Bridge scour describes the process of flowing water eroding the streambed or bank near a bridge. It is important to consider bridge scour because it is the most common cause of bridge failures (FHWA, 2012).
HEC-RAS uses the methodology outlined in the Federal Highway Administration’s Hydraulic Engineering Circular No. 18 (HEC-18) to estimate scour at bridges. Although the FHWA published an updated version of this document in 2012, HEC-RAS uses the procedures from the 2001 version. The rest of this blog post will discuss how to calculate scour using HEC-RAS.
General Information About Bridge Scour
Firstly, develop a one-dimensional hydraulic model of the river containing the bridge. This model should include a sufficient number of cross-sections downstream of the bridge so that the downstream boundary does not affect the hydraulics near the bridge. When you are setting up the Steady Flow Analysis, assign flow distributions.
After running the model, the user can estimate bridge scour, which is comprised of the following components: 1) long-term aggradation or degradation; 2) contraction scour, and 3) local scour at piers and abutments. HEC-RAS does not compute long-term aggradation or degradation. However, it will compute local scour and contraction scour.
Contraction Scour
Contraction scour occurs when the bridge constricts flow and reduces the flow area of a stream. There are two types of contraction scour. Firstly, live-bed contraction scour occurs when flow transports bed material into the contracted bridge section from the upstream part of the river. In contrast, clear-water contraction scour occurs when sediment transport in the uncontracted approach section is negligible. HEC-RAS allows you to choose the type of contraction scour. Alternatively, you can select the “Default” option as shown below. In addition, enter the appropriate D50 and water temperature values.
Pier Scour
Pier scour occurs due to accelerating flow around the pier and the formation of horseshoe vortices. These vortices remove material from the base of the pier and create a scour hole. HEC-RAS allows you to estimate pier scour using the Colorado State University (CSU) equation or the Froehlich equation. However, HEC-18 recommends using the CSU equation. Therefore, the default equation in HEC-RAS is the CSU equation.
Firstly, determine whether you want to perform Maximum V1 Y1 calculations or Local V1 Y1 calculations. When selecting the Maximum V1 Y1 option, HEC-RAS will use the Flow Distribution to obtain the maximum velocity (V1) and flow depth (Y1) at the cross-section just upstream of the bridge. In contrast, the Local V1 Y1 option will find the velocity (V1) and flow depth (Y1) at the cross-section just upstream of the bridge that corresponds to the centerline stationing at each of the piers.
Correction Factors
Next, select the shape of the pier. HEC-RAS will use this to estimate the correction factor for pier nose shape (K1).
Then enter the angle of attack value into the Angle field. The angle of attack is the angle of the flow approaching the pier. HEC-RAS will use this to estimate the correction factor for the angle of attack of flow on the pier (K2).
Next, use the K3 dropdown menu to select the bed condition. The user can select from the following: clear-water scour (K3 = 1.1); plane bed and antidune flow (K3 = 1.1); small dunes (K3 = 1.1); medium dunes (K3 = 1.2); and large dunes (K3 = 1.3).
Finally, enter the D95 value. HEC-RAS will use this value, along with the D50 value, to estimate the K4 correction factor. The K4 factor is used to decrease scour depths to account for armoring. This factor is only estimated when the D50 is greater than 0.2 mm, and the D95 value is greater than 2.0 mm.
Entering Pier Scour Data
Enter the pier scour data.
Abutment Scour
Finally, enter data into the Abutment tab. Abutment scour can be calculated using the HIRE equation or Froehlich’s equation. The HIRE equation is based on field data at the end spurs in the Mississippi River. Froehlich’s equation is based on 170 live-bed scour measurements in lab flumes. Please note that abutment scour is calculated separately for each abutment.
To estimate abutment scour, select the abutment type from the K1 dropdown menu. Then enter the skew (angle of flow against the abutment). A value of 90 degrees is typical since most abutments are perpendicular to flow. Then select the equation from the dropdown menu. HEC-RAS will automatically populate the data needed for both equations.
Generating Reports for Bridge Scour
After you have entered the required information, click Compute at the top of the screen. This will generate data in the bottom portion of the screen.
To generate a report, click the Report… button as shown below. A screen will pop up. Then you can copy the data into a Word Document. Alternatively, you can print the information in PDF.