Trash, trees, and other debris may accumulate on the upstream side of a pier. During high flow events, this debris may block a significant portion of the bridge opening. In order to account for this effect, a pier debris option has been added to HEC-RAS.
The pier debris option blocks out a rectangular shaped area in front of the given pier. The user enters the height and the width of the given block. The program then adjusts the area and wetted perimeter of the bridge opening to account for the pier debris. The rectangular block is centered on the centerline of the upstream pier. The pier debris is assumed to float at the top of the water surface. That is, the top of the rectangular block is set at the same elevation as the water surface. For instance, assume a bridge opening that has a pier that is six feet wide with a centerline station of 100 feet, the elevation of water inside of the bridge is ten feet, and that the user wants to model pier debris that sticks out two feet past either side of the pier and is [vertically] four feet high. The user would enter a pier debris rectangle that is 10 feet wide (six feet for the pier plus two feet for the left side and two feet for the right side) and 4 feet high. The pier debris would block out the flow that is between stations 95 and 105 and between an elevation of six and ten feet (from an elevation of six feet to the top of the water surface).
The pier debris does not form until the given pier has flow. If the bottom of the pier is above the water surface, then there is no area or wetted perimeter adjustment for that pier. However, if the water surface is above the top of the pier, the debris is assumed to lodge underneath the bridge, where the top of the pier intersects the bottom of the bridge deck. It is assumed that the debris entirely blocks the flow and that the debris is physically part of the pier. (The Yarnell and momentum bridge methods require the area of the pier, and pier debris is included in these calculations.)
The program physically changes the geometry of the bridge in order to model the pier debris. This is done to ensure that there is no double accounting of area or wetted perimeter. For instance, pier debris that extends past the abutment, or into the ground, or that overlaps the pier debris of an adjacent pier is ignored.
Shown in the figure below is the pier editor with the pier debris option turned on. Note that there is a check box to turn the floating debris option for this pier. Two additional fields must be filled out, the height and overall width of the pier debris. Additionally, there is a button that the user can use to set the entered height and width for the first pier as being the height and width of debris that will be used for all piers at this bridge location. Otherwise, the debris data can be defined separately for every pier.
After the user has run the computational program with the pier debris option turned on, the pier debris will then be displayed on the cross section plots of the upstream side of the bridge (this is the cross sections with the labels “BR U,” for inside of the bridge at the upstream end). An example cross-section plot with pier debris is shown in Figure 5-15.