Bridge Scour Analysis By HEC-18 Equations – Local Abutment Scour (1 of 2)

The bridge contraction scour depth and pier scour depth calculations are explained in the following three posts – Horizontal Contraction Scour, Vertical Contraction Scour (Pressure Flow Scour), and Local Pier Scour. This post is to introduce the bridge abutment scour (Figure 1) depth calculation using NCHRP 24-20 Approach (HEC-18 Chapter 8.6.3). The two more widely known abutment scour equations – HIRE Equation and Froehlich’s Equation – are briefly explained in here.

NCHRP 24-20 abutment scour approach is physically representative of the abutment scour process and does not require the estimation of L’ – the effective embankment length, which is difficult to determine. More importantly, NCHRP 24-20 abutment scour approach predicts the total scour at the abutment including both local abutment scour and the contraction scour.

NCHRP 24-20 approach starts with calculating the contraction scour depth Yc at left or right overbank area near abutment using a simplified live-bed or clear-water equation (Figure 2) depending on the ratio of upstream overbank area flow velocity V1 and the critical velocity Vc of D50. The calculated contraction scour depth Yc is to be scaled up by an amplification factor (>=1.0) to account for the turbulence flows near abutments (Figure 3).

The amplification factor are determined from HEC-18 Figure 8.9 to Figure 8.12 by first determine Condition A or B using the ratio of L/Bf as indicated in Figure 4. For a skewed embankment, the embankment length L should be corrected to its projected length as shown in Figure 5.

The characteristic velocity V used to calculated unit discharge q2 at the abutment toe is calculated by SBR (set-back ratio) method as explained in Figure 7, Figure 8, and Figure 9 of this post.

FHWA Hydraulic Toolbox is a convenient tool to calculate abutment scour depths by NCHRP 24-20 Approach (Figure 6 and Figure 7).

The input parameters in Figure 6 are the same as those in Figure 7 except D50: in Figure 6, D50=2.0 mm and in Figure 7 D50=20.0 mm. The different D50 sizes made the abutment scour happen under two different mechanisms which in turn resulted in different scour depth Ys values from using different Yc equations.

Using the two example projects which come with HEC-RAS installation, the abutment scour depths by NCHRP 24-20 Approach were compared to those calculated by HEC-18 HIRE Equation and summarized in Table 1. For the two example bridges, the abutment scour depths (including contraction scour depths here for comparison purpose) calculated by HEC-18 HIRE Equation are 2 to 4 times as much as the values by NCHRP 24-20 method. The above statement is, of course, not conclusive, but it does show that the abutment scour depths vary significantly with different methods.