U.S. Geological Survey - Virginia Department of 

Transportation: Bridge Scour Pilot Study

Background

Cost effective and safe highway 

bridge designs are required to ensure the 

long-term sustainability of Virginia’s road 

systems. The streamflows that, over time, 

scour streambed sediments from bridge 

piers inherently affect bridge safety and 

design costs. To ensure safety, bridge 

design must anticipate streambed scour at 

bridge piers over the lifespan of a bridge. 

Until recently Federal Highway Administration (FHWA) guidance provided only 

for scour estimates of granular, noncohesive, highly erosive material yielding overestimates of scour potential in 

instances when streambed materials offer 

some resistance to scour. This study seeks 

to estimate stream power and streambed scour for these more resistive sites, 

with bridge piers potentially established 

in cohesive soil or erodible rock. This 

new knowledge may provide significant 

construction cost savings while ensuring 

design and construction of safe highway 

bridges. 

Purpose of the Study

As research into properties of streambed materials has advanced, methods of 

evaluating scour that incorporate scour 

resistance are now included in new 

FHWA publications. Bridges with piers 

established in, or on, cohesive soil or 

erodible rock may now be designed so 

that the scour resistance of these materials is taken into account. These new and 

All photographs courtesy of Hayley Austin

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2018–3006

March 2018 Printed on recycled paper

more complex design evaluations require 

knowledge of the duration of exposure of 

bed materials to scouring flows over the 

expected life of a bridge. The established 

standards for evaluating hydrologic 

processes employed by the Virginia 

Department of Transportation (VDOT) 

and readily available to the industry do 

not currently provide this type of information. As a step toward meeting this 

need, this pilot study seeks to estimate the 

(1) duration of specific streamflows and 

(2) potential cumulative stream power 

over the design lifespan of a bridge. This 

information may then be used to calculate 

comprehensive projections of anticipated 

scour rates specific to individual bridge 

piers. This new knowledge of cumulative scour may provide significant bridge 

construction cost savings while ensuring 

design and construction of safe highway 

bridges.

Figure 1. Example output for instantaneous streamflow discharge 

measurements associated with daily streamflow discharge measurements. (DV, 

daily discharge)

Outcomes

The U.S. Geological Survey is 

providing compilation, calculation, and 

summation of the hydrologic properties needed to determine potential 

rates of streambed scour at study test 

sites. Changing instantaneous streamflows affect streambed scour. Statistical methods that include Monte Carlo 

sampling processes allow use of instantaneous streamflows measured since 

1990 to estimate instantaneous streamflows across the full range of daily 

streamflow measurements made at each 

study site. These hydrologic statistics 

(fig. 1) and statistical methods (fig. 2) 

provide the data for hydrologic modeling (fig. 3), which will assist in estimating future cumulative streambed scour 

at specific bridge pier locations over 
ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

https://doi.org/10.3133/fs20183006

Figure 2. An overview of statistical methods used when sampling instantaneous flows. (ft3/s, cubic feet per second; inst., instantaneous; 

MLLR, Maximum Likelihood Logistic Regression)

Figure 3. An example of site-specific 

hydrologic modeling using instantaneous 

streamflows. Daily streamflow discharge 

measurements and Monte Carlo 

selected instantaneous streamflow 

discharge measurements are displayed.

their projected design lifespan. VDOT is 

providing the non-hydrologic elements 

essential to determining potential rates of 

streambed scour, such as bridge location and design specifications, attributes 

of cohesive soils associated with bridge 

piers, geotechnical attributes of bridge 

scour computations, and preferred specifications and methods for estimating soil 

scour.

Study Deliverables

• Analysis and evaluation of daily and 

instantaneous stream discharge data.

• Hydrologic design products.

• Hydraulic geometry characteristics.

• Evaluation of discharge probabilities.

• Simulation tools for probabilistic and 

deterministic analyses of discharge–

duration scenarios.

By Samuel H. Austin 

Publishing support provided by the West 

Trenton Publishing Service Center

For more information, contact:

Virginia and West Virginia 

Water Science Center 

1730 East Parham Road 

Richmond, VA 23228

https://www.usgs.gov/centers/va-wv-water.

70,000 80,000 90,000 100,000 110,000 120,000 130,000 140,000 150,000 160,000 170,000

0.00

0.25

0.50

0.75

1.00

Instantaneous Discharge

10,000 15,000 20,000 25,000 30,000 35,000 40,000 45,000 50,000 55,000 60,000 65,000 70,000 75,000 80,000 85,000 90,000 95,000 100,000 105,000 110,000 115,000 120,000 DV

20,000

30,000

40,000

50,000

70,000

110,000

120,000

130,000

~ 0.5 probability of inst. flow > 110,000 ft3

{ /s.

Begin X value of 114,000 ft3

/s.

MLLR 

Ordinal 

Logistic 

Regression

Monte Carlo sampling 

logic randomly selects 

an instantaneous flow 

value from this 

distribution of 

probabilities at each 

iteration of the model.

~ 0.1 probability of inst. flow > 150,000 ft3

/s.

~ 0.4 probability of inst. flow > 120,000 ft3

/s.

At each time step, 

deterministic model logic 

determines whether to 

use the instantaneous 

streamflow discharge

value or the daily

discharge value. 

Deterministic model logic 

increments the time step 

from t to (t + t). 

Deterministic model 

presents a new daily flow 

value (DV) at each model 

iteration (time step). In 

this example the DV is 

114,000 ft3

/s.

Distribution of 

instantaneous values 

when daily flow value 

is 114,000 ft3

/s.

Monte Carlo Selected

Instantaneous Streamflow

Discharge Measurements

Discharge, in cubic feet per second

Time, in 15 - minute intervals