Linear Routing: Floodrouting. HEC-RAS Introduction. Brays Bayou. Uniform Open Channel Flow. v = 1 n R2/3. S S.I. units

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1 Linear Routing: Floodrouting HEC-RAS Introduction Shirley Clark Penn State Harrisburg Robert Pitt University of Alabama April 26, 2004 Two (2) types of floodrouting of a hydrograph Linear Muskingum Reservoir Storage-Indication / Modified Puls Hydrograph (Flow versus Time). Floodrouting input of the inflow hydrograph into a Wedge (Linear) or a Pond (Reservoir). The outflow hydrograph will be dampened such that the outflow hydrograph s peak will be less and delayed. Uniform Open Channel Flow Manning s Eqn for velocity or flow Uniform Open Channel Flow Brays B. Brays Bayou v = 1 n R2/3 S S.I. units v = 1.49 S English units n R2/3 where n = Manning s roughness coefficient R = hydraulic radius = A/P S = channel slope Q = flow rate (cfs) = v A Concrete Channel 1

2 Optimal Channels Non-uniform Flow 2

3 Non-Uniform Open Channel Flow With natural or man-made channels, the shape, size, and slope may vary along the stream length, x. In addition, velocity and flow rate may also vary with x. H = z+ y + α v 2 /2g ( ) Thus, dh dx = dz dx + dy dx + α 2g dv 2 dx Backwater Profiles - Compute Numerically Where H = total energy head z = elevation head, αv 2 /2g = velocity head 3

4 Routine Backwater Calculations 1. Select Y 1 (starting depth) 2. Calculate A 1 (cross sectional area) 3. Calculate P 1 (wetted perimeter) 4. Calculate R 1 = A 1 /P 1 5. Calculate V 1 = Q 1 /A 1 6. Select Y 2 (ending depth) 7. Calculate A 2 8. Calculate P 2 9. Calculate R 2 = A 2 /P Calculate V 2 = Q 2 /A 2 Backwater Calculations (cont d) 1. Prepare a table of values 2. Calculate V m = (V 1 + V 2 ) / 2 3. Calculate R m = (R 1 + R 2 ) / Calculate S = nv m 2 Manning s 1.49R 3 m 2 y 1 + v 1 2g y + v g 5. Calculate L = X from first equation L = S S 0 6. X = X i for each stream reach (SEE SPREADSHEET) Floodplain Bridge Section Watershed Hydraulics Tributary Main Stream D C B A Cross Sections Bridge Q D Q C Q B Q A HEC-RAS: Background River Analysis System model of the U.S. Army Corps of Engineers Input = cross-section geometry and flow rates Output = flood water elevations Cross-Section Schematic Normal Water Surface Floodway Left Bank Station Main Channel Flood Water Surface Floodway Right Bank Station Cross Sections 4

5 HEC-RAS: Cross-Section Description 3-Dimensional River Channel Points describe channel and floodway geometry Bank station locations Water surface elevations and floodplain boundaries Each point has (x,y,z) coordinates ProfileLine CrossSectionLine Example Example Calculate the water surface profiles for two discharges in the channel described in the following tables. Q 1 = cfs Q 2 = cfs Assume a Manning s n of 0.24 for the two overbanks and a Manning s n of 0.04 for the channel. Assume the overbank distances between stations are the same as the distances between the stations in the channel. Section 1 Down L = 0 ft X Y Section 2 Down L = 1500 ft X Y Section 3 Down L = 2100 ft X Y

6 Section 4 Down L = 2000 ft X Y Example Section 5 Down L = 3150 ft X Y Section 6 Down L = 1855 ft X Y Open HEC-RAS (River Analysis System) by double-clicking on the icon (after installing the program). The following screen should appear: Select File New Project This screen should appear. Fill in the needed data. Then click OK. 6

7 The introductory screen should appear with the project name filled in. Need to add geometric data (cross-section data). Click on Geometric Data button (tree). Need to add geometric data (crosssection data). Click on Geometric Data button (tree). Add River Reach by leftclicking on the River Reach button. 7

8 In the tablet area, left-click where you want the reach to start and use the pencil to draw the reach. Double left-click when Reach is completed. Once the reach is drawn (ended by left double-click), a box will appear asking you to name the river and the reach. The result will look like the screen to the right. Next will need to describe cross-sections in the reach. (or can add additional reaches that drain to same outlet). Left-click on the crosssection button on the left. 8

9 The following table should appear. Under Options, select Add New Cross-Section. When the box appears, enter the station number in the reach. Enter the data as required in each of the boxes and then click Apply Data. 9

10 After clicking on Apply Data, the plot should appear. When all cross-sections are entered and the data applied, select Exit Cross-Section Editor. This will return the active screen to the Geometric Data screen. Need to save the geometric data. This will return the introductory (project organization) screen. Want to enter the conditions necessary to perform the steady-state flow example. Click on steady-state flow button. 10

11 The Steady-Flow Data screen will appear. Enter the data (enter number of profiles and the Q values). Select the button Reach Boundary Conditions. The following screen will appear. Click on desired boundary condition. Example will use Normal Depth. 11

12 Normal Depth requires entry of downstream slope at outlet. Use same slope as channel from Stations 2 to 1. Once boundary conditions have been entered, save the flow data and click OK. After saving, return to the introductory screen. The names of the data files for the Project, Geometry and Steady Flow should be showing. After saving, return to the introductory screen. The names of the data files for the Project, Geometry and Steady Flow should be showing. 12

13 Click on the Perform a Steady Flow Simulation button. Select New Plan. Fill in the Plan Name and Short ID. The screen now should look like this. Click on COMPUTE to run the simulation. 13

14 This screen will appear when the simulation is complete. Select View Cross-Sections. Select View Profiles. 14

15 Select View 3D Multiple Cross-Section Plot. Select Profile Table Output button. 15

16 Use Options to show both profiles. Use Options to show detailed cross-section output. Connecting HMS and RAS Discharge at a Particular Cross-Section 16

17 HEC-RAS: Output Graphical Text File HEC-RAS: Data Translation Data translation from HEC-RAS text file to dbase table Bank and floodplain boundaries measured from stream centerline Brays Bayou-Typical Urban System The Floodplain Bridges cause unique problems in hydraulics Piers, low chords, and top of road is considered Expansion/contraction can cause hydraulic losses Several cross sections are needed for a bridge Critical in urban settings 288 Crossing Top Width 17

18 Floodplain Determination The Woodlands The Woodlands planners wanted to design the community to withstand a 100-year storm. In doing this, they would attempt to minimize any changes to the existing, undeveloped floodplain as development proceeded through time. HEC RAS Cross Section 3-D Floodplain 18