Modeling Horizontal Drop Structure in HEC-RAS

[bschaf36]

I have an inline structure with a horizontal drop structure located near the top that enters a culvert that transports flow through the structure. I am trying to model this structure to determine upstream w.s. elevation, but do not have any idea how to enter the horizontal grated inlet in HEC-RAS. Any ideas?

 

[damsafeengineer]

I'm a little confused by your description of your hydraulic structure - I think you are describing what I call a riser and conduit with the water entering through the top (rather than through a side opening), falling down to the bottom of the riser and then exiting via the conduit which goes through your structure. In that case I don't believe you can model it via HEC-RAS. RAS will model weirs and gates and culverts, but not a riser and conduit. A riser and conduit acts first like weir flow and then transitions to orifice flow as the water level rises above the riser and submerges it. That transition has to be plotted so that the point of transition can be determined. Hydraflow by Intellisolve will model this and so will HydroCAD by Applied Microcomputer Systems. If you are looking for a free program because you won't be doing this sort of thing very often, then you could go to the State of Washington's Dam Safety Program and they have a spreadsheet that you can download that will calculate this (use only for small lakes and stormwater detention). Go to:

http://www.ecy.wa.gov/programs/wr/dams/guide.html

and scroll down to drop inlets.
Once you have the stage - discharge info you should be able to set it up in RAS for downstream routing. Hope that helps you.

 

[gbam]

You could model the inlet outside of HECRAS then use a rating curve to compute the headwater in HECRAS

 

[Pete710]

I'm trying to determine the 100-year BFE (base flood elevation) for an unstudied creek. The creek drains to a detention pond which is controlled by a grated riser (inlet) where water enters the top of the inlet structure and then exits the structure through a circular culvert. Once leaving this culvert, the creek picks up again.

I'm familiar with steady flow analysis using HEC-RAS, and have modeled streams with multiple obstructions (bridges, culverts, etc.) but never with a detention pond (which I'm assuming is modeled as a "reservoir"). I can develop a stage/elevation - discharge table rating curve using other software, but where do i go from there?

 

[cvg]

route the hydrograph through the reservoir using a program such as HEC1. This will give you the maximum water surface elevation for the pond during the 100-year storm and will also give you the peak outflow. Then plug this information back into your steady state model

 

[Pete710]

CVG, thanks for the info. The problem I'm having is this:

1) Routing the basin yields that the 100-year storm overtops the basin, thus I don't have an accurate max WSEL.

2) How do I model the entire steady state model in HEC-RAS? Do I model the pond as a reservoir or just cross-sections within the model? And if a reservoir, how do I connect to the downstream?