31 547 1.8 66.6 1.3 0.0

638 Virgin Creek 563 71 563 3.1 28.3 2.9 2.2

639 Glacier Creek 4,920 115 4,920 4.6 120.9 5.0 0.0

724 Ketchikan Creek 3,180 42 3,180 6.3 63.0 6.8 0.0

732 Gold Creek 3,250 70 3,250 5.0 62.2 5.3 0.4

787 Salmon Creek at Twin Lakes 3,290 95 3,172 4.0 78.4 4.6 0.6

989 Sargent Creek 4,230 72 4,177 6.3 64.0 5.1 0.5

990 Russian River 4,730 106 4,618 6.8 99.4 5.6 0.4

992 Salonie Creek 5,090 83 3,710 9.2 90.9 8.5 2.7

1017 Seldovia Slough 200 101 200 0.7 53.6 0.8 0.3

1085 Hartney Bay 2,810 213 2,809 3.9 62.5 5.1 4.1

1121 Knik River northbound 120,000 1,408 120,000 11.9 1,411.9 13.6 -0.0

1124 Matanuska northbound 47,000 1,037 47,000 6.7 1,053.6 8.8 0.0

1149 Kenai at Kenai 42,300 752 42,300 10.0 715.7 9.9 0.3

1188 Salmon Creek at Egan 3,290 51 2,865 7.2 76.2 5.9 0.9

1197 Lemon Creek northbound 6,940 155 6,265 9.4 134.6 8.4 1.7

1274 Monashka Creek 2,210 73 2,038 5.1 92.0 4.5 0.4

1783 Spruce Creek 3,930 154 3,930 4.8 66.0 6.6 3.1

1863 Lemon Creek SB 6,940 155 6,265 9.4 133.8 8.4 1.8

2150 Ship Creek 1,900 54 1,900 3.6 47.5 3.68 0.3

Evaluation of Streambed Scour Computations

Contraction and pier-scour depths were computed for the 

1-percent AEP discharge at the 35 bridges at risk of scour from 

riverine flows. Contraction scour exceeded 2 ft at 5 bridges, 

with a maximum of 4.1 ft at BN 1085 (table 6). Pier scour 

was at least 2 ft at 29 bridges and at least 4 ft at 15 bridges 

(table 7). The validity of the scour computations depends 

on (1) the quality and quantity of data used to develop the 

hydraulic models, (2) the ability of the model to accurately 

extrapolate hydraulic parameters from the calibrated condition 

to the 1-percent AEP discharge, and (3) the accuracy of 

the predictive equations. The amount of data available and 

collected for this study differed from site to site, but was 

sufficient to develop hydraulic models and determine the 

susceptibility of each bridge to contraction and pier scour. 
20 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska Table 7. Computed pier-scour depths and hydraulic variables used in computation for the 1-percent annual exceedance probability (AEP) discharge at selected bridges over tidal waterways in Alaska. Bridge No. Bridge name Froude No. at bridge Approach flow depth at bridge (feet) Pier nose shape Flow angle of attack (degrees) Correction factor for flow angle of attack (K2 coefficient) Pier width (feet) Pier length (feet) Approach flow depth at pier (feet) Computed depth of pier scour (feet) Minimum bed elevation at bridge (feet) 1 percent AEP water-surface elevation (feet) 214 Swanson River 0.35 5.7 round 0 1.0 2.0 34 5.7 4.0 2.8 13.8 385 Salt Creek 0.21 2.8 group of cylinders 0 1.0 1.0 27 2.9 1.6 83.8 87.9 387 Chilkoot River 0.40 9.9 square 0 1.0 1.0 22 10.0 3.7 5.0 17.7 399 King Salmon Creek 0.31 6.9 square 0 1.0 1.0 24 7.0 2.9 -3.0 6.4 400 Leader Creek 0.90 0.8 group of cylinders 0 1.0 1.0 25 0.9 2.0 5.0 6.7 402 Pauls Creek 0.34 5.0 square 0 1.0 1.0 25 4.9 2.7 -1.0 6.1 418 Sheep Creek 1.00 2.8 group of cylinders 0 1.0 1.0 31 2.8 3.2 11.6 15.2 429 Blind River 0.21 7.2 group of cylinders 0 1.0 0.8 16 7.2 1.9 2.4 10.5 444 Salmon River 0.33 10.9 group of cylinders 10 2.1 1.0 25 10.9 6.5 -5.5 10.9 620 Ingram Creek 0.52 5.2 sharp 0 1.0 1.5 40 7.7 4.0 12.7 22.1 627 Placer River Overflow 0.46 6.1 sharp 20 2.9 2.7 35 6.6 16.5 4.4 17.2 629 Placer River Main 0.38 5.1 sharp 0 1.0 2.7 35 5.3 4.4 8.4 17.2 630 Portage #1 0.57 5.8 sharp 0 1.0 1.5 32 6.2 3.8 12.0 20.6 631 Portage #2 0.22 9.0 sharp 0 1.0 1.5 35 9.2 2.9 2.8 17.3 634 Twentymile 0.31 7.6 round 0 1.0 1.5 35 7.6 3.5 -1.0 15.5 636 Peterson Creek 1.00 1.3 sharp 0 1.0 1.5 30 1.5 3.1 16.0 18.8 638 Virgin Creek 0.73 2.9 sharp 15 2.5 2.7 35 3.0 4.8 7.6 11.1 639 Glacier Creek 0.68 5.0 square 0 1.0 2.5 33 5.0 6.5 11.5 17.8 724 Ketchikan Creek 0.50 6.8 none – – – – – – -10.2 1.5 732 Gold Creek 0.68 5.3 round 0 1.0 1.5 88 5.4 4.3 3.1 11.0 787 Salmon Creek at Twin Lakes 0.75 4.6 none – – – – – – 15.9 21.7 989 Sargent Creek 0.80 5.1 group of cylinders 0 1.0 1.2 22 5.8 4.1 2.0 11.7 990 Russian River 0.58 5.6 group of cylinders 0 1.0 1.2 29 5.8 3.6 3.8 11.7

992 Salonie Creek 0.36 8.5 group of cylinders 0 1.0 1.3 29 9.8 3.6 -0.9 13.5

1017 Seldovia Slough 0.89 0.8 group of cylinders 0 1.0 2.0 34 0.8 3.0 3.9 5.5

1085 Hartney Bay 0.70 5.1 group of cylinders 0 1.0 1.0 25 5.2 3.4 -7.8 1.9

1121 Knik River NB 0.30 13.6 round 0 1.0 4.8 40 13.9 9.1 3.7 22.6

1124 Matanuska NB 0.30 8.8 round 0 1.0 4.3 40 9.0 7.2 12.8 27.51149 Kenai at Kenai 0.34 9.9 sharp 0 1.0 4.3 44 9.9 8.0 -16.3 -0.61188 Salmon Creek at Egan 0.51 5.9 group of cylinders 0 1.0 1.5 109 7.3 4.3 10.0 17.81197 Lemon Creek NB 0.37 8.4 group of cylinders 0 1.0 2.0 40 9.5 2.0 8.8 18.8

1274 Monashka Creek 0.42 4.5 group of cylinders 0 1.0 1.0 35 5.3 2.7 0.3 7.3

1783 Spruce Creek 0.62 6.6 None – – – – – – 19.7 27.0

1863 Lemon Creek SB 0.37 8.4 group of cylinders 0 1.0 2.0 40 9.4 2.0 8.2 18.6

2150 Ship Creek 0.66 3.68 group of cylinders 0 1 3 43 4.42 6.3 -5.1 1.9
Riverine Analysis 21

Hydraulic models are sensitive to channel roughness 

coefficients used in the water-surface profile computations. 

If the channel roughness is overestimated (too high), model 

simulations will indicate higher water-surface elevations, 

lower water velocities, and greater flow areas than would 

actually occur. Ideally, the channel roughness coefficient 

is computed using the Manning equation and data from a 

discharge measurement that approximates the flow magnitude 

of interest, and is verified with surveyed water-surface 

elevations. If a discharge measurement of sufficient magnitude 

and surveyed water-surface elevations were not available, then 

a default value of 0.035 was used for the channel roughness 

coefficient. The default value also was used if the measured 

discharge was low enough so that the flow conditions would 

not be representative of the flood conditions. Varying the 

channel roughness coefficient has an equal effect on all the 

variables used in the computation of contraction scour, and 

precision is only critical when the width of the approach 

channel varies with slight changes in water-surface elevation 

(Conaway, 2004). 

Computations of pier scour are more sensitive to channel 

roughness than contraction scour because changes in channel 

roughness affect the flow depth upstream of the pier, the 

approach velocity, and the associated Froude number. The 

other variables in the pier-scour equation are unaffected. 

Sensitivity analysis of the pier-scour equation indicated that 

the approach velocity’s influence on pier scour is second only 

to pier width, which is a constant (Glenn, 1994). Conaway 

(2004) determined that a decrease in channel roughness from 

0.080 to 0.035 resulted in a 23 percent increase in computed 

pier scour at one location. Further analysis is recommended 

for sites that were identified as having unstable channels 

(table 8), have piers with a shallow foundation, and had 

estimated rather than computed channel roughness values. 

Repeated Cross-Section Surveys

Repeated cross-section surveys can provide information 

on long-term aggradation and degradation or stability of 

a stream channel. Biennial bridge inspections made by 

ADOT&PF usually include a survey of a cross section at 

the upstream face of the bridge. Changes in bed elevation 

between surveys (table 5) indicate the relative mobility of 

the streambed and whether scour, fill, or both, are taking 

place. Streambed elevation changes also can indicate changes 

in sediment supply, channel migration, and littoral drift. 

Continued decreases in streambed elevations indicate longterm degradation, which is of particular concern at bridges 

over tidal waterways because an increase in channel area 

results in a higher volume of water being exchanged through 

the bridge during tidal cycles and thus higher water velocities.

Repeated cross-sectional surveys made by ADOT&PF 

or USGS were obtained for 41 bridges and were analyzed 

for channel instability (appendix B). Channel soundings also 

were collected by the USGS for most of the sites in this study 

from the upstream and/or downstream side of the bridge 

deck with sounding weights. Data collected by ADOT&PF 

were synthesized with data collected for this study, and cross 

sections were analyzed visually for any change in shape or 

elevation. Vertical differences less than 2 ft over multiple 

surveys were considered to be an indication of negligible 

channel instability because measurement techniques differed 

between ADOT&PF and USGS. None of the repeated surveys 

indicated long-term channel degradation, but 14 sites showed 

signs of unstable channels (table 8). 

Table 8. Sites where channel instability was identified from 

repeated cross-sectional surveys at the bridge. 

Bridge No. Bridge name

Relative bed elevation

change between 

surveys

(feet)

387 Chilkoot River 2.1

444 Salmon River 3.7

627 Placer River Overflow 4.9

629 Placer River Main 4.2

630 Portage Creek 1 3.9

631 Portage Creek 2 6.1

634 Twentymile River 4.3

638 Virgin Creek 4.9

639 Glacier Creek 2.6

990 Russian River 4.8

1121 Knik River 6.1

1124 Matanuska River NB 7.7

1149 Kenai River at Kenai 6.6

2150 Ship Creek 3.0
22 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

Overall Risk to Bridges over Tidal 

Waterways

Bridges over tidal waterways are subject to a number 

of processes that can threaten the stability of the structure. In 

some instances, an individual process dominates. For example, 

bridges over steep mountain streams that were classified (in 

this study) as tidally influenced are likely at risk of scour only 

from riverine flow. An analysis of risk is more complex for 

sites that are subject to several processes that individually do 

not pose a significant threat to a bridge, but their combined 

and additive effects do. A bridge that was classified as tidally 

affected with upstream storage area and at risk of riverine 

flooding could potentially experience a flood in conjunction 

with a high tide. The combination of these risks could then 

produce potentially hazardous hydraulic conditions at the 

bridge. Each individual risk that was evaluated was assigned 

a value from 0 to 3 to rank the potential risk at each location. 

A risk value of 0 would indicate no risk while a risk value of 3 

would indicate high risk. 

Only three sites considered in this study were identified 

as being subject to storm surges, and all three were assigned a 

risk value of 3. The ranking of the sites where tidal velocities 

were measured was based upon the magnitude of the tidal 

velocity compared to the computed critical velocity for the 

estimated bed material size. A risk value of 3 was assigned 

to the sites where the maximum measured velocity exceeded 

the critical velocity, a value of 2 was assigned if the measured 

velocity was greater than half the critical velocity, and a value 

of 1 was assigned if the measured velocity was less than half 

the critical velocity. The range of values and the assigned risk 

value are summarized in table 9.

Risk values assigned to the upstream storage potential 

are based on visual interpretation of aerial photos and are 

therefore more subjective than the risk values assigned to 

computed scour depths. Bridges that are tidally influenced 

were all assigned a risk value of 0. Tidally controlled and 

affected bridges were assigned a risk value based upon the 

geometry of the channel upstream of the bridge. A risk value 

of 1 was assigned to sites where the tidal exchange was 

confined to the width of the bridge opening. A risk value of 

2 was assigned to sites where the lateral extent of the tidal 

flood upstream of the bridge was greater than the bridge 

opening, but less than twice the width of the bridge opening. 

Sites where the lateral extent of the tidal flood upstream of the 

bridge was greater than twice the width of the bridge opening 

were assigned a value of 3. 

Scour risk associated with channel instability was 

ranked on the basis of the measured range of bed elevations 

determined from repeated channel surveys. Values assigned 

to assess the individual risks from contraction and pier 

scour were based upon the range of computed values from 

the riverine analyses and are summarized in table 9. A risk 

value of 0 was assigned if no riverine analysis was made 

because the site was tidally controlled, the computed scour 

value was actually 0, or if no pier-scour computations were 

made because the bridge did not have piers. The computed 

scour depths were used to determine the risk values before 

considering any mitigating factors such as shallow scour-hole 

depths relative to deep pile-tip elevations.

The six individual risk values were then summed to 

indicate the overall risk to each bridge. Overall or maximum 

cumulative risk values ranged from 0 to 13, (table 10). The 

maximum cumulative risk value alone, however, cannot be 

used to evaluate the risk for a given site. For example, several 

sites have a cumulative risk value of 4, which is considered 

“moderate”, but most of that overall risk can be attributed to 

a high susceptibility to pier scour, which had an individual 

risk value of 3. Individual categories and the cumulative risk 

values were color coded to highlight high susceptibility to 

an individual risk as well as the cumulative risk (table 10). 

Cumulative risk values ranged from 0 to 9 for all of the sites 

except BN 1149, which had a value of 13. The color coding 

for the cumulative risk was limited to a maximum value of 9 

to be more illustrative of the range of values. 

Table 9. Range of values used to assign levels of risk to bridges over tidal waterways in Alaska. 

[Levels are from 0 to 3; 0 indicating no risk or not applicable and 3 indicating a high potential risk. N/A, the risk is not 

applicable to the category; <, less than; >, greater than]

Risk

Level

0 1 2 3

Storm surge N/A N/A N/A Identified risk

Tidal velocity N/A < 1⁄2 critical velocity > 1⁄2 critical velocity > critical velocity

Storage potential none within channel twice bridge opening > twice bridge opening

Channel instability1, in feet none 2 to 3 3 to 5 > 5

Contraction scour, in feet N/A > 0 to 1.5 1.5 to 3 > 3

Pier scour, in feet N/A > 0 to 2 2 to 4 > 4

1

Scour risk associated with channel instability was ranked on the basis of the measured range of bed elevations determined 

from repeated channel surveys.
Overall Risk to Bridges over Tidal Waterways 23

Table 10. Categories and the sum of the assigned levels of risk to bridges over tidal waterways in Alaska.

[Levels are from 0 to 3; 0 indicating no risk or not applicable and 3 indicating a high potential risk, – not evaluated. The cumulative 

risk values ranged from 0 to 9 for all of the sites except BN 1149, which has a value of 13. The color coding for the cumulative risk 

was limited to a maximum value of 9 to be more illustrative of the range of values]

Bridge No. Bridge name Channel

instability

Upstream

storage

Tidal 

velocities

Storm 

surge

Contraction

scour

Pier

scour

Overall

risk

214 Swanson River 0 2 – – 1 2 5

301 Klawock 0 0 – – – – 0

347 Bonanza Creek 0 3 – 3 – 0 6

385 Salt Creek 0 3 2 – 1 1 7

387 Chilkoot River 1 1 – – 1 2 5

399 King Salmon Creek 0 0 – – 1 2 3

400 Leader Creek 0 0 – – 0 1 1

402 Pauls Creek 0 0 – – 1 2 3

418 Sheep Creek 0 0 – – 1 2 3

429 Blind River 0 2 – – 2 1 5

444 Salmon River 2 2 – – 1 3 8

620 Ingram Creek 0 1 – – 1 2 4

627 Placer River Overflow 2 1 – – 1 3 7

629 Placer River Main Crossing 2 1 – – 1 3 7

630 Portage Creek 1 2 1 – – 1 2 6

631 Portage Creek 2 3 1 – – 0 2 6

634 Twentymile River 2 2 3 – 0 2 9

636 Peterson Creek 0 1 – – 0 2 3

638 Virgin Creek 0 3 1 – 2 3 9

639 Glacier Creek 1 1 – – 0 3 5

724 Ketchikan Creek 0 1 – – 0 – 1

732 Gold Creek 0 0 – – 1 3 4

787 Salmon Creek Twin Lakes Drive 0 0 – – 1 – 1

989 Sargent Creek 0 1 – – 1 3 5

990 Russian River 2 1 – – 1 2 6

992 Salonie Creek 0 3 – – 2 2 7

1017 Seldovia Slough 0 3 3 – 1 2 9

1085 Hartney Bay 0 3 1 – 3 2 9

1121 Knik River NB 3 1 – – 0 3 7

1124 Matanuska River NB 3 1 – – 0 3 7

1127 Safety Sound 0 3 2 3 – – 8

1149 Kenai River at Kenai Bridge 3 3 3 – 1 3 13

1188 Salmon Creek at Egan Dr 0 0 – – 1 3 4

1197 Lemon Creek NB 0 3 1 – 2 1 7

1274 Monashka Creek 0 3 – – 1 2 6

1385 Tununak River 0 2 – 3 – – 5

1764 Indian Creek 0 0 – – – – 0

1783 Spruce Creek 0 0 – – 3 – 3

1863 Lemon Creek SB 0 3 1 – 2 1 7

2078 Deer Creek 0 0 – – – – 0

2150 Ship Creek 1 2 2 – 1 3 9

Overall risk 

color code

Low risk 0

1

2

3

4

5

6

7

8

High risk 9
24 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

High-Risk Bridges and Potential Mitigating 

Factors

Bridges identified in the previous section as having a 

high individual factor risk (value of 3) or a relatively high 

cumulative risk (ranking greater than 8) were investigated 

further to identify factors that could mitigate the risks. 

Several mitigating factors that were not included in the risk 

analyses above are discussed in the following sections for each 

high-risk bridge. 

BN 347 Bonanza Creek

BN 347 crosses a small outlet of Safety Sound in 

northwestern Alaska (fig. 6) and was classified as tidally 

controlled. Primary risks to this bridge are the extensive 

upstream storage area and potential for storm surges. The 

upstream storage area is not a concern for normal tidal 

exchanges, because the maximum daily tidal fluctuation is 

only 2.6 ft. Storm surges, however, are common at BN 347 

and can increase the volume and rate of flow through the 

bridge beyond what would result from the tide alone. Woody 

debris deposited by an undocumented storm that occurred 

sometime between field visits made on September 18, 2008, 

and September 28, 2010, was surveyed at an elevation only 

2.5 ft below the low-beam elevation of the bridge. The 

potential for streambed scour at this bridge is mitigated by 

the absence of piers and by riprap armoring of the bridge 

abutments. The primary risks at this site are the potential for 

storm surge waves or ice that could impinge on the bridge. 

BN 1127 Safety Sound

BN 1127 crosses the main outlet of Safety Sound in 

northwestern Alaska (fig. 6). Primary risks to this bridge 

are posed by the extensive upstream storage area and the 

potential for storm surges. Pier and contraction scour were 

not evaluated for the riverine conditions at BN 1127 because 

tidal exchange dominates the flow regime. Flow velocity was 

measured during a tidal exchange, but was not sufficient to 

transport sediment (table 4). The repeated cross-sectional 

surveys did not indicate any signs of channel instability at 

BN 1127 (appendix B). A small storm surge in October 2010, 

increased water-surface elevations at the bridge and at the 

Nome tide gage over two tidal cycles. Water-surface elevations 

increased and decreased at the same rate for the first tidal 

cycle, but the elevations at the bridge were higher than those 

at the tide gage during the second recession (fig. 8). These data 

are not conclusive enough to determine if the bridge constricts 

flow during the recession of a storm surge. Storm-surge 

flooding has washed over the approach road several times in 

the past (Mason and others, 1997), and overtopping of the road 

relieves the contraction of flow through BN 1127 and BN 347. 

The stretch of road along Safety Sound has been armored in 

several locations to protect against damage due to overtopping 

during storms. The armoring can have a deleterious effect, 

however, by concentrating flow in the unprotected areas and 

through the bridges. Further study of the magnitude of storm 

surges and the associated flow velocities through the bridge is 

needed. 

BN 385 Salt Creek

BN 385 crosses a tidal marsh on Kodiak Island that 

is bisected by the road approach to the bridge. The bridge 

and approaching roadway act as a constriction to the tidal 

exchanges and the primary risk identified in this study is from 

the upstream storage potential of incoming tidal flow. BN 385 

was classified as tidally controlled. Repeated surveys of the 

channel cross section do not indicate channel instability at the 

bridge. Shear stress estimated from velocity measurements 

at the bridge during an outgoing tide were less than the 

critical shear stress required for sediment transport (table 4). 

Estimates of pier and contraction scour were both categorized 

as low risk because the minimum streambed elevation is 

nearly 60 ft above the bottom of the pile tips. The risk of 

scour associated with the contraction of the tidal flow through 

the bridge is mitigated by the deep pilings relative to the 

streambed elevations and the low flow velocities during tidal 

exchanges.

BN 444 Salmon River

BN 444 crosses the tidally-affected Salmon River near 

Gustavus, Alaska. BN 444 has a high risk of pier scour 

and moderate risks associated with channel instability 

and potential for upstream storage of incoming tidal flow. 

The bridge piers comprise nine individual pilings spaced 

approximately 3 ft apart. The pilings were treated as an 

individual pier in the event debris was to become lodged in 

the spaces between the pilings. The high risk of pier scour 

is the result of a flow angle of attack of 10 degrees on the 

piers, which was estimated from aerial photos. Pier scour 

was computed to equal 6.5 ft with the flow angle of attack 

considered, and 3.1 ft without the flow angle of attack 

considered. The minimum measured streambed elevation is 

approximately 13 ft above the shallowest pile-tip elevation. 

When the combined scour depths (6.5 ft of pier scour and 

1.4 ft of contraction scour) were subtracted from the minimum 

measured streambed elevation at BN 444, the resulting 

minimum streambed elevation was 5.1 ft above the minimum 

pile-tip elevation. The estimated flow angle of attack on 

the piers needs to be verified at high flows to improve the 

accuracy of the scour computation at this site.
Overall Risk to Bridges over Tidal Waterways 25

Bridges Crossing Rivers Draining into 

Turnagain Arm

The Seward Highway around Turnagain Arm crosses 

12 bridges; 9 of the bridges were investigated as part of this 

study (fig. 10). Turnagain Arm is an approximately 30 mi long 

estuary that is connected to Cook Inlet and experiences some 

of the largest tidal fluctuations in Alaska. Streams that enter 

Turnagain Arm range from high-gradient streams with coarsegrained beds (sand, gravel, and cobble) to lower-gradient 

rivers with mostly silt beds. 

The bed and bank material of the rivers at the end of 

Turnagain Arm is primarily silt that was deposited after the 

area subsided as much as 7.9 ft during the Alaska earthquake 

of 1964 (Bartsch-Winkler and others, 1983). In addition to 

local silt deposits, the rivers transport fine-grained sediment 

from glaciers within each river basin. Glacial-fed rivers in 

this area experience a seasonal cycle of channel aggradation 

and degradation resulting from the fluctuating discharge and 

sediment supply from glaciers (Conaway, 2006). Channel 

instability can be expected at these bridges and becomes a 

concern if the seasonal runoff cycle is large enough to expose 

pier foundations or undermine rip-rap protection at the 

abutments. 

BN 627 Placer River Overflow and BN 629 Placer River 

Main Crossing

The Placer River, which drains approximately 122 mi2 of 

glaciated basin, splits into two channels before it empties into 

Turnagain Arm. BN 629 crosses the current main channel and 

BN 627 crosses a secondary channel. Channel instability and 

pier scour are the greatest risks to these bridges. Both bridges 

were classified as tidally influenced. The distribution of flow 

between the channels varies, and separate hydraulic models 

were developed to analyze scour for each bridge. The total 

1-percent AEP discharge was used in each model to analyze 

scour, rather than estimating the flow distribution in each 

channel. This approach produced conservatively high scourdepth estimates because flow would actually be conveyed in 

some proportion between the two bridges. 

Field data were collected at these bridges on July 23, 

2009. The total measured discharge at the two bridges was 

9,180 ft3/s, with 4,700 ft3/s passing through BN 627 and 

4,480 ft3/s through BN 629. These measurements were 

made after the tide had receded downstream of the bridge 

reaches and, therefore, do not reflect tidal or upstream storage 

conditions. The 1-percent AEP discharge for the Placer River 

is 10,150 ft3/s. Pier scour is evident in the cross sections 

surveyed at both bridges (figs. 11A and 11B). Measured 

pier scour was 5.8 ft at BN 627 and 2.0 ft at BN 629. Pier 

scour was measured from the concurrent ambient streambed 

surface (Mueller and Wagner, 2005), also called the reference 

streambed surface (fig. 11). The reference streambed surface 

was determined from data collected during the survey and 

is an average of several points measured near the pier, but 

beyond the limits of the scour hole (Mueller and Wagner, 

2005). Pier scour was computed for the July 23, 2009, 

measured discharge at BN 627 using the techniques outlined 

in this report. Computed pier scour for this discharge was 

13.2 ft, more than twice the measured scour. The HEC-18 

equation used in this study to compute pier scour is a design 

equation and as such should accurately predict scour for a 

given set of conditions, but errors should overpredict rather 

than underpredict scour (Mueller and Wagner, 2005). The 

discrepancy between the measured and predicted pier scour 

for the discharge on July 23, 2009, indicates that the predicted 

pier scour value of 16.5 ft for the 1-percent AEP also would be 

conservative (table 8).

Channel-instability and pier-scour risks are present at 

both bridges crossing the Placer River, but the predicted and 

measured pier-scour depths are greater at BN 627 because the 

flow angle of attack on the piers was 20 degrees at BN 627. 

No angle of attack was included in the pier scour computation 

for BN 629 because flow was observed to be aligned with 

the piers during the July 23, 2009, field survey. The scour 

analyses for the bridges are conservative because the entire 

Placer River discharge was modeled for each bridge. The 

measured discharge on July 23, 2009, was split nearly equally 

between the two bridges, but no information is available on 

the historical distribution of flow. The high angle of attack 

on the bridge piers at BN 627, flow distribution between 

the two bridges, and mobility of sediment on the Placer 

River will require increased monitoring because of the scour 

susceptibility.

BN 630 and BN 631 Bridges Crossing Portage Creek

Portage Creek flows approximately 7 mi after exiting 

Portage Lake and splits into two channels before entering 

Turnagain Arm. The main channel flows are conveyed by 

BN 631 and BN 630, although BN 630 crosses a secondary 

channel that conveys only high flows. Similar to the Placer 

River sites, the entire 1-percent AEP discharge for Portage 

Creek was modeled through each bridge. Channel instability 

was noted in the cross-sectional surveys at BN 630 and 

BN 631. Both bridges were classified as tidally influenced. 

Field data were collected the two bridges on July 23 

and 24, 2009. Discharges of 5,990 ft3/s were measured on 

July 23 at BN 631 and 990 ft3/s on July 24 at BN 630. Both 

measurements were made when there was no tidal influence 

at the bridges. The 1-percent AEP discharge for Portage 

Creek is 7,200 ft3/s. No pier scour was noted during these 

measurements (appendix B). 

The minimum measured streambed elevation since 1999 

at BN 630 was 11.9 ft, and the piles supporting the bridge 

extend to an elevation of -18 ft. The minimum measured 

streambed elevation since 2001 at BN 631 was 0.8 ft, and the 

piles supporting the bridge extend to an elevation of -22 ft. 
26 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

Figure 10. Bridges over tidal waterways along Turnagain Arm, Alaska. Background image from NASA’s Earth Observatory using data from 

Moderate Resolution Imaging Spectroradiometer (MODIS). 

aktac12-5196_fig10

BN 639

BN 638

BN 636

BN 620 BN 634

BN 631

BN 630

BN 627 BN 629

BN 636

Turnagain Arm

60°56'N60°54'N

60°52'N

60°50'N

149°16'W 149°12'W 149°08'W 149°04'W 149°W 148°56'W

Seward Highway

RoadsBridge and bridge 

number

EXPLANATION

0 300

Chukchi Sea

Beaufort Sea

ALASKA

CANADA

Gulf of Alaska Cook 

RUSSIA

Bering Sea

600 MILES

0 300 600 KILOMETERS

Area of detail

Inlet
Overall Risk to Bridges over Tidal Waterways 27

Figure 11. Surveyed cross sections at (A) bridge number (BN) 627 and (B) BN 629, along Seward Highway, Alaska. Field data 

were collected on July 23, 2009. Locations of bridges are shown in figure 10.

aktac12-5196_fig11

0

5

10

15

20

25

30

0 50 100 150 200 250 300

5

10

15

20

25

30

0 50 100 150 200 250 300 350 400 450

Bridge

Streambed on 07-23-2009 

EXPLANATION

Pier scour

reference surface

Pier scour

reference surface

Elevation, in feet, local bridge datum

Distance from left bank, in feet

BN 627

BN 629

A.

B.
28 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

These measurements indicate sufficient embedment at both 

bridges. Continued biennial channel surveys of each bridge 

by ADOT&PF should be analyzed to determine any trend 

towards degradation of the streambed that could indicate the 

bridge is threatened by scour.

BN 634 Twentymile River

The Twentymile River, which drains a 142 mi2 glacial 

basin, was classified as tidally influenced at BN 634. Large 

tidal exchanges combined with substantial storage capacity 

upstream of the bridge have the potential to generate high 

velocities during the outgoing tide. Channel instability is 

evident in the surveyed cross sections at the bridge. Streambed 

scour is concentrated along the right-bank area of the channel 

because the channel approaches the bridge opening at a 

substantial skew angle along the left-bank approach (fig. 12). 

Field data were collected at BN 634 on July 20–21, 

2009, to quantify the riverine and tidal conditions that might 

affect scour. Shear stress values estimated from velocity 

measurements at the bridge during an outgoing tide were 

greater than the critical shear stress required to initiate 

sediment transport (table 4). Riverine data were collected at a 

discharge of 14,400 ft3/s, which is approximately a 4-percent 

AEP flood. At this discharge, the streambed had scoured to an 

elevation of 1 ft at the right-bank pier. The piles supporting the 

bridge piers were driven to an elevation of -20 ft. 

The observed scour at BN 634 and scour potential 

during tidal exchanges prompted ADOT&PF to increase 

scour monitoring at this bridge in 2011. The USGS installed 

a pier-mounted sonar at the pier nearest the right bank to 

continuously monitor streambed elevations. These data will be 

used in the future by ADOT&PF to evaluate the safety of the 

structure during high flows and periods of scour. 

BN 638 Virgin Creek

Virgin Creek at BN 638 drains a small basin (4 mi2) to 

the south of Glacier Creek and BN 639. BN 638 was classified 

as tidally affected and was ranked as having a high risk of 

scour associated with upstream storage and pier scour. The 

risk of pier scour is high because the site was estimated to 

have a flow angle of attack of 15 degrees, which was used in 

the pier-scour computations. The estimated pier scour for the 

1-percent AEP discharge is 4.8 ft. The minimum surveyed 

streambed elevation at the bridge was 6.3 ft and the elevation 

of the bottom of the pier is -25 ft. The pier would have 26.5 ft 

of embedment after accounting for the scour estimated for the 

1-percent AEP discharge. 

Velocity measurements were made at the bridge on 

September 21, 2009, during an outgoing tide after a high tide 

that was 3 ft higher than the MHHW. The maximum observed 

instantaneous velocity was 1.3 ft/s and the average velocity 

during the measurement was 0.5 ft/s. Shear stress estimated 

from velocity measurements at the bridge during an outgoing 

tide were less than the critical shear stress required for 

sediment transport (table 4), and therefore scour during tidal 

exchanges is not expected. 

The elevation of the bottom of the pier relative to 

potential pier scour depth and low velocities measured during 

the tidal exchange mitigate the scour risks at this bridge. In 

addition, cross sections surveyed at the bridge since 1999 do 

not indicate any channel instability.

BN 639 Glacier Creek

Glacier Creek drains approximately 46 mi2 of glaciated 

basin and was ranked as having a high risk to pier scour 

because the computed scour was 6.5 ft. The bridge was 

classified as tidally influenced. The minimum bed elevation 

surveyed was approximately 27 ft above the pile-tip 

elevations, thus mitigating the risk associated with pier scour. 

Channel instability was not evident in the repeated channel 

soundings at BN 639, and this stability can be considered a 

mitigating factor pier scour that was estimated by the HEC-18 

equation. 

BN 1017 Seldovia Slough

BN 1017 is tidally controlled and crosses a slough that 

connects Seldovia Lagoon with Seldovia Bay. The bridge 

was ranked as high risk because of the substantial area 

available for upstream storage and because high velocities 

were measured during an outgoing tide. Shear stress values 

estimated from velocity measurements at the bridge during 

a tidal exchange were greater than the critical shear stress 

required to initiate sediment transport (table 4). However, 

both piers that support the bridge are founded on bedrock and 

therefore the risk of scour is low. Although the potential for 

streambed scour of bedrock is possible under certain hydraulic 

conditions (Annandale, 2007), velocities generated during 

tidal exchanges at this site were insufficient for this type of 

scour to be a concern.

BN 1085 Hartney Bay

BN 1085 crosses the entrance to Hartney Bay (at 

Cordova) at Orca Inlet and is tidally controlled. This bridge 

was ranked as high risk because of the area available for 

upstream storage and the estimated contraction scour for the 

1-percent AEP discharge. Risks associated with the upstream 

storage area are low for this site because it is not subject to 

storm surges and the shear stress estimated from velocities 

measured during a tidal exchange were less than the critical 

shear stress to initiate sediment transport (table 4).
Overall Risk to Bridges over Tidal Waterways 29

The computed values for contraction and pier scour for 

the 1-percent AEP discharge are 4.1 ft and 3.4 ft, respectively. 

When the combined scour depths were subtracted from 

the minimum measured streambed elevation at BN 1085, 

the resulting minimum streambed elevation was -15.3 ft, 

which is only 4.7 ft above the minimum pile-tip elevation of 

-20 ft. Although the riverine analysis predicted both pier and 

contraction scour, repeat cross-section surveys at the bridge do 

not indicate channel instability. Given the relatively shallow 

pile-tip elevations at BN 1085, however, further monitoring is 

recommended to refine the streambed scour analysis.

BN 1121 Knik River and BN 1124 

Matanuska River

The Knik and Matanuska Rivers merge into a complex 

interconnected system of channels upstream of Knik Arm. 

The Glenn Highway crosses four channels, and two of those 

crossings, BN 1121 and BN 1124, were included in this 

Figure 12. Oblique aerial photograph looking downstream at an Alaska Railroad bridge (upstream) and Seward Highway bridge 

number 634, Twentymile River, Alaska. The head of Turnagain Arm is downstream of the bridges. 

aktac12-5196_fig12

Turnagain Arm 

Flow

study. Both bridges were classified as tidally influenced. 

The distribution of flow through the four channels changes 

over time, but the channel crossed by BN 1121 currently and 

historically conveys the most flow (Lipscomb, 1989). This 

study ranked both bridges as high risk because of channel 

instability and estimated pier scour. 

Streambed scour at BN 1121 was studied in detail by 

Norman (1975), who measured 6 ft of pier scour at this site on 

June 24, 1965, at a discharge of 73,600 ft3/s. The computed 

pier scour for the 1-percent AEP flow of 120,000 ft3/s equaled 

9.1 ft and is considered a reasonable value based upon the 

value measured by Norman (1975). The 9.1 ft of computed 

scour would expose the upper part of the pier footings, but 

would not extend below the bottom of the footings. The 

longitudinal profile surveyed at 73,600 ft3/s indicated dunes on 

the streambed with amplitudes of approximately 4 ft and wave 

lengths of 40 ft. The channel variability noted in the repeated 

cross-section surveys is within the amplitude of the dunes 

measured by Norman (1975). No long-term degradation is 

evident in the repeated surveys (appendix B).
30 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

BN 1124 crosses a smaller distributary channel in 

the Knik-Matanuska River estuary. During a field visit on 

October 16, 2009, the measured discharge in this channel was 

less than 1-percent of the total Matanuska River discharge 

of 2,190 ft3/s measured at the USGS gaging station number 

15284000. Gaging station 15284000 is approximately 12 river 

miles upstream from BN 1124. Currently the Matanuska River 

flows into the Knik River upstream of BN 1124 and BN 1121. 

Historically, however, the entire flow was through BN 1124 

(Lipscomb, 1989). For this study, the entire Matanuska River 

discharge was modeled through BN 1124 to estimate pier 

and contraction scour. The computed pier-scour depth for the 

1-percent AEP flow of 47,000 ft3/s was 7.2 ft, and computed 

contraction scour was 0.0 ft. After subtracting the computed 

pier-scour depth from the minimum measured streambed 

elevation at the bridge, the top of the pier footings would 

still be approximately 2 ft below the streambed. The channel 

instability that was noted at this site was confined to the right 

bank portion of the channel and represents lateral movement 

of the approach channel, but little change was indicated in the 

minimum streambed elevations. The observed distribution 

of flow through the bridge, measured minimum streambed 

elevations, and lack of vertical change in the surveyed cross 

sections should all be considered mitigating factors in the 

analysis of scour at BN 1124. 

BN 1149 Kenai River

BN 1149 crosses the Kenai River approximately 5 mi 

upstream of the river mouth. The flow is tidally controlled 

and the mean tidal range at the bridge is nearly 20 ft. Velocity 

measurements were made on an outgoing tide on November 4, 

2010, when the tidal range was 26.2 ft. Shear stress values 

estimated from velocity measurements at the bridge during 

the outgoing tide were greater than the critical shear stress 

required to initiate sediment transport (table 4). This was the 

only site in the study where the velocity measured during the 

tidal exchange was greater than the measured river velocity 

at low tide. Riverine velocities were measured at a discharge 

of 13,100 ft3/s on July 9, 2009. For comparison, the highest 

mean-monthly discharge occurs in August at a stream-gaging 

station located 15 river miles upstream of the bridge and is 

equal to 14,400 ft3/s (USGS gaging station number 15266300, 

Kenai River at Soldotna). The substantial temporary storage 

of river flow and tidal inflow during the tidal cycle results in 

higher measured flow velocities during the tidal exchange, 

compared to the flow velocities measured at low tide when 

flow through the bridge is only riverine. 

The mobility of the streambed at this site is evident 

in the repeated cross section surveys made at the bridge 

(appendix B). The footing at the second pier from the left 

bank was partly exposed based on two of the soundings and 

nearly 6 ft of vertical change in streambed elevation was noted 

between soundings at this pier. If the footing of a pier is wider 

than the pier and is exposed to the flow, local scour is affected 

by the pier’s footing. For situations in which 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 the side of the 

foundation (Parola and others, 1996; Melville and Coleman, 

2000). Estimated pier scour for the 1-percent AEP discharge 

was equal to 8 ft, but did not account for the possible effects of 

the exposed footing. 

The cumulative risk to scour at BN 1149 was the highest 

of any of the sites studied. Further study and monitoring of the 

streambed at this bridge is recommended to further assess the 

potential for streambed scour. 

BN 1188 Salmon Creek

BN 1188 is on Salmon Creek in Juneau and the flow 

through the bridge was classified as tidally influenced. BN 

1188 was ranked as having a high risk to pier scour because 

the estimated pier scour was 4.3 ft. The minimum streambed 

elevation surveyed was approximately 39 ft above each piletip elevation, thus mitigating the risk associated with pier 

scour. Channel instability was not evident in the repeated 

channel soundings at BN 1188. The lack of any channel 

instability noted in the repeated soundings can also be 

considered a mitigating factor to pier scour.

BN 1197 and BN 1863 Lemon Creek

BN 1197 and BN 1863 are a parallel crossing of Lemon 

Creek along Egan Drive in Juneau. The bridges were ranked 

as having a high risk due to the potential for storage of 

backwater-affected riverine flow upstream of the bridges 

during tidal exchanges. The flow at both bridges was classified 

as tidally affected. Velocity measurements of the outgoing 

tidal and backwater-affected riverine flow were made on 

January 29, 2010, after a high tide that was 3 ft higher than 

the mean high tide. Shear stress estimated from velocity 

measurements at the bridge during the outgoing tide were less 

than the critical shear stress required for sediment transport 

(table 4) and therefore, scour during tidal exchanges is not 

expected. 

The risk of scour from storage of tidal and backwateraffected riverine flow at the Lemon Creek bridges is mitigated 

by the low measured velocities during the tidal exchange, low 

modeled flow velocities for the 1 percent AEP flow, and no 

noted channel instability. Additionally, the streambed through 

the bridges is armored with rip rap to inhibit scour. 
Summary 31

BN 1274 Monashka Creek

BN 1274 crosses Monashka Creek on Kodiak Island and 

was classified as being tidally affected. Monashka Creek has 

a high potential for the temporary storage of tidal and riverine 

waters upstream of the bridge. A surveyed high-water mark 

caused either by tidal effects, riverine effects, or combination 

of the two corresponded to an elevation 0.5 ft greater than 

the modeled water-surface elevation for the 1-percent AEP 

discharge and was 2 ft below the low-beam elevation of the 

bridge (appendix A). Despite the indication of high water 

stage at the bridge, relatively little change in streambed 

elevations were noted in the repeated cross section surveys 

(appendix B). Pile-tip elevations were 15 ft below the lowest 

point in the channel, and 2.7 ft of pier scour was estimated for 

the 1-percent AEP discharge. 

The contraction and pier scour risks at this bridge were 

rated as low, but the evidence of water-surface elevations 

near the bottom of the bridge indicate that the bridge may 

be undersized for the combination of flows that occur. A 

continuous record of water-surface elevations would help to 

further quantify the tidal affect at the bridge and the degree to 

which riverine flow is affected by backwater caused by tides.

BN 1385 Tununak River

The Tununak River bridge (BN 1385) is approximately 

7 mi northwest of the village of Toksook Bay. The bridge is 

on a stretch of coastline that faces north into the Bering Sea 

and is at risk to storm surges. The 1-percent AEPWL is 11.7 ft 

in Toksook Bay and this elevation would be 2.8 ft below the 

lowest elevation of the bridge. The bridge is protected from 

wave action by a spit, but there is potential for upstream 

storage during a storm surge. No field measurements of tidal 

elevations at BN 1385 were made as part of this study. Further 

study of the magnitude of storm surges and the associated flow 

velocities through the bridge is needed.

BN 1783 Spruce Creek

Spruce Creek near Seward is a high-gradient stream 

with a coarse-grained bottom that is minimally influenced by 

tides where it is crossed by BN 1783. The bridge was ranked 

as high risk for contraction scour because 3.1 ft of scour was 

computed for the 1-percent AEP discharge. The minimum 

measured streambed elevation at the bridge was 19.9 ft. The 

risk associated with scour at BN 1783 is reduced because 

there are no piers and the abutments are constructed of sheet 

pile that extends approximately 20 ft below the minimum 

measured streambed elevation.

BN 2150 Ship Creek

Ship Creek at Anchorage is crossed by BN 2150 before 

draining into Cook Inlet, which has a maximum tidal range 

of 40.5 ft. BN 2150 is tidally controlled and has a high 

risk to pier scour and a high overall risk to scour based 

on the combined risk factors that affect the site. Velocity 

measurements were made on the outgoing tide on October 2, 

2009, after a high tide of 11.3 ft (bridge datum). The MHHW 

at the bridge is 12.5 ft (bridge datum). Shear stress estimated 

from velocity measurements at the bridge during an outgoing 

tide were less than the critical shear stress required for 

sediment transport (table 4) and therefore, scour during tidal 

exchanges is not expected. . Estimated pier scour for the 

1-percent AEP flow was 6.3 ft and the minimum measured 

streambed elevation was -6.3 ft. The threat of scour at this 

bridge is offset by the deep pier pilings, which extend to a 

depth of -135 ft. 

Summary

The potential for streambed scour was evaluated at 41 

bridges that cross tidal waterways in Alaska. These bridges are 

subject to several coastal and riverine processes that have the 

potential, individually or in combination, to induce streambed 

scour at the bridge or in the adjacent channel. These processes 

include, but are not limited to, storm surges, long-shore drift, 

tidal exchanges, sea ice, changes in sediment supply, and 

riverine flooding. 

The degree of tidal influence at each bridge was 

determined to select the best approach for evaluating the 

potential for streambed scour. The proximity of the bridge 

to the ocean and water-surface elevation data collected over 

a tidal cycle at 33 of the bridge sites were used to identify 

the flow regime—whether tidal, riverine, or mixed—that 

has the greatest potential to induce streambed scour. The 

degree to which the tides affect flow through the bridge was 

considered to classify each site as either tidally affected, 

tidally influenced, or tidally controlled. Tidally-controlled 

bridges have a full flow reversal at every tide cycle. 

Tidally‐affected bridges have reverse flow during some tide 

cycles, but tidal action is not the dominant flow condition. 

Tidally-influenced bridges are dominated by riverine flow and 

experience backwater at the bridge, but no flow reversal. By 

these criteria, 14 bridges were classified as tidally affected, 

17 as tidally influenced, and 10 as tidally controlled. Watersurface elevations were measured through at least one tide 

cycle at 33 bridges and a correlation was made to the nearest 

tide gage. The shape of the hydrograph from the nearest tide 
32 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

gage was compared to the shape of the hydrograph measured 

at the bridge. The tidal analysis made at 33 bridges identified 

12 sites where the tidal portion of the hydrograph at the bridge 

was asymmetric. The asymmetry is caused by the temporary 

backwater effect and storage of riverine flows during the 

incoming tide and results in a longer duration and higher 

discharge and higher velocities on the outgoing tide at the 

bridge. This scenario can, therefore, lead to scour because 

the potential for sediment transport through the reach on the 

outgoing tide is greater than that of the incoming tide.

Storm surges have the potential to affect three of the 

bridges studied by temporarily increasing water-surface 

elevations beyond the normal extent of the high tides and 

possibly impinging upon the bridge. The 1-percent annual 

exceedance probability water-level predictions for storm 

surge flooding were within 4 ft of the low-beam elevation at 

BN 347, Bonanza Creek near Nome, and BN 1385, Tununak 

River near Tununak. Further study and more detailed 

modeling would be required to evaluate the streambed scour 

potential of storm surges at these three bridges. 

The greatest potential for scour of the streambed during 

the tidal exchange at a bridge is during the outgoing tide. 

Velocity data were collected at 10 tidally-controlled bridges 

during outgoing tides to determine if the velocity was 

sufficient to initiate sediment transport and induce scour at the 

bridge. The measured velocities collected during the outgoing 

tide were used to estimate boundary shear stress, which was 

then compared to the critical boundary shear stress for the 

sediment grain size at the study sites. Shear stress computed 

from velocity data collected during the outgoing tide exceeded 

the critical boundary shear stress required to initiate sediment 

transport at BN 634, Twentymile River near Portage, 

BN 1017, Seldovia Slough, and BN 1149, Kenai River at 

Kenai. The potential for scour at BN 1017 is minimal because 

the bridge piers are founded in bedrock. Streambed scour from 

the combined effects of riverine and tidal flows was observed 

during surveys at BN 634 and BN 1149. 

Repeated surveys of channel cross sections at the bridges 

were compared to determine if the streambed at the site was 

unstable and if there was a trend in degradation or aggradation 

of the channel at the bridge. None of the 41 sites exhibited 

signs of long-term degradation or aggradation, but 14 sites 

were classified as having an unstable channel at the bridge. 

Of the 41 bridges studied, 31 were characterized as either 

tidally affected or tidally influenced. The primary risk for 

streambed scour at these sites is from riverine flows rather 

than tidal fluctuations. Field data including channel cross 

sections, a discharge measurement, and a water-surface slope 

were collected at 35 bridges. The Hydrologic Engineering 

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

calculate the hydraulic variables needed to compute estimates 

of scour for the 1 percent annual exceedance probability 

discharge. The downstream boundary condition for the models 

was either an energy-gradient slope or the minimum lowtide elevation. A high-flow, low-tide scenario represents the 

hydraulic conditions with the steepest water-surface slope 

and highest velocities and is therefore the most conservative 

estimate of scour at a tidal waterway. Contraction and pier 

scour were computed using recommended techniques and 

equations outlined in the Federal Highway Administration 

Hydraulic Engineering Circular No. 18 (HEC-18). Computed 

contraction-scour depths were greater than 2.0 ft at 5 bridges 

and pier-scour depths were 4.0 ft or greater at 15 bridges. 

Bridges over tidal waterways are subjected to a number 

of processes that can threaten the stability of the structure 

individually or in combination. These processes include scour 

during tidal exchanges or storm surges, scour and erosion 

due to hydrodynamic loading from waves, and scour from 

riverine flows. The evaluated processes were ranked and 

summed to determine an overall risk factor for each bridge. 

Bridges that had high individual or overall rankings were 

investigated in more detail for factors that could mitigate 

the scour risks. Mitigating factors for sites with high risk to 

pier and contraction scour included deep pier depths relative 

to predicted scour depths, armored channels, piers founded 

in bedrock, and no observed channel instability in repeated 

channel surveys. Measured velocities during an outgoing tide 

that were less than those required for sediment transport was 

considered a mitigating factor at tidally controlled bridges 

where scour risk is greatest during the tidal exchange. 

After accounting for mitigating factors, additional study 

and monitoring is needed to better quantify the streambed 

scour potential at nine bridges. Determination of the potential 

for streambed scour from storm surges at BN 347, Bonanza 

Creek near Nome, BN 1127, Safety Sound near Nome, and 

BN 1347, Kenai River at Kenai, will require more data 

collection and possibly hydrodynamic modeling. Validation 

of the flow angle of attack of the approaching flow on the 

bridge piers during high-flows is needed to refine the pier 

scour estimates at BN 444, Salmon River near Gustavus, and 

BN 627, Placer River Overflow near Portage. Annual channel 

soundings at BN 1085, Hartney Bay at Cordova, are necessary 

to ensure that the channel is stable and does not scour around 

the shallow piers. A continuous record of water-surface 

elevations at BN 1274, Monashka Creek near Kodiak, would 

help to further quantify the tidal affect at the bridge and the 

degree to which riverine flow is affected by backwatered 

caused by tides. Continuous monitoring of water-surface 

and streambed elevation at one or more piers at BN 634, 

Twentymile River near Portage, and BN 1149, Kenai River at 

Kenai, would improve understanding of the tidal and riverine 

influences on streambed scour. 
References Cited 33

References Cited 

Annandale, G.W., 2007, Prediction of scour at bridge pier 

foundations founded on rock and other earth materials, 

Transportation Research Record: Journal of the 

Transportation Research Board, v. 1696/2000, p. 67–70.

Bartsch-Winkler, S., Ovenshine, A.T., and Kachadoorian, R., 

1983, Holocene history of the estuarine area surrounding 

Portage, Alaska as recorded in a 93 m core: Canadian 

Journal of Earth Sciences, v. 20, p. 802–820. 

Blier, W., Keefe, S., Shaffer, W.A., and Kim, S.C., 1997, 

Storm surges in the region of western Alaska: American 

Meteorological Society, Monthly Weather Review, v. 125, 

p. 3094–3108. 

Brunner, G.W., 2010, HEC-RAS, River Analysis System 

Hydraulic Reference Manual, version 4.1, January 2010: 

Davis, Calif., U.S. Army Corps of Engineers Hydrologic 

Engineering Center (HEC), CDP-69, 411 p.

Chapman, R.S., Kim, S.C., and Mark, D.J., 2009, Storminduced water level prediction study for the western coast 

of Alaska: U.S. Army Engineer Research and Development 

Center, Coastal and Hydraulics Laboratory (ERDC/CHL) 

Letter Report, 120 p.

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.

Conaway, J.S., 2006, Comparison of long-term streambed 

scour monitoring data with modeled values at the Knik 

River, Alaska: Amsterdam, Holland, Proceedings of the 

International Conference on Scour and Erosion, 8 p. 

Curran, J.H., Meyer, D.F., and Tasker, G.D., 2003, Estimating 

the magnitude and frequency of peak streamflows for 

ungaged sites on streams in Alaska and conterminous 

basins in Canada: U.S. Geological Survey Water-Resources 

Investigations Report 03-4188, 101 p.

Douglass, S.L., and Krolak, J., 2008, Highways in the coastal 

environment (2d ed.): Federal Highway Administration 

Hydraulic Engineering Circular No. 25, Publication No. 

FHWA-NHI-07-096, 246 p.

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

equations, in Proceedings of the 1994 National Conference 

on Hydraulic Engineering: Buffalo, N.Y., 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.

Julien, P.Y., 1998, Erosion and sedimentation: Cambridge 

University Press, 280 p.

Laursen, E.M., 1963, An analysis of relief bridge scour: 

Journal of Hydraulics Division, American Society of Civil 

Engineers, v. 89, no. HY3, p. 93–118.

Lipscomb, S.W., 1989, Flow and hydraulic characteristics of 

the Knik-Matanuska River estuary, Cook Inlet, Southcentral 

Alaska: U.S. Geological Survey Water-Resources 

Investigations Report 89-4064, 52 p. 

Mason, O.K., Neal, W.J., and Pilkey, O.H., 1997, Living with 

the coast of Alaska: Durham, N.C., Duke University Press, 

348 p.

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

Highlands Ranch, Colo., Water Resources Publications, 

550 p.

Mueller, D.S., and Wagner, C.R., 2005, Field observations and 

evaluations of streambed scour at bridges: Federal Highway 

Administration Publication FHWA-RD-03-052, 131 p.

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

American Society of Civil Engineers, v. 57, no. 7, p. 66–69.

National Climate Data Center, 2011, Storm events database: 

National Atmospheric and Oceanic Administration database, 

accessed September 28, 2012, at http://www.ncdc.noaa.gov/

stormevents/.

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
34 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

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

at bridges (4th ed.): Federal Highway Administration 

Hydraulic Engineering Circular No. 18, Publication No. 

FHWA NHI 01-001, 378 p.

Richardson, E.V., Harrison, L.J., Richardson, J.R., and Davis, 

S.R., 1993, Evaluating scour at bridges (2d ed.): Federal 

Highway Administration Hydraulic Engineering Circular 

No. 18, Publication No. FHWA-IP-90-017, 131 p.

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

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

American Society of Civil Engineers, 1,040 p.

U.S. Department Transportation, 1988, Scour at bridges: 

Washington, D.C., Federal Highway Administration 

Technical Advisory T 5140–20, 6 p. 

Wilcock, P.R., 1996, Estimating local bed shear stress from 

velocity observations: Water Resources Research, v. 32, no. 

11, p. 3361–3366. 

Wise, J.L., Comiskey, A.L., and Becker, R., 1981, Storm 

surge climatology and forecasting in Alaska: Arctic 

Environmental Information and Data Center Report, 60 p.

Zevenbergen, L.W., Lagasse, P.F., and Edge, B.L., 2004, 

Tidal hydrology, hydraulics and scour at bridges: Federal 

Highway Administration Hydraulic Engineering Circular 

25, Publication No. FHWA-NHI-05-077 HEC-25, 168 p.
Glossary 35

Annual Exceedance Probability (AEP) discharge Annual 

exceedance probability of a peak flow is the probability of 

that flow being equaled or exceeded in a 1-year period and is 

expressed as a decimal fraction less than 1.0. The recurrence 

interval of a peak flow is the number of years, on average, in 

which the specified flow is expected to be equaled or exceeded 

one time. Exceedance probability and recurrence interval are 

mathematically inverse of each other; thus, an exceedance 

probability of 0.01 is equivalent to a recurrence interval of 100 

years. 

Mean higher high water (MHHW) The average of the higher 

high water height of each tidal day observed over the National 

Tidal Datum Epoch. 

Mean high water (MHW) The average of all the high water 

heights observed over the National Tidal Datum Epoch.

Mean sea level (MSL) The average of all the high water 

heights observed over the National Tidal Datum Epoch. For 

stations with shorter series, comparison of simultaneous 

observations with a control tide station is made in order to 

derive the equivalent datum of the National Tidal Datum 

Epoch.

Mean low water (MLW) The average of all the low water 

heights observed over the National Tidal Datum Epoch. For 

stations with shorter series, comparison of simultaneous 

observations with a control tide station is made in order to 

derive the equivalent datum of the National Tidal Datum 

Epoch.

Mean lower low water (MLLW) The average of the 

lower low water height of each tidal day observed over the 

National Tidal Datum Epoch. For stations with shorter series, 

comparison of simultaneous observations with a control tide 

station is made in order to derive the equivalent datum of the 

National Tidal Datum Epoch.

National tidal datum epoch (NTDE) The specific 19-year 

period adopted by the National Ocean Service as the official 

time segment over which tide observations are taken and 

reduced to obtain mean values (for example, mean lower 

low water, and so forth) for tidal datums. It is necessary for 

standardization because of periodic and apparent secular 

trends in sea level. The present NTDE is 1983 through 2001 

and is actively considered for revision every 20–25 years. 

Tidal datums in certain regions with anomalous sea level 

changes (Alaska, Gulf of Mexico) are calculated on a modified 

5-year epoch.

Glossary
36 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

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Appendixes 37

Appendix A. High and Low Tide Elevations and Water-Surface Elevations at 

the 1-Percent Annual Exceedance Probability (AEP) Discharge at Cross Sections 

at 33 Bridges over Tidal Waterways in Alaska

Appendix A contains plots of surveyed channel cross sections and bridge geometry. Elevations of the maximum high and 

minimum low tides (since 1901), mean higher high water (MHHW), and mean lower low water (MLLW) also were plotted 

on cross sections at each of the 33 bridges to determine the tidal elevations relative to the bridge structure. The elevation of 

the modeled water surface at the bridge for the 1 percent annual exceedance probability (AEP) discharge is included for the 

locations where a scour analysis of riverine flow was undertaken. All plots are in the local as-built datum of the bridge.

Appendix A is available in Microsoft Excel format at http://pubs.usgs.gov/sir/2012/5245/.

Appendix B. Multiple Cross-Section Surveys for 42 Bridges over Tidal 

Waterways in Alaska

Appendix B contains plots of repeated survey cross sections at the bridge for 41 sites. Most sites include data from 

three sources; a channel survey completed when the bridge was built (as-built), channel soundings completed by the Alaska 

Department of Transportation and Public Facilities (DOT), and channel soundings completed by the U.S. Geological Survey 

(labeled only with the date surveyed). All surveys are in the local as-built datum of the bridge. 

Appendix B is available in Microsoft Excel format at http://pubs.usgs.gov/sir/2012/5245/.
38 Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska

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Conaway and Schauer— Evaluation of Streambed Scour at Bridges over Tidal Waterways in Alaska—Scientific Investigations Report 2012–5245