## 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**,

**, and**

*Figure 8***of this post.**

*Figure 9*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

**except D50: in**

*Figure 7***, D50=2.0 mm and in**

*Figure 6***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**

*Figure 7***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.

## 2 COMMENTS