Hydrocad Settings? 1

[Wayno64]

I have a unique scenario where a sub area has a mix of A,C & D soil groups. A portion of the soils are removed and redirected from the existing subarea. The remaining subarea, which is undisturbed, now has an increased CN and hence the peak and volume from the reduced area increases from the existing. It is impossible for this to occur but I cannot find the appropriate way to model it or adjust the calculation settings. Any ideas?

 

[psmart]

If I understand correctly, it sounds like the increase in CN is offsetting the reduction in area. This is quite possible, and doesn't necessarily indicate a problem with your results.

Also note that average CN values are normally rounded to the nearest whole number, which can sometimes skew the results of a pre/post comparison. If this is the case, you can add an extra decimal place to the CN value on the Settings|Units screen.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[pjhealy]

Wayno64 - this is a typical result for the situation you described. You can take area away and show a higher rate of flow because the CN is higher, and you can shorten the Tc and get a higher rate of flow. If both occur on the same subcatchment, it is a huge difference. I am working on a site now with exactly the same problem: Predevelopment has a Q=0.01 cfs. Postdevelopment (60% smaller) has a Q=0.17 cfs (17 times the predev rate!). Like me, you will have to adjust your post-development scenario to account for it - divert more area (hopefully some area with higher CN), or add detention, infiltration, etc. Good luck.