1. Interaction between Riprap Layout and Contraction Scour from Vertical-Wall Bridge Abutments Supported by Shallow Foundations
presented by Huang, C., Xie, Z., Suaznabar, O.
Shen, J. and Kerenyi, K.
at the
NHEC Friday, August 12th, 2016 Portland, OR

2. Outline Needed additional guidance—HEC-23 Riprap Countermeasure
Better address the common stream bank conditions
Single-span bridges built close to the main channel
Development of proper Layout for Riprap Countermeasure
Experimental study
CFD study
Preliminary Conclusions

3. HEC-23 Riprap Installation
Top of Shallow Foundation at the same Elevation as Contraction Scour + Long Term Degradation 3

4. HEC-23 Riprap Installation
HEC-23 D.G.14
for Bridge Abutments 4

5. HEC-23 Riprap Installation
HEC-23 D.G.18
for Bottomless Culverts 5

6. Experiment Results Clear-Water Scour – Edge Failure
HEC-23 D.G.14
HEC-23 D.G.18
Side Slope Installation
Edge Failure: Due to narrower erodible width and change of bed roughness:
Increased Contraction Scour
Edge and Slide Failure: combined effect of Local and Contraction Scour:
For Oscar: Edge Failure is only due to change of bed roughness? What is the slide failure for side slope installation?
Riprap Stability – Edge Failure 6

7. Two side effects of riprap installed on the surface:
Edge Failure of Riprap
Why riprap edge fails?
Two side effects of riprap installed on the surface:
Narrower erodible width (volumetric effect)
Change the bed roughness (roughness effect)
Model Without riprap
Model With riprap
𝜏 𝑜
𝜏 𝑅(𝑠𝑎𝑛𝑑)

8. Edge Failure of Riprap Cont’d
Why riprap edge fails?
Two side effects of riprap installed on the surface:
Narrower erodible width (volumetric effect)
Change the bed roughness (roughness effect)
Model Without riprap
Model With riprap

9. Edge Failure of Riprap Cont’d
Less edge failures risk for wider openings
High Shear Lead to Edge Failure
Model without riprap
Model with riprap

10. CFD Modeling – Compound channelsy1
Field Installations
Channel Geometry

11. CFD Modeling – Compound channels
No riprap - compound
Riprap - compound
CFD Models – compound channel

12. Bed shear stress (Riprap size =1’9’’) Case 26 (W2/y0 = 6.2)
No riprap
Riprap
CFD Results - Bed Shear Stress

13. CFD Results - Bed Shear Stress
Case 30 (W2/y0 = 16.0)
No riprap
Riprap
CFD Results - Bed Shear Stress

14. Results for Full Scale Modeling
CFD results
Theoretical
derivation
𝜏 𝑅(𝑠𝑎𝑛𝑑) 𝜏 𝑜
𝑊 2 𝑦 0
Theoretical Derivation – CFD and Curve-fitting

15. Theoretical Derivation – Definition of Narrow Opening
Field Cases
Towlston, VA (W2 =34’, y0 =7.5’)
STH 40, WI (W2 =34', y0 =4.23')
Guthrie Run, DE (W2 =27', y0 =1.5')
Real 𝑊 2 𝑦 0
𝜏 𝑅(𝑠𝑎𝑛𝑑) 𝜏 𝑜
Forestry Service Rd, PA (W2 =33', y0 =4')
Nordyke Road, NV (W2 =57', y0 =11.7')
𝑊 2 𝑦 0
Theoretical Derivation – Definition of Narrow Opening

16. Importance of Riprap
When the bridge abutments are built in the main channel and NOT buried below total scour
Riprap becomes part of bridge structure (riprap failure = bridge failure)
No riprap failure => No surface installation
New layout is needed
Note: The assumption of HEC-23 is that the abutment is built on the flood plain, where most cases will be a wide opening. HEC-23 may no longer be valid for abutment in the main channel, therefore a new layout is needed.

17. Proposed Layout for Riprap Countermeasure for Shallow Foundations
Free Surface Flow
Riprap Layouts based on Scour for Scour Check Flood
Option 1: No riprap for abutments, foundation buried below total scour
Option 2: Riprap buried below long term degradation (LTD) + check flood contraction scour
Option 3: Riprap buried below long term degradation (LTD) + check flood contraction scour , full width protection across the stream bed
Pressure Flow
Riprap Layout based on Contraction Scour for Scour Design Flood
Riprap buried below vertical contraction scour, full width protection across the stream bed

18. Free Surface Flow: Option 1
Cross Section View

19. Free Surface Flow: Option 2
For W2 / y0 (CF) > 6.2
Cross Section View

20. Free Surface Flow: Option 3
Cross Section View

21. Pressure Flow
Cross Section View

22. Free Surface Flow: Option 1
Plan View

23. Free Surface Flow: Option 2
Plan View

24. Free Surface Flow: Option 3
Plan View

25. Pressure Flow
Plan View

26. Buried Full Width Protection W2/y0 = 6.2
Apron extent = 2y
upstream and downstream
Narrow Bridge Openings – Buried Full Width Protection 26

27. Buried Full Width Protection Cont’d W2/y0 = 6.2
No Shear, No Edge failure observed
Narrow Bridge Openings – Buried Full Width Protection 27

28. Bed Shear Stress For Model Without/With Buried Riprap
Case 31 (W2/y0 = 6.2)
No riprap
Buried Riprap
CFD Results - Bed Shear Stress

29. Bed Shear Stress For Model Without/With Buried Riprap
Case 32 (W2/y0 = 8)
No riprap
Buried Riprap
CFD Results - Bed Shear Stress

30. Bed Shear Stress For Model Without/With Buried Riprap
Case 35 (W2/y0 = 16)
No riprap
Buried Riprap
CFD Results - Bed Shear Stress

31. Preliminary Conclusions
For new bridges with shallow foundations built in the main channel, implement proposed layout for riprap countermeasure. No riprap edge failure is allowed when abutments are not buried below the total scour depth of check flood.
HEC-23 riprap may only applies to abutment on the floodplain and wide opening.
Preliminary Conclusions

32. Thank you!