Tuesday, July 31, 2012

How to draw cross sections.

Written by Chris Goodell, P.E., D. WRE
Copyright © 2012. All rights reserved.

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 lines.  In the figure below, the dark blue line represents the main channel.  The brown lines represent the edge of the flood plain.  The light blue lines are my impression of the flow lines through this terrain, if water were flowing appreciably in the floodplain.  The green lines are 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.  It can be very helpful to draw these flow lines before cutting cross sections. 
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. 
Where it can get tricky is at a junction.  The following RAS Bloggery article will help with junctions.