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
Table of Contents
Preface
xiii
Continuing Education Units
xix
About the Software
xxi
Introduction to Floodplain Modeling and
Management
1
1.1
A Brief History of Floodplain Management
1
1.2
Floodplain Modeling
6
1.3
Types of Floodplain Studies
Floodplain Studies
Transportation Facilities
Floodways/Encroachments
Structural Measures
7
7
9
9
10
1.4
Chapter Summary
11
Introduction to Open Channel Hydraulics
13
2.1
Terminology
Depth of Flow
Channel Top Width and Wetted Perimeter
Hydraulic Depth and Hydraulic Radius
Discharge
Velocity
Slopes
13
16
17
18
18
19
22
2.2
Flow Classification
Steady and Unsteady Flow
Uniform and Varied Flow
Gradually and Rapidly Varied Flow
Subcritical and Supercritical Flow
23
23
23
25
26
Chapter 1
Chapter 2
Table of Contents
2.3
Fundamental Equations
The Continuity Equation
The Energy Equation
The Momentum Equation
The Chezy and Manning Equations
29
29
29
30
35
2.4
Energy and Momentum Concepts
Specific Energy and Alternate Depths
Critical Depth
Normal Depth
The Hydraulic Jump
37
38
40
42
43
2.5
Profile Shapes
Governing Equations
Profile Classification
45
45
46
2.6
Computational Methods
Direct Step Method
Standard Step Method
52
52
56
2.7
Chapter Summary
69
Hydraulic Modeling Tools
75
3.1
Uniform Flow
76
3.2
Gradually Varied, Steady Flow
HEC-2
HEC-RAS for Steady Flow
WSP-2
WSPRO(HY-7)
76
77
78
79
80
3.3
Quasi-Unsteady Flow
HEC-1/HEC-HMS
TR20
PondPack
80
80
81
81
3.4
Gradually Varied, Unsteady Flow (One-Dimensional)
HEC-UNET
HEC-RAS, Unsteady Flow
FLDWAV
FEQ
81
83
84
85
87
3.5
Gradually Varied, Unsteady Flow (Two-Dimensional)
RMA2
FESWMS-2DH
87
88
90
3.6
Gradually Varied, Unsteady Flow (Three-Dimensional)
RMA10
90
90
3.7
Sediment Models
HEC-6
SED2D
90
92
92
Chapter 3
Ill
3.8
Physical Models
93
3.9
Selecting a Simulation Program
94
3.10
Chapter Summary
95
Planning for Floodplain Modeling Studies
97
Chapter 4
4.1
Ten Steps of Floodplain Modeling
98
Step 1: Setting Project and Study Objectives
99
100
Step 2: Study Phases
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... 107
Step 9: Performing Project Evaluations
107
Step 10: Preparing the Report
108
4.2
Chapter Summary
Chapter 5
Data Needs, Availability, and
Development
109
111
5.1
Data Sources
Stream Gage Data
Previous Studies
Mapping and Aerial Photos
111
Ill
112
113
5.2
Study Limits and Boundary Determinations
Hydraulic Boundaries
Sediment Boundaries
114
114
120
5.3
Geometric Data
Assessing Existing Topographic Data
Aerial Photographs and Site Visits
Locating and Modeling Cross Sections
Cross-Section Modeling Information
Geometric Data for Obstructions
Reach Length Information
Survey Data Accuracy
121
121
121
122
126
129
130
130
5.4
Discharge Data
Previous Study Information
Gage Data
Statistical Analysis
135
136
136
138
iv
Table of Contents
Regional Analysis
Watershed Modeling
139
142
5.5
Roughness Data
Estimation of Manning's n
Other Techniques to Estimate n
144
144
155
5.6
Other Data
Contraction/Expansion Coefficients
Sediment Data
Future Changes
156
156
157
158
5.7
Routing Data
158
5.8
Calibration and Verification Needs
Calibration Data
159
159
5.9
Chapter Summary
163
Chapter 6
Bridge Modeling
167
6.1
The Effects of a Bridge on Water Flow
167
6.2
Low Flow Through Bridges
Equations for Low Flow
Class A Low Flow
Class B Low Flow
Class C Low Flow
170
170
175
175
176
6.3
High Flow Through Bridges
The Bridge as a Sluice Gate
The Bridge as an Orifice
The Bridge as a Weir
Combination Flow
176
177
178
179
182
6.4
Defining Bridge Cross Sections and Coefficients
Cross-Section Location Techniques
Loss Coefficients for Flow Through Bridges
183
183
194
6.5
Ineffective Flow Areas
Ineffective Flow Area Elevations
Ineffective Flow Area Locations
199
202
204
6.6
Modeling the Bridge Structure with HEC-RAS
Bridge Superstructure
Bridge Piers
Sloping Bridge Abutments
Use of the Bridge Design Editor
Bridge Computation Methods
206
208
209
211
211
213
6.7
Special Situations
Multiple Openings
215
215
Parallel Bridges
Perched Bridges
Low Water Bridges
Bridges on Skew
The Bridge as a Dam
217
217
218
220
221
6.8
WSPRO Bridge Modeling
WSPRO Modeling Procedures
WSPRO Computation Procedures
223
223
226
6.9
Chapter Summary
228
Chapter 7
Culvert Modeling
233
7.1
Terminology
233
7.2
Effects of a Culvert
236
7.3
Culvert Hydraulics - Inlet/Outlet Control
Inlet Control
237
237
Outlet Control
240
7.4
Inlet Control Computations
245
7.5
Outlet Control Computations
248
7.6
Defining Cross-Section Locations and Coefficients
Section Location
Coefficients
Adjustments to Bounding Cross Sections 2 and 3
Culvert Modeling Using HEC-RAS
Roadway Geometry
Inlet Control Data
Outlet Control Data
253
253
255
256
257
258
260
261
7.8
Special Culvert Modeling Issues
Flow Attenuation
Sediment and Debris
Scour at Culvert Outlets
Changing Culvert Shape
Changing Discharge within a Culvert
Changing Materials within a Culvert
Drop Culvert
Fish Passage
Replacing Bridges with Culverts
261
261
265
269
271
272
273
274
274
276
7.9
Chapter Summary
277
7.7
vi
Table of Contents
Chapter 8
Data Review, Calibration, and Results
Analysis
283
8.1
Input Data Checking
Checks Performed by the Modeler
Checks Performed by HEC-RAS
283
284
284
8.2
Analyzing HEC-RAS Output
Program Checks
Graphical Output Review
Tabular Output Review
Mixed Flow Analysis
285
285
288
291
294
8.3
Adjusting HEC-RAS Input
Changing Station ID
Cross Section Points Filter
Reverse Stationing
Cross-Section Interpolation
295
295
296
296
296
8.4
Calibration Procedures
Adopting the Working Model
Comparing Model Output to Actual Data
Adjustments to Model Parameters
Verification
Sensitivity Tests
297
298
298
298
300
301
8.5
Production Runs
Large Changes of Key Parameters
Constraint Elevations and Ineffective Flow Areas
301
302
302
8.6
Developing Hydrologic Routing Data
Routing Reaches
Storage-Outflow Values
Wave Travel Time
Reach Routing Steps
Modifications to Routing Data
303
303
304
308
310
310
8.7
Chapter Summary
311
Chapter 9
The U.S. National Flood Insurance
Program
317
9.1
The U.S. National Flood Insurance Program
317
9.2
Terminology and Concepts
Special Flood Hazard Area
Floodway
Flood Surcharge
Floodway Fringe
319
319
319
319
321
VII
9.3
Publications Used in the NFIP
Flood Hazard Boundary Map (FHBM)
Flood Insurance Rate Map (FIRM)
Flood Insurance Study (FIS)
322
322
322
326
9.4
Criteria for Land Management and Use
330
9.5
Revising Flood Studies and Maps
331
Identification and Mapping of Special Flood Hazard Areas331
Revisions and Amendments
332
CLOMRs - Review of Proposed Projects
339
9.6
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
344
9.7
FEMA Review Software
CHECK-2
CHECK-RAS
345
345
345
9.8
Chapter Summary
346
Chapter 10
Floodway Modeling
10.1
349
Methods of Performing an Encroachment Analysis
Method 1: Specify Encroachment Stations
Method 2: Specify Floodway Top Width
Method 3: Specify Percent Conveyance Reduction
Method 4: Specify Target Surcharge with Equal
Conveyance Reduction
Method 5: Optimization with Two Targets
350
351
351
352
10.2
Developing a Floodway in HEC-RAS
Establishing Base Conditions
Creating a Steady Flow Data File
Downstream Boundary Conditions
Global Options
Reach Options
River Station Options
Computing the Floodway Plan
354
355
355
355
356
357
357
358
10.3
Reviewing the Results
Additional Runs/Methods
Finalizing the Floodway with Method 1
Guidance for Correcting Excessive or Negative
Surcharge
358
359
360
353
354
361
viii
Table of Contents
10.4
Reviewing and Modifying Encroachment Output
Encroachment Tables
Graphics
Key Considerations
Levee Requirements for FEMA Certification
362
362
363
363
365
10.5
Adopting the Floodway
Satisfying Community Needs
Mapping the Floodway
Enforcing the Floodway
367
369
369
371
10.6
Working With an Existing Floodway
Placing Obstructions in the Floodway
Changes to a Floodway
372
372
373
10.7
Chapter Summary
373
Chapter 11
Channel Modification
377
11.1
Channel Stability
A Stream in Equilibrium
A Nonequilibrium Condition—Urbanization
A Nonequilibrium Condition—Channelization
Developing a Stable Channel Modification
Environmental Issues
Positive Effects of Channelization
377
378
381
381
384
385
386
11.2
Channel Modification Methods
Levees
High-Flow Diversion Channel and Weir
High-Flow Cutoff/Diversion Channel
Clearing and Snagging
Compound Channels
Clearing and Enlarging One Side of the Channel
Widening the Upper Channel and Using the Original
Channel for Low Flow
Realigning the Channel
Constructing a Paved Channel
New Channel
Channel Rehabilitation
Permitting Requirements
387
387
387
389
390
391
393
393
394
394
394
396
399
Channel Design Considerations
Flow Regime/Mixed Flow
Air Entrainment
Linings
Freeboard
Channel Transitions
Junctions
400
400
401
402
403
404
405
11.3
IX
Channel Protection
Low Flow Channel
Superelevation
Curved Channels
Drop Structures/Stabilizers
Debris Basins
Bridge Piers
406
408
408
410
410
412
413
11.4
HEC-RAS Input Data for Channel Modifications
Study Watershed/Channel Boundaries
Channel Modification Features
HEC-RAS Channel Improvement Template
414
414
414
414
11.5
Stable Channel Design Using HEC-RAS
Uniform Flow Analysis
Stable Channel Design
Design Parameters
417
417
421
428
11.6
Analyzing Results
Velocity
Energy Grade Line Slope
Top Width
Sensitivity of Manning's n
Sensitivity of Scour/Sediment Deposition on the
Design Profile
Channel Effects Outside of a Modified Reach
Effects on Hydrographs
Plan Comparisons
429
430
430
430
430
11.7
Channel Maintenance Requirements
433
11.8
Chapter Summary
433
Chapter 12
Advanced Floodplain Modeling
430
431
431
431
437
12.1
Levees
Levee Characteristics
Modeling Procedures
437
437
444
12.2
Modeling Obstructions
Without Storage Considerations
With Storage Considerations
451
452
453
12.3
Modeling Tributaries and Junctions
Cross-Section Locations
Computing Losses and Water Surface Elevations
through a Junction
456
457
Inline Gates and Weirs
Types of Weirs and Gated Openings
Governing Equations
459
460
461
12.4
457
Table of Contents
Modeling Procedures
Output Analysis
465
469
12.5
Drop Structures
Modeling of Drop Structure as an Inline Weir
Modeling of Drop Structure Using Cross Sections
470
471
471
12.6
Split Flow
Split Flow Situations
Computational Procedures
Modeling Procedures for Separate Channel Splits
Modeling Procedures for Lateral Weirs
473
473
476
476
477
12.7
Ice Cover and Ice Jam Flood Modeling
Effects on Water Surface Elevations
Data Requirements for Ice Analysis
Ice Modeling Procedures with HEC-RAS
Output Review
Ice Modeling Assistance
483
483
484
487
488
488
12.8
Chapter Summary
489
Chapter 13
Mobile Boundary Situations and
Bridge Scour
501
13.1
Mobile Boundary Analysis
501
13.2
Types of Mobile Boundary Analyses
Base Conditions
Reservoir Projects
Channel Modification Projects
Levee Projects
Diversion Projects
Channel Stability and Protection
502
502
503
506
506
507
509
13.3
Bridge Scour
Key References
Types of Scour
509
510
511
13.4
Bridge Scour Computational Procedures
Initial Preparation
General Bridge Scour Analysis Procedures
Contraction Scour
Pier Scour
Abutment Scour
519
519
520
522
527
538
13.5
Computing Scour with HEC-RAS
Applying the Flow Distribution Option
Bridge Scour Data
Total Scour
545
545
546
548
XI
13.6
Cautions and Concerns for Bridge Scour
550
13.7
Sediment Discharge Relationships
Sediment Transport Equations
Cautions in Applying Sediment Transport Equations
Applying the Equations with HEC-RAS
551
554
556
558
13.8
Chapter Summary
560
Chapter 14
Unsteady Flow Modeling
563
14.1
Why Use an Unsteady Flow Model?
Attenuation
Flow restrictions
Looped ratings
Flow Splits
Time-Based (Transient) Effects
564
564
568
568
569
570
14.2
Unsteady Flow Theory
St. Venant Equations
Steady-State Approximation
Level-Pool Routing
Kinematic Wave Approximation
Diffusion Wave Approximation
Theoretical Applicability of Various Approximations
570
571
576
577
578
578
579
14.3
Solution of Equations
Solving the Diffusion Wave Equation
Solving the Full St. Venant Equations
580
580
587
14.4
Practical Choice of Unsteady Modeling Approach
Routing Models
".
Hydrodynamic Modeling
Hybrid Approach
Troubleshooting Models
590
590
593
602
603
14.5
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.... 626
14.6
Chapter Summary
632
xii
Table of Contents
Chapter 15
Importing and Exporting Files
with HEC-RAS
639
15.1
Imported File Types
HEC-2 Files
HEC-RAS Files
UNET Files
Corps Survey Data Files
GIS/CADD Files
DSS Files
Spreadsheet and Text Files
639
640
640
640
641
641
642
644
15.2
Exporting Files
DSS Files
GIS/CADD Files
645
645
648
15.3
Using HEC-2 Files with HEC-RAS
Importing HEC-2 Files
Data Not Imported
649
649
650
15.4
Program Differences and Review of Imported Data
Program Differences
Comparing HEC-RAS and HEC-2 Output
651
651
657
15.5
Chapter Summary
658
Bibliography
659
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