Chapter 3: BRIDGES 

A. Misalignment

Introduction

Problem:

Damage to a bridge caused by its misalignment with the stream channel. The misalignment may be a result of original design miscalculations and/or subsequent stream migration. (See "Culverts-Misalignment," pp. 40-45)

Mitigation Objective:

To prevent future damage to a bridge by aligning its abutments and piers to the centerline of the stream, by preventing future migration of the stream away from the bridge, and/or by installing additional bridge openings to accommodate future migration of the stream channel.

A.1 Construct Bridge Wingwalls

Install bridge entrance and outlet wingwalls. Design wingwalls to redirect the flow into the bridge opening and eliminate erosion under the bridge piers, abutments and embankment. Use flared wingwalls angled to coincide with the stream. (See "Culverts-Embankment Erosion," pp. 34-39)

Effectiveness:

• Very effective
• Flow volumes may be increased up to 30 percent depending on angle of wingwalls.
• Rounding or beveling of abutment corners may increase flow volumes by 20 percent.
• Consider debris deflectors and embankment slope protection for maximum effectiveness.

Limitations:

• If stream velocities are high, lateral scour of embankments may result from eddies at the ends of wingwalls. Design wingwall shapes and their angles to the stream to minimize the development of eddies.
• Design of debris deflectors needs to account for effect of stream and bridge pier and abutment alignment.

Considerations: 

A.2 Construct Spur Dikes

Spur dikes are embankments designed to direct flood flows into a bridge opening. They are 'tied into' the road embankment at an appropriate point landward from the bridge opening and then extend upstream. The usual shape of a spur dike is either straight or elliptical. Spur dikes should be installed at an angle to redirect the flow into the bridge opening, thereby eliminating the potential for erosion along and under the bridge piers and abutments and along the bridge embankment.

Effectiveness:

• Very effective.  Consider debris deflectors and embankment slope protection for maximum effectiveness.

Limitations:

• Spur dikes should be placed on the stream overbanks so water-surface elevations are not increased significantly.
• If stream velocities are high, scour of spur dike embankments may result from eddies at their upstream ends and along their sides. Design spur dike shapes and angles to the stream to minimize development of eddies.

Considerations:  

A.3 Install Additional Bridge Openings or Spans

Install additional bridge openings or bridge spans. These additional openings or spans should be located at historical and/or potential stream alignments at a crossing site. This measure can be employed to mitigate for the effects of a braiding streambed, or a widening streambed.

Effectiveness:

• Very effective. Consider bridge entrance and outlet wingwalls, debris deflectors, and embankment slope protection for maximum effectiveness.

Limitations:

• Crossing geometry may preclude this option.

Considerations:  

A.4 Realign Piers and Abutments

Realign piers and abutments of the bridge to be parallel to the centerline of the stream, thereby eliminating the potential for erosion along and under the bridge piers and abutments and along the embankment. Realignment of the bridge may include relocating it to the vicinity of the present stream channel and/or aligning the bridge opening to the centerline of the stream.

Effectiveness:

• Very effective
• Flow volumes may be increased up to 30 percent when piers are aligned

  Consider bridge entrance and outlet wingwalls, debris deflectors, and embankment slope protection for maximum effectiveness.

Considerations: 

A.5 Install Approach Berms

(SEE 2.D.3)

A.6 Install Flow Diverters

(SEE 2.D.4)

A.7 Realign the Stream Channel

(SEE 2.D.5)