A very convenient way to troubleshoot instability problems with very complex models is the use of a hotstart run with a stepdown scheme. Creating a stepdown scheme hotstart plan is covered in detail here. In a previous post, I explained some problems with running the stepdown scheme when you have bridges. Here I want to […]
An off-channel storage area in HEC-RAS can be a very useful way to simulate flooding in interior areas, adjacent ponds and lakes, urban areas next to rivers, green storage, or just about any area that you expect to flood but will be better represented as ponded water versus actively conveying water. Connecting rivers to off-channel […]
So many of us know that if you experience instabilities at or near your inline structure, you should bump up the Inline Structure Flow Stability Factor from the default of 1 to the most stable value of 3. But what is really going on when you do this? Here’s an example of a RAS model […]
Just ran into an interesting phenomenon. I was running a dam breach model with fairly typical breach parameters and piping failure mode. After the peak of the breach hydrograph, there appeared a mysterious “spike” as shown in the figure below. This happens to be right at the same time the flow through the developing breach […]
Cross sections must be perpendicular to the flow lines at all locations. And they cannot intersect with each other. That is why it is common to see cross sections snap at different angles outside the main channel (we call this doglegging). The trick is to keep them from intersecting, while also staying perpendicular to flow […]
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 […]
This is a common question. It’s often desirable to have a table of timing for peak stages and flows for dam breach models. This allows you to track the arrival of the flood wave at all locations downstream of the dam. However, the typical output plots in HEC-RAS are not very helpful in providing this […]
The traditional way of changing non-permanent ineffective flow areas to permanent involves going through each cross section, one at a time, in the cross section editor, selecting Options…Ineffective Flow Areas, and then manually checking which ineffective flow areas to make permanent. This can be long and tedious if you have a lot of cross sections […]
Adding to the previous topic on the Theta Implicit Weighting Factor (Theta), this post takes an objective look at how the unsteady-flow option affects model output. Theta is a weighting factor for the spatial derivative used in solving the finite difference forms of the St. Venant equations. Adjusting Theta can improve model stability or increase […]
I’ve mentioned many times on this blog and in class about the importance of having your initial conditions flow match your first time step flow, when running an unsteady flow model. As with any “rule” in RAS modeling, there are exceptions to this, but generally speaking, if your first timestep flow is “X”, then your […]