1 FRONT MATTER
HEC-RAS
River Analysis System
Hydraulic Reference Manual
In addition to the four hydraulic analysis components, the system includes several hydraulic design features that can be activated once the basic water surface profiles have been calculated.
Hydraulic Reference Manual Version 6.0 Beta
December 2020
River Analysis System, HEC-RAS
Terms and Conditions of Use
REPRESENTATIVES AND EMPLOYEES, INCLUDING ITS CONTRACTORS AND SUPPLIERS, ARE LIABLE FOR LOST PROFITS OR SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF OR IN CONNECTION WITH THE USE OF HEC-WAT RELATING TO NEC-WAT. THE LIABILITY OF ITS GOVERNMENT AND THE LIABILITY OF ITS AGENCIES, OFFICERS, REPRESENTATIVES AND EMPLOYEES, INCLUDING ITS CONTRACTORS AND SUPPLIERS, TO YOU OR ANY THIRD PARTY IN ANY COMPLIANCE ARISING. EC-RAS WITH IDENTIFIED ERRORS CORRECTED.
2 FORWARD
Hierarchical Data Format (HDF) – HEC-RAS uses HDF5 libraries in both the User Interface and Calculation engines to write and read data in binary files that follow HDF5 standards. Geospatial Data Abstraction Library (GDAL) – HEC-RAS uses the GDAL libraries in the HEC-RAS Mapper tool.
3 INTRODUCTION
General Philosophy of the Modeling System
Overview of Hydraulic Capabilities
The 2D unsteady flow equation solver was developed at HEC and was directly integrated into the HEC-RAS Unsteady flow engine to facilitate combined 1D and 2D hydrodynamic modeling. The hydraulic calculations for cross-sections, bridges, culverts and other structures that have been developed for the steady flow component have been processed in the unsteady flow module.
HEC-RAS Documentation
Overview of This Manual
Bridge Abrasion Estimation describes the use of HEC-RAS to calculate bridge abrasion. Performing a Dam Break Study with HEC-RAS describes how to perform a dam break study with HEC-RAS.
4 THEORETICAL BASIS FOR ONE-DIMENSIONAL AND TWO- DIMENSIONAL HYDRODYNAMIC CALCULATIONS
1D Steady Flow Water Surface Profiles
Equations for Basic Profile Calculations
The energy head loss between two cross sections consists of friction losses and contraction or expansion losses.
Cross Section Subdivision for Conveyance Calculations
In general, the standard approach to HEC-RAS will yield lower total transport for the same water surface height. The results of the study showed that the standard approach to HEC-RAS will generally yield a higher calculated water surface height.
Composite Manning's n for the Main Channel
Water surface profiles were calculated for a 1% probability event using two transfer calculation methods in HEC-RAS. Further research with observed water surface profiles will be necessary to draw conclusions about the accuracy of both methods.
Evaluation of the Mean Kinetic Energy Head
The calculated value is the composite value of the main channel n in the output and summary tables. The velocity coefficient, α, is calculated based on the transfer in three flow elements: left bank, right bank and channel.
Friction Loss Evaluation
It can also be written in terms of transport and area, as in the following equation: total flow cross-sectional area. flow areas of the left bank, main channel and right bank, respectively.. total transport of the cross-section.. transport of left bank, main channel and right bank. The program also includes an option to select equations depending on the flow regime and profile type (e.g. S1, M1, etc.).
Contraction and Expansion Loss Evaluation
The average transport method (13) is the "default" equation used by the program; that is, it is used automatically unless another equation is selected by the user.
Computation Procedure
Assume a water surface view at the upstream section (or downstream section if a supercritical profile is calculated). The program is limited by a maximum number of iterations (the default is 20) to balance the water surface.
Critical Depth Determination
If the balanced height is on the wrong side of the critical water surface height, the cross-section is assumed to be at a critical depth and the program displays a warning message. The height of the cross section can be doubled five times before the program stops searching.
Applications of the Momentum Equation
If the maximum height of the cross section (highest point to the lowest point) is less than 1.5 times the maximum height of the main channel (from the highest station of the bank of the main channel to the invert), then the program divides the entire cross section into 30 equal intervals. If all local minima occur at discontinuities in the energy curve (caused by slabs and ineffective flow zones), then the program will set the critical depth to the one with the lowest energy.
Air Entrainment in High Velocity Streams
A water surface with air entrainment is calculated and displayed separately in the HEC-RAS tabular output. To display the water surface with air entr, the user must create their own profile table and set the variable "WS Air Entr." within that table.
1D Steady Flow Program Limitations
If HEC-RAS is used on steeper slopes, be aware of the error in the depth calculation introduced by the size of the slope. If you use HEC-RAS to perform the calculations on slopes steeper than 1:10, you will need to divide the calculated depth of water by the cos( ) to get the correct water depth.
1D Unsteady Flow Hydrodynamics
So instead of using d cos( ), the vertical pressure head is approximated as d and used as the vertical water depth. As you can see, for a slope of 1:10 or less, this is a very small error in estimating vertical depth (.5.
Continuity Equation
Also note that very steep slopes can introduce air entrapment into the flow, as well as other possible factors that may not be taken into account within HEC-RAS. Conservation of mass for a control volume means that the net flow rate in the volume is equal to the rate of change of storage within the volume.
Momentum Equation
Where is the net pressure force for the control volume, and is the force exerted on the fluid by the banks in the x-direction. The second integral (multiplied by ) is the pressure force exerted by the fluid on the banks, which is exactly equal in magnitude but opposite in direction to axis.
Application of the 1D Unsteady Flow Equations within HEC-RAS
As the river level drops, water moves upstream from the bank and supplements the flow in the main channel. Pond areas outside the canal can be modeled with storage areas that exchange water with the canal.
Implicit Finite Difference Scheme
If the pointers identify all rare columns to the right of the diagonal, then the number of operations is minimized and the performance is similar to the forward solver algorithm. IRIGHT(IROW) - the number of columns in the main band to the right of the diagonal.
Semi-Implicit Finite-Volume Scheme
The formulation of momentum diffusion based on the Laplacian of the current velocity is discretized as .. 135). Boundary face flows are included in the inner cells as a source term on the right-hand side of the system of equations.
2D Unsteady Flow Hydrodynamics Introduction
The cycle provided by steps 6 to 10 is intended to update the coefficients of the system of equations, so that the solution of the nonlinear system (instead of its linearization) is obtained at each time step. The numerical methods section also details the way in which the various terms of the equations are discretized and how the nonlinear problem is transformed into a system of equations with variable coefficients.
Hydraulic Equations
Since this idea is only related to the mass equation, it can be used independently of the momentum equation version. When the wind is in the same direction as the currents, the wind shear stress is reduced.
Grid and Dual Grid
For example, the double edges intersect the regular edges and the two groups are in a one-to-one correspondence. For example, the duplicate nodes are now in one-to-one correspondence with the set of grid cells and grid boundary edges.
Subgrid Bathymetry
Piecewise differentiation of a linear volume-height curve leads to a piecewise curve of constant area and height. 171). In the image below, the left image represents the shape of the face as seen in the fine mesh and the corresponding function for the surface area of the face relative to the height of the water surface.
Numerical Methods
The Jacobian (derivative) of Ω with respect to is given by another bathymetric relation.. the diagonal matrix of wet cell areas. The momentum diffusion formulation based on the Laplace value of the current velocity is discretized as 195).
5 BASIC DATA REQUIREMENTS
Geometric Data
Study Limit Determination
In general, the water surface at the lower boundary of the model is usually unknown. Using the normal depth will cause an error in the water surface profile at the boundary.
The River System Schematic
A common practice is to use Manning's equation and calculate normal depth as the starting water surface. The river system schematic shown in the figure above demonstrates the ability of HEC-RAS to model flow distributions as well as flow combinations.
Cross Section Geometry
For example, cross-sectional spacing can be maximized when calculating an M1 profile (backwater profile) with the mean friction slope equation or when using the harmonic mean friction slope equation to calculate M2 profiles (subtract profile). Numerous program options are available to enable the user to easily add or modify cross section data.
Optional Cross Section Properties
An example of a cross section with a furrow on the left side is shown in the figure below. This option allows the user to define cross-sectional areas to be blocked permanently.
Reach Lengths
Energy Loss Coefficients
Choosing an appropriate value for Manning's n is very important for the accuracy of the calculated water surface elevations. During data analysis, the energy loss coefficients for contraction and expansion were set to 0.0 and 0.5, respectively.
Stream Junction Data
When the change in effective cross-sectional area is abrupt, such as in bridges, contraction and expansion coefficients of 0.3 and 0.5 are often used. Using contraction and expansion coefficients that would be typical of subcritical flow can result in overestimation of the energy losses and fluctuations in the calculated water surface profile.
Steady Flow Data
The default option is a very simple assumption that the water surface calculated on the downstream side of a flow-combining intersection is used for cross sections upstream of the intersection. If this is not a good assumption (such as for steeper river systems), there is an option to perform an energy balance across the intersection in order to calculate upstream water surface elevations.
Flow Regime
For unsteady flow calculations, HEC-RAS has two options for the hydraulic calculations at an intersection.
Boundary Conditions
Using an estimated water surface will incorporate an error in the water surface profile near the boundary condition. If a supercritical profile is calculated, additional cross sections must be added upstream of the relevant upstream boundaries.
Discharge Information
If a subcritical profile is being calculated, then additional cross sections need only be added below the downstream boundaries. If a mixed flow regime profile is calculated, then the cross sections must be added upstream and downstream of all relevant boundaries.
Unsteady Flow Data
If it is important to have accurate answers to cross-sections close to the boundary condition, additional cross-sections should be added. If the water surface profile converges to the same answer, then by the time the calculations reach the cross sections located in the study area, enough sections have been added and the boundary condition does not affect the answers in the study area.
Boundary Conditions1
To test whether the added cross sections are sufficient for a given boundary condition, the user should try several different initial heights at the boundary condition, for the same discharge. The user can specify the following types of boundary conditions in the internal sections: side inlet hydrograph; uniform lateral inlet hydrograph; groundwater flow; and Hydrograph of internal phase and flow.
Initial Conditions
In addition, all gate structures defined within the system (inline, lateral or between storage areas and/or 2D flow areas) can have the following types of boundary conditions to control the gates: time series of gate openings; height-controlled gate; navigation dam;.
6 OVERVIEW OF OPTIONAL CAPABILITIES
Multiple Profile Analysis
Multiple Plan Analysis
Optional Friction Loss Equations
Any of the above friction loss equations will produce satisfactory estimates provided the reach lengths are not too long. Selection of friction loss equations is accomplished from the Options menu in the Steady Flow Analysis window.
Cross Section Interpolation
This is explained in "Modling Culverts" in the HEC-RAS user manual under cross section interpolation. Interpolated cross-sectional roughness is based on a string model similar to that used for geometry.
Mixed Flow Regime Calculations
Ropes are used to connect breaks in the roughness coefficients of the upstream and downstream sections. The first term is the momentum of the flow passing through the cross section of the channel per unit time.
Modeling Stream Junctions
For the 1D finite difference solution scheme, the user must activate a special calculation mode for mixed flow regime. Please read the HEC-RAS User Manual for how to do mixed flow regime with the 1D Finite difference solution scheme for the unsteady flow equations.
Energy Based Junction Method
If the supercritical sections have a greater specific force than the subcritical sections, the program assumes that. The program then calculates the specific power of the subcritical and supercritical responses in sections 2.0 and 3.0.
Momentum Based Junction Method
The water surface heights at sections 4.0 and 0.0 are solved simultaneously and are assumed to be equal. To solve Equation 4-10 for supercritical flow, assume that the water surface heights at sections 2.0 and 3.0 are equal.
Flow Distribution Calculations
The user can change the number of slices used in each of the cross sections. During the calculations, at each cross section where flow distribution is requested, the program calculates the flow (discharge), area, wetted perimeter, conveyance rate, hydraulic depth and average velocity for each of the user-defined segments.
Split Flow Optimization
To achieve such detail, the user will need to use a three-dimensional hydraulic model, or go out and measure the flow distribution in the field. The results obtained from the flow distribution option may also vary with the number of slices used for the calculations.
Pressurized Pipe Flow
Note: if the user does not make the top of the cover high enough and hydraulic. So the calculated transfer will drop when the water hits the top of the tube.
Estimating Ungaged Area Inflows
To calculate the christened flow, the user must start with a calibrated HEC-RAS river model. Once the data has been entered, HEC-RAS can calculate the uncommitted inflow in a single run of the program (the program will automatically delay the inflows and rework the model).
Theory
The unadjusted flow is optimized to reproduce either a stage hydrograph or a flow hydrograph at the DBC station. The unregulated flow enters between the upstream boundary of the upper reach and cross section j, the downstream boundary.
Optimization of Ungaged Inflow
To use unadjusted inflow in a model, the program delays the flow backward in time and enters it into the model as point and/or uniform lateral inflow(s). The unpaid flow is then optimized from Marseilles TW to Kingston Mines with the unpaid inflow from Lockport to Marseilles TW.
Simultaneous Optimization of Independent Reaches
However, when the unregulated flow is simultaneously optimized, the directed flow hydrograph at cross section j will have an error. Therefore, even after the simultaneous optimization, the program will continue to do a sequential optimization to correct the remaining errors.
Sequential Optimization
To compensate for this, the user can set a time frame for averaging the flows (i.e. a smoothing window). This will limit unused inflow and may be necessary for stability and/or maintaining hydrologically reasonable responses.
Modeling Precipitation and Infiltration
For example, the user can select a three-hour smoothing window to go along with a one-hour hydrograph interval. The unknown inflow is considered to have converged if this flow difference is within the tolerance specified by the user.
Deficit and Constant
This current difference for each time step is squared and then summed for all time steps. If the soil is not saturated, all rain will infiltrate until the soil is.
Curve Number
Publications from the Soil Conservation Service provide further background and details on the use of the model. Another problem with the standard SCS CN method is that as the rainfall increases, the infiltration can become unrealistically small.
Green-Ampt
As the water content of the soil surface increases, the GA model moves the infiltrated water by approaching the wetting front with piston displacement. During the pause period, the surface water content becomes less than the saturated water content.
7 MODELING BRIDGES
General Bridge Modeling Guidelines
Cross Section Locations for Bridges
Cross section 2 is located a short distance downstream from the bridge (ie, generally located at the downstream toe of the embankment). This cross-section should represent the natural ground (main channel and floodplain) immediately downstream of the bridge or culvert.
Defining Ineffective Flow Areas
A practical method of locating the stations in the ineffective flow regions is to assume a 1:1 contraction rate in the immediate vicinity of the bridge. In general, the user should make the active flow area equal to the width of the bridge opening or wider (to account for flow expansion) unless the bridge supports are very steep (vertical wall supports without wing walls).
Contraction and Expansion Losses
The coefficients of contraction and expansion are used to calculate energy losses associated with changes in the shape of river cross-sections (or effective flow areas). In general, the coefficients of contraction and expansion for supercritical flow should be lower than for subcritical flow.
Hydraulic Computations through the Bridge
A summary of this research, as well as recommendations for contraction and expansion coefficients, can be found in "Flow Transitions in Bridge Backwater Analysis." For typical bridges under Class C flow conditions (fully supercritical flow), the coefficients of contraction and expansion should be about 0.03 and 0.05, respectively.
Low Flow Computations
The program then performs a standard walk through the bridge (from section BD to section BU). From the inside of the bridge at the upstream end (BU) to just upstream of the bridge (3); and from just upstream of the bridge (3) to the approach section (4).
High Flow Computations
Pressure flow occurs when the flow contacts the low chord of the bridge. Total discharge through the bridge opening. Discharge coefficients for pressure flow. Net area of the bridge opening at section BU.
Combination Flow
Selecting a Bridge Modeling Approach
Low Flow Methods
High Flow Methods
Unique Bridge Problems and Suggested Approaches
Perched Bridges
Low Water Bridges
Bridges on a Skew
The user should not base the slope angle on the direction of flow upstream of the bridge. The projected width of the pier, perpendicular to the streamlines. The actual length of the pier.
Parallel Bridges
Multiple Bridge Opening
Modeling Floating Pier Debris
The figure below shows the scaffold editor with the scaffold garbage option enabled. Two additional fields must be completed, the overall height and width of the scaffolding waste.
Bridge Modeling in 2D
The detailed modeling approach uses a full 2D mesh to resolve the details of the bridge's hydraulics. The simplified 1D/2D bridge modeling approach is available for all 2D solvers, including the Diffusive Wave Solver.
Simplified 1D/2D Bridge Modeling
This is considered acceptable because the purpose of the simplified 1D/2D bridge modeling approach is to capture the total bridge head losses and not the details of the flow hydraulics through the bridge. The detailed bridge modeling approach should only be applied with the nonlinear solvers of shallow water equations SWE-ELM and SWE-EM.
Detailed Bridge Modeling
The simplified 1D/2D approach can be applied to all types of bridge flows, including pressurized flow and bridge overlay. Another limitation of detailed bridge modeling is that it is currently only applicable for low flow conditions.
8 MODELING CULVERTS
General Culvert Modeling Guidelines
Because of the similarities between culverts and other types of bridges, culverts are modeled in a similar way to bridges. The placement of cross sections, the use of ineffective surfaces, the selection of loss coefficients, and most other aspects of bridge analysis also apply to culverts.
Types of Culverts
