I am working on a one-dimensional (1D) dam-break model in HEC-RAS this week. Typically, I work with 2D HEC-RAS models, so this project has been somewhat challenging. Cross-section placement is an art, and I am slightly out of practice. Anyways, I have been having issues stabilizing the model. This is not an uncommon problem for unsteady 1D HEC-RAS models. This is especially true for dam-break models. HEC-RAS “likes” gradual changes which is not the nature of a dam break. Nonetheless, I am attempting to stabilize the model. Although there are many things you can do to stabilize your HEC-RAS model, this blog post will focus on defining your HTab Parameters.
What are HTab Parameters in HEC-RAS?
First, I will briefly describe how HTab Parameters work in HEC-RAS. The program calculates hydraulic parameters such as conveyance, storage area, and flow area for each cross-section. Storage Area typically refers to the 1D Storage Area element used to represent ponds and reservoirs. However, in this case, the Storage Area parameter represents the ineffective flow area for a cross-section. HEC-RAS computes these hydraulic parameters for the main channel, the overbanks, and the entire cross-section.
The curves shown in the image above are actually comprised of paired data. When HEC-RAS runs a hydraulic model, it picks values from these curves rather than computing hydraulic parameters for each time step. The program uses linear interpolation to determine values that lie between two points. You can view the Hydraulic Property Tables by going to the main menu and clicking View then Hydraulic Property Tables.
Defining HTab Parameters
If you are having problems with your HTab Parameters in HEC-RAS, the first thing you should do is highlight the “Starting El” column then press the “Copy Invert” button. This will set your Starting Elevation as the minimum channel elevation.
Next, adjust the number of points. To do this, change the numbers in the Points (20-500) column. The gray lines in the image below are points. Please note that you can see each point (gray line) in greater detail if you zoom in. More points mean that HEC-RAS can interpolate between more values. Therefore, it is a good idea to increase the number of points when you are having stability problems. HEC-RAS version 5.0 allows up to 500 points for each cross-section. Older versions only allowed up to 100 points for each cross-section. You may be wondering whether more points will slow down your model. HEC-RAS calculates HTab Parameters once for each model run. This happens during the preprocessing part of the calculations. Therefore, it is unlikely that the HTab Curves will noticeably slow down your model.
Finally, adjust the increment values. To do this, change the numbers in the Increment column. The increment is the distance between points (gray lines). Make sure that the gray lines cover the expected maximum water surface elevation. The image above shows an example of a cross-section that does not have points that cover the water surface. Otherwise, you will get an extrapolated error message.
When you have finished entering the appropriate information, click OK.