Using User-Defined Curves for Gates

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants
Copyright © The RAS Solution 2015.  All rights reserved.

HEC-RAS allows for four different gate types that will compute the stage-discharge relationship of that gate for you based on the physical size/shape of the gate and a few empirical coefficients:  Sluice Gates, Radial Gates, Overflow Gates (closed top) and Overflow Gates (open air).  However, sometimes the built-in gate types don’t quite fit the gate that you want to simulate.  Or perhaps you have a unique spillway that can’t be captured adequately with the simple weir equation that HEC-RAS uses.  Either way, if you can come up with your own rating curve (state-discharge relationship) for the gate or hydraulic structure, you can model it accurately in HEC-RAS using the “User Defined Curves” gate type. 

As you can see, the User Defined Curves, although not a gate “type” in the literal sense, is added as the fifth gate type that you can choose from.  To use User Defined Curves, first add a new gate group to your inline structure (can also be used with lateral structures or SA/2D area connections) by entering in some geometric properties.  When using curves, the height and width are inconsequential-they don’t matter, because they won’t be used.  Just make sure that the height is at least as high as the highest gate opening setting you will use.  The invert is important, because that is what determines at what stage HEC-RAS will begin using the rating curves.  The centerline stationing just tells HEC-RAS how many gates you will have.  Again, the actual  stationing is inconsequential, just make sure you have the correct number of stations included.  Finally, enter in the user defined curves by pressing the Enter/Edit User Defined Curves… button. 

After entering the User Defined Gate Performance Curves window for the first time, you’ll notice that you must click on the “New” button   to start a new set of curves.  Click New, then enter a name for the curves set. 

The first column, column 1, starting below the Gate Open Ht\HW entry, put in gate openings you wish to define curves for.  Each gate opening will have its own rating curve.  If you are putting in just a single rating curve (e.g. a spillway-maybe a morning glory type spillway), I always put in a zero gate opening curve in row 2 (with values of zero discharge for each headwater entry) and then the gate opening I wish to define goes in row 3.  No need for any more rows to be entered for single rating curves  The “zero” curve may or may not be necessary.  It seems to change with each new version of HEC-RAS.  Also, with a single rating curve, the gate opening value you enter in the first column really doesn’t matter.  It can be anything as long as it is equal to or less than the arbitrary gate height you defined in the gate editor AND it is the gate opening you use for the unsteady flow editor boundary condition (more on this in just a bit).  For a family of curves, you’ll want to put in a curve for a number of different gate openings, spanning the range of gate openings you plan to use in the simulation. 

Next enter in some headwater elevations (HW) in the first row.  You need at least 2 HW entries so that HEC-RAS can interpolate/extrapolate if necessary.  But the more HW values you enter, the more definition you’ll have.  The first headwater entry should be the invert of the gate (or spillway) and should get all zero values for discharge in that column.  The last headwater entry should be equal to or greater than the largest stage you expect to have in the forebay (although it doesn’t have to be, HEC-RAS will extrapolate). 

Press the Plot… button just to make sure that the curves look correct.

The final step is to go into the unsteady flow editor and set an internal boundary condition for the user defined gates.  You can select from Time Series Gate Openings (T.S. Gate Openings), Elevation Controlled Gates, Navigation Dams, or Rules).  Typically you’ll stick with one of the first two, the latter two are for advanced applications.  If you have a single rating curve, select T.S. Gate Openings and then just enter in the gate opening you have in the user defined curves for the entire simulation.  If you have a family of curves (as illustrated above), you can use T.S. Gate Openings or Elevation controlled gates, which ever suits your needs.  Elevation controlled gates allows HEC-RAS to determine how much to open the gate(s), based on the water surface elevation at a target location.  With T.S. Gate Openings, you simply tell HEC-RAS what the gate opening will be for each computation interval. 

Now you’re ready to compute.

Tuesday Tip of the Week–Axis Zooming for True Peak Discharge

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants

Copyright © The RAS Solution 2014.  All rights reserved.

Another simple, yet very convenient tool not found discussed in the manuals is the ability to zoom in on one of the axes of a graphical plot.  This is especially useful when trying to determine if you’ve captured the true peak discharge in a steeply rising hydrograph.  Take the following example: an unsteady flow dam breach output flow hydrograph.

Here we have a steeply rising flow hydrograph from a breach at the inline structure that appears to have a peak discharge of about 35,000 cfs.  To visually inspect whether or not this hydrograph captures the true peak, we can zoom in on the x axis by clicking and dragging a “bar” on the axis itself.  Simply place your cursor just below the x axis line, on the axis labels (in this plot, the time, in hours), then click and drag a red bar bracketing the portion of the hydrograph you wish to zoom in to.

Once you release the mouse button, the plot zooms in only on the x axis, in effect “spreading” out the hydrograph so that the peak is more apparent.  Notice that the full range of discharges still appear in the plot, we’ve simply truncated the time window to a more visually convenient range. 

Now notice we can see that because the hydrograph output interval is set too coarse (in this case every 10 minutes), we miss out on the true peak of the discharge hydrograph.  It happens somewhere between 1950 hours and 2000 hours.  By rerunning the model with a smaller hydrograph output interval, we can capture a better representation of the true peak discharge.  In the next figure, the discharge hydrograph is shown for the same location with a 2 minute hydrograph output interval.

After zooming in again on the x axis, we can see that we have a much better representation of the true peak of the discharge hydrograph, which is closer to 42,000 cfs, versus the 35,000 cfs shown in the first simulation:

The axis zooming feature works on any plot (input or output) and you can zoom on either the x or y axis.  So next time you’re evaluating hydrographs, or simply wish to have a better view of a curve in any of the plots, give the axis zooming feature a try.

Q & A: Flow Attenuation

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2012. All rights reserved.

Question: 
When running an unsteady flow model with a single inflow hydrograph, why does my discharge decrease in the downstream direction for a given output profile? 

Answer: 
This is called flow attenuation.  You see this to varying degrees in all unsteady flow models and it is a real phenomenon.  The shallower the reach, or the wider the floodplain, the more pronounced this effect will be.  In very steep streams, you may not notice flow attenuation at all. 
The physical process is as follows:  As the flood level rises, water moving downstream fills in available volume.  This volume is called storage.  Water going into storage is taken away from the flow going downstream and that is why you see a decrease in discharge as you move in the downstream direction.  Wider floodplains and shallower reaches have more available storage volume, which is why flow attenuation is pronounced under these situations.  Once the flood wave passes, and you are on the receding limb of the flood hydrograph, the water that had gone into storage now returns to the active discharge.  In this case you’ll see an increase in flow as you move in the downstream direction.  Notice in the figure below that the discharge at time 0042 (before the peak of the flood wave) decreases in the downstream direction, while the flow at time 0124 (after the peak of the flood wave) increases in the downstream direction.

You can also see this effect when viewing hydrographs in the figure below from two cross sections, one upstream (River Station 2500) and one downstream (River Station 2400).  The attenuation of flow is the difference in peak discharge between these two hydrographs-in this case, about 2.3 cms.  The area between these two curves represents a volume of water.  The area to the left, where the upstream discharge is greater than that downstream discharge, represents water going into storage.  The area to the right, where the upstream discharge is less than the downstream discharge, represents water leaving storage.

Attenuation is included in the conservation of mass equation, which is one of the two equations (the St. Venant equations) used to define the movement of water through a reach in HEC-RAS-the other being conservation of momentum.    From the HEC-RAS Hydraulic Reference Manual (Page 2-22), “Conservation of mass for a control volume states that the net rate of flow into the volume be equal to the rate of change of storage inside the volume.”    In other words, Inflow minus outflow equals the change in storage over time.  The equation is:

where A = flow area, Q equals discharge, t = time, and x = length. 
The discretized form of this is more practical to use and may be more familiar: 

Where I = Inflow to a discrete control volume, O = Outflow, DS = Change in Storage, Dt = time duration (i.e. time step).