FLOODPLAIN MODELING USING HEC-RAS

Transcription

1 H A E S T A D M E T H O D S FLOODPLAIN MODELING USING HEC-RAS F i r s t E d i t i o n Authors Haestad Methods Gary Dyhouse Jennifer Hatchett Jeremy Benn Managing Editor Colleen Totz Editors David Klotz, Adam Strafaci, Annaleis Hogan, and Kristen Dietrich Contributing Authors David Ford Consulting Houjung Rhee Peer Review Board Gary Brunner (U.S. Army Corps of Engineers) Donald V. Chase (University of Dayton) Jack Cook (Haestad Methods) Paul Debarry (Borton-Lawson Engineering) Johannes Gessler (Colorado State University) Robert Keller (RJ Keller and Associates) Robert Moore (Washington Infrastructure Services) Ezio Todini (University of Bologna, Italy) Thomas M. Walski (Haestad Methods) Michael Glazner (Haestad Methods) HAESTAD PRESS Waterbury,CTUSA

2 Table of Contents Preface Continuing Education Units About the Software xiii xix xxi Chapter 1 Introduction to Floodplain Modeling and Management A Brief History of Floodplain Management Floodplain Modeling Types of Floodplain Studies 7 Floodplain Studies 7 Transportation Facilities 9 Floodways/Encroachments 9 Structural Measures Chapter Summary 11 Chapter 2 Introduction to Open Channel Hydraulics Terminology 13 Depth of Flow 16 Channel Top Width and Wetted Perimeter 17 Hydraulic Depth and Hydraulic Radius 18 Discharge 18 Velocity 19 Slopes Flow Classification 23 Steady and Unsteady Flow 23 Uniform and Varied Flow 23 Gradually and Rapidly Varied Flow 25 Subcritical and Supercritical Flow 26

3 Table of Contents 2.3 Fundamental Equations 29 The Continuity Equation 29 The Energy Equation 29 The Momentum Equation 30 The Chezy and Manning Equations Energy and Momentum Concepts 37 Specific Energy and Alternate Depths 38 Critical Depth 40 Normal Depth 42 The Hydraulic Jump Profile Shapes 45 Governing Equations 45 Profile Classification Computational Methods 52 Direct Step Method 52 Standard Step Method Chapter Summary 69 Chapter 3 Hydraulic Modeling Tools Uniform Flow Gradually Varied, Steady Flow 76 HEC-2 77 HEC-RAS for Steady Flow 78 WSP-2 79 WSPRO(HY-7) Quasi-Unsteady Flow 80 HEC-1/HEC-HMS 80 TR20 81 PondPack Gradually Varied, Unsteady Flow (One-Dimensional) 81 HEC-UNET 83 HEC-RAS, Unsteady Flow 84 FLDWAV 85 FEQ Gradually Varied, Unsteady Flow (Two-Dimensional) 87 RMA2 88 FESWMS-2DH Gradually Varied, Unsteady Flow (Three-Dimensional) 90 RMA Sediment Models 90 HEC-6 92 SED2D 92

4 Ill 3.8 Physical Models Selecting a Simulation Program Chapter Summary 95 Chapter 4 Planning for Floodplain Modeling Studies Ten Steps of Floodplain Modeling 98 Step 1: Setting Project and Study Objectives 99 Step 2: Study Phases 100 Step 3: Field Reconnaissance 101 Step 4: Determining the Type of Hydrologic/Hydraulic Simulation Needed 103 Step 5: Determining Data Needs 104 Step 6: Defining Hydrologic Modeling Procedures 106 Step 7: Performing Data Input and Calibration 107 Step 8: Performing Production Runs for Base Conditions Step 9: Performing Project Evaluations 107 Step 10: Preparing the Report Chapter Summary 109 Chapter 5 Data Needs, Availability, and Development Data Sources 111 Stream Gage Data Ill Previous Studies 112 Mapping and Aerial Photos Study Limits and Boundary Determinations 114 Hydraulic Boundaries 114 Sediment Boundaries Geometric Data 121 Assessing Existing Topographic Data 121 Aerial Photographs and Site Visits 121 Locating and Modeling Cross Sections 122 Cross-Section Modeling Information 126 Geometric Data for Obstructions 129 Reach Length Information 130 Survey Data Accuracy Discharge Data 135 Previous Study Information 136 Gage Data 136 Statistical Analysis 138

5 iv Table of Contents Regional Analysis 139 Watershed Modeling Roughness Data 144 Estimation of Manning's n 144 Other Techniques to Estimate n Other Data 156 Contraction/Expansion Coefficients 156 Sediment Data 157 Future Changes Routing Data Calibration and Verification Needs 159 Calibration Data Chapter Summary 163 Chapter 6 Bridge Modeling The Effects of a Bridge on Water Flow Low Flow Through Bridges 170 Equations for Low Flow 170 Class A Low Flow 175 Class B Low Flow 175 Class C Low Flow High Flow Through Bridges 176 The Bridge as a Sluice Gate 177 The Bridge as an Orifice 178 The Bridge as a Weir 179 Combination Flow Defining Bridge Cross Sections and Coefficients 183 Cross-Section Location Techniques 183 Loss Coefficients for Flow Through Bridges Ineffective Flow Areas 199 Ineffective Flow Area Elevations 202 Ineffective Flow Area Locations Modeling the Bridge Structure with HEC-RAS 206 Bridge Superstructure 208 Bridge Piers 209 Sloping Bridge Abutments 211 Use of the Bridge Design Editor 211 Bridge Computation Methods Special Situations 215 Multiple Openings 215

6 Parallel Bridges 217 Perched Bridges 217 Low Water Bridges 218 Bridges on Skew 220 The Bridge as a Dam WSPRO Bridge Modeling 223 WSPRO Modeling Procedures 223 WSPRO Computation Procedures Chapter Summary 228 Chapter 7 Culvert Modeling Terminology Effects of a Culvert Culvert Hydraulics - Inlet/Outlet Control 237 Inlet Control 237 Outlet Control Inlet Control Computations Outlet Control Computations Defining Cross-Section Locations and Coefficients 253 Section Location 253 Coefficients 255 Adjustments to Bounding Cross Sections 2 and Culvert Modeling Using HEC-RAS 257 Roadway Geometry 258 Inlet Control Data 260 Outlet Control Data Special Culvert Modeling Issues 261 Flow Attenuation 261 Sediment and Debris 265 Scour at Culvert Outlets 269 Changing Culvert Shape 271 Changing Discharge within a Culvert 272 Changing Materials within a Culvert 273 Drop Culvert 274 Fish Passage 274 Replacing Bridges with Culverts Chapter Summary 277

7 vi Table of Contents Chapter 8 Data Review, Calibration, and Results Analysis Input Data Checking 283 Checks Performed by the Modeler 284 Checks Performed by HEC-RAS Analyzing HEC-RAS Output 285 Program Checks 285 Graphical Output Review 288 Tabular Output Review 291 Mixed Flow Analysis Adjusting HEC-RAS Input 295 Changing Station ID 295 Cross Section Points Filter 296 Reverse Stationing 296 Cross-Section Interpolation Calibration Procedures 297 Adopting the Working Model 298 Comparing Model Output to Actual Data 298 Adjustments to Model Parameters 298 Verification 300 Sensitivity Tests Production Runs 301 Large Changes of Key Parameters 302 Constraint Elevations and Ineffective Flow Areas Developing Hydrologic Routing Data 303 Routing Reaches 303 Storage-Outflow Values 304 Wave Travel Time 308 Reach Routing Steps 310 Modifications to Routing Data Chapter Summary 311 Chapter 9 The U.S. National Flood Insurance Program The U.S. National Flood Insurance Program Terminology and Concepts 319 Special Flood Hazard Area 319 Floodway 319 Flood Surcharge 319 Floodway Fringe 321

8 VII 9.3 Publications Used in the NFIP 322 Flood Hazard Boundary Map (FHBM) 322 Flood Insurance Rate Map (FIRM) 322 Flood Insurance Study (FIS) Criteria for Land Management and Use Revising Flood Studies and Maps 331 Identification and Mapping of Special Flood Hazard Areas331 Revisions and Amendments 332 CLOMRs - Review of Proposed Projects Revision Submittal Steps 341 Step 1 - Obtain FIS, FIRMs, and Backup Data 341 Step 2 - Revise Hydraulic Models 341 Step 3 - Annotation of FIRMs, FIS, and Topographic Map. 343 Step 4 - Fill Out MT-2 Forms 343 Step 5 - Submit to FEMA 343 Step 6 - Wait for a Response 344 Step 7 - Receive Letter or Request for Additional Data FEMA Review Software 345 CHECK CHECK-RAS Chapter Summary 346 Chapter 10 Floodway Modeling Methods of Performing an Encroachment Analysis 350 Method 1: Specify Encroachment Stations 351 Method 2: Specify Floodway Top Width 351 Method 3: Specify Percent Conveyance Reduction 352 Method 4: Specify Target Surcharge with Equal Conveyance Reduction 353 Method 5: Optimization with Two Targets Developing a Floodway in HEC-RAS 354 Establishing Base Conditions 355 Creating a Steady Flow Data File 355 Downstream Boundary Conditions 355 Global Options 356 Reach Options 357 River Station Options 357 Computing the Floodway Plan Reviewing the Results 358 Additional Runs/Methods 359 Finalizing the Floodway with Method Guidance for Correcting Excessive or Negative Surcharge 361

9 viii Table of Contents 10.4 Reviewing and Modifying Encroachment Output 362 Encroachment Tables 362 Graphics 363 Key Considerations 363 Levee Requirements for FEMA Certification Adopting the Floodway 367 Satisfying Community Needs 369 Mapping the Floodway 369 Enforcing the Floodway Working With an Existing Floodway 372 Placing Obstructions in the Floodway 372 Changes to a Floodway Chapter Summary 373 Chapter 11 Channel Modification Channel Stability 377 A Stream in Equilibrium 378 A Nonequilibrium Condition Urbanization 381 A Nonequilibrium Condition Channelization 381 Developing a Stable Channel Modification 384 Environmental Issues 385 Positive Effects of Channelization Channel Modification Methods 387 Levees 387 High-Flow Diversion Channel and Weir 387 High-Flow Cutoff/Diversion Channel 389 Clearing and Snagging 390 Compound Channels 391 Clearing and Enlarging One Side of the Channel 393 Widening the Upper Channel and Using the Original Channel for Low Flow 393 Realigning the Channel 394 Constructing a Paved Channel 394 New Channel 394 Channel Rehabilitation 396 Permitting Requirements Channel Design Considerations 400 Flow Regime/Mixed Flow 400 Air Entrainment 401 Linings 402 Freeboard 403 Channel Transitions 404 Junctions 405

10 IX Channel Protection 406 Low Flow Channel 408 Superelevation 408 Curved Channels 410 Drop Structures/Stabilizers 410 Debris Basins 412 Bridge Piers HEC-RAS Input Data for Channel Modifications 414 Study Watershed/Channel Boundaries 414 Channel Modification Features 414 HEC-RAS Channel Improvement Template Stable Channel Design Using HEC-RAS 417 Uniform Flow Analysis 417 Stable Channel Design 421 Design Parameters Analyzing Results 429 Velocity 430 Energy Grade Line Slope 430 Top Width 430 Sensitivity of Manning's n 430 Sensitivity of Scour/Sediment Deposition on the Design Profile 430 Channel Effects Outside of a Modified Reach 431 Effects on Hydrographs 431 Plan Comparisons Channel Maintenance Requirements Chapter Summary 433 Chapter 12 Advanced Floodplain Modeling Levees 437 Levee Characteristics 437 Modeling Procedures Modeling Obstructions 451 Without Storage Considerations 452 With Storage Considerations Modeling Tributaries and Junctions 456 Cross-Section Locations 457 Computing Losses and Water Surface Elevations through a Junction Inline Gates and Weirs 459 Types of Weirs and Gated Openings 460 Governing Equations 461

11 Table of Contents Modeling Procedures 465 Output Analysis Drop Structures 470 Modeling of Drop Structure as an Inline Weir 471 Modeling of Drop Structure Using Cross Sections Split Flow 473 Split Flow Situations 473 Computational Procedures 476 Modeling Procedures for Separate Channel Splits 476 Modeling Procedures for Lateral Weirs Ice Cover and Ice Jam Flood Modeling 483 Effects on Water Surface Elevations 483 Data Requirements for Ice Analysis 484 Ice Modeling Procedures with HEC-RAS 487 Output Review 488 Ice Modeling Assistance Chapter Summary 489 Chapter 13 Mobile Boundary Situations and Bridge Scour Mobile Boundary Analysis Types of Mobile Boundary Analyses 502 Base Conditions 502 Reservoir Projects 503 Channel Modification Projects 506 Levee Projects 506 Diversion Projects 507 Channel Stability and Protection Bridge Scour 509 Key References 510 Types of Scour Bridge Scour Computational Procedures 519 Initial Preparation 519 General Bridge Scour Analysis Procedures 520 Contraction Scour 522 Pier Scour 527 Abutment Scour Computing Scour with HEC-RAS 545 Applying the Flow Distribution Option 545 Bridge Scour Data 546 Total Scour 548

12 XI 13.6 Cautions and Concerns for Bridge Scour Sediment Discharge Relationships 551 Sediment Transport Equations 554 Cautions in Applying Sediment Transport Equations 556 Applying the Equations with HEC-RAS Chapter Summary 560 Chapter 14 Unsteady Flow Modeling Why Use an Unsteady Flow Model? 564 Attenuation 564 Flow restrictions 568 Looped ratings 568 Flow Splits 569 Time-Based (Transient) Effects Unsteady Flow Theory 570 St. Venant Equations 571 Steady-State Approximation 576 Level-Pool Routing 577 Kinematic Wave Approximation 578 Diffusion Wave Approximation 578 Theoretical Applicability of Various Approximations Solution of Equations 580 Solving the Diffusion Wave Equation 580 Solving the Full St. Venant Equations Practical Choice of Unsteady Modeling Approach 590 Routing Models ". 590 Hydrodynamic Modeling 593 Hybrid Approach 602 Troubleshooting Models Unsteady Flow Modeling Using HEC-RAS 604 Geometric Data Entry and Preprocessor 604 Modeling Floodplain Geometry 610 Unsteady Flow Data Editor 612 Unsteady Flow Analysis 617 Unsteady Flow Simulation Results 620 Other Features in HEC-RAS Unsteady Flow Simulation Chapter Summary 632

13 xii Table of Contents Chapter 15 Importing and Exporting Files with HEC-RAS Imported File Types 639 HEC-2 Files 640 HEC-RAS Files 640 UNET Files 640 Corps Survey Data Files 641 GIS/CADD Files 641 DSS Files 642 Spreadsheet and Text Files Exporting Files 645 DSS Files 645 GIS/CADD Files Using HEC-2 Files with HEC-RAS 649 Importing HEC-2 Files 649 Data Not Imported Program Differences and Review of Imported Data 651 Program Differences 651 Comparing HEC-RAS and HEC-2 Output Chapter Summary 658 Bibliography 659