Contraction and Abutment Scour Introduction

THE ABSCOUR DESIGN METHODOLOGY

The methods presented in this guideline for estimating abutment scour using the ABSCOUR method are based on Laursen's contraction scour equations as presented in (1) the Office of Structures H&H Manual for Hydrologic and Hydraulic Design and (2) the FHWA Publication HEC No. 18, Evaluating Scour at Bridges, Fifth Edition, April 2012 (1). The live bed equation was originally derived by Straub (2) considering that the shear stresses (and thus the rates of sediment transport) in an uncontracted section and a contracted section are the same. It assumes a long contracted channel where the flow is considered to be uniform and the scour depth is constant across the channel section.

The contracting flow at the entrance corner of a channel constriction differs significantly from the conditions described above. The flow velocity across the channel is not uniform. The velocity near the edge of the constriction is faster than that in the midstream. Because of this higher velocity and its associated turbulence, the scour depth near the edge or corner of the constriction is usually deeper than in the center of the channel. The flow pattern at the upstream corner of an abutment will be similar to the flow at the entrance corner of a contracted channel, when the bridge approach roads obstruct overbank flow or the abutment constricts the channel. Local abutment scour can be expected to be deeper than the contraction scour in the center of the channel.

Chang has applied Laursen's long contraction theory to both clear-water and live-bed scour. He developed a "velocity adjustment factor" kv to account for the non-uniform velocity distribution in the contracted section, and a "spiral-flow adjustment factor" kf at the abutment toe that depends on the approach Froude number. The value of kv was based on 2-D potential flow theory, and kf was determined by Chang from the analysis of a collection of abutment scour experiments in laboratory flumes as well as calibration studies conducted by the USGS at bridge abutments in South Carolina.

The ABSCOUR method uses a modification of Neill's methodology for estimating the critical velocity of non-cohesive soils as well as Neill's recommendations for estimating the critical velocity of cohesive soils (See the Office of Structures H&H Manual, Appendices to Chapter 11

THE NCHRP 24-20 METHOD FOR ESTIMATING ABUTMENT SCOUR

Conceptually, this method is very similar to the ABSCOUR method for estimating contraction scour. It uses Laursen's live bed scour procedure as well as Laursen's method for clear water scour to estimate the depth of contraction scour. It uses a factor referred to as the amplification factor (alpha) that is multiplied by the contraction scour to obtain the abutment scour. See HEC-18, fifth edition, 2012 Chapter 8 for the details of the NCHRP 24-20 method. The NCHRP 24-20 method should not be used for the evaluation of cohesive soils.

LAURSEN'S METHOD FOR CLEAR WATER AND LIVE BED CONTRACTION SCOUR

See HEC-18, fifth edition, 2012, Section 6 for the details of the Laursen Method. Laursen's method should not be used for cohesive soils. A particle size of 0.0007 ft (fine sand) is recommended as the smallest particle size for the use of the Laursen method.

ULTIMATE SCOUR IN COHESIVE SOILS

See HEC-18, Fifth Edition, 2012, Section 6.7 for the details of this method. Results from this method can be compared with the results obtained from the ABSCOUR 10 methodology for evaluating contraction scour in cohesive soils.