HECRAS Version 3 1 Unsteady Flow Presented by

HEC-RAS Version 3. 1 Unsteady Flow Presented by: Feb 2003 slides adapted from HEC-RASFlow Version. Course 3. 1 Unsteady 1

Unsteady Flow Documentation l l 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) Feb 2003 HEC-RAS Version 3. 1 2

When to use Unsteady Flow l l l l 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 Feb 2003 HEC-RAS Version 3. 1 3

Steady vs. Unsteady l l l 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 Feb 2003 HEC-RAS Version 3. 1 4

Unsteady Flow Equations Momentum Equation: Continuity Equation: Feb 2003 HEC-RAS Version 3. 1 5

Steady Flow Equations Energy (momentum) Equation: Continuity Equation: Feb 2003 HEC-RAS Version 3. 1 6

Distance vs Time Solution Grid l l X = distance, feet t = time, seconds 2, 2 1, 2 t 1, 1 Feb 2003 HEC-RAS Version 3. 1 x 2, 1 7

Finite Difference Term Q ----- = X Q (2, 2) - Q(1, 2) ------------ + ( 1 X Q(2, 1) - Q(1, 1) ) ----------X = theta weighting factor 0. 5 < Feb 2003 HEC-RAS Version 3. 1 < 1. 0 8

Pre-Computation of Hydraulic Properties 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) Feb 2003 HEC-RAS Version 3. 1 9

Expansion/Contraction Coeffs. • Not used in the momentum formulation (RAS-unsteady) • Should be in the data, however, for use with steady flow analysis Feb 2003 HEC-RAS Version 3. 1 10

Data Requirements (Flow and Boundary Conditions) Steady: Discharge (Q) at each cross section. Unsteady: Inflow hydrograph(s) which are routed by the model. Feb 2003 HEC-RAS Version 3. 1 11

Prepare hydrographs (boundary conditions) Upstream flows Tributary (local flows) Ungaged/unmodeled flows Downstream (rating curve? ) Feb 2003 HEC-RAS Version 3. 1 12

HEC-RAS Main Window Feb 2003 HEC-RAS Version 3. 1 13

Entering Geometric Parameters Feb 2003 HEC-RAS Version 3. 1 14

Cross Section Table Properties Feb 2003 HEC-RAS Version 3. 1 15

Pre-processing Geometry l For unsteady flow, geometry is preprocessed into tables and rating curves n Cross sections are processed into tables of area, conveyance, and storage n Bridges and culverts are processed into a family of rating curves for each structure n Weirs and gated structures are calculated on the fly during unsteady flow calculations n Pre-processor results can be viewed in graphs and tables Feb 2003 HEC-RAS Version 3. 1 16

Cross Section Properties Plot Property Table RS = 138154. 4 700 Legend Conv. Channel Elevation (ft) 690 Conv. Valley Conv. Total 680 Storage 670 660 650 0 1000 2000 3000 4000 5000 6000 7000 Conveyance/1000 (cfs) Storage (cu ft) Feb 2003 HEC-RAS Version 3. 1 17

Geometry Preprocessor l What does it do? Processes geometric data into a series of hydraulic tables and rating curves. l 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. Feb 2003 HEC-RAS Version 3. 1 18

Conveyance Calculations Manning Equation 1/2 Q = K Sf K = Conveyance Sf = friction/energy slope Feb 2003 HEC-RAS Version 3. 1 19

Conveyance Calculations Feb 2003 HEC-RAS Version 3. 1 20

Cross Section Example Feb 2003 HEC-RAS Version 3. 1 21

Geometry Preprocessor Feb 2003 HEC-RAS Version 3. 1 22

Hydraulic Property Plot Feb 2003 HEC-RAS Version 3. 1 23

Cross Section Properties Table Feb 2003 HEC-RAS Version 3. 1 24

Conveyance Subdivisions Feb 2003 HEC-RAS Version 3. 1 25

Conveyance Subdivisions Feb 2003 HEC-RAS Version 3. 1 26

Conveyance Subdivisions Feb 2003 HEC-RAS Version 3. 1 27

Conveyance Subdivisions Feb 2003 HEC-RAS Version 3. 1 28

Boundary and Initial Conditions Objectives: l Know boundary condition options l Know initial condition requirements l Sources of data for both Feb 2003 HEC-RAS Version 3. 1

Unsteady Flow Data l External Boundaries required n Upstream and Downstream ends of the river n Typically flow or stage hydrograph upstream n Typically rating or “normal depth” downstream l Internal Boundaries can be added n Add flow within the river system n Define gate operation l Initial Conditions - at the start of simulation Feb 2003 HEC-RAS Version 3. 1 30

Unsteady Flow Data Editor Feb 2003 HEC-RAS Version 3. 1 31

Boundary Conditions l l l Editor shows required external boundaries Boundary Type shows available options Upstream options: Stage Hydrograph n Flow Hydrograph n Stage & Flow Hydrograph n Feb 2003 HEC-RAS Version 3. 1 32

Boundary Conditions - continued l Downstream Boundary Options: n n n Feb 2003 Stage Hydrograph Flow Hydrograph Stage & Flow Hydrograph Rating Curve Normal Depth HEC-RAS Version 3. 1 33

Flow Hydrograph l Read from DSS Select DSS file n Select Pathname n l Enter in Table Select time interval n Select start date/time n Enter flow data - or cut & paste n Feb 2003 HEC-RAS Version 3. 1 34

Sources of Time-Series Data l Historic Records (USGS) n Stage Hydrographs n Flow Hydrographs Computed Synthetic Floods l Rainfall-runoff modeling l Peak Discharge with assumed time distribution l Others? l Feb 2003 HEC-RAS Version 3. 1 35

Normal Depth l l l Feb 2003 Enter Friction (energy) Slope Program uses Manning’s equation to compute stage Provides semi-dynamic downstream boundary HEC-RAS Version 3. 1 36

Initial Conditions l l l Feb 2003 Requires an initial flow for all reaches Restart file can be read from DSS Enter steady-flow at upstream boundary Can add a flowchange location Pool elevation for storage areas HEC-RAS Version 3. 1 37

File and Options Menus Feb 2003 HEC-RAS Version 3. 1 38

Unsteady Flow 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 Feb 2003 HEC-RAS Version 3. 1 39

Output Selection l Unsteady Flow Output n Stage and Flow Hydrographs n Log File Output l Post Processor n Detailed output – Max Stage – Selected Time Intervals Feb 2003 HEC-RAS Version 3. 1 40

Stage and Flow Hydrographs User Selected Locations Feb 2003 HEC-RAS Version 3. 1 41

Viewing Unsteady Flow Results All of the output that was available for steady flow computations is available for unsteady flow (cross sections, profile, and 3 D plots and tables). l Stage and flow hydrographs l Time series tables l Animation of cross section, profile and 3 dimensional graphic l Feb 2003 HEC-RAS Version 3. 1 42

Stage and Flow Plot Stage Feb 2003 HEC-RAS Version 3. 1 43

Unsteady Flow Rating Curve Feb 2003 HEC-RAS Version 3. 1 44

Log File Output can be generated during computations l information about progression of simulation l can make a large, large file l n are Feb 2003 you sure you want to open it? HEC-RAS Version 3. 1 45

Post Processor Can be run after the unsteady simulation is completed l Provides profiles for the maximum stage and at regular intervals l All regular graphics and tables can be used to view the post process results l Graphics can “animate” the simulation l Feb 2003 HEC-RAS Version 3. 1 46

Profile Animation Feb 2003 HEC-RAS Version 3. 1 47

Accuracy/Stability/Sensitivity Objective For students to have a better understanding of model accuracy, stability, and sensitivity. l To become familiar with the available parameters within HEC-RAS that will allow you to develop a stable and accurate model. l To learn how to detect, find, and fix model stability problems. l Feb 2003 HEC-RAS Version 3. 1 48

Overview l l l Model Accuracy Model Stability Factors Affecting Accuracy and Stability Cross section spacing n Computational time step selection n Practical delta t, hydrograph rise time / 20 n l l l Common Stability Problems Detecting Stability Problems Model Sensitivity Feb 2003 HEC-RAS Version 3. 1 49

Model Accuracy can be defined as the degree of closeness of the numerical solution to the true solution. l Accuracy depends upon the following: Assumptions and limitations of the model (i. e. one dimensional model, subcritical flow only for unsteady flow) n Accuracy of the geometric Data (cross sections, Manning’s n values, bridges, culverts, etc…) n Accuracy of the flow data and boundary conditions n Numerical Accuracy of the solution scheme n Feb 2003 HEC-RAS Version 3. 1 50

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. l Finite difference solutions are approximate. l An exact solution of the equations is not feasible for complex river systems, so HECRAS uses a finite difference scheme. l Feb 2003 HEC-RAS Version 3. 1 51

Model Stability l 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. Feb 2003 HEC-RAS Version 3. 1 52

Feb 2003 HEC-RAS Version 3. 1 53

Factors Affecting Model Stability and Numerical Accuracy Cross Section Spacing l Computation time step l Theta weighting factor l Solution iterations l Solution tolerances l Feb 2003 HEC-RAS Version 3. 1 54

Calculation Options and Tolerances Feb 2003 HEC-RAS Version 3. 1 55

Cross Section Spacing Cross sections should be placed at representative locations to describe the changes in geometry. l Additional cross sections should be added at locations where changes occur in discharge, slope, velocity, and roughness. l Cross sections must also be added at levees, bridges, culverts, and other Feb 2003 HEC-RAS Version 3. 1 56 structures. l

Cross Section Spacing - Slope l Bed slope plays an important role in cross section spacing. n Steeper slopes require more cross sections n Streams flowing at high velocities may require cross sections on the order of 100 feet or less. n Larger uniform rivers with flat slopes may only require cross sections on the order of 1000 ft or more. Feb 2003 HEC-RAS Version 3. 1 57

Cross Section Spacing - How do you know if you have enough XS: l Use the HEC-RAS cross section interpolation. l Make a new plan and run the model. l Compare the before and after. l If no significant difference, then OK! Feb 2003 HEC-RAS Version 3. 1 58

Theta Weighting Factor l l 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. Feb 2003 HEC-RAS Version 3. 1 59

Common Stability Problems Too large of a time step. l Not enough cross sections l Model goes to critical depth – RAS is limited to subcritcal flow for unsteady flow simulations l Bad downstream boundary condition (i. e. rating curve or slope for normal depth) l Bad cross section properties, commonly caused by: levee options, ineffective flow areas, Manning’s n values, etc. . Feb 2003 HEC-RAS Version 3. 1 60 l

Common Stability Problems Continued Cross section properties that do not go high enough, or are way to high (curves are spread to far apart). l Bad bridge/culvert family of rating curves. l Wide and flat lateral weirs/spillways – send to much flow over a given time step. l Gated spillways that are opened or closed to fast. l Feb 2003 HEC-RAS Version 3. 1 61

Detecting Stability Problems l How do you know you have a stability problem? n Program completely blows up during run n Program goes to maximum number of iterations for several time steps in a row. n Program has oscillations in the computed stage and flow hydrographs Feb 2003 HEC-RAS Version 3. 1 62

Detecting Stability Problems Continued l What do you do when this happens? Note the simulation time when the program either blew up or first started to oscillate. n Turn on the “Detailed Output for Debugging” option and re-run the program. n 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. n Find the river station locations that did not meet the solution tolerances. Then check the data in this general area. n Feb 2003 HEC-RAS Version 3. 1 63

Turning on Detailed Output for Debugging Feb 2003 HEC-RAS Version 3. 1 64

Viewing Detailed Log Output Feb 2003 HEC-RAS Version 3. 1 65

Model Sensitivity l Numerical sensitivity: n Computation time step – try a smaller value to see if the output changes significantly. n Theta – start at 1. 0, after you have a working model then try to reduce it towards 0. 6. n Weir/Spillway stability factors – if you are using stability factors, try to reduce them to the lowest value you can get away with. n Weir/Spillway exponential decay factors – in general I would leave them alone, they will not effect the sensitivity of the output much. Feb 2003 HEC-RAS Version 3. 1 66

Model Sensitivity - Continued l Physical Parameter Sensitivity: n n n 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. Feb 2003 HEC-RAS Version 3. 1 67

The End Feb 2003 HEC-RAS Version 3. 1