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 […]
I was recently asked my opinion on a good way to model the following dam breach event: “I …recently completed three consulting projects where I simulated the breach of three proposed coal slurry impoundments. The permitting agency required a RAS model of an “instantaneous” hypothetical breach (over full depth, almost 80 ft for one of […]
In modeling irrigation canals in HEC-RAS, a typical structure that is encountered is a check gate. Check gates are designed to back the water up behind them in an effort to keep the water level immediately upstream of these gates at a constant level. As the flow rates change in the canal, the gate openings […]
An advantage of running a step-down scheme hotstart run is the ability to spatially evaluate stability issues with difficult reaches. One of the elemental features of the step-down scheme is the artificially raising of the downstream boundary during the hotstart simulation to “drown-out” the reach and effectively create a very stable environment. During the hotstart […]
In the current version of HEC-RAS (v 4.1.0) there are two methods of modeling the hydraulics at a junction for unsteady flow. By default RAS selects the Force Equal WS Elevations (Forced) method, which forces the upstream bounding cross-sections’ water surface equal to the downstream water surface. This method may be adequate for some situations […]
By using the Mixed Flow Regime option for Unsteady Flow Analysis, RAS can better handle transitions from subcritical to supercritical flow. This option should be utilized only after determining that a mixed flow situation exists, which requires judgment from the modeler. One application where this could be particularly useful is dam breach modeling, or any […]
I’ve mentioned this a lot in this blog, but I’m finding more and more that a good, solid definition of your HTab parameters up front will go a long way in helping construct a nice stable unsteady flow model in HEC-RAS. First, let me recap how they work. In unsteady flow, HEC-RAS will convert the […]