storage-discharge relationship

[Crystalct1]

I am using HEC HMS to do river reach routing. I have an existing bridge and several proposed models as well. I am trying to compare the existing outflow at 100 year storm (and others) to the proposed outflow at the 100 year storm. I am using Modified Puls routing. I am trying to find out if I can use the rating curve points from HEC RAS as my elevation-flow values to use in the HEC HMS model? If so, from which cross section do I use the rating curve? Upstream of the bridge?, just inside the upstream face?, just inside the downstream face?

 

[francesca]

It was my experience that HEC-RAS is much more conservative performing storage-discharge calculations than was software designed for the purpose (specifically, Hydraflow Hydrographs 2002).

Our justification for using Hydraflow's output in preference to HEC-RAS's for culverts with pressure flow was that HEC-RAS's 1-dimensional calculations were not as accurate in this particular instance as the 3-dimensional volume calculation used by Hydraflow.

Essentially, we modeled small culverts under pressure as normal detention basins, while we retained HEC-RAS's output for bridges with large flows. I should point out, however, that we were designing the culverts and did not want to have any weir flow as the culvert was under a road. When we modeled a bridge, however, we stuck with HEC-RAS as the component of pressure flow was minimal compared to the flow overtopping the abuttments. Also, HEC-RAS's input for bridges is more detailed than that for culverts.

 

[Crystalct1]

Thanks, but that doesn't really answer my question. Maybe I didn't word it well. I have a bridge model in HEC RAS. I have several different profiles in my steady flow file. After I run the computations, I end up with a rating curve that shows a relationship between elevation and flow at different stations in my channel. Now--think of HEC HMS or even HEC1. I need a stage-volume-elevation relationship to do routing. I have my stage-volume relationship from the terrain model and the spreadsheet calculations for accumulated storage. What I am asking is: Is the rating curve I received in HEC RAS an accurate relationship between flow and elevation so that I could take points from that curve and use them as input in my HEC HMS routing model? And if so, at which station's rating curve do I take the points? If not, what's the best way to find that relationship?

 

[cvg]

yes - if your HECRAS model is done accurately...

it would be hard to find another analytical method that can beat HECRAS for bridge hydraulics.

 

[bltseattle]

I've seen the approach you say used to develop a Stage-Volume-Discharge table (F-table) for HSPF modeling. I think accuracy is going to be more tied to the spacing & number of HEC-RAS x-sects than the actual error in the numerical modeling method, so long as you are using subcritical flow when appropriate in the HEC-RAS.

IMO this is a good approach to use when measured flows are not available to correlate stage to discharge, especially if the HEC-RAS is set up carefully. If measured flows are available, be sure to use them to validate the HEC-RAS or to directly develop the relationship in HEC-HMS.

 

[Crystalct1]

So, I could use this relationship instead of using Manning's equation for open channel flow, the orifice equation, and the weir equation? I hope so, because for a natural channel bottom, those equations get real messy in terms of wetted perimeter and area. It would be way too time consuming to establish a elevation-flow relationship that way.

No one ever said if it mattered about which station's rating curve I should use. At and around the bridge station, the curve is a little different. Which station would accurately represent the relationship I need for routing in HEC HMS?

 

[cvg]

I would recommend HECRAS over your manual method which probably does not account for backwater, contraction or expansion losses, overbank flow or variable n-values