Hi everyone. For those of you who are HEC-RAS dam breach enthusiasts, I will be teaching a 3-day course on dam breach modeling with HEC-RAS in Boston in a few weeks. The course will be held next to the historic MIT campus overlooking the Charles River and downtown Boston, May 1-3. In this course you’ll […]
Behind the scenes, HEC-RAS automatically creates a series of input and output files when working with a model. It is important to know what each of these files does and how they fit into the overall scheme of your project. Keeping track of these files in an organized manner is good practice, especially as your […]
Here’s a quick sneak preview of what’s coming in 2-D HEC-RAS. Click either figure below to see an animation of the levee breach simulation. More to come soon…
What are the implications of having a cross section that is too short and doesn’t extend all the way out to the highest computed water surface elevation? Does it affect the results? Take this cross section for example. It is missing much of the left overbank (presumably). Image courtesy of Adam Bohnoff First of all, […]
One of the limitations of HEC-RAS is that it is a one-dimensional model. Simply put, RAS assumes all flow moves along a singular dimension. For a given cross section, all of the flow is assumed to move either downstream, or all of it moves upstream, along the singular dimension (which can be defined as a […]
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 […]