U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2011–1223

Prepared in Cooperation with the 

Crawford County Conservation District and Fairfield Township, Pennsylvania

Simulations of Flow and Prediction of Sediment Movement 

in Wymans Run, Cochranton Borough, Crawford County, 

Pennsylvania
Cover. Photo of upstream opening of PA 28 bridge after the June 5, 2008 storm (Photograph by 

Elizabeth Hittle, U.S. Geological Survey, June 25, 2008) 
Simulations of Flow and Prediction of 

Sediment Movement in Wymans Run, 

Cochranton Borough, Crawford County, 

Pennsylvania

By Elizabeth Hittle

Prepared in Cooperation with the 

Crawford County Conservation District and Fairfield Township, Pennsylvania 

Open-File Report 2011–1223

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: 2011

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reproduce any copyrighted materials contained within this report.

Suggested citation:

Hittle, Elizabeth, 2011, Simulations of flow and prediction of sediment movement in Wymans Run, Cochranton Borough, Crawford County, Pennsylvania: U.S. Geological Survey Open-File Report 2011–1223, 25 p.
iii

Contents

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

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

Purpose and Scope ..............................................................................................................................3

Description of Study Area ...................................................................................................................3

Channel Characterization...........................................................................................................3

PA 285 Bridge ...............................................................................................................................3

Model Selection and Construction .............................................................................................................7

HEC-RAS Model Construction ............................................................................................................7

Channel Bathymetry ...................................................................................................................7

Determination of Manning’s n Values .....................................................................................8

June 2008 Storm Data .................................................................................................................8

Hypothetical Channel Geometries and Bridge Design..........................................................9

MD_SWMS Construction..................................................................................................................11

MD_SWMS Bathymetry ..........................................................................................................12

MD_SWMS Computational Grid and Boundary Conditions ...............................................12

Model Calibration-Drag Coefficient........................................................................................12

Simulation Results .......................................................................................................................................13

HEC-RAS Model Results ...................................................................................................................13

FaSTMECH Model Results–Water-Surface Elevations................................................................13

FaSTMECH Model Results–Velocity Distribution ..........................................................................13

Velocity Distribution for Initial Channel Geometry ...............................................................17

Velocity Distribution for Hypothetical Channel Geometries ...............................................17

FaSTMECH Model Results–Shear-Stress Distribution.................................................................20

Shear-Stress Distribution for Existing Channel Geometry..................................................20

Shear-Stress Distribution for Hypothetical Channel Geometries......................................22

Model Limitations ......................................................................................................................22

Summary and Conclusions.........................................................................................................................22

Acknowledgments.......................................................................................................................................25

References Cited..........................................................................................................................................25
iv

Figures

1. Map showing location of Wymans Run watershed, Crawford County, Pa.........................2

2. Photographs showing channel and bank variation within the Wymans Run watershed, 

Crawford County, Pa. (A) approximately 200 feet upstream from the PA 285 bridge, 

looking downstream, (B) approximately 300 feet upstream from the PA 285 bridge, 

looking upstream, (C) approximately 400 feet upstream from the PA 285 bridge, 

looking upstream, and (D) upper watershed looking downstream......................................4

3. Maps showing locations of cross sections used for the Hydrologic Engineering 

Centers River Analysis System model and sediment surveys, including a detailed 

view in the vicinity of the bridge on Wymans Run, Crawford County, Pa ...........................5

4. Photographs showing Wymans Run, Crawford County, Pa., (A) upstream from the PA 

285 bridge and (B) upstream from the PA 285 bridge after the June 5, 2008, storm..........6

5. Graph showing elevations at cross section 1212, 40 feet upstream from the bridge 

on Wymans Run, Crawford County, Pa., before June 2008 storm determined from 

field-survey and using LIDAR data ...........................................................................................8

6. Graphs showing elevations for Wymans Run study reach, Crawford County, Pa., 

used for modeling the initial channel, expanded bridge or widened channel, and 

dredged channel scenarios. The cross sections surveyed after the June 2008 

storm are also shown for cross sections (A) 1,141, (B) 1,175, (C) 1,212, (D) 1,261, 

(E) 1,341, and (F) 1,378 upstream from French Creek ............................................................10

7. Graph showing water-surface elevations recorded on June 5, 2008, by the installed 

transducer and high-water mark found inside the transducer housing, 

Wymans Run, Crawford County, Pa.........................................................................................11

8. Graph showing elevations of the channel bed, Wymans Run, Crawford County, Pa., 

for the initial channel and the two dredged channel geometries as depicted in the 

Hydraulic Engineering Centers River Analysis System .......................................................11

9. Maps showing Multi-Dimensional Surface-Water Modeling System drag coefficients 

determined for the Wymans Run study reach, Crawford County, Pa., (A) using 

sediment sizes determined from field surveys and depth determined from a model 

run at a streamflow of 1,000 cubic feet per second, (B) actual drag coefficients used 

for the initial channel geometry, (C) close up of drag coefficients used in the widened 

channel geometry, and (D) close up of drag coefficients used in the dredged 50 

channel geometry.......................................................................................................................14

10. Graph showing surveyed high-water marks after the June 2008 storm, and the 

Multi-Dimensional Surface-Water Modeling System simulated initial water-surface 

elevation and various elevations for a streamflow of 1,000 cubic feet per second, 

Wymans Run study reach, Crawford County, Pa...................................................................15

11. Graph showing surveyed high-water marks after the June 2008 storm, the 

simulated Hydrologic Engineering Centers River Analysis System water-surface 

elevation for a streamflow of 1,000 cubic feet per second, and elevation of the left 

bank, Wymans Run study reach, Crawford County, Pa........................................................15

12. Graph showing Flow and Sediment Transport and Morphological Evolution of 

Channels model simulated initial water-surface elevation and the simulated 

water-surface elevations for hypothetical geometries for a streamflow of 

1,000 cubic feet per second, Wymans Run study area, Crawford County, Pa..................16

13. Graph showing variation of the Hjulström diagram showing streamflow in relation 

to grain size .................................................................................................................................17
v

14. Maps showing Flow and Sediment Transport and Morphological Evolution of 

Channels model simulated stream velocities for a streamflow of 1,000 ft3/s for the 

initial channel geometry, Wymans Run study reach, Crawford County, Pa .....................18

15. Maps showing Flow and Sediment Transport and Morphological Evolution of 

Channels model simulated velocity for (A) the expanded bridge, (B) the widened 

channel, (C) the stream channel dredged 50 feet upstream, and (D) the stream 

channel dredged 200 feet upstream for a streamflow of 1,000 cubic feet per 

second, Wymans Run study reach, Crawford County, Pa ...................................................19

16. Maps showing Flow and Sediment Transport and Morphological Evolution of 

Channels model predicted largest mobile sediment size based upon critical 

boundary shear stress values for each sediment class at a streamflow of 

1,000 cubic feet per second for the initial channel geometry, Wymans Run study 

reach, Crawford County, Pa......................................................................................................23

17. Maps showing Flow and Sediment Transport and Morphological Evolution of Channels model largest mobile sediment size based upon critical boundary shear stress 

values for each sediment class at a streamflow of 1,000 cubic feet per second for (A) 

the expanded bridge, (B) the widened channel, (C) the stream channel dredged 50 ft 

upstream from the bridge, and (D) the stream channel dredged 200 ft upstream 

from the bridge, Wymans Run study reach, Crawford County, Pa.....................................24
vi

Tables

1. Results of sediment surveys and Manning’s n calculations at selected cross sections 

in the Wymans Run study reach, Crawford County, Pa..........................................................9

2. Flow and Sediment Transport and Morphological Evolution of Channels model simulated water-surface elevations for the initial channel geometry and elevation differences between the initial geometry and the hypothetical channel geometries for a 

streamflow of 1,000 cubic feet per second at selected cross sections, Wymans 

Run study area, Crawford County, Pa......................................................................................16

3. Flow and Sediment Transport and Morphological Evolution of Channels model maximum simulated velocity for the initial and hypothetical channel geometries for a 

streamflow of 1,000 cubic feet per second at selected cross sections, Wymans 

Run, Crawford County, Pa..........................................................................................................21

4. Sediment grade scale, Shields parameter, and critical shear-stress ranges for 

each sediment class ..................................................................................................................21
vii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (mi)

Area

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

hectare (ha) 0.003861 square mile (mi2) 

square kilometer (km2) 0.3861 square mile (mi2)

Volume

cubic foot (ft3) 0.02832 cubic meter (m3) 

cubic decimeter (dm3) 0.03531 cubic foot (ft3) 

cubic meter (m3) 35.31 cubic foot (ft3)

Flow rate

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

cubic meter per second (m3/s) 35.31 cubic foot per second (ft3/s)

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD 88).

Abstract 

In small watersheds, runoff entering local waterways 

from large storms can cause rapid and profound changes in 

the streambed that can contribute to flooding. Wymans Run, 

a small stream in Cochranton Borough, Crawford County, 

experienced a large rain event in June 2008 that caused sediment to be deposited at a bridge. A hydrodynamic model, 

Flow and Sediment Transport and Morphological Evolution of 

Channels (FaSTMECH), which is incorporated into the U.S. 

Geological Survey Multi-Dimensional Surface-Water Modeling System (MD_SWMS) was constructed to predict boundary shear stress and velocity in Wymans Run using data from 

the June 2008 event. Shear stress and velocity values can be 

used to indicate areas of a stream where sediment, transported 

downstream, can be deposited on the streambed. Because of 

the short duration of the June 2008 rain event, streamflow 

was not directly measured but was estimated using U.S. Army 

Corps of Engineers one-dimensional Hydrologic Engineering Centers River Analysis System (HEC-RAS). Scenarios 

to examine possible engineering solutions to decrease the 

amount of sediment at the bridge, including bridge expansion, 

channel expansion, and dredging upstream from the bridge, 

were simulated using the FaSTMECH model. Each scenario 

was evaluated for potential effects on water-surface elevation, 

boundary shear stress, and velocity. 

Introduction

Wymans Run (local name) in Cochranton Borough, 

Crawford County, Pa., drains an area of approximately 

4 square miles (mi2) and discharges to French Creek approximately 600 ft upstream from the confluence of Little Sugar 

Creek and French Creek in Crawford County, Pa. (fig. 1). Sedimentation and flooding within and along Wymans Run have 

been a persistent problem. In an attempt to mitigate damage to 

property and infrastructure, the reaches immediately upstream 

from the confluence with French Creek were dredged until 

1979 when consistent dredging operations were discontinued. 

Since that time, dredging has been performed occasionally; the 

sediment problems have not abated. On July 21, 2003, a severe 

mesoscale convective weather system delivered more than 

4 inches (in.) of rain within 24 hours; the maximum intensity 

of 2.5 in. fell in 1.5 hours on the Borough of Cochranton, Pa., 

and surrounding areas. Along Steen Hill Road just above the 

junction with PA 285 (fig. 1), severe flooding caused damage to homes and property. Substantial sediment build-up in 

Wymans Run underneath the PA 285 bridge was noted after 

this storm, and residents were concerned that this build-up 

of sediment decreased the conveyance of the bridge causing 

backwater and likely making the flooding worse. 

Simulations of streamflow and backwater can lead to a 

better understanding of how structures, such as bridges, in 

waterways affect the natural streamflow characteristics and 

flood peaks. One aspect of the mission of the U.S. Geological 

Survey (USGS) is to provide reliable scientific information 

that describes natural systems and helps to minimize the loss 

of life and property that may be impacted by natural hazards. 

Studies in small watersheds, such as Wymans Run, follow the 

guidelines within our strategic science direction that the USGS 

works with its cooperative partners to ensure that simulation 

methods can be effectively applied in all situations to better 

understand the natural system. This study by the USGS, in 

cooperation with Crawford County Conservation District and 

Fairfield Township, Pennsylvania, examines the effects of 

possible engineering solutions to decrease the amount of sediment that is built up underneath the bridge in order to reduce 

upstream flooding.

Two models were constructed to represent hydraulic 

conditions in the stream using (1) the U.S. Army Corps of 

Engineers (USACE) Hydrologic Engineering Centers River 

Analysis System (HEC-RAS) and (2) a hydrodynamic model, 

Flow and Sediment Transport and Morphological Evolution of 

Channels (FaSTMECH), which is incorporated into the U.S. 

Geological Survey Multi-Dimensional Surface-Water Modeling System (MD_SWMS). High-water marks from a heavy 

precipitation event occurring on June 5, 2008, were used for 

model calibration. HEC-RAS was used to predict streamflow 

for the 2008 rainfall event and prepare cross sections of the 

stream for input to MD_SWMS. Predicted boundary shear 

stress and velocity from FaSTMECH were used to evaluate 

the potential sediment transport and deposition within the 

stream. 

Simulations of Flow and Prediction of Sediment Movement 

in Wymans Run, Cochranton Borough, Crawford County, 

Pennsylvania

By Elizabeth Hittle
2 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

0 0.25 0.50 1 MILE

0 375 750 1,500 METERS

EXPLANATION

Wymans Run watershed

Stream

Road

Study reach

Cochranton Borough boundary

Wymans Run study area

Crawford County

Pennsylvania

41o

32'

41o

31'

30"

41o

31'

41o

30'

30"

41o

30'

41o

29'

30"

80o06' 80o06'30" 80o05' 80o04'30" 80o04' 80o03'30" 80o03' 80o02'30" 80o02' 80o01'30"

French Creek

Little Sugar Creek

Wymans

Run

Steen

Hill Road

PA 285

US 322

PA 173

Figure 1. Location of Wymans Run watershed, Crawford County, Pa.
Introduction 3

Purpose and Scope

This report discusses the modeling efforts undertaken to 

describe the hydraulic characteristics of Wymans Run, the sediment transport that can occur, and the effects of engineering 

solutions proposed by others that were designed to minimize 

upstream flooding and reduce sedimentation at the bridge. 

The scenarios simulated are expansion of the PA 285 bridge, 

channel widening directly upstream and downstream from 

the bridge, and deepening of the channel upstream from the 

bridge. The main objectives of this report are to describe the 

simulated water-surface elevation, velocity distribution, and 

shear-stress distribution throughout the study reach for four 

channel geometry configurations and a proposed geometry of 

the PA 285 bridge. The modeling results show the potential 

effects that expanding the PA 285 bridge and dredging the 

channel would have on the boundary shear stress and velocity in the channel just upstream from the PA 285 bridge. The 

scope of this report includes definitions of the stream geometry and streambed-sediment characteristics, documentation 

for the construction of the models used for the study reach, 

and a presentation of the results of the models for actual and 

hypothetical channel configurations. 

Description of Study Area

Wymans Run is in the Northwestern Glaciated Plateau 

Section of the Appalachian Plateaus Physiographic Province 

(Sevon, 2000) approximately 0.5 mi west of Cochranton Borough in Fairfield Township, Crawford County, Pa. It drains an 

area of approximately 4 mi2 and discharges to French Creek, 

approximately 600 ft upstream from the confluence of Little 

Sugar Creek and French Creek (fig. 1). 

The headwaters of Wymans Run drain relatively flat open 

fields with poorly drained soils, predominantly of the Cambridge and Venango series (Yaworski and others, 1979). These 

soil series are characterized by low permeability that facilitates 

runoff and concentrates stormwater into various rills and channels. The lower part of the watershed is wooded and has some 

steep valleys that are prone to mass wasting through downslope movement of rock and soil overlying shale bedrock. 

Glacial drift, an unconsolidated erosion-prone mixture of clay, 

silt, sand, and gravel, overlays the bedrock in most places and 

is up to 75 ft thick. Topographic lows along the main channel 

and tributaries are underlain primarily by shale bedrock of the 

Shenango and Cuyahoga Groups, and the ridges are underlain 

by conglomeratic sandstone of the Pottsville Group (Geyer 

and Wilshusen, 1982). 

Channel Characterization

The study reach of Wymans Run extends from French 

Creek to approximately 2,000 ft upstream from the PA 285 

bridge. On the basis of channel-bed elevations surveyed at the 

PA 285 bridge and 600 ft upstream, the slope of the channel 

upstream from the bridge is about 0.014 ft/ft. Downstream 

from the bridge, on the basis of channel-bed elevations surveyed at the bridge and 600 ft downstream, the slope is about 

0.010 ft/ft. The width of the channel is highly variable, but on 

average bankfull width is approximately 28 ft on the downstream side of the bridge and 32 ft on the upstream side of the 

bridge. Throughout the entire channel, but within this reach 

specifically, the composition of the channel bed is extremely 

variable in sediment size, both laterally within a cross section and longitudinally (figs. 2A-C). In the upstream part of 

the watershed, many highly unstable banks (fig. 2D) provide 

nearly unlimited sediment that has the potential to be transported downstream. Depositional features such as point bars 

are visible throughout the channel. The banks of the channel are generally overgrown with vegetation, including thick 

bushes and small trees. In some parts of the stream, the banks 

have been artificially armored with large boulders for erosion 

prevention (fig. 2A).

Sediment sampling was conducted at selected cross 

sections on June 29, 2006 and July 5, 2006, to assist with the 

determination of channel roughness characteristics. Sedimentsize analysis, using 100 particle sampling methods, and 

channel surveys, consisting of a determination of bankfull 

channel elevation, were completed at selected cross sections 

(fig. 3). Note that the study reach is a highly dynamic system; 

the configuration and composition of the channel bottom have 

the potential to be altered during major rain events. Streamchannel conditions observed on June 26, 2006, and on June 

25, 2008, following a storm event that occurred in June 2008 

are shown in figure 4. The sediment surveys conducted in June 

and July 2006 can only indicate the conditions at that time and 

may not represent the stream conditions at a later date. It is 

expected that the size of the sediment will be similar because 

the source material will not have changed; however, the placement of the material may be different.

PA 285 Bridge

The PA 285 bridge is a steel beam bridge sitting atop 

reinforced concrete abutments. The bridge opening is 41 ft 

with a minimum designed underclearance of 3.6 ft. The roadway width is 34 ft. During substantial storms, sediment builds 

up on the upstream side of the bridge, preferentially on the 

left side looking downstream (fig. 4). During the June 2008 

storm, sediment built up so the minimum clearance was 3.4 ft 

compared to a clearance of about 7 ft in the same spot when 

surveys were done 2 years earlier. This bridge is slated to be 

replaced with a concrete spread box bridge with beams on 

integral abutments. The bridge span will be about 15 ft wider 

and 1.5 ft higher (Quality Engineering Solutions, Inc., written 

commun., November 2010), 
4 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

AC

BD

Figure 2. Channel and bank variation within the Wymans Run watershed, Crawford County, Pa. (A) approximately 200 feet upstream from the PA 285 bridge, looking downstream(Photo taken June 30, 2006), (

B) approximately 300 feet upstream from the PA 285 bridge, looking upstream (Photo taken June 30, 2006), (

C) approximately 400 feet upstream from 

the PA 285 bridge, looking upstream (Photo taken June 30, 2006), and (

D) upper watershed looking downstream (Photo taken January 4, 2006).
Introduction 5

Detailed 

view 

632

696 744

803

884

931

955

1011

1041

1084

1102

1175 1141

1261

1227 1212

1325

1341

1378

1428

1453

1475

1496

1523

1542

1586

1102

1141 1175

1195

1227 1212

1261

1302

1325

1341

1356

1378 1402

1428 1453

1475

1496

1523

1542

261 325 382

168

10 41

95

439

505

571

632

696 744

803

884

955

1141

1041

1629

1325

1678 1765

1428 1475

1939 1985

1542

2035

1227

1835

2071

2114

2385

2332

2453

2275

2755

2160

2201

2994

2923

2525

2560

2641

2702

2815

2869

EXPLANATION

Cross-section number is

river feet upstream from 

French Creek

Wymans Run stream

Sediment survey

884

Direction of flow in

Wymans Run

Figure 3. Locations of cross sections used for the Hydrologic Engineering Centers River Analysis System model and sediment surveys, 

including a detailed view in the vicinity of the bridge on Wymans Run, Crawford County, Pa.
6 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

A

B

Figure 4. Wymans Run, Crawford County, Pa., (A) upstream from the PA 285 bridge (Photo taken on January 23, 2006) and (B) upstream 

from the PA 285 bridge after the June 5, 2008, storm (Photo taken on June 25, 2008).
Model Selection and Construction 7

Model Selection and Construction

One model and one modeling system were used for this 

study, HEC-RAS and MD_SWMS, respectively. HEC-RAS, 

version 4.0.0 (U.S. Army Corps of Engineers, 2008) is a onedimensional hydraulic model that was used to estimate streamflow during the heavy June 5, 2008, rain event for which 

field-measured streamflow was not available. HEC-RAS has 

easy to use cross-section editing and interpolation routines that 

were used when designing the channel and bridge modification 

scenarios. HEC-RAS version 4.0.0 also includes a sedimentrouting routine; however, insufficient bedload data prohibited 

the application of that routine. MD_SWMS version 2.3.5b 

(McDonald and others, 2005) is a GUI interface modeling system which incorporates several models. One such model is the 

Flow and Sediment Transport and Morphological Evolution of 

Channels (FaSTMECH) (Nelson and others, 2003), a twodimensional hydraulic model that can be used to calculate the 

distribution of shear stress and velocity continuously across 

a channel. FaSTMECH was used in this study to evaluate the 

potential for sediment erosion and deposition in the stream 

channel, specifically in the area of the PA 285 bridge. Continuous horizontal distribution is important in Wymans Run 

because of the preferential sediment deposition seen on one 

side or the other in the channel. 

HEC-RAS Model Construction

HEC-RAS was designed to perform one-dimensional 

hydraulic calculations for a full network of natural and constructed channel cross-section profiles. The model contains 

river-analysis components for steady-flow water-surface 

elevation computations, unsteady flow simulation, and movable boundary sediment-transport computations. The unsteady 

flow and sediment portions of this software were not used 

because of insufficient field data. For this study, a HEC-RAS 

model was constructed to predict streamflow during a storm 

event in June 2008. Topographic data were collected from 

various sources to be used as input to the hydraulic models. 

Geographic Information System (GIS) coverages of the study 

area and field surveys conducted before the June storm were 

used to develop the model geometries. Field data, including 

particle-size sediment surveys, were used to estimate hydraulic 

characteristics such as Manning’s n. The steady-flow analysis 

routines were utilized with the boundary conditions set to 

normal depth, based upon the calculated slope of the channel 

upstream and downstream from the bridge. The cross-section 

editing and interpolation routines within HEC-RAS were 

used to modify the existing original cross sections to represent the hypothetical stream-geometry scenarios and potential 

bridge design suggested by the Pennsylvania Department of 

Transportation.

Channel Bathymetry 

Hydraulic models require accurate topographic data so 

that stream conditions can be accurately simulated. Light 

Detection And Ranging (LiDAR) data (www.pasda.psu.edu) 

collected in 2007 were used for the base topographic map for 

development of the model geometry. LiDAR data were supplemented with cross-sectional field surveys conducted within 

the channel upstream from the PA 285 bridge. Field surveys 

were conducted downstream from the bridge in July 2006 and 

upstream from the bridge in August and November 2007 using 

a Trimble Total Station and Sokkia laser level and translated 

to North American Vertical Datum of 1988 (NAVD88). These 

field-surveyed cross sections were particularly useful in the 

area of the bridge where large differences between the field 

survey data and LiDAR data were observed because the bridge 

created false ground elevations in the LiDAR data. The average difference between 321 field surveyed points and LiDAR 

derived points was 0.11 ft with a standard error of 0.11 ft. 

However, there were a number of points that differed by more 

than 2 ft. This occurred primarily along the right edge of water 

where the bank is steep and heavily vegetated. An example of 

a cross-section comparison of elevations from the LiDAR data 

and surveyed data 40 ft upstream from the bridge is shown in 

figure 5. 

The USACE HEC-GeoRAS tool for ArcGIS (version 

4.2.92 for ArcGIS 9.2; U.S. Army Corps of Engineers, 2009) 

was used to develop cross sections for use within HEC-RAS. 

HEC-GeoRAS is a set of procedures, tools, and utilities 

that allows for elevation data to be imported directly into 

HEC-RAS from ArcGIS. A digital terrain model of the area is 

required to provide elevations of themes such as cross-section 

cut lines, stream centerline, and main channel banks. If landuse coverage is imported into ArcGIS, Manning’s n values can 

be assigned to different land uses. Manning’s n values can then 

be assigned to different portions of cross-section cut lines. 

LiDAR data were downloaded as a 1-m raster Digital Elevation Model that was brought directly into ArcGIS to develop 

the topographic data. Relevant data, such as the left and right 

banks, land use (for overbank Manning’s n determination), and 

lines for cross sections, were digitized using HEC-GeoRAS 

extensions. Cross-section lines were drawn approximately 25 

to 50 ft apart from each other longitudinally throughout the 

stream reach and extending into the flood plain as shown in 

figure 3. The cross-section designation (for example, 1011) is 

based on distance (in feet) upstream on Wymans Run from the 

confluence of Wymans Run and French Creek along the designated centerline. The elevation and stationing data associated 

with these cross sections were then imported into HEC-RAS 

for model use. 

The cross sections created in HEC-GeoRAS from LiDAR 

data were modified in HEC-RAS based on field-surveyed 

cross sections directly upstream and downstream from the 

bridge. Field surveyed elevation and stationing were used 

at these locations because of the false ground elevations 

observed near the bridge in the LiDAR data. Field surveys 
8 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

also were used to confirm elevations of the top of the bank. 

The differences between the field-surveyed and LIDAR elevations were greater for the top of the bank points than for other 

points because the banks are steep and vegetated. The surveyed point, if available, was used for the left bank elevation 

in the graphs from the bridge up to cross section 1967. The 

average difference in left top of bank elevation was 0.58 ft 

with a standard error of 0.17 ft. Interpolation of cross sections was accomplished using the cross-section interpolation 

routines within HEC-RAS so that there was no more than 2 ft 

between cross sections. These interpolated cross sections were 

imported into MD_SWMS for model construction. 

Determination of Manning’s n Values 

Manning’s roughness coefficient represents the flow 

resistance in a channel. Factors that affect flow resistance are 

(1) size, gradation, and angularity of materials composing 

the streambed; (2) channel shape; (3) type of bed forms (for 

example, channel dunes, antidunes, and ripples); (4) presence 

of sand bars; (5) riparian vegetation; (6) obstructions (for 

example, bridges, causeways, and vegetation); (7) presence of 

suspended sediment and movement of bedload; and (8) degree 

of meandering. In general, resistance decreases as flow depth 

increases; resistance also decreases as the size of the bed material decreases (Arcement and Schneider, 1984). 

 Using data from the sediment surveys, Manning’s n 

values were calculated using the Rosgen method (Rosgen, 

1998). The Rosgen method describes the relation of channel 

bed-particle size to hydraulic resistance for river data from 

a variety of eastern and western streams. It uses the ratio of 

field collected hydraulic mean depth (d) to D84 values (size of 

material, in millimeters, for which 84 percent of the material 

is finer) as an initial field variable to calculate a resistance 

factor (mean velocity/shear velocity ratio) that is then related 

to Manning’s n. Derived values of Manning’s n are presented 

in table 1.

In an effort to use the HEC-RAS model to match watersurface elevation profile data, Manning’s n roughness coefficients were adjusted for selected reaches and streambanks. 

In the straight channel-controlled sections of the study reach, 

the size, gradation, and angularity of the materials composing 

the streambed may be more influential than other factors in 

determining flow resistance. On the basis of sediment surveys, 

three regions of Manning’s n values were field determined 

using equations from Rosgen (1998) and the field surveyed 

D84 values for bed-sediment material. The Manning’s n

value for the reach upstream from the bridge was calculated 

as 0.05, downstream was 0.044, and at the bridge was 0.03. 

Adjustments to Manning’s n were made to the average channel Manning’s n of 0.04 for the banks of the stream because 

of vegetation. A reasonable adjustment for vegetation ranges 

from 0.01 to 0.025, which is added to the base value (Arcement and Schneider, 1984) The left bank tended to be more 

vegetated, especially farther upstream from the bridge. The 

stream banks and flood plain were assigned a consistent value 

of 0.06 for the left bank and 0.05 for the right bank throughout 

the studied reach. 

June 2008 Storm Data

To aid in the modeling of Wymans Run, a pressure transducer data logger was installed on August 28, 2007, inside a 

pipe on the upstream wingwall of the PA 285 bridge to record 

water-surface elevations at determined intervals. Owing to the 

placement of the pipe and the configuration of the channel, 

the transducer was out of the water during low-flow conditions. Stage data were recorded every 5 minutes and retrieved 

monthly. A crest-stage gage (CSG) was installed directly 

downstream from the PA 285 bridge on the left wingwall to 

0 20 40 60 80 100 120 140 160 180 200

Distance, in feet

1,060

1,062

1,064

1,066

1,068

1,070

1,072

1,074

1,076

1,078

1,080

Elevation, in feet above NAVD 88

LiDAR derived cross section (before June 2008 storm)

EXPLANATION

Field-surveyed cross section (before June 2008 storm)

Figure 5. Elevations at cross section 1212, 40 feet upstream from the bridge on Wymans Run, Crawford County, Pa., before June 2008 

storm determined from field-survey and using LIDAR data.
Model Selection and Construction 9

obtain high water marks. On June 5, 2008, 0.98 in. of rain fell 

during a 45-minute period according to weather records at the 

Meadville airport (KGKJ) 11 mi away from the study area 

(Weather Underground, Inc., 2010). The resulting streamflow caused substantial changes throughout the entire channel (fig. 6). The precipitation amount is not from an official 

National Weather Service (NWS) station, but the daily total 

from Linesville (Linesville 1 S), Pa., located about 20 miles 

away was 1.40 inches of precipitation (http://www.ncdc.noaa.

gov/oa/climate/stationlocator.html). Recorded water-surface 

elevations from the transducer indicated the water-surface 

elevation increased about 2.5 ft in 30 minutes. The peak stage 

was obtained from a high-water mark inside the transducer 

instrument housing and was about 0.6 ft higher than the stage 

recorded by the transducer (fig. 7). The elevation data from 

the transducer are considered poor because the instrument was 

found flooded after the event, causing the pressure readings to 

be biased low, but the timing is considered reasonable.

Along with the measurement of high-water marks, cross 

sections located at the bridge and as much as 500 ft upstream 

from the bridge were surveyed after the storm. Elevations 

at selected cross sections were compared between the crosssectional data before and after the storm and are shown in 

figure 6. The cross sections surveyed after the storm were 

not used in the HEC-RAS model construction because the 

pre-storm cross sections are considered to be representative of 

the channel. After the storm, the channel had filled in significantly which most likely occurred during the stream recession. If significant scour had been observed upstream from the 

bridge, the post-storm cross sections would have been more 

representative.

Hypothetical Channel Geometries and Bridge 

Design

In June 2009, roadway engineers began design work to 

replace the existing PA 285 bridge. One of the modification 

scenarios includes raising the bridge deck and expanding 

the width of the bridge 10 ft on the left bank. This proposed 

bridge modification was simulated in HEC-RAS by increasing the width of the channel geometry for two cross sections 

representing the downstream and upstream sides of the bridge 

accordingly. The first cross section 10 ft upstream from the 

bridge also was modified for a more gentle transition from the 

proposed bridge opening to the existing stream. Raising the 

bridge deck in the simulation did not have any effect on the 

water-surface elevation as the modeled water surface never 

reached the bottom of the bridge decking. In the figures and 

in discussions, the modified bridge scenario is presented as 

“Expanded Bridge.” 

A second scenario considered was widening the channel 

upstream and downstream from the bridge to take advantage 

of the proposed expanded bridge width. The channel width 

was increased on the left bank about 15 to 20 ft from cross 

section 1029 through cross section 1370. The cross sections 

were modified so the channel was the same width as the bridge 

and then gradually transitioned back to the natural channel by 

cross section 1370. The modified cross-section bathymetry 

for select cross sections as depicted in HEC-RAS is shown in 

figure 6. The channel-bed elevation was not modified in this 

scenario. In figures and discussions, this scenario is presented 

as “Widened Channel.”

Historically, when sediment was deposited underneath the 

PA 285 bridge, contractors were hired to remove bed material 

in the channel underneath and upstream from the bridge. To 

Table 1. Results of sediment surveys and Manning’s n calculations at selected cross sections in the Wymans Run study reach, 

Crawford County, Pa.

[“Bridge” refers to the PA 285 bridge over Wymans Run. Dnn is the particle-size index where “nn” is a particle size in millimeters. For example, D15 represents the size of material, in millimeters, for which 15 percent of the material is finer. ft, feet; mm, millimeters]

414 ft

above bridge

(1,599 ft)

182 ft 

above bridge

(1,357 ft)

Upstream 

side of bridge

(1,175 ft)

Downstream

side of bridge

(1,141 ft)

143 ft

below bridge

(1,098 ft)

487 ft

below bridge

(654 ft)

Manning’s n 0.049 0.050 0.027 0.029 0.047 0.041

Bankfull mean depth (ft) 1.36 1.46 1

6.38 15.54 1.32 1.37

D15 (mm) 7.66 38.17 0.25 18.63 22.66 8.54

D35 (mm) 47.77 61.30 1.38 37.63 48.29 22.88

D50 (mm) 71.31 82.08 8.00 52.11 62.40 32.00

D84 (mm) 174.71 190.21 74.17 94.07 151.59 100.75

D95 (mm) 401.68 298.55 116.30 192.00 216.75 189.42

1

Values are based on bridge geometry rather than bankfull geometry.
10 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa. Figure 6. Elevations for Wymans Run study reach, Crawford County, Pa., used for modeling the initial channel, expanded bridge (cross sections 1,141 and 1,175 feet upstream from French Creek) or widened channel (cross sections 1,212, 1,261, 1,341, and 1,378 feet upstream from French Creek), and dredged channel scenarios. The cross sections 

surveyed after the June 2008 storm are also shown for cross sections (

A) 1,141, (

B) 1,175, (

C) 1,212, (

D) 1,261, (

E) 1,341, and (

F) 1,378 upstream from French Creek.

EXPLANATION

Dredge 200 cross sectionExpanded bridge cross sectionWidened channel cross sectionField surveyed cross section(After June 2008 storm)Field surveyed cross section(Before June 2008 storm)Lidar derived cross section(Before June 2008 storm)

Distance across channel from left to right looking downstream, in feet

50 70 90 110 130 150

F. Cross section number 1378

1,0601,0621,0641,0661,0681,0701,0721,074

60 80 100 120 140 160

E. Cross section number 1341

Distance across channel from left to right looking downstream, in feet

above NAVD 88 et e in f on, i vat e El

D. Cross section number 1261

1,0601,0621,0641,0661,0681,0701,0721,074

70 90 110 130 150 170 70 90 110 130 150 170

above NAVD 88 et e in f on, i vat e El1,060 1,062 1,064 1,066 1,068 1,070 1,072 1,074

C. Cross section number 1212

1,0601,0621,0641,066

1,068

1,0701,0721,074

70 90 110 130 150 170

B. Cross section number 1175

1,060

1,0621,0641,0661,0681,0701,0721,074

20 40 60 80 100 120

et above NAVD 88 e in f on, i vat e El

A. Cross section number 1141

1,0601,0621,0641,066

1,068

1,0701,0721,074
Model Selection and Construction 11

simulate this scenario, two additional models were constructed 

by modifying the channel geometry of cross sections from the 

bridge to (1) 50 ft upstream, and (2) 200 ft upstream. For all 

modified cross-section scenarios beginning at the bridge and 

continuing upstream 100 ft (cross sections 1140–1302), the 

elevation of the channel bottom was set to the bed elevation 

underneath the bridge. From 100 to 150 ft upstream from the 

bridge (cross sections 1324–1356), the elevation of the channel bottom was changed to 1 ft above the elevation under the 

bridge and 2 ft above for cross sections 150 to 200 ft upstream 

from the bridge (cross sections 1378–1402). The channel sides 

were not modified. Actual dredging would probably not create 

the uniform channel bottom depicted in the models. A comparison between the initial channel geometry and the hypothetical 

geometry is shown in figure 6, and a comparison of channelbed elevations is shown in figure 8. In the figures presented, 

these scenarios are represented as “Dredged.”

MD_SWMS Construction

Multi-dimensional hydrodynamic models provide a physically based method for simulating hydraulic characteristics 

in complex flow environments for a range of discharges. For 

this investigation, the MD_SWMS version 2.3.5b, (McDonald 

and others, 2005), which incorporates the Flow and Sediment Transport and Morphological Evolution of Channels 

(FaSTMECH) 2- and 2.5-dimensional flow models (Nelson 

and others, 2003), was used. MD_SWMS is a graphical user 

interface for applying the FaSTMECH model that allows for, 

among other things, editing and visualizing of model input 

data, examining model output, and both visual and statistical 

evaluation of observed and predicted parameters (McDonald 

and others, 2005). FaSTMECH is a finite-difference, steadystate surface-water computer modeling system that simulates 

vertically averaged two-dimensional streamflow velocities on 

1,063.5

1,064.0

1,064.5

1,065.0

1,065.5

1,066.0

1,066.5

Elevation, in feet above NAVD 88

12:00 12:30 13:00 13:30 14:00 14:30 15:00 15:30 16:00 16:30 17:00 17:30

June 5, 2008

Pressure transducer readings 

EXPLANATION

Transducer housing water mark 

Figure 7. Water-surface elevations recorded on June 5, 2008, by the installed transducer and high-water mark found inside the 

transducer housing, Wymans Run, Crawford County, Pa.

1,059

1,060

1,061

1,062

1,063

1,064

1,065

1,066

1,067

1,068

Elevation, in feet above NAVD 88

1,130 1,180 1,230 1,280 1,330 1,380 1,430 1,480 1,530 1,580 1,630

Distance from French Creek, in feet

Dredged 200 channel-bottom elevation

Dredged 50 channel-bottom elevation

Initial channel-bottom elevation

Extent of bridge (low steel elevation)

EXPLANATION

Figure 8. Elevations of the channel bed, Wymans Run, Crawford County, Pa., for the initial channel and the two dredged channel 

geometries as depicted in the Hydraulic Engineering Centers River Analysis System (HEC-RAS).
12 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

the basis of input data and boundary conditions. Input data 

include surface topography (stream and overbank crosssection profiles), streamflow, water-surface elevation at the 

downstream boundary, and surface-material roughness entered 

as non-dimensional drag coefficients. FaSTMECH outputs 

shear-stress values from the two-dimensional velocity solution 

(Kenney, 2005). Model output from MD_SWMS was exported 

as point coverages and imported into ArcMap. For display purposes, the point coverages were interpolated using the inverse 

distance weighting (IDW) ArcMap algorithms. This modeling 

system has been used in a number of alluvial environments to 

predict hydraulic and sediment transport characteristics (Kenney, 2005; Barton and others, 2005).

MD_SWMS Bathymetry 

The bathymetry data for use within MD_SWMS were 

imported from HEC-RAS. The bridge decking was not modeled within FaSTMECH because it cannot be supported within 

the computational grid. This was not a problem because the 

HEC-RAS simulations showed the decking was not a factor for streamflow simulations at the flows modeled because 

the water never reached the low steel elevation. The HECRAS cross sections representing the bridge were modified 

to simulate the bridge geometry. This was accomplished for 

all model geometries. MD_SWMS interpolates a continuous 

surface from the input topography with a triangulated irregular 

network (TIN) for the grid-defined model boundary.

MD_SWMS Computational Grid and Boundary 

Conditions

The computational grid used in MD_SWMS is a curvilinear orthogonal coordinate system with a user-defined 

centerline. The grid centerline is defined interactively to 

approximate the mean flow streamline of the modeling reach 

(Nelson and others, 2003). The model boundary is defined 

by the creation of a curvilinear grid in which the number of 

points along the stream length (streamwise) and the number 

of points across the stream (cross stream) are defined by the 

user. The computational grid used to model Wymans Run was 

approximately 1,312 ft in length along the centerline with 801 

streamwise nodes and 131 ft wide with 80 cross-stream nodes. 

The cell sizes vary with a curvilinear grid, but a 1.64 x 1.64 ft 

grid was created along the centerline. The model input water 

surface at the downstream boundary was determined using 

the HEC-RAS simulations for the same channel and discharge 

conditions. In order to represent lateral momentum exchange 

resulting from turbulence or other variability not generated 

at the bed, FaSTMECH incorporates a lateral eddy viscosity 

(LEV) (Nelson and others, 2003), which was set to 0.05 for all 

simulations. The following equation can be used to compute 

the initial lateral eddy viscosity:

LEV = .01 * avg(u) * avg(D) , (1)

where

LEV is the lateral eddy-viscosity coefficient, in 

meters per second; 

Avg u is the average channel velocity, in meters per 

second; and 

Avg D is the mean depth of flow, in meters. 

Model Calibration-Drag Coefficient

Model calibration consists of adjusting certain parameters 

in the model so the model output matches observations. In this 

study, the observations consisted of high-water marks for the 

June 2008 peak streamflow. The dimensionless drag coefficient, which represents the flow resistance in a channel, is one 

parameter that can be adjusted to calibrate the FaSTMECH 

model. The equation used by FaSTMECH for the determination of drag coefficient is

Drag coefficient = coefficient

* depth * grainsize (D50) , (2)

where 

coefficient is a constant adjustment multiplier; 

depth is the depth of water, in meters; and 

grainsize (D50) is the particle-size index,

where 

(D50) is a particle size in millimeters for which 

50 percent of the material is finer.

For equation 2, grainsize (D50) is expressed in meters.

Using estimated flow depths for the simulated June 2008 

peak streamflow (with a constant drag coefficient throughout 

the channel) and D50 determined from field-sediment surveys, 

the dimensionless drag coefficient was initially calculated 

within MD_SWMS. To assign sediment sizes for the initial 

drag coefficient determination, the cross sections where sediment surveys were conducted were assigned the previously 

determined D50 value. The entire section downstream from 

the bridge was assigned the D50 determined at cross sections 

1098 and 654. The bridge section was assigned the average 

of the D50 at the cross sections directly upstream and downstream from the bridge. The reach upstream from the bridge 

(cross sections 1175 to 1495) was assigned the D50 from cross 

sections 1357 and 1495 to 1629; the coefficient was assigned 

from cross section 1599. Determining the areas where 

sediment size values would be assigned was accomplished 

during the calibration process; an iterative process was used 

by assigning the sediment sizes to a range of cross sections, 

computing a water-surface elevation, comparing the surveyed 

high-water marks and the computed water-surface elevation, 

and then adjusting the range to better match the high-water 

marks. For cross sections 1261 to 1495, the D50 was increased 

0.018 m in order to increase the drag coefficient because the 

difference between the water-surface elevation and high-water 
Simulation Results 13

marks was large using the computed D50. The input of sediment sizes into the FaSTMECH model resulted in variable 

drag coefficients throughout the channel based upon depth and 

sediment size. 

 The initial drag coefficients were then adjusted by a constant factor so the water level was comparable to the observed 

high-water marks. A coefficient of 0.9 was used for all simulations. On the basis of this drag coefficient “map” (fig. 9A), 

generalized regions of drag coefficients were developed 

(fig. 9B-D) and used in the model runs. 

Simulation Results

HEC-RAS was used to estimate the peak streamflow for 

the June 2008 event that was simulated with FaSTMECH. 

Simulated water-surface elevations, velocity, and computed 

shear-stress values were obtained from FaSTMECH. Results 

are given for the initial channel geometry and the hypothetical 

bridge and channel geometry changes. Water-surface elevation 

results and boundary shear-stress distribution are given only 

for the highest streamflow simulated because these results are 

considered the most relevant. 

HEC-RAS Model Results 

HEC-RAS simulations were run using the steady-flow 

analysis routine with a normal depth and slope downstream 

boundary condition of 0.01 ft/ft. In order to estimate streamflow for the June 2008 event, Manning’s n values were 

estimated using sediment-survey data and entered into the 

HEC-RAS model. Streamflow was adjusted until the watersurface elevation reasonably matched the surveyed high-water 

marks (fig 10). Matching the simulated water-surface elevations to surveyed high-water marks upstream from the bridge 

produced an estimated streamflow for the June 2008 event of 

1,000 ft3/s. For cross sections 1378 to 1496, the initial watersurface elevation simulation did not reasonably match the 

surveyed high-water marks. Because of the modest curvature 

in this section, a reasonable increase in Manning’s n from the 

straight-channel-section values would be 30 percent (Chow, 

1959; Arcement and Schneider, 1984) and would result in a 

Manning’s n value of 0.065. Even with this adjustment, the 

water-surface elevation did not match the high-water marks 

observed at these cross sections; the average difference 

between observed and simulated water-surface elevations for 

this section is 0.93 ft. Although the HEC-RAS water-surface 

elevation generally matches the upstream marks, the downstream high-water mark on the CSG was more than 1 ft higher 

than that calculated by the model. This could be caused by 

turbulence downstream from the bridge, causing an artificially 

high mark in the CSG. 

FaSTMECH Model Results–Water-Surface 

Elevations

Simulated water-surface elevations for the entire Wymans 

Run study reach for the June 2008 storm are shown in 

figure 11 along with the elevation of the top of the left bank. 

In the vicinity of the study reach, the right bank is higher than 

the left and not subject to overflow. High-water marks and 

photographs taken during the June 2008 storm show water 

flowing out of the channel on the left bank about 1,418 to 

1,475 ft upstream from French Creek. The FaSTMECH model 

was calibrated to match these observations at a streamflow 

of 1,000 ft3/s. The simulated water-surface elevations for 

the hypothetical bridge and channel geometries are shown 

in figure 12. The water-surface elevation generated with the 

expanded bridge geometry shows the largest water-surface 

elevation difference to be directly upstream from the bridge, 

a decrease of 0.7 ft. The simulations of a deeper channel from 

the bridge to 200 ft upstream indicate a decrease in the watersurface elevation at 1,402 ft from French Creek (about 225 ft 

upstream from the bridge where the left bank was overtopped) 

of about 1.0 ft; farther upstream, the dredging rapidly loses 

effectiveness in lowering the water-surface elevation from the 

initial simulation. Simulations of the expanded channel configuration resulted in a lower the water-surface elevation farther 

upstream from the dredging, which could prevent the water 

from overtopping the left channel bank at these points during a 

storm of similar size. The simulated water-surface elevation at 

selected cross sections for the initial geometry and the watersurface elevation difference for the hypothetical geometries 

at a streamflow of 1,000 ft3/s are listed in table 2. Channel 

cross sections not shown in figure 12 were not affected by the 

hypothetical channel changes, and the simulated water-surface 

elevation was unchanged.

FaSTMECH Model Results–Velocity Distribution

Stream velocity can show the potential for sediment 

deposition. Two sediment-movement diagrams, the Hjulström 

diagram (Hjulström, 1939) and the Sundborg diagram (Sundborg, 1956) are generated using stream velocity and sediment size to graphically depict regions of sediment erosion, 

transport, and deposition. A variation of the original Hjulström 

diagram from Karrok (2009) is shown in figure 13.

The lines shown in figure 13 could be shown as broad 

regions of uncertainty because there are many variations of 

this diagram in the literature with broad ranges of deposition 

velocity (Hjulström, 1939; Sundborg, 1956; Karrok, 2009). In 

light of this uncertainty, the analysis of sediment movement 

that was completed in this study with shear stress does not 

use velocity as a factor. The conclusion that can be reached 

from the literature is that the velocity required to pick up bed 

material is more than that required to keep the material moving downstream. As might be expected, the largest particles 

will deposit first as the velocity in the stream decreases. The 
14 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

0 100 200 FEET

0 50 METERS

0 100 200 FEET

0 50 METERS

Detailed View Detailed View

High : 0.075

MD_SWMS

Drag coefficient

Low : 0.01

High : 0.075

MD_SWMS

Drag coefficient

Low : 0.01

High : 0.075

MD_SWMS

Drag coefficient

Low : 0.01

High : 0.075

MD_SWMS

Drag coefficient

Low : 0.01

0 100 FEET

0 20 METERS

0 100 FEET

0 20 METERS

A

C

B

D

0 100 FEET

0 20 METERS

0 100 FEET

0 20 METERS

Figure 9. Multi-Dimensional Surface-Water Modeling System (MD_SWMS) drag coefficients determined for the Wymans Run 

study reach, Crawford County, Pa., (A) using sediment sizes determined from field surveys and depth determined from a model run 

at a streamflow of 1,000 cubic feet per second, (B) actual drag coefficients used for the initial channel geometry, (C) close up of drag 

coefficients used in the widened channel geometry, and (D) close up of drag coefficients used in the dredged 50 channel geometry.
Simulation Results 15

Figure 10. Surveyed high-water marks after the June 2008 storm, and the Multi-Dimensional Surface-Water Modeling System

(MD_SWMS) simulated initial water-surface elevation and various elevations for a streamflow of 1,000 cubic feet per second, 

Wymans Run study reach, Crawford County, Pa.

Figure 11. Surveyed high-water marks after the June 2008 storm, the simulated Hydrologic Engineering Centers River Analysis System

(HEC-RAS) water-surface elevation for a streamflow of 1,000 cubic feet per second, and elevation of the left bank, Wymans Run study 

reach, Crawford County, Pa.

1,064

1,065

1,066

1,067

1,068

1,069

1,070

1,071

1,072

1,073

1,074

Elevation, in feet above NAVD 88

Distance from French Creek, in feet

1,130 1,180 1,230 1,280 1,330 1,380 1,430 1,480 1,530 1,580 1,630

High-water marks, left edge of water

High-water marks, right edge of water

High-water marks, downstream crest

stage gage

Top of left bank elevation

HEC-RAS water-surface elevation

Extent of bridge (low steel elevation)

EXPLANATION

1,050

1,060

1,070

1,080

1,090

1,100

1,110

Elevation, in feet above NAVD 88

0 500 1,000 1,500 2,000 2,500 3,000

Distance from French Creek, in feet

MD_SWMS initial water-surface elevation

Top of left bank elevation

Channel-bed elevation

Extent of bridge (low steel elevation) 

High-water mark, left edge of water

High-water mark, right edge of water

EXPLANATION
16 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

1,064

1,066

1,068

1,070

1,072

1,074

1,076

Elevation, in feet above NAVD 88

1,000 1,100 1,200 1,300 1,400 1,500 1,600 1,700

Distance from French Creek, in feet

MD_SWMS dredged 200 water surface

EXPLANTION

MD_SWMS widened channel water surface

MD_SWMS expanded bridge water surface

MD_SWMS initial water surface

MD_SWMS dredged 50 water surface

Extent of bridge

Top of left bank

Figure 12. Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH) model simulated initial water-surface 

elevation and the simulated water-surface elevations for hypothetical geometries for a streamflow of 1,000 cubic feet per second, 

Wymans Run study area, Crawford County, Pa.

Table 2. Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH) model simulated water-surface 

elevations for the initial channel geometry and elevation differences between the initial geometry and the hypothetical channel

geometries for a streamflow of 1,000 cubic feet per second at selected cross sections, Wymans Run study area, Crawford County, Pa.

[Elevations are referenced to the North American Vertical Datum of 1988. ft, feet]

Cross-section designation 

(distance above French 

Creek (ft))

Initial channel 

geometry

Elevation difference (ft)

Expanded 

bridge

Widened 

channel Dredged 50 Dredged 200

1,102 1,065.26 0.27 0.32 0.33 0.41

1,141 1,065.32 -0.10 -0.29 0.40 0.61

Bridge (1,155) 1,066.43 -0.57 -0.55 -0.65 -0.39

1,175 1,067.16 -0.58 -0.60 -1.34 -1.06

1,212 1,067.56 -0.35 -0.65 -1.70 -1.37

1,261 1,068.03 -0.14 -0.90 -0.38 -1.88

1,356 1,068.58 -0.05 -1.43 -0.17 -1.94

1,356 1,069.30 -0.03 -1.51 -0.08 -2.59

1,402 1,070.08 -0.02 -0.97 -0.04 -1.02

1,459 1,071.49 -0.01 -0.25 -0.01 -0.05

1,496 1,071.58 0.00 -0.15 0.00 -0.03

1,542 1,072.00 0.00 -0.06 0.00 0.00
Simulation Results 17

computed velocity distributions can show where the sediment 

may be deposited by looking at areas of different velocity 

magnitudes in the stream, not necessarily the absolute velocity. 

Areas of comparatively slower velocities in the stream have 

a higher potential for deposition than those in the areas with 

faster velocities. Velocities for a streamflow of 1,000 ft3/s for 

the initial and hypothetical stream geometries are described in 

the sections below. 

Velocity Distribution for Initial Channel Geometry

At a streamflow of 1,000 ft3/s, the main in-channel 

velocity is such that all variations of the Hjulström diagrams 

indicate the water is moving fast enough that sediment is being 

transported but is not being deposited during the time period 

when that flow (or higher) is occurring (fig. 14). Simulated 

flows show that sediment would not build up underneath the 

bridge during the peak flow, thus the buildup seen after a 

storm is not occurring until the recession (after the flooding 

potential). As a result, the sediment in the channel does not 

appear to be an additional factor in flooding. Focusing on the 

reach immediately upstream from the bridge, the velocity is 

lower, compared to that of the surrounding area. This could 

indicate areas where sediment will preferentially deposit, 

which are consistent with the sediment deposition observed 

after the June 2008 storm (fig 6). 

Velocity Distribution for Hypothetical Channel 

Geometries

A comparison of velocity differences between the initial 

geometry and the hypothetical geometries provides an indication of areas within the study reach where velocities change 

because of the channel modifications. These differences may 

indicate possible streambed changes, but the actual sedimentation patterns would not be evident. Velocity distributions 

for the hypothetical channel geometries at a streamflow of 

1,000 ft3/s are provided in figure 15. Results of simulations 

using a streamflow of 1,000 ft3/s at the expanded bridge 

(fig. 15A) and at the dredged channel 50 ft upstream from 

the bridge (fig. 15C) show that the velocity at the bridge is 

fast enough to transport most sediment sizes. Thus, sediment 

would not be deposited and affect conveyance during the 

flows greater than or equal to a streamflow of 1,000 ft3/s. A 

Grain size, in millimeters

0.001 0.01 0.1 1 10 100 1,000

Velocity, in centimeters per second

1,000

100

10

1

0.1

Erosion

Transport Deposition

Delineation of sediment transport

based upon velocity and grain size

EXPLANATION

Figure 13. Variation of the Hjulström diagram showing streamflow in relation to grain size (Modified from Karrok, 2009).
18 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

0 .0 0 - 0 .4 9

0 .5 0 - 0 .9 9

1 .0 0 - 1 .4 9

1 .5 0 - 1 .9 9

2 .0 0 - 2 .4 9

2 .5 0 - 2 .9 9

3 .0 0 - 3 .4 9

3 .5 0 - 3 .9 9

4 .0 0 - 4 .4 9

4 .5 0 - 4 .9 9

5 .0 0 - 5 .4 9

5 .5 0 - 5 .9 9

6 .0 0 - 6 .4 9

6 .5 0 - 6 .9 9

7 .0 0 - 7 .4 9

7 .5 0 - 7 .9 9

8 .0 0 - 8 .4 9

8 .5 0 - 8 .9 9

9 .0 0 - 9 .4 9

9 .5 0 - 9 .9 9

Greater than 10.00

Stream velocity,

in feet per second

0 100 200 300 FEET

0 20 40 60 80 METERS

Detailed view

Direction of flow in

Wymans Run

Figure 14. Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH) model simulated stream velocities for a 

streamflow of 1,000 ft3/s for the initial channel geometry, Wymans Run study reach, Crawford County, Pa.
Simulation Results 19

0.00-0.49

0.50-0.99

1.00-1.49

1.50-1.99

2.00-2.49

2.50-2.99

3.00-3.49

3.50-3.99

4.00-4.49

4.50-4.99

5.00-5.49

5.50-5.99

6.00-6.49

6.50-6.99

7.00-7.49

7.50-7.99

8.00-8.49

8.50-8.99

9.00-9.49

9.50-9.99

Greater than 10.00

Stream velocity,

in feet per second

0.00-0.49

0.50-0.99

1.00-1.49

1.50-1.99

2.00-2.49

2.50-2.99

3.00-3.49

3.50-3.99

4.00-4.49

4.50-4.99

5.00-5.49

5.50-5.99

6.00-6.49

6.50-6.99

7.00-7.49

7.50-7.99

8.00-8.49

8.50-8.99

9.00-9.49

9.50-9.99

Greater than 10.00

Stream velocity,

in feet per second

0.00-0.49

0.50-0.99

1.00-1.49

1.50-1.99

2.00-2.49

2.50-2.99

3.00-3.49

3.50-3.99

4.00-4.49

4.50-4.99

5.00-5.49

5.50-5.99

6.00-6.49

6.50-6.99

7.00-7.49

7.50-7.99

8.00-8.49

8.50-8.99

9.00-9.49

9.50-9.99

Greater than 10.00

Stream velocity,

in feet per second

0.00-0.49

0.50-0.99

1.00-1.49

1.50-1.99

2.00-2.49

2.50-2.99

3.00-3.49

3.50-3.99

4.00-4.49

4.50-4.99

5.00-5.49

5.50-5.99

6.00-6.49

6.50-6.99

7.00-7.49

7.50-7.99

8.00-8.49

8.50-8.99

9.00-9.49

9.50-9.99

Greater than 10.00

Stream velocity,

in feet per second

0 100 FEET

0 20 METERS

0 100 FEET

0 20 METERS

0 100 FEET

0 20 METERS

0 100 FEET

0 20 METERS

A

C

B

D

Figure 15. Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH) model simulated velocity for (A) the 

expanded bridge, (B) the widened channel, (C) the stream channel dredged 50 feet upstream, and (D) the stream channel dredged 

200 feet upstream for a streamflow of 1,000 cubic feet per second, Wymans Run study reach, Crawford County, Pa.
20 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

comparison of the results of simulations of the initial channel geometry with the results for the hypothetical expanded 

bridge geometry indicates that the expanded bridge geometry 

allows water to move at a higher velocity upstream from the 

bridge; this may lead to flushing of sediment downstream 

and under the bridge. The widened channel geometry generates much slower velocities in the widened area as a result 

of the increased channel area. The stream channel dredged 

50 ft upstream from the bridge results in faster velocities in 

the middle of the channel than those in the expanded bridge 

scenario upstream from the bridge. However, the velocity on 

the left side of the channel appears to be about 3 ft/s slower 

directly upstream from the bridge, implying sediment may still 

be deposited on that side. The scenario of the stream channel 

dredged 200 ft upstream from the bridge shows water slowing 

down in the dredged region, compared to the channel without 

dredging, indicating the potential for sediment deposition. 

This is consistent with the concept of a sediment trap that 

would most likely be formed by the dredged channel until the 

channel slope is returned to the slope of the upstream reach. 

These simulations do not produce an estimate of time scale 

for sediment deposition, only the potential for deposition. The 

velocity values for all scenarios at selected cross sections are 

listed in table 3.

FaSTMECH Model Results–Shear-Stress 

Distribution

Kenney (2005) describes a method of graphically representing shear-stress distribution in a stream in terms of the size 

of material that may be transported. Boundary shear stress can 

be used to predict when different sizes of bed material begin 

to move in the water column by assuming motion is initiated 

when the predicted boundary stress exceeds the critical shear 

stress for a particle class size. Sediment class, sediment size in 

metric and English units (Sturm 2001), Shields parameter, and 

the range of values for critical shear stress for the sediment 

class (Berenbrock and Tranmer, 2008) are listed in table 4. 

Channel boundary shear stress as predicted by FaSTMECH is calculated using the following equation

τ b = ρ Cd(u2 + v2) , (3)

where

τ b is the boundary shear stress, in pounds per 

square foot (lb/ft2);

ρ is the fluid density, in pounds per cubic foot 

(lb/ft3);

Cd is the nondimensional drag coefficient;

u is the vertically averaged x component of 

velocity, in ft/s; and

v is the vertically averaged y component of 

velocity, in ft/s.

Critical shear stress for each sediment class is calculated 

using the equation 

τbc = τ*bc((γs / γ) – 1)γd , (4)

where 

τbc is the dimensional critical boundary shear 

stress, in lbs/ft2;

τ*bc is the non-dimensional critical boundary 

shear stress;

γs is the specific weight of sediment, in lbs/ft3;

γ is the specific weight of water, in lbs/ft3; and

d is grain size, in feet.

Non-dimensional critical shear-stress values for the 

grain sizes were obtained from the Shields diagram (see 

Sturm (2001) for a comprehensive explanation of the Shields 

diagram) after first computing non-dimensional sediment grain 

sizes using the equation

d* =

γ

γs −1 gd^3

v^2

 , (5)

where

d* is the nondimensional grain size;

γs is the specific weight of sediment, in pounds 

per cubic foot;

γ is the specific weight of water, in pounds per 

cubic foot;

g is the acceleration of gravity, in feet per 

second squared;

d is grain size, in feet; and

v is the kinematic viscosity of water, in square 

feet per second.

To aid in the evaluation and presentation of predicted 

boundary shear-stress values, Kenney’s plots were constructed 

with colors developed from empirical critical shear-stress 

values of specific grain sizes. Kenney (2005) presents only 

critical shear-stress values up to the size of very coarse gravel 

(32–64 mm or 1.3–2.5 in.). In a similar study, Berenbrock and 

Tranmer (2008) give critical shear-stress values up to the large 

cobble size (128–256 mm or 5–10 in.). The sediment grade 

scale (Sturm, 2001) is presented in table 4 for reference. It is 

important to emphasize that this analysis determines whether 

or not a given grain size is likely to become mobile but does 

not provide information regarding scour, deposition, or the 

volume of material that might be transported. 

Shear-Stress Distribution for Existing Channel 

Geometry

The predicted boundary shear-stress values for the 

June 2008 peak streamflow of 1,000 ft3/s were assigned a 

color on the basis of the maximum grain size available to be 
Simulation Results 21

Table 3. Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH) model maximum simulated velocity for 

the initial and hypothetical channel geometries for a streamflow of 1,000 cubic feet per second at selected cross sections, Wymans 

Run, Crawford County, Pa.

[Velocity is in feet per second. ft, feet]

Cross-section designation

(distance above French

Creek (ft))

Maximum simulated velocity

Initial channel 

geometry

Expanded 

bridge

Widened 

channel Dredged 50 Dredged 200

1,102 12.4 9.8 8.8 8.1 6.8

1,141 11.4 10.4 9.8 8.2 6.7

Bridge (1,155) 9.0 9.1 8.3 8.4 6.5

1,175 8.0 8.9 7.0 8.7 6.7

1,212 7.3 8.2 5.8 9.3 7.1

1,261 7.9 8.6 6.0 11.8 8.2

1,356 8.4 8.7 6.7 9.2 9.8

1,356 8.4 8.5 10.4 8.6 10.3

1,402 7.5 7.5 10.3 7.6 12.4

1,459 8.1 8.1 10.4 8.3 9.5

1,496 8.6 8.6 9.5 8.6 8.8

1,542 9.6 9.7 9.8 9.7 9.7

Table 4. Sediment grade scale, Shields parameter, and critical shear-stress ranges for each sediment class.

[Grade scale from Strum (2001) Shields parameter and critical shield stress ranges from Berenbrock and Tranmer (2008). lbs/ft2, pounds per feet squared]

Sediment

material

Grain diameter

(millimeters)

Grain diameter

(inches)

Shields

parameter 

(dimensionless)

Range of critical 

shear stress 

(lbs/ft2)

Very fine sand 0.0622 – 0.125 0.0025 – 0.0049 0.109 – 0.072 0.0029 – 0 .0038

Fine sand 0.125 – 0.250 0.0049 – 0.0098 0.072 – 0.048 0.0038 – 0.0042

Medium sand 0.25 – 0.50 0.0098 – 0.0197 0.048 – 0.033 0.0042 – 0.0054

Coarse sand 0.5 – 1.0 0 .020 – 0.039 0.033 – 0.029 0.0118 – 0.0304

Very coarse sand 1 – 2 0.039 – 0.079 0.029 – 0.039 0.0304 – 0.0608

Very fine gravel 2 – 4 0.079 – 0.157 0.039 – 0.042 0.0608 – 0.1216

Fine gravel 4 – 8 0.16 – 0.31 0.042 – 0.044 0.0608 – 0.1216

Medium gravel 8 – 16 0.31 – 0.63 0.044 – 0.047 0.1216 – 0.2432

Coarse gravel 16 – 32 0.63 – 1.26 0.047 – 0.05 0.2432 – 0.4865

Very coarse gravel 32 – 64 1.3 – 2.5 0.05 – 0.052 0.4865 – 0.9729

Small cobbles 64 – 128 2.5 – 5.0 0.054 – 0.052 0.9729 – 1.9458

Large cobbles 128 – 256 5.0 – 10.1 0.054 – 0.054 1.9458 – 3.8916
22 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

transported, based on the non-dimensional computed critical 

shear-stress values (fig. 16). At a streamflow of 1,000 ft3/s, the 

boundary shear stress in the stream is such that large cobbles 

(grain size of 5–10 inches) can be moved at most locations 

in the channel. One noticeable exception is directly upstream 

and downstream from the PA 285 bridge where the boundary 

shear stress is less than in the reaches farther upstream and 

downstream from the bridge. Velocity plots for the simulation 

(fig. 14 and table 3) also show that the area upstream from 

the bridge has slower average velocities than the surrounding areas. This confirms the observation that larger sediment 

deposited upstream from the bridge from prior storms will not 

necessarily wash out.

Shear-Stress Distribution for Hypothetical 

Channel Geometries

Shear-stress distributions of sand, gravel, and cobbles 

for the hypothetical channel geometries for a streamflow of 

1,000 ft3/s are shown in figure 17. For the expanded bridge 

geometry (fig. 17A), a noticeable difference is an increase 

in boundary shear stress directly upstream from the bridge, 

indicating an increased capability to move sediment, possibly 

under the bridge. Widening the channel upstream and downstream from the bridge (fig. 17B) causes a decrease in boundary shear stress (compared to the initial channel) throughout 

much of the widened reach. At a streamflow of 1,000 ft3/s, 

the dredged channel 50 and 200 ft upstream from the bridge 

shows a decrease in boundary shear stress in the dredged area 

of the channel (fig. 17C, D). Larger sediment in these areas 

would not become mobile.

Model Limitations 

Specific limitations of the constructed model restrict the 

extent of interpretations and results presented in this report. 

Valid interpretation of the results presented in this report 

requires a clear understanding of these limitations. The models 

used in this analysis should be considered not fully calibrated 

because field data, such as streamflow during the June 2008 

storm and high-water marks downstream from the PA 285 

bridge, were not available to verify the calibration. The most 

critical factor is the extreme variation in channel geometry 

from before the June 2008 rain event to after the event. The 

changes in the cross-sectional area at the upstream side of 

the bridge before and after the June 2008 storm are shown in 

figure 6A. The models are based on the surveyed geometry 

of the channel before the storm; however, there is no assurance that the geometry used represents the geometry at the 

time of the storm. The storm occurred over a few hours, and 

a streamflow measurement was not made. Streamflow was 

determined indirectly using surveyed cross sections, measured 

water-surface elevations, and selected Manning’s n values. 

For the scenarios that are described, the proposed bridge 

expansion, widened channel, and dredged channel geometries 

are hypothetical. At the time of this writing (2010), the plans 

for the PA 285 bridge have not been finalized and may not 

match the dimensions of the hypothetical bridge geometry that 

was used in this analysis. Many variables exist in the hypothetical dredged channel geometries such as how far upstream 

the dredging may go and how far across the channel will be 

dredged. Most importantly, the depth of the dredging will 

greatly affect both stream velocity and elevation in the stream 

channel. 

The HEC-RAS model was run as a steady-state simulation although the extremely fast rise and fall of the stream 

would indicate the flow was unsteady. A temporary gage was 

installed at the site; however, the data were considered unreliable because the instrument was over-topped during the rain 

event, and the gage data were not used.

The bathymetry used for MD-SWMS is a combination of 

surveyed field data and LiDAR data that were processed for 

HEC-RAS using HEC-GeoRAS routines available in ArcGIS. 

Once the bathymetry was in HEC-RAS, interpolation routines 

were used, and the interpolated data were imported into MDSWMS. The use of LiDAR data and interpolated bathymetry 

could decrease the accuracy of the stream-channel elevations 

and affect the resulting water-surface elevations, shear stress, 

and velocity data. 

Summary and Conclusions

A hydraulic model, U.S. Army Corps of Engineers onedimensional Hydrologic Engineering Centers River Analysis 

System (HEC-RAS) and a hydrodynamic model, Flow and 

Sediment Transport and Morphological Evolution of Channels 

(FaSTMECH), which is incorporated into the U.S. Geological 

Survey Multi-Dimensional Surface-Water Modeling System 

(MD_SWMS), were constructed and calibrated for a selected 

reach of Wymans Run in Cochranton Borough, Crawford 

County, Pa. Inputs for these models include field surveyed 

cross sections, sediment surveys, a digital elevation model 

developed from LiDAR data, and high-water cross-section 

data collected following a storm in June 2008. 

After the models were calibrated using the channel 

conditions observed in 2007, scenarios were generated with 

hypothetical channel geometries, including an expansion of 

the existing PA 285 bridge, expansion of the channel upstream 

and downstream from the bridge, and dredging the channel 50 

and 200 ft upstream from the bridge. Frequent changes in the 

channel geometry created by storms made it difficult to establish a normal channel condition. The models produced values 

for boundary shear stress and velocity, which are indicators of 

sediment transport at the maximum streamflow estimated for 

a June 2008 rain event. The values of boundary shear stress 

produced by FaSTMECH for the simulated streamflow of 

1,000 ft3/s indicate that most sediment in the channel can be 

moved. The velocity during these conditions shows sediment 

is moving at the simulated flow of 1,000 ft3/s, thus the issue 
Summary and Conclusions 23

Figure 16. Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH) model predicted largest mobile 

sediment size based upon critical boundary shear stress values for each sediment class at a streamflow of 1,000 cubic feet per second 

for the initial channel geometry, Wymans Run study reach, Crawford County, Pa. 

0 100 200 300 FEET

0 20 40 60 80 METERS

PREDICTED LARGEST

MOBILE

SEDIMENT SIZE

Smaller than medium sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

Coarse gravel

Very coarse gravel

Small cobbles

Large cobbles

Larger than large cobbles

Detailed view
24 Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.

PREDICTED LARGEST

MOBILE

SEDIMENT SIZE

Smaller than medium sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

Coarse gravel

Very coarse gravel

Small cobbles

Large cobbles

Larger than large cobbles

PREDICTED LARGEST

MOBILE

SEDIMENT SIZE

Smaller than medium sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

Coarse gravel

Very coarse gravel

Small cobbles

Large cobbles

Larger than large cobbles

PREDICTED LARGEST

MOBILE

SEDIMENT SIZE

Smaller than medium sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

Coarse gravel

Very coarse gravel

Small cobbles

Large cobbles

Larger than large cobbles

PREDICTED LARGEST

MOBILE

SEDIMENT SIZE

Smaller than medium sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

Coarse gravel

Very coarse gravel

Small cobbles

Large cobbles

Larger than large cobbles

PREDICTED LARGEST

MOBILE

SEDIMENT SIZE

Smaller than medium sand

Medium sand

Coarse sand

Very coarse sand

Very fine gravel

Fine gravel

Medium gravel

Coarse gravel

Very coarse gravel

Small cobbles

Large cobbles

Larger than large cobbles

0 100 FEET

0 20 METERS

0 100 FEET

0 20 METERS

0 100 FEET

0 20 METERS

0 100 FEET

0 20 METERS

A

C

B

D

Figure 17. Flow and Sediment Transport and Morphological Evolution of Channels (FaSTMECH) model largest mobile sediment size 

based upon critical boundary shear stress values for each sediment class at a streamflow of 1,000 cubic feet per second for (A) the 

expanded bridge, (B) the widened channel, (C) the stream channel dredged 50 ft upstream from the bridge, and (D) the stream channel 

dredged 200 ft upstream from the bridge, Wymans Run study reach, Crawford County, Pa.
References Cited 25

of sediment filling the bridge opening is not a concern during 

peak flows of major events. A proposed bridge redesign could 

allow for higher velocities directly upstream from the bridge, 

perhaps decreasing the amount of sediment deposited directly 

upstream from the bridge opening. Results of the simulation 

for the hypothetical dredged channel show that less sediment 

was deposited at the bridge and as much as 200 ft upstream for 

this scenario than for the other scenarios, indicating that sediment will likely continue to be deposited in these manmade 

sediment traps until the stream reaches its equilibrium slope. 

The most effective hypothetical channel geometry change for 

reducing flooding and over-topping of the left bank upstream 

from the bridge was the widening of the channel about 15 to 

20 ft on the left bank upstream from the bridge to match the 

hypothetical bridge expansion. However, the simulations show 

velocities will be reduced in the widened areas and sediment 

has a high potential of depositing. 

Acknowledgments

The author would like to thank the residents who live 

near Wymans Run and Bob Rose for the pictures, stories, and 

history of the area. The author appreciates the work of Jeff 

Chaplin who provided photographs and other data for the 

study reach. The author is grateful to Charles Berenbrock for 

his technical assistance with MD_SWMS. Terry Kenney and 

Mark Roland of the USGS provided technical reviews of the 

manuscript.

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For additional information, write to:

Director

U.S. Geological Survey

Pennsylvania Water Science Center

215 Limekiln Road

New Cumberland, PA 17070

or visit our Web site at:

http://pa.water.usgs.gov/
Hittle—Simulations of Flow and Prediction of Sediment Movement in Wymans Run, Cochranton Borough, Crawford County, Pa.—Open-File Report 2011–1223

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