Basic Hydraulics: Bridges. Definition & terminology Abutments Flow contraction.

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