Introduction

Welcome to the Hydrologic Engineering Center's River Analysis System (HEC-RAS). This software allows you to perform one-dimensional steady, one- and two-dimensional unsteady flow hydraulics, sediment transport/mobile bed computations, water temperature modeling, and generalized water quality modeling (nutrient fate and transport).

This manual documents the hydraulic capabilities of the Steady and unsteady flow portion of HEC-RAS, as well as sediment transport computations.

This chapter discusses the general philosophy of HEC-RAS and gives you a brief overview of the hydraulic capabilities of the modeling system. Documentation for HEC-RAS is discussed, as well as an overview of this manual.

General Philosophy of the Modeling System

HEC-RAS is an integrated system of software, designed for interactive use in a multi-tasking, multi-user network environment. The system is comprised of a graphical user interface (GUI), separate hydraulic analysis components, data storage and management capabilities, graphics and reporting facilities.

The HEC-RAS system contains four one-dimensional river analysis components for: (1) steady flow water surface profile computations; (2) unsteady flow simulation (one-dimensional and two-dimensional hydrodynamics); (3) Quasi unsteady or fully unsteady flow movable boundary sediment transport computations (1D and 2D); and (4) water quality analysis. A key element is that all four components use a common geometric data representation and common geometric and hydraulic computation routines. In addition to the four river analysis components, the system contains several hydraulic design features that can be invoked once the basic water surface profiles are computed. HEC-RAS also has an extensive spatial data integration and mapping system (HEC-RAS Mapper).

The current version of HEC-RAS supports Steady and Unsteady flow water surface profile calculations; combined 1D and 2D hydrodynamics; sediment transport/mobile bed computations; water temperature analysis; water quality analyses (Nutrient transport and fate); and spatial mapping of many computed parameters (Depth, water surface elevation, velocity, etc…). New features and additional capabilities will be added in future releases.

Overview of Hydraulic Capabilities

HEC-RAS is designed to perform one-dimensional (1D), two-dimensional (2D), or combined 1D and 2D hydraulic calculations for a full network of natural and constructed channels. The following is a description of the major hydraulic capabilities of HEC-RAS.

Steady Flow Water Surface Profiles. This component of the modeling system is intended for calculating water surface profiles for steady gradually varied flow. The system can handle a single river reach, a dendritic system, or a full network of channels. The steady flow component is capable of modeling subcritical, supercritical, and mixed flow regime water surface profiles.

The basic computational procedure is based on the solution of the one-dimensional energy equation. Energy losses are evaluated by friction (Manning's equation) and contraction/expansion (coefficient multiplied by the change in velocity head). The momentum equation is utilized in situations where the water surface profile is rapidly varied. These situations include mixed flow regime calculations (i.e., hydraulic jumps), hydraulics of bridges, and evaluating profiles at river confluences (stream junctions).

The effects of various obstructions such as bridges, culverts, weirs, spillways and other structures in the flood plain may be considered in the computations. The steady flow system is designed for application in flood plain management and flood insurance studies to evaluate floodway encroachments. Also, capabilities are available for assessing the change in water surface profiles due to channel improvements, and levees.

Special features of the steady flow component include: multiple plan analyses; multiple profile computations; multiple bridge and/or culvert opening analysis, and split flow optimization at stream junctions and lateral weirs and spillways.

Unsteady Flow Simulation. This component of the HEC-RAS modeling system is capable of simulating one-dimensional unsteady flow; two-dimensional unsteady flow; or combined 1D and 2D unsteady flow modeling through a full network of open channels. The 1D unsteady flow equation solver was adapted from Dr. Robert L. Barkau's UNET model (Barkau, 1992 and HEC, 1997). This 1D unsteady flow component was developed primarily for subcritical flow regime calculations. The 2D unsteady flow equation solver was developed at HEC and was directly integrated into the HEC-RAS Unsteady flow engine in order to facilitate combined 1D and 2D hydrodynamic modeling.

The hydraulic calculations for cross-sections, bridges, culverts, and other hydraulic structures that were developed for the steady flow component were incorporated into the unsteady flow module. Additionally, the unsteady flow component has the ability to model storage areas and hydraulic connections between storage areas; 2D Flow Areas; and between stream reaches.

Sediment Transport/Movable Boundary Computations. This component of the modeling system is intended for the simulation of one-dimensional and two-dimensional sediment transport/movable boundary calculations resulting from scour and deposition over moderate time periods (typically years, although applications to single flood events will be possible).

The sediment transport potential is computed by grain size fraction, thereby allowing the simulation of hydraulic sorting and armoring. Major features include the ability to model a full network of streams, channel dredging, various levee and encroachment alternatives, and the use of several different equations for the computation of sediment transport.

The model is designed to simulate long-term trends of scour and deposition in a stream channel that might result from modifying the frequency and duration of the water discharge and stage, or modifying the channel geometry. This system can be used to evaluate deposition in reservoirs, design channel contractions required to maintain navigation depths, predict the influence of dredging on the rate of deposition, estimate maximum possible scour during large flood events, and evaluate sedimentation in fixed channels.

Water Quality Analysis. This component of the modeling system is intended to allow the user to perform riverine water quality analyses. The current version of HEC-RAS can perform detailed temperature analysis and transport of a limited number of water quality constituents (Algae, Dissolved Oxygen, Carbonaceous Biological Oxygen Demand, Dissolved Orthophosphate, Dissolved Organic Phosphorus, Dissolved Ammonium Nitrate, Dissolved Nitrite Nitrogen, Dissolved Nitrate Nitrogen, and Dissolved Organic Nitrogen). Future versions of the software will include the ability to perform the transport of several additional water quality constituents.

HEC-RAS Documentation

The HEC-RAS package includes several documents, each are designed to help the modeler learn to use a particular aspect of the modeling system. The documentation has been divided into the following three categories:

Documentation	Description
User's Manual	This manual is a guide to using HEC-RAS. The manual provides an introduction and overview of the modeling system, installation instructions, how to get started, a simple example, entering and editing geometric data, detailed descriptions of each of the major modeling components, and how to view graphical and tabular output.
2D User's Manual	This document describes how to use the 2D modeling capabilities that are included in this version of the software. It also describes how to use RAS Mapper in support of 2D modeling (mesh generation) and inundation mapping for models containing 2D flow areas.
HEC-RAS Mapper	This document describes how to use HEC-RAS Mapper to do the following: establish a horizontal coordinate system; develop an HEC-RAS terrain model; layout the geometric data model and extract terrain data; visualize results in the form of maps, plots, and tables.
Sediment Transport User's Manual	This manual describes how to perform sediment transport modeling. The document describes 1D quasi unsteady; 1D unsteady flow, and 2D sediment transport modeling. Additionally sediment impact analysis (SIAM) and bank stability using BSTEM is also described.
Hydraulic Reference Manual	This manual describes the theory and data requirements for the hydraulic calculations performed by HEC-RAS. Equations are presented along with the assumptions used in their derivation. Discussions are provided on how to estimate model parameters, as well as guidelines on various modeling approaches.
Applications Guide	This document contains a series of examples that demonstrate various aspects of HEC-RAS. Each example consists of a problem statement, data requirements, and general outline of solution steps, displays of key input and output screens, and discussions of important modeling aspects.

Overview of This Manual

This manual presents the theory and data requirements for hydraulic calculations in the HEC-RAS system. The manual is organized as follows:

Theoretical Basis for One-Dimensional and Two-Dimensional Hydrodynamic Calculations provides an overview of the hydraulic calculations theory in HEC-RAS for 1D steady and unsteady flow calculations, as wells as 2D unsteady flow calculations.
Basic Data Requirements describes the basic data requirements to perform the various hydraulic analyses available.
Overview of Optional Capabilities is an overview of some of the optional hydraulic capabilities of the HEC-RAS software.
Modeling Bridges, Modeling Culverts, Modeling Multiple Bridge and/or Culvert Openings, and Modeling Gated Spillways, Weirs and Drop Structures provide detailed discussions on modeling bridges; culverts; multiple openings; inline structures (weirs and gated spillways), and lateral structures.
Floodplain Encroachment Calculations describes how to perform floodway encroachment calculations.
Estimating Scour at Bridges describes how to use HEC-RAS to compute scour at bridges.
Modeling Ice-covered Rivers describes how to model ice-covered rivers.
Stable Channel Design Functions describes the equations and methodologies for stable channel design within HEC-RAS.
Performing a Dam Break Study with HEC-RAS describes how to perform a Dam break study with HEC-RAS.
References provides a list of all the references for the manual.
Appendix - Flow Transitions in Bridge Backwater Analysis is a summary of the research work on "Flow Transitions in Bridge Backwater Analysis."
Appendix - Computational Differences Between HEC-RAS and HEC-2 is a write up on the computational differences between HEC-RAS and HEC-2.
Appendix - Computation of the WSPRO Discharge Coefficient and Effective Flow Length is a write up on the "Computation of the WSPRO Discharge Coefficient and Effective Flow Length
Appendix - Sediment Transport Functions – Sample Calculations