From the previous post, we saw that water was not leaving the model, causing a build up of water at the downstream end (although the downstream end is the Caroni Swamp where we expect water to be). To fix this, we add a Boundary Condition Line at the downstream end of the model.
in RAS Mapper, start editing the Geometry. Turn on Boundary Condition Lines and select it to draw a new feature. I turned on the max Depth results grid to use as a guide. Draw the Boundary Condition Line left to right, looking downstream. Ensure that the maximum extent of the floodplain is covered so that it can properly drain. I have named the line Outlet.
Now we need to set the type of Boundary Condition to apply to the newly drawn line. Save edits and close RAS Mapper. In the Unsteady Flow File, the line will be added to list of boundary conditions. We will set the boundary condition to Normal Depth, which allows the water to flow freely from the model.
Double clicking on Normal Depth allows us to set the value to 0.0001. This represents a very flat slope, which represents the flat topography of the swamp where water is not expected to move very quickly.
the Normal Depth Boundary Condition Line is ready to go! Run the model again. Interestingly, the model runs slightly faster than previous, possibly because there are less wet cells as water can now exit the model.
Now to compare results. below the previous results are on the left and the results with the Normal Depth applied are on the right. The Normal Depth shows less water in the model as we can see that the Highway is no longer submerged and the general floodplain extent is smaller.
We have successfully added a Normal Depth Boundary Condition at the downstream extent of our model. In the next step, we will be adding more detail to the 2D mesh using Breaklines. In the previous post there was a screenshot of the particle tracing showing water going through a road (as opposed to over or around it as one would expect). It would be great it that didn’t happen.
Mapping T&T 