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

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

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

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

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

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

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)

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

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

CFD Modeling – Compound channels y1 Field Installations Channel Geometry

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

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

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

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

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

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.

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

Free Surface Flow: Option 1 Cross Section View

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

Free Surface Flow: Option 3 Cross Section View

Pressure Flow Cross Section View

Free Surface Flow: Option 1 Plan View

Free Surface Flow: Option 2 Plan View

Free Surface Flow: Option 3 Plan View

Pressure Flow Plan View

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

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

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

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

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

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

Thank you!