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H A E S T A D M E T H O D S
FLOODPLAIN MODELINGUSING HEC-RAS
F i r s t E d i t i o n
AuthorsHaestad Methods
Gary DyhouseJennifer Hatchett
Jeremy Benn
Managing EditorColleen Totz
EditorsDavid Klotz, Adam Strafaci,
Annaleis Hogan, and Kristen Dietrich
Contributing AuthorsDavid Ford Consulting
Houjung Rhee
Peer Review BoardGary 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 PRESSWaterbury,CTUSA
Table of Contents
Preface xiii
Continuing Education Units xix
About the Software xxi
Chapter 1 Introduction to Floodplain Modeling andManagement 1
1.1 A Brief History of Floodplain Management 1
1.2 Floodplain Modeling 6
1.3 Types of Floodplain Studies 7Floodplain Studies 7Transportation Facilities 9Floodways/Encroachments 9Structural Measures 10
1.4 Chapter Summary 11
Chapter 2 Introduction to Open Channel Hydraulics 13
2.1 Terminology 13Depth of Flow 16Channel Top Width and Wetted Perimeter 17Hydraulic Depth and Hydraulic Radius 18Discharge 18Velocity 19Slopes 22
2.2 Flow Classification 23Steady and Unsteady Flow 23Uniform and Varied Flow 23Gradually and Rapidly Varied Flow 25Subcritical and Supercritical Flow 26
Table of Contents
2.3 Fundamental Equations 29The Continuity Equation 29The Energy Equation 29The Momentum Equation 30The Chezy and Manning Equations 35
2.4 Energy and Momentum Concepts 37Specific Energy and Alternate Depths 38Critical Depth 40Normal Depth 42The Hydraulic Jump 43
2.5 Profile Shapes 45Governing Equations 45Profile Classification 46
2.6 Computational Methods 52Direct Step Method 52Standard Step Method 56
2.7 Chapter Summary 69
Chapter 3 Hydraulic Modeling Tools 753.1 Uniform Flow 76
3.2 Gradually Varied, Steady Flow 76HEC-2 77HEC-RAS for Steady Flow 78WSP-2 79WSPRO(HY-7) 80
3.3 Quasi-Unsteady Flow 80HEC-1/HEC-HMS 80TR20 81PondPack 81
3.4 Gradually Varied, Unsteady Flow (One-Dimensional) 81HEC-UNET 83HEC-RAS, Unsteady Flow 84FLDWAV 85FEQ 87
3.5 Gradually Varied, Unsteady Flow (Two-Dimensional) 87RMA2 88FESWMS-2DH 90
3.6 Gradually Varied, Unsteady Flow (Three-Dimensional) 90RMA10 90
3.7 Sediment Models 90HEC-6 92SED2D 92
Ill
3.8 Physical Models 93
3.9 Selecting a Simulation Program 94
3.10 Chapter Summary 95
Chapter 4 Planning for Floodplain Modeling Studies 97
4.1 Ten Steps of Floodplain Modeling 98Step 1: Setting Project and Study Objectives 99Step 2: Study Phases 100Step 3: Field Reconnaissance 101Step 4: Determining the Type of Hydrologic/Hydraulic
Simulation Needed 103Step 5: Determining Data Needs 104Step 6: Defining Hydrologic Modeling Procedures 106Step 7: Performing Data Input and Calibration 107Step 8: Performing Production Runs for Base Conditions... 107Step 9: Performing Project Evaluations 107Step 10: Preparing the Report 108
4.2 Chapter Summary 109
Chapter 5 Data Needs, Availability, andDevelopment 111
5.1 Data Sources 111Stream Gage Data I l lPrevious Studies 112Mapping and Aerial Photos 113
5.2 Study Limits and Boundary Determinations 114Hydraulic Boundaries 114Sediment Boundaries 120
5.3 Geometric Data 121Assessing Existing Topographic Data 121Aerial Photographs and Site Visits 121Locating and Modeling Cross Sections 122Cross-Section Modeling Information 126Geometric Data for Obstructions 129Reach Length Information 130Survey Data Accuracy 130
5.4 Discharge Data 135Previous Study Information 136Gage Data 136Statistical Analysis 138
iv Table of Contents
Regional Analysis 139Watershed Modeling 142
5.5 Roughness Data 144Estimation of Manning's n 144Other Techniques to Estimate n 155
5.6 Other Data 156Contraction/Expansion Coefficients 156Sediment Data 157Future Changes 158
5.7 Routing Data 158
5.8 Calibration and Verification Needs 159Calibration Data 159
5.9 Chapter Summary 163
Chapter 6 Bridge Modeling 1676.1 The Effects of a Bridge on Water Flow 167
6.2 Low Flow Through Bridges 170Equations for Low Flow 170Class A Low Flow 175Class B Low Flow 175Class C Low Flow 176
6.3 High Flow Through Bridges 176The Bridge as a Sluice Gate 177The Bridge as an Orifice 178The Bridge as a Weir 179Combination Flow 182
6.4 Defining Bridge Cross Sections and Coefficients 183Cross-Section Location Techniques 183Loss Coefficients for Flow Through Bridges 194
6.5 Ineffective Flow Areas 199Ineffective Flow Area Elevations 202Ineffective Flow Area Locations 204
6.6 Modeling the Bridge Structure with HEC-RAS 206Bridge Superstructure 208Bridge Piers 209Sloping Bridge Abutments 211Use of the Bridge Design Editor 211Bridge Computation Methods 213
6.7 Special Situations 215Multiple Openings 215
Parallel Bridges 217Perched Bridges 217Low Water Bridges 218Bridges on Skew 220The Bridge as a Dam 221
6.8 WSPRO Bridge Modeling 223WSPRO Modeling Procedures 223WSPRO Computation Procedures 226
6.9 Chapter Summary 228
Chapter 7 Culvert Modeling 2337.1 Terminology 233
7.2 Effects of a Culvert 236
7.3 Culvert Hydraulics - Inlet/Outlet Control 237Inlet Control 237
Outlet Control 240
7.4 Inlet Control Computations 245
7.5 Outlet Control Computations 248
7.6 Defining Cross-Section Locations and Coefficients 253Section Location 253Coefficients 255Adjustments to Bounding Cross Sections 2 and 3 256
7.7 Culvert Modeling Using HEC-RAS 257Roadway Geometry 258Inlet Control Data 260Outlet Control Data 261
7.8 Special Culvert Modeling Issues 261Flow Attenuation 261Sediment and Debris 265Scour at Culvert Outlets 269Changing Culvert Shape 271Changing Discharge within a Culvert 272Changing Materials within a Culvert 273Drop Culvert 274Fish Passage 274Replacing Bridges with Culverts 276
7.9 Chapter Summary 277
vi Table of Contents
Chapter 8 Data Review, Calibration, and ResultsAnalysis 283
8.1 Input Data Checking 283Checks Performed by the Modeler 284Checks Performed by HEC-RAS 284
8.2 Analyzing HEC-RAS Output 285Program Checks 285Graphical Output Review 288Tabular Output Review 291Mixed Flow Analysis 294
8.3 Adjusting HEC-RAS Input 295Changing Station ID 295Cross Section Points Filter 296Reverse Stationing 296Cross-Section Interpolation 296
8.4 Calibration Procedures 297Adopting the Working Model 298Comparing Model Output to Actual Data 298Adjustments to Model Parameters 298Verification 300Sensitivity Tests 301
8.5 Production Runs 301Large Changes of Key Parameters 302Constraint Elevations and Ineffective Flow Areas 302
8.6 Developing Hydrologic Routing Data 303Routing Reaches 303Storage-Outflow Values 304Wave Travel Time 308Reach Routing Steps 310Modifications to Routing Data 310
8.7 Chapter Summary 311
Chapter 9 The U.S. National Flood InsuranceProgram 317
9.1 The U.S. National Flood Insurance Program 317
9.2 Terminology and Concepts 319Special Flood Hazard Area 319Floodway 319Flood Surcharge 319Floodway Fringe 321
VII
9.3 Publications Used in the NFIP 322Flood Hazard Boundary Map (FHBM) 322Flood Insurance Rate Map (FIRM) 322Flood Insurance Study (FIS) 326
9.4 Criteria for Land Management and Use 330
9.5 Revising Flood Studies and Maps 331Identification and Mapping of Special Flood Hazard Areas331Revisions and Amendments 332CLOMRs - Review of Proposed Projects 339
9.6 Revision Submittal Steps 341Step 1 - Obtain FIS, FIRMs, and Backup Data 341Step 2 - Revise Hydraulic Models 341Step 3 - Annotation of FIRMs, FIS, and Topographic Map. 343Step 4 - Fill Out MT-2 Forms 343Step 5 - Submit to FEMA 343Step 6 - Wait for a Response 344Step 7 - Receive Letter or Request for Additional Data 344
9.7 FEMA Review Software 345CHECK-2 345CHECK-RAS 345
9.8 Chapter Summary 346
Chapter 10 Floodway Modeling 34910.1 Methods of Performing an Encroachment Analysis 350
Method 1: Specify Encroachment Stations 351Method 2: Specify Floodway Top Width 351Method 3: Specify Percent Conveyance Reduction 352Method 4: Specify Target Surcharge with Equal
Conveyance Reduction 353Method 5: Optimization with Two Targets 354
10.2 Developing a Floodway in HEC-RAS 354Establishing Base Conditions 355Creating a Steady Flow Data File 355Downstream Boundary Conditions 355Global Options 356Reach Options 357River Station Options 357Computing the Floodway Plan 358
10.3 Reviewing the Results 358Additional Runs/Methods 359Finalizing the Floodway with Method 1 360Guidance for Correcting Excessive or Negative
Surcharge 361
viii Table of Contents
10.4 Reviewing and Modifying Encroachment Output 362Encroachment Tables 362Graphics 363Key Considerations 363Levee Requirements for FEMA Certification 365
10.5 Adopting the Floodway 367Satisfying Community Needs 369Mapping the Floodway 369Enforcing the Floodway 371
10.6 Working With an Existing Floodway 372Placing Obstructions in the Floodway 372Changes to a Floodway 373
10.7 Chapter Summary 373
Chapter 11 Channel Modification 37711.1 Channel Stability 377
A Stream in Equilibrium 378A Nonequilibrium Condition—Urbanization 381A Nonequilibrium Condition—Channelization 381Developing a Stable Channel Modification 384Environmental Issues 385Positive Effects of Channelization 386
11.2 Channel Modification Methods 387Levees 387High-Flow Diversion Channel and Weir 387High-Flow Cutoff/Diversion Channel 389Clearing and Snagging 390Compound Channels 391Clearing and Enlarging One Side of the Channel 393Widening the Upper Channel and Using the Original
Channel for Low Flow 393Realigning the Channel 394Constructing a Paved Channel 394New Channel 394Channel Rehabilitation 396Permitting Requirements 399
11.3 Channel Design Considerations 400Flow Regime/Mixed Flow 400Air Entrainment 401Linings 402Freeboard 403Channel Transitions 404Junctions 405
IX
Channel Protection 406Low Flow Channel 408Superelevation 408Curved Channels 410Drop Structures/Stabilizers 410Debris Basins 412Bridge Piers 413
11.4 HEC-RAS Input Data for Channel Modifications 414Study Watershed/Channel Boundaries 414Channel Modification Features 414HEC-RAS Channel Improvement Template 414
11.5 Stable Channel Design Using HEC-RAS 417Uniform Flow Analysis 417Stable Channel Design 421Design Parameters 428
11.6 Analyzing Results 429Velocity 430Energy Grade Line Slope 430Top Width 430Sensitivity of Manning's n 430Sensitivity of Scour/Sediment Deposition on the
Design Profile 430Channel Effects Outside of a Modified Reach 431Effects on Hydrographs 431Plan Comparisons 431
11.7 Channel Maintenance Requirements 433
11.8 Chapter Summary 433
Chapter 12 Advanced Floodplain Modeling 437
12.1 Levees 437Levee Characteristics 437Modeling Procedures 444
12.2 Modeling Obstructions 451Without Storage Considerations 452With Storage Considerations 453
12.3 Modeling Tributaries and Junctions 456Cross-Section Locations 457Computing Losses and Water Surface Elevations
through a Junction 457
12.4 Inline Gates and Weirs 459Types of Weirs and Gated Openings 460Governing Equations 461
Table of Contents
Modeling Procedures 465Output Analysis 469
12.5 Drop Structures 470Modeling of Drop Structure as an Inline Weir 471Modeling of Drop Structure Using Cross Sections 471
12.6 Split Flow 473Split Flow Situations 473Computational Procedures 476Modeling Procedures for Separate Channel Splits 476Modeling Procedures for Lateral Weirs 477
12.7 Ice Cover and Ice Jam Flood Modeling 483Effects on Water Surface Elevations 483Data Requirements for Ice Analysis 484Ice Modeling Procedures with HEC-RAS 487Output Review 488Ice Modeling Assistance 488
12.8 Chapter Summary 489
Chapter 13 Mobile Boundary Situations andBridge Scour 501
13.1 Mobile Boundary Analysis 501
13.2 Types of Mobile Boundary Analyses 502Base Conditions 502Reservoir Projects 503Channel Modification Projects 506Levee Projects 506Diversion Projects 507Channel Stability and Protection 509
13.3 Bridge Scour 509Key References 510Types of Scour 511
13.4 Bridge Scour Computational Procedures 519Initial Preparation 519General Bridge Scour Analysis Procedures 520Contraction Scour 522Pier Scour 527Abutment Scour 538
13.5 Computing Scour with HEC-RAS 545Applying the Flow Distribution Option 545Bridge Scour Data 546Total Scour 548
XI
13.6 Cautions and Concerns for Bridge Scour 550
13.7 Sediment Discharge Relationships 551Sediment Transport Equations 554Cautions in Applying Sediment Transport Equations 556Applying the Equations with HEC-RAS 558
13.8 Chapter Summary 560
Chapter 14 Unsteady Flow Modeling 56314.1 Why Use an Unsteady Flow Model? 564
Attenuation 564Flow restrictions 568Looped ratings 568Flow Splits 569Time-Based (Transient) Effects 570
14.2 Unsteady Flow Theory 570St. Venant Equations 571Steady-State Approximation 576Level-Pool Routing 577Kinematic Wave Approximation 578Diffusion Wave Approximation 578Theoretical Applicability of Various Approximations 579
14.3 Solution of Equations 580Solving the Diffusion Wave Equation 580Solving the Full St. Venant Equations 587
14.4 Practical Choice of Unsteady Modeling Approach 590Routing Models ". 590Hydrodynamic Modeling 593Hybrid Approach 602Troubleshooting Models 603
14.5 Unsteady Flow Modeling Using HEC-RAS 604Geometric Data Entry and Preprocessor 604Modeling Floodplain Geometry 610Unsteady Flow Data Editor 612Unsteady Flow Analysis 617Unsteady Flow Simulation Results 620Other Features in HEC-RAS Unsteady Flow Simulation.... 626
14.6 Chapter Summary 632
xii Table of Contents
Chapter 15 Importing and Exporting Fileswith HEC-RAS 639
15.1 Imported File Types 639HEC-2 Files 640HEC-RAS Files 640UNET Files 640Corps Survey Data Files 641GIS/CADD Files 641DSS Files 642Spreadsheet and Text Files 644
15.2 Exporting Files 645DSS Files 645GIS/CADD Files 648
15.3 Using HEC-2 Files with HEC-RAS 649Importing HEC-2 Files 649Data Not Imported 650
15.4 Program Differences and Review of Imported Data 651Program Differences 651Comparing HEC-RAS and HEC-2 Output 657
15.5 Chapter Summary 658
Bibliography 659
