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 Centers 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 Users 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 FEMAs 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 Mannings 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: 

 Mannings 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 Administrations (FHWAs) 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 Users 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 Agencys 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 
Users 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 Users 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 Mannings roughness 
coefficients 
 HEC-2 option to compute Mannings 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 Users 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 Users 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 

Mannings 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 Mannings 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 
Mannings 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 Mannings 
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 

Hortons 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 Users 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 Users 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 
 Mannings 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 Mannings 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 
Mannings 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 Administrations 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.