feb 2003hec-ras version 3.11 slides adapted from hec unsteady flow course unsteady flow course

Feb 2003 HEC-RAS Version 3.1 1

HEC-RAS Version 3.1HEC-RAS Version 3.1Unsteady FlowUnsteady Flow

Slides adapted from HECSlides adapted from HEC Unsteady Flow CourseUnsteady Flow Course


Unsteady Flow Documentation

Technical/theoretical - Chapters 2 and 5 from EM 1110-2-1416

http://www.hnd.usace.army.mil - click on TECHINFO, then Engineering Publications, then Engineering Manuals, scroll for manual

HEC-RAS User’s Manual - Chapter 8 (data input and window operations)

HEC-RAS Application Guide - Chapter 17 (example application)


When to use Unsteady Flow

Tidal/estuary fluctuation Off-channel storage Dam breach routing Channels with flat slopes Levee overtopping Hydraulic structures affected by changing

backwater Large amounts of storage behind roads or

culverts


Steady vs. Unsteady

Difference in handling boundary friction and other losses

Difference in numerical solution algorithm Difference in handling non-flow areas Difference in flow and boundary condition data

requirements Difference in calibration strategy Difference in application strategy


2

o f

( /A)Q Q h gA( ) 0S St x xα

Momentum Equation:

Continuity Equation:

Q A + = 0x t

Unsteady Flow Equations


2

o f

( /A)Q h + gA( - S + S ) = 0x x

α

Energy (momentum) Equation:

Continuity Equation:

Q = VA

Steady Flow Equations


Distance vs Time Solution Grid

X = distance, feet t = time, seconds

t

x1,1 2,1

1,2 2,2


Finite Difference Term

Q Q (2,2) - Q(1,2) Q(2,1) - Q(1,1) ----- = ----------------------- + ( 1 - ) --------------------- X X X

= theta weighting factor 0.5 < < 1.0


Steady – Compute exact hydraulic properties at a section for each trial water surface elevation from the elevation/station points, n-values.

Unsteady – Hydraulic properties are pre-computed for all possible water surface elevations at each cross section (hydraulic table)

Pre-Computation of Hydraulic Properties


Data Requirements(Flow and Boundary Conditions)

Steady: Discharge (Q) at each cross section.

Unsteady: Inflow hydrograph(s) which are routed by the model.


Prepare hydrographs

(boundary conditions)

Upstream flows

Tributary (local flows)

Ungaged/unmodeled flows

Downstream (rating curve?)


HEC-RAS Main Window


Entering Geometric Parameters


Cross Section Table Properties


Pre-processing Geometry

For unsteady flow, geometry is pre-processed into tables and rating curves Cross sections are processed into tables of

area, conveyance, and storage Bridges and culverts are processed into a

family of rating curves for each structure Weirs and gated structures are calculated

on the fly during unsteady flow calculations Pre-processor results can be viewed in

graphs and tables


Cross Section Properties Plot

0 1000 2000 3000 4000 5000 6000 7000650

660

670

680

690

700

Property TableRS = 138154.4

Conveyance/1000 (cfs) Storage (cu ft)

Ele

vatio

n (f

t)

Legend

Conv. Channel

Conv. Valley

Conv. Total

Storage


Geometry Preprocessor

What does it do? Processes geometric data into a series of

hydraulic tables and rating curves. Why do we use it for unsteady flow? Instead of calculating hydraulic variables for

each cross-section during each iteration, the program interpolates the hydraulic variables from the tables.


Conveyance Calculations

Manning Equation

1/2 Q = K Sf K = Conveyance Sf = friction/energy slope


Conveyance Calculations

nch n1 n2 n3

A1 P1 A2 P2 A3 P3 Ach Pch

Klob = K1 + K2

Kch

Krob = K3


Cross Section Example


Geometry Preprocessor


Hydraulic Property Plot


Cross Section Properties Table


Conveyance Subdivisions

Boundary andInitial Conditions

Objectives: Know boundary condition options Know initial condition requirements Sources of data for both


Unsteady Flow Data

External Boundaries required Upstream and Downstream ends of the river Typically flow or stage hydrograph upstream Typically rating or “normal depth” downstream

Internal Boundaries can be added Add flow within the river system Define gate operation

Initial Conditions - at the start of simulation


Unsteady Flow Data Editor


Boundary Conditions

Editor shows required external boundaries

Boundary Type shows available options

Upstream options: Stage Hydrograph Flow Hydrograph Stage & Flow

Hydrograph


Boundary Conditions - continued

Downstream Boundary Options: Stage Hydrograph Flow Hydrograph Stage & Flow Hydrograph Rating Curve Normal Depth


Flow Hydrograph

Read from DSS Select DSS file Select Pathname

Enter in Table Select time interval Select start date/time Enter flow data - or

cut & paste


Sources of Time-Series Data

Historic Records (USGS) Stage Hydrographs Flow Hydrographs

Computed Synthetic Floods Rainfall-runoff modeling Peak Discharge with assumed time

distribution Others?


Normal Depth

Enter Friction (energy) Slope

Program uses Manning’s equation to compute stage

Provides semi-dynamic downstream boundary


Initial Conditions

Requires an initial flow for all reaches

Restart file can be read from DSS

Enter steady-flow at upstream boundary

Can add a flow-change location

Pool elevation for storage areas


File and Options Menus


Unsteady Flow Simulation Simulation Manager

1. Define a Plan

2. Select which programs to run

3. Enter a starting and ending date and time

4. Set the computation settings

5. Press the Compute button


Output Selection

Unsteady Flow Output Stage and Flow

Hydrographs Log File Output

Post Processor Detailed output

– Max Stage– Selected Time Intervals


Stage and Flow Hydrographs User Selected Locations


Viewing Unsteady Flow Results

All of the output that was available for steady flow computations is available for unsteady flow (cross sections, profile, and 3D plots and tables).

Stage and flow hydrographs Time series tables Animation of cross section, profile and 3-

dimensional graphic


Stage and Flow Plot

Stage


Unsteady Flow Rating Curve


Log File Output

can be generated during computations information about progression of

simulation can make a large, large file

are you sure you want to open it?


Post Processor

Can be run after the unsteady simulation is completed

Provides profiles for the maximum stage and at regular intervals

All regular graphics and tables can be used to view the post process results

Graphics can “animate” the simulation


Profile Animation


Accuracy/Stability/SensitivityObjective

For students to have a better understanding of model accuracy, stability, and sensitivity.

To become familiar with the available parameters within HEC-RAS that will allow you to develop a stable and accurate model.

To learn how to detect, find, and fix model stability problems.


Overview

Model Accuracy Model Stability Factors Affecting Accuracy and Stability

Cross section spacing Computational time step selection Practical delta t, hydrograph rise time / 20

Common Stability Problems Detecting Stability Problems Model Sensitivity


Model Accuracy

Accuracy can be defined as the degree of closeness of the numerical solution to the true solution.

Accuracy depends upon the following: Assumptions and limitations of the model (i.e. one

dimensional model, subcritical flow only for unsteady flow) Accuracy of the geometric Data (cross sections, Manning’s

n values, bridges, culverts, etc…) Accuracy of the flow data and boundary conditions Numerical Accuracy of the solution scheme


Numerical Accuracy

If we assume that the 1-dimensional unsteady flow equations are a true representation of flow moving through a river system, then only an analytical solution of these equations will yield an exact solution.

Finite difference solutions are approximate. An exact solution of the equations is not

feasible for complex river systems, so HEC-RAS uses a finite difference scheme.


Model Stability

An unstable numerical model is one for which certain types of numerical errors grow to the extent at which the solution begins to oscillate, or the errors become so large that the computations can not continue.


Factors Affecting Model Stability and Numerical Accuracy

Cross Section Spacing Computation time step Theta weighting factor Solution iterations Solution tolerances


Calculation Options and Tolerances


Cross Section Spacing

Cross sections should be placed at representative locations to describe the changes in geometry.

Additional cross sections should be added at locations where changes occur in discharge, slope, velocity, and roughness.

Cross sections must also be added at levees, bridges, culverts, and other structures.


Cross Section Spacing - Slope

Bed slope plays an important role in cross section spacing. Steeper slopes require more cross sections Streams flowing at high velocities may

require cross sections on the order of 100 feet or less.

Larger uniform rivers with flat slopes may only require cross sections on the order of 1000 ft or more.


Cross Section Spacing - How do you know if you have enough XS:

Use the HEC-RAS cross section interpolation.

Make a new plan and run the model.

Compare the before and after.

If no significant difference, then OK!


Theta Weighting Factor

Theta is a weighting applied to the finite difference approximations when solving the unsteady flow equations.

Theoretically Theta can vary from 0.5 to 1.0. However a practical limit is from 0.6 to 1.0

Theta of 1.0 provides the most stability. Theta of 0.6 provides the most accuracy.

The default in HEC-RAS is 1.0. Once you have your model developed, reduce theta towards 0.6, as long as the model stays stable.


Common Stability Problems Too large of a time step. Not enough cross sections Model goes to critical depth – RAS is limited

to subcritcal flow for unsteady flow simulations

Bad downstream boundary condition (i.e. rating curve or slope for normal depth)

Bad cross section properties, commonly caused by: levee options, ineffective flow areas, Manning’s n values, etc..


Common Stability Problems - Continued

Cross section properties that do not go high enough, or are way to high (curves are spread to far apart).

Bad bridge/culvert family of rating curves. Wide and flat lateral weirs/spillways – send

to much flow over a given time step. Gated spillways that are opened or closed to

fast.


Detecting Stability Problems

How do you know you have a stability problem? Program completely blows up during run Program goes to maximum number of

iterations for several time steps in a row. Program has oscillations in the computed

stage and flow hydrographs


Detecting Stability Problems - Continued

What do you do when this happens? Note the simulation time when the program either blew

up or first started to oscillate. Turn on the “Detailed Output for Debugging” option and

re-run the program. View the text file that contains the detailed log output of

the computations. Locate the simulation output at the simulation time when the solution first started to go bad.

Find the river station locations that did not meet the solution tolerances. Then check the data in this general area.


Turning on Detailed Output for Debugging


Viewing Detailed Log Output


Model Sensitivity Numerical sensitivity:

Computation time step – try a smaller value to see if the output changes significantly.

Theta – start at 1.0, after you have a working model then try to reduce it towards 0.6.

Weir/Spillway stability factors – if you are using stability factors, try to reduce them to the lowest value you can get away with.

Weir/Spillway exponential decay factors – in general I would leave them alone, they will not effect the sensitivity of the output much.


Model Sensitivity - Continued Physical Parameter Sensitivity:

Manning’s n Values – What if the true n values were 10% higher or Lower?

Cross Section Spacing – Test by interpolating Cross Section Storage – What if there is really more or

less storage in the cross sections (I.e. ineffective flow areas, etc…)

Weir/Spillway coefficients – For lateral weirs/spillways the coefficient selected can have a great impact on the results.

Bridge/Culvert Parameters – normally only effect the locally computed stages, unless it is a flat area in which the bridge causes great backwater.

The End

feb 2003hec-ras version 3.11 slides adapted from hec unsteady flow course unsteady flow course

Documents

culvertshecras version

tableshecras version

unsteady hydraulic properties

unsteady flowslides

hydraulic structures

continuity equation

boundary conditionssteady

handling boundary friction