How to model a long steep ravine in Hydrocad

[DPAJR]

I am working on a project where I need to model a ravine with an intermittent stream at the bottom. The stream meanders with a total length of just over 1600 feet. The soil is extremely course sand with gravel and only a few tumbleweeds.

The side slopes are between 50 and 65 percent and over 120 feet high. The stream drops 168 feet before reaching a four foot diameter concrete pipe. The bottom is up to 100 feet wide in places but the stream is very small.

It seems to me that this area is too large to model as a single subcatchment. In Hydrocad you can’t have one subcatchment flow into another yet it seems like that would be a realistic way to depict this situation since the stream would be getting deeper and faster along its length. Would using a Link be appropriate for connecting subcatchments in this situation?

It also seems that combining the side slopes with the bottom as two or three areas of a single subcatchment is unrealistic since the relatively flat bottom would lower the overall Tc by averaging the slopes.

I know about the average slope equation Y=CI/A but it seems like that would be the same as combining two or three areas in a single subcatchment.

Any ideas would be appreciated.

Dave Adkins

 

[psmart]

Try several alternatives and see how the results compare...

BTW: A subcatchment doesn't contain any routing details, which is why you cannot route one subcatchment to another. However, you can use a reach to handle the channel routing. In your case, the stream could be modeled as one or more reaches end-to-end, with the contributing subcatchments entering at the junctions.


Peter Smart
HydroCAD Software

http://www.hydrocad.net