Calculating Velocity in Cross Section

[RRWeare]

I have cross sectional geometry in the form of x-y station-elevation data as well as a rating curve for the cross section.

Therefore, for a flow that I am interested, I can determine the depth of flow in the cross section.

I need to divide the cross section into three divisions and determine the average velocity in each section.

I don't have enough information to build a RAS model, is there another simple program or method to find these velocities?

Thanks for your help!

Rich

 

[froude]

RRWeare,

Is there a particular reason you need the flow divided into the 3 sections? If you're looking for another simple program, FlowMaster (Bentley Systems International formerly Haestad) will do the job. However, I am not positive if a trial or free version of this is available or not.

I believe this program uses manning's equation to solve. Have you considered using your normal depth and substituting it into Manning's Equation?

 

[Drew08]

Are the sections you refer to vertically stacked, horizontally across, or irregular?

If they are horizontal, you can use HEC-RAS by defining the boundaries of the three sections with bank stations. I disagree you don't have enough information. You have cross-sectional area (A), depth (d), and flow rate (Q); and missing only slope (S) and roughness (perhaps you even have one of those).

But you know: S^(1/2)= Qn/[aAR^(2/3)] from Manning's, so really your only missing one parameter.

Build a HEC-RAS model with two identical cross-sections at an arbitrary distance apart, say 10'. Set the downstream section at a lower elevation to achieve slope S. Vary S and n such that S^(1/2)/n = Q/[aAR^(2/3)] till you achieve the specified rating curve. Set downstream and upstream boundary conditions to the specified depth for the discharge.

You can probably guess a typical n and thus S to start at to avoid to many iterations.

If the sections are vertically stacked, HEC-RAS, or any other programs wont do you much good. Take V = Q/A to get an average velocity, and do a little research for vertical distribution of flow. There is something in almost every text book on stream hydraulics.

If the sections are irregular than make something up. It will likely be just as close to any analysis you can do.

 

[RRWeare]

The simple RAS model did the trick since I was indeed looking at vertical breaks in the section.

I hadn't thought to use the program for a simple investigations like this.

Thanks for the help!