Prepared for CIVE 401 Hydraulic Engineering By Kennard Lai, Patrick Ndolo Goy & Dr. Pierre Julien Fall 2015

Transcription

1 Prepared for CIVE 401 Hydraulic Engineering By Kennard Lai, Patrick Ndolo Goy & Dr. Pierre Julien Fall 2015

2 Contents Introduction General Philosophy Overview of Capabilities Applications Computational Procedure Quick Start Guide Examples Advantages & Disadvantages Resources References

3 Introduction Hydrologic Engineering Center River Analysis System Developed by US Army Corps of Engineers (USACE) Previously HEC-2 (1990) Latest version 4.1 (2010) Free Download at:

4 General Philosophy of the Modeling System Integrated Software Package includes: - Graphical User Interface (GUI) - Analysis Components - Data Storage & Management - Graphics - Report Generation Using a common geometric data representation and common geometric and hydraulic computation routine

5 Overview of Capabilities One-Dimensional (1D) River Analysis: 1. Steady Flow Water Surface Profile Computations 2. Unsteady Flow Simulation 3. Movable Boundary Sediment Transport Computations 4. Water Quality Analysis and Several Hydraulic Design Features

6 Applications Bridge/Culvert Modeling Inline Structures (Dams, Weirs and Gated Spillways) Lateral Structures (Weirs, Spillways) Flood Management (Floodplain encroachment, Flood Insurance Studies) Sediment Transport & Management Water Quality Analysis (Temperature, Algae, DO, BOD, etc.) Various Flow Analysis

7 Computational Procedures Steady Gradually Varied Flow Water Surface Profile Calculations Basic Equations: 1D Energy Equation

8 Computational Procedures Conveyance Calculation: From Manning Equation 2 Q = 1 R 1 n h 3AS f 2 (SI) 2 K = 1 R n h 3A 1 Q = KS f 2 Where: Q = flow n = Manning s coefficient A = cross sectional area R H = hydraulic radius S f = friction slope K = conveyance

9 Computational Procedures Energy Loss, h e : > Friction - Manning Equation > Contraction/Expansion loss coefficient h e = LSf + C a 1V 1 2 a 2 2V 2 2g 2g (SI) Where: L = reach length A = cross sectional area C = contraction/expansion coefficient a = velocity weighing coefficient S f = friction slope h e = head loss g = gravity V = flow velocity

10 Computational Procedures HEC-RAS utilizes the Standard Step Method: Iterative computation of water surface elevation between two known cross sections Steady flow Does not require uniform cross-section or channel conditions Requires known channel cross sections along reach Computation of energy equation section by section Control Section: > subcritical flow: evaluate from downstream to upstream > supercritical flow: evaluate from upstream to downstream

11 Standard Step Procedure: Computational Procedures 1. Assume initial water surface (WS) elevation at 2 2. Based on assumed WS elevation, determine the corresponding total conveyance and velocity head 3. With values from step 2, compute S f and solve for h e 4. With values from steps 2 and 3, solve 1D energy equation for new WS elevation at 2 5. Compare WS elevations of step 1 and step 4. Iterate until values agree to within 0.1 feet (0.003 m) or to user-defined tolerance

12 Computational Procedures The program is constrained by a maximum number of iterations (default = 20) If max iterations is reach before balanced WS is achieved, then calculate critical depth Then check if error of minimum error WS is within predefined tolerance If within predefined error tolerance and at the correct side of critical depth, then minimum error WS will be used and set a warning message Or else critical depth will be used and set a warning message While both are not valid solutions, minimum error WS is better than critical depth Common sources of error: > Inadequate number of cross sections (reach length too long) > Bad cross section data > Wrong boundary condition specified (subcritical/supercritical/critical)

13 For Steady Flow Simulation: Basic Data Requirements 1. Geometric Data: > River System Schematic > Cross Section Geometry > Reach Length > Hydraulic Structures/Obstructions 2. Energy Loss Data: > Manning n > Contraction/Expansion Coefficients 3. Flow Data: > Flow Rate > Profiles > Boundary Conditions Quick Start Guide

14 HEC-RAS Main Window Quick Start Guide

15 Quick Start Guide 1. Start HEC-RAS 2. Click File > New Project 3. Select drive and path to store project in 4. Enter project title, ensure the project File Name has the extension.prj 5. Click OK

16 6. Enter Geometric Data i) River system schematic > Draw from upstream to downstream Quick Start Guide > River Station number increases upstream

17 Quick Start Guide ii) Cross Section Data > Cross Section is looking downstream > Station (different from River Station) is from left to right > To add new cross sectional data: click Options > Add a new Cross Section

18 ii) Cross Section Data > Manning n > Contraction/Expansion Coefficient Quick Start Guide

19 Quick Start Guide iii) Hydraulic Structures: Bridge, Culverts, Dams, Weirs, etc

20 Quick Start Guide 7. Enter Steady Flow Data i) Profiles: > Enter number of profiles > Name each profile e.g. 25 yr, 100 yr, May 74 flood etc. ii) flow data > Enter flow data for each River Station > It is assumed flow remains constant until another flow value is encountered on next River Station

21 Quick Start Guide iii) Boundary Conditions > Necessary to establish the starting water surface at the ends of the river system > Subcritical flow > boundary conditions required at downstream end > Supercritical flow > boundary conditions required at upstream end > Mixed flow > boundary conditions required at all open ends of river system > Four types of boundary conditions: > Known Water Surface Elevation > Critical Depth > Normal Depth > Rating Curve

22 Quick Start Guide 8. Perform Steady Flow Simulation > Click File > New Plan > Enter plan name and Short ID > Select Flow Regime

23 Quick Start Guide 9. View Results: > Cross Section Plots > Profile Plots > General Profile Plot > Rating Curves > X-Y-Z Perspective Plots > Tabular Results

24 i) Cross Section Plot Quick Start Guide

25 ii) Profile (WSP) Plot Quick Start Guide

26 Quick Start Guide iii) General Profile (WSP) Plot

27 Quick Start Guide iv) Rating Curves (Water Surface Elevation vs Discharge):

28 v) X-Y-Z Perspective Plots (3D): Quick Start Guide

29 vi) Tabular Results Quick Start Guide

30 Examples HEC-RAS GIS Output for Floodplain Mapping

31 Bridge Scour Analysis Examples Ice Covered River Analysis

32 Advantages vs Disadvantages Advantages: FREE! Simple to Use Accepted by most government and private agencies Extensive Support by USACE Add-on packages available Disadvantages: Numerical instability during unsteady analyses Problems modeling highly dynamic rivers and streams 1D modeling is less detailed

33 Resources HEC-RAS User s Manual HEC-RAS Reference Manual HEC-RAS Applications Guide Available at:

34 References Djokic and Maidment Introduction to HEC-RAS and Floodplain Mapping Ndolo Goy and Julien, CIVE 401 HEC-RAS Presentation. Available at: US Army Corps of Engineers. HEC-RAS

35 Thank You! Questions?