Basic Hydraulics: Bridges

1. Basic Hydraulics: Bridges

2. Definition & terminology • Abutments • Flow contraction

3. Definition & terminology • Bridge opening

4. Definition & terminology • Piers in normal and skew crossings • Obstruct flow • Create instability

5. 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

6. Bents, abutments, & embankments • Two common types of revetments used to protect abutments are rigid (concrete riprap) and flexible (stone protection).

7. 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.

8. 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)

9. 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.

10. Bridge scour USGS

11. 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.

12. 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

13. Hydraulics as pre-requisite • Generally use computer models • WSPRO • HEC-RAS • Flow Zones Loss methods in Zone 2 HDM 9-20

14. Hydraulics as pre-requisite • If water reaches the lower chord of the bridge, then pressure flow is used. Still have a free surface