Chapter 3:

BRIDGES 

Introduction

Flood damage to bridges are typically caused by water overtopping decks, erosion of the streambed under piers and abutment footings, erosion of the embankments, and impact and accumulation of floating debris on the decks, piers, and abutments. These damages may be related to the inadequate hydraulic capacity of the bridge, misaligned piers and/or abutments, or accumulation of debris.

Assessing the Causes of Bridge Damage

Inadequate hydraulic capacity of a bridge may result in streambed scour on both sides of the piers and abutment. Misalignment of the piers and/or abutments may result in streambed scour on the exposed side of the piers, and deposition of streambed materials on the lee sides. To help identify excessive flood flow velocities that caused undermining and subsequent damage to bridge piers and abutments, determine: The location of streambed scour, and

Deposition of streambed materials.

Debris can threaten bridge facilities, whether it is carried in flood flows that are rising or receding. In rising waters, debris may become caught on bridge piers and abutments, which decreases the size of the bridge opening. The bridge can then become damaged by flow impacting the decks, piers, and abutments, and by washouts of piers and abutments due to streambed scour.

Damage caused by debris impact and accumulation is verified by observing debris jammed into bridge members, debris piles deposited around the upstream side of piers and abutments, and streambed scour below and adjacent to debris piles. If debris was carried through receding flood flows, the debris will be deposited on the stream overbanks or around the base of piers and abutments, but are usually not the cause of damage.

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 sit

E. 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 the bridge piers and abutments 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: 

B. Insufficient Capacity (Decks)

Introduction

Problem: Damage to bridge decks and associated superstructures (railings and truss) as a result of overtopping due to insufficient capacity for flow through the bridge opening.

Mitigation Objective: To prevent damage to bridge decks and associated superstructures by increasing the design capacity for the bridge opening, and/or modifying the bridge deck design to allow for controlled overflow.

B.1 Elevate the Bridge Deck

The bridge deck and associated superstructure should be elevated to a level sufficient to pass anticipated flood flows. Approach sections to the bridge may likewise need to be raised

effectiveness:

• Most effective mitigation for passing flood flows.

Limitations:

• The pier and abutment supports may need to be redesigned to accommodate and support the elevated bridge deck and associated superstructure

Considerations: 

B.2 Replace a Steel Truss Bridge With an Open Deck Bridge

Replacing a steel truss bridge with an open deck bridge will reduce the backwater conditions upstream, and eliminate the accumulation of debris should the bridge become over-topped during flood events.

effectiveness:

• Generally very effective
• An open deck bridge does not trap floating debris to the same extent that a steel truss bridge will when overtopped
• effectiveness is increased if open deck bridge is elevated

Limitations:

• Bridge piers and abutments may require extensive redesign to accommodate open deck bridge

Considerations: 

B.3 Replace Multi-Spans With a Single Span Bridge

Replace the multiple spans of a bridge with a single clear-span to eliminate the need for piers. This will increase the flow through the bridge and reduce upstream backwater conditions.

effectiveness:

• Very effective
• Increases the effective size of the bridge opening and flow capacity.
• Reduces debris accumulation.
• Decreases the backwater conditions upstream from the bridge and the effects of drawdown through it.
• Consider relief openings, wingwalls, realignment of piers and abutments, embankment slope protection, and abutment debris deflectors for maximum effectiveness.

Limitations:

• Length of span may be limited by strength of materials.

Considerations:

B.4 Increase Bridge Opening Size

Increase the size of the bridge opening(s) by lengthening the opening or raising the bridge deck. Increasing bridge opening size will decrease any backwater conditions upstream from the bridge and reduce the effects of drawdown through the bridge

effectiveness:

• Very effective.  Particularly effective where damage was caused by overtopping of the bridge due to excessively high water surface elevations upstream or by excessively high water velocities eroding the pier and abutment foundations.
• Degree of effectiveness varies with the difference of the water surfaces upstream and downstream from the bridge, and with the water velocities through the bridge
• Consider relief openings, wingwalls, realignment of piers and abutments, embankment slope protection, and debris deflectors for maximum effectiveness.

Limitations:

• Crossing and stream channel geometry may preclude this option.

Considerations: 

B.5 Construct a Relief Opening

Construct one or more relief openings through the road prism at a location that will carry excess floodwaters. The relief opening may be a culvert or bridge, or multiple culverts or bridges. The openings should be located at natural side channels and in line with heavy flow areas located on the stream overbanks. (See "Culverts-Plugging," pp. 29-33)

effectiveness:

• Generally very effective, particularly if combined with appropriate culvert and/or bridge entrance and outlet treatments.
• Consider wingwalls, embankment slope protection, and debris deflectors for maximum effectiveness.

Limitations:

• Geometry of drainage area may preclude this option.

Considerations:  

C. Erosion (Approaches)

Introduction

Problem:

Damage to bridge approaches resulting from overtopping with subsequent erosion of the road surface, shoulder and embankment, and from impact of flood flows and debris with subsequent erosion of the embankment.

Mitigation Objective:

To prevent future damage to bridge approaches (embankments, road shoulder, and road surface) by eliminating overtopping and erosion.

D. Scour (Piers & Abutments)

Introduction

Problem:

Damage to bridge piers and abutments resulting from scouring of the streambed along and under their footings.

Mitigation Objective:

Reduce flood flow velocities along bridge piers and abutments, thereby eliminating scouring of the streambed along and under their footings.

D.1 Increase Footing Depth

The depth of pier and abutment footings should be extended below the expected depth of streambed scour or to bedrock. The expected depth of scour depends on the flood flow velocities along the footing and the nature of the streambed materials.

effectiveness:

• Very effective, particularly when flood flow velocities are relatively high.
• Consider flow deflectors, debris deflectors, or replacing multi-spans with a single span for maximum effectiveness.

Limitations:

• The depth of pier and abutment footings may be limited by streambed characteristics.
• Footings should be inspected periodically after floods for streambed erosion.

Considerations:  

D.2 Install Flow Deflectors

Install "V" shaped flow deflectors on or immediately upstream from the upstream sections of piers and abutments to reduce flow velocities and protect footings from scouring. Install a concrete collar on lower section of piers immediately above the footing. Also extend lower sections of abutments and the wingwalls, if present. This will assist in deflecting flood flows away from the piers and abutments, and will eliminate streambed scour along and under them.

effectiveness:

• Flow deflectors are very effective, particularly for flood flows with high velocities.
• Pier collars and abutment sub-walls are moderately effective
• Pier collars and extended abutment and wingwalls may provide additional protection from impact of rocks and debris.

Limitations:

• Flow deflectors should be inspected periodically after floods for impact damage and for streambed erosion.

Considerations:

D.3 Install Semicircular or Triangular Endnoses

Semicircular or triangular endnoses may be installed on the upstream ends of the piers to redirect flood flow velocities. Pier endnoses are a protection measure, such as sheet metal attached to the pier to redirect flow. Endnoses should also be designed to both prevent debris accumulation and to protect the piers and abutments from floating debris impact.

effectiveness:

• Moderately effective where flood flow velocities are relatively high.
• Less effective when flood flow velocities are relatively low.

Limitations:

• Piers should be inspected periodically after floods for impact damage and for streambed erosion.
• Bridge decks need to be high enough to pass floating debris.
• Any debris that accumulates in the bridge opening needs to be removed during the flood or immediately after the flood peak has passed

Considerations:

E. Debris Impact (Piers & Abutments)

Introduction

Problem: Damage to bridge piers and abutments resulting from the impact and accumulation of debris.

Mitigation Objective: To prevent future debris damage to bridge piers and abutments by directing debris around and away from them, by providing clear passage of debris through the bridge opening, and by minimizing amount of debris catching on the structural elements of the bridge

E.1 Install Debris Deflectors

Debris deflectors or debris fins should be installed on the upstream ends of piers and abutments and angled so as to direct floating debris into areas of high flood flow velocities. The debris deflectors and fins should be "V" shaped and extend upstream a sufficient distance to orient the floating debris for easy passage through the bridge. Debris deflectors or fins should be designed to both prevent debris accumulation and to protect the piers and abutments from floating debris impact.

effectiveness:

• Very effective in areas that have significant debris loading in the upstream drainage and flood flow velocities are high.
• Less effective when flood flow velocities are low.

Limitations:

• Bridge decks need to be high enough to pass floating debris.

Considerations:

E.2 Install Batters

Install batters (steel plates) on the upstream ends of concrete piers with semicircular or "V" shaped endnoses, or on wingwall ends and wingwall/abutment junctions to protect them from the impact of floating debris.

effectiveness:

• Very effective in protecting piers from debris impact damage

Considerations:

E.3 Replace Wood Pile Bent Pier Structure With

Solid Concrete Column Pier

Replace a pier constructed with wooden piling with a solid concrete column pier. This measure will prevent debris from becoming caught and accumulating in the pile bent pier configuration, and will protect the pier from debris impact.

effectiveness:

• Very effective in areas that have significant debris loading in the upstream drainage
• effectiveness increases with debris deflectors or debris fins, semicircular or "V" shaped endnoses, and/or batters.

Considerations:

E.4 Construct Debris Catchments

Debris catchment structures, such as debris barriers (trash racks) or low height dams, may be constructed on small tributary streams upstream from the bridge. The catchment structures should be designed to trap debris while passing the stream flow. If a debris catchment dam is constructed, it must include an emergency spillway.

effectiveness:

• effective where the source of debris is from highly vegetated drainage areas upstream from the bridge and where there are adequate storage areas upstream from the catchment structures.
• Less effective on larger tributary streams.

Limitations:

• Any debris that accumulates upstream from the catchment structures needs to be removed during the flood or immediately after the flood peak has passed

Considerations: