In the Viewing the Results post, we saw the neat Particle Tracing feature in RAS Mapper, which us to easily visualize the movement of water. However, if we look closely enough, things look a bit strange. The particles seem to be going straight through that road embankment (The Priority bus Route) as if it doesn’t exist.
Now we know that the Priority Bus Route does in face exist but the model does not know that because the current cell faces do not align with the embankment. The model only sees what the cell faces see, and so since the faces do not fall on top of the road embankment, it pretty much does not exist in the model. This means our model is not very accurate, which we already know.
To get the cell faces to align with the the road embankment, Breaklines are used. Breaklines force the mesh to follow the user-defined line, and can be added at any time during the mesh development process. Breaklines should be added to any location that is a barrier to flow, or controls flow/direction such as levees, roads, and natural berms in the terrain. In the screenshot below, I have added (and enforced) two breaklines over the two main road embankments. Now the particle tracing and floodplain shape look very different.
First we notice that there is faster velocity through the structure opening as expected as water speeds up to squeeze through. We also see that water does not seem to be just passing through the embankment like before (expect on the left side which suggests the road is overtopping). Lastly, we notice that the floodplain upstream of the road is much larger than previous, while the downstream is smaller as water builds up behind the road.
It is obvious that Breaklines make a big impact on model and definitely help to improve its accuracy. The next questions is: where do breaklines come from and how to process them to import them into RAS. To be continued…
Mapping T&T 