# SMS:Bridge Scour Methodology

The various bridge scour computations use the following extraction methodology:

The process requires the specification of one particle size. If only one size is specified, SMS verifies that this is the D50. If there is more than one particle size specified, SMS extrapolates the data to determine the D50, D84, and D90. SMS exports these values to Hydraulic Toolbox.
Computing Banks and Abutment Toe Locations
SMS intersects the user specified approach section and contracted section arcs with the bank arcs to find the station values of the left and right banks on the approach and contracted section. It will also intersect the abutment toe arcs with the contracted arc to determine the stationing of the abutment toes. If there are too many points of intersection or not enough, then SMS reports errors. There should be two bank intersections on the approach arc, two bank intersections on the contracted arc, and two abutment toe intersection on the contracted arc.
Cutting the Cross Sections from the Mesh Datasets
SMS extracts the stations, elevations, water surface elevations, and velocities along the approach and contracted arcs. It then analyzes this data to determine the hydraulic parameters for the entire section and within the main channel. Principally, SMS computes the wetted perimeter, flow area, average velocity, hydraulic radius, and hydraulic depth for both sections.
• Hydraulic depth is determined by dividing the flow area by the width and is used for the average depth. The width is adjusted for skew and reduced for blockage by piers when applicable on the contracted section.
• Flows are determined by using the depth and velocity datasets. The unit discharges are determined by determining the flow at each location and the width between the data points.
Computing the Critical Velocity
SMS determines the critical velocity using Equation 6.1 of HEC-18.
${\displaystyle V_{c}=K_{u}y^{1/6}D^{1/3}}$
where
VC = Critical velocity above which bed material of size D and smaller will be transported, ft/s (m/s)
y = Average depth of flow upstream of the bridge, ft (m)
D = Particle size for Vc, ft (m)
D50 = Particle size in a mixture of which 50 percent are smaller, ft (m)
Ku = 6.19 SI units
Ku = 11.17 English units
SMS uses the hydraulic radius of the approach channel and the specified D50 to determine the critical velocity. The user can specify the critical velocity to use instead of a computed value.
Computing the EGL
SMS determines the point of intersection with the approach and the centerline arcs then the contracted and centerline arcs. the distance between the approach and contracted arcs is measured along the centerline arc. The energy gradeline slope is computed as the change in the water surface elevation between the two sections divided by this distance along the centerline arc between the two sections.
Determining Skew Angle of the Contracted Section
SMS computes a simple average of the flow direction of each point on the contracted section in the main channel.
SMS also computes the orientation of the contracted section from the left bank intersection to the right bank intersection.
The skew angle is computed as the variation of the flow direction from perpendicular to the contracted section orientation. This angle is used to correct the width of contracted section.
Computing the Angle of Attack on Abutments
The flow direction determined passing through the main channel along the contracted arc is then compared to the orientation of the contracted arcs outside of the abutment toes. (Note: this value is not necessary for the NCHRP Abutment scour method.)
Computing Local Approach Arc
The local approach arc is an arc that SMS generates upstream of the piers. The objective is to find a point just upstream of the piers, where the local flow direction and magnitude has not yet been influence by the pier itself. By default (enter an offset width of 0.0), SMS uses the length of the longest pier, and creates an arc that is a copy of the contracted arc offset that distance upstream. This distance can be specified by the user in the coverage properties dialog.
SMS performs a streamtrace upstream from each peir to intersect to this local approach arc. These locations are used to compute the local velocity and depth for each pier.
This arc is also analyzed to determine the point of highest unit discharge. The velocity and depth at this locations is used for the pier scour analysis of any piers set to a scour reference of "Thalweg".
Computing Pier Angle of Attack
SMS determines the angle of attack on each pier by comparing the velocity angle at the local approach arc upstream from the pier to the pier orientation.