Advanced HEC-RAS Features

What we will coverChannel ModificationBridge DesignScour at Bridges

Channel modificationThe Channel Modification option allows trapezoidal cuts and fills into the existing geometryCuts can be place at any location in the cross section

Channel modification analysisGeneral Modeling GuidelinesFirst develop a hydraulic model of the existing river reach containing the channel modification area This model should include several cross sections downstream from the study reach, such that the downstream boundary condition does not affect the hydraulic results inside the channel modification region. The model should also include several cross sections upstream of the study reach, in order to evaluate the effects of the channel modification on the water surface profile upstream.

Channel Modification optionThe Channel Modification option allows trapezoidal cuts and fills into the existing geometry. Once the user has performed all of the desired channel modifications, then the modified geometry data is saved into a new geometry file. The user can then create a new plan, which contains the modified geometry and the original flow data that was used under the existing conditions planComputations can then be performed for the modified condition, and the user can compare the water surface profiles for both existing and modified conditions.

Channel modification option (1)Channel modification option allows for: Multiple trapezoidal cuts (up to three)Independent specification of left and right trapezoidal side slopesAbility to change the Mannings n value for the trapezoidal cutSeparate bottom widths for each trapezoidal cutAbility to set new channel reach lengthsMultiple ways of locating the main channel centerline

Channel modification option (2)Channel modification option also allows for: User can explicitly define the elevation of the new channel invert, or it can be based on the original channel invert, or it can be based on projecting a slope from a downstream cross section or an upstream cross sectionThe centerline of the trapezoidal cut can be entered directly, or it can be located midway between the original main channel bank stationsOption to fill the existing channel before performing cutsCut and fill areas and volumes are computed

Channel modified window

Single cross section

Comparison of plans

X-Y-Z perspective plot

Profile Output Table

Bridge DesignThe bridge design editor allows the user to enter or modify bridge data quickly and conveniently.

Bridge DesignSteps for putting together a bridge with this editor:1. From the Geometric Data window, open the Bridge/Culvert data editor. Select the River and Reach in which you would like to place the bridge.2. Go to the Options menu and select Add a Bridge and/or Culvert from the list. An input box will appear prompting you to enter a river station identifier for the new bridge.3. Open the Bridge Design editor by pressing the Bridge Design button on the lower left side of the Bridge/Culvert Data editor.4. Enter the required data for the bridge deck/roadway, sloping abutments (optional), and piers (optional).

Bridge Design EditorThe bridge design editor allows the user to enter or modify bridge data quickly and conveniently. With this editor the user can enter the deck/roadway data, sloping abutments, and pier information.

Bridge Design Editor

Building the bridgeThe user only has to enter a minimal amount of information to build or edit the bridge. To create the bridge deck/roadway, the user must enter: a high chord elevation (top of road), and a low chord elevation (maximum elevation inside of the bridge opening). The user enters the number of piers, the upstream and downstream stationing of the left most pier, the spacing between the centerline of the piers, and the width of the piers.

Bridge Design EditorThe bridge data can be changed at any time by either going back into the Bridge Design editor and entering new valuesYou can also go to the more detailed editors for the bridge deck/roadway, sloping abutments, and piers and change data

Bridge design example

Computing Scour at Bridges

Computing Scour at BridgesThe computation of scour at bridges within HEC-RAS is based upon the methods outlined in Hydraulic Engineering Circular No. 18 (FHWA, 2001). Before performing a scour analysis with the HEC-RAS software, the engineer should thoroughly review the procedures outlined in the Hydraulic Engineering Circular No. 18 (HEC 18) report. This section presents the data input required for computing contraction scour and local scour at piers and abutments. For information on the bridge scour equations, see Chapter 10 of the HEC-RAS Hydraulic Reference Manual.

General Modeling GuidelinesIn order to perform a bridge scour analysis, the user must first develop a hydraulic model of the river reach containing the bridge to be analyzed. The model should include several cross sections downstream from the bridge (downstream boundary conditions do not affect bridge hydraulic results ).The model should also include several cross sections upstream of the bridge, in order to evaluate the long term effects of the bridge on the water surface profile upstream.

Flow distributionThe flow distribution option in HEC-RAS should be turned on This option allows for additional output showing the distribution of flow for multiple subdivisions of the left and right overbanks, as well as the main channel The output of the flow distribution option includes the following items for each flow slice: percentage of flow; flow area; wetted perimeter; conveyance; hydraulic depth; and average velocity The user must request the flow distribution output for the cross sections inside the bridge, the cross section just upstream of the bridge, and the approach section

Bridge scour evaluationAfter performing the water surface profile calculations for the design events and computing the flow distribution output, the bridge scour can then be evaluated. The total scour at a highway crossing is comprised of three components: long-term aggradation and degradationcontraction scour local scour at piers and abutments

Entering Bridge Scour DataThe bridge scour computations are performed by opening the Hydraulic Design Functions window and selecting the Scour at Bridges function.

Bridge Scour DataOnce this option is selected the program will automatically go to the output file and get the computed output for the approach section, the section just upstream of the bridge, and the sections inside of the bridgeThe user is required to enter only a minimal amount of input and the computations can be performed. If the user does not agree with any of the data that the program has selected from the output file, the user can override it by entering their own values

Bridge/Culvert Data Editor

Set Locations for Flow Distribution

Bridge Scour Editor

Contraction ScourFor contraction scour analysis the user must provide:D50 for the bed materialWater temperatureThe equation to be usedRAS provides the additional information (EG slope, velocity, etc.)

Pier Scour

Abutment Scour

Total Bridge ScourThe total scour is a combination of the three types of scour at each location: Contraction scourPier scour Abutment scour

Summary Table

ResultsThe graphic and the tabular results can be sent directly to the default printerThey also can be sent to the Windows Clipboard in order to be pasted into a report. A detailed report can be generated, which shows all of the input data, computations, and final results.

Plot with Total Scour

Saving ResultsThe bridge scour input data can be saved by selecting Save Hydraulic Design Data As from the File menu of the Hydraulic Design Function window. The user is required to enter a title for the data. The computed bridge scour results are never saved to the hard disk. The computations can be performed in a fraction of a second by simply pressing the compute button.Therefore, when the Hydraulic Design Function window is closed, and later re-opened, the user must press the compute button to get the results.

Advanced HEC-RAS FeaturesAdvanced HEC-RAS Features