Text Box: HEC-RAS Procedures for HEC-2 ModelersText Box: Federal Emergency Management Agency
Mitigation Directorate
500 C Street, SW
Washington, DC 20472
April 2002
Text Box: FLOODPLAIN MODELING MANUALFloodplain Modeling Manual: HEC-RAS Procedures for HEC-2 Modelers - Manual Cover
Floodplain Modeling Manual
HEC-RAS Procedures for HEC-2 Modelers
Prepared by
Dewberry & Davis LLC
for the
Federal Emergency Management Agency
April 2002
TABLE OF CONTENTS
CHAPTER 1: INTRODUCTION..........................................................................1
1.1 Organization of the Manual..........................................................................2
1.2 Acknowledgment.........................................................................................2
CHAPTER 2: FLOODPLAIN MODELING - HEC-RAS
AND HEC-2 PROCEDURES.............................................................................3
2.1 Governing Equations and the Solution Procedure.........................................4
2.2 Analysis of Subcritical, Supercritical, and Mixed Flows................................4
2.3 Analysis of Flow Junctions..........................................................................4
2.4 Cross Section Conveyance Computation......................................................5
2.5 Critical Depth Computation.........................................................................5
2.6 Ineffective Flow Areas and Levees at Cross Sections.....................................6
2.7 Geometric Cross Section Interpolation.........................................................6
2.8 Analysis of Flow Distribution at a Cross Section...........................................7
2.9 Analysis of Special Types of Flows................................................................8
CHAPTER 3: ANALYSIS OF FLOODPLAINS WITH BRIDGES AND CULVERTS..............9
3.1 Analysis of Floodplains with Bridges............................................................9
3.1.1 Low Flow Analysis.......................................................................................9
3.1.2 High Flow Analysis......................................................................................9
3.1.3 Parallel Bridges (bridge openings in series).................................................10
3.2 Analysis of Floodplains with Culverts.........................................................10
3.2.1 Culvert Shapes..........................................................................................10
3.2.2 Culverts in Series......................................................................................11
3.2.3 Parallel Culverts with Same Invert Elevation..............................................11
3.3 Analysis of Floodplains with Multiple Bridge Openings...............................11
3.4 Analysis of Flow Through Gated Spillways and In-line Weirs......................12
3.5 Analysis of Flow Over Drop Structures.......................................................12
CHAPTER 4: FLOODWAYS - ENCROACHMENT ANALYSES..................................13
CHAPTER 5: ANALYSIS OF COMPLEX FLOODPLAINS.........................................14
5.1 Wide Floodplains.......................................................................................14
5.2 Split Flow Modeling...................................................................................14
5.3 Analysis of Flow through Long Pipes – Storm Sewer Analysis......................15
CHAPTER 6: QUALITY ASSURANCE SOFTWARE – CHECK-2 AND CHECK-RAS.......16
CHAPTER 7: DEVELOPMENT OF THE HEC-RAS MODEL....................................17
7.1 Schematic of the River System...................................................................17
7.1.1 River Junctions.........................................................................................17
7.1.2 Cross Sections...........................................................................................17
7.2 The Input Data..........................................................................................18
7.2.1 Locating the Input Data – HEC-RAS Index.................................................18
7.3 Geometric Data of Cross Sections..............................................................18
7.4 Calibrating to Observed High-water Marks.................................................18
7.5 Special Features to Review the Results.......................................................19
CHAPTER 8: PROFILE PLOTTING SOFTWARE - RASPLOT.................................20
CHAPTER 9: CONVERSION OF HEC-2 INPUT DATA FILES
TO HEC-RAS............................................................................................21
9.1 Importing HEC-2 Input Data......................................................................21
9.2 The HEC-RAS Index...................................................................................21
9.3 Comparison of Solution Methods and Computational Procedures...............25
9.4 Precautions to be Taken when Importing HEC-2 Input Data.......................31
9.4.1 Conveyance Computation..........................................................................31
9.4.2 Imported Bridge Modeling Data..................................................................32
9.4.3 Critical Depth Computation.......................................................................32
9.4.4 Other Observations....................................................................................32
CHAPTER 10: ACQUIRING HEC-RAS............................................................33
APPENDIX A: USE OF HEC-RAS FOR FLOOD INSURANCE STUDIES...................A-1
APPENDIX B: CONVERSION OF HEC-2 INPUT DATA
TO HEC-RAS (EXAMPLES) .........................................................................B-1
Example 1: Importing HEC-2 Natural Floodplain Model......................................B-2
Example 2: Importing HEC-2 Bridge and Culvert Models....................................B-8
Example 3: Importing HEC-2 Model with a Tributary.......................................B-19
Example 4: Importing HEC-2 Ice-jam Model.....................................................B-23
Example 5: Importing HEC-2 Encroachment Model..........................................B-26
Example 6: Importing HEC-2 Split Flow Model.................................................B-28
CHAPTER 1: INTRODUCTION
The U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center’s River
Analysis System (HEC-RAS) supersedes its HEC-2 program, widely used in the
preparation of Flood Insurance Studies. The Federal Emergency Management Agency
(FEMA) has adopted the guidance that hydraulic analyses for newly contracted Flood
Insurance Studies (FISs) and restudies of entire watersheds (with detailed HEC-2
hydraulic analysis) should be conducted using HEC-RAS instead of HEC-2. Details of
this guidance are documented in FEMA Memoranda dated March 14, 1997, and
October 19, 2000. FEMA has also issued guidance that encourages the use of HEC-
RAS to revise floodplains with effective HEC-2 analyses. This guidance is documented
in a FEMA memorandum dated April 30, 2001. Copies of these memoranda are
included in Appendix A.
In order to ensure the accurate modeling of floodplains analyzed for FISs, FEMA's
study contractors and the Flood Map Production Coordination Contractors (FMPCCs)
have to familiarize themselves with the capabilities and limitations of the HEC-RAS
program to model floodplains.
This floodplain modeling manual is prepared as a guide to introduce the HEC-RAS
Version 3.0 modeling procedures to engineers who have experience in preparing
floodplain models with the HEC-2 computer program. The first version 1.0 of HEC-
RAS was released in July 1995 and there have been six releases since then. The
major advancement of Version 3.0 over the previous releases is the inclusion of
unsteady flow routing. The unsteady flow equation solver is adopted from the program
UNET, developed by Dr. Robert L. Barkau and supported by the USACE. However, the
discussion provided in this manual is limited to the steady flow water-surface profile
calculations and complements the following three HEC-RAS documents:
• HEC-RAS River Analysis System Version 3.0 User’s Manual (UM);
• HEC-RAS River Analysis System Version 3.0 Hydraulic Reference Manual
(HRM); and
• HEC-RAS River Analysis System Version 3.0 Applications Guide (AG), all dated
January 2001, and published by the USACE.
Before preparing a HEC-RAS model of a floodplain, it is recommended that the user
read and understand the aforementioned HEC-RAS documents.
1.1 Organization of the Manual
The hydraulic principles and computational procedures used in the HEC-RAS and
HEC-2 programs are summarized in this manual, and the differences are identified.
Chapter 2 discusses the issues relevant to modeling floodplains unobstructed by
structures. Issues related to modeling the floodplains obstructed by structures are
discussed in Chapter 3. Floodway considerations are discussed in Chapter 4.
Chapter 5 of this manual summarizes the HEC-RAS features that can be used in
modeling complex floodplains such as multiple bridges, culverts in series, flow
junctions, and split flow. A brief summary of the quality assurance software, CHECK-
RAS, recently published by FEMA, is included in Chapter 6 of the manual. Issues
related to the general development of a HEC-RAS model are discussed in Chapter 7.
In Chapter 8, the manual summarizes the graphical capabilities and the post
processing programs available for HEC-RAS. The issues related to the conversion of
HEC-2 input data to HEC-RAS are discussed in Chapter 9.
Chapter 9 of the manual provides quick reference facts about HEC-RAS program.
These facts include, information necessary to purchase the program, tables
summarizing the capabilities of HEC-RAS program with reference to pages where
detailed discussion on the topic is provided in the HEC-RAS documentations, and the
variations between HEC-RAS and HEC-2 modeling procedures. In addition, a
HEC-RAS index included in this section summarizes the different locations where
hydraulic data are stored by HEC-RAS computer program. Chapter 10 of this manual
provides information on acquiring the HEC-RAS program.
A copy of FEMA’s memoranda providing guidance on the use of HEC-RAS is included
in the Appendix A. Appendix B summarizes six sensitivity analyses conducted using
the HEC-RAS import feature to import HEC-2 files. The HEC-2 input data with
floodplain modeling, bridge and culvert modeling, tributary modeling, ice-jam
modeling, and encroachment modeling are included in the sensitivity analysis. The
enclosed computer disk includes the models prepared for each of the six sensitivity
analyses.
1.2 Acknowledgment
This report was prepared by Dewberry & Davis LLC, under contract with the Federal
Emergency Management Agency.
CHAPTER 2: FLOODPLAIN MODELING - HEC-RAS AND HEC-2 PROCEDURES
HEC-RAS, operated in the MS Windows. environment, consists of:
• A graphical user interface
• Separate hydraulic analysis components
• Data storage and management capabilities
• Graphical and report preparation capabilities
In addition, FEMA has published the post-processing program RASPLOT to prepare
flood profiles suitable for publication in FISs. The quality assurance software CHECK-
RAS is published by FEMA to facilitate the preliminary technical review of a HEC-RAS
floodplain model.
The governing equations and the solution methods used by HEC-2 and HEC-RAS to
model steady flow in a floodplain unobstructed by structures are generally similar.
The theory and data requirements for the hydraulic computations performed by the
HEC-RAS computer program, Version 3.0, are documented in the Hydraulic Reference
Manual. Major differences between the solution methods used by HEC-2 and HEC-
RAS are found in the following computations:
• Cross section conveyance
• Critical depth
The following computational capabilities are newly added (or enhanced) in HEC-RAS:
• Analysis of flow in a mixed flow regime and the location of the hydraulic jump
• Ineffective flow area and levees at cross sections
• Geometric cross section interpolation
• Analysis of flow distribution at a cross section
• Analysis of the effect of air entrainment in high velocity streams
• Split flow optimization at stream junctions and lateral weirs
• Analysis of flow over Inline weirs and bridges with multiple openings
• Unsteady flow analysis (this capability will not be discussed in this report)
The governing equations used by HEC-2 and HEC-RAS to model floodplains and the
solution methods used are summarized in this section.
2.1 Governing Equations and the Solution Procedure
HEC-2 can analyze the flow in simple dendritic systems. HEC-RAS, Version 3.0, is
capable of performing one-dimensional steady-state hydraulic computations for a full
network of natural and constructed channel. The basic solution procedure is based
on the solution of one-dimensional energy equation. The energy losses are evaluated
by considering the friction and expansion and contraction losses between the cross
sections that represent the topography of the floodplain.
2.2 Analysis of Subcritical, Supercritical, and Mixed Flows
HEC-RAS have the capability to conduct backwater and forewater computations
necessary to analyze channel flows in subcritical, supercritical, and mixed flow
regimes. HEC-RAS conducts the backwater and forewater computations and assigns
the appropriate flow regime. HEC-2 has the capability to conduct backwater and
forewater computations necessary to analyze channel flows in subcritical, and
supercritical, flow regimes. However, the HEC-2 requires the user to prepare a
different input data set from downstream to upstream to analyze subcritical flows and
from upstream to downstream to analyze supercritical flows. In order to analyze
mixed flows using HEC-2, both the supercritical and subcritical flow models of the
stream must be prepared. However, hand computations are needed to compute the
exact location of the hydraulic jump.
2.3 Analysis of Flow Junctions
HEC-2 has the capacity to analyze simple dendritic channel systems. HEC-RAS have
the capability to analyze a full network of channels, including a dendritic system. In
order to model a channel network, the program must have the capability to analyze
the flow at junctions. Generally, a flow junction consists of a flow-split; where, a main
upstream channel splits to form two or more tributaries downstream; or a flow-
combination, where, two or more upstream tributaries join to form a downstream
main channel.
The “Tributary” option available in HEC-2 is capable of analyzing flow-combinations.
However, HEC-RAS has the capability to analyze flow-splits and flow combinations.
When a flow-split at a junction is analyzed, a trial and error procedure is necessary to
determine the discharge in each branch.
The HEC-RAS junction solution procedure also has the capability to compute flood
elevations at flow-combinations by considering the energy losses at the junction.
HEC-RAS offers two solution methods to compute this energy loss. First, the energy
method is suitable for subcritical flow combinations and is based on the conservation
of energy principles. Second, the momentum method is suitable for supercritical flow
combinations and is based on the conservation of momentum principles. The flow
combinations frequently considered in the FISs are those of a main river and its
tributary. In these cases, it is generally assumed that the flooding in the main river
and its tributary will not occur at the same time. Therefore, the flow-combination
solution procedure available in the HEC-RAS may not be generally used for FISs. The
HEC-2 program does not have the option to compute energy losses at junctions.
Both, HEC-2 and HEC-RAS offer split flow optimization procedures that offer the
capability to model flows over lateral weirs, spillways or overtopping of watershed
divides. In addition, both HEC-2 and HEC-RAS allows the use of a lateral rating curve
to determine the split flow discharge. The optimization procedure determines the split
flow discharges and flood elevations along the split flow path. Unlike HEC-2, HEC-
RAS is capable of defining split flows that occur on either or both sides of the channel
banks. In HEC-RAS, split flow location can be defined on left or right channel banks
or at left or right overbanks.
2.4 Cross Section Conveyance Computation
The total conveyance of the overbank floodplain is computed in HEC-2 by adding the
individual conveyance computed between each coordinate point used to define the
cross section. The conveyance of a cross section is a function of the different
roughness coefficients assigned. The channel conveyance for the channel in HEC-2 is
computed by estimating one equivalent roughness coefficient for the entire channel (if
more than one roughness values are assigned).
The HEC-2 method is used by HEC-RAS to compute the conveyance of the channel.
The individual conveyance is computed for each portion of the floodplain with a
different Manning’s roughness coefficient is computed by HEC-RAS, when assessing
the overbank conveyance. However, HEC-RAS also supports the HEC-2 method of
computing overbank conveyance; i.e., computing for smaller areas (between each
coordinate point) and then adding these individual conveyance estimates.
2.5 Critical Depth Computation
HEC-2 and HEC-RAS compute the critical depth at a cross section by minimizing the
total energy computed at that section. There are differences in the computational
algorithms used to locate the minimum total energy of the cross section.
HEC-2 uses a “parabolic” method (HRM, Appendix C, Page C-4) to find the critical
depth associated with the minimum total energy. HEC-RAS has two methods to find
the critical depth by minimizing the total energy. The first method is similar to HEC-2
parabolic method. If the parabolic method did not yield satisfactory results, HEC-RAS
uses a more robust “secant” method to arrive at a solution. The HEC-RAS secant
method is capable of finding up to three minimum values from the energy verses depth
relationship. Whenever, multiple minimum values are found, the HEC-RAS secant
method has the capability to locate the lowest of all the minimum values.
Alternatively, when multiple minimum values are inherent in an energy verses depth
relationship, the parabolic method will terminate at the first minimum value found.
A large variation can be found between the critical depths computed by HEC-2 and
HEC-RAS models of the same cross section. The computational features available in
HEC-RAS are superior to compute a more reliable minimum total energy estimate and
the corresponding critical depth.
2.6 Ineffective Flow Areas and Levees at Cross Sections
HEC-2 and HEC-RAS have the capability to model ineffective flow areas. Ineffective
flow areas are frequently used where wide floodplains have the capacity to store water
with minimal conveyance. In HEC-2 large roughness coefficients are assigned to
model the storage effect of an ineffective flow area. Large roughness coefficients can
also be used in HEC-RAS to model ineffective flow. In addition, HEC-RAS has the
capability to define portions of a cross section as not contributing in the flow but
where flow can be stored.
Levees are generally modeled in HEC-2 using X3 statements. X3 statements allow the
water to be constrained within a portion of a cross section until it reaches a water level
specified by the user. HEC-RAS offers the user the option to identify levees at cross
sections. In addition, HEC-RAS can place vertical walls at specified locations of a
cross section to simulate the effect of levees. When levees are used in a cross section,
HEC-RAS restricts the flow to the riverside of the levee until overtopped.
2.7 Geometric Cross Section Interpolation
HEC-2 and HEC-RAS have the capability to create interpolated cross sections. The
difference in velocity head between adjacent cross sections is one criterion used for
interpolating additional cross sections in HEC-2. When the velocity head difference
exceeds a user specified maximum value, up to three cross sections are interpolated.
However, additional cross sections will not be interpolated for the following scenarios:
• When length between cross sections is smaller than 50 feet
• When encroachments are specified at the cross sections
• When downstream cross section is also used to define the bridge geometry for a
HEC-2 Special Bridge or a Special Culvert analysis
In HEC-2, the option to interpolate cross sections is selected for the entire stream
reach modeled.
In HEC-RAS, interpolated cross sections are created in response to requests by the
user. Additional cross sections are added between two adjacent cross sections or
within a specified stream reach. The HEC-RAS Hydraulic Reference Manual states
that HEC-RAS computes the topography of the interpolated cross section by
interpolation. In addition, HEC-RAS computes the following hydraulic parameters by
interpolation:
• Manning’s roughness coefficients
• Reach lengths
• Channel bank stations
• Contraction and expansion coefficients
• Normal ineffective flow areas, when upstream and downstream cross sections
have ineffective flow areas
• Levees, when upstream and downstream cross sections have levees
• Normal blocked obstructions, when upstream and downstream cross sections
have obstructions
Interpolated cross sections created by HEC-RAS are accepted for hydraulic analyses
prepared for the NFIP.
2.8 Analysis of Flow Distribution at a Cross Section
HEC-2 and HEC-RAS has the capability to compute the flow distribution within a
cross section. In HEC-2, the flooded portions of the channel and overbanks are
divided into subsections. The depth of flow, flow area, velocity, and the discharge
conveyed through each portion is computed and printed out. The HEC-2, flow
distribution option, when selected, will apply to all of the cross sections used in the
model. The number of subsections used for flow distribution is internally determined
by HEC-2. Three to thirteen subsections are used by HEC-2 for flow distribution.
Unlike in HEC-2, in HEC-RAS, the user selects the individual cross sections where
flow distribution computations are requested. In addition, flow distribution
parameters (velocity, flow, and flow area) are computed for the subsections specified
by the user. In HEC-RAS, the flow distribution results are viewed as tables or velocity
distribution plots.
2.9 Analysis of Special Types of Flows
HEC-2 and HEC-RAS have capabilities to analyze ice-covered stream flows, determine
floodways and to determine the effects of channel modifications as well as sediment
deposition. In addition, HEC-RAS has the capability to analyze the effects of air
entrainment in high-velocity stream flows and bridge scour.
Chapter 3: Analysis of Floodplains with Bridges and Culverts
HEC-2 and HEC-RAS have the capability to compute the energy losses associated with
the low and high flows through bridges and culverts. HEC-RAS offers a solution
routine to analyze flows through culverts and bridges located at different invert
elevations.
3.1 Analysis of Floodplains with Bridges
HEC-RAS has the capability to analyze flows through single and multiple bridges.
Similar to the HEC-2 Special Bridge analysis, the HEC-RAS Bridge analysis requires
four cross sections and the bridge geometry to model the flow through a bridge.
The HEC-2 offers two types of bridge analyses, the Special Bridge analysis and the
Normal Bridge analysis. HEC-RAS bridge analysis is designed differently than the
HEC-2 bridge analysis. HEC-2 offers the “Normal Bridge” and “Special Bridge”
routines to analyze flow through bridges. The Normal Bridge analysis is generally
used to model flow situations in which bridge is a small obstruction to the flow (may
exist in both, low and very high bridge flows), and the flow is similar to open channel
flow. Similarly, Special Bridge analysis is generally used with high bridge flows, where
the flow in the bridge barrel is pressurized and there is a possibility of weir flow over
the bridge deck. The names, “Normal Bridge,” and “Special Bridge,” used by HEC-2
are not followed in HEC-RAS. Instead, the bridge flow analyses are classified as “Low
Flow” and “High Flow.”
3.1.1 Low Flow Analysis
HEC-RAS offers four different user selected methods for the analysis of low flow
through bridges, including: Energy balance method, Momentum balance method,
Yarnell equation, and Federal Highways WSPRO method. The Energy method is
similar to the HEC-2 Normal Bridge analysis. HEC-2 has the Momentum balance
method and the Yarnell equation method for analysis of low flow through bridges. The
assignment of one of these two methods is done internally by the HEC-2 Special
Bridge analysis as a function of the momentum computed just down stream of the
bridge and inside the bridge opening.
3.1.2 High Flow Analysis
The Energy Method, and Pressure and Weir flow methods are available in HEC-RAS to
perform high flow computations. As mentioned earlier, the energy method is similar to
the HEC-2 normal bridge analysis. When upstream and the downstream low chords
of the bridge is submerged, the HEC-RAS Pressure and Weir flow analysis is similar to
the HEC-2 Special Bridge analysis. When the upstream low chord of the bridge is
submerged and the downstream low chord is not submerged, HEC-RAS analysis is
similar to the analysis of a sluice gate flow.
3.1.3 Parallel Bridges (bridge openings in series)
Flow through parallel bridges, situated close to each other, can be modeled using
HEC-RAS bridge analysis. The upstream and downstream cross sections are shared
by the bridges and low flows are not allowed to expand between two bridges. This
modeling procedure is similar to that adopted by HEC-2.
3.2 Analysis of Floodplains with Culverts
The Federal Highways Administration’s (FHWA’s) inlet control and outlet control
procedures described in the 1985 hydraulic design series report Number 5 entitled,
“Hydraulic Design of Highway Culverts” is used by the HEC-2 and HEC-RAS to
analyze the flow through culverts. Inlet control flow occurs when the flow capacity of
the culvert entrance is less than that of the culvert barrel. Outlet control flow occurs
when the flow carrying capacity of the culvert is limited by downstream conditions or
the size of the culvert barrel. The inlet control equations developed by the FHWA are
used by the HEC-2 and HEC-RAS. For outlet control flow, the required upstream
energy to maintain the flow is computed using energy balance equations.
3.2.1 Culvert Shapes
HEC-RAS has the capability to model the eight most commonly used culvert shapes.
They are:
• Circle
• Box (Rectangular)
• Arch
• Pipe arch
• Low profile arch
• High profile arch
• Elliptical
• Semicircle
• ConSpan
The HEC-RAS culvert solution routine is similar to the HEC-2 special culvert analysis.
3.2.2 Culverts in Series
Culvert analysis of the HEC-RAS program has the ability to model culverts in series.
Any change in shape, slope, roughness, or FWHA chart and scale number will require
the definition of a new culvert. However, data is scarce to determine the transition
loss when culverts of different shapes and dimensions are used. This application is
similar to the use of the HEC-2 special culvert analysis to model culverts in series.
3.2.3 Parallel Culverts with Same Invert Elevation
Culvert analysis of the HEC-RAS program has the ability to model up to 9 types of
culverts in parallel and with the same invert elevation. Any change in shape, slope,
roughness, or FWHA chart and scale number will require the definition of a new
culvert. In addition, HEC-RAS can analyze up to 25 identical culvert barrels (of each
type). The HEC-RAS culvert computations are done for the flow through one barrel
only. The flow rate is equally divided among the identical culvert barrels specified.
HEC-2 Special culvert analysis is less flexible than HEC-RAS and has the ability to
model up to 10 parallel culverts of one type.
3.3 Analysis of Floodplains with Multiple Bridge Openings
HEC-RAS has the capability to analyze the different types of flows (low, pressure,
pressure and weir) through multiple bridge and/or culvert openings at one cross
section. This often occurs with wide floodplains, where bridges and culverts at
different elevations are built to convey the main river discharge as well as the flood
discharges on the overbanks. Seven types of bridge openings can be analyzed at one
cross section using the HEC-RAS multiple bridge opening analysis.
The types of openings at a bridge location can be a combination of:
• Bridges
• Culverts
• Open conveyance areas
Multiple opening approach and divided flow options are available to analyze the flow
through multiple bridges at one cross section. In the multiple opening approach, a
constant “energy” elevation is computed for upstream and downstream cross sections
of the structure. This approach allows the water-surface elevations to vary. The
divided flow option will compute different flood elevations to the flow paths used. The
user must provide the different discharges through each flow path.
In HEC-2, multiple bridge/ culvert openings has to be analyzed by preparing separate
HEC-2 model for each opening and obtaining the combined discharge rating curve at
the upstream cross section.
3.4 Analysis of Flow Through Gated Spillways and In-line Weirs
HEC-RAS has the capability to analyze flows over gated spillways such as radial gates
(or under sluice gates), inline weirs, or a combination of both. HEC-2 does not have
the capability to model weir flows and flows through sluice gates. However, the broad-
crested weir computations done by the HEC-2 special bridge analysis is sometimes
used to model weir flows.
HEC-RAS has the capability to analyze flows over spillways ogee shape or broad-
crested weir shape. The program has the ability to account for the effects of
submergence. Up to 10 sets of spillways and weirs can be analyzed for one cross
section. Similar to its weir analyses, the HEC-RAS program requires four cross
sections to model the flow through control structures (cross sections 1 and 2 in the
downstream expansion reach and 3 and 4 on the upstream contraction reach). HEC-
RAS applies the crest information of the weir and the spillway to cross section 3.
3.5 Analysis of Flow Over Drop Structures
In HEC-RAS program, the flow over a drop structure can be analyzed as an inline weir
or as a series of cross sections. HEC-2 Special Bridge analysis can be used to flow
over drop structures.
CHAPTER 4: FLOODWAYS - ENCROACHMENT ANALYSES
Similar to HEC-2, the HEC-RAS floodplain encroachment procedure is based on
calculating the flood elevations reflecting the existing topographic (natural) conditions
as well as floodplain encroachments. The first step in conducting an encroachment
analysis is to select the locations of the encroachment boundaries (encroachment
stations) at the left and right overbanks. The encroachment analysis assumes that the
cross section area within the encroachment stations (floodway) is available to convey
the flood discharge. The encroachment analysis computes the flood elevations using
the encroached topography (floodway elevations). The encroachment stations are
selected such that the surcharge value, the difference between the natural flood
elevation and the floodway elevation, remains smaller than a maximum surcharge
value selected by the user. The floodplain encroachment analyses are conducted with
an objective of determining an optimum floodway that will yield the maximum possible
surcharges within the floodplain, and therefore, are iterative in nature.
HEC-RAS has five floodway determining procedures available.
• Method 1: The left and right encroachment stations are provided as input.
• Method 2: The desired floodway width is provided as input.
• Method 3: The desired reduction in conveyance is provided as input.
• Method 4: The desired surcharge is provided as input.
• Method 5: The desired surcharge and maximum energy increase are provided
as input.
The first three methods are similar to Methods 1, 2 and 3 available in the HEC-2
program. HEC-RAS encroachment Method 4 is conceptually similar to HEC-2 Method
4. However, the HEC-RAS iterative solution procedure used for Method 4 is more
efficient than that used in HEC-2. HEC-RAS Method 5 is a combination of HEC-2
encroachment methods 5 and 6.
In HEC-2 and HEC-RAS, the encroachment at bridges and other structures in the
floodplain could be made effective or ineffective. However, unlike HEC-2, the default
option in HEC-RAS, at bridges and structures, is to perform the encroachment
analysis. In HEC-2 encroachment analysis, fixed encroachments specified in X3
record will override the encroachments computed using the ET record. However, in
HEC-RAS, blocked obstructions (equivalent to HEC-2 X3 record) and encroachments
(similar to HEC-2 ET record) can be simultaneously applied.
Chapter 5: Analysis of Complex Floodplains
HEC-RAS and HEC-2 have the capability to analyze one-dimensional steady-state
open channel flows. In this analysis, the river channels are assumed to have small
slopes. In addition, the flow is assumed to vary gradually, except at hydraulic
structures. The flow in the vicinity of the hydraulic structures are analyzed using
momentum equation or by empirical equations. In complex floodplains one or more of
these basic assumptions are violated. As a result the appropriateness of the HEC-RAS
and HEC-2 analyses to predict the floodplain properties becomes highly questionable.
Examples are wide floodplains, two-dimensional flow near bridges, flow through long
pipes, and storm sewer flows. The availability of data and funds often decide the
choice of the hydraulic model. There may be occasions, where it is necessary to use
the HEC-RAS model to analyze these flow situations. Some techniques that can be
applied to improve the ability of the HEC-RAS model to analyze such complex
floodplains are discussed in this chapter.
5.1 Wide Floodplains
Wide floodplains are caused by flat topography of the overbanks. The surveyed cross
sections will allow the floodplains to extend to very large lengths before the flood can
be contained by high grounds. The assumption of one-dimensional flow is often
violated in very wide floodplains. In reality, a floodplain area further away from the
channel will store flood waters rather than convey them as efficiently as the floodplain
adjoining the channel. Therefore, the wide floodplain is divided into two areas; one
area which would contribute to the conveyance of water and the other area which will
only store water. Calibration to historic events as high as the 100-year flow will help
to estimate the effective flow areas of the floodplain.
Because of the flood storage available on the overbanks, the steady flow assumption
used in the HEC-RAS program may not reflect the flow conditions of a wide floodplain.
An unsteady flow model has the capability to reflect the effects of the conveyance as
well as the flood storage potential of floodplains. Therefore, more accurate answers
may be obtained by using the unsteady flow routing available in HEC-RAS.
5.2 Split Flow Modeling
The split flow analysis option available in HEC-2 and HEC-RAS provides for the
automatic determination of the main flow and spill-over flow discharges in situations
where flow is lost from the main channel. The HEC-2 and HEC-RAS split flow models
can estimate the flow lost by side weirs and watershed divides. HEC-2 split flow
model does not relate the lateral weir length to the channel and overbank distances of
the cross sections used to model the floodplain. The HEC-RAS split flow model
provides means to accurately specify the locations where split flow is likely to occur.
Unlike HEC-2, HEC-RAS is capable of defining split flows that occur on either or both
sides of the channel banks. In HEC-RAS, split flow location can be on the left or right
channel banks or at the left or right overbanks. Long lateral weirs that cross as many
as eight cross sections can be defined in HEC-RAS. Unlike HEC-2, in HEC-RAS, the
split flow modeled using lateral rating curve cannot be brought back into the main
channel. Similar to HEC-2, HEC-RAS split flow computations can also be based on
equating the water-surface or energy elevations within a tolerance of 2%.
5.3 Analysis of Flow Through Long Pipes - Storm Sewer Analysis
The unsteady flow models such as SWMM, FEQ, and adICPR are more suitable to
model urban runoff where storm drainage is collected by inlets and transmitted
through long pipes. However, if a steady state analysis is to be performed, the series
culvert modeling procedures and the “lid” option available in HEC-RAS to generate
closed cross sections can be used to approximate the flow through storm sewers and
long pipes. These procedures can be approximated with the bridge and culvert
options available in the HEC-2 program.
CHAPTER 6: QUALITY ASSURANCE SOFTWARE – CHECK-2 AND CHECK-RAS
FEMA has developed quality assurance software that have the capability to provide an
initial assessment of HEC-2 and HEC-RAS analyses. CHECK-2 is the quality
assurance software that will scan HEC-2 input and output files and highlights areas
that need further review. CHECK-RAS is the quality assurance software for HEC-RAS
analyses. This software can be downloaded from FEMA web site at:
http://www.fema.gov/fhm/frm_soft.shtm
CHECK-RAS extracts information from the HEC-RAS input and output data files to
generate a summary output. This summary output highlights areas that need further
review. CHECK-RAS also provides help screens to assist HEC-RAS users in correcting
potential errors. CHECK-RAS will run under Windows 95/98/NT operating systems.
Both CHECK-2 and CHECK-RAS have the ability to check the suitability of the
following five areas of the hydraulic analysis.
• Roughness coefficient and transition loss coefficients
• Cross sections
• Structure modeling
• Floodway modeling
• Multiple profiles
CHAPTER 7: DEVELOPMENT OF THE HEC-RAS MODEL
The main objective of preparing a HEC-RAS model of a floodplain is to compute water-
surface elevations of the floodplain for a given discharge. In order to achieve this
objective, the program must be supplied with geometric data of the floodplain, steady
flow data defining the discharge rates for the floodplain and a criteria to define the
starting water-surface elevation. The same data are required to prepare a HEC-2
model of a floodplain. Since, HEC-RAS has the capability to analyze flow junctions,
the geometric data will include the connectivity of the river system, as well as the
cross section data, reach lengths, energy loss coefficients, and the hydraulic structural
data necessary to model floodplains obstructed by structures.
7.1 Schematic of the River System
The connectivity of the river system is referred to as the schematic of the river system
and generally would be the first task in creating a HEC-RAS model. The schematic of
the river system is developed using the geometric data editor available in HEC-RAS.
The different river reaches are determined and the schematic is drawn from an
upstream to downstream direction on a reach by reach basis.
HEC-RAS also has the capability to import an existing HEC-2 input data file and
create an equivalent HEC-RAS input data. During this procedure, the river schematic
is created by HEC-RAS.
7.1.1 River Junctions
As reaches are connected together, junctions are automatically formed by the
interface. Each river reach is assigned with a name given by the user. Similarly, each
junction used in the schematic is also given a name.
7.1.2 Cross Sections
The next stage of the development of a schematic would be to define the locations of
the cross sections and enter the geometric data associated with each cross section.
Each cross section used in a HEC-RAS model will be associated with a river, reach,
and a river station (cross section) label.
Similar to HEC-2 program, the river station label should consist of numbers.
However, for HEC-RAS models, the numbering system should be consistent and the
labels should be higher for upstream cross sections (increasing from downstream to
upstream).
7.2 The Input Data
Unlike HEC-2, the topographic data, discharge data, and the boundary conditions are
entered using the Graphical User Interface (GUI) of the HEC-RAS. The cross sectional
data and structural data are entered from the geometric data window. The discharges
and boundary conditions are entered from the steady flow data window. Up to 100
topographic data points (to provide station and elevation data) can be used to define
HEC-2 cross sections. HEC-RAS cross section data can have up to 500 topographic
data points.
7.2.1 Locating the Input Data – HEC-RAS Index
Data for each cross section are displayed under one window and include River Name,
Reach Name, River Station, Reach Lengths, Roughness Coefficients,
Contraction/Expansion Coefficients, Ground Stations and Elevations. The user can
also access the reach lengths, n-values, and contraction/expansion loss coefficients at
all the cross sections under one window by clicking “Geometric Data” button and
“Tables” menu. The input data in the HEC-RAS model can be checked by clicking
“Steady Flow Analysis” button, “Options” menu, and “Check data before execution”
command.
The HEC-RAS Index is given in Section 9.2 of this document. This index will help the
user to find the locations of the variables and coefficients used in the HEC-RAS
analysis.
7.3 Geometric Data of Cross Sections
Cross sections can have up to 500 data points for HEC-RAS and 100 for HEC-2. The
data points should be surveyed from left to right looking downstream. Negative
station numbers can be used in a cross section. The channel banks, reach lengths
and Manning's roughness coefficients should be entered for each of the cross sections
defined.
7.4 Calibrating to Observed High-water Marks
HEC-2 and HEC-RAS have the ability to plot observed high water marks against
computed water-surface profiles. Calibration to high-water marks is generally done
manually by the user.
7.5 Special Output Features to Review Results
HEC-2 and HEC-RAS have the ability to present the hydraulic properties computed
during a flow simulation. HEC-2 out put is limited to 40 flow and topographic
variables to describe the floodplain. HEC-RAS has the ability to print up to 250 flow
and topographic variables to describe the floodplain. A complete list of these variables
can be found in the Appendix C of the HEC-RAS River Analysis System User’s Manual.
The hydraulic properties can be reviewed as graphs or tables. In HEC-RAS, the flow
distribution within a cross section could be requested and reviewed in a graphical or a
tabular format.
CHAPTER 8: PROFILE PLOTTING SOFTWARE - RASPLOT
FEMA has developed the software RASPLOT to create profiles for Flood
Insurance Studies (FISs). RASPLOT will run under Windows 95/98/NT
operating systems and this software can be downloaded from FEMA web site at:
http://www.fema.gov/fhm/frm_soft.shtm
RASPLOT extracts data from HEC-2 and HEC-RAS files and automatically
creates the profile database necessary to generate FIS flood profiles. In
addition, RASPLOT provides a blank profile table database into which data from
input and output files of other programs, such as WSP-2 or WSPRO, can be
manually inserted. After creating the profile database, RASPLOT creates a DXF
file that meets the Federal Emergency Management Agency’s specifications for
FIS flood profiles. The user can use a DXF Viewer/Edit program to open the
DXF file and plot the profiles.
Chapter 9: Conversion of HEC-2 Input Data Files to HEC-RAS
HEC-RAS has the capability to import HEC-2 input files. Chapter 3 of HEC-RAS
User’s Manual provides information on what user should be aware of before importing
HEC-2 file. It also provides step by step instruction on how to import an HEC-2 input
file. The User’s Manual identifies the HEC-2 features that are not imported in HEC-
RAS. Eight HEC-2 features that are not available in HEC-RAS are listed below:
• HEC-2 SF statement for split flow optimization
• HEC-2 IC statement for ice jam modeling
• HEC-2 NV statement for modeling vertical variations in Manning’s roughness
coefficients
• HEC-2 option to compute Manning’s roughness coefficients using high water
marks (J1, 3)
• HEC-2 statement RC to define internal rating curves
• HEC-2 statement AC to create archives
• HEC-2 command FR allowing the input data file to be in free format
• HEC-2 statement J4 to create storage – outflow data
HEC-2 input data files with the above mentioned statements will be imported into HEC-
RAS. However, these features will not be converted to HEC-RAS modeling. HEC-RAS
does not have the ability to create HEC-2 files from a HEC-RAS input data set.
9.1 Importing HEC-2 Input Data
A HEC-2 input data set can be imported in to a HEC-RAS project that is already open.
Alternatively, a new HEC-RAS project can be opened using the “File” command of the
HEC-RAS main window and the “Import HEC-2 data” feature available at the “File”
command can be used to import a HEC-2 input data file.
HEC-2 input data files created in free format cannot be generally imported into
HEC-RAS. However, EDIT-2 program available in the HEC-2 package has the
capability to create a formatted echo of the HEC-2 file (Tape 16). This file can be
imported by HEC-RAS.
9.2 The HEC-RAS Index
After importing a HEC-2 input file, the user will want to know where HEC-RAS stores
all the data. In HEC-2, the user can search for or edit the data within one file.
However, in HEC-RAS the data from HEC-2 is stored under different windows. Table 1
shows a HEC-RAS index that relates different hydraulic parameters and the
corresponding location in the HEC-RAS input file.
This index has two columns in which the hydraulic parameter and its HEC-RAS
location are tabulated. This index will assist the user to access the hydraulic data in
HEC-RAS, starting from HEC-RAS main window. The topographic data and the
hydraulic parameters of a cross section (given in HEC-2 records; X1, GR, and NC) are
stored in the “Cross Section” screen, which is accessed through the “Geometric Data”
screen. The “Geometric data screen can be found on the main HEC-RAS main screen.
For example, the information about the channel bank stations of a cross section is
given on the seventh row of the HEC-RAS index. This information is summarized in
the HEC-RAS index as follows:
Channel Bank Stations: From “Geometric Data” screen, select “Cross Section”;
“Main Channel Bank Stations” located on this screen.
Figure 1, HEC-RAS Main Window (copied from the HEC-RAS User’s Manual),
illustrates the HEC-RAS main screen which shows the various menus.
HEC-RAS Main Window
Figure 1: HEC-RAS Main Window
Table 1: HEC-RAS Index
Feature/ Parameter
Location
Add a New Cross Section :
From "Geometric Data" in main screen, select "Cross Section"
screen, then "Options", and to, "Add a new Cross Section"
Bank Erosion :
From "Hydraulic Design" in main screen, select "Functions", and
then "Bank Erosion"
Blocked Obstructions :
From "Geometric Data" in main screen, select "Cross Section"
screen and then "Options", and to "Blocked Obstructions"
Boundary Conditions :
From "Steady Flow Data" screen, select "Reach Boundary
Conditions"
Bridge Design Editor :
From "Geometric Data" screen, select "Brdg/Culv".
Bridge Modeling Approach :
From "Geometric Data" screen, select "Brdg/Culv", "Bridge
Modeling Approach”
Channel Bank Stations :
From "Geometric Data" screen, select "Cross Sections"; "Main
Channel Bank Stations" located on this screen
Channel Modification :
From "Geometric Data" screen, select "Options", and to
"Channel Modification".
Cont/Exp Coefficients :
From "Geometric Data" screen, select "Cross Sections" screen,
and
to "Cont/Exp Coefficients" or "Tables" from main screen to
"Coefficients"
Copy Current Cross
Section :
From "Geometric Data" screen, select "Cross Section", under
"Edit", select "Copy"; "Paste" section where needed
Cross Section
Interpolation :
From "Geometric Data" screen, under "Options", select "XS
Interpolation”
Cross Section Stationing :
From "Geometric Data" screen, select "Cross Section"
Culvert Data :
From "Geometric Data" screen, select "Brdg/Culv",
"Culvert".(included on this screen are the culvert chart #,
diameter, D/S invert elev., exit loss coeff., length, "n" value, scale
#, shape, U/S invert elev)
Deck/ Roadway Data :
From "Geometric Data" screen, select "Brdg/Culv",
"Deck/Roadway"
Delete Cross Sections :
From "Geometric Data" screen, select "Cross Section", then
"Options", to "Delete Cross Section"
Discharge Changes :
From "Steady Flow Data" screen, select "Add a Flow Change
Location”
Discharges :
Select “Edit” on HEC-RAS main screen, “Steady Flow Data”
Drag Coefficient, Cd :
From “Geometric Data” screen, select “Brdg/Culv”, Bridge
Modeling Approach”
Encroachment Stations :
From "Steady Flow Analysis" screen, "Options",
"Encroachments"
Equivalent Roughness K :
From "Geometric Data" screen, select "Cross Section"
Errors, Warnings,& Notes :
“Summary of Errors, Warnings and Notes” is located under
“View” in the HEC-RAS main screen
Table 1: HEC-RAS Index (continued)
Feature/ Parameter
Location
Floodway Data :
Press "Steady Flow Analysis" button, "Options", "Encroachments"
Flow Dist'n (QL, QR, Qch) :
From "Steady Flow Analysis" screen "Options" "Flow Distribution
Locations"
Flow Regime :
From "Steady Flow Analysis" screen
Friction Slope Method :
From "Steady Flow Analysis" screen, "Options", "Friction Slope
Method"
Gated Spillway Data :
Press "Geometric Data" button, select "Inline Weir/Spillway"
Horiz. Variation in "n"
Values :
From "Geometric Data" screen, select "Cross Sections", "Options",
"Horizontal Variation in n Values"
Import Geometric Data :
From "Geometric Data" screen, "Import Geometric Data" is located
within "File" menu
Import HEC-2 :
"Import HEC-2 Data" is located in the "File" option from main
screen
Ineffective Flow Areas:
From "Geometric Data" screen, select "Cross Sections", "Options",
"Ineffective Flow Areas"
Inline Weir Data :
From "Geometric Data" screen, select "Inline Weir/Spillway"
Junction Data :
From "Geometric Data" screen, select "Junct"
Levees :
From "Geometric Data" screen, select "Cross Section", "Options",
"Levees"
Low Chord :
From "Geometric Data" screen, select "Brdg/Culv", select
"Deck/Roadway”
Manning's n Values :
From "Geometric Data" screen, select "Cross Section", or select
"Tables", "Manning's n or k values"
Multiple Opening
Analysis :
From "Geometric Data" screen, select "Brdg/Culv", select "Multiple
Opening Analysis"
Pier Data :
From "Geometric Data" screen, select "Brdg/Culv", select "Pier"
Pier Shape "K" :
From "Geometric Data" screen, select "Brdg/Culv", "Bridge
Modeling Approach”
Rating Curves :
Under the "View" menu in the HEC-RAS main menu, select
"Rating Curves"
Reach Lengths :
From "Geometric Data" screen, select "Cross Section", or select
"Tables", "Reach Lengths"
Rip-Rap Design :
From "Hydraulic Design" screen, select "Functions", "Rip-Rap
Design"
Scour at Bridges :
From "Hydraulic Design" screen from HEC-RAS main screen
Sediment Elevations:
From "Geometric Data" screen, select "Tools", "Fixed Sediment
Elevation"
Sloping Abutment :
From "Geometric Data" screen, select "Brdg/Culv", "Sloping
Abutment”
Starting WSEL :
From "Steady Flow Data" screen, select "Reach Boundary
Conditions"
Weir Crest Shape :
From "Geometric Data" screen, select "Brdg/Culv", select
"Deck/Roadway”
X-Y-Z Perspective Plots :
In "View" option in HEC-RAS main screen, select "X-Y-Z
Perspective Pots"
9.3 Comparison of Solution Methods and Computational Procedures
One-dimensional steady flow analysis forms the basis of the computations done by the
HEC-RAS (Version 3.0) computer program. The HEC-2 computer program also uses
the same analysis to compute water-surface profiles.
The types of flows analyzed by both these programs, along with the general solution
methods, are compiled from the HEC-RAS documentation and provided as a reference
in Table 2 entitled “Solution Methods Used for Analysis”. Similarly, a brief summary
of the computational procedures used by HEC-2 and HEC-RAS is given in Table 3
entitled “General computational Procedures Used for analysis”.
Table 2 and 3 contain the following information:
Column 1: Type of flow (Table 2) and hydraulic parameter (Table 3)
Column 2: Solution methods (Table 2) and computational procedures (Table 3) used
by HEC-2 and HEC-RAS
Column 3: Reference to sections of the HEC-RAS reference documentation that
contain discussions on the solution method.
The following annotations are used in column 3:
• UM: HEC-RAS River Analysis System Version 3.0 User’s Manual
• HRM: HEC-RAS River Analysis System Version 3.0 Hydraulic Reference
Manual
• AG: HEC-RAS River Analysis System Version 3.0 Applications Guide (AG)
• All of these documents are dated January 2001, and published by the USACE.
• In addition, following notations are used in column 3:
• Ch: - Chapter
• Ex: - Example
• p - Page number
Column 4: Remarks, related to the HEC-2 and HEC-RAS solution methods, are
noted in this column.
Table 2: Solution Methods Used for Analysis
A: Floodplains Unobstructed by Bridges and Culverts
Type of flow
Solution Method
HEC-2 & HEC-RAS
Reference from
HEC-RAS Manuals
Remarks
Open channel flow:
subcritical flow
Backwater analysis
downstream to upstream
HRM Ch: 2
AG Ex: 1, P1-1
None
Open channel flow:
supercritical flow
Draw down analysis
upstream to downstream
HRM Ch:4, P4-6,
HRM Ch: 3, P3-22
UM Ch:3, P3-11
AG Ex:1, P1-10
HEC-RAS: the flow regime is specified in the
steady flow analysis window
HEC-2: rearranging of the cross sections is
done manually
Open channel flow:
Mixed flow
Backwater analysis
HRM Ch: 4, P4-6,
HRM Ch: 3, P3-22
UM Ch:3, P3-11
AG Ex:1, P1-10
HEC-RAS: the flow regime is specified on the
steady flow analysis window
HEC-2: independent sub and supercritical
runs are used
Water-surface level with
air entrainment
EM 1110-2-1601 Method
HRM Ch:2, p2-20
Available in HEC-RAS only
Modeling energy losses
Friction loss and
transition loss
HRM Ch:3, P3-12
None
Modeling friction loss
Friction loss is evaluated
as the product of friction
slope and discharge
weighted reach length.
HRM Ch:2,
UM Ch:4, P4-5
None
Boundary roughness
Manning’s roughness
coefficients (n) and
roughness heights (k)
can be used; horizontal
variation
HRM Ch: 2, p2-6
HRM Ch:3, p3-12
AG Ex: 2, P2-6
UM Ch:6, P6-16
UM Ch:6, P6-74
HEC-RAS: based on main channel slope, a
single composite n value is computed for
channel, k-values are not generally used in
FISs
Expansion and
contraction coefficients
Coefficients are
multiplied by the mean
kinetic energy head of
the two cross sections
concerned.
HRM Ch:3, p3-19
UM Ch: 3, p3-8
Ch: 6, p6-9
None
Ineffective flow: water is
stored, not conveyed
Water will be stored but
will not be conveyed.
HRM Ch:3, p-3-7
UM Ch:6, p6-12
High Manning’s roughness coefficients are
used in HEC-2. Instead, blocked ineffective
flow option available in HEC-RAS can be
used.
Ineffective flow:
water flow is blocked by
high ground
Water will not be stored
or conveyed. High
grounds will cause
contraction and
expansion of flow.
HRM Ch:3, p3-8 to 11
UM Ch:6, p6-12 to 16
None
Levee on floodplain
Wetted perimeter
computed
HRM Ch:3, p3-8
AG Ex:2, p2-7
In HEC-2, X3 is used to model in effective
areas caused by levees. HEC-RAS has a
levee option to model the flow.
Floodplain with a
meandering channel
Reflected in the
Manning’s roughness
coefficient.
None
None
Table 2: Solution Methods Used for Analysis (continued)
B: Flow Junctions
Type of flow
Solution Methods
HEC-2 & HEC-RAS
Reference from
HEC-RAS Manuals
Remarks
Flow combination:
subcritical flow
Default is energy balance
method. Momentum
balance method is also
available
HRM Ch:4, p4-9
UM Ch:6, p6-3, 18
AG Ex:10, p10
AG Ex:8, p8-3
Similar to HEC-2 Tributary option.
Momentum balance is not available in HEC-
2. Unless coincident peaks occur; not used
for FISs.
Flow split:
subcritical flow
Default is energy
balance.
HRM Ch:4, p4-11
UM Ch:6, p6-3, 18
None
Flow combination:
supercritical flow
Default is energy
balance.
Momentum balance
method is more suitable.
HRM Ch.4, p4-12
UM Ch. 6, p6-3, 18
AG Ex:10. p10-11
Generally, not used for FISs.
Flow split: supercritical
flow
Default is momentum
balance. Momentum
balance is more suitable.
HRM Ch:4, p4-13
UM Ch:6, p6-18
None
Flow combination: mixed
flow
HEC-RAS uses the
appropriate method
(momentum or energy).
HRM Ch:4, p4-14
UM Ch:6, 6-18
AG Ex:10, p10-5
Not available in HEC-2
Flow split: mixed flow
HEC-RAS uses the
appropriate method
(momentum or energy).
HRM Ch:4, p4-15
UM Ch:6, 6-18
Not available in HEC-2
Split flow discharge
computation
HEC-RAS and HEC-2
have the capability to
model the flow over
levees or side weirs and
the overtopping of
watershed divides.
AG Ex:8, p8-5
In both programs, lateral weirs and lateral
rating curves are used to model split flows.
Looped stream
Discharges in each
branch are computed by
trial and error.
AG Ex:8, p8-5
HEC-2 cannot model a looped stream
system. In HEC-RAS, iterative procedures
are used by the user to determine the
discharges in each branch.
Table 2: Solution Methods Used for Analysis (continued)
C: Flow Obstructed by Structures
Type of Flow
Solution Method
HEC-2 & HEC-RAS
Reference From
HEC-RAS Manuals
Remarks
Bridges and culverts
Flow compression upstream
of the structure, flow across
the structure, and flow
expansion downstream of the
structure are considered.
HRM Ch:5, p5-1
HRM Ch:6, p6-1
UM Ch:6, p6-19
None
Topographic data
necessary to model
bridge flow
Four cross sections and the
bridge geometry are given as
input data. Cross sections4
and 3 are upstream of the
bridge; 1 and 2 are
downstream.
HRM Ch:5, p5-2,11
UM Ch:6, p6-20
UM Ch:6, p6-30, 35,
UM Ch:6, p6-39, 47
AG Ex:2, p2-3-12
Similar to HEC-2 special bridge model.
However, using the input data, HEC-RAS
can create internal bridge cross sections
inside of the bridge structure. These cross
sections can be changed/edited by the user.
Upstream contraction
of flow
Ineffective flow areas defined
at cross section 3,
immediately upstream of the
bridge. Guidance is given to
select the contraction reach
length. Recommended
contraction coefficient is 0.3.
HRM Ch:5, p5-8
UM Ch:6, p6-22
AG Ex:2, p2-26
AG EX:3, p3-19
None
Downstream expansion
of flow.
Ineffective flow areas defined
at cross section 2, just
downstream of bridge.
Guidance is given to select
an expansion reach length.
Recommended expansion
coefficient is 0.5.
HRM Ch:5, p5-8
UM Ch:6, p6-22
AG: Ex:2, p2-24
AG EX:3, p3-19
None
Ineffective flow areas
Ineffective flow area is
modeled as normal
ineffective area (no wetted
perimeter is added) or
blocked ineffective area
(wetted perimeter is added).
HRM Ch:3, p3-7
HRM Ch:5, p5-5 to 8
UM Ch:6, p6-12, 30
AG Ex:2, p2-12
AG Ex:3, p3-4
In HEC-2 X3 statements are used to define
ineffective flow areas at two locations on the
cross section. HEC-RAS has option to
model many blocked obstructions.
Low flow through the
bridge
Energy equation, momentum
balance, Yarnell equation,
and FHWA WSPRO method
are available. Momentum
balance is suitable for super
critical low flow passes
through the bridge.
HRM Ch:5, p5-9 to 18
HRM Ch:5, p5-26
UM Ch:6, p6-34
AG Ex:2, p2-14
Energy equation is similar to HEC-2 Normal
bridge analysis. Yarnell equation and
Momentum balance analyses were available
in HEC-2 Special bridge analysis. HEC-
RAS gives the user to choose the method of
computation.
High flow through the
bridge
Energy equation, pressure
flow analysis, and pressure
and weir flow analysis are the
three methods available.
HRM Ch:5, p5-18 to23
UM Ch:6, p6-34
AG Ex:2, p2-16
Similar to HEC-2 Special bridge analysis.
Subcritical flow regime is used for FIS
analyses.
Table 2: Solution Methods Used for Analysis (continued)
C: Flow Obstructed by Structures (continued)
Type of flow
Solution Method
HEC-2 & HEC-RAS
Reference to
HEC-RAS Manuals
Remark
High flow : perched
bridges
Iterative methods will be
used to analyze the low
flow in the bridge and
weir flow on the over
banks.
HRM Ch:5, p5-28
HEC-2 Special bridge analysis is used with
iterative method to compute flood elevations.
HEC-RAS Multiple Opening Option
automates the iterative procedure.
High flow : perched
bridge
Pressure and weir flow
through the high bridge
and low flow on both
overbanks.
None
None
High flow : low water
bridges
Energy equations
available in HEC-RAS
are suitable to model
highly submerged bridge
flows.
HRM Ch:5, p5-29
Similar to HEC-2 Normal bridge analysis.
Flow through bridges in a
skew
Adjustments are done to
the bridge dimensions to
reflect the effective flow
area.
HRM Ch:5, p5-29
Bridge data are revised by the user to reflect
effective area. HEC-2 and HEC-RAS can do
this adjustment internally.
Parallel bridges
Low flow analysis or high
flow analysis.
HRM Ch:5, p5-31
Cross section immediately upstream of one
bridge (#3) will be used as the downstream
cross section of the next bridge (#2).
Flow through culverts:
subcritical flow
Inlet control, outlet
control computations
based on the 1985
FHWA publication titled,
“Hydraulic Design of
Highway Culverts.”
HRM Ch:6, p6-9
UM Ch:6, p6-40
Similar to HEC-2 special culvert analysis.
This analysis is not suitable for supercritical
flows through culverts.
Flow through culverts:
supercritical flow
Results of culvert
analysis for super critical
flow regime should be
carefully checked with
those of mixed flow and
sub critical flow regime.
Alternatively, energy
equation can also be
used to analyze super
critical flows through
culverts.
HRM Ch:6, p6-18
HEC-2 cannot model super critical flow
through culverts.
Table 3: General Computational Procedures Used for Analysis
A: Floodplains Unobstructed by Structures
Parameter
Computation method
HEC-RAS & HEC-2
Reference from
HEC-RAS Manuals
Remarks
Conveyance
Cross section is
subdivided to compute
the conveyance.
HRM Ch:2, p2-5
UM Ch:7, p7-10, 13
Overbanks of the cross sections are
subdivided at locations where the Manning’s
roughness coefficients change. In HEC-2,
these subdivisions occurred at each
coordinate point. The HEC-2 method of
computation is also available in HEC-RAS.
Composite roughness
coefficient
Horton’s method is used.
HRM Ch:2, p2-6
When main channel side slope is larger than
5:1 (vertical to horizontal), a composite
channel roughness coefficient is computed in
HEC-RAS.
Reach lengths
Weighted average reach
length.
HRM Ch:2, p2-3
None
Mean kinetic energy
head
Discharge-weighted
velocity heads computed
for overbanks and the
channel are summed.
HRM Ch:2, p2-8
None
Friction loss evaluation
Default is the average
conveyance equation.
HRM Ch:2, p2-9
Ch:4, p4-2
Option to choose the friction loss equation
based on flow is not used in FISs.
Contraction and
expansion loss
Difference of mean
kinetic energy head
between the two cross
sections is multiplied by
the coefficient.
HRM Ch:2, p2-11
None
Computation of the
water-surface elevation
Iterative procedure is
used.
HRM Ch:2, p2-11
Default tolerance in metric system is 0.003
meter. This tolerance is 0.01 meter in HEC-2.
Checking the flow regime
Critical depths are
computed for flows with
Froude number >0.94
HRM Ch:2, p2-13
None
Critical depth
computation
Elevation with the
minimum specific energy.
HEC-RAS: secant
method available to
determine critical depth
when multiple minimum;
specific energy points
exist.
HRM Ch:2, p2-13
HRM Ap:C, pC-1
AG EX:1, p1-9
UM Ch: 7, p7-14
Multiple critical depth search option is
available in HEC-RAS.
Ineffective flow area
computations
No additional wetted
parameter is computed.
HRM Ch:3, p3-7
None
Flow distribution within a
cross section
Hydraulic properties are
computed for segments
within a cross section.
HRM Ch:4, p4-19
Up to 45 segments can be defined. Similar
feature is available in the J2 statement of
HEC-2.
9.4 Precautions to be Taken when Importing HEC-2 Input Data
The observations from a series of sensitivity tests conducted with importing HEC-2
input files are summarized in this section. Detailed observations are provided in
appendix B and the models are available in the enclosed computer disk. Six HEC-2
input files are selected to test the import feature of HEC_RAS. The tests were done on
the following models:
• Natural Floodplain Model (Appendix B-1)
• Floodplain Model with Structures (Appendix B-2)
• Model with Tributaries (Appendix B-3)
• Ice-jam Model (Appendix B-4)
• Encroachment Model (Appendix B-5)
• Split Flow Model [Appendix B-6]
The sensitivity tests indicated that water-surface elevations computed by using raw
HEC-RAS input data, created from HEC-2 (using the “Import” feature), were generally
different than the HEC-2 results. In many cases, these differences are small and of
the order of 0.2 foot. Since there are differences in the computational methodology
between HEC-2 and HEC-RAS, a HEC-RAS input data created by importing a HEC-2
input has not produced water-surface elevations that are identical to HEC-2 results.
However, many of the differences can be eliminated by identifying the cause and
selecting HEC-RAS computational procedures that are similar to HEC-2. Following
HEC-RAS computations were observed to cause differences in computed results:
• Conveyance Computation
• Imported bridge modeling data
• Critical depth computation
9.4.1 Conveyance Computation
In all cases observable differences in water-surface elevations were created by the
different conveyance computation methods used by HEC-RAS and HEC-2. The HEC-
RAS method appears to compute higher water-surface elevations than the HEC-2
method. However, as pointed out in section 2.4, the HEC-2 conveyance computation
method is available in HEC-RAS. Changing the conveyance computation method in
HEC-RAS to the HEC-2 method always produced water-surface elevations that are
closer to HEC-2 results. HEC-2 conveyance computation method can be selected from
the “Steady Flow Analysis” screen. The “Options” feature of the “Steady Flow Analysis”
screen contains the menu bar to change the conveyance computation method from
HEC-RAS to HEC-2.
9.4.2 Imported Bridge Modeling Data
Adjustments were necessary to the imported HEC-2 Bridge and Culvert input data to
obtain correct modeling in HEC-RAS. Either one or more of the following adjustments
were included to the HEC-RAS bridge modeling data that was imported from HEC-2:
• Revision of the internal cross section data (accessed through “options” feature
available in the “Bridge Culvert Data “screen)
• Revision of the bridge deck data (accessed through the “Deck/ Roadway”
feature available in the “Bridge/Culvert Data” screen)
• Revision of the pier data (accessed through “Pier” feature in “Bridge/ Culvert
Screen”
The skew factor applied to the bridge and bridge deck geometry was imported correctly
into the HEC-RAS model.
9.4.3 Critical Depth Computation
Generally, the critical depth computed by HEC-2 and HEC-RAS matched closely.
However, in some cases, the critical depth computed by HEC-2 was smaller than that
computed by the HEC-RAS program.
9.4.4 Other Observations
The sensitivity tests included importing HEC-2 input data with tributary modeling, ice
jam modeling, and encroachment modeling. HEC-2 model with tributary option was
imported into the HEC-RAS; minor modifications were necessary to model flow
junction accurately according to HEC-RAS guidelines. Even though, HEC-RAS
imported the Ice-jam model into HEC-RAS, the input data needed adjustments before
it could be analyzed (by HEC-RAS). Generally, the encroachment data was imported
correctly into HEC-RAS. However, minor differences in the surcharge values were
observed for HEC-2 and HEC-RAS cross sections located near the bridges. The split
flow records were not imported into the HEC-RAS model. However, when the lateral
weir and lateral rating curves were manually included HEC-RAS computed
corresponding split flow discharges and water-surface elevations. There were minor
differences in the results computed by HEC-RAS and HEC-2.
CHAPTER 10: ACQUIRING HEC-RAS
The HEC-RAS computer program and the documentation, HEC-RAS User’s Manual,
HEC-RAS Hydraulic Reference Manual, and HEC-RAS Applications Guide, can be
downloaded from the Internet from the Hydrologic Engineering Center (HEC) home
page at:
http://www.hec.usace.army.mil
This home page has useful information on installation problems and ways to resolve
these problems.
The program and documentation can be purchased from the official vendors of the
HEC as well as from the NTIS. The installation procedure is discussed in Chapter 2 of
the HEC-RAS User’s Manual.
HEC-RAS is a federally developed program and available to users; there are no
copyright restrictions.
APPENDIX A: USE OF HEC-RAS FOR FLOOD INSURANCE STUDIES
The Federal Emergency Management Agency (FEMA) has adopted guidance that
hydraulic analyses for contracted Flood Insurance Studies and restudies of entire
watersheds (with detailed HEC-2 hydraulic analysis) should be conducted using
HEC-RAS instead of HEC-2. Details of this guidance are documented in FEMA
Memoranda dated March 14, 1997, October 19, 2000, and February 8, 2000. In
addition, FEMA issued a guidance that encouraged the use of HEC-RAS for revisions
to effective HEC-2 analyses. This guidance is documented in a FEMA memorandum
dated, April 30, 2001. Copies of these memoranda are included in Appendix A.
Policy for Use of HEC-RAS Memorandum
Policy for Use of HEC-RAS Memorandum - Page 2
Procedure Memorandum No. 16 - Use of HEC-RAS Version 2.2 - Page 1
Implementation of HEC-RAS Version 2.2 for NFIP Mapping - Page 1
Implementation of HEC-RAS Version 2.2 for NFIP Mapping - Page 2
Policy for Use of HEC-RAS in the NFIP - Page 1
Policy for Use of HEC-RAS in the NFIP - Page 2
APPENDIX B: CONVERSION OF HEC-2 INPUT DATA TO HEC-RAS (EXAMPLES)
HEC-RAS can import data from HEC-2 and calculate water-surface elevations based
on that data. Some HEC-2 features, however, are treated differently in HEC-RAS.
This appendix lists the important observations of sensitivity tests conducted with
importing HEC-2 input data sets into HEC-RAS to compare the results.
The HEC-2 model data sets used for examples are as follows:
Example 1: Importing HEC-2 Natural Floodplain Model
Example 2: Importing HEC-2 Bridge and Culvert Models
Example 3: Importing HEC-2 Model with a Tributary
Example 4: Importing HEC-2 Ice Jam Model
Example 5: Importing HEC-2 Encroachment Model
Example 6: Importing HEC-2 Split Flow Model
Example 1: Importing HEC-2 Natural Floodplain Model
INTRODUCTION
This sensitivity test used the HEC-2 model fplnhc.dat. This model is given in the
attached computer disk. The HEC-RAS model fplain30.prj is created by importing the
HEC-2 model. Few modifications were added to the imported HEC-RAS data in order
to obtain results that matched closely with HEC-2.
HEC-2 MODEL OF NATURAL FLOODPLAIN
The floodplain contained no structures. The following HEC-2 features were used in
the HEC-2 model fplnhc.dat:
• Title Records, T1, T2, T3
• Comment Records
• Job control records, J1, J2, J3, J5, EJ, and ER
• Manning’s roughness coefficient records NC and NH
• Discharge record QT
• Cross section records, X1, GR, X2, X4, and X5
The schematic diagram created by HEC-RAS for this HEC-2 input data is shown in
Figure 1.1.
Figure 1.1: Schematic Diagram - Natural Floodplain Model
Figure 1.1: Schematic Diagram – Natural Floodplain Model
Figure 1.1: Schematic Diagram - Natural Floodplain Model
OBSERVATIONS
The water-surface elevations computed by the HEC-2 model and the HEC-RAS models
are compared in Table 1.1. With some revisions to the imported HEC-RAS input data
set, HEC-2 and revised HEC-RAS produced comparable results. The differences in
HEC-2 and the initial HEC-RAS (raw input data created by HEC-RAS import feature)
results appear to be caused by the different algorithms used for the computation of
the critical depth of the flow and the conveyance of a cross section. This section
summarizes our observations on the performance of the HEC-RAS import feature in
importing the HEC-2 model fplnhc.dat.
Table 1.1: Comparison of Water-Surface Elevations – Natural Floodplain Models
Cross Section
Elevation (Feet NGVD)
HEC-2
HEC-RAS
imported
HEC-RAS
Revised
76.8
644.69
644.86
644.69
77
644.89
645.06
644.89
77.37
645.25
645.26
645.25
77.4
645.29
645.31
645.29
77.59
645.36
645.38
645.36
77.8
645.40
645.42
645.40
77.97
645.46
645.49
645.46
78.3
645.60
645.63
645.60
78.41
646.38
645.75
645.71
78.62
645.78
645.82
645.78
78.82
645.86
645.90
645.86
79.11
746.03
746.08
646.03
79.22
646.12
646.16
646.11
79.32
646.25
646.29
646.25
79.89
646.61
646.67
646.61
80.1
646.86
646.91
646.85
Title Records
The alphanumeric data given the first title record T1 are displayed in the project
description window of the HEC-RAS main window. The first six lines of the HEC-2
title data (T1 to T6) are imported as the detailed project summary and can be viewed
by expanding the project summary window. When “Generate Report” command (HEC-
RAS main window under the “File” menu) is used all the title records used in the HEC-
2 file are printed.
Comment Records
The HEC-2 comment records placed (using *) within the first six lines of the HEC-2
input data are imported into the detailed project summary available on the HEC-RAS
main window. All of the HEC-2 comment records can be viewed in the project
summary report generated using “Generate Report” command. The HEC-2 comment
record created using record C is reproduced in the description area of the cross
section data window. In addition, whenever a cross section is repeated, HEC-RAS
creates a note in the description area of the cross section data window. However, in
Example 2, “Importing HEC-2 Bridge and Culvert Models”, we noted that comment
records placed in between HEC-2 GR and BT records interfered with the function of
the HEC-RAS import feature. The topographic data entered in HEC-2 GR and BT
records subsequent to a comment record were not imported into the HEC-RAS input
data.
Job Control Records
HEC-2 Job control records define the following features:
• Flow regime (J1, field 4)
• Boundary condition (J1, field 5)
• Starting water-surface elevation (J1, field 9)
• English/ Metric Units (J1, field 6)
• Interpolated Cross sections (J1, field 7)
• Discharge (J1, field 8)
• Flow distribution (J2, field 10)
• Multiplying factors for discharge (J1, field 10), and Manning’s roughness
coefficients (J2, field 6)
• Number of profiles (J2, field 1)
• End job and end run (EJ and ER)
Flow Regime (J1, field 4)
Supercritical and subcritical flow regimes defined in HEC-2 are reflected in the
imported HEC-RAS input. Subcritical and supercritical flow regimes are defined in the
“Steady Flow Analysis” screen accessed through the “Simulate” command available in
the HEC-RAS main screen.
Boundary Condition (J1, field 5)
HEC-RAS imports all starting water-surface conditions that HEC-2 uses. Except for
the rating curve, HEC-RAS can also apply different starting water-surface conditions
for individual profiles. If HEC-RAS imports a multi-profile HEC-2 model where one of
the profiles uses a rating curve, HEC-RAS will assign that rating curve to all other
profiles. Boundary conditions are defined in the “Steady Flow analysis” screen
accessed through the “Simulate” command available in the HEC-RAS main screen.
English/ Metric Units (J1, field 6)
The definition of units in HEC-2 input data set (J1, 6) is imported correctly into HEC-
RAS. The unit system is defined in the “Options” menu available in the main screen.
Interpolated Cross Sections (J1, field 7)
HEC-2 provides a cross section interpolation option, where the interpolation is
performed between cross sections with a change in velocity head greater than the
value assigned in field 7 of the J1 record. HEC-RAS import feature did not import the
HEC-2 input data requesting automatic interpolated cross sections (J1, 7). In HEC-
RAS, interpolated cross sections can be created using the “Option” menu in the
“Geometric Data” screen. This window is accessed through the HEC-RAS main
screen.
Multiplication Factors for Discharge (J1, field 10) and
Manning’s Roughness Coefficients (J2, field 6)
Discharge multiplication factor was not imported into HEC-RAS. Manning value
multiplication factor was imported into HEC-RAS. However, this factor was used to
adjust the roughness coefficient values for all of the profiles. This application is
different from HEC-2, where the factors alter the roughness coefficients of the first
profile only. In HEC-RAS, the roughness coefficients can be viewed from the “Cross
Section Window”. This window is accessed through the “Edit” menu of the main HEC-
RAS screen.
Discharges (J1, field 8)
The discharge defined in the J1 record of the HEC-2 model is reflected correctly in the
imported HEC-RAS input data set. Discharges used in HEC-RAS analysis are defined
in the “Steady Flow analysis” screen. This screen is accessed through the “Simulate”
menu available in the HEC-RAS main screen.
Number of Profiles (J2, field 1)
The maximum number of profiles computed by the HEC-2 program is 15. Sometimes
in HEC-2, this number, which is read in the J2 record (field 1), is used to terminate a
multiple profile execution. HEC-RAS ignores this application and proceeds to compute
all the profiles originally requested in the imported HEC-2 input.
End Job and End Run (ER and EJ)
In HEC-2 input data set, an ER record placed after J2 record at the end will terminate
the HEC-2 computation process with that execution. However, HEC-RAS ignores the
ER record and proceeds to complete the runs listed below the ER record (in the
imported HEC-2 input file).
Roughness, and Expansion, Contraction Coefficients - NC and NH Records
HEC-RAS imports NC and NH cards from HEC-2 and assigns the roughness
coefficients and the expansion, contraction coefficients to the appropriate cross
sections except in one case. A NC record added after an NH record to define new
expansion and contraction coefficients has made the HEC-RAS to use the roughness
coefficients used in the previous NC record in the HEC-2 data set (instead of the data
defined in the NH statement for that cross section). A summary of the roughness
coefficients and expansion, contraction coefficients used in the HEC-RAS model for all
the cross sections can be viewed by using the “Tables” option available at the
Geometric Data screen or in the “Cross Section Data” screen for individual cross
sections.
Discharge Record QT
HEC-RAS assigns discharge values to cross sections in a downstream direction as
opposed to an upstream direction in HEC-2. The discharges assigned in the HEC-2
model are imported correctly into the HEC-RAS input data set. A summary of the
discharges used in the HEC-RAS model can be viewed at the “Steady Flow Data”
screen.
Cross Section Records, X1, GR, X2, X4, and X5
X1Record
X1 record assigns a numeric name to a cross section and defines the following:
• number of points used in the topographic data (X1, 2);
• channel bank stations (X1, fields 3 and 4);
• distance between cross sections (X1, fields5, 6, and 7); and
• horizontal and vertical adjustments to surveyed stations and elevations (X1,
fields 8 and 9).
These data are imported correctly into HEC-RAS and can be viewed through the
“Cross Section Data” screen of individual cross sections. The horizontal and vertical
adjustments defined in X1 (fields 8 and 9) would have been applied to the HEC-RAS
cross section data.
GR Record
Topographic data defined in the GR record are imported correctly into HEC-RAS and
can be viewed through the “Cross Section Data” screen of individual cross sections.
However, in Example 2, “Importing HEC-2 Bridge and Culvert Models”, we noted that
comment records placed in between HEC-2 GR and BT records interfered with the
function of the HEC-RAS import feature. The topographic data entered in HEC-2 GR
and BT records subsequent to a comment record were not imported into the HEC-RAS
input data.
X2 Record
Observed water-surface elevations specified in X2 cards are included in HEC-RAS
output tables. These elevations can be viewed from the “Options” menu available in
the “Steady Flow Data” screen.
X4 Record
Cross section station and elevation adjustments specified in X4 records are imported
into HEC-RAS cross section values. Similar to HEC-2, HEC-RAS will not use the
specified vertical adjustments (X1, field 9) to modify the data given in an X4 record.
Information provided in X1, GR, X2, and X4 records can be viewed using the Cross
Section Data screen (through Geometric Data screen).
X5 Record
The known water-surface elevations specified in X5 cards are imported correctly into
HEC-RAS. This data can be viewed at “Set Changes in WS and EG” command of the
“Options” menu available at the “Steady Flow Data” screen.
CONCLUSION
This sensitivity test indicates that with appropriate changes, the HEC-RAS model
computed water-surface elevations identical to those computed by the imported
HEC-2 model.
Example 2: Importing HEC-2 Bridge and Culvert Models
INTRODUCTION
HEC-2 models, structure.dat and struct2.dat (provided in the enclosed computer disk)
are used in the sensitivity test. These models are identical, except for the removal of
unnecessary comment records in struct2.dat. In this sensitivity test, we noted that
the HEC-RAS did not import the topographic data (such as the data entered on HEC-2
GR and BT records) entered after HEC-2 comment (*) records in the corresponding
HEC-2 model. The str2.prj data file contains the HEC-RAS model created by
importing HEC-2 input data in struct2.dat. The HEC-2 model used Normal Bridge,
Special Bridge, and Special Culvert procedures to model the flow through 18 bridges
and 2 culverts.
HEC-2 MODEL OF FLOODPLAIN WITH BRIDGES AND CULVERTS
In addition to the records necessary to model flows through natural floodplains (which
are discussed in Example 1), the following HEC-2 features were used in Struct2.dat:
• Definition of the bridge geometry and piers, BT, X2, GR
• HEC-2 Normal bridge analysis: low flow
• HEC-2 Special bridge analysis: low flow SB
• HEC-2 Special bridge analysis: pressure and weir flow
• HEC-2 Special culvert analysis:
• HEC-2 long pipe flow: SB
• Bridge crossing with a skew
The schematic diagram for this example is shown in Figure 2.1. This diagram is
created by HEC-RAS using the imported HEC-2 input data.
Figure 2.1: Schematic Diagram - Model with Bridges and CulvertsFigure 2.1: Schematic Diagram – Model with Bridges and Culverts Figure 2.1: Schematic Diagram – Model with Bridges and Culverts
OBSERVATIONS
The water-surface elevations computed by the HEC-2 model and the HEC-RAS models
are compared in Table 2.1. The imported HEC-2 geometric data needed some
revisions to correctly model the flow through these structures with the HEC-RAS.
Except for the HEC-2 Special Bridge Analysis of low-flows in bridges, the modified
HEC-RAS model computed flood elevations that compared well with the HEC-2
results. This section summarizes our observations on the performance of the HEC-
RAS import feature in importing the HEC-2 model struct2.dat and the modifications
included into the raw imported HEC-RAS input data. These modifications were
necessary to model the bridge and culvert flows according to HEC-RAS guidelines. A
brief summary of methods used by HEC-2 and HEC-RAS to analyze bridge and culvert
flows precedes the discussion on modifications included to the HEC-RAS data.
Analysis of Flow Through Bridges and Culverts
In HEC-2, bridge flow is analyzed using Special Bridge, and Normal Bridge analyses.
Low flows through bridges are generally analyzed using HEC-2 Normal Bridge
analysis. Friction losses and contraction and expansion losses occurring in the
vicinity of the bridge structure are considered in computing the water-surface
elevations. The pressure and weir flow parameters are defined in the HEC-2 SB and
X2 records. The bridge geometry (bridge opening, bridge deck, piers, and over banks)
is generally defined using the BT, GR, and X2 records. The culvert flow is analyzed
using the input data defined in HEC-2 SC and X2 record.
Table 2.1: HEC-2 to HEC-RAS – Bridge and Culvert Models
Bridge
Cross
Section
HEC-2
Method
Changes to HEC-RAS Data
Elevation (feet NGVD)
HEC-2
HEC-RAS
Imported
HEC-RAS
Revised
Private
Drive
1.9
Normal
Bridge
Define pier
Delete cross sections 2&3
Revise the bridge distance
410.63
410.57
410.57
2
410.64
410.59
410.58
3
410.67
410.64
410.61
3.1
410.72
410.65
410.65
Table 2.1: HEC-2 to HEC-RAS – Bridge and Culvert Models (continued)
Bridge
Cross
Section
HEC-2
Method
Changes to HEC-RAS Data
Elevation (feet NGVD)
HEC-2
HEC-RAS
Imported
HEC-RAS
Revised
Visco
Drive
4.9
Normal
Bridge
Delete cross sections 5&6
Revise the bridge distance
Define the piers in “Pier Data”
Remove the piers from
Internal Cross Section or
Road/ Deck data
411.41
411.35
411.35
5
411.41
411.35
411.36
6
411.43
411.38
411.37
6.1
411.6
411.55
411.54
Rail Road
7.9
Normal
Bridge
Delete cross sections 8&9
Add bridge distance at “Road/
Deck” screen
Define the piers in “Pier Data”
Remove the piers from
Internal Cross Section or
Road/ Deck data
412.56
412.51
412.51
8
412.51
412.46
412.46
9
412.59
412.54
412.53
9.1
412.76
412.72
412.71
Unnamed
Road
10.9
Normal
Bridge
Delete cross sections 11&12
Add bridge distance at “Road/
Deck” screen
413.73
413.69
413.69
12
412.33
415.86
415.96
12.1
416.94
416.9
416.90
Interstate
24-40
14.9
Normal
Bridge
Delete cross sections 15&16
Add bridge distance at “Road/
Deck” screen
418.65
418.63
418.62
15
418.62
418.6
418.6
16
418.76
418.74
418.74
16.1
418.87
418.85
418.87
Table 2.1: HEC-2 to HEC-RAS – Bridge and Culvert Models (continued)
Bridge
Cross
Section
HEC-2
Method
Changes to HEC-RAS Data
Elevation (feet NGVD)
HEC-2
HEC-RAS
Imported
HEC-RAS
Revised
Rail Road
18.9
Normal
Bridge
Delete cross sections 19&20
Add bridge distance at “Road/
Deck” screen
420.55
420.54
420.55
19
420.41
420.57
420.41
20
420.57
420.97
420.57
20.1
420.72
420.9
420.72
Murfrees.
21.9
Normal
Bridge
Delete cross sections 22&23
Add bridge distance at “Road/
Deck” screen
Change roughness coefficients
of the internal bridge cross
sections to match HEC-2 data
421.83
421.91
421.84
22
420.24
420.41
420.24
23
421.08
424.18
421.10
23.1
430.91
432.97
430.92
Hart St.
26.9
Normal
Bridge
Delete cross sections 27&28
Add bridge distance at “Road/
Deck” screen
432.12
433.59
432.14
27
432.14
433.59
432.17
28
432.15
433.6
432.18
28.1
432.14
433.6
432.18
Factory
Street
30
Normal
Bridge
Same as other normal bridges
432.38
433.7
432.38
31
432.4
433.73
433.43
RR
32.9
Special
Bridge
Adjust pier center line for
internal cross sections
X5 used to correct HEC-2
433.63
434.3
433.64
34
433.63
441.2
440.90
34.1
441.13
441.41
441.13
Moor Av.
39.9
Special
Bridge
None
442.90
443.01
442.90
40
442.90
443.15
443.06
Nolensvlle
43
Special
Bridge
None
445.63
445.64
445.64
44
446.91
448.09
448.05
Table 2.1: HEC-2 to HEC-RAS – Bridge and Culvert Models (continued)
Bridge
Cross
Section
HEC-2
Method
Changes to HEC-RAS Data
Elevation (feet NGVD)
HEC-2
HEC-RAS
Imported
HEC-RAS
Revised
Private Dr.
46.9
Normal
Bridge
Same as other normal bridges
451.23
451.25
451.25
47
453.8
452.77
453.76
48
454.58
455.08
454.55
48.1
454.46
455.07
454.44
Private Dr.
49.9
Special
Culvert
None
457.51
457.61
457.51
50
457.51
457.6
457.58
Private Dr.
53
Normal
Bridge
and
Culvert
Same as other normal bridges
456.58
456.69
456.58
54
458.06
458.08
457.84
54.1
458.7
458.74
458.42
Private Dr.
55.9
Normal
Bridge
Delete cross sections 56&57
Add bridge distance at “Road/
Deck” screen
461.21
461.25
461.26
56
461.22
461.25
461.27
57
461.22
461.25
461.27
57.1
461.22
461.26
461.28
Private Dr.
58.9
Normal
Bridge
Delete cross sections 59&60
Add bridge distance at “Road/
Deck” screen
461.23
461.27
461.29
59
461.24
461.27
461.29
60
461.25
461.27
461.30
60.1
461.25
461.28
461.31
Bransford
62
Special
Culvert
None
461.52
461.55
461.57
63
462.16
462.4
462.42
Table 2.1: HEC-2 to HEC-RAS – Bridge and Culvert Models (continued)
Bridge
Cross
Section
HEC-2
Method
Changes to HEC-RAS Data
Elevation (feet NGVD)
HEC-2
HEC-RAS
HEC-RAS
Craighead
64.9
Normal
Bridge
Delete cross sections 65 & 66
Add bridge distance
Define the piers in “Pier Data”
Remove the piers from
Internal Cross Section or
Road/ Deck data
464.46
464.46
464.48
65
464.51
464.49
464.53
66
464.14
464.63
464.64
66.1
464.64
464.63
464.64
RR
70
Special
Bridge
None
470.05
470.05
470.05
71
472.92
472.75
472.75
72
477.74
477.71
477.71
73
477.81
477.78
477.78
I-440
74
Special
Bridge
Revise bridge deck
definition for internal
cross sections
477.57
477.54
477.54
75
511.19
499.23
511.32
Normal Bridge
In HEC-2 six cross sections are used to define flow through a bridge. Cross sections 1
and 6 represent stream flows that are unaffected by the structure. Cross sections 5
and 2 reflect stream flows just upstream and downstream of the bridge; contraction of
flow occurs upstream of the bridge (between cross section 6 and 5) and expansion of
flow occurs downstream of the bridge (between cross sections 2 and 1). Cross sections
3 and 4, at the downstream and upstream faces of the bridge, and the bridge deck and
road information available in HEC-2 BT records define the geometry of the bridge
opening.
Special Bridge
HEC-2 uses four cross sections to reflect a special bridge. Cross sections 1 and 4
represent stream flows that are unaffected by the structure. Contraction of flow
occurs upstream of the bridge (between cross sections 4 and 3) and expansion of flow
occurs downstream of the bridge (between cross sections 2 and 1). Cross sections 3
and 4 are at the downstream and upstream of the bridge, respectively and the bridge
deck and road information available in HEC-2 BT records define the geometry of the
bridge opening. The coefficients provided in HEC-2 SB records are used to analyze
pressure and weir flow through the bridges.
HEC-RAS Analysis of Flow Through Bridges
HEC-RAS used four stream cross sections and two internal bridge cross sections to
define the flow through a bridge. Cross sections 1 and 4 represent stream flows that
are unaffected by the structure. Contraction of flow occurs upstream of the bridge
(between cross sections 4 and 3) and expansion of flow occurs downstream of the
bridge (between cross sections 2 and 1). The bridge geometry is defined through the
“Bridge Culvert Data” screen accessed through the Geometric Data editor. To define
the bridge geometry, HEC-RAS requires two internal cross sections (defining the bridge
opening), bridge deck data, and pier data. The HEC-RAS internal bridge cross
sections are automatically altered to match the cross sections immediately upstream
and downstream of the bridge (HEC-RAS cross sections 3 and 2). However, this
automatic alteration feature is terminated once the internal cross section is manually
revised.
HEC-RAS does not distinguish between a ‘normal’ and ‘special’ bridge as HEC-2 does.
For low flows through bridges, the user can select from four methods: Energy,
Momentum, Yarnell, and WSPRO. For, high flows, two methods are available. They
are Energy method, and Pressure and Weir method. Generally, Energy method is
assigned to HEC-RAS bridge flow data imported from HEC-2 Normal Bridge data.
Pressure and Weir method is assigned to HEC-RAS bridge flow data imported from
HEC-2 Special Bridge data.
Analysis of Flow through Culverts
Both, HEC-2 and HEC-RAS uses the Federal Highways Administration’s inlet control
and outlet control analysis method to compute the flood elevations for flow through
culverts.
Skew Angles
HEC-RAS reflected the skew factor in defining the bridge and bridge deck geometry for
the bridge at cross section 28. HEC-RAS internally adjusted the ineffective flow areas
on the internal bridge cross sections were also adjusted to reflect the skewed geometry
of the bridge.
Summary of Modifications to HEC-RAS Bridge and Culvert Models
General
Roughness Coefficients:
For the bridge at cross section 23, roughness coefficients defined at HEC-2 cross
section just inside the downstream face of the bridge (Cross section 3 for HEC-2
normal bridge model) were not imported into the HEC-RAS model. The correct
roughness coefficients of the internal bridge cross sections were revised to match
those defined in the HEC-2 model. In HEC-RAS, these roughness coefficients are
accessed through “Options” feature available in the “Bridge Culvert Data “screen.
Imported HEC-2 Normal Bridge Model
Piers:
HEC-RAS model, created by the imported HEC-2 input data, reflected the piers
through the bridge deck data or internal cross section data. In this example, four
Normal Bridge models used pier data and needed revision. Generally, the revision of
pier data in HEC-RAS included one or all of the changes listed below:
• Revision of the internal cross section data (accessed through “options” feature
available in the “Bridge Culvert Data “screen)
• Revision of the bridge deck data (accessed through the “Deck/ Roadway” feature
available in the “Bridge/Culvert Data” screen)
• Revision of the pier data (accessed through “Pier” feature in “Bridge/ Culvert
Screen”)
Bridge Cross Sections:
HEC-RAS created the internal cross sections from the upstream and downstream
cross sections (HEC-2 cross sections 4 and 3) used in the normal bridge analysis.
However, these cross sections are also repeated as stream cross sections (HEC-RAS
Bridge Sections 3 and 2) in the HEC-RAS model at zero distance from the faces of the
bridge, respectively. In order to comply with the HEC-RAS bridge modeling guidance,
the stream cross sections at the bridge faces should be deleted (using the “Options”
feature available in the “Cross Section Data” screen). The pier data can be revised
using the “Pier Data Editor” available in the “Bridge and Culvert” Screen of the HEC-
RAS.
Imported HEC-2 Special Bridge Model
Piers:
HEC-RAS assumed that the pier defined in the SB record is located at the center of the
channel bank stations. If the location of the pier is different, the HEC-RAS model will
need a revision. It has to be noted that the actual number of piers present in the
bridge structure may be more than one. It is recommended that the number of piers
and their location, if known, should be included in the HEC-RAS input. In this
example, the bridge at cross section 34 needed to be revised.
Class A low Flow:
Large differences in water-surface elevations were observed between the HEC-2 class A
low flow computation for the bridge at cross section 34 and the HEC-RAS result. In
this case, the HEC-2 computation appears to use unrealistically large flow areas.
Therefore, for comparison purposes, the computed water-surface elevation at cross
section 34 of the HEC-2 model struct2.dat was revised to match the HEC-RAS result.
HEC-2 X5 statement was used for this purpose.
Discharge Coefficients:
Weir and pressure flow coefficients used in the HEC-2 model are imported accurately
into the HEC-RAS model.
Long Pipe Flow:
The downstream bridge deck data of the HEC-RAS model for the bridge at cross
section 75 was revised to match the downstream cross section geometry. HEC-2
defines the bridge deck geometry at the upstream cross section location only. Due to
the large distance between the cross sections, it was necessary to revise the upstream
bridge opening geometry to match the downstream topography.
Bridge Opening Area:
In the special bridges considered for the sensitivity test model, the bridge opening area
computed by HEC-RAS from the Deck/Roadway data generally matched that used in
the HEC-2 special bridge analysis. The maximum difference was 10%, and the bridge
opening area computed by the HEC-RAS was larger. However, according the HEC-2
Special Bridge modeling guidelines, the HEC-2 cross sections 2 and 3 (upstream and
down stream of the bridge) will reflect the valley sections. These sections, when
combined with the Deck/Roadway geometry may not reflect the bridge opening
accurately, as required for the HEC-RAS internal bridge cross sections. It is
recommended that, if the bridge opening area between the HEC-2 and HEC-RAS differ,
additional data to represent the bridge opening should be entered into the HEC-RAS
input files.
Cross Section 44:
A difference of 1.22 feet was observed between the bridge flow computations at section
44. The HEC-2 elevation for this cross section is computed by assuming critical flow.
However HEC-RAS does not assume critical flow conditions to prevail at this cross
section. HEC-RAS uses energy method to compute the elevation. The cause of this
difference is explained in a letter by the USACE as follows:
“Both programs are trying to use the Pressure and Weir flow method to get a solution
for the bridge. The solution that both models come up with yields an energy elevation
lower than the downstream energy grade line. This is not a valid solution, so HEC-2
just defaults to critical depth. However, HEC-RAS is more advanced, in that when
pressure and weir method fails to come up with a valid solution, it will automatically
defaults back to trying the energy based method. In this case, HEC-RAS is able to get
a solution with energy method, but HEC-2 defaults to critical depth. The HEC-RAS
answer is more appropriate, as critical depth is not the right answer.
Imported HEC-2 Special Culvert
Culvert models were imported correctly from HEC-2 Special Culvert model input data.
CONCLUSION
The results of the sensitivity test indicate that with appropriate changes to the
imported HEC-RAS model, both the HEC-2 and HEC-RAS models, can compute
comparable water-surface elevations. One major difference was noted between the
water-surface elevations computed by the HEC-2 special bridge analysis for low bridge
flows. In this case, it appeared that the HEC-RAS computations were more
appropriate than the HEC-2 results.
Example 3: Importing HEC-2 Model with a Tributary
INTRODUCTION
HEC-2 model, prop.dat (provided in the enclosed computer disk) is used in the
sensitivity test. The HEC-2 model reflected a floodplain with one tributary and a long
culvert. The Tributary option available in HEC-2 was used to determine the overland
flow that existed in the vicinity of the long culvert. In the HEC-2 model, an internal
rating curve is used to determine the flow through the culvert.
HEC-2 MODEL WITH A TRIBUTARY
In addition to the records necessary to model flows through natural floodplains and
culverts, the following HEC-2 features were used in prop.dat:
• Definition of the tributary flow with a negative station number, X1
• Internal rating curve, RC
The schematic diagram for this example is shown in Figure 3.1. This diagram is
created by HEC-RAS using the imported HEC-2 input data.
OBSERVATIONS
Table 3.1 compares the flood elevations computed by HEC-2 and HEC-RAS models.
The results of the sensitivity test indicated that, with some revisions, the HEC-2 and
HEC-RAS models computed comparable water-surface elevations. Major differences
(up to 0.7 foot) were created when the computed water-surface elevations were
obtained from the critical depth computations.
The HEC-2 input data used identical cross sections to model the main flow path as
well as the over flow path. In HEC-RAS, different cross section numbers are necessary
to define tributaries. Therefore, imported HEC-RAS model created its own cross
section numbers to identify cross sections. These cross section numbers varied from 1
to 24.
Figure 3.1: Schematic Diagram - Model with a Tributary
April 2002Figure 3.1: Schematic Diagram – Model with a Tributary
Table 3.1: HEC-2 to HEC-RAS – Model with a Tributary
HEC-2
Cross Section
HEC-RAS
Cross Section
Elevation (feet NGVD)
HEC-2
HEC-RAS
Revised
1
1
844.20
844.20
20
2
844.20
844.20
108
3
844.20
844.20
200
4
844.07
844.07
330
5
844.02
844.02
630
6
843.89
843.89
690
7
844.00
844.00
870
8
845.48
845.48
1470
9
852.34
852.39
1485
10
854.00
854.00
1535
11
853.98
853.98
1687
12
854.00
854.00
1805
13
854.01
854.02
1895
14
854.25
854.25
2042
15
854.39
854.39
2167
16
854.54
854.54
2090
17
855.21
855.21
2420
18
857.02
857.03
Tributary
-690
19
844.00
--
890
20
851.51
851.98
1070
21
853.45
853.44
1170
22
853.76
853.75
1360
23
853.79
853.77
1486
24
853.79
853.77
The river system schematic reflected the origin of the tributary correctly. The river
system can also be modeled as looped network using the junction option available in
the HEC-RAS. However, HEC-RAS has the capability to compute the discharges in
different branches of the loop. However, the imported HEC-RAS model was not
modified to model a looped network.
The distances of the cross sections at the junction were not correctly defined in the
HEC-RAS model. The distance of the most downstream cross section of the tributary
from the junction was imported incorrectly. The correct distance of 200 feet was typed
in. Differences in computed water-surface elevations between HEC-2 and HEC-RAS
models appear to be at locations where these elevations were equal to critical depths.
It has been generally observed that HEC-2 and HEC-RAS programs compute different
critical depths for the same flow at a cross section.
CONCLUSION
This sensitivity test illustrates that a HEC-2 model with tributary option was imported
into the HEC-RAS. However, minor revisions were necessary to model flow junctions
according to HEC-RAS guidelines.
Example 4: Importing HEC-2 Ice Jam Model
INTRODUCTION
The HEC-2 model icejam.dat provided in the enclosed computer disk is used in the
sensitivity test. This model has four floodplain cross sections, two of which are
modeled to reflect ice cover in the channel. The imported HEC-RAS model is stored in
icejam.prj. Computed water-surface elevations computed by HEC-2 and HEC-RAS
agreed well.
HEC-2 MODEL WITH ICE-JAM MODELING
In addition to the records necessary to model flows through natural floodplain, the
HEC-2 model contained the HEC-2 record IC to reflect ice cover thickness. The
schematic diagram for this example is shown in Figure 4.1. This diagram is created
by HEC-RAS using the imported HEC-2 input data.
OBSERVATIONS
HEC-RAS imported the ice analysis data correctly from the HEC-2 input file. However,
zeros and repeated ice parameters of the HEC-2 data set had to be retyped in the
imported plan. Both programs computed similar water-surface elevations. Table 4.1
compares the water-surface elevations computed by these two programs.
Figure 4.1: Schematic Diagram - Ice Jam Model
Figure 4.1: Schematic Diagram – Ice Jam Model
April 2002
Table 4.1: HEC-2 to HEC-RAS – Ice Jam Model
Cross
Section
Remarks
Elevation (feet NGVD)
HEC-2
HEC-RAS
Revised
198730
known
1103.10
1103.10
199930
ice
1103.45
1103.40
201130
ice
1104.24
1104.30
202540
no ice
1104.52
1104.58
CONCLUSION
The results of this sensitivity test indicate that the ice analysis data was imported
correctly into HEC-RAS and similar flood elevations are computed by HEC-2 and HEC-
RAS.
Example 5: Importing HEC-2 Encroachment Model
INTRODUCTION
This sensitivity test used the HEC-2 model enc-br.dat (provided on the enclosed disk).
The encroachment analysis methods 4 (equal conveyance reduction method) and 1
(known encroachment stations), available in the HEC-2 program were used in enc-
br.dat. This model analyzed the flow in a floodplain with one bridges. The bridge was
modeled using HEC-2 special bridge modeling method.
HEC-2 MODEL WITH ENCROACHMENT MODELING
In addition to the records necessary to model flows through natural floodplain, and
bridges, the HEC-2 model enc-br.dat contained the HEC-2 Encroachment record ET.
The schematic diagram for this example is shown in Figure 5.1. This diagram is
created by HEC-RAS using the imported HEC-2 input data (encbrdge.prj).
Figure 5.1: Schematic Diagram - Encroachment Model
Figure 5.1: Schematic Diagram – Encroachment Model
OBSERVATIONS
HEC-RAS imported the encroachment analysis data correctly from the HEC-2 input
file enc-br.dat. However, the ineffective flow modeling done using ET records for the
natural profile were not imported into the HEC-RAS model. In the HEC-2 model
encrs.dat, the ineffective flow modeling is conducted using X3 records. This allowed
the ineffective flows to be correctly imported into the HEC-RAS input data using the
options, Ineffective Areas and Blocked obstructions. These features are accessible
through the “Options” menu available in the “Geometric Data” screen.
In the sensitivity test, the HEC-RAS input was modified (as explained in Example 1) so
that the HEC-2 method was used to compute the conveyance. The flood elevations
computed for the natural flood profile generally matched with the HEC-2 results, the
maximum difference being 0.22 foot. However, the flood elevations computed by the
HEC-RAS for the encroached floodplain matched more closely with those computed by
HEC-2. The differences of approximately 0.2 foot were observed between the
surcharge values computed by HEC-2 and HEC-RAS programs.
CONCLUSION
Encroachment data was imported correctly into HEC-RAS. The minor differences in
the water-surface elevations observed between the HEC-2 and HEC-RAS computations
appear near the bridge locations.
Example 6: Importing HEC-2 Split Flow Model
INTRODUCTION
This sensitivity test used simple HEC-2 models split.dat with six cross sections and
splitwr.dat with four cross sections (provided on the enclosed disk). The HEC-2 rating
curve option was used in split.dat. The HEC-2 model splitwr.dat used weir flow option
to model split flow. These two models analyzed the flow in a floodplain with out any
bridges. In the sensitivity test, it was necessary to redefine the lateral weir geometry in
HEC-RAS from the topographic data. Split flows modeled using the lateral weir flow
option as well as the rating curve option available in HEC-2 (splitwr.dat and split.dat)
and HEC-RAS (splwweir.prj and splitrat.prj) computed comparable results.
HEC-2 MODEL WITH SPLIT FLOW MODELING
In addition to the records necessary to model flows through natural floodplain, the
HEC-2 models split.dat and splitwr.dat included the HEC-2 split flow records SF, TW,
TC, WS, WC, CS, CR, and EE. In addition, X3 records were used to model ineffective
flow on the over banks. The schematic diagram for the imported HEC-2 model
splitwr.dat (lateral weir example) is shown in Figure 6.1. The schematic diagram for
the imported HEC-2 model split.dat (rating curve example) is shown in Figure 6.2.
These diagrams are created by HEC-RAS using the imported HEC-2 input data and
manual inclusion of lateral weir flow and rating curve data necessary to model the
split flow.
Figure 6.1: Schematic Diagram - Split Flow Model, Lateral Weir Example
Figure 6.1: Schematic Diagram – Split Flow Model, Lateral Weir Example
Figure 6.2: Schematic Diagram - Split Flow Model, Rating Curve Example
Figure 6.2: Schematic Diagram – Split Flow Model, Rating Curve Example
OBSERVATIONS
The split flow parameters are not imported by HEC-RAS. However, the floodplain
modeling data of the HEC-2 input file, i.e., the data below the HEC-2 title records T1,
T2, and T3 were imported. In order to match the HEC-2 results without split flow, the
HEC-RAS model cross section data needed to be modified. When the lateral weir
dimensions and the rating curve were manually inserted into the input, HEC-RAS
computed split flow discharges and flood elevations that are comparable to HEC-2
results.
In HEC-RAS, the lateral weir can be defined more accurately along the left or right
over bank of the channel than it is possible with HEC-2. This may be one factor that
contributed in causing different results. HEC-2 lateral weirs can have lengths larger
than the overbank flow distance. For the most downstream cross section, HEC-RAS
expected the weir lengths to be shorter than the overbank flow distance.
In HEC-2, the weir geometry is defined from the downstream cross section. HEC-RAS
defines the weir geometry from upstream to downstream. This fact should be
considered when HEC-2 split flow data are converted into HEC-RAS.
OBSERVATIONS
HEC-RAS imported the floodplain modeling data correctly from the HEC-2 input file.
HEC-RAs conducted split flow analysis when the lateral weir and rating curve
information was manually included. Differences in results were observed between the
HEC-2 and HEC-RAS outputs. Table 6.1 compares the water-surface elevations and
discharges computed by these two programs.
Table 6.1: HEC-2 to HEC-RAS – Split Flow Model, Lateral Weir Example
Cross
Section
Remarks
Elevation
(feet NGVD)
Discharge
(cubic feet/ second)
HEC-2
HEC-RAS
HEC-2
HEC-RAS
1
20.07
20.77
24,409
24,594
2
Lateral weir
24.75
25.47
26,765
24,594
3
Lateral weir
26.34
26.11
39,816
39,966
4
Lateral weir
28.48
28.05
40,000
40,000
Table 6.2: HEC-2 to HEC-RAS – Split Flow Model, Rating Curve Example
Cross
Section
Remarks
Elevation
(feet NGVD)
Discharge
(cubic feet/ second)
HEC-2
HEC-RAS
HEC-2
HEC-RAS
1
25.26
25.26
38,281
35,258
2
31.00
30.99
38,281
35,258
3
33.74
33.73
38,281
35,258
4
Rating Curve
33.73
33.83
40,000
40,000
5
34.50
34.40
40,000
40,000
6
35.22
35.14
40,000
40,000
CONCLUSION
Split flow data was not imported into HEC-RAS. However, when these data were
manually typed into the imported HEC-2 model, HEC-RAS results compared well with
HEC-2 results.