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Basic Hydraulics: Bridges
Definition & terminology
• Abutments• Flow contraction
Definition & terminology• Bridge opening
Definition & terminology• Piers in normal and skew crossings
• Obstruct flow• Create instability
Bents, abutments, & embankments• Abutments: Protective measures used at
abutments consist of the following: • riprap header slopes and deep toe walls (stone protection is generally preferred to
• vertical abutment walls • sheet pile toe walls • deep foundations of piles or drilled shafts
• Revetment is usually placed at the abutment on the slopes under the bridge end and around the corners of the embankment to guard against progressive embankment erosion
Bents, abutments, & embankments• Two common types of revetments used to
protect abutments are rigid (concrete riprap) and flexible (stone protection).
Bents, abutments, & embankments• Embankments: Embankments that encroach on
floodplains are most commonly subjected to scour and erosion damage by overflow and by flow directed along the embankment to the waterway openings
• The embankment may need to be protected if significant overtopping of the approach embankment is anticipated during the life of the crossing
• The embankment can be protected with soil cement or revetments, rock, wire-enclosed rock, or concrete.
Contraction• The contraction of flow caused by the bridge
opening (and piers) increases velocities in the contraction.
• The contraction also causes a backwater drawdown (in subcritical flow)
Bridge scour
• A scour analysis is required for new bridges, replacements, and widenings
• Where a scour analysis indicates high depths of potential contraction scour, a structure larger than that required by the basic velocity and backwater criteria may be more cost effective than to designing foundations and armoring to withstand the scour
• The potential for deep local scour can be reduced by enlarging the structure, but designing foundations and armoring to withstand local scour depths may be more cost-effective.
Bridge scour
USGS
Scour at highway bridges
Three areas at bridges are affected by scour:• At pier and abutment foundations. Commonly
water flows faster around piers and abutments, making them susceptible to local scour
• Across a bridge opening. Contraction scour occurs when water accelerates as it flows through a bridge opening that is narrower than channel and floodplain upstream from bridge
• Throughout a long reach of the stream, both upstream and downstream of the bridge. This degradation in a stream is usually a long-term process that results in lowering the streambed.
Hydraulics as pre-requisite• Determine backwater caused by the bridge
abutments and piers• Determine flow distribution and velocities• Estimate scour potential
• HDM Chapter 9
• Culvert Design and Analysis is subject of DES604
Hydraulics as pre-requisite• Generally use computer models
• WSPRO• HEC-RAS
• Flow Zones
Loss methods in Zone 2HDM 9-20
Hydraulics as pre-requisite• If water reaches the lower chord of the
bridge, then pressure flow is used.
Still have a free surface