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...but my Tailwater is not fixed

...but my Tailwater is not fixed

...but my Tailwater is not fixed

My pond riser outfalls into a stream channel.  I'd like to create a rating curve for the outfall channel because it influences the riser.  Adding a reach like in TR-20 doesn't seem appropriate because there's no attenuation, but this seems to be the only way.  With the old HY-8 it was pretty easy to input an irregular channel shape, slope, and manning's roughness.  Any suggestions?

RE: ...but my Tailwater is not fixed

You might look into using a link with the TIDAL option on the Elevation tab.  You would need to come up with a  peak time/stage for your receiving channel, then shape the "tide" parameters accordingly.  Not perfect, but likely approximate tailwater at the critical times.

A custom tailwater table might be a nice wishlist item.

RE: ...but my Tailwater is not fixed

I don't understand why you don't use a Custom Wetted Perimeter Reach (with appropriate slope and Manning's "n" under Profile) with a Baseflow, then a new Subcatchment for additional storm runoff inflows.

Remember: The Chinese ideogram for “crisis” is comprised of the characters for “danger” and “opportunity.”

RE: ...but my Tailwater is not fixed

Good point, Steve.  I suppose it depends on what data one has readily available.  

Sometimes a project does not warrant determining data for the entire contributing watershed to the stream reach at the outfall.  If one is familiar with the reach and can approximate wse for specific events.

Another option that came to mind is the "below headwater" option on the tailwater tab of the pond editor dialog.

I believe all these methods could be suitable depending on the project and engineering judgement.

RE: ...but my Tailwater is not fixed

The upcoming HydroCAD 7.1 release will also let you enter an arbitrary table of elevation vs. time with a link.  You can then use the link to define the pond's tailwater.

But if the tailwater is a function of the pond outflow into a channel, then using a reach is the best solution, as suggested above.  If you set a very short length (10') you'll get the desired rating curves without any significant routing effects.

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