Numerical Simulation: River Modelling
Case Study River Rhine Section: Ruhrort - Mehrum - Wesel

HEC-RAS Tutorial 1: set-up and analysis of a simple channel model

This tutorial deals with a simple channel model as preparation for more complex modelling tasks.

Basic steady model properties:

• homogeneous channel of 5000m length
• prismatic rectangular cross section of 50m width and 10m height
• known water surface elevation upstream and downstream: 4 m
• estimated channel roughness of 30 (Strickler) / 0.0333 (Manning).
• no slope
• no weir

Step 1: Basic steady HEC-RAS model

Please download the basic steady model from the course server: Extract the model to a new folder. Run the steady simulation and analyse the results by physical interpretation.

Step 2: Set-up 1st unsteady model

Please create a unsteady simulation for the basic model.

• same geometry as the steady model
• same channel roughness as the steady model
• simulation window: 2 hours with 1 minute time step
• initial conditions: no flow, constant water level of 4 m
• boundary conditions: upstream known water level h(t) = 4 m
• boundary conditions: downtream known discharge q(t) = 0 m3/s (no flow, vertical wall)

step	action/screenshot thumbnails with links to full size images		remark
1.	start HEC-RAS project form:		open the basic steady model file SimpleChannel.prj
2.	open and edit Unsteady Flow Data unsteady flow data form: stage hydrograph form: flow hydrograph form:		save the Plan in a new file with suitable name always save the file after any change select for the river station (RS) 5000 the boundary condition Stage Hydrograph  specify the stage in the table:     using a Data time interval of 1 Minute and No. Ordinates to 121 (-> 2 hours)     specify for the 1st and last Stage (m) value 4,     all other values can be interpolated by Interpolate Missing Values select for the river station (RS) 0 the boundary condition Flow Hydrograph   specify the flow in the table:     using a Data time interval of 1 Minute and No. Ordinates to 121 (-> 2 hours)     specify for the 1st and last Flow (m3/s) value 0,     all other values can be interpolated by Interpolate Missing Values     set the Initale Stage as initial conditional to 4 m
3.	perform the Unsteady Flow Analysis unsteady flow anlaysis form:		activate Geometry Preprocessor, Unsteady Flow Simulation and Post Processor set Starting Time to 00:00 set Ending Time to 02:00 set Computation Time (time step) to 1 Minute set Hydrograph Output Interval to 1 Minute keep Mapping Output Interval on a high value (e.g. 2 hours, we are not generating maps) set Detailed Output Interval to 1 Minute save the Plan in a new file with suitable name finally run the simulation by the Compute button
4.	analyse the results hydrograph diagram: profile plot: general profile: cross section table profile table:		use Stage and Flow Hydrographs to analyse the h(t) and Q(t) time series upstream and downstream use Water Surfave Profile to analyse the water surface in a longitudal profile h(x) use General Profile to analyse the result values R in a longitudal profile R(x) use Detailed Output Tables to get access to all relevant numbers for cross sections use Detailed Output Tables to get access to all simulation numbers in time and space
5.	initiate a wave stage hydrograph: profile plot: hydrograph diagram:		change the Stage Hydrograph upstream: increase the water surface elevation after 2 min from 4 m to 5 m analyse the results (step 4.) and specify the travel time of the wave from the result numbers compare your result analytic insight from hydraulics the speed of a gravity wave in shallow water is sqrt(g*h) the time to run 5000 m is 5000 / sqrt(g*h) for 4 m this is 5000 / sqrt(9.81*4) = 798 sec = 13,3 min

Step 3: Variation of the basic unsteady model

Please adapt your model with a variation of parameters.

• change the upstream water surface elevation from 4m/5m to 8m/9m (1m increase on 8m)
  this requires the change of the Stage Hydrograph and the Initale Stage value of the Flow Hydrograph
  and analyse the impact of the new water level (8m instead of 4m )
  to the travel time in the channel
• change the cross section width 50 m to other values (e.g. 20 m or 100 m)
  and analyse the impact to the results (e.g. hydrograph diagram)
• change the calculation time step (1 sec, 10 sec, 5 min, 30 min)
  and analyse the impact to the results (e.g. hydrograph diagram) using physical/numerical insight
• change the cross section distance to 50 m and 1000 m (XS interpolation)
  and analyse the impact to the results (e.g. hydrograph diagram) using physical/numerical insight

• change the discharge at the boundary to introduce a basic velocity
• introduce a slope for the channel
• fix the water level and introduce a discharge time series
• change the bed resistance values