Modeling reaches in series

[JTugwell]

I have a 440 acre watershed that I have broken up into 5 subcatchments. Each subcatchment reports to a reach, and all reaches are connected in series. I am attempting to model a long trapezoidal drainage ditch (8880 feet) to take the stormwater runoff out of the system. There is quite a bit of flow coming out of this system (260 cfs) and I am getting some strange results from the program. The only way i can get this model to "work" (I am still getting error code 62, but I think its ok) is to have the first 2 reaches 10 feet wide and the last three at 25 feet wide. The strange thing is that if I make the first two reaches wider, say 15 feet, then I start getting problems (error 63). This just doesn't make any sense to me as I would think the wider ditch would flow more water, and 25 feet seems excessively wide for the last segments. I am using the dynamic storage-indication reach routing method. Is there any explaination for this?

 

[psmart]

You need to review the warning messages in detail. Click each message for further information. Remember that a HydroCAD reach always operates under normal Manning's flow, and is not tailwater sensitive, regardless of the routing procedure. For details see

http://www.hydrocad.net/reach.htm

Although making a reach wider will reduce the depth of flow, it will increase the tailwater effect caused by the next reach downstream, hence the occurrence of additional warnings.

Basically, you will get some warnings when doing any reach-to-reach routing, and you need to review each situation in detail to determine if you need to use a river modeling system to generate a full water surface profile.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[JTugwell]

Thanks for the reply. You lost me when you said making a reach wider will increase the tailwater effect on the next reach. Could you explain that since you said that reaches are not tailwater sensitive?

 

[psmart]

For example: Reach A has a flow depth of 2 feet and flows into reach B with a flow depth of 1.5 feet. If you make reach A wider it's Normal flow depth will be reduced, perhaps to 1 foot, so the depth and WSE at the lower end of reach A is now 1/2 foot lower than the WSE in reach B. This situation is clearly incorrect, hence the warning message issued by HydroCAD.

Remember: Each reach is calculated separately without regard to tailwater conditions. So HydroCAD checks the results and warns you of any abnormal conditions that may require closer scrutiny. Messages 61, 62, and 63 indicate progressively more serious tailwater conditions.






Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[JTugwell]

Thank you for the explaination. I do have another question. I have taken the time to draw out each reach on paper showing the inlet and outlet depths. I then took the flow depths from each reach report and added them to the inlet and outlet elevations to get the water elevations for each reach. I wanted to compare the water depths at each reach to determine if there was a flooding situaion. Everything looks ok after i've drawn it on paper, however, I am now getting warning 63 for one of the reaches. It says that the depth exceeds the INLET depth of the previous reach by 0.48 feet, but I don't get the same results from my sketch. Everything would be contained under the bank full depth. Does this mean that the model would still work even with the warning? I am not familiar with HEC-RAS, so i am trying to do the best i can with this model.

 

[psmart]

According to your message 63, the WSE in the downstream reach is 0.48 feet higher than the WSE at the inlet of the upstream reach. Basically, the "upper" reach is innundated by the lower reach.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

There may be a disconnect here with the warning. I haven't seen the model, but it's possible that the peak elevations of each reach in the routing would not provoke a tailwater situation, but that some other time before or after the peak does produce a tailwater. I believe in that case HydroCAD would issue you a warning, but that the reason for the warning would not be obvious by comparing peaks.

Peter, correct me if I'm wrong about that.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[psmart]

beej67: That's possible. As you point out, the warnings indicate the time of maximum tailwater influence, which may not correspond to the peak. Like you, I would need to see the file to make an informed comment.


Peter Smart
HydroCAD Software

http://www.hydrocad.net