HEC RAS split flow lateral weir with perpendicular ditch
HEC RAS split flow lateral weir with perpendicular ditch
(OP)
I have an interesting situation and am not sure how to approach it.
I have two parallel streams that share flow in a few areas so I am using lateral weirs to account for the flows. No problem. However in one instance I have this ditch. It is located between the two streams. It is perpendicular and connected to the left reach. Generally, it is a dry ditch however for the 100 year event the right reach shares flow at this point. Most if not all of the right reach right bank Q is going to end up in that ditch. What would be the appropriate way to model this? The next cross section downstream in the right reach is also sharing flow to the left reach. I am thinking if I have a cross section both just before and after the ditch I could run the weir up to the ditch, pull the right Q from the right reach and add it to the left reach then pick up the weir again beyond the ditch.
Any brilliant ideas out there??
I have two parallel streams that share flow in a few areas so I am using lateral weirs to account for the flows. No problem. However in one instance I have this ditch. It is located between the two streams. It is perpendicular and connected to the left reach. Generally, it is a dry ditch however for the 100 year event the right reach shares flow at this point. Most if not all of the right reach right bank Q is going to end up in that ditch. What would be the appropriate way to model this? The next cross section downstream in the right reach is also sharing flow to the left reach. I am thinking if I have a cross section both just before and after the ditch I could run the weir up to the ditch, pull the right Q from the right reach and add it to the left reach then pick up the weir again beyond the ditch.
Any brilliant ideas out there??





RE: HEC RAS split flow lateral weir with perpendicular ditch
Have you evaluated the capacity of the ditch? Could the OB flow skip over it, thus ignore it?
RE: HEC RAS split flow lateral weir with perpendicular ditch
RE: HEC RAS split flow lateral weir with perpendicular ditch
I have modelled flows that would exit one channel fill up the parallel and then flow back into the main channel further down stream. I had to trigger something and I cannot for the life of me remember what it was.
Can this reach be modelled as a single reach with two thalwegs? How important is this ditch between the overbanks? Some thing you should consider when modeling.
RE: HEC RAS split flow lateral weir with perpendicular ditch
The ditch really isn't important at all. It is dry unless there is more than normal flow in the two streams. My next attempt is to delete the dip representing the ditch in the weir station and elevation chart to see if I can get the WS elevs to balance.
RE: HEC RAS split flow lateral weir with perpendicular ditch
You may want to step back and think about the overall result and what you are trying to find in terms of the big picture. Is this a floodplain study or what?
RE: HEC RAS split flow lateral weir with perpendicular ditch
Thanks for your help to date.