Excess Rainfall Hyetograph (Pe) as input to generate runoff hydrograph

[Tradtimbo]

I know there is a way to manually input hydrographs using "links", but I'm wondering if there is a way I can manually input excess rainfall data and have HydroCAD run it through the SCS unit hydrograph to generate a runoff hydrograph. Right now I have an excess rainfall hyetograph and a time of concentration. I'm trying to get to runoff hydrograph generated from an SCS unit hydrograph. Seems like this should be possible. If something like this is done by hand I'd put the hyetograph data through the SCS unit hydrograph using the method of convolution, but my data has hundreds of steps. I also need to incorporate the hydrograph I produce into a project with typical HydroCAD subcatchments.

I hope this is clear and makes sense. I"ll be around to clarify if not. Any help is greatly appreciated.

Regards,
Tim

 

[psmart]

That's a pretty unusual approach. So you're basically trying to bypass the SCS runoff equation (which is normally used to calculate the excess) and enter the excess directly. Why not let HydroCAD calculate the excess as it normally does? Are you trying to use a different runoff equation?

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Tradtimbo]

Peter,

I appreciate the prompt response (and from you no doubt!)

I've used the SCS curve number method to tabulate initial abstraction (Ia), continual abstraction (Fa), and excess rainfall (Pe) from gross rainfall of a design storm (type III, 24hr). From this data I have added a portion of the abstraction back into the excess rainfall to account for subsurface collection via a interceptor drain at a specified depth. The determination of the portion of abstraction is based on the porosity, field capacity, initial moisture content, and depth of soil above the interceptor.

If we were not incorporating this model into a larger scheme of subcatchments and just wanted to find out the flow from the interceptor, we would calculate the water collected in the interceptor using an infiltration method (like Green-Amp or Horton's). In this case, however, we'd like to add what the interceptor collects to the excess rainfall on the surface. In this scenario they are going to the same place, so the outlet hydrograph will essentially be the addition of the hydrograph produced from the surface and the hydrograph produced by the interceptor. Since SCS CN method accounts for infiltration through abstraction, the value collected in the interceptor can be calculated. Since the surface runoff, and the intercepter runoff will both be collected and conveyed at the same point, we thought it would be appropriate to add them prior to applying the SCS unit hydrograph.

If for some reason this can't be done, I suppose an alternative would be to add the interceptor flow after the surface excess rainfall has been through the unit hydrograph. This might even be a better solution. I'd love to hear your thoughts.

Thank you for the quick reply. I'm available by phone if something isn't making sense.

Regards,
Tim

 

[psmart]

Tim - Thanks for the detail. I would tend to favor the approach of calculating the runoff and interceptor flow separately and adding them together. This approach would:
1) permit the use of different Tc values for the overland and interceptor flow (I assume the overland will be much shorter)
2) permit other "regular" subcatchments in the model that need the complete rainfall distribution, rather than just the excess.
3) make you input parameters (CN, etc) easily visible to a reviewer, and easy to modify.

BTW, if you decide to enter the Pe directly, HydroCAD can handle this by using a CN of 100, so the SCS runoff equation essentially drops-out, with all the precipitation appearing as excess.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Tradtimbo]

Peter - Thank you very much. You've made me realize the better approach. I wasn't thinking enough about the Tc values. That point brought it home. Also, I'll keep in mind the trick with CN=100 as well.

I appreciate your willingness to help. You seem to be a constant presence in this forum and that is incredibly valuable when it comes to things like this.

Enjoy your day,
Tim