Simulation of Flow and Evaluation of Bridge Scour at 

Horse Island Chute Bridge near Chester, Illinois

Water-Resources Investigation Report 01–4176

Prepared in cooperation with the

Missouri Department of Transportation

U.S. Department of the Interior

U.S. Geological Survey
Cover Photographs: Horse Island Chute Bridge, facing south, January 19, 2000, and July 25, 1993.
U.S. Department of the Interior

U.S. Geological Survey

Simulation of Flow and Evaluation of Bridge Scour at 

Horse Island Chute Bridge near Chester, Illinois

By Richard J. Huizinga and Paul H. Rydlund, Jr.

Water-Resources Investigations Report 01–4176

Prepared in cooperation with the

Missouri Department of Transportation

Rolla, Missouri

2001
U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does not constitute 

endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased from:

District Chief

U.S. Geological Survey, WRD U.S. Geological Survey

1400 Independence Road Branch of Information Services

Mail Stop 100 Box 25286

Rolla, Missouri 65401 Denver, CO 80225-0286
Contents iii

CONTENTS

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

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

Purpose and Scope ................................................................................................................................................... 2

Description of Study Reach..................................................................................................................................... 3

Description of the 1993 Flood................................................................................................................................. 3

Simulation of Flow ............................................................................................................................................................ 5

Model Development ................................................................................................................................................ 6

Calibration of Model to Observed Conditions........................................................................................................ 6

Simulation of August 6, 1993, Maximum Discharge .............................................................................................. 8

Simulation of Impendent Discharge ........................................................................................................................ 13

Evaluation of Scour........................................................................................................................................................... 16

Scour Depth Computations in Finite Element Surface-Water Modeling System ................................................... 16

Scour Depth Computations using Finite Element Surface-Water Modeling System Results ................................. 17

Summary............................................................................................................................................................................ 22

References Cited................................................................................................................................................................ 28

FIGURES

1. Photograph of Horse Island Chute bridge (structure L 1004R1) on Missouri State Highway 51, in Perry

County, Missouri, near Chester, Illinois, as viewed from the Bois Brule Levee on the upstream right 

(southwest) floodplain. The Mississippi River bridge (structure L 135A) and bluff on the Illinois side

of the Mississippi River are in the background................................................................................................ 2

2. Map showing location of the study reach near Chester, Illinois....................................................................... 4

3. Shaded relief image of study reach near Chester, Illinois, in an oblique perspective facing downstream ....... 5

4. –5. Maps showing:

4. Finite-element mesh used in flow simulations near Chester, Illinois.................................................. 7

5. Land-use coverages used in flow simulations near Chester, Illinois................................................... 9

6. Graph showing observed and simulated water-surface elevation profiles for the measured discharge of 

944,000 cubic feet per second along the railroad embankment on the Illinois side of the Mississippi

River near Chester, Illinois............................................................................................................................... 10

7. –9. Maps showing:

7. Simulated water-surface elevations for measured discharge of 944,000 cubic feet per second 

near Chester, Illinois............................................................................................................................ 11

8. Simulated water-surface elevations for maximum discharge of 1,000,000 cubic feet per 

second near Chester, Illinois................................................................................................................ 12

9. Simulated water-surface elevations for impendent discharge of 894,000 cubic feet per second 

near Chester, Illinois............................................................................................................................ 14
iv Contents

FIGURES—Continued

10. Graph showing relation between measured discharge in Horse Island Chute (QChute) and total

measured discharge (QTotal) for the Mississippi River at the U.S. Geological Survey gaging

station at Chester, Illinois.................................................................................................................................. 15

11.–12. Maps showing:

11. Simulated velocity magnitude using maximum discharge of 1,000,000 cubic feet per

second near Chester, Illinois ................................................................................................................ 18

12. Simulated depths and velocity vectors for maximum discharge of 1,000,000 cubic feet 

per second and flux lines used for scour parameter determimations at Horse Island Chute 

bridge near Chester, Illinois ................................................................................................................. 19

13.–14. Graphs showing:

13. Contraction and local pier scour depths for simulated maximum discharge of 1,000,000 cubic

feet per second through Horse Island Chute bridge near Chester, Illinois .......................................... 23

14. Total scour depths for three discharge simulations through Horse Island Chute bridge near 

Chester, Illinois.................................................................................................................................... 24

15. Photograph showing stone revetment on abutment face under left (north) end of Horse Island Chute 

bridge near Chester, Illinois, as viewed from downstream face of bridge ........................................................ 25

16. Graph showing channel bed profiles for: A) small, and B) large flood events at Horse Island Chute

bridge near Chester, Illinois .............................................................................................................................. 26

17. Photograph showing downstream face of Horse Island Chute bridge near Chester, Illinois, as viewed from

downstream face of left (north) embankment. Note debris trapped on upstream face of bents........................ 27

TABLES

1. Manning’s n-values used in model calibration ...................................................................................................... 10

2. Final simulated results for three alternative discharges......................................................................................... 13

3. Simulated and calculated discharges through Horse Island Chute bridge ............................................................. 16

4. Contraction scour depths and parameters used to compute scour ......................................................................... 20

5. Local pier scour depths and parameters used to compute scour ............................................................................ 21

6. Local abutment scour depths and parameters used to compute scour ................................................................... 25

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic 

datum derived from a general adjustment of the first-order level nets of both the United States and Canada, formerly

called Sea Level Datum of 1929.
Introduction 1

Simulation of Flow and Evaluation of Bridge Scour at 

Horse Island Chute Bridge near Chester, Illinois

By Richard J. Huizinga and Paul H. Rydlund, Jr.

ABSTRACT

The evaluation of scour at bridges throughout the State of Missouri has been ongoing since 

1991, and most of these evaluations have used 

one-dimensional hydraulic analysis and application of conventional scour depth equations. Occasionally, the conditions of a site dictate that a more 

thorough hydraulic assessment is required. To provide the hydraulic parameters required to determine the potential scour depths at the bridge over 

Horse Island Chute near Chester, Illinois, a twodimensional finite-element surface-water model 

was used to simulate flood flows in the vicinity of 

the Missouri State Highway 51 crossing of the 

Mississippi River and Horse Island Chute.

The model was calibrated using flood-flow 

information collected during the 1993 flood. A 

flood profile along the Illinois side of the Mississippi River on August 5, 1993, with a corresponding measured discharge of 944,000 cubic feet per 

second was used to calibrate the model. Two additional flood-flow simulations were run: the flood 

peak that occurred on August 6, 1993, with a maximum discharge of 1,000,000 cubic feet per second, and the discharge that caused impending 

overtopping of the road embankment in the vicinity of the Horse Island Chute bridge, with a discharge of 894,000 cubic feet per second 

(impendent discharge).

Hydraulic flow parameters obtained from 

the simulations were applied to scour depth equations to determine general contraction and local 

pier and abutment scour depths at the Horse Island 

Chute bridge. The measured discharge of 944,000 

cubic feet per second resulted in 13.3 feet of total 

combined contraction and local pier scour at Horse 

Island Chute bridge. The maximum discharge of 

1,000,000 cubic feet per second resulted in 15.8 

feet of total scour and the impendent discharge of 

894,000 cubic feet per second resulted in 11.6 feet 

of total scour.

INTRODUCTION

In 1991, the U.S. Geological Survey (USGS) 

began a study in cooperation with the Missouri Department of Transportation (MoDOT) to analyze and 

assess nearly 3,300 bridges throughout Missouri for 

scour susceptibility. Approximately 270 bridges 

throughout the state were identified as “scour-susceptible,” requiring a detailed hydraulic evaluation to determine potential scour depths. The term “scoursusceptible” describes a bridge that is deemed potentially unstable because abutment or pier foundations, or 

both, have the potential to be undermined because of 

erosion to the channel bed or banks (Federal Highway 

Administration, 1988). Most of these bridges have 

been assessed using techniques outlined in Hydraulic 

Engineering Circular No. 18 (Richardson and others, 

1995), which includes a one-dimensional hydraulic 

analysis and application of predictive scour-depth computations.

Occasionally, the simplifying assumptions that 

must be made to apply a one-dimensional hydraulic 

analysis to a two-dimensional flow scenario are 

deemed unreasonable. This is the case at structure L 

1004R1 on Missouri State Highway 51, crossing Horse 

Island Chute in Perry County, Missouri, near Chester, 

Illinois, hereinafter referred to as Horse Island Chute 

bridge. Horse Island Chute bridge (fig. 1) is built on a 

horizontal curve, and is near a bridge over the Missis-
2 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois

sippi River (structure L 135A, hereinafter referred to as 

Mississippi River bridge) on a constricted section of 

the floodplain bounded by the Bois Brule Levee on the 

right floodplain (as viewed facing downstream) and a 

high bluff on the Illinois side of the Mississippi River. 

This unique configuration is beyond the bounds of reasonability for a one-dimensional hydraulic analysis, 

and requires a two-dimensional hydraulic analysis of 

the site.

The flood of 1993 in the central United 

States—recognized as one of the most dramatic and 

devastating floods in U.S. history—occurred in the 

Lower Missouri and Upper Mississippi River Basins, 

which includes the Mississippi River and Horse Island 

Chute at Missouri State Highway 51. Commonly, the 

hydraulic analysis of a scour-susceptible site must proceed without the benefit of major flood information, 

against which the hydraulic model of the site can be 

calibrated. However, the duration of the flood and the 

flow scenario at the site before, during, and after the 

peak of the flood on August 6, 1993, along with documented flood information, provided nearly ideal 

hydraulic conditions for the determination of worstcase scour depths at Horse Island Chute bridge. The 

data collected at the USGS streamflow-gaging station 

on the Mississippi River at Chester, Illinois (07020500) 

were available for use in calibrating the two-dimensional hydraulic model at this site. At the gaging station, the Mississippi River had a flood-peak elevation 

of 390.79 ft (feet) above sea level on August 6, 1993, 

which exceeded the April 1973 flood-peak elevation by 

more than 6 ft.

Purpose and Scope

This report gives the results of a study in which 

the numerical, two-dimensional, Finite Element Surface-Water Modeling System (FESWMS-2DH, Froehlich, 1989) was used to determine the hydraulic 

parameters needed for an evaluation of the contraction 

and local pier and abutment scour that might occur at 

Horse Island Chute bridge on Missouri State Highway 

51 in Perry County, Missouri, near Chester, Illinois. 

The report also documents the flow distribution and 

velocities of floodwaters in the Mississippi River, 

Horse Island Chute, and associated floodplains for a 

discharge measurement on August 5, 1993 (hereinafter 

Figure 1. Horse Island Chute bridge (stucture L 1004R1) on Missouri State Highway 51, in Perry County, 

Missouri, near Chester, Illinois, as viewed from Bois Brule Levee on the upstream right (southwest) floodplain. The Mississippi River bridge (structure L 135A) and bluff on the Illinois side of the Mississippi River 

are in the background.
Introduction 3

referred to as measured discharge), the flood-peak discharge on August 6, 1993 (hereinafter referred to as 

maximum discharge), and the discharge that causes 

impendent overtopping of Missouri State Highway 51 

(hereinafter referred to as impendent discharge).

Description of Study Reach

The study reach begins near river mile 108 and 

ends near river mile 111 on the Mississippi River near 

Chester, Illinois (fig. 2). The total width of the Mississippi River floodplain throughout this reach generally 

is about 5 mi (miles), but is dissected by various levee 

districts. Upstream from the study reach, the Mississippi River is isolated from the Old River channel by 

Kaskaskia Island. The Old River floodway is confined 

between the northwestern edge of the Bois Brule Levee 

and the southeastern edge of the Kaskaskia Levee. 

Horse Island Chute splits from the Old River approximately 3,000 ft upstream from the mouth of the Old 

River (approximately 3,000 ft upstream from the Missouri State Highway 51 bridge over the Mississippi) 

and flows into the Mississippi River approximately 

1,400 ft downstream from the Mississippi River bridge. 

Horse Island is bounded by the Mississippi River on 

the northeast, the Old River on the northwest, and 

Horse Island Chute on the south. Near the point where 

Horse Island Chute separates from the Old River, the 

Bois Brule Levee trends toward the east, parallel to 

Horse Island Chute, and approaches the southern bank 

of the Mississippi River. The Bois Brule Levee then 

turns toward the southeast and essentially follows the 

southern bank of the Mississippi River for several 

miles. The Bois Brule Levee creates a constriction on 

the floodplain of the Mississippi River that narrows to 

a minimum width of 2,230 ft approximately 3,500 ft 

downstream from the Mississippi River bridge. During 

the 1993 flood, the Bois Brule Levee was not overtopped in the study reach; thus, the study reach was 

constrained between the Bois Brule Levee on the Missouri side and the railroad embankment along the toe of 

the bluffs on the Illinois side. A shaded relief image of 

the study reach in an oblique perspective facing downstream is shown in figure 3.

Missouri State Highway 51 crosses the Mississippi River in a northeast direction between Bois Brule 

Levee and the Illinois bluffs near river mile 110, 

approximately 8,400 ft upstream from the downstream 

boundary of the study reach. Missouri State Highway 

51 is bearing to the northwest inside (south of) the Bois 

Brule Levee and begins to curve to the northeast as it 

crosses the levee. The Missouri State Highway 51 

curve continues as it crosses Horse Island Chute and 

terminates just before the Mississippi River bridge 

(figs. 2 and 3). The Horse Island Chute bridge (structure L 1004R1) is 464 ft long and was built on a horizontal curve (fig. 2). The Mississippi River bridge 

(structure L 135A) is 2,827 ft long and is raised substantially above normal water-surface elevations for 

barge traffic on the Mississippi River. A short section 

of raised road embankment exists between the southern 

end of the Horse Island Chute bridge and the Bois 

Brule Levee (fig. 3). Another section of raised and 

curved road embankment extends northward from the 

Horse Island Chute bridge, rising to meet the sloped 

approach spans of the Mississippi River bridge.

Description of the 1993 Flood

A USGS streamflow-gaging station has been 

collecting continuous record on the Mississippi River 

at Chester (station 07020500; fig. 2) since October 

1927. As the floodwaters rose during July 1993, several 

levees in the Mississippi River floodplains were 

breached, often resulting in flood levels that were temporarily lowered in the vicinity of the breach and 

upstream of the breach. The Bois Brule Levee, in combination with the raised and super-elevated road 

embankment of Missouri State Highway 51, permitted 

access to at least one lane of traffic across the Mississippi River floodwaters into late July. However, on 

July 22, 1993, a breach in the Bois Brule Levee 

occurred approximately 12 river miles downstream 

from the Mississippi River bridge. The river level 

upstream of the breach and at the gage dropped noticeably for a period of time as the area behind the levee 

flooded; however, Missouri State Highway 51 had to 

be closed because of inundation of the highway inside 

the levee.

The floodwaters again continued to rise until a 

flood-peak elevation of 390.79 ft above sea level was 

measured at the gage on August 6, 1993. This peak elevation corresponds to a discharge of 1,000,000 ft 3

/s 

(cubic feet per second). The 1993 peak surpassed the 

previous flood-peak elevation of 380.85 ft on June 30, 

1844, but was slightly less than the 1844 peak discharge of 1,050,000 ft 3

/s. The constricting effect of the
4 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois
Bois Brule Levee, which was not present in 1844,

caused the 1993 peak elevation to be larger than the

peak elevation in 1844, but caused the peak discharge

to be less.

On August 5, 1993, priorto the peak, a flood profile was obtained along the railroad embankment on the

Illinois side of the river from approximately 12,200 ft

upstream from the Mississippi River bridge to 15,280

ft downstream (fig. 2). This profile indicated a river

surface slope of 1.7 ft/mi (feet per mile) on August 5,

1993. An Acoustic Doppler Current Profiler (ADCP)

measurement made concurrent with the flood profile

downstream of the bridges and road embankment indicated the discharge at that time was approximately

944,000 ft

3/s. Water was over Missouri State Highway

51 and Horse Island Chute bridge at that time.

SIMULATION OF FLOW

The FESWMS-2DH model simulates flows in

two dimensions in the horizontal plane. It uses a finiteelement mesh and the Galerkin finite-element method

of solving three partial-differential equations representing conservation of mass and momentum (Froehlich,1989). Thistwo-dimensional model can simulate

longitudinal and lateral variations in water-surface eleSimulation of Flow 5

MISSISSIPPI RIVER

Old River

Horse Island

Kaskaskia Island

Bois

Brule

Levee

Road

embankment

Figure 3. Shaded relief image of study reach near Chester, Illinois, in an oblique perspective facing downstream.

Bluff

Horse Island Chute
6 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois

vations and velocities and can accommodate geometric 

features such as highway embankments, channel 

bends, and bridge piers. A graphical user interface 

called Surface-Water Modeling System (SMS, Environmental Modeling Research Laboratory, 1999), was 

used for converting survey data into a scatter data set, 

developing the finite-element mesh from the scatter 

data, assigning roughness coefficients and other 

parameters to the mesh elements, and evaluating the 

FESWMS-2DH model output.

Model Development

The finite-element mesh (fig. 4) was designed to 

closely represent the non-uniform characteristics of the 

study reach inundated by the 1993 Mississippi River 

flood at Chester. The mesh extended upstream and 

downstream from Missouri State Highway 51 to eliminate any errors in flow computations because of estimated boundary conditions. The reach begins about 

8,400 ft downstream from the Mississippi River bridge, 

and continues to about 7,600 ft upstream from the 

bridge (fig. 4). The bluff along the left floodplain (as 

viewed facing downstream) determines the lateral 

extent of the mesh on the Illinois side, and the Bois 

Brule Levee determines the lateral extent of the mesh 

on the right floodplain (as viewed facing downstream) 

on the Missouri side. In November 1999 and January 

2000, five cross sections were surveyed across the 

channel and floodplains using global positioning satellite (GPS) technology combined with conventional surveying techniques. These methods were used to 

correct, refine, and correlate topographic contours from 

a 1:24,000 scale digital raster graphic (DRG) image of 

the study area to define the topography of the reach.

In areas where substantial change in topography 

exists, the mesh was refined to accurately represent 

existing structures and channels, and to accommodate 

the model’s sensitivity to wetting and drying of elements along boundaries. Elements became “wet” or 

“dry” as the water surface changes to cover or expose 

them. Examples of topographic features in this study 

reach that required refinement were levees, roadway 

embankments, and river banks. Furthermore, in areas 

where velocity, depth, and water-surface elevation 

changes were expected to be large, mesh details were 

increased to better facilitate simulation by the model. 

An example of this refinement is near the Horse Island 

Chute and Mississippi River bridges and the adjoining 

road embankments (fig. 4). The refinement also 

improves the precision of the parameters needed for the 

scour computations at the Horse Island Chute bridge. 

The finite-element mesh used in this study has 6,576 

elements. Each element has a node at each corner, at 

the mid-point of each side, and in the center, for a total 

of 22,060 nodes. 

A particular flow scenario cannot be simulated 

directly because the simulation cannot be started with 

boundary conditions that exactly represent the true 

conditions. The true conditions that are to be simulated 

are reached in a process called “spindown.” In a subcritical flow reach (a reach where flow is deep and 

velocities are low, typical of large rivers), spindown 

involves starting the simulation with the desired discharge as the upstream boundary condition and a 

downstream tailwater elevation that is higher than the 

highest land-surface elevation in the mesh. This condition must be met so that all nodes and elements in the 

model are “wet,” or have a positive depth of flow. The 

simulation is run with these conditions for a sufficient 

number of iterations to cause the water-level elevation 

changes between iterations to be minimized within a 

pre-set limit. The downstream tailwater elevation is 

then decreased by some finite amount, and the simulation is run again using the results of the previous 

run—called a “hot start”—as the starting point for the 

first iteration of the new run. This process is repeated 

until the desired tailwater elevation is reached, as dictated by high-water marks, flood profiles, or other 

known site parameters.

During the spindown process, if the simulated 

water-surface elevation at a particular node is less than 

the land-surface elevation assigned to the node, the 

node is said to “go dry.” If one or more of the nodes for 

a particular element go dry, then the element goes 

“dry,” and the element is not included in the computations during that iteration. As the simulation proceeds 

through the assigned number of iterations, it is possible 

for an element to oscillate between wet and dry; the 

user sets a tolerance on the depth of flow over a node. 

However, if an element goes dry and stays dry for several iterations, it should be disabled or the simulation 

may become unstable and not converge.

Calibration of Model to Observed Conditions

Typically, a model is calibrated to an extreme 

condition such as a flood peak, because flood-peak elevation marks can be identified after an event, and a 

steady-state condition is assumed. However, accurate
Simulation of Flow 7
8 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois

flood-peak elevations were not obtained in 1993 within 

the study reach. Elevations were estimated several 

months after the peak from high-water marks along the 

levees and generalized comments of local observers; 

however, accurate placement of the marks could not be 

established. For this study, a supplemental attempt to 

obtain additional flood-peak elevations in buildings 

along the bluff on the Illinois side resulted in very limited success. Therefore, the non-peak flood profile 

obtained on August 5, 1993, with a measured discharge 

of 944,000 ft3/s and a water-surface elevation at the 

gage of 390.20 ft above sea level, were used as the calibration parameters.

A variety of land-use coverages were applied to 

elements in the mesh (fig. 5), and depth-dependent 

Manning’s roughness coefficients (n-values) were 

assigned to each coverage type (table 1). In the depthdependent n-value method, the “lower depth” n-value 

is applied when the water depth over the element is less 

than the lower depth, and the “upper depth” n-value is 

applied when the water depth is greater than the upper 

depth; when the water depth is between the lower and 

upper depths, the n-value is interpolated linearly from 

the upper and lower n-values (Froehlich, 1989). Depthdependent n-values were used to account for changes in 

the roughness with increased depth of flow; for most 

land-use types, the roughness decreases with increased 

depth because the effect of the physical features causing the roughness decreases as the depth of flow 

increases (for example, grass and crops that lie over in 

high flows). For a few of the land-use types, the roughness increases with increased depth (for example, flow 

through overhanging tree branches in “Old River and 

Horse Island Chute” or the transition from stems and 

trunks to branches in “trees and brush”). The “spur 

dike” category was created to represent the effect of 

spur dikes along the banks; actual elevations of the 

dikes were unavailable, but the n-values assigned to the 

lower depth were increased to impede flow at lower 

depths of flow.

A discharge of 944,000 ft3/s was used as the 

upstream boundary condition, and spindown of the 

model occurred until a tailwater elevation of 388.35 ft 

was reached at the downstream boundary. During calibration of the model, the roughness coefficients were 

adjusted within reasonable limits until simulated flood 

elevations satisfactorily matched the flood elevations 

determined during the profile on August 5, 1993 (fig. 

6). The calibrated model results are within +/- (plus or 

minus) 0.20 ft of the surveyed flood elevations, except 

the farthest upstream profile point and the profile 

points at the constriction (fig. 6). The larger differences at the constriction are thought to be the result of 

drawdown caused by the combined effects of the constriction and a shelf of sediment deposition at the 

mouth of Horse Island Chute during the channel survey 

in 1999, which presumably was not present during the 

1993 flood. Simulated water-surface elevations from 

the measured discharge simulation are shown in figure 

7. The results indicate the discharge through the Mississippi River bridge was 886,000 ft3

/s (93.9 percent of 

the measured discharge), and discharge through the 

Horse Island Chute bridge and over the road embankment between the bridges was 57,500 ft3

/s (6.1 percent 

of the measured discharge).

Simulation results also indicated that the maximum velocity in the Mississippi River was 11.5 ft/s 

(feet per second) and occurred at the levee constriction; 

the maximum depth was 79.2 ft and occurred near the 

Mississippi River bridge. The maximum velocity and 

depth in Horse Island Chute were 4.3 ft/s and 40.3 ft, 

and occurred near the bridge. The maximum velocity 

and depth over the road embankment were 2.6 ft/s and 

3.9 ft.

Simulation of August 6, 1993, Maximum 

Discharge

The peak discharge of 1,000,000 ft3/s was used 

as the upstream boundary condition for the maximum 

discharge simulation. The final results of the measured 

discharge simulation were used as the hot start for the 

first iteration of the maximum discharge simulation, 

and spindown of the simulation occurred until the tailwater elevation (the downstream boundary condition) 

approximately matched a flood-peak elevation mark 

obtained about 1,500 ft downstream from the downstream end of the reach. The same model configuration 

established in the calibration simulation was used in the 

maximum discharge simulation, except for a few elements near the north end of the road embankment that 

had been disabled (they had gone dry in the calibration 

simulation) and were re-enabled to permit flow 

because the water-surface elevations were higher than 

in the calibration simulation.

Simulated water-surface elevations from the 

maximum discharge simulation are shown in figure 8. 

The results indicate the discharge through the Mississippi River bridge was 936,000 ft3

/s (93.6 percent of 

the maximum discharge), and discharge through the
Simulation of August 6, 1993, Maximum Discharge 9
10 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois

Table 1. Manning’s n-values used in model calibration

[ft, feet]

Land-use coverage

Lower depth Upper depth

Manning’s n

Depth 

(ft) Manning’s n

Depth

(ft)

Channel and bank

Mississippi River main channel 0.025 15 0.031 20

Spur dikes (on main channel banks) .500 20 .050 25

Commercial and railroad embankment .075 3 .050 8

Old River and Horse Island Chute .040 2 .045 5

Floodplain

Row crop .100 3 .065 8

Timber and brush .160 2 .220 5

Open timber and brush .075 5 .120 10

Brush (small willows, scrub) .170 5 .130 8

Pasture (native grass) .055 1 .045 2

Road and embankment .045 1 .040 2

Bois Brule Levee (crab grass, bluegrass) .055 1 .045 3
Simulation of August 6, 1993, Maximum Discharge 11
12 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois
Simulation of August 6, 1993, Maximum Discharge 13

Horse Island Chute bridge and over the road embankment between the bridges was 63,500 ft3

/s (6.4 percent 

of the maximum discharge).

Simulation results also indicated that the maximum velocity in the Mississippi River was 12.1 ft/s and 

occurred at the levee constriction; the maximum depth 

was 80.0 ft and occurred near the Mississippi River 

bridge. The maximum velocity and depth in Horse 

Island Chute were 4.5 ft/s and 41.0 ft, and occurred 

near the bridge. The maximum velocity and depth over 

the road embankment were 2.5 ft/s and 4.6 ft. The simulated water-surface elevation at the gage was 390.83 

ft, which compares favorably with the gaging station 

elevation of 390.79 ft measured at the peak.

Simulation of Impendent Discharge

The impendent discharge was determined by a 

step process. As a starting point, the average depth of 

flow over the road in the calibration simulation (3.6 ft) 

was subtracted from the water-surface elevation at the 

gaging station, which resulted in an approximate elevation of 386.60 ft at the gaging station for the impendent 

discharge. This gage elevation, related to a discharge 

based on the stage-discharge relation of the gaging station, corresponded to a discharge of approximately 

880,000 ft3/s for the upstream boundary condition for 

the first step in the impendent discharge simulation.

The final results of the measured discharge simulation were used as the hot start for the first step of the 

impendent discharge simulation. As the tailwater elevation (the downstream boundary condition) was incrementally lowered in the spindown process, the 

elements along the top of the road embankment began 

to go dry as more of the embankment was extended 

above the simulated water surface. The tailwater elevation was lowered until most of the elements at the top 

of the road embankment were dry, except those immediately adjacent to the Horse Island Chute bridge, 

which remained wet because of the backwater caused 

by the contraction at the bridge.

The simulated water-surface elevation at the 

gaging station under the conditions of the first step 

(387.08 ft) was related to a new discharge of 894,000 

ft3/s for the second step in the impendent discharge 

simulation. The final results of the first step were used 

as the hot start for the second step, and the tailwater elevation again was lowered until the elements immediately adjacent to the Horse Island Chute bridge went 

dry.

The simulated water-surface elevation at the 

gaging station under the conditions of the second step 

changed by 0.01 ft from the results of the first step, so 

the results of the second step were assumed to be an 

accurate simulation of the impendent discharge. 

Therefore, the impendent discharge was 894,000 ft3/s 

with a water-surface elevation of 385.15 ft at the downstream end of the study reach. These simulated results 

compare reasonably with the corresponding parameters 

from the measured discharge and maximum discharge 

simulations, as shown in table 2.

Simulated water-surface elevations for the 

impendent discharge simulation are shown in figure 9. 

The results indicate the discharge through the Mississippi River bridge was 846,000 ft3

/s (94.7 percent of 

the impendent discharge), and discharge through the 

Horse Island Chute bridge was 47,800 ft3/s (5.3 percent 

of the impendent discharge). Simulation results also 

indicated that the maximum velocity in the main channel was 11.7 ft/s and occurred at the levee constriction; 

the maximum depth was 76.3 ft and occurred near the 

Mississippi River bridge. The maximum velocity and 

depth in Horse Island Chute were 4.2 ft/s and 37.2 ft, 

and occurred near the bridge.

Table 2. Final simulated results for three alternative discharges

[ft3/s, cubic feet per second; ft, feet]

Discharge type

Discharge

(ft3/s)

Water-surface

elevation at gage

(ft)

Water-surface 

elevation at downstream 

boundary

(ft)

Difference 

in water-surface 

elevations

(ft)

Measured 944,000 390.18 388.35 1.83

Maximum 1,000,000 390.83 388.85 1.98

Impendent 894,000 387.09 385.15 1.94
14 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois
Simulation of August 6, 1993, Maximum Discharge 15

Historical records at the Mississippi River gaging station show that the discharge measured through 

the Horse Island Chute bridge is related to the total discharge measured for the Mississippi River at the gaging 

station. A graph of discharge measurements through 

Horse Island Chute (fig. 10) indicates that once the 

total discharge measured at the the gaging station 

exceeds approximately 452,000 ft3/s (with approximately a 2-year discharge recurrance interval), water in 

the Mississippi River main channel overtops the banks 

and begins to flow toward Horse Island Chute. At total 

discharges larger than 452,000 ft3/s, the discharge in 

Horse Island Chute is approximated by the relation: 

where QChute is 

the discharge in the Horse Island Chute and QTotal is 

the total discharge measured at the gaging station. As 

determined by the simulation, flow over the road 

embankment exists for total discharges greater than 

894,000 ft3/s, so the discharge in Horse Island Chute 

calculated using the regression equation for a total discharge above 894,000 ft3

/s will not include the quantity of discharge over the road. Using this relation, the 

discharge in Horse Island Chute for the three discharges was compared with the simulation results, 

excluding road overflow (table 3).

QChute 0.105QTotal = – 47, , 500
16 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois

EVALUATION OF SCOUR

Scour is the general term used to describe the 

erosion of channel material and can be divided into 

three primary components:

 • Degradation (or aggradation), which refers to the 

general lowering (or raising) in elevation of a channel 

bed with time because of sediment transport;

 • General contraction scour, which describes the erosion caused by increased velocities and turbulence that 

occur during flooding or result from the contraction of 

flow by bridge approach embankments or other constrictions that encroach on a floodplain; and

 • Local scour, which describes the localized removal 

of material by increased velocities and turbulence 

resulting from local disturbances to flow such as bridge 

piers, abutments, and spur dikes/guidebanks (Richardson and others, 1995).

Although scour processes continuously are at 

work, they are accelerated during high-flow conditions, 

thereby increasing the potential for scour-related problems at a bridge.

Using techniques outlined in Hydraulic Engineering Circular No. 18 (HEC-18), entitled “Evaluating Scour at Bridges” (Richardson and others, 1995), 

the various components of scour can be estimated from 

hydraulic parameters determined by numerical modeling and bridge parameters obtained from bridge plans 

and surveys taken of the site. Long-term degradation 

or aggradation can be determined from a comparison of 

the present channel cross section at the bridge with the 

channel cross section at the time of bridge construction. General contraction scour depths can be estimated 

using the ratio of the discharge in the approach channel 

to the discharge in the bridge opening and the ratio of 

the width of the approach channel to the width of the 

bridge opening. Local pier scour depths can be estimated at each pier or bent using flow velocity, flow 

depth, pier width, pier length, and flow approach angle. 

Local abutment scour depths can be estimated for the 

approach embankment on each side of the bridge using 

the amount of discharge and flow area on the floodplain that is blocked by each approach embankment.

Because the Bois Brule Levee in the vicinity of 

Missouri State Highway 51 was not breached, the funneling effect of the levee downstream from Missouri 

State Highway 51 caused the velocities of water in the 

Mississippi River, Horse Island Chute, and in the 

adjoining floodplains to be greater during the 1993 

flood than they would have been had the levee been 

breached. Therefore, the conditions during this record 

flood were almost ideal for determining the most 

extreme scour effects at the Horse Island Chute bridge. 

Another situation for maximum scour exists when 

adjacent road embankments are just about to be overtopped, as is the case in the impendent discharge scenario, because all of the flow on adjoining floodplains 

is forced through the bridge opening, instead of flowing directly over the roadway.

Scour Depth Computations In Finite Element 

Surface-Water Modeling System

The FESWMS-2DH model is able to compute 

general contraction scour at each node, using a critical 

bed shear stress (τ0, the stress that must be overcome to 

move a soil particle by the flow) and various other 

parameters computed by the model during the simulation. Chow (1959) defines τ0

 as “permissible tractive 

force” or “permissible shear force” and provides several tables for estimating τ0

 based on material type, parTable 3. Simulated and calculated discharges through Horse Island Chute bridge

[ft3/s, cubic feet per second]

Discharge

Total

discharge (ft3/s)

Simulated 

discharge through 

Horse Island Chute 

bridge 1

(ft 3/s)

Calculated 

discharge through 

Horse Island Chute 

bridge based on 

regression

(ft3/s)

Simulated minus 

calculated 

discharge 

(ft3/s)

Percent 

difference

from calculated 

discharge

Measured 944,000 54,700 51,600 3,100 6.0

Maximum 1,000,000 59,200 57,500 1,700 3.0

Impendent 894,000 47,800 46,400 1,400 3.0

1

 Flow over road embankment is not included.
Evaluation of Scour 17

ticle size, and cohesion. Furthermore, Froehlich 

(1989) provides guidance for estimating τ0 based on 

soil cover. However, the critical bed shear stress was 

difficult to estimate with confidence. The critical bed 

shear stress for the Mississippi River was estimated as 

0.09 lb/ft 2 (pounds per square foot) based on the 

median particle diameter (D 50, the particle diameter at 

which 50 percent of the material is coarser and 50 percent is finer) of the material found in the main channel 

[D50 is 0.76 mm (millimeter)], but specific information 

about the channel and floodplain conditions during the 

flood was not available. Therefore, the general contraction scour depths computed by the model were not 

used.

However, the constriction in the floodplain 

downstream of the Mississippi River bridge caused a 

substantial amount of general contraction scour 

because of increased velocities in this part of the study 

reach. A contour plot of the velocity magnitudes in the 

study reach for the maximum discharge simulation are 

shown in figure 11. The velocity magnitude reaches a 

maximum just upstream from the point of maximum 

constriction by the levee and diminishes downstream. 

This velocity increase and subsequent downstream 

expansion is likely the cause of the relatively flat 

water-surface profiles seen at and downstream from the 

constriction in figures 6 through 9, because an increase 

in velocity results in a decreased flow depth in a subcritical flow reach. Furthermore, general contraction 

scour depths computed at nodes in this part of the study 

reach reached 35 ft at the point of maximum velocity in 

the maximum discharge simulation, indicating the 

shelf of sediment deposited at the mouth of Horse 

Island Chute may not have been present in the 1993 

flood. Thus, the flood-profile marks in the vicinity of 

the constriction are substantially higher that the simulated water-surface elevations.

Local pier scour also can be computed by the 

model when the user defines the location, size, alignment, and various other parameters for each pier or 

bent. However, the pier scour depths computed by the 

model are for the existing channel configuration. Currently (2001), the scour assessments performed for the 

MoDOT are for most extreme conditions, which allow 

for a dynamic channel configuration such as a shift in 

the channel thalweg during a flood. Therefore, the pier 

scour depths computed by the model were not used.

A magnification of the finite-element mesh in 

the vicinity of the Horse Island Chute bridge is shown 

in figure 12 with results from the maximum discharge 

simulation. The depth of flow is indicated with shaded 

areas, and the velocity magnitude computed at each 

corner node is indicated with a vector. The velocity 

vectors indicate the line of separation of flow on the 

embankment between the two bridges occurs approximately 115 ft north of the left (north) end of the Horse 

Island Chute bridge for the maximum discharge.

In the impendent discharge simulation, the line of 

flow separation on the upstream face of the left (north) 

road embankment was expected to shift more to the 

north, toward the Mississippi River bridge, as flow over 

the road diminished. However, the velocity vectors for 

the impendent discharge simulation indicate that the flow 

separation line remained at approximately the same place 

as for the maximum discharge simulation and also the 

measured discharge simulation. The effects of the road 

embankment on the flow distribution through the Horse 

Island Chute bridge are minimal; therefore, the worstcase conditions for scour are not created in the impendent 

discharge simulation as was expected.

Scour Depth Computations Using Finite 

Element Surface-Water Modeling System 

Results

To obtain the parameters required for scour 

depth computations from the simulation results, flux 

lines were delineated at critical locations of interest in 

the finite-element mesh. The various node strings used 

as flux lines are indicated in figure 12. Flux lines were 

delineated at: 1) the upstream face of the Horse Island 

Chute bridge (upstream bridge face); 2) the downstream face of the Horse Island Chute bridge (downstream bridge face); 3) across the upstream channel of 

Horse Island Chute, from the top of the left (north) 

bank to the top of the right (south) bank, one bridgelength upstream from the bridge (approach); 4) along 

the upstream toe of the road embankment from the 

point of flow separation to the left (north) abutment toe 

(left embankment toe); 5) along the upstream toe of the 

road embankment from the right (south) abutment toe 

to the levee (right embankment toe); 6) along the 

upstream top edge of the road embankment from the 

line of flow separation to the left (north) end of the 

bridge (left road overflow); and 7) along the upstream 

top edge of the road embankment from the right (south) 

end of the bridge to the levee (right road overflow). The 

velocity magnitude, depth of flow, and water-surface 

elevation assigned to each node in the flux lines were 

obtained from the simulations and used to determine
18 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois
Evaluation of Scour 19
20 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois

the cross-sectional area of flow, average depth, and 

average flow velocity at each flux line. Additionally, 

the average elevation of the energy grade line (the sum 

of the land-surface elevation, the depth of flow, and the 

velocity head, V

2

/2g, where V is the mean velocity of 

flow and g is the acceleration of gravity), was determined at the upstream flux line and the flux line at the 

downstream bridge face for use in the contraction scour 

computations.

The estimated contraction scour depths for 

Horse Island Chute (table 4) were calculated using 

Laursen’s clear-water contraction scour equation 

(Richardson and others, 1995). Based on the velocities 

and depths obtained from the FESWMS-2DH analysis 

and the median diameter of the material in the Mississippi River (D50 is 0.76 mm), a critical velocity of 

incipient motion, Vc, for each of the three simulations 

was calculated using Laursen’s equation (Richardson 

and others, 1995). Because the critical velocity was 

greater than the mean velocity in the approach channel 

for all three of the discharge simulations used in this 

analysis and because of the extensive vegetation found 

in the upstream reach, a clear-water scour scenario was 

assumed to exist in the Horse Island Chute channel.

The local pier scour depths (table 5) were calculated using the Colorado State University (CSU) pier 

scour equation (Richardson and others, 1995). Each of 

the 13 bents consisted of 5 octagonal concrete piles 

1.334 ft in diameter. Therefore, each bent has an overall length of 6.67 ft, according to HEC-18 guidelines 

(Richardson and others, 1995). Each bent was analyzed 

using the maximum velocity magnitude and the maximum depth of flow in the channel at the upstream face 

of the bridge, to account for possible changes in the 

thalweg during a flood. The flow angle of attack as 

determined by FESWMS-2DH for each bent was 

included in the local pier scour depth computation.

The contraction and local pier scour depths computed for the maximum discharge are shown in figure 

13, and the total scour (sum of contraction scour and 

local pier scour) for the three simulations is shown in 

figure 14. Scour depths are shown in relation to the land 

surface on January 19, 2000. The channel shows longterm aggradation, as indicated by the difference in the 

land surface in 1942 (from MoDOT plans) and the land 

surface at the time of the onsite field survey (January 

19, 2000).

Table 4. Contraction scour depths and parameters used to compute scour

[ft/s, feet per second; ft 3/s, cubic feet per second; ft, feet]

Parameter required to compute scour depth

Measured 

discharge

Maximum 

discharge

Impendent 

discharge

Critical velocity of incipient motion (ft/s) 2.69 2.70 2.65

Average velocity in approach channel (ft/s) 2.22 2.35 2.13

Type of scour Clear water Clear water Clear water

Approach section (fig. 12)

Discharge (ft3/s) 23,130 24,952 20,270

Channel width (ft) 296.6 296.6 296.6

Average depth (ft) 35.1 35.8 32.0

Energy grade line elevation (ft) 391.0 391.8 388.0

Downstream bridge face (fig. 12)

Discharge (ft3/s) 54,830 59,297 47,797

Channel width (ft) 460.8 460.8 460.8

Average depth (ft) 33.4 34.1 30.2

Energy grade line elevation (ft) 390.5 391.2 387.4

Computed contraction scour depth (ft) 8.1 10.3 6.7
Evaluation of Scour 21

Table 5. Local pier scour depths and parameters used to compute scour

[MoDOT, Missouri Department of Transportation; ft, feet; ft/s, feet per second]

MoDOT bent 

number 1

Stationing along 

centerline from left 

(north) end of bridge2

(ft)

Maximum depth 

at upstream face3

(ft)

Maximum 

velocity at 

upstream face3

(ft/s)

Angle of attack 

computed by 

FESWMS-2DH 

(degrees)

Computed pier 

scour depth 

(ft)

Measured discharge

14 34.0 40.14 4.12 0.2 3.8

13 67.0 40.14 4.12 1.9 4.2

12 100.0 40.14 4.12 3.2 4.5

11 133.0 40.14 4.12 4.5 4.7

10 166.0 40.14 4.12 5.6 4.9

9 199.0 40.14 4.12 6.1 5.0

8 232.0 40.14 4.12 6.4 5.1

7 265.0 40.14 4.12 6.6 5.1

6 298.0 40.14 4.12 6.7 5.1

5 331.0 40.14 4.12 6.8 5.1

4 364.0 40.14 4.12 7.0 5.2

3 397.0 40.14 4.12 7.4 5.2

2 430.0 40.14 4.12 7.0 5.2

Maximum discharge

14 34.0 40.99 4.34 0.9 4.1

13 67.0 40.99 4.34 1.4 4.2

12 100.0 40.99 4.34 2.9 4.5

11 133.0 40.99 4.34 4.2 4.8

10 166.0 40.99 4.34 5.4 5.0

9 199.0 40.99 4.34 6.1 5.1

8 232.0 40.99 4.34 6.4 5.2

7 265.0 40.99 4.34 6.7 5.2

6 298.0 40.99 4.34 6.8 5.3

5 331.0 40.99 4.34 7.1 5.3

4 364.0 40.99 4.34 7.3 5.3

3 397.0 40.99 4.34 7.9 5.5

2 430.0 40.99 4.34 7.6 5.4

Impendent discharge

14 34.0 37.23 4.08 0.5 3.9

13 67.0 37.23 4.08 2.0 4.2

12 100.0 37.23 4.08 3.2 4.4

11 133.0 37.23 4.08 4.3 4.6

10 166.0 37.23 4.08 5.3 4.8

9 199.0 37.23 4.08 5.7 4.9

8 232.0 37.23 4.08 5.8 4.9

7 265.0 37.23 4.08 5.8 4.9

6 298.0 37.23 4.08 5.6 4.8

5 331.0 37.23 4.08 5.4 4.8

4 364.0 37.23 4.08 5.1 4.8

3 397.0 37.23 4.08 4.9 4.7

2 430.0 37.23 4.08 2.7 4.3

1

Pier/bent number corresponds to Missouri Department of Transportation bridge plans. 

2 Stationing is the distance from the left abutment (as viewed facing downstream).

3 Maximum depth and velocity are used for all bents to account for possible changes in the thalweg during a flood event. 
22 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois

Local abutment scour depths were computed for 

the three simulations (table 6) using the Froehlich livebed abutment scour equation (Richardson and others, 

1995). The local abutment scour depths assume no protection on the abutment faces; however, the abutment 

and upstream embankment faces are covered with large 

riprap boulders and stone revetment (fig. 15), which is 

expected to provide some resistance to abutment scour. 

Furthermore, there is no evidence of substantial scour 

occurring at the abutments. Therefore, no abutment 

scour is shown on figures 13 and 14.

Based on the computations of total scour depths 

(fig. 14), the scour depths indicated that the bridge 

bents were not substantially exposed during the maximum flood discharge on August 6, 1993. The computed contraction scour depths expose bents 5 through 

10 to approximately the same depths as when the 

bridge was constructed in 1942. The computed scour 

depths for the measured discharge and the impendent 

discharge are less than those computed for the maximum discharge.

A channel bottom profile of Horse Island Chute 

at the bridge was constructed from discharge measurements obtained during the flood of 1993, using the lowest streambed elevation obtained at a given station 

along the bridge. This profile, shown in figures 13 and 

14, indicates that the predicted scour had not occurred 

as of July 30, 1993, the last date a discharge measurement was obtained from the Horse Island Chute bridge 

before the peak on August 6, 1993.

In fact, an examination of the historical discharge measurements obtained at the Horse Island 

Chute bridge indicates that the channel bed was 

dynamic through April of 1960, but has since remained 

essentially stable at it’s current configuration, despite 

several large flood events after 1960. Figure 16A 

shows bed profiles for small flood events, and figure 

16B shows bed profiles for large flood events, the 

breakpoint between “small” and “large” being a flood 

that results in approximately 22,000 ft 3/s through 

Horse Island Chute bridge (approximately a 5-year 

recurrence interval). Before 1960, the channel bed was 

dynamic, with measured bed elevations as much as 13 

ft lower than the as-built bed elevation during the 1943 

and 1947 floods (fig. 16B). On April 11, 1960, a discharge of 24,700 ft3

/s was measured through the Horse 

Island Chute bridge, and the channel bed was approximately equal to the as-built bed (fig. 16B). However, 

on April 13, 1960, a discharge of 17,300 ft3/s was measured through the Horse Island Chute bridge, and the 

channel bed had aggraded nearly 10 ft to the current 

bed configuration (fig. 16A).

The constriction caused by the Bois Brule Levee 

may have contributed to the aggradation of the channel 

bed at Horse Island Chute bridge. The MoDOT records 

for Horse Island Chute bridge indicate that the Bois 

Brule Levee was considerably lower when the bridge 

was constructed in 1942, and the levee was apparently 

overtopped several times in the 1940’s. As a result of 

the large flood on the Mississippi River in 1947, the 

levee was raised to it’s present day level during the 

1940’s (Francis Toohey, Attorney for Bois Brule Levee 

District, oral commun., 2001). The channel began to 

stabilize, as is shown by the bed profiles on July 10, 

1951 (fig. 16A) and July 23, 1951 (fig. 16B), and 

reached an equilibrium after substantial sediment deposition during the flood in April 1960.

Debris was trapped on the upstream face of the 

bents at the time of the onsite visit (fig. 17), and measurement notes indicate that debris trapped on the 

upstream face of the bridge is a typical occurrence during floods. The local pier scour depths can be increased 

by debris lodged on the bents, but HEC-18 (Richardson 

and others, 1995) indicates that limited research has 

been done on local scour with debris and, therefore, 

scour equations do not include the effect of debris. The 

effects of debris in floods on bed profiles since 1960 

apparently have been minimal (fig. 16), but additional 

study would be required to determine the effects of 

debris on the total scour that might occur at this site.

SUMMARY

A two-dimensional Finite-Element SurfaceWater Modeling System (FESWMS-2DH) was used to 

simulate flow conditions at the Missouri State Highway 51 crossing of the Mississippi River and Horse 

Island Chute in Perry County, Missouri, near Chester, 

Illinois. The model was used to simulate flow conditions for three discharges: one simulation was for a discharge measurement made on August 5, 1993, at the 

time of a flood profile defined along the Illinois side of 

the river (measured discharge); another simulation was 

for the record flood peak on August 6, 1993 (maximum 

discharge); and the third simulation was for the discharge 
Summary 23
24 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois
Summary 25

Table 6. Local abutment scour depths and parameters used to compute scour

[ft3/s, cubic feet per second; ft, feet; ft2, square feet; ft/s, feet per second]

Parameter required for computation of scour depth

Measured 

discharge

Maximum 

discharge

Impendent 

discharge

Left (north) abutment/embankment (from left abutment toe to flow separation line)

Discharge blocked by embankment1 (ft3/s) 1,246 1,428 715

Embankment length (ft) 143 143 143

Area of blocked flow (ft2) 2,520 2,539 2,447

Velocity at upstream toe (ft/s) 2.41 2.63 2.13

Computed abutment scour depth (ft) 25.4 26.1 20.6

Right (south) abutment/embankment (from right abutment toe to levee)

Discharge blocked by embankment1 (ft3/s) 2,928 3,337 1,160

Embankment length (ft) 278.1 278.1 278.1

Area of blocked flow (ft2) 3,140 3,178 2,998

Velocity at upstream toe (ft/s) 2.96 3.16 2.56

Computed abutment scour depth (ft) 34.4 35.3 28.7

1Discharge blocked by embankment is the difference in total approach discharge blocked and amount of flow 

over the road embankment.

Figure 15. Stone revetment on abutment face under left (north) end of Horse Island Chute bridge near 

Chester, Illinois, as viewed from downstream face of bridge.
26 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois
Summary 27

that caused impending road overtopping at the road 

embankment in the vicinity of the Horse Island Chute 

bridge (impendent discharge).

Using a measured discharge of 944,000 cubic 

feet per second and elevation data collected on August 

5, 1993, the model was calibrated to the flood profile 

along the railroad embankment on the Illinois side of 

the Mississippi River. The simulated flow conditions 

for the measured discharge indicated that 93.9 percent 

of the flow was through the Mississippi River bridge, 

and 6.1 percent was through the Horse Island Chute 

bridge and over the road embankment. The maximum 

velocity was 11.5 feet per second in the Mississippi 

River at the levee constriction, 4.3 feet per second in 

Horse Island Chute at the bridge, and 2.6 feet per second over the road. The simulated flow conditions for 

the maximum discharge of 1,000,000 cubic feet per 

second indicated that the percentage of flow through 

the Mississippi River bridge decreased slightly to 93.6 

percent, with a corresponding increase to 6.4 percent 

through the Horse Island Chute bridge and over the 

road embankment. The maximum velocity increased to 

12.1 feet per second in the Mississippi River at the 

levee constriction, and to 4.5 feet per second in Horse 

Island Chute at the bridge, and decreased slightly to 2.5 

feet per second over the road. The impendent discharge 

was determined to be 894,000 cubic feet per second, 

with 94.7 percent of the flow going through the Mississippi River bridge, and 5.3 percent through the Horse 

Island Chute bridge. However, the maximum velocity 

decreased to 11.7 feet per second in the Mississippi 

River at the levee constriction and 4.2 feet per second 

in Horse Island Chute at the bridge.

The flow conditions determined in the simulations were used to estimate the contraction, local pier, 

and local abutment scour depths for the three discharges. The contraction scour depth calculated for the 

maximum discharge was the greatest at 10.3 feet. The 

contraction scour depths calculated for the measured 

and impendent discharges were 8.1 and 6.7 feet. The 

calculated local pier scour depths varied from bent to 

bent depending on angle of attack of flow, but the maximum pier scour depth for the maximum discharge was 

5.5 feet. The maximum pier scour depths calculated for 

Figure 17. Downstream face of Horse Island Chute bridge near Chester, Illinois, as viewed from downstream 

face of left (north) embankment. Note debris trapped on upstream face of bents.
28 Simulation of Flow and Evaluation of Bridge Scour at Horse Island Chute Bridge near Chester, Illinois

the measured and impendent discharges were 5.2 and 

4.9 feet. The maximum calculated total scour depth 

(contraction and local pier scour) was 13.3 feet for the 

measured discharge, 15.8 feet for the maximum discharge, and 11.6 feet for the impendent discharge. 

Abutment scour depths were calculated for the site, but 

are expected to be minimized by the presence of riprap 

boulders and stone revetment on the abutment and 

embankment faces.

REFERENCES CITED

Chow, V.T., 1959, Open-channel hydraulics: New 

York, McGraw-Hill Publishing, 680 p.

Environmental Modeling Research Laboratory, 1999, 

Surface-water modeling system reference manual, 

version 6.0: Brigham Young University, Provo, 

UT, 315 p.

Federal Highway Administration, 1988, Scour at 

bridges: Federal Highway Administration, Department of Transportation Technical Advisory 

5140.20, 103 p.

Froehlich, D.C., 1989, HWO.31.D—Finite element 

surface-water modeling system: Two-dimensional 

flow in a horizontal plane—Users manual: U.S. 

Federal Highway Administration, FHWA-RD-88-

177, 285 p.

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

Davis, S.R., 1995, Evaluating scour at bridges: 

U.S. Federal Highway Administration, FHWA-IP90-017 HEC-18, 204 p.