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Publication Number:  FHWA-HRT-15-033    Date:  May 2015
Publication Number: FHWA-HRT-15-033
Date: May 2015


Scour in Cohesive Soils

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Scour in cohesive soils has been a challenge for engineers and designers. Unlike noncohesive soils, practical measurement techniques and well accepted guidance on the scourability of cohesive soils are severely lacking. This report summarizes a study through which an erosion testing device that simulates open channel flow on a small scale was developed and tested. In addition, a recommended design approach is provided that can be used for estimating scour for a range of cohesive soils. The study described in this report was conducted at the Federal Highway Administration Turner-Fairbank Highway Research Center J. Sterling Jones Hydraulics Laboratory.

Jorge E. Pagán-Ortiz
Director, Office of Infrastructure
Research and Development


This document is disseminated under the sponsorship of the U.S. Department of Transportation in the interest of information exchange. The U.S. Government assumes no liability for the use of the information contained in this document. This report does not constitute a standard, specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or manufacturers’ names appear in this report only because they are considered essential to the objective of the document.

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1. Report No.


2. Government Accession No. 3 Recipient's Catalog No.
4. Title and Subtitle

Scour in Cohesive Soils

5. Report Date

May 2015

6. Performing Organization Code
7. Author(s)

Haoyin Shan, Jerry Shen, Roger Kilgore, and Kornel Kerenyi

8. Performing Organization Report No.


9. Performing Organization Name and Address

2 Eaton Street, Suite 603
Hampton, VA 23669

10. Work Unit No. (TRAIS)

11. Contract or Grant No.
12. Sponsoring Agency Name and Address

Office of Infrastructure Research and Development
Federal Highway Administration
6300 Georgetown Pike
McLean, VA 22101-2296

13. Type of Report and Period Covered

Laboratory Report
Feb. 2008–April 2014

14. Sponsoring Agency Code


15. Supplementary Notes

The Contracting Officer’s Technical Representative (COTR) was Kornel Kerenyi (HRDI-50).

16. Abstract

This study of scour in cohesive soils had two objectives. The first was to introduce and demonstrate a new ex situ erosion testing device (ESTD) that can mimic the near-bed flow of open channels to erode cohesive soils within a specified range of shear stresses. The ESTD employs a moving belt and a pump to generate a log-law velocity profile in a small test channel to simulate open channel flow. Successful testing requires careful preparation of soil specimens to avoid slaking. Preparation of erosion test samples by compaction usually leads to soil slaking, which cannot be tolerated to generate meaningful erosion function data. Therefore, cohesive soil specimens with different percentages of clay, silt, and non-uniform sands were mixed and de-aired in a pugger mixer to prevent slaking. The testing confirmed that the ESTD is capable of determining erosion characteristics of cohesive soils for bed shear stresses within the range of 0.063 to 0.31 lbf/ft2 (3 to 15 Pa). Its capability of directly measuring bed shear stresses enhances the understanding of the erosion process in cohesive soils.


The second objective was to develop a method for estimating the critical shear stress and erosion rates for a limited range of cohesive soils in the context of the Hydraulic Engineering Circular 18 scour framework. The method is based on more easily obtained soil parameters so that direct erosion testing is not needed in all cases. General relations are proposed for both best-fit and design applications. Estimates of critical shear stress are based on the water content, fraction of fines, plasticity index (PI), and unconfined compressive strength. In addition, an equation for estimating erosion rates when bed shear stress exceeds critical shear stress is proposed. For application, the designer must determine the critical shear stress of the soil (from the previous relation), the unconfined compressive strength, and the PI. The guidance may be used for engineering design within limits based on the range of values in the current data set and to a lesser extent the range from Illinois field data on which parts of the methodology were validated. A Texas data set on which additional validation was attempted represents a distinct data set. The recommendations apply to fine grained cohesive soils within a range of plasticity and liquid limit (LL) characteristics. The PI should be within the range of 4 to 25 percent and the LL between 15 and 50 percent. The fraction of fines should fall between 10 and 90 percent. These methods best apply to soils with at least 90 percent saturation but can be used with lower degrees of saturation.

17. Key Words

Cohesive soils, erosion testing, slaking, direct force gauge, log-law velocity profile, bed shear stress, critical shear stress, erosion rate

18. Distribution Statement

No restrictions. This document is available to the public through the National Technical Information Service, Springfield, VA 22161.

19. Security Classification
(of this report)


20. Security Classification
(of this page)


21. No. of Pages


22. Price
Form DOT F 1700.7 Reproduction of completed page authorized

SI* (Modern Metric) Conversion Factors



Figure 1. Graph. Generalized relationships for erosion in cohesive materials
Figure 2. Graph. Division of materials into zones for specific devices
Figure 3. Equation. Ultimate contraction scour
Figure 4. Equation. Time rate of contraction scour
Figure 5. Equation. Ultimate pier scour
Figure 6. Equation. Critical velocity and critical shear stress
Figure 7. Equation. Linear erosion law of cohesive soils
Figure 8. Equation. Exponential erosion law of cohesive soils
Figure 9. Equation. Wan and Fell equation for cohesive soil erosion
Figure 10. Equation. Instantaneous shear stress equation for a jet
Figure 11. Photo. The ESTD
Figure 12. Diagram. Schematic of the ESTD
Figure 13. Diagram. Dimensions of the moving belt
Figure 14. Diagram. Principles of force measurements in the horizontal direction
Figure 15. Diagram. Principles of force measurements in the vertical direction
Figure 16. Photo. The PIV system
Figure 17. Photo. The PIV illuminated laser
Figure 18. Photo. The PIV measurement plane
Figure 19. Graph. Velocity profiles of conduit flow with P220 sandpaper bed
Figure 20. Graph. Velocity profiles for belt-only tests with a P100 sandpaper bed
Figure 21. Graph. Combination velocity profiles at 0.58 gal/s (2.2 L/s) with a P80 sandpaper bed
Figure 22. Graph. Shear stress measurements of conduit flow on each sandpaper bed
Figure 23. Equation. Bed-specific relation for conduit flow bed shear stress in the ESTD
Figure 24. Graph. Shear stress measurements of conduit flow
Figure 25. Equation. Equation of bed shear stress for conduit flow in the ESTD
Figure 26. Graph. Shear stresses for combination tests on a P150 sandpaper bed
Figure 27. Equation. Dimensionless roughness height
Figure 28. Equation. The law of the wall
Figure 29. Equation. Friction coefficient
Figure 30. Graph. Comparison of conduit flow point velocities on a P220 sandpaper bed
Figure 31. Graph. Conditions used for erosion testing in the ESTD
Figure 32. Photo. The pugger mixer used for soil preparation
Figure 33. Graph. Particle-size distributions for Red Art clay, silt, and non-uniform sands
Figure 34. Graph. Particle-size distributions of the cohesive soils tested in the ESTD
Figure 35. Graph. Erosion curves of soils from a New Orleans levee
Figure 36. Graph. Gradation of compacted clays
Figure 37. Photo. Slaking test on a compacted soil specimen
Figure 38. Graph. Slaking test results for soils 2 and 3 in still water
Figure 39. Graph. Example data recorded for sample with soil index 4
Figure 40. Photo. Erosion soil sample 1W183 with increasing shear
Figure 41. Graph. Representative plots of erosion rate versus shear stress
Figure 42. Equation. Power relationship between shear stress and erosion rate
Figure 43. Graph. Measured and fitted power model for soil index 1
Figure 44. Equation. Linear relationship between shear stress and erosion rate
Figure 45. Graph. Estimated critical shear stress comparison
Figure 46. Graph. Comparison of FHWA, Illinois, and Texas soil data
Figure 47. Equation. USDA equation for permissible shear stress
Figure 48. Graph. Comparison of permissible and critical shear stress
Figure 49. Equation. Briaud equation for critical shear stress lower bound
Figure 50. Equation. Briaud equation for critical shear stress upper bound
Figure 51. Graph. Comparison of critical shear stress with Briaud bounds
Figure 52. Equation. General power model
Figure 53. Graph. Critical shear versus unconfined compressive strength
Figure 54. Equation. Predictive relation for critical shear stress
Figure 55. Graph. Comparison of predicted versus estimated critical shear stress
Figure 56. Equation. Water content at 100-percent saturation
Figure 57. Graph. Critical shear stress comparison with FHWA, Illinois, and Texas data
Figure 58. Equation. Predictive relation for C1
Figure 59. Graph. Predictive relation for the multiplier coefficient
Figure 60. Equation. Erosion rate model
Figure 61. Graph. Predicted versus measured erosion rates
Figure 62. Graph. Predicted versus measured erosion rates with Illinois data
Figure 63. Equation. Design equation for critical shear stress
Figure 64. Graph. Design equation for critical shear stress compared with data
Figure 65. Equation. Design equation for C1
Figure 66. Graph. Design equation for erosion rate compared with data
Figure 67. Graph. Relationship between PI and clay percentage
Figure 68. Equation. Linear relationship between PI and clay percentage


Table 1. Bed materials
Table 2. Time interval validation tests
Table 3. ESTD shear stress parameters for rectangular conduit tests
Table 4. Consecutive day validation shear tests
Table 5. Relationship between f, r, ks, and ks+
Table 6. Particle size and modeled roughness height
Table 7. Equations for the constant B
Table 8. Test conditions for the ESTD.
Table 9. Classification and composition of soils for testing
Table 10. Mass and moisture properties of soils prepared by pugger mixer
Table 11. Mass and moisture properties of soils prepared by compaction
Table 12. Additional geotechnical properties of soils prepared by pugger mixer
Table 13. Additional geotechnical properties of soils prepared by compaction
Table 14. Properties of four soils from a New Orleans levee
Table 15. Erosion test matrix and results for soils prepared by pugger mixer
Table 16. Estimated critical shear stress parameters for the power model
Table 17. Estimated critical shear stress parameters for the linear model
Table 18. Data set parameter summary


ASSETAdjustable shear stress erosion and transport flume
CCFEDCircular Couette flow erosion device
EFAErosion function apparatus
ESTDEx situ scour test device
HEC-18Hydraulic Engineering Circular 18
HETHole erosion test
JETJet erosion test
LLLiquid limit
PIPlasticity index
PLPlastic limit
PIVParticle image velocimetry
ReReynolds number
SEDflumeSediment erosion at depth flume
SERFSediment erosion rate flume
USCSUnified Soil Classification System
USDAUnited States Department of Agriculture


αPier diameter (or width), ft (m)
cCohesion, lbf/ft2 (N/m2 or Pa)
D50Median grain size, inches (mm)
eVoid ratio, dimensionless
FFraction of soil passing #200 sieve (0.075 mm)
gAcceleration due to gravity, 32.2 ft/s2 (9.81 m/s2)
hNozzle height above the soil surface, ft (m)
JpPotential jet core length, ft (m)
JiInstantaneous jet orifice height, ft (m)
kSlope of the erosion curve, dimensionless
K1Correction factor for pier nose shape, dimensionless
K2Correction factor for angle of attack of flow, dimensionless
kfEmpirical floc erosion rate, lb/ft2/s (g/m2/s)
kLEmpirical erosion constant, lb/ft2/s (g/m2/s)
ksRoughness height, inches (µm)
nManning's roughness coefficient, dimensionless
QFlow rate in the ESTD test channel, gal/s (L/s)
quUnconfined compression strength, lbf/ft2 (N/m2 or Pa)
SDegree of saturation, dimensionless
SgSediment specific gravity, dimensionless
tDuration of flow or test, h
uVelocity at a depth y, ft/s (m/s)
u*Shear velocity, ft/s (m/s)
umaxMaximum flow velocity at the boundary layer thickness, ft/s (m/s)
V1Mean velocity of flow directly upstream of the pier, ft/s (m/s)
V2Average flow velocity in the contracted section, ft/s (m/s)
VcCritical velocity, ft/s (m/s)
wWater content, dimensionless
y1Upstream average flow depth, ft (m)
ys,uUltimate scour depth, ft (m)
ys(t)Scour depth at time, t, ft (m)
zDepth from soil surface, inches (mm)
Erosion rate, inches/h (mm/h)Erosion rate, inches/h (mm/h)
Erosion rate, inches/h (mm/h)iInitial rate of scour, ft/h (m/h)
Erosion rate, inches/h (mm/h)MMass erosion rate, lb/ft2/s (g/m2/s)
αUnit conversion constant, value and dimensions are equation-specific
δBoundary layer thickness, ft (m)
κVon Karman constant
vKinematic viscosity of water, ft2/s (m2/s)
ρDensity, lb/ft3 (kg/m3)
𝜏Hydraulic shear stress, lbf/ft2 (N/m2 or Pa)
𝜏cCritical shear stress for the initiation of erosion, lbf/ft2 (N/m2 or Pa)
𝜏c(z)Critical shear stress at a depth of z, lbf/ft2 (Pa)
𝜏iInstantaneous peak boundary shear stress, lbf/ft2 (N/m2 or Pa)
𝜏pSoil permissible shear stress, lbf/ft2 (Pa)
φFriction angle, degree (radian)


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