Now…how does HEC-RAS know the meandering characteristics of the river? The ONLY effects of meandering that HEC-RAS accounts for in the computations is the varying reach lengths (LOB, Channel, and ROB) for a given cross section. Some people like to call the "0" point the channel centerline, and so all station points to the left the centerline will have negative stationing and all points the right will have positive stationing. Most people will start their stationing at 0, but you don't have to. One cross section could start its stationing at 500, and the very next one can start its stationing at -200. No…the cross sections do not have to always begin their stationing the same distance from the imaginary centerline. If this start point varies how does HEC-RAS know how the channel meanders?Īlso where are LOB and ROB downstream reach lengths measured from?
The following RAS Bloggery article will help with junctions. Where it can get tricky is at a junction.
#Hec ras deck/roadway location trial
It takes a little bit of practice to do this correctly, and most of the time some trial and error, but as long as you remain perpendicular to the flow lines and don’t intersect, you’ll have a good set of cross sections. It can be very helpful to draw these flow lines before cutting cross sections. Notice that the cross sections are drawn so that they are not only perpendicular to the main channel, but also to my perception of the flow lines in the floodplain.
The light blue lines are my impression of the flow lines through this terrain, if water were flowing appreciably in the floodplain. The brown lines represent the edge of the flood plain. In the figure below, the dark blue line represents the main channel. The trick is to keep them from intersecting, while also staying perpendicular to flow lines.
That is why it is common to see cross sections snap at different angles outside the main channel (we call this doglegging). And they cannot intersect with each other. Cross sections must be perpendicular to the flow lines at all locations.