HECRAS Unsteady Flow Boundary Conditions Simulation and Post

HEC-RAS Unsteady Flow ~ Boundary Conditions, Simulation and Post Processing Jon Fripp NDCSMC 2016

Module: 1 -D Boundary Conditions, Simulation and Post Processing § Boundary Conditions • What, Why, Where § Simulation • How to § Post processing • What did you run and how to look at it 2

Boundary Conditions § Must be established at all ends of the river system (As with Steady flow, the unsteady flow data file defines flow and starting conditions for the simulation) § External Boundaries required at the Upstream and Downstream ends of the river. • • • Flow Hydrograph Stage Hydrograph Flow and Stage Hydrograph Rating Curve Normal Depth § For most situations, the simulation will route an upstream flow hydrograph, using a downstream rating curve or normal depth assumption. 3

Initial Conditions A big difference with unsteady flow is that we must define the Initial Conditions at the start of simulation § Initial conditions for entire system – required • Initial Flows in reach • Storage § Read in flow and stage from a previous run (“hot start”) § Internal boundary conditions in river system – optional • • • Lateral inflow into a node Uniform lateral inflow across a reach Groundwater inflow Gate openings (Time or elevation) Observed internal stage/flow hydrograph 4

Q: How is this done? A: All from the Unsteady Flow Data editor. Unsteady Flow Data Editor on main menu …or click Unsteady Flow Data button

Unsteady Flow Data Editor The user is required to enter • Boundary conditions at all of the external boundaries of the system • Any desired internal boundary conditions • Initial flow • Storage area conditions

Boundary Conditions § Editor shows required external boundaries § Boundary Type shows available options § Upstream options: • Stage Hydrograph • Flow Hydrograph • Stage & Flow Hydrograph Inappropriate options are grayed-out

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?

Flow Hydrograph § Read from DSS • Select DSS file • Select Pathname OR § Enter in Table • Select time interval • Select start date/time • Enter flow data - or cut & paste

Enter Flow Data The user has the option of entering a flow hydrograph directly into a table. Note DATA TIME INTERVAL

Flow Min & Multiplier 1. The flow hydrograph for model boundaries can be monitored to ensure that the flow values do not drop below an input minimum value. If the value is less than the minimum, the value is set to the minimum. 2. The flow hydrograph for model boundaries can be easily modified by a constant ratio. This allows easy testing of model sensitivity to input flow values, or an analysis of model results with larger flood events.

Boundary Conditions § Downstream Boundary Options: • • • Stage Hydrograph (gage data on the stream, or tidal cycle) Flow Hydrograph (gage data converted to flow) Stage & Flow (combined observed stage and forecasted flow) Rating Curve (rating at a gauged location, or steady-flow rating) Normal Depth (average slope of stream to estimate energy slope)

Stage Hydrograph A Stage Hydrograph is typically from a stream gage or tidal cycle. (time vs stage) § Read from DSS • Select DSS file • Select Pathname § Enter in Table • Select time interval • Select start date/time • Simulation time • Fixed starting time • Can paste from spreadsheet

Rating Curve § A Downstream rating Curve Boundary Condition is typically from a gage § Enter by hand or from a DSS file § It is a single valued relationship (no loop) which may be an issue in mild gradient streams It is important to make sure that the ratering curve boundary condition is a sufficient distance downstream of the study area, such that any errors introduced by the single valued rating curve does not affect the study reach.

Normal Depth § Normal depth is typically used as a downstream boundary condition but it can also be upstream. § Enter Friction (energy) Slope § Usually the average stream slope § Program uses Manning’s equation to compute stage As with steady flow, it is important to make sure that the starting point is a sufficient distance downstream of the study area, such that any errors introduced by the normal depth assumption does not affect the study reach.

Internal Boundary Conditions § Not required § Examples: • Lateral Inflow Hydrograph • Uniform Lateral Inflow • Groundwater Interflow • Storage Area Inflow • Gate openings • Elevation controlled gates • Navigation Dams

Initial Conditions The user must also establish then initial conditions of the system at the beginning of the unsteady flow simulation Two options: 1. Initial Flow and Storage Levels 1. Restart File

Initial Conditions Data § Initial Flow data • Should be nearly steady -state conditions • Starting flow from upstream hydrograph • Flow distribution from Steady Flow profile § Initial Elevation in Storage areas • Stage from historic data • Initial early in event, dry conditions

Options Menus § § Delete an added Boundary Condition Internal RS Initial Stages Flow Minimum and Flow Ratio Table Add Observed Data

Observed (Measured) Data 1. Time Series in DSS 2. High Water Marks 3. Rating Curve (Gage) ? n o i t ra C b i l a

Save your data Just a good thing to do Note the ‘u’ in the filename

Perform the Hydraulic Computations Enter the Steady Flow Analysis window from the main menu or select the Unsteady Flow Analysis button:

Unsteady Flow Simulation Manager Define a Plan Select files Select which programs to run Enter a starting and ending date and time Set the computation settings Press the Compute button

Unsteady Flow Simulation Manager Programs to Run 1. Geometry 2. Unsteady Flow Simulation 3. Post Processing 1. 2. 3. 4. Geometric Preprocessor: Processes the geometric data into a series of hydraulic property tables Unsteady Flow Simulation: Uses the Barkau matrix solver to perform unsteady flow calculations. Can also perform unsteady sediment analysis Post Processor: Computes detailed hydraulic info per user specifications Floodplain Mapping: Computation of static flood inundation maps (Depth Grid). Requires Static Map Layer in HEC-RAS Mapper

Unsteady Flow Simulation Manager Mixed Flow – Be Careful! Only use mixed flow if you really think you have a mixed flow regime situation. This increases model instability. 25

Unsteady Flow Simulation Manager The user is required to enter a time window that defines the start and end of a simulation period. The time is entered in military style

Unsteady Flow Simulation Manager Enter the computational interval we often end up setting this time on the order of 1 to 10 minutes.

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. It has to do with computational problems that get worse until the model gives up 28

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 29

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 rerun the program (options -> Output Options). • 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.

Factors Affecting Model Stability and Numerical Accuracy § § § Solution iterations Solution tolerances Theta weighting factor Cross Section Spacing Computation time step

Calculation Options and Tolerances Weir and spillway stability factor 1. 0 is most accurate but 3. 0 is most stable. Increasing the factor provides greater damping of the flows but less accuracy. Solution Tolerance: user can set water surface (0. 02 default) and storage are elevation (0. 05 default). Larger ones can be good for most systems but can reduce stability. Smaller tolerances can make the program 32 hit the max number of iterations.

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.

Cross Section Spacing - How do you know if you have enough XS: Steeper slopes require more sections l Look for big changes in KRATIO l Rapid changes need more sections l Use the HEC-RAS cross section interpolation. l Make a new plan and run the model. l Compare the before and after. l

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 affect 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 affect the locally computed stages, unless it is a flat area in which the bridge causes great backwater.

Viewing Unsteady Flow Results All of the output that was available for steady flow computations is available for unsteady flow § Stage and flow hydrographs § Time series tables § Animation of cross section, profile and 3 dimensional graphic 37

Unsteady Flow Simulation Manager The Hydrograph Output Interval is used to define at what intervals the computed stage and flow hydrographs will be written to HEC-DSS. Therefore it must be larger than the computational interval. The Detailed Output Interval allows the user to specify specified time intervals during the simulation where profiles of WSE and flows will be written.

Stage and Flow Hydrographs User Selected Locations the program automatically specifies the locations at the beginning and end of every reach

Post Processor § The Post Processor takes the results from the UNET unsteady flow run and writes them to a HEC-DSS file § 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

Program Output Flow Chart Unsteady Simulation Log File Stage & Flow Hydrographs DSS File DSS Viewer XS Plots Post Processor Output File PF Plots PF & XS 41 Tables

Everything Available for Steady is in Unsteady Stage and Flow Plot Unsteady Flow Rating Curve Profiles data and animation Sections Stage

Unsteady modeling can be difficult Be careful May 2009 150 Unsteady Post 43

The End May 2009 150 Unsteady Post 44